Cooling Power

CT
the Debate on with
Success Power
Surge testing
motors P6 in India P2 DB P8-9
Published by Megger
July 2019

ELECTRICAL
TESTER The industry’s recognised information tool

Comprehensive analysis of Single CT in Bus coupler

compared to Dual CT for Bus Bar protection scheme
Dr K N Dinesh Babu - Application Manager
breaker. When the fault is located in c, it’s a clear
Power System Protection Megger zone 2 fault and zone 1 need not trip. This trip did
not occur in single CT arrangement as shown in
Introduction: In this document, the pros & cons Table 1. Hence an unwanted trip of a healthy zone
of using single CT (current transformer) compared can be eliminated by using single CT arrangement
to the conventional dual CT are discussed. Earlier only.
days, in bus bar protection relays, two CTs were
Analysis: The use of single CT has reduced the
used on either side of the bus coupler breaker due
cost and also met all the power system protection
to the following reasons
principle by the use of advanced logic in numerical
1. CT current direction has to be matched in the relays. More over dual CT has dis advantages
relay. It’s a common practice to earth the CT compared to single CT as hi-lighted. With the
towards the protected zone direction. In case of availability of this flexibility in numerical relays, dual
two CTs, one will be earthed towards zone 1 and CT for Bus coupler and Bus section is no longer
one towards zone 2

Figure 2: Zone segregation for Dual CT arrangement

a mandatory requirement for low impedance bus K N Dinesh Babu is a

bar protection relays. However, In Table 1, we professional power system
observed that there was a delay in tripping of the protection engineer in
bus bar protection, if the fault is located at position India with extensive
b. This delay is the breaker operation time delay experience in HV / EHV sub
which may not be acceptable in some cases. Bus stations and industries. His
bar protection relays are supposed to trip in 15ms work background includes
to 25ms as per majority of the specification and experience in working for
hence this delay of CB operation time may act as a Alstom, ABB, GE and Megger in various roles like
dis-advantage. To better analyze the requirement, testing; commissioning; design and application
we have to choose based on the application and engineering. A major portion of this experience
identify the number of incoming bays available has been in the area of protective relaying with a
in the substation. If the substation is close to a major focus on the application and coordination of
Figure 1: Zone segregation for single CT arrangement
generation station and the bus fault can feed a protective facilities on electrical power systems. His
huge amount of fault current, then dual CT would experience includes the development of protection
be beneficial as we cannot compromise on timing. philosophies, implementation of new technologies
2. To provide zone overlap and to prevent the blind Fault Zone Zone
However, if the bus does not have major source, in the field; control logic requirements for
spot between the CT and the CB. Location 1 2
and the operation philosophy permits only one bay protective systems; development of specifications
In modern relays, the above two concepts can be a Trip No Trip through the Bus coupler, then there would not be for protective relay settings; and analysis of
achieved using the internal logics and flexibility of any difference between the fault current of a bus disturbances and faults in electric power systems.
Trip
the relay. Figure 1 shows the single CT arrangement b Trip bar and the fault current of a feeder bay. In such He also trains protection engineers, field engineers
(Delayed)
with faults located in various places. The color scenarios, the specification permits 35ms to 40 ms and O&M engineers on IEDs, SCADA, power
shows the zone limit. In this example, green color c No Trip Trip for a feeder tripping and then why should the user system protection and synchrophasors for major
represents zone 1 and blue color represents zone 2. worry about a delay tripping of a bus where the utilities and industries in India and Abroad.
d No Trip Trip
The CT defines the boundary for both the zones. bus itself acts as an extension of a feeder with the
same fault current? In such scenarios single CT He holds a doctorate in electrical engineering and
Table 1 shows the trip matrix for various fault Table 1: Trip matrix for various fault location would be a cost effective solution. is a DFSS certified six sigma green belt. He is a
locations of Figure 1. It can be observed that the in single CT arrangement senior member of IEEE and member of CIGRE. His
relay trips the faulty zone and there is no blind spot Conclusion: Bus Couplers with single & Dual areas of contribution are electric traction, power
in this scheme. For fault located in b, Zone2 trips Fault Zone Zone CT application has been described in this paper, system protection and renewable energy utilization
and opens the bus coupler circuit breaker. Opening Location 1 2 and both the types have merits and demerits and have published several articles in international
of bus coupler circuit breaker will eliminate the CT as hi-lighted. The application depending upon journals and conferences.
a Trip No Trip
from zone 1 which in turn will trip zone 1 thereby the operation philosophy and the fault current
b Trip Trip
clearing the fault. magnitude would be the best input to decide
c Trip Trip the better option based on which the user is fee
Table 2 shows the trip matrix for various fault to choose the type of CT arrangement for their
d No Trip Trip
locations of Figure 2. It can be observed the relay station.
trips the faulty zone and there is no blind spot in
this scheme too. For fault located in b and c, both Table 2: Trip matrix for various fault location
the zones trips and opens the bus coupler circuit in dual CT arrangement

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 ELECTRICAL
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Developments in protection relay testing
Constant power output test being performed. test results automatically with the time curve, and
Stan Thompson - Product manager determines pass/fail.
The latest high-power voltage-current amplifiers Automatic ramp, pulse ramp and pulse ramp
deliver maximum compliance voltage to the load binary search capabilities Dynamic sequencing test capability
constantly during the test, and range changing is
done automatically under load. This ensures better The best test sets have all of these capabilities This feature provides multi-state dynamic testing to
test results and saves time, as the outputs don’t and they also allow users to use the control knob include trip and reclose timings tests for, typically,
have to be turned off to change ranges, as was to ramp outputs up and down manually. When up to 15 states, including lockout. Each binary
typically the case in older instruments. using the automatic ramp functions, users can input and output can be set with each state along
easily adjust the ramp increment to suit their with voltage, current amplitudes and phase angles.
In many cases, constant power output eliminates the requirements. With each state, the user may input values of
need to parallel- or series-connect channels when voltage, current, phase angle, frequency and phase
testing high-burden relays. Outputs are regulated Click-on-fault impedance relay test screen angle, as well as setting the binary input sensing.
and, therefore, they are unaffected by changes in Another key feature found in the latest instruments With some instruments, three-pole tripping as well
load impedance which, for example, occur with is a click-on-fault test screen facility for easy and as single-pole can be simulated.
saturating loads like inverse and extremely inverse convenient testing of impedance relays. Typically,
time characteristic electromechanical overcurrent Simplified testing of three phase transformer
all the user needs to do is to select from a menu differential relays
relays. the generic operating characteristics of the relay
Higher output current to be tested, or to select its exact operating In order to correctly test new three phase
characteristics from the instrument’s relay library. transformer differential relays, three
Best-in-class current amplifiers now provide 30 A compensations must be taken into consideration
at 200 VArms per phase continuous and up to 60 Click-on-fault impedance relay library by the relay; current magnitude compensation,
A for 1.5 seconds. Six of these amplifiers can be An integral relay library means that users simply pick phase angle compensation and the zero sequence
paralleled to provide 180 A at 1,200 VA or up to from a list of predefined operating characteristics current compensation. By performing transformer
360 A at 1,800 VA for 1.5 seconds. for popular relays from manufacturers such as differential tests for each sequence component
Recent developments have transformed the field verifies that the differential protection will be stable
of protection relay testing, with the latest multi- Low range with higher accuracy ABB, AREVA, General Electric and SEL. They then
enter the relay settings and select the type of for all symmetrical and non-symmetrical external
phase test sets in particular offering features and faults, and through-load conditions. In addition,
benefits far beyond those provided by even their The availability of a 1 A range provides enhanced fault desired – line-to-earth, line-to-line or three-
accuracy at low test currents. Low current ranges phase. The test will then progress down a line as tests will also confirm that the differential relay
recent predecessors. But exactly what can users will trip for any internal fault. The most modern
expect from these new test sets? Let’s take a look. typically also have reduced switching noise, which a ramp, pulse ramp of pulse ramp binary search,
improves regulation and accuracy with test currents looking for the relay to operate. With the shot test test systems can not only provide the six currents
Flexibility as low as a few tens of milliamps. method, users can also select a test point within required for testing two winding transformer
and outside the characteristic to quickly check the differential relays, but they also should include both
Test sets with fixed configurations deny their Constant power output voltage amplifiers relay under test. ANSI and IEC transformer models to accurately
users future proofing. In contrast, those that offer calculate the required test currents and phase
flexible configurations mean that future changes The new voltage amplifiers provide higher current Real-time click-on-fault impedance test screen
output at low test voltages, from 30 V to 150 V. angles relationships that get applied to the relay.
in requirements can be accommodated easily, From the transformer model, the test system should
conveniently and at modest cost. Flexible multi- This is important for users who want to test a This feature allows users to see, on the screen, the
complete panel of relays as a single operation, as operating characteristic of the relay under test with be able to take into consideration the transformer
phase test sets are now available that allow users vector groups, be able to eliminate zero sequence
to choose up to four voltage/current generators this is often impossible with older test equipment the test pointer moving in the impedance plane
that has voltage amplifiers with a limited VA rating. to indicate exactly where the test impedance is, when testing single phase to earth faults. Using a
(VIGENs) to provide four voltages and four currents, touch screen to select test lines, it should also be
or to combine the four VIGENs with one double- The new amplifiers also solve the problem of in real time. When the relay operates, a coloured
testing old-style electromechanical distance relays test point is displayed showing where the relay able to perform dual slope characteristic tests using
current generator to provide four voltages and one of the seven different Biasing equations, and
six currents simultaneously, or even to combine that can only be tested using a high-power voltage operated – the test point is shown in red if the
source. relay failed the test, green if it passed. The user can one of the four different slope characteristics that
three VIGENs with a single-channel voltage source the modern microprocessor based relays use today.
(VGEN) to provide four voltages and three currents. also see the test vectors changing in real time, or
Compact construction and light weight see positive-, negative- and zero-sequence vector Manually defined harmonic waveforms
Other options available with some of the most The best of modern test sets are very much smaller values. When the test is complete, the test results
versatile test sets include using five VIGENs with and lighter than their predecessors. Specifiers and are displayed for each defined test point with pass/ A recently introduced feature on top-of-the-range
both of their channels configured as current intending purchasers should, however, exercise a fail indication. test sets, this allows users to easily create harmonic
outputs to provide a total of 10 current channels. little caution in this respect. There are compact, waveforms consisting of a fundamental plus
This allows the latest multi-phase differential Binary inputs and outputs superimposed second, third and fourth harmonic
lightweight units available that have achieved their
protection relays to be tested conveniently. diminutive dimensions and weight by sacrificing User-configurable binary inputs to monitor contacts waveforms. Individual harmonic amplitudes or
performance, particularly in terms of output and high-current binary outputs are invaluable, and phase angles can be easily ramped as required.
In addition, the ability to interconnect multiple This feature provides harmonic restraint testing
units together, along with the convertible voltage/ power. These units are best avoided as, for most in leading instruments they can even be allocated
users, their limitations outweigh their benefits. user-defined names, such as Trip1 or Recl1, to with a single current channel and is also useful for
current channels, can provide up to 18 currents checking the performance of power quality meters.
for testing multi-phase bus differential protection make them even easier to work with.
Large, high-resolution colour display
schemes. Overcurrent testing with IEC/IEEE curve With the latest relay test sets offering so many new
An important innovation is the introduction of algorithms and relay-specific curves and useful features, now is a very good time for
Flexibility not only means that users can configure large, high-resolution touch-screen colour user those involved with protection system testing to
their test sets so that they match their exact interface systems that provide intuitive automatic, The test set evaluates timing tests by automatically take a long hard look at the test equipment they
requirements, but also that they can initially semi-automatic and manual control of the test comparing actual and theoretical operating times. are currently using. Undeniably, a new test set
purchase a unit with a modest specification safe set. In leading instruments, the screens are The user doesn’t need to refer to the manufacturer’s represents a significant investment, but if the new
in the knowledge that they can upgrade it to a full complemented by a context-aware control knob time curve to determine error, which means that test equipment prevents just one significant power
four- or even five-channel unit in the future, should that allows easy and convenient adjustments to be this feature is a big time saver. The test set displays outage it will, for most users, have paid for itself
they need to do so. made to key parameters related to the particular the time curve for the relay under test, plots the many times over.

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Meet IDA:
An interview with Megger’s
Dr. Peter Werelius
AB. When we discussed a name for our first
Jill Duplessis - Global technical marketing
product, we talked about a few requirements:
manager and Editor
preferably it should be a name as well as an
acronym and ideally the acronym should work
On September 27, 2018, IEEE-SA Standards Board
in Swedish and in English. My now grown
approved IEEE C57.161-2018 – Guide for Dielectric
up baby girl, born in 1995, is named Ida and
Frequency Response test. This work follows that of
besides being very personal to me, her name
other standards organizations, such as CIGRE’s TB
met our other requirements. In Swedish, it was
254, TB 414 and TB 445 – all technical brochures
IsolationsDiagnostikApparat, which was later
that address aspects of dielectric frequency
changed to IsolationsDiagnostikAnalysator, and
response (DFR) testing.
in English it was Insulation Diagnostic Apparatus
A Dielectric Frequency Response (DFR) test is an and later Insulation Diagnostic Analyzer. When Pax
insulation health assessment tool that is more Diagnostics took over the IDA from GE-Programma
discriminating than traditional dielectric screening in 2006, the instrument’s name was modified
tests such as power factor/dissipation factor. slightly to IDAX (Insulation Diagnostic Analyzer
DFR testing can be used, for example, to assess by Pax). Both IDA and IDAX are pronounced as
the insulation health of transformers, bushings, names, not acronyms.
instrument transformers and cables. In a complex
insulation system such as that of a transformer,
DFR testing provides a moisture assessment of the A true frequency domain
solid insulation, conductivity of the liquid insulation
method that speeds
and information about the thermal behavior of the
insulation system. up measurement is
accomplished by applying
With increasing attention being paid to the DFR
method, it seemed a very appropriate time for several frequencies Baby Ida, the eponymous hero of the IDAX
Megger’s Jill Duplessis to talk to Dr Peter Werelius simultaneously, and
who developed the first commercially available extracting measurement JD: There has been some talk about the need to compensation algorithm, this may rightly cause
DFR test set for field use. Here’s what he had compensate DFR test results for ageing byproducts; discerning users to grow uneasy.
data from the applied
to say. otherwise, in some cases, moisture results may be
frequencies simultaneously misleading. For perceptive users, this “red herring”
How can a user go about validating an instrument
JD: Thanks Peter. I’d like to begin with manufacturer’s database? Also, is there a more
congratulations to you and your peers for your of sorts puts focus squarely back on the database. pragmatic approach to take when a user has
collaborative work on the newly published IEEE JD: Can you tell us a little about the history Fundamentally, DFR analysis and the results it’s
legitimate concerns that aging of the asset may be
guide! Back in 1996, the first commercially of the IDAX instruments and how they have skewing moisture assessment results?
intended to provide, such as moisture content in
available DFR instrument designed for field use, developed over the years since the first model solid insulation and oil conductivity, is based on PW: The fact that aging byproducts, or more
which is known today as the Megger IDAX, was was introduced? fitting the measured DFR curve to its best match specifically low molecular weight acids, affect DFR
introduced. As its developer, what advice would
in a database of curves. The test software does results has been known by the industry for more
your “scientist self” of today give to your “scientist PW: The initial development of the DFR
this through mathematical modeling – adjusting than 10 years ( See: Dielectric response of mineral
self” 22 years ago? instrument was part of my Ph.D. project that
parameters that affect the shape of the model oil impregnated cellulose and the impact of aging,
began in early 1992 at KTH (Royal institute of
PW: I am fortunate that my personal guiding curve until the software finds the best match. The D.Linhjell et. al. IEEE Transaction of Dielectrics and
Technology, Stockholm). In 1994, Björn Jernström
principles have served the development of the DFR parameters include: Electrical Insulation, 2007). Our database and
joined me and helped me to turn my student set-
technology well. So I would offer to others my others are typically applied without any attempt
up into a practical instrument for field use. Late in „„ Relation of solid (cellulose) vs. liquid (oil)
firsthand experience that being generous to my to directly compensate the results. One database
1996, we sold the first commercial unit, IDA#1, insulation;
first and subsequent business partners (with whom attempts to compensate for the effect of aging
to Vattenfall, Sweden. The second unit, IDA#2,
I founded WaBtech and later Pax Diagnostics) and products by using measured oil conductivity but
which we delivered in spring 1997 to ABB Inc. in „„ moisture in solid insulation;
being honest and helpful to my former employer in my opinion this approach is not successful.
the USA, was already significantly changed from
along the way as well as early customers has „„ and oil conductivity. It was reported in the paper referenced above
IDA#1, so that it would fit into a case about half
come back to me in unimaginably, positive ways. that low molecular weight acids influence DFR
My partners, my employees at that time, my the size of the previous one. After Programma I think that it is important for users to recognize
response, but high molecular weight acids, which
former employer GE Programma, Megger, early Electric acquired WaBtech in late 1998, the IDA that there is not one universal database shared by
also can affect oil conductivity, do not affect the
customers, and KTH (Royal Institute of Technology) 200 instrument was developed and released in all instrument manufacturers. When you acquire
DFR response.
have been very committed and supportive. 2000. IDAX 206 followed (2006) and IDAX 300 a DFR instrument, you are selecting a database.
entered the market in 2008. At about this time, The database determines the accuracy of moisture In my opinion, the influence of low molecular
JD: I was really moved when I learned of the
competitors started to appear. Since then, we have assessment results. If differences that are observed weight acids is not a big problem. For dry
origin of the name “IDAX”. I understand that it is insulation, the DFR interpretation will give dry
improved the hardware, software and digital signal in moisture assessments using different DFR
quite personal to you. Can you explain? insulation. If the insulation is moderately moist,
processing algorithms. An important milestone instruments (or in results from a DFR instrument as
PW: Björn Bengtsson, who is now Björn was the introduction of the 2000V peak amplifier compared to other moisture assessment tools) are say with moisture content of 2%, the level of low
Jernström, and I were the co-founders of WaBtech accessory in 2011. explained away as being due to some undefined Continued on page 4...

3 ELECTRICAL TESTER - July 2019 www.megger.com

 ELECTRICAL
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Contents JD:
method is particularly susceptible to error due to Speaking of time, if I’m testing a three-
Continued from page 3...
interference. Is there a way to perform DFR tests winding transformer and I want to measure two
molecular weight acids is usually very low. The quickly without reverting to an approach that insulation components concurrently (e.g., CHL
only case where low molecular weight acids may combines FDS with PDC?
and CLT), this requires an instrument with dual
influence the moisture assessment is that of a very
Comprehensive analysis of Single CT in Bus PW: Any time that you can perform an entire measuring channels. If an instrument claims to
coupler compared to Dual CT for Bus Bar aged and wet transformer. However, acids are
measurement in only FDS mode, you have a better, deliver DFR test results for two components in
protection scheme........................................... p1 also very bad for the insulation, so a high content
Dr K N Dinesh Babu - Application Manager
more reliable measurement. The DC method has the same amount time it takes to test a single
of acid with low molecular weight combined with
Power System Protection Megger two major disadvantages. The first is fundamental component, am I truly getting the same number
what turns out to be an actual moisture content
and is that a DC current interference will appear as of measured values per insulation component
that is slightly less than estimated still requires
Meet IDA: An interview with increased losses (power factor/dissipation factor), as I would if I had measured each insulation
actions to be planned.
Megger’s Dr. Peter Werelius........................... p3 especially at lower frequencies. This may lead to component separately?
Jill Duplessis - Global technical marketing Before investing, decide how to treat the under- or, more commonly, over-estimation of
manager and Editor transformer. Complementing analyses, including moisture content. In a true frequency domain PW: In some dual channel instruments, both
insulation liquid analyses, are usually performed measurement, DC current interference will not channels share a single ammeter and, with these,
Big success for CT testing in India.................. p4 to get a more complete picture about the affect the measured losses. Secondly, you can either half as many measurements are made or
Ajay Goyal - Managing Director insulation condition. look at a DC (PDC) measurement as applying all no timesaving is realized. An instrument must
frequencies at the same time. This may shorten the have two ammeters if it is to perform two truly
Surge Testing Electric Motors – JD: Some years ago, a DFR test on a healthy
concurrent measurements.
measurement time but, by spreading the energy
Cooling the debate ..................................... P6,7 transformer may have taken well over one hour.
across all frequencies, it makes the method more JD: Most DFR test instruments provide a
Mike Teska - Product Line Manager Naturally, this was the source of some concern
sensitive to AC interference. maximum 140 V RMS signal (200 V peak),
in the industry since adoption of a test requires
Work safer on medium voltage systems........ p7 that its value outweigh its cost – and the cost, A true frequency domain method that speeds primarily because this voltage is sufficient to
Damon Mount - UK Power Sales Manager of course, is greatly affected by the time the test up measurement time is accomplished simply provide an accurate moisture assessment of the
takes to perform. by applying several frequencies simultaneously, solid insulation. In addition, the voltage source
Maximising Asset Reliability........................ p8,9 and extracting measurement data from the lends itself to a lightweight, easily transported
An early attempt to reduce DFR test time
Mark Meyer - Product Manager, PowerDB applied frequencies simultaneously. Of course, instrument. However, in some instances, a 1400V
combined the preferred frequency domain
Brad Perry - National Sales Manager, PowerDB the voltage level of each frequency needs to be RMS is required to improve accuracy of the
spectroscopy (FDS) method, which has better
reduced, which makes the measurements a bit measurement – or even to acquire intelligible test
immunity to interference, with the polarization
RADAR – IN 1904!............................................ p8 more sensitive to AC interference, but DC current results. Please share more about this.
and depolarization current (PDC) method by
Keith Wilson - Electrical Engineer interference will still not affect the measurements.
arranging for the instrument to switch to the use PW: The most commonly used voltage level
This multi-frequency approach is an advance over
of DC voltage for measurement of polarization is about 140 V RMS and this is enough for
Questions & Answers - the older approach of combining FDS with PDC,
current (PC) at low frequencies. These are the measurement of the CHL-insulation in most
Winding resistance testing........................... p10 which is more sensitive to AC interference and
frequencies where the FDS measurement is slow conditions. However, in situations where there is a
also quite sensitive to DC interference.
because it has to wait for the completion of high level of interference or when measuring CH,
POWERED BY… Hein Putter......................... p10
several very time-consuming cycles of the applied The time savings of the multi-frequency method CL, reactors, bushings and current transformers
test voltage. However, reliability suffered with are significant. For example, using three (CT’s), a 140V RMS signal does not provide a high
A guide for configuration enough measurement signal-to-noise ratio to get
this combined approach, especially in electrically frequencies simultaneously reduces measurement
& Testing of Blocking logic in
noisy field environments, because the PDC time by about 40% meaningful results. Using a higher test voltage
Bus Bar protection IEDs................................. p11

Big success for CT testing in India

visited private customers such as Sterlite Grid.
The rights of the individuals attributed in Electrical Tester to However, they encountered numerous problems,
be identified as authors of their respective articles has been
including the difficulty of taking the plant out
asserted by them in accordance with the Copyright, Designs
of service so that the tests could be performed,
and Patents Act 1988. © Copyright Megger. All rights
reserved. No part of Electrical Tester may be reproduced in a
sites in hard-to-reach locations, and the need
retrieval system, or transmitted in any form or by any means, for multiple validation sessions to confirm that
electronic, mechanical, photo-copying, recording or otherwise the instrument was delivering accurate and
without the prior written permission of Megger. repeatable results.
To request a licence to use an article in Electrical Tester,
More technical discussions were then held, as a
please email electricaltester@Megger.com, with a brief
outline of the reasons for your request. result of which Megger upgraded the MRCT to
incorporate 4 kV knee-point voltage testing, a
All trademarks used herein are the property of their respective
DC option, nameplate guessing and a turret CT
owners. The use of any trademark in this text does not imply
testing feature. Adding these features was a
trademark ownership rights in such trademarks, nor does use
specifications. Before it was ready to place an major challenge, but the Megger team in Dallas
of such trademarks imply any affiliation with or endorsement Ajay Goyal - Managing Director, Megger India
order for the instruments, however, PGCIL needed managed to complete the work in just two weeks.
of Electrical Tester by such owners.
to be sure that the instruments fully matched strict
Current transformer test sets have long been Megger India then tested and validated the new
technical and commercial qualification criteria, and
in high demand in India, but users had only a features on around 35 CTs (765 kV and 400 kV)
that they could be shown to operate satisfactorily
‘Views expressed in Electrical Tester are not necessarily handful of brands to choose from. However, in preparation for a final demonstration to PGCIL.
the views of Megger.’ on one of the company’s 765 kV sites.
Megger recently entered this challenging market For turret CT testing in particular, Megger made
The word ‘Megger’ is a registered trademark
and, in 2016, the company achieved a notable When the Megger team examined the eight site visits and tested close to 40 turret CTs in
success by securing a single order from Power qualification criteria in detail, it found that, as 765 kV substations. This exercise revealed not only
Grid Corporation of India Limited (PGCIL) for no things stood, it was unable to meet some of these. that the features had been correctly implemented,
fewer than 106 of its latest MRCT test sets. For example, the bidding documents stipulated but also that they delivered better results than
that the instrument must have a performance instruments from other manufacturers.
During 2014, PGCIL planned to double the
number of its 765 kV and 400 kV substations to certificate from a 400 kV substation in India.
Finally, Megger carried out a formal demonstration
over 200, with each substation having a minimum But after Megger had provided demonstrations
of the MRCT for PGCIL, which confirmed that
of 15 CTs. To ensure correct operation, it was at a number of PGCIL sites and engaged in in-
it fully met the PCGIL technical requirements
essential for the key parameters of each CT to depth discussions with the company’s engineers,
and also showed that the multi-tap testing
be verified against the manufacturer’s data as PGCIL relaxed some of its bidding requirements.
capability of the MRCT delivered big savings in
part of the commissioning process. To carry out Megger’s Dallas factory then arranged for the
test time compared with standard instruments.
this verification, PGCIL considered using primary remaining qualification documents to be supplied.
The exceptional performance of the MRCT,
injection testing, but this involves time-consuming
The next requirement was for Megger to the willingness of Megger to enhance the
set up procedures and the use of large, heavy test
demonstrate successful operation of the MRCT in standard design so that it complied with the
sets. The company decided therefore to adopt
a high noise 765 kV PGCIL substation but this was tender specification and the company’s ability to
secondary injection testing.
far from easy to arrange. To locate a site where demonstrate that all of PGCIL’s key requirements
A market survey of the available CT test sets the demonstration could be carried out, technical had been met, combined with very competitive
showed that the Megger MRCT offered a and product experts from Megger India travelled pricing, allowed Megger to secure the order for
number of key features that met PGCIL’s technical extensively to various regions of PGCIL and also 106 test sets.

4 ELECTRICAL TESTER - July 2019 www.megger.com

 ELECTRICAL
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The first commercially available DFR instrument designed for field use as a result of Peter Werelius’ Dr. Peter Werelius is the man behind IDAX – the
Ph.D. project, known today as the Megger IDAX, was introduced in 1996 The Megger IDAX today first field instrument for Frequency Domain
Spectroscopy (FDS) measurements.
(1,400 V RMS/2,000 V peak) in these instances every time data is needed, an alternate means and in some cases, this introduces quite large
will improve the measurement accuracy and is to determine an insulation’s equivalent behavior errors. Fortunately, by using our knowledge of Roland Eriksson, and later my colleagues and I
recommended. And, referring back to your earlier at a baseline temperature of 20°C is extremely DFR interpretation and the fact that modern made was to take DFR testing out of the laboratory
question, using a higher test voltage also makes valuable. A lot of information about an insulation’s power factor instruments can sweep over a range into the field. As you say, I have remained – and
it possible to successfully use multi-frequency thermal behavior can be accessed from DFR test of frequencies, we can now determine a much still remain – very closely involved with DFR
measurements to shorten measurement times in results. Can you explain a little more about this? more accurate temperature correction factor by testing and I am delighted to see how widely the
medium interference conditions. carrying out a limited frequency sweep on the
PW: The DFR interpretation used in IDAX – techniques that my colleagues and I devised and
and in other instruments – is based on the so actual test object.
JD: No discussion about insulation is complete developed are now being used. Of course, we no
without addressing the important topic of called master curve technique, where the DFR longer have the field to ourselves, but they do say
JD: From what you’ve told us, Peter, it’s apparent
temperature. It is widely understood that the response of the impregnated cellulose material that imitation is the sincerest form of flattery. With
that you were a true pioneer of DFR testing and
electrical characteristics of insulating materials can be shifted to another temperature by simply
that you’re still a very active innovator in the field. this in mind, I’m sincerely flattered by how many
changing the frequency axis based on the
change with temperature. Since diagnostic Is this a valid assessment? of our ideas other instrument manufacturers have
temperature difference while keeping the curve
tests on insulation rely on detecting changes in now adopted!
shape unchanged. The analysis is then performed PW: It’s a good summary, but it’s only fair to
the material(s)’s electrical characteristics, it is
at actual measurement temperature, e.g. 32°C.
important that only data that has been obtained acknowledge that my early work on DFR testing JD: Thank you for your time, Peter. I know you’re
at the same temperature be compared. This way, However, network frequency (50/60 Hz) power built on work that had already been carried out very busy but I’m sure your time has been well
any change in the measured electrical parameters factor measurements have traditionally been by others, notably by A K Jonscher in the UK and used and readers will be fascinated by the insights
can be attributed to a change in the state of the corrected to 20°C by using an approximate by Professor Uno Gäfvert, Dr. Bo Nettelblad and
you’ve given us into DFR testing past and present.
material and not a change in temperature. Since correction factor. It is well-known that this Kenneth Johansson in Sweden. The big advance
it is not practical to wait for same conditions correction factor depends on insulation status, our research group at KTH, headed by Professor PW: It’s a pleasure!

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Surge Testing Electric Motors –

This arc shows up as a pattern change in the Evolution of the Surge Test for finding insulation faults, and it opened up
Mike Teska - Product Line Manager surge waveform. numerous possibilities for improving insulation
Surge testing of motor coils has been an industry
testing equipment as well as insulating methods.
The surge test is performed with an impulse practice since 1926, when J.L. Rylander published
For many years, predictive maintenance and motor Even in these early days of surge testing, the
generator with a display that continuously shows “A High Frequency Voltage Test for Insulation of
reliability experts have debated the merits of surge the surge waveform. The surge waveform is benign nature of the test was stated in an article
Rotating Electrical Apparatus” in Transactions
testing. Questions about the benefits and findings the voltage present across the test leads of the written by D.J. Reynolds, R.J. Alke, and L.W.
of the AIEE. In 1926, an indication of a turn-to-
of the test have fuelled heated discussions on instrument during the test. The indication of turn insulation failure was a drop of coil voltage Buchanan of Westinghouse: “Because the energy
forums and in boardrooms around the world. a turn-to-turn fault is a rapid shift to the left or amplitude. This amplitude was determined by a of the surge is extremely limited, the current
Some in the motor reliability community think that a decrease in amplitude of the waveform. The vacuum tube rectifier circuit using an apparatus through the faulty insulation is so small that no
the very process of finding insulation weakness observed waveform is directly related to the coil’s the size of a large workbench with rotating spark severe burning occurs at the point of weakness.”
in an electric motor by surge testing damages inductance. In effect, the coil becomes one of two gaps for switches and large step-up transformers With the amount of testing and information
the remaining dielectric insulation so the motor elements in a tank circuit. This circuit is a LC-type, to charge large high-voltage capacitors. As gathering that Westinghouse was doing, one
will not continue to operate. Others believe that made up of the coil’s inductance (L) and the surge time wore on, there was a need for a compact, would think that they would not make this
testing above operating voltage is absolutely tester’s internal capacitance (C). portable machine that would produce a high statement lightly.
necessary to verify insulation integrity, and does voltage at relatively low currents. At this time,
The inductance (L) of a coil is determined by its There have been many advances in high-voltage
not precipitate insulation weaknesses. surge technology was not new; however, the
geometry (number of turns of wire) and the type testing since the 1980s. Broad markets and
So what is the truth? Can surge testing, done of iron core it rests in. The frequency of the wave machines used for transformer testing were technology developments in the electronics
properly, help industry reduce costly downtime, pattern is determined by: expensive and cumbersome. To meet a growing industry have helped manufacturers of motor
or is it just smoke and mirrors? Does the fact need for testing, the surge comparison tester was testing equipment make great strides in
that the motor manufacturing and motor repair developed. This compared the wave shape of two
modernization, reliability, and tester sensitivity.
industries specifically require surge testing to verify or more identical windings against a known good
Today’s high-voltage testers use advanced
insulation integrity outweigh the few that feel that This formula implies that when the inductance winding. This comparison made it possible to
high-speed electronic evaluation of changes in
surge testing undermines reliability? Or has there decreases, the frequency increases. study insulation faults as well as find them.
resistance, leakage current, leakage current versus
been so much misinformation published that the A surge test detects a fault between turns by In the 1950s, the surge comparison test was time, voltage, step voltage, dielectric absorption,
true benefit of the test has been muddied? observing a jump in the resonant frequency of still considered fairly new. It was very useful frequency response, wave shape, corona inception
This two-part article outlines the principle, proper this LC tank circuit. If the voltage produced by
performance and insulation effects of surge the surge is greater than the weakened dielectric
testing, and the second part includes motor test strength of the turn insulation, one or more turns
results that should settle the debate. may be shorted out by arcing. This effectively
decreases the number of turns in the coil. Fewer
Principle of the Surge Test working turns reduces the inductance of the coil
Whereas the insulation resistance (IR), polarization and increases the frequency of the surge test
index (PI) and HiPot tests are used to detect ringing pattern. The voltage or amplitude of a
ground wall problems, the surge test is used to surge wave pattern is determined by:
find turn-to-turn insulation weakness. Motor
winding insulation failures often start as turn-to-
turn failures that eventually damage the ground
Where:
wall insulation and lead to catastrophic failure.
L = coil inductance
The main claim of surge testing is that it can detect
di = Delta i (instantaneous change in current)
the early stages of a problem before it becomes
dt = Delta T (amount of time to change)
severe, providing an opportunity to repair or
Therefore, just as the reduction in L (due to a
replace without unscheduled downtime.
faulty coil) causes a frequency change, it can also
Turn-to-turn insulation problems can be cause an amplitude change. But this assumes di/
definitively found with the surge test. The surge dt is constant, which is not usually the case in a
test applies a fast rise-time, high-voltage impulse modern surge test. Typically, when we see the
to a winding, which produces a voltage difference small breakdowns that are useful for predictive
between adjacent loops of wire. If the insulation maintenance, the amplitude of the surge remains
between the two loops of wire is damaged or has the same, and only the frequency shifts. The surge
been somehow weakened through operation, tester has enough energy to keep the voltage the
and if the voltage difference between the wires is same into the slightly smaller L caused by the
Testing insulation with a surge comparator (Westinghouse article, July 1951)
high enough, an arc will form between the wires. breakdown.

6 ELECTRICAL TESTER - July 2019 www.megger.com

 ELECTRICAL
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– Cooling the debate

voltage (CIV) and more, to detect faults at or The old surge comparison test, where two repair facility, and even then some don’t want of coils, chemical attack, partial discharge,
below the levels of energy the motor is exposed to windings are compared to a known good it done at all. So why would you do it? What exposure to damaging transients, exposure to
during operation. “master” winding, has been modernized. Now is the essential benefit of surge testing electric radiation and variable frequency drive operation.
every surge pulse is digitized and compared to the motors? The surge test is the most efficient way For operators of electric motors, confidence
Microprocessor-controlled instantaneous trips previously applied pulse. This type of comparison to find turn insulation weaknesses. For those that a motor’s insulation is sound is necessary
allow winding conditions to be evaluated was impossible without computer-analyzed who manufacture or rewind motors, being able to maintain a productive and profitable process.
without compromising dielectric integrity. The waveforms. If any weakness is detected, the
to manufacture coils free of insulation defects is Properly performing a surge test to verify insulation
addition of field-developed PASS/FAIL test test is instantaneously stopped, preserving the
paramount to reliability. Therefore the surge test integrity is the easiest and fastest way to confirm
criteria makes this type of testing extremely dielectric integrity. This gives the test the ability to
is used universally in the manufacture of coils for motor viability.
repeatable. One of the major advances is the find micro-faults without human interpretation,
both small and large motors.
replacement of heavy step-up transformers and provides a higher level of repeatability. The second part of this article, which will
with compact and much lighter solid-state, Weaknesses are recorded and stored in a For motors in service, the dielectric strength of appear in a future edition of Electrical Tester,
high-voltage power supplies, leading to great memory bank or database for future reference the turn insulation slowly decreases with time will look at how to perform surge tests
improvements in equipment portability. Test and further evaluation. as the insulation ages. Some factors causing properly, how motors fail, and the benefits
equipment that once tipped the scale at more than Even with these advances, critics still feel that the insulation to age include thermal cycling, that modern surge testing techniques
500 kg now normally weighs less than 25 kg. the surge test can only be safely done at a motor vibration, abrasion due to mechanical movement can provide.

Work safer on medium voltage systems

a live environment, so that you don’t need to
Damon Mount - UK Power Sales Manager
take equipment out of service to carry out your
surveys. You won’t, however, need sophisticated
When you work on medium voltage functionality – you can always bring in other
switchgear, your first and overriding priority equipment when you want to investigate potential
will always be to maximise safety. But what problems more fully – but you will need versatility,
can you do to protect yourself against hidden so that you can deploy the widest possible range
hazards that are not readily apparent? Damon of PD detection techniques. A PD detector can
Mount of Megger looks into this issue and offer the versatility you need by employing
offers some useful suggestions. multiple sensors – both internal and external – so
let’s take a look at some of the possibilities.
If you’re professionally involved with MV
switchgear, you’ll know that there are many ways You will find an internal acoustic sensor –
in which it can fail. You’ll also know that the most essentially a microphone that listens for the
common is insulation breakdown, and that the characteristic noise produced by partial discharges
results can be catastrophic. Even a modest fault – is a good choice for PD testing on air-insulated
in an MV installation releases enough energy to MV switchgear and equipment that’s in direct line
wreck equipment and, if the circumstances are of sight. If you need more flexibility in where you
truly unfortunate, to kill. Which means that, when can place the sensor, you can achieve this by using
you’re working with or even near MV equipment an external acoustic sensor that connects to the
you want to be as sure as you possibly can be that detector via a cable.
such a fault is not likely to occur.
For fully enclosed switchgear, you’ll want a contact
Unfortunately, equipment that’s about to fail rarely probe that is sensitive to vibrations produced by
gives you an advance warning. Or does it? In fact, partial discharge, while for switchgear where the
insulation breakdowns are very often preceded by components are not visually accessible – mostly
partial discharge (PD) events. If you can detect and vacuum and GIS equipment – an internal TEV
monitor these events, you will, therefore, have a (transient earth voltage) sensor will pick up RF
strong indication of equipment and cables that radiation from the PD via the switchgear’s metallic
may soon fail. You can then investigate further enclosure. An external TEV sensor performs a experience to interpret. But you don’t need that and which can also be used to read QR codes
and, if necessary, take the equipment out of similar function but will also help you to localise level of detail for a safety survey. So how about an to identify equipment and documentation. It
service before a really serious problem develops. the PD source. instrument that provides you with “traffic light” has a humidity and temperature sensor so that
results? Green for all is OK, amber for caution – you can easily add these key parameters to your
At the very least, you’ll know that it’s a good idea If you have a PD detector that allows you to connect
consider investigating further, and red for danger data, and there’s provision for you to download
to keep your distance from the suspect equipment an HFCT (high-frequency current transformer)
– further investigation definitely needed. your results easily into a PC and quickly generate
if you want to stay safe. That’s reason enough for sensor, this will usefully extend its functionality to
owners and operators of MV systems to insist include simple on-line PD surveying of MV cables. comprehensive reports.
Of course, as you gain experience, you may well
that PD checks are ALWAYS carried out on their Finally, a parabolic acoustic sensor will let you want to be able to alter the thresholds for amber For all of its versatility and functionality, this
equipment before any work takes place on or carry out outdoor surveys to detect, for example, and red indications to suit your own specific instrument is as simple to use as a smartphone.
near it. corona and surface discharge in terminations, CTs, requirements, and you’ll almost definitely want a It has just three buttons, and a large colour
PTs and isolators. little more detail in the results so you can record
That’s all very well, but how do you carry out touchscreen that is intuitive in use and shows only
and trend them. A phase-pattern PD display those options you need to deal with the current
these surveys? Equipment to detect PD is nothing Let’s remember now that if you’re going to be
option will also be useful, as it provides a very operation, rather than forcing you to plough
new but, for the most part, it has been designed routinely carrying out quick pre-work safety
useful way to distinguish between true partial
not only to detect PD but also to investigate and surveys, your PD detector must be fast and through a rat’s nest of complicated menus.
discharge effects and random noise.
characterise it. That’s excellent in its place, but easy to use, so what should you need to do to
Every one of us in the electrical sector is constantly
the consequence of all this functionality is that configure it appropriately for the type of sensor in Hopefully by now you’re convinced of the
aware of the need for safe working, which is
the equipment requires skill to operate and to use? The answer is nothing! The detector should benefits of routinely carrying out PD surveys on
why many safety precautions are “baked” into
correctly interpret the results. Also, if you invest automatically set itself up for the type of internal MV equipment, especially before you start work
our routines: locking out circuits under test, for
in this equipment, it will undoubtedly serve you sensor you select, or for the type of external on or near it, and so you’ll be interested to know
example, and grounding equipment that’s being
well in demanding applications, but if you only sensor you plug in. This not only saves you time that a PD survey instrument that meets all of the
worked on. Now the easy-to-use equipment
intend to use it for MV switchgear surveys, you’ll and trouble, but also makes it near impossible requirements we’ve discussed has recently been
discussed in this article provides us with the
be paying for a lot of features you’ll never use. for you to make mistakes that might produce introduced to the market. This is the new PD Scan
opportunity to adopt another safety routine: that
misleading results. from Megger.
What you really need is a compact instrument of carrying out PD surveys before working on MV
– ideally handheld – that is very easy to use And what about those results? You may have In fact, this innovative device does rather more. equipment. The process is fast, easy and non-
and that provides clear, unambiguous go/no-go heard – or even know from your own experience It includes, for example, a camera that allows disruptive; the results can save not just money,
results. You’ll need an instrument that works in – that PD results can sometimes require skill and photographs to be easily attached to test reports, but lives.

7 ELECTRICAL TESTER - July 2019 www.megger.com

 ELECTRICAL
The industry’s recognised information tool TESTER

Maximising Asset Reliability

Mark Meyer - Product Manager, PowerDB
the end of their useful service lives, whether or The capabilities of PowerDB, however, go far These forms can be used as they stand, or users can
Brad Perry - National Sales Manager, PowerDB not this coincides with the end of their nominal beyond simply collecting, organising and storing easily customise them to suit their own particular
service lives. As well as the type of asset under data, although these are the central elements requirements. Users can also design new forms
consideration, the formulation of the testing of its functionality. Three key requirements from scratch if they have special requirements for
Regular maintenance on a fixed schedule is the
program will take into account the criticality of for an effective program of regular testing are which no similar form is available. No matter how
key to ensuring that assets operate reliably. At
the asset, the criticality of its function, and its scheduling, consistency and documentation. the forms are generated, their use enforces a high
least that’s the traditional view and it’s a view
operating conditions. Critical assets operating in PowerDB facilitates all of these. It supports, for degree of standardisation on the test procedure
that might have been justified in the days when
severe conditions will, for example, be scheduled example, automatic generation of work orders and the recording of results, thereby achieving the
the true condition of many types of assets could
for the most frequent testing. for testing based on asset maintenance periods consistency that is essential if the full benefits are
only be determined by dismantling them. But
and features extensive job listing with powerful to be gained from routine testing.
in today’s world, the shortcomings of regular This approach to maximising asset reliability clearly
filtering options for easy retrieval.
maintenance carried out at fixed intervals are too has a lot to recommend it but there are challenges PowerDB can also help with scheduling and
great to ignore, especially as modern technology to its implementation. Many of these relate to To help ensure consistency in work practices and compliance. The optional compliance view
makes possible a much more efficient and cost- the data produced by regular testing. First of all, reports, and to provide an intuitive user interface, makes it easy to monitor compliance with NERC-
effective approach. there’s a lot of it, especially in a large organisation PowerDB makes extensive use of forms for data mandated testing schedules or simply to track
like a power utility, so just collecting, recording entry, data display and for producing printed regular testing of assets or groups of assets.
But what are the shortcomings of fixed-interval
and storing it can be problematic. Then there’s reports. An important benefit is that the data Users can create compliance categories and sub-
maintenance? The biggest is unnecessary
the needle-in-the-haystack effect: when a large entry screens are exactly the same as the printed categories and assign these to individual assets or
expenditure – maintenance work is carried out on
amount of test data is available, it’s very difficult reports, which greatly simplifies interpretation groups. Powerful filtering capabilities then make
the asset whether it’s needed or not, and in many
to spot the occasional unexpected result that may of the test results. Close to 400 predefined test it a simple task to determine the compliance or
cases it’s not needed, because the maintenance
well be indicative of a problem. Analysing and forms are provided in PowerDB covering many maintenance status of these assets or groups. The
intervals have to be set based on worst-case
reporting on the mountain of data and providing types of electrical apparatus such as bushings, display screen for the compliance view includes a
scenarios. Unnecessary costs escalate further if
easy access to specific sets of data when they’re batteries, cables, generators, transformers, relays colour-coded calendar that provides an at-a-glance
the maintenance program includes automatic
needed – for example, when comparing a new
replacements of assets that have reached the end and motors. overview of asset status. The default colours are
result with historic results for the same asset – are
of their nominal service lives, as many of these will
other challenges.
be perfectly capable of giving further service.

All of that being said, it’s also true that studies It is with all of this in mind that Megger has
have shown that the most frequent causes of asset developed PowerDB, an asset and test data
failure relate to poor or inadequate maintenance. management system that’s specifically designed
So what’s the solution? The answer is to replace to make life easier for engineers and technicians
routine maintenance carried out at regular working with electrical power systems and also to
intervals with maintenance carried out only when help users maximise the reliability of their assets
needed and as determined by a carefully structured while minimising lifetime costs.
program of testing. Modern test equipment and
A key feature of this versatile software package
techniques allow the condition of almost every
is that it runs on a readily available platform – a
type of asset to be accurately assessed without
modestly specified PC with a recent version of
the need for dismantling or long periods out of
Microsoft Windows is all that’s needed – and
service, so this is a much more effective and much
that it can easily import data from a wide variety
less costly option.
of sources, including direct from many Megger
Routine testing will form a major part of the instruments, for which it provides a simple and
program, especially for assets that are already consistent user interface. It can also acquire data
in service, but the program will also include from selected third-party test sets and import
acceptance testing for new assets and pre-service existing test data in many popular formats, a
tests for assets that have undergone repair. And feature that is particularly useful for adding
there will inevitably be a need from time to historical data. Data can also be entered manually.
time for diagnostic testing to determine the
Once captured by PowerDB, test data is stored in a
cause of the problem when an asset is not
database, and the databases from multiple devices
performing correctly.
running PowerDB can be synchronised into a
The testing program will be designed to ensure single central database for added security and to
that, as far as possible, incipient faults are detected ensure wider accessibility. It is even possible to
before they progress to failures and that assets merge data imports from other sources – such as
are replaced when they have genuinely reached external databases – to aid analysis and trending.

8 ELECTRICAL TESTER - July 2019 www.megger.com

 ELECTRICAL
The industry’s recognised information tool TESTER

green for compliant, yellow for a warning period comparing new readings with historical values documentation including, for example, cover almost any standard internet browser running
and red for non-compliant. and with results from similar assets. Statistical pages, table of contents, comment and deficiency on a PC, a tablet or even a smartphone and it
information is produced which aids the evaluation summaries, test forms populated with test results opens with a summary view of activities, including
Another PowerDB option is an alert notification of test results and, therefore, assessment of the statistics about data entry and usage. Users can
and field service reports. This documentation,
system, or alert manager, will automatically send condition of the asset under test. then search and review entries in the database,
which can be printed or saved in the form of
email or text messages to pre-defined recipients which are presented in forms which are identical
Adobe Acrobat pdf files for easy electronic storage
when any time or activity threshold has been A wide range of analytical functions are available with those used elsewhere in the program.
and transmission, is an exceptionally useful aid for
reached, based on the information held in the for processing test results. These include the
planning future activities and schedules. Carefully planned testing is undoubtedly the surest
master database. This facility can be used, for generation of an asset ‘health index’ based on
example, to produce a reminder when an asset is information acquired from multiple tests, and Even though it offers wide-ranging functionality, route to maximising asset reliability, but it is only
becoming due for maintenance. The alert manager failure studies that can provide revealing insights PowerDB is intuitive in operation and easy to use. effective if the tests are carried out on schedule
into the failure rates for a population of assets. As has already been mentioned, it uses exactly the using prescribed procedures, and the results are
can also be configured to create notifications for
The intuitive PowerDB data query tool can be used same forms for data capture and for producing properly organised, stored, analysed and reported.
database entries that reveal deficiencies, invalid
to extract data from the database and present it These requirements might seem onerous, but
compliance status, out-of-limit test values and any reports, and there is a high level of uniformity
in a format that is optimised for use in further PowerDB, a powerful software package designed
other situation that can be determined from the across different applications.
studies or for processing by third-party diagnostic specifically to meet the needs of the electrical
data held in the database.
The simplicity and convenience of the standard power sector, provides a complete and cost-
software.
Versatile data trending and analysis functions are user interface can be further enhanced with the effective solution. To put it in a nutshell, a modest
further key features of PowerDB. Trending can be PowerDB can be configured to pass data to optional dashboard add-on, which is a web-based investment in software unlocks big gains in asset
performed on the fly while tests are being carried other computerised maintenance management portal that gives access to the data in the master reliability which, in turn, means money saved and
out. This offers a convenient and efficient way of systems (CMMSs) and to generate comprehensive database. The dashboard can be viewed using disruption eliminated.

RADAR – IN 1904!
It seems incredible that this system could be made
Keith Wilson - Electrical Engineer
to work given the technology available in 1904 – the
Telemobiloscope used a spark transmitter (essentially an
Most engineers and many non-engineers will have at electric arc connected to an aerial) and a coherer receiver.
least a vague idea about the beginnings of radar. It was
invented in Britain during the Second World War, wasn’t A coherer is a glass tube with metal filings loosely
it? And didn’t it help the Allies to win the war? The name contained between two metal plugs. Normally, the
electrical resistance between the plugs is high, but if the
Watson-Watt may even come to mind as one of the
device is exposed to RF energy (radio waves) the metal
leading pioneers. Well, all of this is true but it’s not quite
filings stick together (cohere) and the resistance falls
the full story. In fact, the course of history could have
to a much lower value. The device must then be reset
been very different because a form of radar was invented
by tapping it to disturb the filings so that it is ready to
and demonstrated in Germany in 1904!
respond to the next burst of radio waves.
The fascinating story is fully told in an article written by
Unfortunately, this means that coherers are inherently
V J Phillips and published in the July 1978 issue of the
sensitive to mechanical vibration so, as V J Phillips notes
British magazine, Wireless World. This article is, in turn,
in his article, they were far from ideal for use on board a
based on an item published in The Electrical Magazine
ship! Conversely, they were very insensitive to RF, so a lot
(London) in 1904.
of reflected energy from the “radar” target would have
The inventor was Christian Hülsmeyer of Düsseldorf, been needed to get any response at all.
and he called his invention the Telemobiloscope. The
Given these limitations, did the system actually work? It
Electrical Magazine notes that he demonstrated his
seems so, because The Electrical Magazine concludes
apparatus to North German Lloyd (a major maritime its piece by saying, “Experiments made so far on small
insurer), before going on to say: instruments designed for shorter distances [probably
less than a kilometre in this context] have given every
“The new invention is based on the principles of wireless
satisfaction.” British patent 13,170 was granted in 1904
telegraphy and is intended for viewing ships and
for the original apparatus, and British patent 25,608
metallic objects at sea. [No aircraft in 1904, of course!]
later in the same year for “certain improvements.”
In wireless telegraphy, the transmitter and receiver are
used separately on different ships, but both are on the Fortunately for the Allies in the Second World War, the
same ship with the Telemobiloscope. The electric waves, Telemobiloscope appears to have received little further
not being able to reach directly the receiver, must be attention, most probably because of the limitations of
reflected by metallic objects on the sea (that is ships) so the technology used. That left the field clear for Watson-
as to reach the receiver on a broken path. The advantage Watt and his team to develop the first practical radar
afforded by this invention is mainly that ships fitted with system, which had such an enormous influence on the
the transmitter and receiver will be able to view any course of the war. But, as we’ve seen, it could so easily These two diagrams are taken from the patent application. The top one shows
other ship devoid of these apparatus.” That’s a pretty have been the Germans, not the Allies, who had the the general principle, and the bottom one (presumably!) shows the arrangement
reasonable description of how radar works, isn’t it? advantage of radar in that conflict. of the apparatus.”

9 ELECTRICAL TESTER - July 2019 www.megger.com

 ELECTRICAL
The industry’s recognised information tool TESTER

Q&A
Winding resistance tests are used to assess the condition of the current carrying path between transformer bushing
terminals. Experts recommend that this test is carried out routinely as, arguably, it can reveal more incipient failure
modes than most other tests. Problems such as loose, defective or incorrect connections; open, partially-open (i.e.,
broken strands); short-circuited turns in windings; or high contact resistance in tap changers will result in a change
in resistance, and therefore be indicated by an unexpectedly high, low (for short-circuits) or unstable resistance
measurement. Here are some of the questions the Megger helpline frequently receives about this invaluable test.

Q: Some more experienced colleagues has to be dissipated before leads can be safely that the clamps make good contact with the A: It is often thought that higher test currents
have told me that we have only disconnected, which can take several minutes for bushing terminals. The transformer core must speed up the saturation of large transformers but,
recently started performing routine a large transformer. To safeguard personnel, the be saturated to obtain representative results; in the majority of cases, this isn’t true. Voltage
winding resistance tests because of asset under test, and the test instrument, Megger if it isn’t, errors in excess of 20% are possible. determines the saturation rate of the test, as
the dangers involved. Can you explain winding resistance test instruments are equipped Having an understanding of which instrument is given by: (flux) ɸ = Volts * t (seconds). Therefore,
these dangers and suggest how they with multiple safety features. These include: the best to do each job is helpful. Generally, the when selecting a test instrument, compliance
can be mitigated? „„ A safety circuit that provides an “escape test current should be greater than 1% of the voltages above 40V dc are preferred. Make sure
A: A transformer winding is an inductor and, if path” for energy dissipation. (This uses winding current rating but should not exceed
however, that the test current is greater than 1%
an inductor is suddenly open-circuited while dc current and potential leads to provide an 15%, as given in IEEE C57.152, to avoid heating
of the current rating of the winding under test. To
current is flowing through it, it will try to oppose alternate discharge path if leads are accidently of the winding. If the test current is too high, this
disconnected) improve saturation on transformer windings when
the change in current by developing a high voltage. may cause an unquantifiable error as winding
The voltage is given by V = IR + Ldi/dt, where I charging time is slow, connecting the primary
„„ Protection/automatic discharge when input resistance is temperature dependent. Check
is the dc test current, R is the resistance of the power is lost or when the test current is and secondary windings in series may help. This
that your results compare well (within a couple
winding, L is the inductance of the winding, and accidentally changed before discharging the speeds up the test by providing more volt-turns
of percent) between phases and with previous
di/dt is the rate of change of current. If a test lead existing test current. of charge. It is important, however, to ensure that
test results. Note, however, that when you make
falls off or is pulled off a bushing terminal during the windings are connected so that the current
„„ A test current indicator that remains comparisons with previous results, your measured
the test, a dangerously high voltage will develop. through them produces a unidirectional flux in the
functional even when instrument power is lost results, Rm, must be converted to temperature-
For example, if the winding has an inductance core. With this setup, the test current will flow
„„ An emergency off switch compensated results, Rs, to take into account the
of 100 H, the test current is 25 A and di/dt =
temperature difference between the two tests. through the high voltage winding and the low
25/0.1 (that is, the time taken for the test lead Q: How can I be sure that my
winding resistance test results are This is particularly important if the previous test voltage winding in series, while the dc voltage
to disconnect and the current to fall from 25 A to
representative? results are factory measurements. drop will only be measured across the low voltage
0 A is 0.1 s), the voltage developed will be more
than 25,000 V! In addition, when dc test current A: Make sure that the voltage (measuring) leads Q: My test results are not stabilizing winding, for example. Since current will not flow
is injected into a winding for the duration of a are “inside” the current (source) leads – using and I’m questioning whether the into the X1 bushing terminal, be aware that
winding resistance test, a lot of energy is stored Kelvin clamps will remove any doubt about transformer core is saturated. What the X1 bushing connection integrity will not be
in a transformer’s magnetic field. This energy test connections but you’ll still need to be sure should I do? assessed during this variation of the test.

POWERED BY… Hein Putter

Thanks to its team of extraordinary people, of the contact and discussions I have with our Tell us something about you that not many
Megger is able to bring to market exceptional customers, sales teams, R&D people and others. people know.
innovations and great products that make life The wide range of people I deal with makes my
easier for everyone whose work involves electrical job really interesting; every day is different, and Rules and regulations are everywhere, and the
measurement. One of those great people is every day brings new challenges. Netherlands, where I was born, is no exception.
Hein Putter, and we’re very pleased to interview When I was doing my Master’s degree, I rented
him for the latest instalment of our “Powered What’s been your biggest work-related a flat with a friend and, nine months before
by …” series. challenge to date? we finished our studies, we were visited by
a representative of the local municipality. He
Hello Hein! Please tell us a little about what Adapting to local cultures! Every country has its
inspected our flat and, after about five minutes,
you do at Megger. own culture and when you work in a country or
he left. We thought that was it but, a couple of
even with a country it’s essential to learn about
weeks later, we had a letter from the municipality
I’m Product Manager for Megger’s German site, and understand the local culture. If you don’t,
which means I’m responsible for the test and doing business, getting information and building saying that we were using the flat illegally and
diagnostic equipment that the company develops relationships becomes very difficult. we must leave. Eventually we found out what the
and manufactures in Germany. problem was – the flat was designated for use by
What do you like to do in your spare time? families only, but not even our landlord knew. To
What is the thing you most enjoy about get around this, my friend and I had to say that
As I travel a lot, I like to spend most of my spare entire world. One of the first things that comes to
your job? we were gay and the municipality even made us
time with my family in and around our home. I mind is free vaccinations for all so that everyone
put it in writing before they would accept it and
Well, being a product manager is not the world’s have two daughters, who are one and three years has a chance to live a full life. And it might help
let us stay in the flat!
easiest job. You’re “in the middle” between all old, and love spending time with them – except us to completely eradicate some diseases. Also,
departments and, if something goes wrong, you’re at night-time when they won’t go to sleep! They If you were President of the World, what I would ban single-use plastic bags. I travel a lot
the one who gets the blame! You need to have give me a lot of energy and inspiration and, to would be your first executive order? and in my travels, I see evidence that these bags
broad shoulders, which I don’t, but hopefully my be honest, they give me a good excuse to be are major source of pollution everywhere in the
height makes up for this. Being serious though, young again and do things that adults shouldn’t This is tough question, as it’s really not easy to world. We need to think about the future and
I really enjoy being a product manager because really do. come up with something that would benefit the stop using them.

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September 2018

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Continued on page 2
www.megger.com
ELECTRICAL TESTER - September 2018 1
ELECTR
ICAL
TESTER
- Jan
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2018
1

10 ELECTRICAL TESTER - July 2019 www.megger.com

 ELECTRICAL
The industry’s recognised information tool TESTER
A guide for configuration & Testing of Blocking
logic in Bus Bar protection IEDs
Dr K N Dinesh Babu - Application Manager
Power System Protection Megger

CT ISSUES AND BB BLOCKING LOGICS

The CT trouble logic in bus bar protection relay

is used to identify the flow of differential current
in the IED due to errors in the secondary system
and not a bus fault. The IED can discriminate
a bus bar fault and a secondary system fault by
the comparison of additional security measures
like the directional principle and the bus voltage
supervision check. Check zone cannot identify this
issue unless it is using a separate CT or a different
core from the same CT. Even different core of
the same CT may not meet the requirement since
open CT conditions affect the CT itself and it does
not reproduce the primary current properly.
Based on the above verification measures, the CT
trouble function is detected on failure of a CT and
differential is blocked. However in case of a real
bus bar fault followed by the CT trouble; the relay
will not trip, since the CT trouble functionality
is blocking its operation. In IEDs, this issue may
be handled by implementing the logic shown in
Fig.1. In case of a CT trouble, the IED verifies the
healthiness of the bus bar voltage and bus bar Fig. 1. Zone 1 block logic – DC changeover security
protection is blocked only during voltage healthy ADDITIONAL SECURITY FOR BATTERY the DC switching is completed. The relay will K N Dinesh Babu is a
conditions. When there is a bus fault following still be powered ON during this changeover as professional power
FAILURE DURING MAINTENANCE
a CT trouble, the bus voltage collapses and the it normally takes more than 100ms to switch off system protection
blocking command is unblocked dynamically, During maintenance of any bus, the differential due to the use of capacitors in the power supply. engineer in India with
thereby executing a tripping command protection will be blocked using a switch. Let Alternatively, this issue can be handled by utilizing extensive experience in
successfully as desired. In installations where bus us assume that some activities are being carried a non-volatile latch with dual inputs from the cut- HV / EHV sub stations
PT is not available, or in situations when the PT out using CT secondary shorting mechanism off switch. The latch will be reset only when the and industries. His
fuse fails, a ‘NOR’ logic of ‘instantaneous over and PT fuse removal such that, the IED measures reset input is high by the change in state of the work background
current (IOC) is used as a backup as shown. a differential current during healthy condition switch. includes experience in working for Alstom,
During a healthy (no fault) scenario, the current in which results in picks up in check zone. If the During DC failure scenarios as explained above, ABB, GE and Megger in various roles like
all the IOC elements will be below the threshold bus bar is having very less no of bays in service both the inputs will be low and the latch will testing; commissioning; design and application
settings which will ensure that the second input (say 2 or 3), there are more chances that the main maintain its previous state thus preventing such engineering. A major portion of this experience
of the second ‘AND’ gate is high, thus permitting zone directional principle is also satisfied. In this unwarranted operation. has been in the area of protective relaying with a
a block during CT trouble conditions. The CT situation, the block input is the only source that major focus on the application and coordination
The above sections ensure proper tripping and
circuitry issues will increase the differential is holding the bus bar from tripping. During this of protective facilities on electrical power systems.
blocking of Bus Bar protection under different
current but not the IOC element. Any of the IOC condition, if there is a disturbance or a fault in His experience includes the development of
scenarios. These conditions may be updated in
elements will pick up during a fault which in turn the DC source, the DC changeover switch, will protection philosophies, implementation of
the testing procedure to verify the performance
blocks the CT trouble element, thus permitting a switch the DC from source 1 to source 2. The new technologies in the field; control logic
of the IED operations using the State sequencer
tripping. In this manner, the blocking operand switching time may be less than 10 ms, however requirements for protective systems; development
options available in RTMS software. To reproduce
will block bus bar protection. during this period, the block input will be low and of specifications for protective relay settings; and
a real bus differential scenario, the need for more
In certain situation, it has also been observed that as the other conditions for tripping are already analysis of disturbances and faults in electric
number of current inputs plays a vital role and
during a rare scenario of battery failure during satisfied, it would result in bus bar tripping. To power systems. He has also experience in training
Megger introduces the option to connect multiple
maintenance activity, the blocking logic fails to prevent this kind of scenario, a drop out timer protection engineers, field engineers and O&M
test sets in series and the software treats all the
block bus bar resulting in an unwanted tripping. as shown in Fig.1 is included in the block logic. engineers on IEDs, SCADA, power system
units as a single test set with which multiple
This issue is address using a logic which is detailed This time should be adjusted to twice that of protection and synchrophasors for major utilities
current inputs can be simulated. Figure 2 shows
below. the DC changeover time thereby assuring that and industries in India and Abroad.
the testing of Bus bar differential relay using 18
the input will be latched for a duration when He holds a doctorate in electrical engineering and
current inputs.
is a DFSS certified six sigma green belt. He is a
The sophistication of Megger test sets to play
senior member of IEEE. His areas of contribution
multiple test sets helps the user to even reproduce
are electric traction, power system protection and
a real bus fault by playing back the oscillograph
renewable energy utilization and have published
file where many current inputs would be present
several articles in international journals and
in reality. The software can also playback the
conferences.
digital signals and hence scenarios like local
breaker backup (LBB) can also be simulated and
verified.

Conclusion:

The above article may provide the user to

configure the blocking logic of Bus Bar protection
to differentiate between a secondary system
fault and a real fault to act accordingly. In
addition the facility available in sophisticated
relay testing equipment would further assist the
user to test and verify the developed logic in a
proper manner ensuring proper operation of
the bus bar protection relay in accordance with
the specification of the utility and prescribed
Fig. 2. Megger Software (RTMS) used to test Bus Bar with 18 currents standards

11 ELECTRICAL TESTER - July 2019 www.megger.com

 ELECTRICAL
The industry’s recognised information tool TESTER
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Protection Testing
Onsite Training Service
Megger India now offers an onsite technical
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Megger India now provides an
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onsite protection relay testing and
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The course comprises a series of
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12 ELECTRICAL TESTER - July 2019 www.megger.com