ISSUE 16

December 2021 transformer-technology.com ISSN 2642-2689

T r a n s f o r m e r
L i fe b l o o d :
O i l s & F l u i d s
PA RT I

Interview with Javiera McGuiggan Global Business Leader at Cargill

Degradation of Insulating Liquids: Myths and Facts
Interview with Giovanni Cattani General Manager of Siemens Energy Transformers, Trento, Italy
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4 TABLE OF
CONTENTS

Index Contents

Table of
Table of Contents_04

Editors & Impressum_10

Editor’s Letter_14

Interview with Javiera McGuiggan,

Global Business Leader

at Cargill_16

Improving Routine Insulation Line-

Frequency Power Factor Test with

Measurement at 1 Hz_26 16
Q&A: Interview with Giovanni Interview with Javiera McGuiggan
Global Business Leader at Cargill
Cattani, General Manager of

Siemens Energy Transformers s.r.l.

Trento, Italy_32

Degradation of Insulating Liquids:

Myths and Facts_36

Women of Note - Melisa

Carmine-Zajc_45
Transformer Technology December 2021 5

Issue 16

26
Improving
Routine
Insulation Line-
Frequency Power
Factor Test with
Measurement
at 1 Hz

36
What methods are available
to improve the quality of
assessment of line-frequency
power/dissipation factor (LF
PF) without affecting the
Degradation of
productivity and efficiency of Insulating Liquids:
maintenance and field staff? Myths and Facts
Can this new method be easily
adapted within routine testing This article discusses
practices? This article aims to the role of natural ester
answer these questions. liquids in helping reduce
the degradation rate of

32 transformer insulation.

Q&A - Interview with Giovanni Cattani

General Manager of Siemens Energy
Transformers s.r.l. Trento, Italy
45
Women of Note:
Melisa Carmine-Zajc
6 TABLE OF
CONTENTS

Index Contents

Table of
The State of Ester Filled

Transformers in Europe’s

Transformer Market: An Overview

and Outlook_46

Condition Monitoring of

Transformers Using Oil Analysis

Data, Vital Parameters and Critical

Values_54

Importance of Proper Test

Protocols and Procedures for Ester

Liquids_64

Unique Properties of Transformer

Insulating Oils Containing Hydrogen

Donor Compounds_72

Coming in February_78
Transformer Technology December 2021 7

Issue 16

46
The State of Ester
Filled Transformers in
Europe’s Transformer
Market: An Overview
and Outlook
As the world tackles a global problem of
climate change, electrical industry is playing
an important role in the transition towards
sustainable future. This brief market
overview discusses a current status of ester
fluids adoption in the European market. 64
Importance of Proper Test
54 Protocols and Procedures
Condition Monitoring for Ester Liquids
of Transformers Using This article aims to review the differences, and
the similarities, between the liquids in terms of
Oil Analysis Data, testing and results interpretation to ensure that it is
Vital Parameters and appropriate to the dielectric liquid being assessed.

Critical Values
In her new article, Corné Dames focuses
on the types of tests to determine the
condition of the transformer, outlining
the critical values and the recommended
actions.

72
Unique Properties
of Transformer
Insulating Oils
Containing
Hydrogen Donor
Compounds
This article discusses gassing
properties of transformer insulating
oils, correlating the Hα content
of a number of gassing inhibitor
compounds with the gassing tendency
of mineral transformer oils.
8

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Transformer Technology December 2021 9

Issue 16

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10 EDITORS &
IMPRESSUM

Impressum Editors

Editor in Chief Editor in Chief

Alan M. Ross, CRL, CMRP Alan M. Ross CRL, CMRP
Transformer maintenance
Associate Editor in Chief and reliability
Corné Dames 26 years industry
Independent transformer consultant experience

Contributing Editors
Diego Robalino, PhD, PMP
IEEE Senior Member Associate Editor in Chief
Jon Trout, PE and Chair of the Technical
Electric utility Advisory Board
Alan Sbravati, ME, MBA Corné Dames
Transformer insulating materials Independent transformer
Marco Tozzi, PhD consultant
Diagnostics and asset monitoring Transformer oils
Curtus Duff 20+ years industry
Power transformer design experience

Chair of the Technical Advisory Board

Corné Dames
Independent transformer consultant
Contributing Editor
Technical Advisory Board Member Diego Robalino PhD, PMP
Edward Casserly, PhD IEEE Senior Member
Senior Scientist, Transformer oils Transformer condition
Maria Lamorey assessment and
Industrial OEM manufacturing diagnostics
20+ years industry
Graphic design experience
BE Koncept Communication Boutique

Photo Cover
Shutterstock

Sales & Marketing Technical Advisory

Kevan Sears Board Member
kevan.sears@apc.media Edward Casserly, PhD
Senior Scientist,
Sales & Marketing Americas Transformer oils
Maria Salamanca 35 years industry
maria.salamanca@transformer-technology.com experience

Sales & Marketing Mexico

Fernando Campos
fernando.campos@transformer-technology.com

Sales & Marketing Brazil Technical Advisory

Marcelo Braga Board Member
marcelo.braga@transformer-technology.com Maria Lamorey
Industrial OEM
Marketing Global manufacturing
Marin Dugandzic 35 years industry
marin.dugandzic@apc.media experience
Transformer Technology December 2021 11

Issue 16

ISSN 2642-2689 (Print)

ISSN 2642-2697 (Online)
Contributing Editor
DIGITAL Membership Jon Trout PE
Free Electric utility
14 years of experience
Transformer Technology magazine is a quarterly
magazine published by APC MEDIA LLC, 11210
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content does not represent official position of APC
MEDIA LLC. Responsibility for the content rests
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Contributing Editor
maintains the right to keep the textual and
Alan Sbravati ME, MBA
graphical documents submitted for publication.
Transformer insulating
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Copyright and reprint permission
18 years of experience
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Publisher: Contributing Editor

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1317 Winding River Trail Diagnostics and asset
Woodstock GA 30075, USA monitoring
15+ years of experience
transformer-technology.com

Contributing Editor
Curtus Duff
Power transformer design
4 years of experience
12

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Transformer Technology December 2021 13

Issue 16
14 LETTER FROM
THE EDITOR

Dear Readers,

I remember the first issue of Transformer Tech- While technology is changing rapidly to a digi-
nology: Oils & Fluids and how we were amazed talized world, you would think that the chemistry
at how quickly we were able to curate the content of oils and fluids would not change, but as they do,
that was the correct combination of quality and to adapt to new applications, one thing is certain:
quantity. For this issue, we have been over- data collection and machine learning must keep
whelmed with both the quality, but also the up. Over time we will be gathering tremendous
quantity of articles and interviews, so much so amounts of testing data that will determine the
that we are splitting Oils & Fluids into two parts, condition and thus, the life cycle of these critical
with this issue being Part 1 and in January we will assets. Making the right decision of which fluid
publish Part 2. to use will depend on the specific requirements
of the transformer, and here is where I get most
As we develop our Body of Knowledge (BoK) for excited. Since all reliability of an asset and thus
the TT Community, we look for several things for of a system in which the asset is deployed begins
Oils & Fluids: a variety of technical content about at design, the specification of the fluid must also
the different types of fluids, whether it is mineral begin at the design stage.
or vegetable, application information about how
these fluids are being used, and comparative If we are to build more reliability into the electrical
information between different types of fluids. We system, whether that be for the grid or for a data
add in an interview that was transcribed from a center or steel mill, then it stands to reason that
video-cast we did with interesting and informative the selection, testing and treatment of the fluid
subject matter experts, all in the hopes that we within the transformer is a critical decision.
inform, educate and inspire. We expect you will
find that both this December issue and the future
January issue will do just that.
If we are to build more
reliability into the
Over the past decade, I have watched as natural electrical system, whether
and synthetic esters were changing the industry.
Their unique qualities and chemistry have a grow- that be for the grid or for
ing list of applications that make for a changing a data center or steel mill,
landscape. But, not to be left behind, mineral oil
technology is also changing to adapt to unique
then it stands to reason
applications as well. What does all this change that the selection, testing
mean to the transformer fluid testing world? and treatment of the fluid
The way we extract condition information must
change, and the way we gather and evaluate the
within the transformer
data from that testing is changing as well. Change! is a critical decision.
I love that word.
Transformer Technology December 2021 15

Issue 16

At Transformer Technology we are committed to

give you, our community, actionable information
that can help you make key decisions for all stages
of the life cycle of transformers; from design, to

Alan M Ross
manufacturing, to installation, to testing and
finally to maintaining and disposal. With this
issue focusing on Oils & Fluids and with the
upcoming January 2022 Part 2 issue, we believe
we are bringing insight that educates, informs
and inspires. Enjoy!

Finally, our February issue of Transformer Tech-

nology is themed, Transformers: The Heart of the
Generation, Transmission and Distribution System.
We will also preview the upcoming IEEE PES
Conference and Exhibition coming in April in New
Orleans.

If you, or someone from your organization wishes

to contribute to the February issue and add your
voice to our BoK, please contact me at:

alan.ross@transformer-technology.com
Curator of the Community
Transformer Technology
Alan M Ross
Editor in Chief

President of EPRA
CRL, CMRP
16 FEATURE
INTERVIEW

Javiera McGuiggan
Photo: Cargill

Issue 16
Transformer Technology

a time.
the world, one
transformer at
We are changing
Global Business
Interview with Javiera McGuiggan Leader at Cargill
December 2021
17

Alternative Fluids: Making Smart Choices for a Better Future

18 FEATURE
INTERVIEW

Javiera McGuiggan is a hydraulic engineer, fluid expert and Global

Business Leader in Cargill’s bioindustrial business, passionate about the

optimal more sustainable solutions for transformer fluids. In her interview

with Transformer Technology, Javiera shares insights on the bioindustrial

market and her views on the present and future of alternative transformer

fluids as well as Cargill’s efforts to bring the optimal biodegradable

insulating solutions.
 Transformer Technology December 2021 19

Issue 16

Alan Ross: Javiera, you are the Cargill’s global

business leader for the bioindustrial market,
Using alternative
including the biodegradable FR3® fluid. Could transformer fluids can
you tell us about the department’s mission with
transformer fluids?
change the way transformer
owners do their operations,
Javiera McGuiggan: The main mission is to
take products that are either chemicals, that
the way they manage their
are not necessarily good for the world or they assets, financial returns,
are petroleum-based solutions for industrial
purposes, and change them into renewable bio-
and influence overall peace
based solutions. I run one of the product lines in of mind with grid reliability
this group, but it is a broader group, with lines in
consumer and other industries.
and safety and protection of
the environment.
AR One of the things we realized is the
importance of sustainability. Safety
and environmental health are becoming a big
It can literally change the way transformer
owners do their operations and the way they

area, but alternative transformer fluids are still manage their assets. It is influencing their overall
not widely accepted and there is need for more peace of mind with the reliability of the grid, the
and better information. I want to talk about safety and the protection of the environment.
different industry segments. We know there is a It can also strongly influence financial returns
safety issue about using FR3 fluid compared to in terms of asset management, performance
using any mineral oil. What can you tell us about quality, the cost-reduction of maintenance and
the overall cost and safety, as we see that more the asset longevity. There’s a broad set of impacts
and more people justify the cost because of a that people forget to put into the equation. That
safety issue? is why we do a lot of education on understanding
the broader picture in reliability.

JM
Photo: Shutterstock

This goes to my point that people are

not seeing the broader picture of the
impact that using this fluid can have. It’s not just
an ingredient in the transformer.
AR In reliability, we focus on the total cost
of ownership as opposed to the cost of
an asset. What is the advantage of FR3 fluid for a
20 FEATURE
INTERVIEW

total cost of ownership? Does it last longer? Is it Our fluid doesn’t. We have had transformers
less damaging? literally working for 25 years, and, when you
open them, the fluid is almost as new. Therefore,

JM There are two things that directly

impact the long-term value equation.
The main one is a property inherent to the fluid
there is much less maintenance required to
refresh and put additives in. Just even for
inspection, there should be significant changes in
related to its behavior with water. If there is maintenance.
excess water in a transformer, which is generated
by moisture because of usage and environmental
conditions, FR3 fluid will grab that water and
We have never had
take it away from the paper, the solid insulation. a reported fire in a
To put it that way, transformers end up dying
when the solid paper insulation dies. If you take
transformer using FR3®
away that moisture and the liquid takes care fluid. Even if it fails,
of it, it is protecting the paper and naturally
extending the life of the asset. Depending on the
there wouldn’t be an
size of the transformer and the type of paper explosion or a fire.
used, the paper’s life is extended by five to eight
times. That is the first advantage, something very And the advantage that surprised me the most is
basic that most people aren’t familiar with. related to understanding the possibility of your
flexible loading capability, and that is that you
Once someone told me “I hate your fluid because can run your transformers a little hotter and
when I have a mineral oil transformer, I open it then get some more power out of them. People
and I immediately know what's going on with it don't realize that a large utilities, for example, can
Photo: Shutterstock

just by looking at it.” And my response was “Yes, save huge amounts of money just by inventory
that’s because mineral oil kind of burns and it’s management. You don't have to have every size of
nasty after a couple of years. You know exactly transformer. You can skip a size and don't have to
what it's been through.” have the whole set of inventory just in case there
Transformer Technology December 2021 21

Issue 16

is a storm, or for every different unit have all environmental impact of using biodegradable
the little parts and pieces that go with it. fluids?
That way your inventory can be much more
efficiently managed. And, of course, there are
many financial benefits of using FR3 fluid. JM The beauty of the whole picture is
that using biodegradable fluids is

AR You mentioned moisture, and you

are absolutely right – the life of the
transformer is the life of the paper. I call it a
the smartest choice, not just because of safety
concerns, but also because you don’t have to
compromise. You don’t have to spend more
wicking propensity that FR3 fluid has. For those money to take care of the planet and have
among us who are not chemists – what happens all the other benefits. When you look at the
to the moisture? total cost, it is actually saving money. It is an
initial transformation, but you are not really

JM Briefly, there is a chemical reaction

with the dielectric fluid that, so to
speak, disintegrates the water and separates it
compromising, you are making the smartest
choice. This goes back to my point about
educating people who are scared and have not
into its two components. That way the water tried this before – that is one of our main goals.
doesn’t stay trapped in the transformer, but
it decomposes into oxygen and hydrogen,
becoming neutral and has no negative effects.
There is no limit on the
size of the unit when it
AR I was once involved with a company
that had six transformers located
on a river running through the city – two
comes to using FR3®
fluid. My team has even
primaries, three secondaries and one tertiary.
The city decided to turn the area into a tourist
come up with a fun
destination, and the company was asked challenge about who is
to relocate because their mineral oil-filled
transformers posed an environmental risk.
getting to work on the
It was a multi-million-dollar deal and a great biggest transformer.
publicity problem for the company. They
solved it by retrofilling the transformers with
a biodegradable fluid, and later relocating
their facility. What can you tell us about the
AR In the case of a leak, there is an
environmental risk because mineral oil
is considered a hazardous material.
22 FEATURE
INTERVIEW
 Transformer Technology December 2021 23

Issue 16

JM
quickly.
Because of the lower viscosity, mineral
oil could get to the waterbed very
then it was Brazil, etc. It is a fun challenge to see
who is getting the largest transformer, but there
is no limit. In fact, the larger the transformer, the
greater the financial benefits you get because you

AR When we speak of environmental

hazard, we mean a hazard from a
leak. And leaks in transformers are much more
are extending the life of this giant asset that you
can expect to last longer.

prevalent than a catastrophic failure. Every year

we hear of some transformer explosion and when
One of our main goals is
they blow up, they're like bombs, particularly to educate people who
bushing explosions.
are scared about using
alternative fluids on
JM Apart from the environmental issue,
another hidden cost that people
forget about is their corporate image, how the
why this is the smartest
community sees your company. I am sure it is choice.
hard to put a price on your peace of mind before
you have had an accident or before you have
had a spill or a fatal fire. Because it isn’t fire.
It is a literal explosion when a transformer fails,
AR I believe we have just dispelled the
myth about bio fluids. We have hit
many reasons why FR3 fluid is a growing choice
and it keeps burning until all the oil is gone. This for a lot of people. If somebody is looking to make
peace of mind is like insurance. It is just hard the change, especially those that are looking to
to get people to understand it until sometimes retrofill their transformers or buy transformers
it is too late. And that is part of our education with alternative fluids, how does your team help
journey, letting people understand how you them?
might be ignoring this or not seeing it because
it is inconvenient, and you are naturally scared.
People are just scared of change. Overall, it is a
natural thing for people to think “If I don't do a
JM We offer a lot of information, and we
especially help in cases of retrofills.
That is when people worry that their asset might
big change in my job, if I don’t put my neck out, fail and they want a warranty that the fluid
I shouldn't get fired” rather than have the vision won’t cause any damage. We help them every
of “I should be the one pushing for this because step of the way – we help them understand the
I want the future to be better.” current status of the transformers they want
to retrofill, recommend the best partners that
We have never had a reported fire in a we have to do the work, and we are present to
transformer using FR3 fluid. It can fail, but this do the change if they want to make sure that
wouldn’t result in explosion or fire. everything was done correctly. We perform trials,
if you have many units that you want to retrofill,

AR I would like you to address a criticism.

FR3 fluid started with smaller
transformers, and some people think that the
and you want to just start with one for your own
peace of mind. So, we grab people's hand to go
through that journey.
higher up you go in terms of the power of the
transformer, the less likely it is that FR3 fluid is
available. AR Excellent. Well, we grab hands with
you, we appreciate it, and we will be
following the journey of biodegradable fluids in
How high do you go? What size of transformer is the industry.
your limit for FR3 fluid?

JM Thank you, because this is a very good

question. We started focusing on
JM Thank you. It is a really fantastic
mission we are doing. Our main focus is
to keep growing, building plants everywhere so
distribution transformers exactly because that these biodegradable solutions are accessible
companies were driven by the fear of change. to everyone, and no one is missing out on
Distribution transformers are smaller, you change understanding what this can do for them. We are
them more frequently, less cost is involved if changing the world, one transformer at a time.
anything goes wrong, and that is why this was our (laughs)
first window into this change. But FR3 fluid can
be used in any size of transformer. There is no
limitation for the use of the fluid related to the AR That is brilliant! Thank you so much for
joining us, Javiera, and for sharing your
Photo: Shutterstock

kV rating or any other aspect of the transformers. knowledge and your enthusiasm.
So, there is no limit. My team is almost in an
internal competition about who has worked on
the biggest transformer lately – it was China, JM Thank you for inviting me.
24 TOPIC
MISSING

IT’S EASY
TO KEEP IN
THE LOOP!
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Transformer Technology December 2021 25

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26 INSULATION
CONDITION
ASSESSMENT

Improving Routine Insulation

Line-Frequency Power Factor
Test with Measurement at 1 Hz
by Diego M. Robalino
Vince Oppedisano
and Kenneth Petroff

Photo: Megger
Transformer Technology December 2021 27

Issue 16

Diego Robalino is the Business Development Kenneth Petroff is a Product Manager for Vince Oppedisano is the Transformer Product
Director – Power Transformers at Megger. Transformer Products at Megger in Valley Specialist at Megger in the Valley Forge, PA
Diego is a Senior Member of IEEE, a member Forge, PA. Ken is a graduate of Western factory. Vince has dedicated more than 35 years
of IEEE/PES Transformers Committee, a cer- Michigan University, and brings both field to the electrical testing industry. He has re-
tified Project Management Professional with experience as well as unique skills related to presented Megger at several technical and
the Project Management Institute (PMI), and the modern requirements of testing methods commercial events worldwide. He is a dedi-
the General Chairman for the IEEE/DEIS 2022 and the complex computer operations needed cated product development leader devoted
40th Electrical Insulation Conference. He is the to perform them efficiently. Ken is a product to power and instrument transformer testing
author and co-author of over 40 technical arti- quality champion with a wide understanding and diagnostics.
cles related to power, distribution, and instru- of the transformer testing challenges and
ment transformer condition assessment. needs of testing personnel in the field.
Diego received his Ph.D. in Electrical Engineer-
ing from Tennessee Technological University.

Introduction encountered situations where a

transformer or bushing with a
Line-frequency (50/60 Hz) insulation recently tested good %PF value failed
dissipation factor (DF) or power in the field shortly thereafter.
factor (PF) is commonly used in the
field for general insulation condition Then the question for most asset
assessment of substation equipment. managers is: What methods are
This common approach to measure available to improve the quality
dielectric losses in the insulation and of assessment of line-frequency
quantify as a percentage PF or DF power/dissipation factor (LF PF)
has been performed using portable without affecting the productivity
field equipment with a voltage and efficiency of maintenance and
source up to 12 kV. Measurement field staff? Can this new method be
of dielectric losses depends on easily adapted within routine testing
dielectric properties of the insulation practices?
material, insulation temperature,
geometrical design, as well as aging
and contamination that might be Line-frequency
present within the insulating medium.
Most important to consider is that the insulation dissipation
insulation system is not perfect, and factor or power factor
some losses will be identified.
(LF PF) is commonly
General limits have been established used in the field for
for line-frequency (LF) PF/DF
measured in liquid-impregnated general insulation
power and distribution transformers condition assessment
and guidelines have been given to
field users. It is important to mention of substation
that PF/DF test equipment does not equipment.
only measure %PF but also provides
a measurement of capacitance,
watt losses, and resultant current.
All of them are important for overall Line-Frequency Power
condition assessment. Factor (LF PF)

Nevertheless, field users from In a liquid-immersed

different parts of the world have transformer, LF PF is measured
28 INSULATION
CONDITION
ASSESSMENT

10%
winding-to-winding and winding-to-
ground, typically at 10 kV (or below
r_0°C r_20°C r_40°C
rated voltage of the winding under
test) and at line-frequency or at
frequencies very close to it.
As mentioned above, LF PF is
temperature-dependent which leads

% DF
1%
to an additional requirement in the
assessment process – normalization
to a base temperature value (20°C).

In the last 25+ years, the electrical

power industry has seen added value
provided by the analysis of dielectric 0%
1 10 100 1000
losses at frequencies different from
Frequency [Hz]
50 or 60 Hz (Dielectric Frequency
Response). 0°C 20°C 40°C
Figure 1.
Is DFR a viable path to help clarify Resonant frequency shift in a dielectric response at different temperatures
doubts related to LF PF measure-
ments? What are the lessons learned 10 times more sensitive to insulation Power Factor at 1 Hz
and how can this tool be used to degradation. For dielectric analysis,
support routine testing in the field? the amount of information in each For liquid-immersed systems tested
logarithmic decade (1 Hz to 10 Hz; at or close to 20°C, the frequency
Looking into the alternatives and the or 10 Hz – 100 Hz) is of similar range between 10 and 500 Hz
physical capabilities of the PF/DF test importance. displays a very linear characteristic
set, the ability to measure dielectric when losses are low. Measuring PF
losses in a spectrum of frequencies Therefore, the use of an additional at lower frequencies, the dielectric
from 1 Hz and up to 500 Hz was frequency value while performing the response encounters a frequency
implemented. Measurement in this LF PF/DF test is a practical approach range represented by high losses
spectrum of frequencies does not to leverage technical assessment and greater dispersion. The point of
imply significant addition of testing on two points within the dielectric transition is called resonant frequency
time but opens the opportunity to spectrum of the insulation under test. and it will shift depending on
also measure PF/DF at any other temperature (see Figure 1), insulation
frequency in this range as part of the materials, and level of contamination
routine LF PF test. The use of an or degradation of the insulation.

Now that the test set can measure

additional frequency
The variation of LF PF as a function
dielectric losses within this frequency value while perform- of temperature is very small as
spectrum, the traditional LF PF test
takes a major step further towards
ing the LF PF/DF compared to that observed at 1 Hz,
especially for a healthy and dry
improvement and ease for condition test is a practical specimen (<0.5% moisture in the solid
assessment. LF PF/DF is not any
more a one-point/single reference
approach to leverage insulation) immersed in good liquid
insulation (conductivity <0.37 pS/m).
that may or may not be influenced by technical assessment When the condition of the insulation
degradation or contamination in the
complex insulation system under test.
on two points changes, the thermal response of LF
PF also changes, and these changes
Between 10 and 100 Hz, the dielectric within the dielectric may be overseen when temperature
response is typically very linear
(especially at temperatures between
spectrum of the correction is not performed properly.

0 and 25°C) whereas the range insulation under test. The research work carried out in
between 1 and 10 Hz is at least liquid-immersed transformers and

1 Hz DF/PF 1 Hz DF/PF
OIP bushing insulation condition OIP transformer insulation condition
at 20°C at 20°C
As new 0.2 – 0.5 As new 0.2 – 0.75
Good 0.5 – 0.75 Good 0.75 – 1.25
Aged 0.75 – 1.25 Aged 1.25 – 2.0
Investigate >1.25 Investigate >2.0

Table 1. OIP bushings assessment for 1 Hz DF/PF at 20°C Table 2. OIP transformers assessment for 1 Hz DF/PF at 20°C
 Transformer Technology December 2021 29

Issue 16

(ITC) algorithm showing a “Good”

Insulation Measured 20°C Measured 20°C overall condition of the insulation
tested %LF PF %LF PF 1 Hz PF 1 Hz %PF (Figure 2). Without any additional
information, it would be assumed
CHG 0.52 0.39 11.7 4.25
that no additional work needs to be
CHL 0.42 0.39 10.6 3.54 performed on the transformer to
improve its lifespan.
CLG 0.45 0.35 10.6 3.56
As part of the routine test, 1 Hz PF
Table 3. Overall LF and 1Hz PF results for 1978 Tx was measured and corrected at the
same time LF PF was carried out. The
Bushing Measured 20°C 𝚫% Measured 20°C 1 Hz PF values obtained are much
tested %LF PF %LF PF LF PF at 20°C 1 Hz PF 1 Hz %PF greater than those considered in
Table 2 for service-aged transformers
H1 1.04 0.88 0.6 15.1 12.3 and the system calls for Investigation
[I] as presented in Table 3.
H2 0.72 0.63 0.34 8.77 7.12
Capacitance C1 of the HV bushings
H3 0.73 0.64 0.35 9.69 7.92
are tested at an estimated bushing
temperature of 22.5°C (as an
Table 4. HV bushings C1 capacitance LF and 1 Hz DF/PF results
average between ambient and top-oil
temperatures).
bushings using wide spectrum team completed the required repairs
dielectric frequency response testing and conducted a complete protocol Line-frequency power factor (LF
techniques is extensive. From this of routine commissioning tests PF) on all HV bushings shows
research, recommendations were including a regulatory 10 kV line- values doubling or nearly tripling
made for 1 Hz PF limits for OIP frequency (LF) power factor (PF) test. the nameplate reference values
transformers (Table 1) and OIP HV (see Table 4). These elevated values
bushings (Table 2). The overall capacitance and line- warrant investigation.
frequency power factor (LF PF)
Field Experience test at 10 kV is carried out using Again, ITC is used to normalize LF
Routine testing on a Megger’s DELTA4000 power factor and 1 Hz PF results to 20°C in Table 4.
service-aged transformer (dissipation factor) test set at a top-
oil temperature (TOT) of 30°C. Results in Table 4 lead to a decision
A 1978, 20 MVA, 69/13.09 kV, Dyn1 to replace the HV bushings. The
transformer was taken out of service The overall line-frequency PF results transformer oil was drained and the
to repair a pump in the cooling are normalized to 20°C using the HV bushings were removed from
system. The technical maintenance individual temperature correction the unit. Figure 2 shows a layer of
contamination on the bottom section
of the bushings.

Based on these findings, the owner

decided to filter the oil to ensure the
removal of all particulates.

New bushings were installed on

the HV side of the transformer, and
they tested at an estimated bushing
temperature of 24.5°C. Excellent LF
and 1 Hz PF values were obtained on
the C1 capacitance as presented in
Table 5.

With the HV bushings replaced, the

oil filtered, and under the same test
conditions, the overall LF PF test was
carried out and results show clear
improvement, as presented in Table 6.

As with LF PF values, 1 Hz PF did

Photo: Megger

Figure 2. improve but assessment indicates

HV bushing removed from the transformer – surface contamination that the unit requires further
 30 INSULATION
CONDITION
ASSESSMENT

investigation. Why would the 1 Hz PF

values still be high after HV bushings
replacement and oil filtering? Full-spectrum DFR shows that
the interwinding solid insulation
To answer this question, and based contains a typical % moisture (1.7%) times to remove contamination.
on the 1 Hz PF results, the owner for a service-aged transformer but Sampleswere taken and sent to the
of the equipment requested a the liquid insulation reports high laboratory for analysis.
definitive analysis of the insulation conductivity (11.8 pS/m).
system using Megger’s Insulation It is important to mention that
Diagnostics Analyzer IDAX300S Further internal inspection of the historical DGA data did not alarm the
to obtain a wide spectrum unit shows that tank walls and paper operator. In Duval Triangle 1, values
(1 mHz – 1 kHz) dielectric frequency insulation retained contamination fluctuated within the T1 region.
response (DFR) and determine the material in areas difficult to be
condition of the entire insulation removed. It was recommended now to Conclusions and
system inside the transformer. flush the core-coil assembly several Recommendations

Line frequency PF (DF or tangent

delta) measurements by themselves
Bushing Measured 20°C Measured 20°C may or may not reflect the true
tested %LF PF %LF PF 1 Hz PF 1 Hz %PF condition of the insulation system
inside a transformer.
H1 0.24 0.25 0.11 0.19
• It is a fundamental practice to
H2 0.24 0.25 0.14 0.18
record a benchmark signature
H3 0.24 0.25 0.1 0.12 of the dielectric condition of a
power transformer before any
Table 5. HV Bushings (new replacements) C1 capacitance LF and 1 Hz PF results maintenance work is carried out.
• Identify deviations from practical
references besides LF. 1 Hz
complements, confirms, and
Insulation Measured 20°C Measured 20°C verifies the information obtained
tested %LF PF %LF PF 1 Hz PF 1 Hz %PF by LF PF.

CHG 0.36 0.26 10.8 1.75 The use of LF DF together with 1 Hz

DF results properly corrected to 20°C
CHL 0.45 0.33 13.6 2.19
using the Individual Temperature
Correction (ITC) algorithm provides
CLG 0.51 0.34 13.7 2.29
higher sensitivity to changes in the
Table 6. Overall LF PF results after oil process and HV bushings replacement insulation system of HV equipment.

100 1.00E+02
The use of LF DF
together with 1 Hz
DF results properly
10
corrected to 20°C
using the Individual
Capacitance [pF]
% Power Factor

1.00E+01
Temperature
1
Correction (ITC)
algorithm provides
higher sensitivity
0.1 1.00E+00 to changes in the
0.01 0.1 1
Frequency
10 100 1000
insulation system of
%PF Capacitance
HV equipment.
Figure 3.
Dielectric Response after oil-processing
Transformer Technology December 2021 31

Issue 16

Figure 4.
DGA Trending - analysis in Duval triangle 1

1 Hz measurement is a valuable condition and thermal behavior of

addition to the “ROUTINE” testing PF. End-users should consider the 1 Hz measurement
procedure for insulation condition implementation of the ITC algorithm
assessment of liquid-impregnated for reliable assessment of results is a valuable addition
transformers and bushings. and proper trending over time. to the “ROUTINE”
The combined analysis of LF DF + The traditional 10 kV LF DF together testing procedure for
1 Hz DF (ITC corrected) allows for with the 1 Hz DF (ITC corrected) insulation condition
numerical condition assessment of marginally increases the testing time
new and service-aged transformers (les than one minute). The testing assessment of
and bushings as suggested by the approach presented in this paper liquid-impregnated
authors in Tables 1 and 2. The 1 Hz helps extend the life of HV and EHV
DF with ITC assessment does not assets providing sufficient support for transformers and
require trending analysis, although future sound technical and financial bushings.
it is also possible to trend this value. decisions or future investigation
Moreover, average factors used for and definitive analysis using DFR
temperature correction which are technology.
obtained from generic tables do not
represent theimmediate dielectric

[4] D.M. Robalino, R.C. Breazeal,

[3] D. Robalino, R. Alvarez, “Evaluation of Distribution
[2] I. Güner, D.M. Robalino. P. “Advances of Dielectric Class Transformers Using
Werelius, “HV and EHV bushing Frequency Response Testing Narrowband Dielectric
References condition assessment – field for HV OIP Bushings,” Frequency Response
[1] CIGRE TB 755, Transformer experience,” Proceedings of the Proceedings of the CIGRE Measurements,“ Proceedings
bushing reliability, CIGRE WG 2016 CIGRE-IEC Colloquium, Session 48, paper A2-206, of the IEEE 2020 Electrical
A2-43, 2019 Montreal, Canada, 2016 Paris 2020 Insulation Conference, 2020
32 EXPERT
Q&A

INTERVIEW WITH GENERAL MANAGER

GIOVANNI OF SIEMENS ENERGY

TRANSFORMERS S.R.L.
CATTANI TRENTO, ITALY

Photo: Siemens
Transformer Technology December 2021 33

Issue 16

Giovanni, thank you for taking the time to share your knowledge and in-
sights about Siemens Energy Transformers factory in Trento and your very
successful career in the industry.
First of all, congratulations to Siemens Energy Transformers in Trento on
100 years of history. You became General Manager in 2014. Tell us a little bit
about the Trento factory, which is one of the oldest factories, isn’t it?
Giovanni Cattani: That’s right, I became General Manager in 2014 and
together with my team I manage the business, driving the factory towards the
future. We’re a long-established factory with a 100-year tradition of delivering
high-quality transformers across the world
The factory was founded in Trento (Northern Italy) on the 3rd February 1921,
and it kept its name of S.T.E.M (Società Trentina Elettro Meccanica) until 1986.
In 2005 it became part of Siemens AG and starting from 2020, after the spin-off
of Siemens AG, we’ve been part of Siemens Energy.
We are an international company with a strong local connection with the
Trentino territory, but at the same time, we are highly oriented to foreign markets
with more than 85% of our orders coming from the export market – mainly
Europe, USA and Middle East.
Our highly motivated workers, employees, technicians and engineers are
our real asset that helped us achieve this level and thanks to them we will
successfully overcome the challenges of the new century.

Giovanni, what is the specialty focus of the Trento plant? What range of
transformers are manufactured there?
I’m proud to work GC: Our main business is the development and production of innovative
power transformer solutions for special customer demands. Nearly every
for a company that is transformer delivered out of Trento is unique and tailor made according to the
innovating, looking for customer specifications.
new technologies and The products that we manufacture are:
helping customers in • Single/three-phase transformers (generator step-up transformers, unit
the energy transition in auxiliary transformers, grid network transformers, mobile substation trans-
formers)
a more sustainable way. • Single/three-phase autotransformers
It’s a factory with strong • Railway feeder transformers
competences, local roots Our design range comprises transformers, both mineral oil as well as synthetic
and international vision and natural ester filled units, from 10 MVA to 150 MVA and up to 300 kV.
and breadth that is One of the changes we see taking place in our industry is the focus on
contributing to a better reliability engineering by design. How does Siemens Energy Trento integrate
energy distribution, all aspects of the process from design to engineering, and from engineering
to integrating production, quality control, testing and commissioning?
making a real difference GC: We know that reliability of our products is extremely important to our
in the world. customers, so we have invested a lot over the years to improve all processes.
It all starts with reliable design, which is ensured by using consolidated and
harmonized solutions that are developed by central R&D teams and subjected
to test programs. The feedback is collected before and after their applications
on dedicated platforms.
Technical risk evaluation is performed at different levels, with suitable
methodologies (such as Complexity Matrices, FMEAS, etc.) for detecting
problems before they happen. Safety and environmental aspects are also
evaluated.
Check, review and validation points are well established (according to ISO),
as well as the entire process from design, through manufacturing (including
manufacturing, engineering and quality experts) up to the final tests. Analyses
are performed, in case of failures in the factory or claims from the field, by
interdisciplinary teams (e.g. with 8D technique). Non-conformities (NC) are
constantly monitored, and statistics are maintained for feeding a continuous
improvement loop and increasing reliability.
34 EXPERT
Q&A

What are some of the biggest challenges that the global transformers
market will be facing in the years and decades ahead and what is Siemens
Energy Trento doing to overcome those challenges?
GC: One the biggest challenges will be, of course, the decarbonization, which
will play a central role in the decades to come. At Siemens Energy, there is a
clear commitment to carbon-neutral operations.
At our Trento facility, decarbonization and the carbon-footprint of our
products is seen as a step of upmost importance towards sustainability. In
2019, we obtained a certification for our systematic approach according to
the latest ISO 14067: 2018 (Annex C), and thanks to this, we can respond to
our customer’s requests ON TIME because we are able to autonomously
create a carbon-footprint register of our power transformers and monitor the
environmental impact, the CO2 emissions, without the necessity of providing
each time a third-party verification statement.
Another challenge that I see is the integration of energy generated by
renewables in the existing power grids and providing solutions for a smooth
transition to a more decentralized power supply.
We’ve been actively working on this challenge for years because we are
convinced that renewables are the right path towards a sustainable electrical
system, as the world seeks long-term economic solutions. At Trento, we started
manufacturing power transformers for renewable applications back in 1997,
and since then, we have successfully delivered about 500 units to more than 40
different countries all around the world.
Finally, the areas that are seeing growing development are digitalization
and resiliency. We are focusing on these areas together with Siemens Energy,
including the innovative solution called “Sensformer”. Thanks to this innovation,
we equipped our products with a new device developed by Siemens Energy
which allows real-time and remote monitoring of transformer parameters with
units connected to the Cloud service. Using this technology, we can collect
the data to analyze and simulate the operating condition of the transformers,
allowing our customers to make informed decisions about asset management,
predictive maintenance planning and improving operation of their electricity
network to ensure continuity of service.

You must have seen a great deal during your career with Siemens and
Siemens Energy. What would be one or two of the significant highlights of
your career thus far?
GC: I started working at the Trento factory in 1990 and for Siemens in 2005,
holding different positions.
My personal highlight was in 2012, when I started my position with Siemens
as Global Commodity Manager for grain-oriented steel, negotiating volumes
for all power transformer factories. It was a great opportunity for me to deal
with worldwide supplier base and international teams who supported me in
reaching ambitious goals.
Another significant highlight was in 2014, when I was appointed General
Manager of the Trento factory, where I started my carrier after university and
where now I see implemented and realized many of the ideas that I developed
over the years with my teams and colleagues.
I’m proud to work for a company that is innovating, looking for new technologies
and helping customers in the energy transition in a more sustainable way –
a factory with strong competences, with local roots and international vision
and breadth, which is contributing to a better energy distribution making a real
difference in the world.

How has Covid-19 affected or changed the approach you are taking regard-
ing these challenges?
 Transformer Technology December 2021 35

Issue 16

GC: As you well know, Italy was one the countries that suffered a lot in the
beginning of the outbreak of COVID-19. Nevertheless, even during the lockdown
that was imposed by the Italian Government in March 2020, our business was
classified as “essential”, so we did not close the factory. We have implemented
stringent measures to stay fully operational through the crisis.
In the past we introduced the remote inspection and FAT, and we took the
occasion to additionally improve this technology. Our clients appreciate this
huge advantage that gives them the possibility for home office inspection.
Additionally, before COVID-19, we had introduced the smart-working activities
which allowed us to adapt easily to the “new normal” and successfully deal with
this challenging situation.

How do you advocate for diversity, equity, and inclusion in your company?
GC: Siemens Energy believes that inclusion and diversity create more
opportunity for success. Regardless of gender, age, ethnic or other differences,
everyone has an equal part to play in the energizing society.
At Trento we believe that through diversity we generate power, and this is
the belief and practice supported by our headquarters. We are proud to have in
our factory employees coming from 11 different countries and some of the key
positions are headed by women, such as CFO, Head of HR, Head of Business
Administration and Head of Logistics.

Giovanni, what is your vision for the future?

GC: We have six topics that we are putting at the core, which can be translated
into real actions as a partner and driver of the green energy transition. They
include:
Digitalization: Focus on customer value from information via application on
demand and anywhere from connected grid assets in a secure manner.
Efficiency: Providing products and solutions with state-of-art technologies to
enable a highly efficient grid ensuring affordable energy.
Decarbonization: Becoming C02 neutral by building renewable and environ-
mental power grids.
Resilience: Delivering high quality products and solutions for uninterrupted
power supply, independent of the operating environment.
Innovation: Continuing to innovate together with our customers, creating a
sustainable and greener world.
People: Investing in people and their capabilities, knowledge, competence
and expertise, because our employers are the real asset of the company.

Any final thoughts or advice that you would give to young engineers just
starting their careers, thinking back to when you had just graduated from the
University of Padova?
GC: The advice that I usually give to young engineers at University conferences
that we participate in is to stay curious and have a flexible mindset to cope with
ever faster evolution of technologies and markets.
The real asset of our company are the workers, employees, technicians
and engineers. We highly believe in this and for this reason we started years
ago direct collaboration with universities promoting our field of business and
looking for new graduates willing to accept challenges and to improve their
skills directly in the factory.
Photo: Siemens

We invest a lot to make them grow with us, learning directly on the job and
preparing the base for our future.
36 NATURAL
ESTERS

Degradation of Insulating Liquids:

Myths and Facts
by Alan Sbravati

With the main focus today shifting to strengthening

Photo: Cargill, Incorporated

of the electrical grid, natural ester filled transformers

can provide advantages for both the reliability and
resilience of electrical power networks.
Transformer Technology December 2021 37

Issue 16

Insulating materials are some of How Does a Fluid Degrade?

the most critical components of a
transformer. Typically, they are divided Degradation processes are not the
into solid and liquid insulation. The life same for all insulating liquids. The
of a transformer is often defined by chain of reactions usually included
the life of the solid insulation since its in the oxidation process have several
replacement is practically non-viable. similarities when occurring in mineral
And the liquid insulation plays an oils and ester fluids. The differences
essential role in the ageing process are in the final step of the reaction,
of the solid insulation, especially the where the natural ester molecules
Alan Sbravati started his career working for cellulose-based materials (paper). – having a free electron – bond to
a transformer manufacturer, mainly develop- Although it is possible to recondition each other instead of generating non-
ing calculation and design tools for power the liquid insulation, there are aspects soluble/polar byproducts. Thus, the
transformers. After almost 9.5 years in the of its interaction with the solid insulation main effect of oxidation in natural
same company, he was the R&D&E manager that need close watching. ester is an increase of fluid viscosity,
for power transformers in Brazil and respon- which is prevented by the actuation
sible for two global R&D projects directly While most people associate the of the oxidation inhibitor additive.
related to transformer design and thermal degradation of liquid insulation with Even in a free breathing transformer,
calculation. After three years in a commer- oxidation, the wide use of sealing the increase of viscosity should
cial role, he moved back to a more techni- systems and a potential impact not exceed 10% within 10 years of
cal position at Cargill. Over the last six years of other degradation processes continuous operation.
he has been working with the development challenge this direct link.
and application of alternative insulating However, there are other degradation
liquids, especially natural ester fluid (FR3® It is a common understanding that water processes whose timeframe is much
fluid), holding the position of Global Tech- is a transformer’s worst enemy. shorter than that of oxidation. For all
nical Manager since 2018. Alan chaired the Its presence has a range of undesirable insulating liquids, the most relevant
Brazilian Standards Committee from 2012 to effects, affecting the dielectric capacity, one is the absorption of moisture
2016, prior to moving to USA. He participates and thus the reliability of the transformer, from the ambient air. Water is the
in IEC TC 14 and Cigre working groups. Cur- and leading to the degradation of main enemy of insulation systems
rently he is a member of IEEE Transformers other materials, such as the insulating because it typically leads to a
Committee, as chair of task forces and secre- paper. A rule of thumb says that the depletion of the dielectric capacity.
tary of the Insulating Fluids Subcommittee. rate of paper degradation doubles as
the moisture content in paper doubles, In a mineral oil immersed transformer,
starting at 0.5%. Thus, the degradation being fluid “hydrophobic”, moisture
rate in a transformer where the tends to accumulate in the paper
Paraphrasing Cindy moisture content is 2%, which is not insulation. While the water content
an uncommon condition for mineral oil in the paper in a new transformer is
Joseph quote “Ageing filled transformers that have been in expected to be around 0.5%, it may
service for a couple of years, will be four quickly reach values of 2-3% in a free
is just another word for times higher than that estimated by the breathing unit and after 10-15 years
Arrhenius curve. The second enemy in a sealed mineral oil unit, as water is
living”, all materials is oxygen, which leads to oxidation of a byproduct of cellulose degradation.
liquid and paper. The latest revision of A wet insulation system leads to
inside a transformer IEC 60076-07 [1] brings charts and more high power factor (and dielectric
accurate data on the influence of water dissipation factor – DDF), accelerated
start ageing and and oxygen on paper degradation. paper degradation and low dielectric
capacity.
degrading the moment Early degradation processes in a
transformer can also be identified As a rough estimation, we can
the transformer is by the laboratorial analysis of the consider an equivalent relative
insulating liquid. Generating mostly moisture content as the equilibrium
built. While much of polar byproducts, tests such as condition for the balance of moisture
Interfacial Tension (IFT) and Dielectric between the paper and the fluid.
the ageing process Dissipation Factor (DDF, also Considering moisture saturation in
associated with Power Factor – PF) the paper to be around 6.5-7%, a
can be influenced by can accurately measure the effects 2% water content would represent
of the presence of polar compounds a relative content close to 30% (2%
the transformer design in the insulating liquid. Within a range divided by 6.5%). Applying the same
of variation, these properties have no relative saturation to the insulating
and use, it cannot be impact on transformer’s performance, liquid would lead to 18 ppm, as the
being exclusively early indicators of water saturation of mineral oil at 40°C
stopped. other degradation processes. is around 60 ppm. In a free breathing
38 NATURAL
ESTERS

unit, a typical moisture content would hydrolysis reaction. In synthetic ester be higher [2], since the ester groups
be in the range of 25-30 ppm in the liquids, the ester groups are hindered are more exposed. As the long-chain
fluid, and about 3% in the paper. in the molecule structure, typically free fatty acids generated by the
leading to a lower rate of hydrolysis consumption of water are completely
In the case of synthetic ester liquids, reaction in comparison to the natural soluble in natural ester and mild (low
whose saturation point is around ester liquids. Due to the absence reactivity/corrosiveness), this fluid
1800 ppm at 40°C, the water content of double bonds in the typical “degradation” process has a positive
in the fluid could be as high as 800- molecules, the oxidation will lead to outcome of preventing the increase of
900 ppm in free breathing units. polar byproducts also affecting other water content in paper.
This would lead to very high DDF fluid properties.
values (also known as tan delta or In a recently published paper [3],
fluid power factor), and, potentially, In natural ester liquids, the rate of samples of the three fluids – mineral
to increased acidity through the hydrolysis reaction is expected to oil, synthetic ester and natural

Two years ago, over 2.5 million transformers were

estimated to have already been produced with just one
brand of natural esters. The expectation is that today
this number would have exceeded 3 million units.

Day 0 Day 1 Day 2 Day 3 Day 7 Day 14

FR3 fluid sample performance under test: ageing and oxidation

Day 0 Day 1 Day 2 Day 3 Day 7 Day 14

Mineral oil sample performance under test: ageing and oxidation

 Transformer Technology December 2021 39

Issue 16

ester – were artificially aged in the

lab under two different conditions,
both simulating free breathing
transformers. Open bottles of the
fluids were aged in hot-air
circulation ovens at 130°C (an
ambient moisture) in one case, and
in a “climatic chamber” at 80°C and
80-100% moisture content. While the
higher temperature in the first case
increased the thermal degradation/
oxidation, the lower temperature
in the climatic chamber favored
moisture absorption.

The conclusions of the study

clearly indicated that oxidation, transformers have been available Long-term Behavior
by far, is not the priority concern for years for natural ester liquids
for any of the liquids. Variations [4] [5] [6]. Conversely, there are A large service provider in the United
in other properties reached still several discussions on which States has reported in a webinar that
continuous operation limits much parameters would be relevant approximately 50% of the insulating
faster. Natural ester was, in most to indicate the degradation of liquid samples they have been
cases, the last fluid to exceed the synthetic ester liquids, with very testing are samples of FR3 fluid.
maintenance thresholds, proving limited published data. Having a Even if we were to consider that this
to be the most robust solution for standardized framework for fluid percentage may vary according to
sealed units which may get some application is a major aspect for the type of customer each service
eventual exposure to ambient air. such highly regulated markets provider focuses on, it indicates that
because this removes uncertainty the number of datapoints of fluid
Standardization Framework or unclear responsibilities when it properties is growing exponentially.
comes to transformer assessment. Some of the largest transformer
Being the most traditional solution, service providers of lab analysis in
mineral oil has the advantage of the A task force within the IEEE the U.S. have shared their databank
availability of information of its long- Transformers Committee is of fluid testing with the IEEE
term behavior and valid international performing a study, doing rounds working groups that are reviewing
standards. Nevertheless, occurrences of accelerated thermal aging of the and developing natural ester
such as the presence of corrosive three insulating liquids. Renown standards, allowing for a significant
sulfur are still causing significant laboratories and institutions are improvement of the assertiveness of
financial losses to many utilities participating in this investigation, the defined limits.
around the world, so the assumption and the results of the first round
of “no surprises” has been proven to of tests have been published [7]. The very first natural ester filled
be biased. The test results clearly indicate commercial transformer, sold by
that ester fluids can be subjected Cooper Power System (currently
Natural ester liquids were developed to a significantly higher continuous Eaton) back in 1997, remains in
in the early 1990s as the latest operating temperature in com- continuous operation in a large
generation of less-flammable liquids, parison to mineral oil. amusement park in Florida.
also known as alternative liquids.
Although synthetic ester liquids were
developed in the early 1980s, based
on market estimations, the number
of transformers produced with natural
ester liquids exceeds the number of
synthetic ester filled transformers by
approximately one order of magnitude.
Two years ago, over 2.5 million
transformers were estimated to
have already been produced with
just one brand of natural esters. The
Photo: Cargill, Incorporated

expectation is that today this number

would have exceeded 3 million units.

International standards applicable

even to higher voltage classes of
40 NATURAL
ESTERS

This 1,500 kVA, 12.47 kV – 480/277 V

transformer has remained in service
under continuous operation without
any maintenance intervention since
installation. Confirming the expected
behavior, the results of the laboratorial
analysis confirmed all properties are
still within the acceptance limits for
a sample of natural ester in a new
transformer, prior to energization.
Not bad for a transformer which is
getting close to 25 years in service.
This data is included in a paper to be
presented in 2022.

Application in Power
Transformers

Possibly due to the focal market of

the transformer manufacturer that
developed the FR3 fluid, the use of
this fluid has been developed with
more focus on distribution trans-
formers than power transformers.
Yet, the estimated number of power
transformers using this fluid is
between 30,000 and 50,000 units.
Natural ester power transformers are
in operation in almost every climatic
condition, including extremely cold
and extremely hot locations, even
remote locations, and under very
different maintenance practices.
The same good practices applied
in traditional transformers are
recommended for the same-sized
natural ester units, which may
eventually be subjected to internal
inspections, maintenance intervent-
ions and even refurbishment, only
with minor adjustments when the
duration of the intervention exceeds
two weeks of coils exposure to
ambient air.

The first ever transformer in the

voltage class of 420 kV (in Europe,
which would be similar to a 550 kV
unit in the U.S.) filled with ester used
a natural ester FR3 fluid, back in
2013. It is a 300 MVA unit, with forced
circulation of the insulating liquid
(KDAF, which would correspond to
the ODAF in a mineral oil unit). This
transformer remains in continuous
operation and the transmission
Photo: Cargill, Incorporated

system operator (TSO) already

installed additional units which are
also filled with natural ester.

Other TSOs also adopted natural

ester liquids for their transformers
Transformer Technology December 2021 41

Issue 16

in the same voltage class, including

some single-phase transformers of
200 MVA (three-phase bank of 600
MVA) and more than two dozen of
250 MVA autotransformers [8]. In the
United States, the highest voltage
class transformers filled with ester
liquids are also using natural ester,
and they are generator step-up
transformers in a large hydropower
dam. They are single-phase,
two-winding, 13.8 kV to 345 kV, 125
MVA power transformers.

The most significant advantage of

using natural ester liquids in power
transformers is the continuous drying
of the insulation system, as described
in [9] and [10]. As the moisture
migrates from the paper to the liquid,
the excessive moisture is consumed
by the hydrolysis reaction thus
keeping the moisture content of the
coils (insulating paper) in the range of
approximately 0.5 to 1%, throughout
the transformer life.

Over the years, the benefits of

continuous drying on paper
degradation rate have been widely
explored. International standards that
support the increase of the thermal
class of thermally upgraded paper
to 20°C were published almost a
decade ago [11] [12]. However, most
significantly, continuous drying for
power transformers helps avoid
deterioration of the insulating system
dielectric capacity and reduces the
need for maintenance and removing
moisture.

Careful drying of the coils in a new

transformer is performed to ensure
the required dielectric capacity for
the high voltage tests, such as
applied and induced voltage and,
especially, the impulse voltage test.
When moisture content in the
insulating paper exceeds 2%, the
probability of withstanding an
impulse test is severely reduced in
comparison to a transformer where
the moisture content is preserved at
the 0.5 – 1% range.

Conclusions

The excellent history of application

of natural ester liquids in both
distribution and large power
transformers confirms not only
42 NATURAL
ESTERS

their suitability for use, but also With the main focus today shifting to resilience is deployed from the
the effectiveness of the claimed strengthening of the electrical grid, superior loading capacity since both
advantages over traditional units. natural ester filled units can provide paper insulation and the natural
advantages for both the reliability ester itself can ensure continuous
Today, the triggers for utilities and resilience of electrical power operation at higher temperatures
to adopt natural ester liquids go networks. compared to those in traditional units.
beyond the initial motivations for
the use of alternative liquids, which While the improved reliability Simply put, the engineers adopting
were improved fire safety and results from the preservation of natural ester are seeking peace of
environmental benefits. the dielectric capacity, the higher mind.

References [8] V. Vasconcellos, A. Sbravati,

[1] IEC 60076-7 Ed 2.0, "Power L. C. Zanetta Jr., K. Rapp,
transformers - Part 7: Loading L. Lombini, S. Nazzari, F.
guide for mineral-oil-immersed Scatiggio and A. Valant,
power transformers," Technical "Increased Loadability of
Committee TC14, International Transformers Using Natural
Electrotechnical Committee, Ester and Cellulosic Materials
2018 as High Temperature Insulation
[2] S. Boyde and U. Wilton, Systems," IEEE Electrical
"Hydrolytic Stability of Insulation Magazine, pp. 8-17,
Synthetic Ester Lubricants," September/October - Vol. 34,
Journal of Synthetic No. 5 2018
Lubrication, vol. 16, no. April, [9] A. Lemm, K. Rapp and J.
pp. 297-312, 2000 Luksich, "Effect of Natural Ester
[3] A. Sbravati, K. Wirtz and L. B. (Vegetable Oil) Dielectric Fluid
d. Oliveira, "Insulating liquids on the Water Content of Aged
at free breathing conditions," Paper Insulation," in EIA/IEEE
in 2021 Electrical Insulation - 10th Insucon International
Conference (EIC), Virtual Event, Electrical Insulation
2021 Conference, Birmingham, UK,
[4] IEEE C57.147, "IEEE Guide for May 24-26, 2006
Acceptance and Maintenance [10] K. Rapp, C. P. McShane
of Natural Ester Fluids in and J. Luksich, "Interaction
Transformers," Institute of Mechanisms of Natural Ester
Electrical and Electronics Dielectric Fluid and Kraft
Engineers, Inc, New York, USA, Paper," in IEEE/DEIS 15th
2008 International Conference on
[5] IEEE C57.155, "IEEE Guide Dielectric Liquids, Coimbra,
for Interpretation of Gases Portugal, June 26-July 1, 2005
Generated in Natural Ester [11] IEEE C57.154, "IEEE Standard
and Synthetic Ester-Immersed for the Design, Testing, and
Transformers," 2014 Application of Liquid-Immersed
[6] IEC 62975 Ed 1.0, "Natural Distribution, Power, and
esters - Guidelines for Regulating Transformers Using
maintenance and use High-Temperature Insulation
in electrical equipment," Systems and Operating at
Technical Committee TC10, Elevated Temperatures,"
International Electrotechnical Institute of Electrical and
Committee, 2021 Electronics Engineers, Inc,
[7] A. Sbravati, E. Casserly, H. New York, USA, 2012
Wilhelm, P. Su, A. Levin, A. [12] IEC 60076-14 Ed 1.0, "Power
Gyore, M. A. M. Cheema, K. transformers – Part 1: Liquid-
Wirtz and N. Lukenda, "Initial immersed power transformers
investigation of a thermal using high-temperature
Photo: Cargill, Incorporated

performance qualification insulation materials," Technical

method for transformer Committee TC14, International
insulating liquids," in 2021 Electrotechnical Committee,
Electrical Insulation Conference 2013
(EIC), Virtual Event, 2021
Transformer Technology December 2021 43

Issue 16

Better
transformers.
Vibrant
communities.
Your community counts on you for power that is always safe and always

on, whatever the demand. Count on us to deliver better transformer

performance when you use FR3® fluid by Cargill, for the safest, most

reliable and most flexible grid possible, without compromises.

Count on us.

FR3fluid.com
© 2021 Cargill, Incorporated. All rights reserved.
A BETTER ENERGY FUTURE IS
A FUTURE OF COLLABORATION
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 Transformer Technology WOMEN 45

Issue 16 OF NOTE

Melissa
Carmine-Zajac

the
ine-Z ajac is
Melissa Carm oratory Services
ab
Direc tor of L eering Company
D o b le E n gin
for rate
se d at the corpo h,
and is ba in Marlborou
g
headquarters s. A self-proclaimed
tt
Massachuse elissa is passionate
n e rd ”, M
“s u p e r nd knew
th in gs science a anted
about all e w
g age that sh
from a youn st someday.
ti
to be a scien
maceutical
udying phar
Originally st elissa realized the best do
chemistr y, Me out what she wanted toa lab
way to figur rew up” was to get into ce.
when she “g and learn from experien
immediately

d
tal science an
ca re er in environmen lab and in the field.
ted her in the
Melissa star aterials, working both e was promoted to a er goal
hazardous mst year of her career, shuce her to her next care ist
Within the firrole which would introd into an analytical chemk and
supervisor y . She eventually moved excitement of fieldwor . She
- leadership kly began to miss the lenging consulting role quite
role, but quic to take on a very chal boratory, which proved st into
was inspired revamp a floundering lan, Melissa dove head-firithin
was hired to Instead of backing dow e lab rebuild project w d
a challenge. ccessfully completing thb was destroyed in a floourately
the work, su rtunately, the rebuilt la be both literally and fige
weeks. Unfo believed her career to different role within th e nex t
and Melissa . She was then offered a t in New Jersey. For th gly
under water zation at a chemical planb, becoming increasin 15
same organi e managed the plant lad plant operations. In 20re
few years, shprocess engineering angineering Company wheg
involved in led Melissa to Doble En . After proving a stron to
this journey as a Laboratory Manageran ex traordinary ability to
she star ted technical aptitude, and , she made the transition sible
work ethic, plex managerial issues rrent role, she is respon al
handle com Director in 2019. In her curies that perform critic
Labor ator y S la bo ra to th e po w er
ions of four U services for nd
for the operat d advanced diagnosticectric utility clients arouelissa
g an
lab testin e works directly with have reliable energy. M ce
el
industry. Sh suring that all people entor and the importan er
the world, en the impact of a good m n works directly with h g by
understands young talent . She ofted career growth. Leadinomen
of developingencourage learning an ency, and suppor ting w not
lab team to oting career transpar ions. When Melissa is rance
example, prom a few of her
current passg in hobbies like endu k at
Photo: Melissa Carmine-Zajac, rawpixel.com

or
in STEM are u can find her par takinrts. In addition to her w own
in the lab, yo ing, and making dese ted official in her homet Source: Melissa
spor ts, travelsa is also a publicly elec areness. Carmine-Zajac
Doble, Melis cate for mental health aw
and an advo
46 MARKET
OVERVIEW

The State of Ester Filled Transformers

in Europe’s Transformer Market:
An Overview and Outlook
by Aleena Ahmad

Technology has taken huge leaps climate change, there is an urgent

in the previous decades and the need for shift in our processes
advancements have benefitted all to make them more sustainable.
spheres of life with the biggest Electrical industry is also playing its
impact on economic development. role in this transition. Among other
In the recent years, however, there green initiatives and directives, the
has been a growing concern about development of SF6 alternatives
the environmental impact of certain in switchgear and vegetable
technological advances with effects ester oil as an insulation fluid for
becoming more and more pronounced transformers, and their slow but
Photo: Shutterstock

through rising temperatures, floods, increasing adoption by utilities and

droughts and more. As the world manufacturers, is a step in the
tackles the growing problem of right direction.
Transformer Technology December 2021 47

Issue 16

of ester oils over mineral oil as an

insulating fluid across different
parameters.

Ester has an improved environmental

footprint owing to biodegradability.
This means that the cost of the
clean-up procedure and spill
prevention mechanisms that is
associated with the use of mineral
Aleena Ahmad is is a Market Analyst at oil is considerably reduced. In case
Power Technology Research. She is involved of a spill or breakdown, ester fluid
in projects on the transformer topic at will not contaminate the surrounding
Power Technology Research and is responsi- environment as is the case with
ble for data collection and analysis in various mineral oil. Mineral oil oxidizes readily,
areas including the structure of distribution leaving behind sludge precipitates
utilities, the installed base of T&D equipment, that give rise to the need for periodic
and future market trends. As a market ana- cleaning and maintenance, but ester
lyst at PTR, she performs in depth analy- is resistant to oxidation degradation,
sis of the different technologies within the which is another added advantage.
transformer market and their impact. Prior to
joining PTR, Aleena worked at Nestle as an With mineral oil, fire safety is a
electrical and automation engineer. Aleena concern because temperature of the
comes from a technical background and has oil rises with transformer loading.
a B.Sc. in electrical engineering. Ester fluid has almost twice the

Although the percentage of installed

ester filled transformers is still low, this
number is expected to rise in the future
as countries race towards achieving their
2030 climate goals.

Ester Fluids vs. Mineral Oil

as an Insulating Fluid

Ester fluid is derived from 100% fire point making it relatively better
renewable vegetable oil or inorganic than mineral oil in that aspect. This
feedstock. It was originally developed reduced risk of fire in turn lowers the
in 1996 as an alternative to need for fire protection equipment and
polychlorinated biphenyl (PCBs) and fire safety systems. Since ester oil
high molecular weight hydrocarbons has a high fire and flash point, it can
but has gradually become the fluid of perform better in higher temperatures
choice for transformer insulation and and has better loading capabilities
an environmentally friendly alternative as well. Ability to withstand higher
to mineral oil due to its properties. temperatures means that ester has a
Table 1 compares the properties of longer life span than mineral oil. Life
natural and synthetic esters with cycle expansion in turn directly relates
mineral oil indicating the advantages to cost and maintenance optimization.
48 MARKET
OVERVIEW

efforts that are being made to make

Despite its advantages over other their operations more environmentally
sustainable and to achieve the
transformer fluids, initial capex for ester ambitious climate goals being set
out by their respective countries.
can still be up to 20% higher than for Europe has a well-developed grid
infrastructure and is in the position to
mineral oil. take the lead in this regard, which it
has been taking with numerous laws
and initiatives in place.

Cost of Ownership and transformers, these transformers For instance, UK has announced
Current Applications are also gradually replacing dry that it intends to achieve 65% clean
type transformers because of their energy by 2030 and zero carbon
Despite ester having the above enhanced fire safety and ecofriendly emissions by 2050. Similarly,
stated advantages, its initial capex properties. In addition to this, Germany plans to reduce its
can still be up to 20% higher than ester is not only being adopted for emissions by 65% by 2030 as
for mineral oil, which can be a new installations, but existing oil compared to the 1990 level and 88%
concern for companies adopting immersed transformers that have by 2040. France’s current law states
this technology. It should be noted, reached their end of life or are that it will decrease emissions by
however, that this high capital overloaded are being retro-filled with 40% by 2030 as compared to the
cost is compensated by the lower ester fluids to improve grid reliability 1990 level. Talking about the entire
maintenance costs and longer life and stability until replacements can region, EU claims that the bloc will
spans in the long term. be carried out. reduce its carbon emission by 55%
by 2030 and become climate neutral
Although most of the discussions Globally, more and more electric by 2050.
around ester filled transformers utilities are installing ester filled
focus on replacing mineral oil transformers to complement the There are several green initiatives
being adopted by European utilities
that include the decarbonization of
economy: an initiative by Endesa,
Mineral Natural Synthetic a Spanish DSO, to develop 23
Property renewable hydrogen projects; Green
Oil Ester Ester
Finance Framework by the Norwegian
DSO Elvia plans on adding new hydro
and wind power infrastructure to
Fire Point 180°C 360°C 310°C
increase renewable capacity in the
system; and Fluvius in Belgium plans
Flash Point 160°C 330°C 260°C to install EV charging infrastructure
to facilitate the penetration of zero-
emission vehicles.
Biodegradability No Ultimately Readily
Adoption of ester filled transformers
is in line with the direction that
Toxicity Toxic Non-Toxic Less Toxic
the utilities in Europe are taking to
achieve environmental sustainability.
Viscosity 12 37 29 The increase in renewable power
generation and the penetration
of electric vehicles has given rise
Thermal Aging Good Better Best to a lot of gaps in the distribution
grid with areas for improvement in
the existing infrastructure as well
Table 1. Comparison of ester oils vs mineral oil properties as potential for expansions and
additions to cater with this increased
and dynamic load. The renewable
installed capacity has grown with a
EU claims that the bloc will reduce its CAGR of 5.4% from 2015-2020.
With regards to EVs, Europe
carbon emission by 55% by 2030 and accounted for 30% of the global
electric vehicle passenger fleet which
become climate neutral by 2050. amounts to approximately 3.1 million
electric vehicles.
 Transformer Technology December 2021 49

Issue 16
Photo: Shutterstock
50 MARKET
OVERVIEW

State of Adoption
in Europe According to Power Technology Research,
Europe has always been keen in Europe, the ester filled transformer
at adopting and manufacturing
latest technologies and the same market capacity accounts for about
is being observed when it comes
to the environment friendly and 1.29% of the total installed base, with
sustainable insulation fluid for
transformers. Some of the top OEM UK, Netherlands and Spain being the top
markets for ester filled transformers
in the region are Turkey, Poland, countries adopting ester fluid.
and Switzerland. UK, Germany,
and France are some of the user
markets in the region with ester oil
transformers constituting a growing Others (Oil-immersed, Dry-type)
percentage of the total installed Ester filled transformers
base of distribution transformers.

According to Power Technology

Research, in Europe, the ester 98.70%
filled transformer market capacity
accounts for about 1.29% of
the total installed base, with UK,
Netherlands and Spain being the
top countries adopting ester fluid 1.30%
(Figure 1).

Although the percentage of installed

ester filled transformers is low, this
number is expected to rise in the
future as countries race towards Figure 1. Ester filled transformers penetration in Europe
achieving their 2030 climate goals.
UK, however, is breaking away
Indoor/underground
from the generally slow adoption
rate observed in the region with a Pole & ground mounted
comparatively higher percentage
of ester filled transformers. Some
utilities in the UK are exclusively
installing ester filled transformers 87.00%
and others are using ester to retro-
fill existing mineral oil transformers
that have reached their end of
life. Furthermore, ester filled
transformers are being installed 13.00%
in historical buildings to ensure
safety, prevent fire risk and to avoid
fire suppression costs. Another
observation is that utilities are more
inclined towards adopting synthetic
ester in comparison to natural ester Figure 2. Segmentation of ester filled transformers by installation type
since the former can be tailored to
specific needs of the customers.
Another statistic of importance
is that utilities are adopting ester Utilities are more inclined towards
oil transformers in underground
substations where the risk of adopting synthetic ester than natural ester
mineral oil fires in transformers is
higher than outdoors. 86.65% of the since the former can be tailored to specific
Photo: Shutterstock

total installed base of ester filled

transformers is installed indoors needs of the customer.
and underground.
Transformer Technology December 2021 51

Issue 16

Future Outlook

Looking ahead, according to Power

Technology Research, the penetration
of ester insulated transformers will continue
to increase, especially in Western Europe. Even
though the installed base of these transformers
is quite low at the moment with some utilities even
reporting no ester insulated transformers in their
grid, their market is expected to grow in the future.
This increase is directly tied to utilities upgrading
their infrastructure to incorporate the additional
renewable power generation and install new
charging facilities for the growing market of
electric vehicles, which will consequently
result in a rise of the installed base of
distribution transformers, especially
sustainable options like ester filled
transformers.

86.65% of the total installed base of ester

filled transformers is installed indoors and
underground.
52 TECH
TALKS

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AND KNOWLEDGE
ARE IMPORTANT
TO US

JOIN OUR
#COLLABORATIVE COMMUNITY

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Transformer Technology December 2021 53

Issue 15

Tech Talks

To watch and listen visit

www.transformer-technology.com/
community-hub/tech-talks
54 TECHNOLOGY
INSIGHTS BY
CORNÉ DAMES

Condition Monitoring
of Transformers Using Oil Analysis Data,
Vital Parameters and Critical Values
Condition monitoring is the
frequent collecting, measuring,
recording and analysis of relevant
data. If we interpret the data
correctly, it can give us great
insight into the condition of
an asset.

This frequent monitoring of a

transformer can result in less
maintenance required, or more
extended periods without any
maintenance required at all.

It is crucial to identify the key

parameters that are needed to
give us a complete picture of the
actual status of the transformer
and the actions we need to take
to ensure the continued reliability
of the transformer to achieve
and maximize its life cycle.
Photo: Shutterstock
Transformer Technology December 2021 55

Issue 16

It is vitally important to identify

clear goals as part of your
transformer monitoring strategy.

Corné Dames is the Managing Director of

Independent Transformer Consultants, al-
ways striving to keep on top of new devel-
opments and research. She has expertise
as Laboratory Manager in the analysis of
transformer oils and as diagnostician identi-
fying problem areas in transformers, as well
as profiling of transformers according to
available results thus empowering the cus-
tomer to take preventative steps in mainte-
nance. Corné has vast practical and theo-
retical knowledge on reliability maintenance
programs. Coming from a strong chemical
background she has insight in all the chemi-
cal processes that are part of the transform-
er system. Coupled with technical insight,
her knowledge and experience help custom-
ers optimize their reliability maintenance and
electrical asset lifetime.
56 TECHNOLOGY
INSIGHTS BY
CORNÉ DAMES

What is the data telling us? Category Type of equipment

• Has the condition of the

Power transformers/reactors with a nominal system voltage of
unit changed since the last Category O
400 kV and above
maintenance or testing period?
• Is it safe to operate the unit? Power transformers/reactors with a nominal system voltage above
• Are there signs of deterioration? 170 kV and below 400 kV. Also, power transformers of any rated
Category A
• Is it safe to load the unit above the voltage where continuity of supply is vital and similar equipment for
nameplate rating for some period? special applications operating under onerous condition.
• Am I required to implement action
Power transformers/reactors with a nominal system voltage above
to ensure the continued reliability
Category B 72.5 kV and up to and including 170 kV (other than those in
of the unit? Category A)
• How long can we use the unit
before considering replacement? Power transformers/reactors for MV/LV application: e.g. nominal
• Are the identified problems of a system voltages up to and including 72.5 kV and traction transformers
recurring nature? (other than those in Category A). Oil-filled circuit breakers with a
Category C
nominal system voltage exceeding 72.5 kV. Oil-filled switches, a.c.
metal-enclosed switchgear and control gear with a nominal system
Effective condition voltage greater than or equal to 16 kV.
monitoring outline
Instrument/protection transformers with a nominal system voltage
Category D
It is vitally important to identify clear above 170 kV
goals as part of your transformer
monitoring strategy. What do you Instrument/protection transformers with a nominal system voltage up
Category E
to and including 170 kV
want to achieve by implementing
this condition-monitoring plan? Is
Diverter tanks of on-load tap-changers, including combined selector/
it in-service failure prevention, life Category F
diverter tanks
extension or maintenance deferral?
By stipulating the outcome and what Oil-filled circuit breakers with a nominal system voltage up to an
you want to accomplish, it would be Category G including 72.5 kV, Oil-filled switches, a.c. metal-enclosed switchgear
much easier to identify the required and control gear with a nominal system voltage less than 16 kV.
parameters.
NOTE 1. Separated selector tanks of on-load tap-changers belong to the same category as the associated
transformer.
Health indexing of assets is becoming
an important tool to obtain a clearer NOTE 2. Oil-impregnated paper bushings and other hermetically sealed equipment may be placed in
Category D or E if a routine monitoring program is desired. The manufacturer's instructions should be
picture of the condition of your referred to.
transformer. Test parameters carry
NOTE 3. Regardless of size or voltage, a risk assessment may justify condition-monitoring techniques
a numerical value-adding to the total usually appropriate to a higher classification.
value of the Health Index Value of the
transformer. These parameter weight NOTE 4. For practical and economical reasons, some electrical utilities may decide that their small
transformers up to 1 MVA and 36kV are not included in this classification. Routine monitoring may not
values were calculated based on the
Photo: Shutterstock

be considered economical for this type of equipment. Where a monitoring program is required for these
international standards for mineral transformers, the guidelines in category C should be adequate.
oils, indicating the critical values
stipulated in the various standards. Table 1. Equipment categories [1]
Transformer Technology December 2021 57

Issue 16

Property Groupa Subclause Method

The scope of oil analysis,
interpretation of the data and
critical values
Color and appearance 1 5.2 ISO 2049

It is important to realize that we deal

Breakdown voltage 1 5.3 IEC 60156
with different size transformers in
Water content 1 5.4 IEC 60814
the industry, where transformers are
divided into classes according to
Acidity (neutralization value) 1 5.5
IEC 62021-1 or the kV ratings of the equipment (see
IEC 62021-2
Table 1 [1]). It is up to the reliability or
Dielectric dissipation factor (DDF) asset manager to use the guidelines
1 5.6 IEC 60247
and resistivity
for larger equipment, thereby
Inhibitor contentb 1 5.7.3 IEC 60666 implementing shorter increments
of oil analysis and electrical testing.
Sediment
Sludge
2 5.8 Annex C of this standard Table 2 explains which oil analyses
ASTM D971
are recommended, how often or
Interfacial tension (IFT)c 2 5.9 under which circumstances [1].
EN14210

Particles (counting and sizing)c 2 5.1 IEC 60970 Color and appearance
Oxidation stabilityc 3 5.7 IEC 61125
This is a routine inspection applied to
every oil sample. When an oil sample
Flash point d
3 5.11 ISO 2719
arrives at the laboratory, one of the
Compatibilityd 3 5.12 IEC 61125
“tests” is a visual inspection of the oil
sample in a clear vessel to determine
Pour pointd 3 5.13 ISO 3016 the color, turbidity and possible
particle identification.
Densityd 3 5.14 ISO 3675
Dark oils might indicate chemical
Viscosityd 3 5.15 ISO 3104 degradation or contamination of the
oil. When there is a lot of turbidity, it
Polychlorinated biphenyls (PCBs) 3 5.16 IEC 61619 might indicate high-water content in
IEC 62535
the oil.
Corrosive sulphurc 3 5.17 ASTM D1275, METHOD B
DIN 51353 If the drain valve was not wiped clean
Dibenzyl disulfide (DBDS) content 3 5.18 IEC 62697-1
by the sampler, the dirt particles in the
drain valve might be incorporated into
Passivator contentb 3 5.19
ANNEX B OF the sample. If particles are identified
IEC 60666:2010
as carbon, this might indicate a
a
Group 1 are routine tests, Groups 2 are complementary tests, Group 3 are special investigative tests. possible electrical fault in the unit.
b
Restricted to inhibited and or passivated oils. The DGA analysis of the oil will
c
Only needed under special circumstances, see applicable subclause.
d
Not essential but can be used to establish type identification. confirm if this is the case.

Clear oils without contamination will

Table 2. Tests for in-service mineral insulating oil oils [1]
indicate a good condition, and no
action is recommended.
Property Category Good Fair Poor
When oils are dark or turbid, further
analysis will confirm if there are any
O, A, D >60 50 to 60 <50 problems. The oil analysis results will
also determine the degree and type of
B, E >50 40 to 50 <40 action.

Breakdown C >40 30 to 40 <30 Breakdown voltage

voltage (BV)
This is a routine inspection.
<30 kV for OLTC in star point application
F
<40 kV for OLTC in delta or line-end application
Breakdown voltage (Table 3) will
indicate the water content or the
G <30 presence of foreign particles, or both
in the oil being analyzed. As the oil in
Table 3. Breakdown voltage test transformers acts as an insulation
58 TECHNOLOGY
INSIGHTS BY
CORNÉ DAMES

A Poor result will require immediate

Property Category Good Fair Poor
action from the asset manager. This
might include taking another sample
O, A <15 15 to 20 >20 to confirm the results from the first
analysis. If it is confirmed that the
B, D <20 20 to 30 >30 water content is high, the oil should
Water Content be filtered; a process that can remove
(mg/kg at a large portion of the moisture from
transformer C, E <30 30 to 40 >40
the oil if applied correctly. (Editor’s
operating
temperature) Note: It is recommended that passive
F Action necessity >40 drying over a prolonged period of time
is a better solution than filtering, since
there is more moisture in the paper
G Not a routine test
than in the oil.)

Table 4. Water content test Follow-up samples need to be taken to

ensure that the moisture content is still
within the required limits. The reason is
that the most significant portion of the
medium to avoid flashover in the unit, Water content (mg/kg at water is caught up in the paper system
the breakdown voltage must be high. transformer operating in the transformer. This moisture will
temperature) move from the paper into the oil under
If the values are Good, it is recom- conditions that favor this movement.
mended to continue with the current This is a routine test for all classes It might be found later that the oil in
sample interval action plan. If the of electrical equipment, except class the water has increased again without
values are Fair, more frequent G (Table 4). The results of this test
sampling is recommended in should always be considered in
collaboration with other parameter collaboration with the breakdown
results like the water content, DDF, strength. If it is found that the water
and acidity. If values are Poor, it is content is high and the breakdown
recommended to recondition the oil strength is low, further action needs
via oil reconditioning processes. to be taken. It is recommended that
a second sample from the same unit
If alternative tests indicate severe is tested to confirm the results.
aging, the oil can be replaced with
new or reclaimed oil. Another In the case of switching equipment,
option would be to perform on- where there is no paper present, the
site reclamation of the oil using a breakdown voltage is the determining
reclamation plant. factor.

Reclamation of oil has the advantage It should be noted that the limits
that the color of the oil is restored, indicated by IEC 60422 Edition 4 [1]
and the polar components are apply to transformers with operating
removed from the oil. This process temperatures between 40 and
will remove acid and water as well 70°C. If it is found that the unit's
as some other compounds. Another operating temperature is outside this
advantage is that the oil can be temperature range, it is best to refer
re-used, and in most situations, this to Annex A of the standard.
can be done without switching off
the unit, which contributes to cost- When the value obtained through
saving. If in doubt – switch off the analyses is Good, the normal
unit during this treatment process. sampling interval can be maintained,
requiring no further action.
If the values are Poor, it is advisable
to take action as soon as possible When the value returns a Fair
and not delay the maintenance result, more frequent sampling is
process. Excess water in the recommended. It is also helpful
transformer system decreases to consider other parameters like
the projected transformer lifetime the breakdown voltage, particle
Photo: Shutterstock

significantly; extremely high water content and DDF/resistivity, and

content can cause flashover in the acidity to decide the action to be
unit resulting in loss of the asset. implemented.
Transformer Technology December 2021 59

Issue 16

any apparent reason, but the source

would be the paper in the transformer. Property Category Good Fair Poor

A visual inspection is also recom- O, A, D <0.10 0.10 to 0.15 >0.15

mended to determine if any water
might move into the transformer or B, E <0.10 0.10 to 0.25 >0.20
electrical equipment through leaks. Acidity
This problem might be more severe if mgKOH/g oil
C <0.10 0.15 to 0.30 >0.30
the transformer or electrical equipment
is outside and not in a covered area.
F, G Not a routine test
Acidity (mgKOH/g oil)
neutralization number Table 5. Acidity test

This is a routine test for all classes and will increase paper degradation. In case of a Fair result, the sampling
except F and G (Table 5). If left untreated in transformers, interval should be increased to fit
this can lead to sludge formation, the situation. Future analysis should
The acids in oils are formed due usually around the lower parts of include a visual inspection of the oil
to chemical reactions between the transformer core. The sludge for sediment and sludge. If the result
the oil, water, and paper. Higher will eventually form a semi-solid is Poor according to the prescribed
temperatures or increased load will substance that is extremely difficult values in IEC 60422 Edition 4.0 [1], the
increase the formation of these to remove. asset manager may decide to reclaim
acids. Because acids are polar the oil or replace it with new or
compounds, it will adversely affect If the result is Good, the regular reclaimed oil, whichever option might
the insulation properties of the oil sampling interval can continue. suit their requirements the best.
60 TECHNOLOGY
INSIGHTS BY
CORNÉ DAMES

Dielectric dissipation factor Property Category Good Fair Poor

at 40 Hz to 60 Hz at 90°C
O, A <0.10 0.10 to 0.20 >0.20
This is a routine test for all classes of
electrical equipment, except F and G
(Table 6). B, C <0.10 0.10 to 0.50 >0.50
Dielectric
dissipation
The dielectric dissipation factor or tan factor at 40 Hz D <0.01 0.01 to 0.03 >0.03
delta test provides information regarding to 60 Hz at
the extent of the dielectric losses in 90°C
E <0.10 0.01 to 0.30 >0.03
transformer oil. This test measures the
inefficiency of insulating material. F, G Not a routine test

When oil ages, we have the formation

of polar compounds, leading to
phase displacement and dielectric Table 6. Dielectric dissipation factor test
losses. Some other impurities that
might influence the dissipation factor
include water, dissolved insulating Property Category Good Fair Poor
resin, and paper. When the result is
Fair, more frequent sampling and O, A >200 20 to 200 <20
checking additional parameters is Dielectric
recommended. When the result is dissipation B, C >60 4 to 60 <4
Poor, reclamation or an oil change is factor at 40 Hz
recommended. to 60 Hz at D >800 250 to 800 <250
90°C

Resistivity (GΩm) at 20°C E >60 7 to 60 <7

or 90°C
O, A >10 3 to 10 <3
This is NOT a routine test (Table 7). DC
resistivity of the oil is one of the key B, C >3 0.2 to 3 <0.2
Resistivity
parameters to assess the transformer (GΩm) at 90°C
D >50 10 to 50 <10
insulation condition; this is based on
the fact that DC resistance is sensitive
E >3 0.4 to 3 <0.4
to oil degradation.

When the result is Fair, more frequent

sampling and checking additional Table 7. Resistivity test
parameters is recommended. When
the result is Poor, reclamation or an oil
change is recommended. Property Category Good Fair Poor

Inhibitor content % Restricted to

Inhibitor Inhibited oils, 40% to 60% of <40% of original
All
This test is restricted to oils with content % consult oil original value value
supplier
inhibitor added (Table 8). It would be
advisable to contact the oil supplier to
verify the details regarding additives.
Table 8. Inhibitor content test
The two most common oxidation
inhibitors for transformer oils are It is advisable to use a field profes- a built-in oil preservation system;
2,6-di-tertiary-butyl para-cresol sional trained in the procedure to this is designed to keep dissolved
(DBPC) and 2,6-di-tertiary butyl-phenol perform this task. oxygen at levels below 1000 ppm.
(DBP). The purpose of the inhibitor is This would be in the form of a
to prevent oxygen from reacting with If the result obtained is Poor, the nitrogen system, a nitrogen tank or
the oil. This significantly slows the recommendation for this scenario generator, or a conservator tank
aging process in the oil as well as in would suggest that the end user equipped with a rubber diaphragm
the solid insulation. continue to use the oil “uninhibited”, (bladder). Using inhibited oils under
but this may lead to more rapid these circumstances is not required,
Photo: Shutterstock

If the result is Fair, it is advised to top degradation of both the liquid and although it might add additional
up the inhibitor level to the prescribed solid insulation. It should be noted protection against oil oxidation if the
level per supplier instructions. that some transformers already have preservation system ever fails [2].
Transformer Technology December 2021 61

Issue 16

Passivator content Property Category Good Fair Poor

Passivators, also known as metal

50-70mg/kg or
deactivators, react with reactive metal >70 and
<70 mg/kg, with a <50 and
surfaces and dissolved metals such Passivator O, A, B, C, stable (rate of
significant rate of decreasing at
content (mg/kg) D, E, F decrease <10/
as copper and silver and reduce their decrease of >10 mg/ >10 mg/kg/year
mg/kg/year)
rate of reaction with compounds kg/year
in the oil. This includes oxidation
reactions with organic compounds Table 9. Passivator content test
and reactions with corrosive sulfur.
Passivators are composed of two The results need to be less than 0.02% there is more polar compound present
basic types: sulfur-based and by mass to be negligible. If the results in the oil, decreasing the ability of
nitrogen-based. The first suggested return a value of more than 0.02% an oil to serve as an insulator in the
use of passivators in transformer oil, by mass, it is suggested that it be transformer system. There is a direct
which the author is aware of, was in reclaimed. Otherwise, an oil change is correlation between interfacial tension
1967 by J.J. Melchiore and I.W. Mills recommended. and neutralization number. Therefore,
of the Sun Oil Company [3]. the interfacial tension becomes
Interfacial tension a quality criterion: the oil must be
As the oil ages, the passivator might changed below a predefined limit.
deplete more rapidly; this depletion This is not a routine test (Table 10).
might accelerate when the oil is un- If the results are Good, continue
inhibited. During the aging process, the the regular sampling interval. If the
interfacial tension between results are Fair, increase the sampling
With Good results, regular sample transformer oil and water reduces. interval. If the results are Poor, Check
intervals can be maintained (Table What this means in practical terms is the oil for sediment and/or sludge.
9). With Fair results, maintain regular
monitoring. When the results are
Property Category Good Fair Poor
Poor, it is advised to remove the oil or
remove the source of corrosivity from
O, A, B, C, D
the oil via special oil treatment. Inhibited
>28 22 to 28 <22

Sediment and sludge Interfacial Uninhibited >25 20 to 25 <20

tension
(mN/m)
This is not a routine test. It is advised E Not a routine test
that this test is performed when the
oil results indicate a high acid value F, G Not Applicable
and the dissipation factor is near the
acceptable limit. Table 10. Interfacial tension test

Corrosive sulfur

This is not a routine test.

Oil is either corrosive or non-corrosive.

The presence of corrosive sulfur in
transformer oil and its effect on the
transformer system can be significant.
The extent of the corrosion damage
caused by the sulfur can be so severe
that it might cause the failure of the
equipment if not checked. The addition
of a copper passivator can reduce
the impact of this compound on the
transformer system.

CIGRE Brochure no. 378, 2009

stipulates the necessity of corrective
actions based on this institute's risk
assessment study [4].
62 TECHNOLOGY
INSIGHTS BY
CORNÉ DAMES

Particles counting and Flashpoint °C This value might differ in different

sizing countries.
This is not a routine test.
Table 11 outlines typical contamination It is advised to perform this test when
levels (particles) encountered on power If there is a maximum decrease in an unusual odor is noticed, the unit has
transformer insulating oil as measured flashpoints by 10%, the equipment been refilled, or an internal fault has
using IEC 60970 [5]. might require further inspection. occurred.

Maximum count
PCB
Adoption of per 100 ml
ISO 4406 This test is not to determine the
ISO 4406
(Edition Contamination
(Edition 5 μm 15 μm Notes condition of the transformer. It is a
1987) class designation
1999) class (equal (equal health and safety impact test. PCB
[8]
[7] to 6 μm to 14 is hazardous to both humans and
(c)) μm (c))
the environment, and it is therefore
Cleanliness vital to test for PCBs after the retrofill
Background requirement for of a transformer. The test is also
Up to 10/8/5 Up to 8/5 250 32
contamination sample bottles filled required whenever any maintenance
with clean solvent
has been done on the unit and the
Oil cleanliness possibility of contamination is present.
11/9/6 to
encountered during If PCB content is exceeding the
9/6 to 10/7 1,000 130 Low factory acceptance recommended limits, the appropriate
13/10/7
test and transformer
commissioning* action needs to be taken.

Contamination The limits are defined by local

14/11/8 to 11/8 to level typical for
17/15/12 15/12
32,000 4,000 Good
transformer in
regulatory bodies.
service
DGA
Contamination
level found on a
18/16/13 to 16/13 to As DGA (Dissolved Gas in Oil) is an
130,000 16,000 Fair significant number
19/17/14 17/14
of transformers in intricate science with a lot of data and
service interpretation, we will discuss this
Contamination phenomenon in part II of the article.
level rare and The limits for the different gases
20/18/15 18/15 and
and above above
Poor usually indicative of and the interpretation of this data
abnormal operating
according to international standards
conditions
will be discussed in detail, forming
NOTE: Considering the field measurements reported by various countries, it is recommended that the part of the overall health rating
contamination level encountered in service should be classified as indicated in this table. A more refined
classification would not be realistic in view of the variations that can occur in this type of measurement. determination of the transformer.
*Statistical survey has shown that values 15/11/9 are more realistic. The cleanliness requirements may
depend on the rating and shall be clarified between the customer and the manufacturer.
Conclusion

This is an interlaced, highly exciting

Table 11. Typical contamination levels (particles) encountered on transformer insulating oil field of study. In this article, we focused
as measured using IEC 60970 [5]. Source: CIGRE Technical Brochure 157, June 2004 [6]
on the types of tests to determine
the condition of the transformer, the
References critical values, and the recommended
[1] IEC 60422 Edition 4.0 2013-01 liquids - Methods for counting actions. In the future articles, we will
International Standard, Mineral and sizing particles focus on determining the Health Index
insulating oils in electrical [6] CIGRE Technical Brochure 157, for each unit using the test data results
equipment – Supervision and 2000, Effect of particles on and each test's weight in the Health
maintenance guide transformer dielectric strength Index determination.
[2] A. Shkolnik, “Oxidation inhibitor [7] ISO 4406:1999, Hydraulic
and reinhabiting oil-filled fluid power — Fluids — The Health Index indication will make it
transformers” Method for coding the level possible to see the supposed reliability
[3] L. Lewand, Passivators, “What of contamination by solid of a specific unit at that specific date
They Are and How They Work,” particles and time. This will make it possible
Doble Engineering Company [8] ISO 4406:1987, Hydraulic fluid to ensure best practice application
[4] CIGRE technical brochure power — Fluids — Method for and optimized maintenance. With this
information, it would be easier to draw
Photo: Camlin

378, 2009, Copper sulfide in coding level of contamination

Transformer Insulation by solid particles up a maintenance plan and action
[5] IEC 60970:2007, Insulating plan.
Transformer Technology December 2021 63

Issue 16
64 TESTING
PROCEDURES

Importance of Proper Test

Protocols and Procedures
for Ester Liquids
by Rosie Lawton
and Andrew Glanville

+
Natural and synthetic esters used as transformer dielectric liquids are
established alternatives to mineral oil, but there is a need in the industry
for better understanding of these liquids and how they are analysed.
 Transformer Technology December 2021 65

Issue 16

Rosie Lawton is an Application Scientist at

M&I Materials. Her role involves providing tech-
nical support to customers regarding the
quality of the MIDEL range of ester transform-
er liquids and their compatibility with other
materials. Rosie holds a Master’s in Chemis-
try from Manchester Metropolitan University,
UK and is pursuing a PhD in Tribology at the
University of Leeds, UK. Prior to working for
M&I Materials, Rosie worked as a Develop-
ment Chemist at Offshore Environmental Oils;
a company producing environmentally friendly
fluids for subsea applications.

Andrew Glanville is a Development Chem-

ist at M&I Materials. He holds a Bachelor of
Science in Biochemistry from the University
of Manchester as well as a PGCE in Second-
ary Chemistry with QTS. His previous roles
include the development and deployment of
Specialised Industrial Products at Infineum
UK, and Metalworking Fluid technical roles at
Afton Chemical.

Although both mineral oil and ester In the case of DDF, cleaning and
liquids are now established in the sample preparation is significant
transformer industry, it is important in producing reliable results.
to note that their chemistries are Relatively small amounts of
quite different. As such, testing contamination can lead to changes
procedures outlined in various in flash and fire point. In addition,
standards must be taken into inadequate cleaning leads to a
consideration. Even subtle build-up of carbonaceous material
differences between synthetic and on the sample holder that may
natural esters must be considered. cause a depression in the flash
Furthermore, variation in results and fire point. This article aims
provided by different laboratories to review the differences, and the
Photo: M&I Materials

can often be seen. Some of the similarities, between the liquids

key areas for these discrepancies in terms of testing and results
include flash and fire point, dielectric interpretation to ensure that it is
dissipation factor (DDF) and appropriate to the dielectric liquid
dissolved gas analysis (DGA). being assessed.
66 TESTING
PROCEDURES

AROMATIC HYDROCARBONS
ISO-PARAFFINIC
properties. The degree of saturation
HYDROCARBONS
of an ester refers to the amount of
double bonds present in the fatty acid
NAPTHENIC HYDROCARBONS chain, and these double bonds are
susceptible to oxidation. Saturation
also influences pour point, with
highly saturated natural esters such
PARAFFINIC HYDROCARBONS
as coconut oil having a pour point
too high for use in a transformer.
Currently, canola, soybean and
sunflower oils are most commonly
employed for use in transformers
Figure 1. Structure of mineral oils

+ (Figure 3) [1].

High fire safety and Synthetic esters are a popular choice

as dielectric liquids. They are created
O biodegradability by reacting select acids and alcohols

C are some of the by the process of esterification. Using

a synthetic ester provides a high fire
R OR´ key properties that point and readily biodegradability,

Figure 2. Structure of an ester

have successfully and they have better low temperature
behaviour without the oxygen stability
established ester weakness when compared to natural

Introduction
liquids in the esters. Synthetic ester can also
offer higher water tolerance due to
transformer industry, its additional ester linkage in the
High fire safety and biodegradability
are some of the key properties that
but their chemical structure, as shown in Figure 4.

have successfully established ester differences compared Interpretation of test

results based on insulating
liquids in the transformer industry, but
their chemical differences compared
to mineral oil are liquid type
to mineral oil are not always fully not always fully
recognised and understood. This
article will highlight how care is
recognised and With multiple liquid types now
in widespread use, correct
required to ensure the test method understood. identification of the type of
and results interpretation are insulating liquid is vital to obtaining
appropriate to the dielectric liquid Ester liquids can be synthetic or meaningful results. This section
being assessed. natural, and there are differences in outlines some common tests and
their chemical structures. highlights the importance of liquid
Mineral oil is a mixture of paraffinic, identification. A small range of
naphthenic, or aromatic hydrocarbon Natural esters are also known as tests is commonly used to evaluate
structures (Figure 1), and the ratio triglycerides, containing a glycerol the quality of both unused and
of these structures varies leading backbone and three fatty acid in-service insulating liquids for
to property differences including chains, with the structure of the fatty transformers. The results generated
viscosity, oxidation stability and more. acid chains influencing the ester are an invaluable diagnostic tool.
New mineral oil has minimal polar
molecular content, however, during
the aging process, acids and ketones CH₃
are produced, increasing the overall
polarity of the liquid. This leads to
changes in the liquid properties such H₃C
as the dielectric dissipation factor
(DDF) and interfacial tension. O
O
An ester is a chemical compound O
containing carbon, hydrogen and O O
oxygen bonds arranged as in Figure 2,
where R represents a hydrocarbon O
chain. Unlike mineral oils, ester liquids
are polar to begin with, which gives
esters beneficial properties such as CH₃
their excellent water tolerance. Figure 3. Structure of natural ester
Transformer Technology December 2021 67

Issue 16

Common diagnostic tests for

transformer liquids include:

• Water content
• Acid value
• Dielectric breakdown strength
• Dielectric dissipation factor (DDF)
• Dissolved gas analysis (DGA)

Flash point and fire point analyses

are elaborated on below as
they have significant safety
ramifications. Other tests, which
are performed less frequently for Figure 4. Structure of synthetic ester
esters, will not be explored in this
article. These include interfacial
tension, polychlorinated biphenyl + could lead to misleading results
(PCB) content, corrosive sulphur
content and viscosity.
With multiple and unnecessary maintenance
liquid types now intervention.
Water content is typically
analysed via Karl Fischer titration
in widespread use, DDF, or tan delta, is a measure of
instrumentation and expressed in correct identification the dielectric losses in an electrical
insulating liquid in an alternating
parts per million (ppm). Due to their
differing chemical structures, mineral
of the type of electric field and of the energy
oil and esters behave markedly insulating liquid is dissipated as heat. Any contaminants
present such as cellulose fibres,
different in the presence of water.
Mineral oil does not readily interact
vital to obtaining polar breakdown compounds,
with water, so relatively low levels meaningful results. water, etc. increase the tendency
of water negatively impact the of the liquid to conduct electricity
dielectric properties of the liquid. In This benefit alongside the superior seen as an increase in tan delta.
comparison, esters readily absorb water tolerance of esters has been Esters are more polar and much
water, and therefore higher levels demonstrated to be less harmful more hygroscopic than mineral oil,
may be detected (especially in to cellulose materials resulting in hence have inherently higher DDF
service) before the same effect is an increased transformer lifespan, without compromising dielectric
apparent. For example, at 20°C, the and led to a revision of industry performance.
same loss of breakdown strength standards so that higher levels of
is seen in mineral oil at water levels acid are acceptable inside ester filled DGA quantifies the level of
below 50 ppm, compared to over transformers versus those filled with atmospheric and fault gases
300 ppm for natural esters, or over mineral oil [3]. dissolved in transformer liquids.
600 ppm in synthetic esters [2]. Levels of these gases serve as a
Dielectric breakdown strength is powerful diagnostic tool to track
Acid value is expressed as the the minimum voltage, typically the health of transformers whilst in-
quantity (mg) of Potassium expressed in kilovolts (kV), at which service. The transformer liquid type
Hydroxide (KOH) needed to the liquid is forced to conduct has an impact on the interpretation
neutralise 1g of the liquid. Therefore, electricity between two electrodes at of results as the chemistry of mineral
liquids with a higher acid value a specified distance apart (e.g., 1 or oils and esters dictates that they
(mgKOH/g) contain higher levels of 2mm for ASTM D1816). Despite their interact differently with different
acidic components. As discussed chemical differences this breakdown gases. Whilst some general trends
previously, the chemical structures voltage of mineral oils and esters are true for the various types of
of mineral oil and esters are different, is similar. However, to ensure the liquid, other results are liquid specific.
affecting the way in which these integrity of results generated, liquid- For example, in mineral oils certain
liquids age. Mineral oil produces specific differences in the testing levels of carbon oxides is indicative
short chain (<C4), water soluble protocols must be followed. For of cellulose paper degradation
carboxylic acids in service that example, ASTM D1816 states that whereas for esters the presence of
migrate with water into the solid esters require a 15-minute stand time these gases is possible even in the
insulation where they can cause prior to testing whereas mineral oils absence of cellulose paper from the
harm. Esters produce longer chained require only five minutes. Failure to breakdown of the ester linkage in the
carboxylic acids with very low water follow this test method requirement liquid itself, so as with other analysis
solubility. due to misidentification of the liquid care needs to be taken and the liquid
 68 TESTING
PROCEDURES

320

300

280
Temperature oC

260

240

220

200

180
1 2 3 4 5
Mineral oil content %

Fire point Flash point (open) Flash point (closed) Target fire point Target flash point

Figure 5. Fire point and flash points of MIDEL 7131 with mineral oil contamination at low concentrations

type given high precedence when an ester, residual mineral oil above a short time. On the other hand,
interpreting results [3]. This has led recommended levels can impact the reproducibility refers to change
to ester specific revisions to Duval’s inherent safety benefits of an ester measured for results obtained under
triangles and pentagons – liquid and so must be managed varying conditions, for example
well-established diagnostic tools accordingly. different testing laboratories,
which plot levels of gases simultane- instruments or even an extended
ously to aid interpretation of results. Test procedure, repeatability, timeframe.
and reproducibility
Flash point is the lowest temperature
at which vapours ignite in the In addition to interpretation of results,
+
presence of an ignition source testing procedures are important Strict adherence
whereas fire point is the lowest
temperature at which vapours
in differentiation between different
insulating liquids. Repeatability and
to test procedures
continue to burn for five seconds after reproducibility are also extremely including cleaning and
the ignition source is removed [4].
Due to chemical differences in
important terms in the world of
science and engineering. The amount
sample preparation
structure the flash and fire points of error considered acceptable can be crucial for
are significantly lower for mineral oil
than that of natural and synthetic
greatly depends on the test and
the application in question. For
producing reliable
esters. This is an important safety example, large error is not acceptable results.
consideration. As discussed in the when under strict legislation or
following section, excessive amounts where there may be concerns for Strict adherence to test procedures
Photo: M&I Materials

of mineral oil cross-contamination safety. By definition, repeatability including cleaning and sample
can reduce the flash and fire point of measurements refers to change preparation can be crucial
significantly. In retrofilling situations measured for tests conducted for producing reliable results.
where mineral oil is replaced with under the same conditions and over Transformer liquid test laboratories
Transformer Technology December 2021 69

Issue 16

commonly analyse more than one a different mineral oil with higher in fire point measurements of fresh
substance – mineral oil, synthetic flash & fire point indicated that up synthetic ester when the cup was not
ester, natural ester, silicone liquid or to 3.5% mineral oil contamination cleaned in between tests, even after
less flammable hydrocarbons. Low was acceptable in retaining a fire excessive use seen in Figure 5. On the
levels of residual contamination point above 300°C. The repeatability other hand, fire point measurements
between consecutive tests could in this study was very good overall. were depressed by ~7°C when cups
easily lead to variation in results. An The repeatability of the flash containing mineral oil had been
investigation into the effect of mineral point measured on the closed cup tested but had not been cleaned
oil contamination in synthetic ester apparatus is much better than when before changing liquid type. A similar
on flash and fire point was recently measured using the open cup. effect may be seen if any sample
carried out. Interest was shown This is expected and likely why the material is left on the cup rim during
around the lower concentrations measurement using the closed cup is the analysis. Test methodology, or
of mineral oil, particularly around often suggested in various standards. various apparatus brands could be
the concentration that determines In some situations, the margin of responsible for the inconsistencies
less flammable (fire point ≥300°C) error due to the test procedure or seen among different test sites.
or K class (fire point >300°C) equipment, 8˚C as stated in ISO 2592,
specification. The repeatability may explain why the flash point Users often test DDF in new mineral
of these investigations was also and/or fire point result is lower oil or used dielectric liquids, which
considered. A mineral oil, used for than anticipated. In that situation, it may have a detrimental effect if they
transformer applications, was mixed would be recommended to retest the then test unused ester. The polarity
with MIDEL 7131 synthetic ester at sample to confirm the analysis and of ester enables the liquid to be
increasing concentrations. The fire interpretation [4]. extremely tolerant of water, but the
and flash point were measured on presence of water may increase DDF
both open and closed cup apparatus Flash and fire points of unused values which can be falsely viewed
and each mixture was tested three synthetic ester are often not tested as poor liquid condition. It is known
times. The results in Figure 4 shows however low results have been that DDF values can be higher in
that MIDEL 7131 fire point falls below reported on several occasions, ester liquids even when other testing
less flammable liquid specification thought to be caused by improper parameters such as breakdown
between 3% and 4% residual mineral cleaning of the pans. Contrary to voltage indicate the insulating liquid is
oil content. Previous work using expectation, no difference was found in good working condition [5].

Figure 6. Left: Cup cleaned using wire wool and solvent. Right: No cleaning of cup in between tests
70 TESTING
PROCEDURES

sensitive in esters and should only

Synthetic Natural be considered together with other
Gas Mineral oil
ester ester liquid properties. DGA is a useful tool
to detect and prevent potential faults
Nitrogen N2 0.091 0.091 0.074 in transformers however the right
calibration is important to account for
Oxygen O2 0.172 0.152 0.134
differences in gas solubility. Although
Hydrogen H2 0.0504 0.0479 0.0471 standards specifically designed for
ester liquids exist, care must be
Methane CH4 0.423 0.378 0.341 taken to adhere to the correct liquid
method for some testing parameters,
Ethane C2H6 2.88 2.2 2.14 while other parameters should be
considered with a cautious approach.
Ethylene C2H4 1.81 1.85 1.67

Acetylene C2H2 1.25 4.26 2.58 References

[1] M. P. Schneider, “Review Plant-
Carbon monoxide CO 0.125 0.13 0.108 oil-based lubricants and hydraulic
fluids.,” J. Sci. Food Agric. J Sci,
Carbon Dioxide CO2 1.1 2.08 1.54 no. 86, pp. 1769–1780, 2006, doi:
10.1002/jsfa.
Table 1. Gas solubility coefficients in insulating liquids from CIGRE Technical Brochure 443 [8] [2] Technical Brochure 446,
Experiences in Service with New
The small sample volume of the test mineral oil and esters will influence Insulating Liquids, CIGRE, 2010
cell makes the liquid susceptible to results and their interpretation. As [3] M. Lashbrook, H. Al-Amin, and
unreliably high results as even trace with mineral oil, correct sampling, R. Martin, “Natural Ester and
contamination will affect the DDF. storage, and handling is required to Synthetic Ester Fluids, Applications
Recent investigations [6] showed avoid losing hydrogen or introducing and Maintenance.,” 2017 10th
that mixture of esters with oxidised air bubbles. It has been shown that Jordanian Int. Electr. Electron. Eng.
aromatic oils resulted in abnormally air bubbles >8% volume can reduce Conf., 2017.
high DDF. No explanation for this the hydrogen value by 35% [7]. The [4] “Determination of flash and fire
phenomenon was provided though it same test methods are used for the points - Cleveland open cup
emphasises how DDF values could different insulating liquids; however, method. EN ISO 2592,” pp. 0–14,
be unreliable when tested alongside analysts must ensure the correct 2001.
used and different insulating liquids, calibration standards are used. [5] P. Livesey, M. Lashbrook, and
especially esters. For these reasons, The gas solubility of the liquid is R. Martin, “Investigation of the
DDF is not recommended for an important parameter related to factors affecting the dielectric
monitoring the condition of an ester the extraction method (headspace dissipation factor of synthetic
as a stand-alone test. analysis) and later determination of and natural esters,” Proc. - IEEE
the gas concentrations. The solubility Int. Conf. Dielectr. Liq., vol. 2019-

+ of most of the gases listed in Table 1

are relatively similar for each of the
June, pp. 2–5, 2019, doi: 10.1109/
ICDL.2019.8796834.
Although standards insulating liquid types however it [6] M. Lyutikova, S. Korobeynikov, and

specifically designed should be noted that the solubility

of acetylene and carbon dioxide
A. Konovalov, “Evaluation of the
Properties of Mixtures of Aromatic
for ester liquids exist, is much higher in ester liquids. Mineral Oil and Synthetic Ester for

care must be taken to Incorrect calibration may ultimately

lead to misdiagnosis of faults in the
High-Voltage Equipment,” IEEE
Trans. Dielectr. Electr. Insul., vol. 28,
adhere to the correct transformer which may prove costly. no. 4, pp. 1282–1290, 2021, doi:

liquid method for some A recommendation would be to have

online monitoring in conjunction with
10.1109/tdei.2021.009636.
[7] S. Tenbohlen, J. Aragon-Patil, M.
testing parameters, periodic laboratory testing to generate Fischer, M. Schäfer, Z. D. Wang, and

while other parameters a reliable trend to support operational

decisions.
I. Höhlein Atanasova, “Investigation
on sampling, measurement and
should be considered interpretation of Gas-In-Oil analysis
Conclusion
cautiously. for power transformers,” 42nd Int.
Conf. Large High Volt. Electr. Syst.
Natural and synthetic esters used 2008, CIGRE 2008, pp. 1–8, 2008.
Although DGA is not a novel as transformer dielectric liquids are [8] Technical Brochure 443, DGA in
technique, it remains a topic of great established alternatives to mineral Non-Mineral Oils and Load Tap
interest. As with other analyses, oil. The industry calls for better Changers and Improved DGA
it must be recognised that the understanding of these liquids and Diagnosis Criteria, CIGRE, 2010
chemistry differences between how they are analysed. DDF is highly
 Transformer Technology December 2021 71

Issue 16

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72 MINERAL OIL
PROPERTIES

Unique Properties of Transformer

Insulating Oils Containing
Hydrogen Donor Compounds
by Maryam Mohagheghi

Photo: HollyFrontier
Transformer Technology December 2021 73

Issue 16

Introduction

Transformers are critical and

expensive pieces of equipment
in the power generation and
distribution network. Any failure
In transformers, oil in could have drastic consequences
combination with paper is to an entire electrical network.
used to provide insulation To protect and increase the life
to the transformer’s of transformers, insulating oils
windings and other internal are used in combination with
components. The majority of solid insulation. Together, they
transformer insulating oils provide dielectric insulation to the
are composed of base oils transformer’s windings and internal
and specific performance components and, also, dissipate
additives. Gassing inhibitors heat generated by the transformer’s
may be added to higher windings and core [1, 2]. Different Maryam Mohagheghi received her Ph.D. in
quality base oils to improve types of insulating liquids exist, Chemical Engineering from Western Universi-
the gassing properties of the including petroleum derived mineral ty in London, ON, Canada. In 2014, she joined
finished insulating oil. An oils, natural and synthetic esters, HollyFrontier Lubricants & Specialties, which
insulating oil with silicone, and synthetic hydrocarbons. includes Petro-Canada Lubricants brand, as
a negative gassing tendency Mineral oil based transformer/ research & development product specialist.
reduces hydrogen gas insulating oils are commonly She is currently R&D specialist for Petro-Canada
bubbles which result from composed of base oil and additives Lubricants isoparaffinic transformer oil,
electrical and thermal fault that enhance oxidation stability, LUMINOL.
conditions in the transformer. cold temperature performance, and
A negative gassing tendency gassing properties of the base oil. Although the hydrocarbons in finished
also reduces the risk of The base oil may be an isoparaffinic mineral oils are carefully refined
transformer failure and oil or naphthenic oil. Isoparaffinic and highly stable, the molecules
explosion due to hydrogen oils are produced through a severe may decompose through thermal
gas generation, providing hydrotreating process; as a result or electrical stresses. During a
an extra margin of safety. of the process, deleterious sulfur, thermal or electrical fault, energy
As a chemical class, nitrogen, and oxygen compounds are is released and absorbed by the
gassing inhibitors are also eliminated, and undesirable aromatic mineral oil’s hydrocarbon molecules.
known as hydrogen donors. compounds are converted to more This excess energy breaks down
Structurally, chemical stable compounds. This study used chemical bonds in the hydrocarbons,
compounds containing Hα isoparaffinic mineral transformer producing hydrogen gas alongside
protons, protons attached to oils for their superior dielectric low molecular weight hydro-
non-aromatic carbon atoms and oxidative stability, as well as carbons [3]. It’s these characteristic
that are attached directly to established performance hydrocarbons that are monitored by
an aromatic ring structure, with gassing inhibitors. routine dissolved gas analysis (DGA)
are potential hydrogen testing to monitor the health and
donors. In this study, the performance of the transformer.
Hα content of a number of The tendency of an oil The remaining part of the
gassing inhibitor/hydrogen
donor compounds were
to absorb or evolve decomposed hydrocarbon is a
free radical, which can react with
measured using 1H-NMR. gas under electrical a similar free radical and produce
The correlation between
Hα proton content and
stress can be higher molecular weight species
which are no longer soluble in the
the gassing tendency of a measured by ASTM oil (i.e. sludge). When this process
transformer insulating oil
was then studied using the
D2300 (standard test happens, the evolved hydrogen
gas exacerbates oil aging, and is a
ASTM D2300 test method. method for gassing of potential threat to the operational
electrical insulating safety of the transformer.

liquids under
electrical stress and
ionization).
74 MINERAL OIL
PROPERTIES

The value reported by ASTM D2300 is

the net value of these two competing
reactions. A positive result indicates
gas is evolved following electrical
stress. If gas is absorbed, however,
the result is a negative gassing

The tendency of an oil to absorb or

tendency. During this equilibrium,
saturated hydrocarbons are mainly
Negative gassing
evolve gas under electrical stress responsible for the hydrogen gas tendency is a
can be measured by ASTM D2300
(standard test method for gassing
produced. On the other end, aromatic
molecules, or unsaturated bonds
unique property
of electrical insulating liquids under within a molecule, tend to absorb for the transformer
electrical stress and ionization). In
this test method, electric stresses are
ree hydrogen.
oil because it
simulated by ionic bombardment of Since isoparaffinic mineral oils reduces the risk of
the oil molecules producing hydrogen
and other breakdown species. Should
are highly refined to enhance their
oxidative stability, they tend to evolve
transformer failure
they be present, gassing inhibitors hydrogen under stress as they lack and explosion due to
can absorb the generated hydrogen
gas in the insulating oil.
reactive unsaturated and aromatic
molecules. To improve the gassing
hydrogen generation,
tendency of stable isoparaffinic providing an extra
Table 1.
Hydrogen types and
mineral oils without compromising
their oxidation stability, gassing
margin of safety and
chemical shift
inhibitors are added to the oil. performance.
Chemical Examples include partially saturated
Parameter Type of Protons
shift, ppm aromatic compounds such as
tetrahydronaphthalene, alkyl
HAr 6.0-9.0 Aromatic hydrogen substituted tetrahydronaphthalenes,
and alkylated benzenes [4]. The
ability of a donor material to donate
Hα 2.0-4.0 Aliphatic hydrogen on Cα to aromatics rings a hydrogen bond can be expressed in
terms of specific types of hydrogen
Aliphatic hydrogen on Cβ and the CH2 beyond content. There are four hydrogen
Hβ 1.0-2.0 types according to the 1H-NMR
the Cβ to aromatic rings
(Nuclear Magnetic Resonance)
Aliphatic hydrogen on Cγ and the CH3 beyond spectral analysis of hydrocarbons,
Hγ 0.5-1.0
the Cγ to aromatic rings as shown in Table 1. For a better
visualization, the four hydrogen types
As mentioned, gassing inhibitors are shown in Figure 1 [5, 6].
are also known as hydrogen donors,
Gassing inhibitors, as they can donate a hydrogen The HAr protons are attached to
also known as bond with free hydrogen molecules. aromatic rings and are a measure
A hydrogen donor may be any of the aromaticity of a material. Hα
hydrogen donors, are compound which contains labile protons are labile and are potential
used to improve the or easily displaced hydrogen. By hydrogen donors. They are attached
readily displacing existing hydrogen to non-aromatic carbon atoms
gassing tendency of bonds, the gassing inhibitor is attached directly to an aromatic ring
stable isoparaffinic able to absorb new, free hydrogen. structure.
mineral oils without Figure 1. Hβ -protons are attached to carbon
compromising their Hydrogen types
in a molecule
atoms in a second position away
from an aromatic ring. Protons
oxidation stability. attached to carbon atoms three or
α β γ
CH2 CH2 CH3 more bonds away from an aromatic
ring structure are Hγ protons.
HAr As such, the performance of any
hydrogen donor was expected to
be linked to the type and amount of
aromatic structures, alongside higher
Hα proton content [5].
Transformer Technology December 2021 75

Issue 16

Results and Discussions

Experimental For 1H-NMR interpretation, the typical

1
H-NMR signal positions reported in
To study the effectiveness of different Table 1 were used. The percentage
hydrogen donor compounds as of Hα and HAr protons was calculated
gassing inhibitors, 1H-NMR was first for the hydrogen donor compounds
used to measure HAr and Hα proton tested in this study and is shown The elevated amount of HAr for
content in four different compounds. in Table 2. Three different samples the compounds with higher Hα
The gassing tendency of transformer of the HD4 stream were tested to content indicates the link between
oils made with each compound was check the variation between different hydrogen donor performance and
then measured using the ASTM production batches. aromatic structures, alongside with
D2300 test method to assess higher Hα proton content. Figure 3
the correlation between the two The gassing tendency of transformer shows the correlation between Hα
properties. oil blends made using each hydrogen content of a hydrogen donor and the
donor as the gassing inhibitor were gassing tendency of an isoparaffinic
Table 3. then measured by the ASTM D2300 transformer oil containing 2 vol.%
Gassing test method. hydrogen donor material.
tendency results

Transformer oil 1 Transformer oil 2 Transformer oil 3 Transformer oil 4

with HD1 with HD2 with HD3 with HD4

Gassing tendency, µL/min -13 25 17.2 -4

Hα percentage, % 33.0 5.2 12.2 28.9

HAr percentage, % 32.0 0.3 12.6 22.6

Four hydrogen donor streams (HD1 Table 3 shows the gassing tendency Results confirm that the addition of
to HD4) were assessed in this study results. The treat rate of the hydrogen a superior hydrogen donor additive,
of which three were pure compounds donor/gassing inhibitor was the same based on Hα content, provides
and one was a mixed hydrocarbon in all the blends at 2 vol.%. Results superior negative gassing properties
refinery byproduct. 1H NMR spectra show that hydrogen donor material to the oil.
were recorded at room temperature on with Hα content greater than 28%
a Bruker Avance II 400 spectrometer. provides the blend with a negative
Approximately 5-10 mg of sample was gassing tendency.
dissolved in approximately 1 mL
of CDCl3 (Cambridge Isotope Without negative gassing agent: With negative gassing ( ) agent:

X
Laboratories, 99.8% D, w/ 1% V/V TMS)
in a 5 mm NMR tube. 1H (400.32 MHz)
experiments were run with an interscan
delay of 4.83 s and 32 scans were
collected. Figure 2 shows the 1H-NMR H2
spectra for each hydrogen donor.
H2

H2

H2

H2 H2
H2

H2 H2 H2
H2
H2
76 MINERAL OIL
PROPERTIES

Figure 2.
1
H-NMR
Spectra

[rel]
[rel]
HD1 HD2
1d_1H_16_scans CDCl3 1d_1H_16_scans CDCl3

15
15

10
10

5
5
0

0
224.9796
20.2424

80.0035

57.0692
1.0000

1.0295

1.0909

1.0000

8.4903
8
HAr 6 4 Hα 2 0 [ppm] 8
HAr
6 4
Hα 2 0 [ppm]

[rel]
HD3
[rel]

1d_1H_16_scans CDCl3
HD4
1d_1H_16_scans CDCl3
15

14
12
10
10

8
6
5

4
2
0

0
0.0417

1.0000

0.0214

1.3037

1.4781

0.6531
1.0000

0.9729

5.9862

8 6 4 2 0 [ppm] 8 6 4 2 0 [ppm]
HAr H Ar

Table 2.
Hydrogen content of four different hydrogen
donor compounds using 1H-NMR data

HD4
HD1 HD2 HD3
HD4-1 HD4-2 HD4-3

Hα content, protons integration 1.030 20.242 0.973 13.251 12.597 13.077

HAr content, protons integration 1.000 1.000 1.000 10.417 10.000 10.000

Total Hydrogen, protons integration 3.120 391.785 7.959 44.98 44.412 45.338

Hα percentage, % 33.0 5.2 12.2 29.5 28.4 28.8

HAr percentage, % 32.0 0.3 12.6 23.2 22.5 22.1

Photo: Shutterstock
Transformer Technology December 2021 77

Issue 16

Figure 3.
Gassing tendency of a transformer oil vs Hα content of
hydrogen donor compound used in the transformer oil

30

25

20

Gassing tendency, µL/min 15

10

0
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0
Conclusions
-5
The effectiveness of different
hydrogen donor compounds as -10
gassing inhibitors in isoparaffinic
transformer oils was studied. -15
The 1H-NMR technique was used Hα content
to assess the Hα and HAr content
of different hydrogen donor References
compounds. The gassing tendency [1] I. Fofana and J. Sabau, [5] L. R. Rudnick, “Process for
of a representative transformer oil “Application of Petroleum- Visbreaking resins in the
blend containing 2 vol% of each based oil in Power Presence of Hydrogen-Donor
hydrogen donor was then measured Transformer,” Natural Gas Materials and Organic Sulfur
using the ASTM D2300 test method. Research Progress, Editors: Compounds,” US Patent 4, 587,
The correlation between Hα content N. David and T. Michel, Nova 007, 1986
and gassing tendency indicates Science Publishers, Inc., ISBN: [6] C. O. Rossi, P. Caputo,
higher Hα content provides negative 978-1-60456-700-7, 2008 G. De Luca, L. Maiuolo,
gassing tendency to the transformer [2] D. Peterchuck and A. Pahwa, S. Eskandarsefat and
oil. Negative gassing tendency is a “Sensitivity of transformer’s C. Sangiorgi,“ 1H-NMR
unique property for the transformer hottest-spot and equivalent Spectroscopy: A Possible
oil because it reduces the risk of aging to selected parameters,” Approach to Advanced
transformer failure and explosion due IEEE Trans. Power Delivery Vol. Bitumen Characterization
to hydrogen generation, providing 17, pp. 996-1001, 2002 for Industrial and Paving
an extra margin of safety and [3] J. S. N’Cho, I. Fofana, A. Applications,” Applied Science,
performance. Beroual, T. Aka-Ngnui and J. 8, 229, 2018
Sabau, “The Gassing Tendency
of Various Insulating Fluids
under Electrical Discharge,”
IEEE Transactions on
Dielectrics and Electrical
Insulation, Vol. 18, No. 5,
pp.1616-1625, 2011
[4] M. Fefer and T. Ruo, “Dielectric
Fluid,” US patent 67, 790, 386
B2, 2004
78 COMING
IN FEBRUARY

Transformers:
The Heart of the
Power System

In February 2022 we zoom out our focus from the

transformer components to the transformer as the
most critical component of the power system, looking
at it from a range of application perspectives and
technologies, including:

• Generation, Transmission and Distribution

Systems
• Green Energy
• Distributed Energy Resources
• Battery Technology
• Digital Twins

If you, or someone from your organization wishes

to contribute to the February issue and add your voice
to our compilation of knowledge that we distribute
through our community channels, please contact
alan.ross@transformer-technology.com.
Photo: Shutterstock
Transformer Technology December 2021 79

Issue 16

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