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functions of insulation and cool-
ing, the oil serves as an important
carrier of information

ABSTRACT
As blood tests provide information The diagnostic
power of oil
about a person’s state of health, oil
analyses provide information about
the condition of a transformer.
Transformer oil testing has proven to

analysis for power

be an excellent tool for early defect
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transformers:
has permitted extended service life
by giving a measure of both oil and
solid insulation condition, and by
identifying the presence of incipient

New
thermal and dielectric faults. When
performed regularly, oil analysis
will help the asset manager run a
cost-effective maintenance program

developments
for transformers.
This column addresses some recent
developments in oil analysis, such as
sustainable alternatives for mineral
oil, an alternative marker for analy- 1. Introduction
lishing the status of ageing and corrosion
sis of paper degradation, and online
Over the last decades a large variety of oil for replacement and refurbishment issues
transformer oil monitoring.
analyses techniques have been developed [1]. In the majority of oil-filled transform-
and standardized. These techniques in- ers mineral oil is used, and physical oil
KEYWORDS volve physical or chemical oil properties, samples are taken in service for analysis
electrical and environmental properties, in the laboratory. This column addresses
oil analysis, ester, cellulose paper, and DGA (Dissolved Gas Analysis). The some recent developments in oil analysis.
degree of polymerization application of oil analysis may be linked to The first one is related to sustainable alter-
maintenance, lifetime extension, and estab- natives for mineral oil, in particular esters.

66 TRANSFORMERS MAGAZINE | Volume 5, Issue 4 | 2018

 Jos WETZER

The second one deals with methanol as an basis, and have built a history that serves properties, and interacts very well
alternative marker for analysis of paper as a reference for trending and analysis. with mineral insulating oil. Under the
degradation, and the third one with the For esters, the methods for assessing the combined effects of temperature, oxygen
benefits and drawbacks of online trans- condition can only be applied to a limited and moisture, the cellulose polymer chains
former oil monitoring. extent using IEC standards. IEC did issue will break up and result in a weak and
a standard on new and unused natural es- brittle material that has lost its mechanical
ters, such as FR3. But so far there are no strength. Solid insulation in transformers
2. Natural esters as an guidelines for esters in operating electrical not only serves as an insulator, but also
alternative for mineral oil equipment. New knowledge rules need to ensures mechanical stability, creation of
From the start, mineral oils have formed be applied, and a new history needs to be space, the proper direction of oil flow, and
the insulation and cooling medium of built up. IEC TC 10 is presently working so on.
liquid-filled transformers. For most ap- on such a standard. It is to be expected
plications, mineral oil is a proven, accept- that, in the longer run, a similar standard New insulating paper typically has a DP
able, cost-effective insulating and cooling will be developed for the analysis of paper value between 1000 and 1200, which
fluid. It has good thermal and dielectric ageing through ester transformer oil. corresponds to a high tensile strength. As
properties, and there is a lot of experience paper ages, its electrical properties do not
gained over the years. change significantly, but the mechanical
3. Methanol as a marker for properties are reduced drastically. As a
In recent years, manufacturers have intro- paper degradation result, the mechanical strength of the
duced transformers with biodegradable Transformer insulation paper is generally paper is reduced with time, and when a
liquids, in particular synthetic and nat- recognized as the most significant lim- DP value of about 200 to 300 is reached,
ural esters. The advantages are pri- iting factor in the thermal operating life the mechanical strength has gone. The
marily related to safety properties of a transformer [2]. Not as much because rate at which the DP value decreases
(thermal stability, flashpoint, smoke it is the dominant failure mechanism – it in time during service depends mainly
generation) and environmental properties is not, but because paper degradation is on the temperature in the transformer,
(biodegradable). It is further claimed that an irreversible mechanism that will even- on the amount of moisture and oxygen
they can absorb water without having a tually destroy the transformer and does present, and on the acidity of the oil. The
detrimental effect on dielectric properties, (practically) not allow repair or refurbish- main cellulose degradation process is acid
and that they have a longer life expectancy ment. hydrolysis, which requires water and acids.
due to the reduced ageing rate of solid in- The process is accelerated by temperature,
sulation. Transformers with such fluids The winding insulation consists of oil degradation byproducts and possible
are being introduced more and more, impregnated cellulose. Cellulose has electrical fields. Figure 1 schematically
especially in applications where fire excellent electrical and mechanical illustrates the degradation process [3].
safety and environmental protection
are important, and have reached a state
of maturity. There is a trend to use them
at ever increasing voltages and power
ratings, and today there are examples of
biodegradable fluids being used in high-
voltage and high-power transformers.

Apart from fulfilling the primary functions

of insulation and cooling, the oil serves as
an important carrier of information. From
physical and chemical analysis we assess
whether the oil still meets the required
specifications for continued use. With
DGA (Dissolved Gas Analysis) dormant
errors can be detected at an early stage.
This gives us, as it were, a glimpse into the
interior of the transformer without having
to open it and even without having to take
it out of operation. DGA is the most widely Figure 1. Schematic representation of the interaction between oil and paper
used technique worldwide for monitoring
the operating status of transformers.
Oil testing gives a measure of both oil
A drawback of changing from mineral
oil to esters is that is has an impact on and solid insulation condition,
dition, and helps
he
transformer monitoring and diagnosis.
Most utilities have experience with min-
identify the presence of incipient
ncipient thermal
therm
eral oil analysis as a standard diagnostic and dielectric faults
w w w . t ra n s fo r m e r s - m a g a z i n e . co m 67
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In the last decade, research has indicated It does, by its nature, not allow immediate
feedback or alert.
that methanol is a promising alternative as
a tracker for paper degradation These days, however, modern sensing
and diagnostic equipment allows to re-
place sampling, transport and laborato-
ry analysis of oil samples. Mobile on-site
test methods are presently commercially
available. By applying continuous on-line
oil testing, in combination with intelligent
electronic devices which can record and
trend results and trigger alarms, we now
have predictive diagnostic tools which
can eliminate the risks associated with
interval testing. On-line testing also elimi-
nates the necessity of oil sampling and the
associated issues related to contamination
during sampling and storage. After all, the
test results are only as reliable as the sam-
ples obtained. One might suggest that this
will eventually lead to the end of sampled,
interval-based, oil testing practice [7].
Will we exclusively use on-line predictive
oil analysis in the future? To answer this
question we need to understand the prin-
)LJXUH7\SLFDOFRUUHODWLRQVEHWZHHQ'3YDOXHDQGPDUNHUFRQFHQWUDWLRQVUDWLRV>@ ciples of both methods.

The off-line analysis provides a wealth

For the above reasons, operators are The CO2 and furfural concentrations are of information from all kinds of analyses
keen to know the degree of polymeri- more sensitive to DP variations in the low (physical and chemical properties and
zation (the DP value) of the insulation DP range (below 400, late degradation DGA), and needs to be performed on a
paper. Sampling paper for analysis is stages), whereas the MeOH concentration regular basis. According to CIGRÉ [7], a
not a feasible option, not only because is more sensitive to DP variations in typical maintenance interval for DGA,
it requires opening the transformer, but the high DP range (above 400, earlier dependent on load, age and importance,
more importantly, because the sample degradation stages). The methods may varies from three months to two years.
would have to be taken from the hot- be regarded complementary. For early Once a defect is observed, the same sam-
spot location. The breakdown strength warning, the MeOH approach would be ple may be used for additional analysis,
of the insulating paper is determined better, but close to the end of paper life, the and the sample frequency may be in-
by the weakest link in the chain, which furfural content may give a more accurate creased. Interpretation is performed by an
is the hotspot location. This is located picture. It should be noted, however, expert (or a team of experts).
inside the winding package close to the that research is still ongoing to arrive
core. at a reliable empirical relation between The on-line analysis is predominantly
MeOH and DP value. At present, all three based on DGA monitoring, and ranges
Therefore, the DP value is determined methods have some challenges based from simple fault detectors to full DGA
indirectly from the byproducts in the on their advantages and disadvantages. monitoring systems, and may be performed
oil that result from breaking up of the Table 1 gives an overview of the three continuously. The first interpretation can
cellulose chains. The byproduct para- methods discussed [3]. be made by knowledge rules embedded in
meters used so far for assessing the DP software. In order to fully benefit from this
value are the CO2 concentration, the feature it is important to ensure that oil is
CO2/CO ratio and the furfural content.
4. Online monitoring of drawn from and returned to chosen points
In the last decade research has indicated transformer oil in the oil circulation circuit so as to ensure
that methanol (MeOH) is a promising For critical transformer units where pre- representative sampling. It is also impor-
alternative as a tracker for paper degra- mature or unexpected failure would have tant that the oil is suitably conditioned
dation [4, 5]. significant consequential costs one may prior to testing to ensure measurement
consider to increase the oil sampling consistency. As mentioned earlier, on-line
The assessment is based on the strong frequency for coping with defects that analysis may be used for fault detection or
correlation between CO2 concentration, develop fast. This allows maintenance for a full DGA analysis.
the CO2/CO ratio, the furfural content engineers to timely recognize incubating
(2-FAL) and the MeOH content on the defects and take corrective action before Fault detectors usually operate on only
one side, and the DP value on the other they mature, however at the expense of one or two key parameters. As hydro-
[4]. This is illustrated in Figure 2. cost, manpower and logistic complexity. gen is produced in all fault mechanisms,

68 TRANSFORMERS MAGAZINE | Volume 5, Issue 4 | 2018

It is anticipated that on-line oil analysis will monitoring will find its way on a limited
scale, focusing on strategic applications.
increasingly be applied for high-value stra- It is unlikely that on-line monitoring will
tegic transformers, but will not be able to replace off-line analysis at a significant
scale in the foreseeable future, unless the
replace laboratory oil analysis in the fore- parameters of the business case dramatic-
ally change.
seeable future
Conclusion
measuring dissolved hydrogen serves sampling and laboratory analysis) and Transformer oil testing has proven to be
as an excellent fault indicator. Similarly, the benefits (e.g. the failures prevented an excellent tool for early defect iden-
moisture is a very useful indicator of the by timely detection). Expensive and stra- tification in preventive maintenance of
overall condition of (and risks associated tegic high-voltage transformers will have oil-filled transformers. It has permitted
with) both oil and solid insulation. These a better monitoring business case then extended service life by giving a measure
are therefore regarded as key parameters. medium-voltage commodities. of both oil and solid insulation condition,
Fault detectors combined with intelligent and by identifying the presence of incipi-
logic systems allow to timely intervene in Also, one has to take into account that on- ent thermal and dielectric faults. When
critical situations, but do not have the pre- line systems need maintenance and cali- performed regularly, oil analysis will help
dictive power of a full analysis. bration, and tend to have a lower accuracy the asset manager run a cost-effective
then laboratory analysis. maintenance program for transformers.
Complete DGA monitoring benefits from
the DGA analysis advantages to the full. Transformer oil testing is firmly entren- New sustainable fluids are being de-
However, it does not monitor all physi- ched in the condition-based maintenance veloped as an alternative for mineral oils.
cal and chemical properties, the costs rise practice. The results of oil analyses form Esters have become available with sim-
with the number of key gases to be ana- an important part of the condition infor- ilar dielectric and thermal properties, but
lysed, and complex situations still requi- mation feeding maintenance and replace- with superior safety and environmental
re an expert interpretation. The decision ment decisions. As such, oil analysis is in- properties. Further work is needed in or-
to use an on-line monitoring system in- creasingly integrated in decision support der to develop guidelines for using ester
volves a trade-off between the costs (costs systems such as health and risk indexing analysis for diagnostic and maintenance
of automated monitoring versus interval software [8]. It is very likely that on-line applications.

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Test Advantages 'LVDGYDQWDJHV Challenges

Laboratory or online test; Limited information on cellulose 'LI¿FXOWWRGHWHUPLQHVRXUFH

ageing; location, type of degradation,
degree of egress/ingress;
CO2 & CO &DQEHXVHGDVDÀDJWR Origin can be solid insulation or oil; May be dependent on
indicate cellulose ageing manufacturer / design
Gases may escape, depending on
WKHEUHDWKLQJFRQ¿JXUDWLRQ
Laboratory test; Thermal instability furans; Proper interpretation requires
knowledge on the insulation
Estimates DP value; Detection limited at higher DP system
values
2-FAL
Empirical relation between
2-FAL and DP;
Sensitive for late
degradation (low DP)
Laboratory test; Requires knowledge on paper (VWDEOLVKLQJXQLWVSHFL¿FHPSLULF
type, partitioning factors, size, equations still in development
Estimates DP value; GHVLJQEUHDWKLQJFRQ¿JXUDWLRQ

Methanol
Empirical relation between
MeOH and DP;
Linear & sensitive for early
degradation (high DP)

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For determining the ageing status of paper [3] N. Perjanik, Weidmann Introduces [6] CIGRÉ, Guide for transformer
insulation, methanol analysis has been Methanol and Ethanol in Oil Testing, maintenance, CIGRÉ brochure 445,
developed as an alternative for furfural NETA Powertest Conference, 2018 WG A2.34, 2011
analysis. It turns out that methanol
has superior properties for identifying [4] M. Duval, Transformers with low [7] M. Kuises, W. Morse, Has transfor-
the early phases of paper degradation. degree of polymerization, Transformers mer oil testing reached its sell by date?,
However, further development is Magazine, Vol. 1, Issue 3, 2014 Energize, April 2005
needed to arrive at empiric relations for
interpretation. [5] A. Schaut and S. Eeckhoudt, Iden- [8] J. Wetzer, Transformer health and
tification of early-stage paper degrada- risk indexing, Transformers Magazine,
It is anticipated that on-line oil analysis tion by methanol, CIGRÉ Conference, Vol. 5, Issue 2, 2018
will increasingly be applied for high-value 2012, paper A2-107
strategic transformers, but will not be able
to replace laboratory oil analysis in the
foreseeable future: for large numbers of
medium-voltage commodities the bene- Author
fits will not balance the costs. Dr. Jos Wetzer from the Netherlands is senior principal
consultant “Asset Management” with DNV GL - Energy,
stationed in Arnhem, The Netherlands. He obtained
Acknowledgement a M.Sc. degree in Electrical Engineering (cum laude)
The author gratefully acknowledges the and a Ph.D. degree in Technical Sciences, both from the
support of Walter Hulshorst and Pieter Eindhoven University of Technology. After obtaining his
de Bijl of DNV GL. They have delivered Ph.D. degree he worked as an associate professor at the
valueable contributions to the field of Eindhoven University of Technology for 14 years. His
power transformer oil analysis. activities included research, education and consultancy in the field of High-
Voltage Engineering and EMC. In 1998 he joined KEMA (now DNV GL) as a
senior consultant, in the field of condition assessment, asset management and
Bibliography EMC. From 2001 to 2006 he held a management position, and from 2006 on
[1] N. Robinson, Transformer oil ana- he has been active as a (senior) principal consultant, and has served numerous
lysis: a basic introduction, in: Transfor- clients in Europe, North America, the Middle East and Asia Pacific on asset
mer and substation handbook 2014, management issues. From 2004 to 2007 he also held a position as a part-time
Electricity &Control Spot On, Crown professor at the Eindhoven University of Technology in the Netherlands,
Publications engaged in Asset Management of Electrical Infrastructures. His expertise
includes asset and maintenance management, condition assessment and
[2] CIGRÉ, Thermal performance of diagnostics, dielectrics and electrical insulation, electromagnetic fields and
transformers, CIGRÉ brochure 393, EMC. He has published over 150 scientific and professional publications.
WG A2.24, 2009

70 TRANSFORMERS MAGAZINE | Volume 5, Issue 4 | 2018