Tesla Solar Test

... p8 flares vans

...p7 ...p4 Published by Megger
November 2015

ELECTRICAL
TESTER The industry’s recognised information tool

Pushing past the limitations of power

factor testing

„„ Use a core ground, if easily accessible, as a all medium-power transformers sent in for evaluation winding while measuring the leakage current flowing
Robert Breazeal - Southern California Edison test point; many transformer manufacturers and refurbishment, further segmenting CH and from the insulated primary winding to the secondary
Jill Duplessis - Global technical marketing now bring the core ground out through the CHL components into 3 parts each. It was found winding or ground. In this scenario, a voltage gradient
manager and Editor tank wall for test convenience. that in 5% of these 95 and 150 kV BIL class units is established across the winding, whereby the
10 years old or less, significant differences existed voltage at H1 is at 100% of the applied voltage and
„„ Use the DETC (de-energized tap changer)
Introduction in “per-phase” CHL values (e.g., CHL1; CHL2; and the potential at H2 is essentially zero.
set between on-tap positions. Note, however,
Line-frequency power factor tests – also known as CHL3). While GST test-derived values did not seem to
that many transformer owners are reluctant (1) For an overall CH test of the winding as given in
tan δ tests – are among the most widely performed to allow the DETC to be moved, so this significantly change with age, UST test-derived values
(i.e., CHL) drifted upwards significantly as insulation Fig 3,
field tests on power assets. It is not always completely approach is unlikely to be suitable for routine
understood, however, that these tests have a serious testing. It can nevertheless be a useful option began to deteriorate. “Per-phase” CHL differentials of CH Capacitance Current = ICA + ICB + ICC + ICD + ICE
limitation – they are averaging tests. In other words, when investigating a known problem. up to 0.25% were found. Some units had incipient + ICF + ICG + ICH + ICI
they deliver a result that represents the average anomalies, while others appeared to have been
„„ Use the cross-check test. This has been subjected to unbalanced loads. (2) If a single “per-phase” test is performed by
condition of the total insulation under test, however
popularized in North America for testing energizing H1 and guarding H2, test capacitive
big or small that might be.
potential (voltage) transformers, but can also Physical Arrangement of Components current equals:
This averaging affects the sensitivity of power factor be applied to power transformers with a – Core Form 3 Phase Winding
ICA + (7/8)ICB + (3/4)ICC + (5/8)ICD + (1/2)ICE + (3/8)
tests to problems, particularly in assets like power delta-wye (delta-star) configuration. Assemblies
ICF + (1/4)IC0 + (1/8)ICH + (0)ICI
transformers where the insulation system is large and The winding assemblies of the subject SCE
The second method of overcoming the limitations of
the problem affects only a small part of the system. transformers were of core form construction that uses If it is assumed that the condition of insulation is
line-frequency power factor testing is that of using a
It also means that power factor tests are unable to multiple frequency test, wherein one option is to use a a three-leg configuration (versus, for example, the five- homogenous for the entire winding, mathematically
discriminate between severe, localized problems power factor test set capable of operating over a wide leg configuration often used in the shell type core). it may be assumed, from the second equation, that
that demand immediate attention and widespread frequency range. Test sets in the Megger Delta 4000 As illustrated in Figures 1 and 2, the three individual the measured capacitance and watt losses in the
general deterioration that may only require regular family support testing at frequencies from 1 Hz to 500 coil packages are concentrically arranged around crosscheck will be very close to 50% of the measured
monitoring. Hz. Another option is to carry out dielectric frequency the vertical members of the core. The individual coil capacitance and watt losses in the overall test. If the
response (DFR) testing – also known atsfrequency assemblies are constructed so that the low-voltage
There are two principal ways of addressing the Top core iron

domain spectroscopy – using a test set of the type winding is located on the inside of the assembly. This
sensitivity problem of line frequency power factor
Secondary winding

Secondary winding

Secondary winding

Secondary winding
Secondary winding

Secondary winding
Primary winding

Primary winding

Primary winding

included in Megger’s IDAX family.

Primary winding

arrangement isolates the primary coil packages from

Centre core leg

Primary winding
Primary winding

Right core leg

Left core leg

testing. One way is to reduce the amount of insulation

A phase

B phase

C phase
A phase

A phase

B phase

C phase
B phase

C phase
A phase

C phase
B phase

Multiple frequency insulation testing will be covered in the vertical (ground potential) core legs and facilitates
under test by (electrically) dividing the asset under
a future article in this series, but the remainder of this efficient phase/tap lead routing.
test into as many sections as possible. The other is to
perform the test over a range of frequencies rather article will discuss how Southern California Edison (SCE)
Basis of the cross-energization process
Bottom core iron

than just at line frequency. has successfully adopted methods based on electrically Figure 1: Mid Sagittal cross section view
dividing the insulation of power transformers into even When a standard overall power factor test is
Looking at the first approach of dividing the asset further sections than typical to facilitate diagnosis of performed on a single-phase winding, whereby the Primary Primary Primary
under test into sections, the technique adopted suspect transformers, specifically relying on the cross- winding terminals are short-circuited, the full applied Secondary Secondary Secondary

will depend on the type and configuration of the check and the open DETC methods. test voltage is impressed across all points of the Core Core Core

asset. With transformers, for example, some of the energized winding. When the “per-phase” cross-
possibilities for sectionalizing the insulation beyond Background to SCE experience check test is performed, the H1 terminal is energized
the typical segmentation (e.g., CH, CHL and CL on a In early 2013, SCE Distribution Apparatus Repair and the return lead is connected to the H2 terminal
Figure 2: Transverse cross sectional view
two-winding unit), are: began using a “per-phase” power factor protocol on to guard the excitation current flowing through the
..continues on p2

www.megger.com ELECTRICAL TESTER - November 2015 1

 ELECTRICAL
The industry’s recognised information tool TESTER
Contents Pushing past the lim
..continued from page 1

test is repeated by energizing the H2 terminal, the

sums of the watt losses and capacitances from both
cross excitation tests will nearly exactly match the
values from the overall test.
Transcending the limitations of power
factor testing........................................p1-3 Application
Robert Breazeal - Southern California If a high loss anomaly exists on coil G of the
Edison and Jill Duplessis - Global technical transformer shown in Figure 3, it will be detected in
marketing manager and Editor the overall test. The overall test is therefore useful,
but will not provide information about the location of
New editor at Electrical Tester...............p2 the anomaly. When the cross-check test is performed
Isobel Fraser-Underhill - Group graphic by energizing the winding from the Coil A side, the
designer voltage at Coil G will be dissipated to a level where Figure 3: Primary winding
the measured losses from the anomaly at coil G will be
Settlement metering - a new code of practice...p3 very low. Conversely if the test is performed from the two primary terminals to shunt the excitation as well as three CHL results, it is not known to be
Coil I side, the power factor at coil G will be measured current around the metering circuit. This creates possible to calculate the current and loss for each
Tony Wills, Technical support group leader
at 75% of applied voltage and losses measured at a voltage gradient whereby 100% of the applied phase alone. Rather, if a problem is isolated to a
coil G will be significant. An anomaly located in the voltage is applied at the energized primary terminal single phase, this problem should be evident to
Transformer condition assessment with
center of the winding will result in equal watt losses and essentially zero voltage exists at the return lead varying degrees on two of the three cross-check
an integrated test van.........................p4-5
as measured from each side. connections. measurements but completely absent in the third test
Denis Denissov - Business development in which the questionable phase is short-circuited.
manager Because of the voltage gradient, most of the current
Physical Geometry of Delta Wye Robert Breazeal of SCE suggests that there is more
passing through the metering circuit is from the
Configurations to consider: “Properly interpreting UST test data
Current transformers in electrical region in proximity to the energized terminal. If the
The cross-check method works most reliably on a requires taking into account the physical geometry of
diagnostics...............................................p6 assumption is made that the condition of the winding
Delta-Wye configured transformer with a three leg insulation is homogenous, the third of the winding the individual coil assemblies. The primary winding is
Dr Stan Zurek, MSc, PhD, SMIEEE - located on the outer portion of the coil assembly. In
core as given in Figure 4. segments nearest to the energized terminal accounts
Manager of magnetic development for approximately 60% of the metered current. The the delta primary configuration, each terminal lead
Figure 5 provides an illustration of a three-phase core- center third of the windings account for 33%, and splits with one leg connected to the outside end of a
GIC: a major risk factor for the power form construction discussed previously, and given in the third of the windings adjacent to the return leads primary coil, and the other leg connected to the inside
grid...........................................................p7 Figure 2, but with the physical connections of the account for less than 8%. end of the primary winding on a different phase.
Jill Duplessis - Global technical marketing delta-wye winding superimposed. The low-voltage Because the HV/LV barrier is physically located on the
Any anomaly in the winding insulation which is in
manager and Editor coil packages are concentrically arranged around inside edge of the primary coil, the applied voltage
the portion of the windings adjacent to the voltage
the vertical core legs, an interwinding barrier with oil which enters the primary winding from the outside
source results in a higher power factor than when the
Get up to date with the latest in ducts is installed around the outside edge of the low- edge of the primary is 95% dissipated by the time
other terminals are energized. In cases where tests
cable test..................................................p7 voltage package, and the primary winding package is the applied voltage reaches the barrier. Conversely,
indicate the anomaly is located near the center of the
installed tightly around the interwinding barrier. The winding, the DETC may be opened to sectionalize the the voltage which is applied on the inside edge of the
Tony Wills, Technical support group leader
lead from the outside layer of the primary winding in winding to determine where the anomaly is located primary coil on the opposite leg is essentially at 100%
the center winding assembly is connected to the H2 in relation to the DETC. A significant anomaly in a as seen from the barrier because the inside primary
Q&A.......................................................................p8
bushing. The center winding assembly is designated winding is detectable from either end of the winding, lead enters the winding at the barrier.
as B phase. The leads from the outside layer of the but in cases where the anomaly is small and located
The perpetually inspiring The significance of this arrangement is that when per-
other 2 winding packages are connected to the H1 at the extreme end of one winding, the power factor
Nikola Tesla.............................................p8 phase UST measurements are performed, virtually all
and H3 terminals. The outer lead connected to the deviation as seen from the other end may be only
Keith Wilson, electrical engineer of UST current and watts are derived from a single
H1 terminal is designated as A phase, while the outer several hundredths of a percent. On this basis the
winding despite the fact that two windings are
location of an anomaly can be determined in terms
lead connected to the H3 terminal is designated as energized. In GST measurements, a CH anomaly may
of its phase relationship and by its location in the
C phase. be detectable from both directions depending on
winding relative to the terminals.
A jumper is connected from the A phase lead on the the location and nature of the anomaly, but a CHL
The Effect of Physical Geometry on UST anomaly detected in a UST test will be nearly invisible
inside of the winding to the lead connected to the H3 and GST measurements
terminal. A second jumper is connected from the B as seen from the outer portion of the winding. When
phase lead on the inside of the winding to the lead Generally, even though three CH values are generated the open DETC test is performed, the winding that
connected to the H1 bushing. The final delta jumper is !# !$ !"
When you have finished with connected from the C phase lead on the inside of the
this magazine please recycle it.
winding to the lead connected to the H2 terminal. Not
%$
shown in Figure 5 is the gang operated de-energized ' ( ) !$

tap changer (DETC) on each phase. %# %&

The rights of the individuals attributed in Electrical Tester to be

Theory of the GST Cross-Check Test for !# !" %"
identified as authors of their respective articles has been asserted CH Anomalies
by them in accordance with the Copyright, Designs and Patents
The delta-wye (or delta-star) “per - phase” cross-
Act 1988. © Copyright Megger. All rights reserved. No part %# %$ %" %&
check test is performed by applying a test voltage
of Electrical Tester may be reproduced in a retrieval system, or Figure 4: Delta Wye winding configuration
on one primary terminal while guarding the other
transmitted in any form or by any means, electronic, mechanical, Figure 4 Delta Wye Winding Configuration
photo-copying, recording or otherwise without the prior written !

New editor at Electrical Tester

permission of Megger.

To request a licence to use an article in Electrical Tester, please

email electricaltester@Megger.com, with a brief outline of the
Jill is particularly quick to speak of the value of her
reasons for your request. Isobel Fraser-Underhill - Group graphic designer experiences whilst working for electrical utilities at the
All trademarks used herein are the property of their respective onset of her career so without further ado, here is an
owners. The use of any trademark in this text does not imply
introduction note from Jill herself:
Faithful readers of Megger’s Electrical Tester magazine
trademark ownership rights in such trademarks, nor does use “I would like to begin by thanking our readers for
will recall the first edition of the publication back in
their continuous support and for making ET the
of such trademarks imply any affiliation with or endorsement of 2007. After years of providing electrical engineers
with insightful and exciting news, we are now proud successful resource for our industry that it is today. I
Electrical Tester by such owners.
to announce that ET has a new editor at its helm: welcome you all to our newest edition and pledge my
Jill Duplessis, Global Technical Marketing Manager passion and dedication to providing readers with the
A printed newsletter is not as interactive as its email equivalent
for Megger. same high standard of information, articles and news.
Jill joined Megger in March of this year. Before that, she This is an exciting and transformative time in our
so to help you find items quickly on www.megger.com, we have
was Director of Power Programs for SmartSenseCom,
underlined key search words in blue.
industry. I am grateful that my path has allowed
where she was responsible for developing, testing
me to gain multiple perspectives around operation,
and deploying new applications of the company’s
state-of-the-art optical monitoring systems (based on maintenance and test of the grid and its components;
‘Views expressed in Electrical Tester are not necessarily data; emerging test technologies and future grid
newly declassified technology from the United States
the views of Megger.’
Navy) for transmission and distribution utilities. technologies; and even more appreciative that the
The word ‘Megger’ is a registered trademark too in test solutions for a multitude of components
With over 17 years of experience in the condition journey increasingly holds me in a creative and open-
and applications that together comprise our electrical
assessment of substation assets, first as Principal minded space. systems, from generation to consumption. Megger
Editor Jill Duplessis Engineer at Doble Engineering Company and then is doing a lot and offers a lot of solutions so it may
I have been privileged to meet and work with diversely
in her capacity of Primary Manager and Regional be that your first impression doesn’t go much further
E electricaltester@megger.com experienced and talented individuals and couldn’t be
Application Specialist, Transformers, for Omicron
Megger Limited more thrilled to find myself now working among than this. But there is much beneath this shell and it is
electronics USA, Jill is considered a specialist in
truly a company worth getting to know!
Archcliffe Road Dover Kent CT17 9EN power transformer diagnostics, including emerging some of the brightest here at Megger.
test technologies. In addition to a number of papers, As I embark on this fun, new chapter, I encourage your
T +44 (0)1304 502100 Megger truly embodies “all things insulation” in its
she is the author of the industry acclaimed book “letters” and feedback as I am similarly very keen to
E electricaltester@megger.com history, depth of understanding of dielectrics and get to know you, our readers, too!” Correspondence
titled, “Electrical Field Tests for the Life Management
www.megger.com of Transformers”. behavior, and direction but are experts and leaders may be sent to electricaltester@megger.com.

2 ELECTRICAL TESTER - November 2015 www.megger.com

 ELECTRICAL
The industry’s recognised information tool TESTER
limitations of power factor testing
Primary Primary Primary During the “per-phase” CHL tests, similar extraneous
X2 contributions due to induced voltages on the LV
Secondary Secondary Secondary windings are included in the measurement. These
too tend to stay invisible (since they cancel with
X0 H2
Core Core Core each other) until there is a problem in a LV winding
which upsets balance in the contributions from each
LV winding. In such a case, the test results will be
X0 affected much like those of the CH “per-phase” tests.

Table 1 contains data from “per-phase” testing

performed on a medium-power transformer in good
H3 X3
C B A H1 condition. In this example, a 2.7 MΩ resistor was
connected from the X1 terminal to ground in order
to simulate a ground fault on the outside edge of the
X1 X0
H3 H2 H1 A phase secondary winding. The test specimen in this
CHL case is a 750 kVA unit with a 12000Δ primary and a
Figure 5: Winding conneciton diagram CH
Meter 4160Y/2400 secondary.
contributes the smaller portion of the current/watts is is impossible due to low primary winding insulation Figure 6: CH1 Test with Ground Fault on X1 end of Upon examination of the “per-phase” data, it is
removed from the circuit. Even if a significant problem value and/or high excitation current from shorted LV Winding evident that the values for the “per-phase” cross-
exists on the winding which is dropped out, the net turns. In cases involving a secondary fault, “per-
becomes visible, that is: during the CH cross-check
1 check tests closely follow the pattern outlined
change from the standard per-phase UST test for that phase” test data is seemingly difficult to interpret due
test, voltage is induced in low voltage windings X1 – previously. Conversely, the open DETC “per-phase”
phase will only be a few hundredths of a percent.” to what appear to be erratic values.
X0 and X2 – X0, because flux is moving in opposite test data exhibits no influence of the ground fault on
Effect of CL Ground Faults on In Figure 6, a ground fault is shown to exist in the the UST and GST measurements.
directions in A and B core legs during this test, the
“Per Phase” Test Data (Cross-Check and vicinity of the X1 bushing of the A phase secondary
voltage induced from X0 to X1, with respect to X0 During open DETC “per-phase” tests, each low
DETC Open) winding. The GST test voltage is applied to the H1
(where a measuring return lead is placed), is 180°
terminal while guarding H2 and H3 (and X0). In this voltage winding terminal (X1, X2, X3, and X0) is
SCE’s Distribution Asset Repair shop asserts that in out of phase with the voltage induced from X0 to
CH1 test, the watts and power factor will be elevated guarded so the low voltage windings are effectively
performing failure analysis on transformers, the “per-
well above what would be consistent with the CH1 X2. Given healthy winding insulation conditions,
phase” protocol has proven to be a preferred method short-circuited. Therefore very little voltage is present
value in the absence of the CL fault. This is because a the dielectric current and loss associated with the
of insulation assessment. This, however, has been on the low voltage windings during tests. The
physical process that is present in the measurement, induced voltage across X1 – X0 (which is unavoidably
challenging at times because, in many cases, when extraneous contributions that may or may not affect
but typically concealed, falls out of balance and included in the measurement given the test’s circuit
a winding has failed, standard “per-phase” testing the “per-phase” cross-check tests are not factors in
configuration) tends to cancel with that which is also
an open DETC “per-phase” test. As a result the open
Test Test Description kV mA Watts p.f. Cap (pf) measured and results from the voltage induced in X2 –
X0. But in this example, the X1 – X0 loss contribution DETC test data accurately reflects the condition of the
1 GST Energize H1 (ICH1) 5.0 3.335 8.336 24.993 856.6
with a low resistance faulted path to ground is much CH and CHL insulation.
2 GST Energize H2 (ICH2) 5.0 1.073 0.014 0.129 284.7
higher than the X2 – X0 contribution so the influence As the location of the secondary ground fault is
3 GST Energize H3 (ICH3) 5.0 2.892 -8.113 -28.056 736.1
is seen in the CH1 test results. moved closer to the X0 end of the secondary winding,
7.300 0.237 1877.5
When the CH3 test is performed by energizing the H3 the resultant loss contribution decreases. If the
4 UST Energize H1 (ICHL1) 5.0 10.845 -7.58 -6.986 2869.8
terminal and guarding terminals H1 and H2 (and X0), ground fault resistor is moved from the X1 terminal
5 UST Energize H2 (ICHL2) 5.0 10.332 0.605 0.586 2740.5 the test results will be abnormally low. During this to the X0 terminal in the above scenario, the resultant
6 UST Energize H3 (ICHL3) 5.0 10.937 8.775 8.024 2891.6 test, voltage is induced in windings X1 – X0 and X3 “per-phase” data will be identical to the baseline per-
32.114 1.80 8501.9 – X0 and the preceding explanation similarly applies phase data for the test specimen before the resistor
here, albeit with a net current contribution seen by is installed.
1 Open DETC (ICH ) 1
0.5 3.288 0.138 0.419 872.21
the meter that is of the opposite polarity. When the
2 Open DETC (ICH ) 2
0.5 1.017 0.02 0.196 269.75 The concluding part of this article will appear in
CH2 test is performed by energizing H2 and guarding
3 Open DETC (ICH7) 0.5 2.779 0.111 0.399 737.17 the next issue of Electrical Tester and will include
terminals H1 and H3 (and X0), voltage is induced
in LV windings X2 – X0 and X3 – X0. As neither an overview of supplementary tests carried out
7.084 0.269 1879.13
of these windings is affected by the ground fault, by SCE, discussion of transformer configurations
4 Open DETC UST (ICHL1) 0.5 10.825 0.577 0.533 2871.26
their resultant current and loss contributions that and limitations, and a case study involving a 69 kV
5 Open DETC UST (ICHL2) 0.5 10.386 0.565 0.544 2754.82 substation transformer with a secondary ground fault.
are unavoidable seen by the meter remain more or
6 Open DETC UST (ICHL3) 0.5 10.82 0.562 0.519 2869.97 less equal but of opposite polarity and will therefore A further article, scheduled for a future issue, will
32.031 1.704 8496.05 cancel each other. Consequently, the resulting data look at multiple frequency insulation test techniques
will accurately reflect the CH value of the insulation in for transformers, and explore the benefits of this
Table 1: Per Phase Data for Simulated 2.7 GΩ Ground Fault on X1
proximity of the H2 terminal. approach.

Settlement metering – a new code of practice

the Code of Practice for the Calibration, Testing and
Tony Wills, Technical support group leader Commissioning Requirements of Metering Equipment
for Settlement Purposes. The current edition of this
One of the consequences of today’s relatively open document is Issue 6, Version 9.0, which was published
energy market is that there is a vital need to monitor on 6th November 2014. It’s a long and detailed
exactly how much energy is produced by each and document that runs to almost 40 pages so discussing
every one of the numerous energy suppliers to ensure it in depth is beyond the scope of this short article.
In addition to their use in the test and commissioning In addition to a wide range of other applications, it can
that the revenue from energy sales is distributed Section 5.2 of the document is, however, of particular
of potential and current transformers in measurement be used to record ratio errors for current transformers
correctly and fairly. This onerous task falls to Elexon, interest as this deals with commissioning tests. to an accuracy of ± 0.1%, making it ideally suited for
applications, these versatile instruments are also ideal
the organisation that oversees the operation of the for the routine testing of single- and three-phase use in demanding settlement metering applications.
Among other requirements, Section 5.2 states that, as
so-called Balancing and Settlement Code (BSC) in power and distribution transformers. They perform This compact hand-held transformer tester weighs
part of the commissioning tests carried out on site,
England and Wales. all tests automatically, with three-phase transformers just 890 grams including batteries, making it easy to
the polarity and ratios of both voltage and current
tested on a phase-by-phase basis. The results are carry around and use, yet its durable ABS construction
The work of Elexon is vital to the smooth operation of transformers must be measured and recorded.
shown on a large, clear liquid crystal display and are protects it effectively against the knocks and bangs of
the wholesale electricity market. Essentially its role is to Many instruments are available for this purpose, of
also available for printing in real time on a thermal everyday on site usage.
compare that amount of energy that generators and course, but among the most affordable, convenient
printer, or for uploading to a PC for reporting and
suppliers they are going to produce with the volumes and dependable are those that make up the latest When the amount of money at stake is considered, it’s
analytical purposes.
of energy that they actually produce. Having made generation of handheld turns ratio testers. small wonder that Elexon lays down strict requirements
this determination, Elexon calculates a price for the The Megger TTR25, which is an excellent example for the testing and commissioning of settlement
The best of these instruments, typified by the Megger of this class of instrument, is capable of measuring metering installations. These requirements may at
difference and transfers funds accordingly. This involves
TTR25, are powered by readily available AA batteries turns ratios from 0.8:1 to 20,000:1 with five-digit first seem daunting, but satisfying them can be made
taking 1.25 million meter readings every day and
and feature simple one-handed single-button resolution. It uses a low excitation voltage and, to much easier by choosing appropriate modern test
handling sums that total over £1.25 billion every year.
operation. They measure turns ratio and excitation ensure a consistent high level of accuracy, it performs equipment and, as we have seen, the latest handheld
To help ensure that its settlement calculations are current and, for single-phase transformers, they can an automatic self-calibration operation at power-up transformer turns ratio testers are an indispensible
based on accurate and reliable data, Elexon issues also indicate polarity. before each test. tool for those engaged in such work.

www.megger.com ELECTRICAL TESTER - November 2015 3

 ELECTRICAL
The industry’s recognised information tool TESTER

Transformer condition assessme

Figure 3: User interface for switchbox control

Figure 4: Report view provided by the software

than the simple visual inspection that is prescribed at

regular intervals. Thus there is a need for preventative
testing on transformers taken out of service.
Upon completion of each measurement, results are
Most commonly, measurements of the insulation
automatically transferred into the test protocol.
parameters of transformers and bushings are made
Database software allows calculation of the differences
with a megohmmeter and a dissipation factor (tan
between measured values as well as comparison of
delta) test set. The results provide defined values for the
measurement results with the nameplate and previous
insulation condition, which can indicate the presence
data. At the heart of the system is a switch box that
provides automated (software-driven) selection of of major defects, and in some cases, locate the defect
HV and LV methods and test schemes. This article origin. To measure the insulation resistance of power
describes test methods and report structures to show transformers a DC voltage of several kilovolts is used.
how testing time is reduced and human error avoided. Capacitance and dissipation factor measurements
for winding insulation are conducted with an AC
Measurement techniques provided voltage up to 10 kV using circuit arrangements
Four instruments are integrated into the core of the similar to those used for the DC insulation resistance
system: measurements. This means that an HV switch can be
used for commutating the test leads between the
„„ Insulation tester (5 kV megohmmeter)
Figure 1: Transformer test van with equipment rack and cable drum compartment megohmmeter and the dissipation factor test set, as
„„ Capacitance and tan delta test set shown in Figure 2.
(exciting current)
analysis, winding resistance measurement and on- The user interface provided by the auxiliary software
Denis Denissov - Business development load tap changer tests. Confirmation of ratio, vector „„ Winding resistance and .. that controls the switch box is shown in Fig.3. The
manager group, no-load and short-circuit losses is desirable user works with two sets of test leads (for HV and LV
„„ Turns ratio meters
after repairs to ensure that these have been carried testing). When the test leads have been connected
out to a high standard. Oil samples are routinely taken The test equipment is mounted in the rack as shown to the test object as shown in Fig. 8, an instrument
Introduction for breakdown tests and gas analysis. in Fig.1. can be selected (powered on). The instrument is
Commissioning tests and periodic on-site maintenance then controlled by a dedicated software package to
Cable drums of 30m length are located in the back
checks are essential for the safe and outage-free Performing all these tests on-site requires many perform tests, collect results and manage them within
of the vehicle. If this length is not enough, the
operation of power transformers and substations. A different devices, each with its own test leads. Often instruments can be removed from the drawers and a database as shown in Fig.4.
combination of routine electric tests and advanced such testing is time consuming and tricky because used standalone with standard test leads.
of the numerous test arrangements that have to
Ratio testing and vector group
diagnostic techniques in accordance with the IEC
Optional further test capabilities can be added: verification for three-phase
60060-3, IEC 60076, IEEE Std. C57.12.00 , IEEE Std. be set up, and the need to climb repeatedly to the
transformers
C57.152 - 2013, and GOST 11677-85 standards can top of the transformer to change connections. The „„ Short circuit impedance
be performed with a dedicated test system. These latter can easily lead to accidents during testing. A To perform turns ratio measurements and verify the
tests are also valuable in troubleshooting in case test van can easily accommodate all of the above- „„ Power loss for no-load and short circuit vector group of winding connections a transformer
of an outage. Deterioration of electrical insulation mentioned methods and instruments, and organize conditions turns ratio tester is used. To measure the DC winding
can be identified with the high voltage insulation them to provide an automated test flow. This article „„ Frequency response analysis resistance a transformer ohmmeter is deployed. Both
tests (insulation resistance, dissipation factor and discusses a test van solution with centralized control devices share the same multi-core leads, so it is practical
capacitance measurement, dielectric frequency and reporting software and a single connection to „„ Moisture assessment with the DFR technique to develop an LV switch that allows the switching of
response). Mechanical damage due to the transport the test object, which is shared among instruments. test leads between these two instruments, as shown
„„ Single-phase HV source up to 100 kV AC and
or the effect of faults, malfunctions and winding Automated test circuit arrangement and switching in Figures 5 and 6. The test leads (one multicore cable
70 kV DC for withstand tests
shorts are typically found using frequency response processes to enable safe test flow are also provided. for the HV side and another for the LV side) are 30
„„ Oil breakdown test set m long. Each has four Kelvin clamps with separate
current and potential contacts.
Insulation testing
HV cable, 30m This arrangement means that measurements are
Delta 4000
Insulation testing is a crucial component of electrical
made with a four-wire circuit that compensates for
unit plant condition assessment. During its lifetime,
lead length. Depending on the standards specified,
insulation is affected by thermal, mechanical, electrical
Measure the ratio measured during commissioning should not
and environmental stresses. This leads to acceleration
«Red» 30m differ by more than 0.5% between windings or from
of chemical processes (oxidation), changes in
the nameplate value. The ratio should be measured
material structure, decrease of mechanical strength
at all tap positions of the on-load tap changer
and sometimes delamination. Moisture and surface
Measure (OLTC) or/and de-energized tap changer (DETC) for
«Blue» 30m contamination are especially harmful for power
all phases. Simultaneously with the ratio testing, the
transformer insulation. Moisture ingress has a harmful
instrument determines the vector group of winding
S1-568 unit impact and may ultimately lead to a breakdown. As
interconnection in three-phase power transformers.
a result, insulation is exposed to accelerated ageing.
Some general deterioration of parameters (uniformly Winding resistance measurement
HV Switch distributed) occurs and local defects appear. Dissolved-
Winding resistance measurements on service-aged
gas-in-oil analysis (DGA) is a very powerful tool for
transformers are performed to identify defects in
Figure 2: HV switch for switching between AC and DC insulation test sets condition assessment, but even DGA results are not
the windings, in welded connections and in the tap
always able to identify all insulation defects, any more

4 ELECTRICAL TESTER - November 2015 www.megger.com

 ELECTRICAL
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ment with an integrated test van

the results. It is acceptable to measure no-load losses This information is derived from the comparison of the
at frequencies close to the rated value of 50 Hz ±3%. test results with the initial Zk value measured by the
For aged transformers no-load losses are not specified
manufacturer. In the manufacturer’s documentation
in the standards, so when measuring at a frequency
the initial value is given as the average of three phases,
outside ±3% tolerance band there is no need to
MTO3xx TTR3xx make corrections. For three-phase transformers however the use of this parameter as a reference is not
the no-load loss is measured phase-by-phase. This advised, since the deformation in one of the phases
allows the losses for the phases to be compared to may be missed; it may be hidden within the average
reveal a faulty phase, and also allows the results to for the three phases. It is recommended to compare
be compared with data provided by the manufacturer. phase values for Zk, using as a reference the values
For example, phase a is shorted, windings b and c
measured during commissioning. To check the short
Figure 5: Lead set for LV measurements. excited, current IBC and loss power PBC are measured
circuit impedance of single-phase transformer it is
possible to use the data provided by manufacturer.

The short circuit test is carried out with a low test voltage
(380, 220 V). The arrangement for short circuit testing
on aged transformers is to excite the HV windings and
short the LV side as shown in Figure 10. For three-
H multicable
A-B-C-N, 30m phase transformers, three-phase excitation is used,
but current and voltage are measured in individual
MTO300 phases sequentially. Voltage and current values are
measured along with frequency. Measured values for
short circuit impedance should be corrected to line
frequency test conditions. Condition assessment is
done by comparing DZk with the maximum acceptable
according to the standards.

For transformers and autotransformers equipped with

TTR300 an OLTC, the test involves measuring current and
voltage at the nominal tap and two extreme taps.
X multicable When testing at maximum tap, the regulating winding
a-b-c-n, 30m is also tested. When testing at minimum tap, the
regulating winding is excluded. This helps to identify
LV Switch
the faulty winding if the deviation against the reference
Figure 6: LV switch for switching between resistance and turns ratio test sets (where a,b,c are the LV phases and A,B,C are the HV exceeds the limits. During tests it is recommended
phases). With no defect in a three-phase transformer to avoid frequent reconnection of shorts. For three-
wires of OLTC devices. These measurements can be interrupting the current at other than a zero transition.
Demagnetization is accomplished by injecting into the losses PBC and PaB are nearly equal with an phase winding transformers the following procedure
also conducted during commissioning, after transport,
one of the HV windings a cyclic DC current flow of acceptable tolerance of ± 5 %. The loss PaC is in
after long storage or repair, to confirm maintenance is quite practical: HV-LV, MV-LV, HV-MV. To short the
changing polarity with amplitude decreasing from general between 20% and 25% higher (depending
quality, or even after a fault to investigate the type and phases, flexible copper or aluminium wires are used.
maximum to zero, as shown in Figure 9. on the design and number of yokes) than PBC and PaB.
scope of damage to parts of the transformer. Winding
There is no need to correct the power loss measured The cross-section of the wire must be at least 30% of
resistance measurements should be performed for all
Power loss measurement at low voltage to the rated voltage. Instead, it is the winding wire cross-section.
tap positions.
compared to the loss measured by the manufacturer
No-load losses
Simultaneous connection to the HV and LV terminals or during commissioning under the same conditions Conclusion
Some standards, including CIGRE TB 445 – Guide (low voltage). Transformer manufacturers usually
allows the use of the dual-magnetization method. The transformer test van combines routine electrical
for Transformer Maintenance, prescribe a no-load
This is especially valuable with large transformers that measure losses at both rated and low (380 V) voltage.
test performed at low voltage as the main method tests and advanced diagnostic techniques to allow a
have a delta connected LV side. The advantages of Differences from the manufacturer’s values should
for measuring excitation current and no-load loss complete transformer check in field. Field experience
the dual magnetization method are shown in Figure be less than 10% for single-phase transformers and
on site. No-load loss measurements are usually done has shown that automated selection of the instrument
7. The core of the transformer is magnetized by the less than 5% for three-phase transformers. The losses
during commissioning and after repairs on service
“effective flux”, which is 10 times higher than the should be measured before winding resistance tests and switching through all necessary test arrangements
aged transformers in order to identify an inter-turn
flux developed during a single LV side measurement. (to avoid heating). It is worth noting that similar provides a substantial time saving – over 70% – as
shorts, core sheet shorts and core-ground faults. It is
Typical test currents are between 0.1% and 5% of transformers (same steel and test voltage) demonstrate well as minimising the risk of accidents. The central
recommended that the no-load test is carried out at
the rated winding current. Currents more than 10% similar loss values for each phase, no matter whether
380/220 V. The test voltage is applied to an LV winding, computer collects all measurement results and
of the rated value may cause heating, which could the losses are measured at low or rated voltage.
with the other windings left open. It is preferable the integrated database allows remote accessing,
compromise accuracy. For comparison purposes,
to excite windings with phase-to-phase voltage of Short circuit losses reporting and comparison of the data to the previous
temperature correction can be performed using the
380 V. This is because a phase-to-ground voltage
formulas for copper and aluminium built into the This test is used to determine the complex test results, eventually building a trend of transformer
could be subject to harmonics and would not be
software. short circuit impedance (Zk) of transformers to condition over time. Multiple field tests have proven
perfect sine curve, which could lead to inaccuracies in
identify possible deformations with winding that the measurement accuracy is in line with that
For field tests, the values of the winding resistances in
damages due to through-fault currents. achieved when using individual instruments.
a three-phase transformer measured at the same taps
and temperature should not differ by more than 1%
between phases. Absolute readings after temperature + 100%
correction should be within 5% of the values of + 10%
provided by the manufacturer. After finishing winding
+ 0.1% Positive polarity
resistance testing, it is recommended to perform
- 0.01% Negative polarity
core demagnetization (or removal of remaining - 1%
- 100%
magnetization). The best transformer ohmmeters
provide facilities for doing this.
Figure 9: Demagnetization cycle
Before measuring no-load losses or carrying out Figure 7: The effect of using the
frequency response analysis on a transformer taken dual-magnetization method
TL
out of service, the core must be demagnetized to T
eliminate magnetization that may have been caused by
LV Test connection for MTO / TTR HV Test connection for DELTA / S1
A a
5 A Am
4 4
Hz V
B b
380 V
Bm
2 3 1 1

C c
0 Cm
1 Protective earth 2 FU cable (earth spike) 3 External safety device 4 HV connection 5 Power supply

Figure 8: The Basic connection setup Figure 10: Circuit for short-circuit loss measurement

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Current transformers in electrical diagnostics
resolution required by some standards for the leakage
current is at the level of 10 μA. This means that the
differential resolution should be ideally be at the level
of 0.0001% or 1 ppm (10 μA in 10 A).

For this reason the measurement cannot be performed

by measuring two separate currents, one in L and
another one in N, using, for instance, series current
shunts. Such absolute measurement accuracy is perhaps
possible to attain in high-precision calibration laboratory
like NPL, but not in everyday portable equipment!

As mentioned above, the solution is to perform the

measurement with a differential current transformer.
A high permeability magnetic core automatically
performs the task of summing the contributions from
L and N conductors threaded through the core as the
primary winding. As a result the secondary winding
produces signal proportional directly to the difference
of the two large currents, because they are made to
flow in opposite directions. Any additional conductors
like PE are routed outside the differential CT, so the
currents in these is ignored.

The differential CT technique is also commonly used in

Many of the requirements focus on the safety of In the UK, according to the Health and Safety
Dr Stan Zurek, MSc, PhD, SMIEEE other applications. For instance, most RCDs (residual
people, which can be evaluated in many different regulations, all mains-powered portable equipment
- Manager of magnetic development must be kept in a safe condition. One of the ways to current devices) operate on the same principle. The L
ways, depending on application, power and voltage
ensure this is to carry out regular tests on all portable and N conductors are the primary windings, and the
level, point of connection in the installation, etc.
equipment – so-called Portable Appliance Testing or secondary winding detects difference between them.
Introduction An important test, however, is the measurement of PAT testing.
insulation resistance, the basic technique for which However, an RCD typically detects currents around 10
All electrical installations, devices and machines rely was invented by Sidney Evershed at the end of 19th With commonly used mains-powered equipment mA (at the level of 5 mA or 22 mA), which is three
on the quality of electrical insulation. A failure of century. the person carrying out the test might have electrical orders of magnitude higher than that needed in a
insulation usually leads to a failure of the system in access only to the mains plug, which has connections PAT tester, so that the magnetic performance of the
which it is used, often with catastrophic consequences. Leakage Current and Insulation to live (L), neutral (N) and protective earth (PE) pins. core is not as critical as it is in PAT testers. In addition,
Apart from the damage to equipment, there could be Resistance The internal wiring is unknown and insulation quality
the CT in an RCD can have the secondary winding
risks of injury or death, as well as severe environmental The underlying principle of insulation testing is a direct cannot be easily evaluated from a direct measurement
wound only around a part of the core. This is because
consequences if chemical substances are involved. application of Ohm’s law. Voltage is applied to a given of the type discussed earlier.
detecting 10 mA in 10 A is not as demanding (0.1 %).
There are national and international standards circuit and the very small resulting current is measured For this reason, PAT test instruments are designed to
prescribing techniques and practices used for (for good insulation it can be at the nanoamp level). accept the mains plug from the unit under test, and All current transformers are affected by conductor
design and installation of wires and cables, as well The unknown resistance can be then calculated as some tests are actually carried out when the unit is positioning. If the secondary winding is non-uniform,
as earthing/grounding. For instance, in the UK the Rins = V/I. powered (Fig. 1). For some well-isolated devices, the a conductor placed closer to denser part of the
currently relevant standard is BS 7671, IET Wiring However, there are cases where the test voltages and quality of the insulation can be assessed by direct winding will induce more signal than a conductor
Regulations 17th Edition. In North America, the U.S., currents cannot be applied or measured in such a measurement of the leakage current that flows carrying identical current placed farther away.
Mexico and some South American countries use the direct way because, for instance, of a leakage current through the protective earth (PE) pin.
Performance is improved if good quality magnetic
National Electrical Code (NEC). Other countries usually between the source and the object under test, or the But for non-isolated devices (Fig. 1) this may not be cores are used. However, using very high permeability
have their own national regulations, specific to each configuration of the internal circuitry.
possible, because there could be an additional leakage materials (like mumetal or nanocrystalline) is costly,
country. path to earth, which might be unintentional (e.g. and cannot be always justified commercially. Materials
building flooded with water) or intentional (a dedicated
with lower permeability force designers to use other
earthing or bonding connection). In these cases the
methods instead.
direct measurement of current in the PE pin does not
give full information about the total leakage current. For example, the differential CTs in PAT testers are
However, the leakage current can be measured required to have uniform winding, and the two
indirectly by means of a differential current transformer conductors (L and N) are aimed at being positioned at
(CT in Fig. 2). The L and N wires are passed through the exact centre using, for example, physical spacers.
the CT. Any current flowing into the instrument in It can also be beneficial to “equalise” the contributions
L and flowing back in N means real energy used by
from each conductor. Each of the conductors is split in
the device. But current flowing in any different path
two parts, with each part carrying 50% of the current.
means leakage due to non-ideal insulation. The
This configuration reduces the conductor positioning
difference between L and N currents is, therefore,
directly proportional to all the leakage currents. effect so that lower permeability and lower cost cores
can be used.
Such differential measurement is quite challenging.
For high-power devices there could be several The second part of this article, which will appear in
amperes of the main current, for instance for an future issue of Electrical Tester, will go on to discuss
Fig 1 - Measuring leakage current in an earthed appliance. (brown = L, blue = N, green-yellow = PE, ordinary 13 A socket the load could be 10 A, yet the the use of CTs in the measurement of earth resistance.
black = return path through earth.)

Fig 2 - Differential measurement of leakage current Fig 3 - A differential CT with multiple conductors in the comprehensive PAT400 series tester

6 ELECTRICAL TESTER - November 2015 www.megger.com

 ELECTRICAL
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GIC: a major risk factor for the power grid
In accordance with Faraday’s law of induction, these
Jill Duplessis - Global technical marketing
magnetic field fluctuations induce currents in the
manager and Editor
earth’s surface which, in turn, give rise to potential
differences – ESPs (earth surface potentials) – between
Geomagnetically Induced Current (GIC) has the grounding points. The distances over which these
potential to seriously disrupt the operation of the effects are felt can be quite large.
power transmission grid over a wide area. And, if
a GIC event damages key components like power The field, then, essentially behaves as an ideal voltage
transformers, the effects may continue to be felt for source between remote neutral ground connections
months or even years. This article examines what GIC of transformers in the a power system, causing a GIC
is and why it occurs, before discussing its effects in to flow through these transformers, the connected
general terms. A subsequent article will further explore power system lines and the neutral ground points.
the impact of GIC on the power grid, with particular The susceptibility of a power system to geomagnetic
reference to the vulnerability of transformers. storms – and hence GIC – varies and depends on a
The phenomenon of GIC is well documented in number of contributing elements. These include:
technical sources but is not necessarily well-known „„ The characteristics of the transformers on the
or well-understood outside specialist circles. For this system, as these serve as the entry and exit
reason, and to provide a sound foundation for the rest points for the GICs. Relevant factors are:
of the material that will be presented in these articles,
it is worthwhile starting with a brief examination of ƒƒ Transformer winding configuration: Any
GIC and its causes. transformer with a grounded-wye
(grounded-star) connection is susceptible other types of core. Most GIC problems „„ The strength of the geomagnetic storm:
The primary source of GIC events is activity on the to a quasi-DC current flowing through its are associated with single-phase the more powerful the storm, the greater the
sun’s surface, in the form of sunspots and solar flares. windings; an autotransformer, where core- or shell-form units, three-phase intensity of the auroral electrojet currents, and
Solar flares produce coronal mass ejections, x-rays the high- and low-voltage windings are shell-form designs and three-phase five- the closer these are likely to be to the equator.
and charged particles that form a plasma cloud – a partly shared, permits GIC to pass through The impact of GIC on transformers and power
gust of solar wind –that can reach the earth in as leg core form designs.
the high-voltage power lines, but a delta- systems is well understood in general terms.
little as eight minutes. Depending on its orientation, wye (delta-star) transformer does not. ƒƒ Transformer ground construction: However, because so many variables influence
the magnetic field produced by the electric currents (See Figure 1). Transformers on extra-high voltage (EHV) vulnerability, it is almost impossible to predict in
within the plasma cloud can interact with the earth’s transmission systems are particularly quantitive terms the impact of a GIC event on a
magnetic field, causing it to fluctuate, resulting in a ƒƒ Transformer core construction. The design particular power system. In fact, most attempts
vulnerable as these systems are very
geomagnetic storm. of the core determines the magnetic at quantification to date have essentially been
solidly grounded, creating a low-resistance
reluctance of the DC flux path, which anecdotal.
Geomagnetically Induced Current (GIC) is produced influences the magnitude of the DC flux preferential path for GIC. Additionally,
when auroral electrojet currents that flow in circular shift that will occur in the core. Three- EHV transformers are usually not three- The next article in this series will examine in more
paths around the earth’s geomagnetic poles at phase transformers with a three-leg phase, three-leg core form designs. detail the first order effects of GIC on transformers
altitudes of around 100 km become energised by the core are the least vulnerable to GIC, and the second order effects on the power system.
„„ The geographical location of the power It will also discuss how current transformers and
arrival of a plasma cloud. This energisation results in because they have an order of magnitude
system. The closer the power system is to the protective relays are likely to be affected by GIC, as
slow, time-varying fluctuations in the earth’s normally higher DC reluctance in the core-tank
earth’s magnetic poles, the nearer it is likely
unvarying magnetic field. magnetic circuit than transformers with well as briefly reviewing regulations introduced by
the Federal Energy Regulatory Committee (FERC) and
In this standard transmission National Energy Reliability Committee (NERC) in the
line setup, geomagnetically USA in response to the threats posed by GIC.
induced current (GIC) flows
from the earth into the
gorunded neutral of a three
phase autotransformer, where
Get up to date
it divides evenly in each phase
of the transformer. It then 3 - Phase
with the latest in
proceeds into the transmission
autotransformer
3 - Phase Δ−Y cable test
lines and flows to other transformer
transformers, returning from Tony Wills, Technical support group leader
Geomagnetically Transmission line
them to earth.
induced current
Geomagnetically Offering power professionals an unparalleled
Note that an auto- transformer induced current opportunity to discover and discuss the most
- one in which windings are recent developments in cable test technology,
shared - permits GIC to pass the latest in Megger’s biennial series of cable
through, while a delta-wye test seminars is being held at the National
transformer does not. Grounded transformer neutrals
Motorcycle Museum near Birmingham, UK,
on Thursday, 26th November.
Figure 1: Conducting paths for GICs in a power transmission system
to be to the auroral electrojet currents and The topics scheduled for coverage include
consequently the greater their effect. Note, a discussion on distributed generation
however, that the earth’s magnetic poles do and connections with respect to the latest
not coincide exactly with its geographical poles. Ofgem RIIO requirements, an examination
This means that in the USA, for example, East-
of cable types and terminations, and the
coast geographic mid-latitude locations are
latest developments in testing, diagnostics
more vulnerable than the equivalent West-coast
and cable fault location. Also featured in the
geographic locations, as the former are closer to
preliminary programme are a presentation
the magnetic pole.
focussing on the most common problems
encountered by cabling contractors, and
„„ Ground conductivity: Power systems in a case study on the comparison of test
areas where ground conductivity is poor (see results achieved using three different voltage
Figure 2) are more vulnerable to the effects of sources: damped AC (DAC), very low
geomagnetic activity because any geomagnetic frequency cosine-rectangular (VLF-CR), and
disturbance will produce a larger gradient in the very low frequency sine wave (VLF Sin).
earth surface potential it induces into the ground Throughout the day, delegates attending the
and also because the high ground resistance seminar will have an opportunity for hands-
encourages more current to flow through on examination of a wide range of cable test
alternative paths such as power transmission equipment, and to discuss their requirements
lines. and problems with Megger’s experts.
„„ Orientation of the power system lines: The There will also be ample opportunities for
Regional gradient of the earth surface potential is usually, networking and informal discussions with
conductivity though not invariably, greater in the east-west other delegates.
direction than the north-south direction.
10-4 - 10-3 s/m Lunch and light refreshments are included.
„„ The length of the power system lines: The The venue is close to both the road and
10-3 - 10-2 s/m longer the transmission lines the greater their rail networks, and ample free parking is
vulnerability. This was all-too-convincingly available. There is no charge for attending
10-2 - 10-1 s/m demonstrated in March 1989 when power
the seminar, but all places must be booked
systems operated by Hydro Quebec in Canada
10-1 - 1 s/m were ravaged by a GIC event – the Hydro Quebec in advance as demand for these events is
system includes generators that are 1,000 km invariably high. Please reserve your place by
Figure 2. Earth conductivity in the USA and Canada away from the main populated load centres. emailing uksales@megger.com.

www.megger.com ELECTRICAL TESTER - November 2015 7

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Q&A
Low resistance measurement is an invaluable diagnostic aid in a wide variety of applications and,
most of the time, it’s fast and easy to carry out. There are, however, a few pitfalls that often result in
calls to our technical support team. Here are some of the questions the team is regularly asked.

Q: Why is a four terminal connection current and voltage test leads are electrically so that the correct result is displayed without the instrument will need to offer facilities for
frequently used when making low separate, they’re usually combined in the same operator intervention even if there is a standing dynamic resistance measurement (DRM).
resistance measurements? lead set, which may be terminated with Kelvin EMF on the circuit under test. Q: Why does it take so long to
clamps. These are, in effect, modified crocodile measure the resistance of transformer
A: This arrangement is used so that the
clips that provide separate voltage and current Q: What type of instrument should I windings?
resistance of the test leads and connections
connections to the device under test. use for measuring circuit breaker contact
doesn’t influence the results obtained. Such A: The windings are highly inductive so,
Q: When making low resistance resistance?
influence is usually unimportant in the measure when a DC voltage is applied, the current in the
measurements, how do I overcome
of resistances from, say, a few tens of ohms A: Ideally an instrument that’s designed winding increases exponentially, which means
problems with standing EMFs
upward, but with lower values the resistance for the job! A general-purpose ohmmeter is it increases rapidly at first but takes a long
(Electric and Magnetic Fields) on circuits
of the leads can be similar to or even greater time (theoretically an infinite time!) to reach
involving connections between different unlikely to give satisfactory results, even if it can
than the resistance of the device under its final value. Only then can the resistance be
metals? nominally measure the low values of resistance
measured. In practice, the best transformer
test. Also any resistance of the connections
A: These problems can be overcome involved. It’s important for the instrument to be ohmmeters, such as the Megger MTO250,
from the test leads to the test piece can vary
fairly easily by making a measurement, then capable of delivering a high test current – 50 offer ways of reducing the time needed for
depending on the condition of the conductor.
reversing the polarity of the test leads and A or greater is required by most standards – it tests. One method is to provide a high test
Four terminal connections use separate current
making a second measurement. The required should also support four-terminal testing, and current capability, which means that the rate
and voltage leads between the test instrument of increase of current in the winding will never
resistance value is the arithmetic average of
and the device under test. This means that the ideally it should be suitable for use with Dual
the measurements. Some instruments, such as be limited by the test set. Another approach
voltage is measured directly at the device and Ground test techniques. In cases where the is to provide dual-channel operation, so that
those in the Megger DLRO10 range of digital
the measurement is unaffected by any voltage low resistance ohmmeters, feature automatic measurement is being carried out to determine measurements can be performed on two
drop in the current leads. Even though the current reversal, the remaining length of the arcing contact, windings simultaneously.

The perpetually inspiring Nikola Tesla

Keith Wilson, electrical engineer

Ominously born during a lightening storm in

modern day Croatia, Nikola Tesla is credited with
numerous inventions and scientific discoveries. It
has been claimed that his inquisitive mind and thirst
for knowledge fuelled the discovery of AC current
(as a cheaper and safer alternative to Edison’s
DC), the predecessors of fluorescent light bulbs,
electromagnetic and ionizing radiation (x-rays), the
radio, the remote control, the laser – just to name
a few. It is only fair to say that many of these claims
are disputed, but nevertheless, there can be no doubt
that Tesla was a true genius.

Tesla’s most controversial development was the

“Tesla Coil,” which he devised in 1891, and which is
believed to have paved the way for many of today’s
wireless technologies.

A Tesla Coil is a resonant transformer circuit used to

produce high voltage, high-frequency alternating-
current electricity. Tesla used these coils to conduct
innovative experiments in electrical lighting, X-ray
Double exposed publicity photo of Serbian-American inventor Nikola Tesla in December 1899 sitting in his laboratory in Colorado Springs next to his
generation, electrotherapy and, most significantly,
magnifying transmitter high voltage generator while the machine produced huge bolts of electricity. Image use courtesy of Wikipedia Public Domain library.
wireless transmission of electrical energy. His ultimate
aim was to be able to deliver electrical power from These technical difficulties were only a part of Tesla’s The Wardenclyffe tower project continued, and it He foresaw the existence of the devices we use
power stations to consumers without the need for problems. The Wardenclyffe Tower took incredible was even tested a few times during construction today that allow us to “witness and hear events--the
wired connections. In other words, he wanted to amounts of money and resources to build and that, with encouraging results. But Tesla realised that his inauguration of a President, the playing of a world-
eliminate what we would now call the transmission together with the economic crash of 1900, meant competitor’s success with simple wireless telegraphy series game, the havoc of an earthquake or the terror
and distribution grids. that it became increasingly difficult for him to source had greatly diminished his own chances of attracting of a battle--just as though we were present”. He also
the funds he needed to continue his work. any more investors. Ultimately, Wardenclyffe was said that, “when the wireless transmission of power
Understanding the potential of Tesla’s research, J.P.
abandoned even though Tesla himself remained is made commercial, transport and transmission will
Morgan backed him with $150,000 in 1900 to build As if the situation wasn’t bad enough for Tesla and
convinced for the rest of his life that the dream of be revolutionized.” No doubt he was right, but even
a tower (later known as Wardenclyffe Tower) that his team, another inventor – Guglielmo Marconi –
wireless power transmission would have been within today we’re still waiting for this to be achieved.
would allow large-scale practical experiments to be managed to steal much of the limelight from Tesla
by successfully receiving the world’s first trans- his grasp if only it had been possible to raise the Tesla’s intellect, his ethos and will to create a better
carried out into wireless power transmission.
Atlantic wireless telegraph signal. He did this using no finance needed to continue his researches. future for all mankind are indubitable. His legacy,
Around this time, Tesla was, in fact, achieving some
less than seventeen of Tesla’s patents and although In the event, he went on to patent many great evidenced by almost 300 patents, has paved the way
limited success – he was able, for example, to light
wireless telegraphy was a much-needed invention, it inventions, as well as predict social, economical for many of the modern conveniences we enjoy today.
three incandescent light bulbs at a distance of about
was far less ambitious than Tesla’s idea of transmitting and scientific developments he believed to take I leave you with this thought that sums up his idea of
30 m with no wired connection. Unfortunately,
power without wires. progress and his undefeated belief that science will
these results were achieved using near-field effects, place in the future. One such successful prediction
improve the lives of everyone if given free reign:
where the energy transmitted falls off very rapidly as Tesla’s financial supporters were now becoming very was the envisaging of modern day smartphones.
the distance between the transmitter and receiver anxious because of Marconi’s development. Though With uncanny accuracy, Tesla claimed that his work “All that was great in the past was ridiculed,
increases. This makes it impossible to scale the results his plans were considerably smaller scale than Tesla’s, would lay the foundations for the development of condemned, combatted, suppressed — only to
to work over the larger distances necessary for Marconi’s apparatus was also considerably less communication devices “a man will be able to carry emerge all the more powerfully, all the more
practical wireless power distribution. expensive. one in his vest pocket”. triumphantly from the struggle.”

8 ELECTRICAL TESTER - November 2015 www.megger.com