Insulation
tester
for substations
Limiting
catastrophes
Cable
faults
see page 3
see page 5
see page 7
ELECTRICAL
TESTER
Published by Megger
July 2013
The industry’s recognised information tool
Phase evaluation in power networks
Dr Frank Petzold and Alexander Stanischa
Megger Baunach
The spread of green micro-generation systems
and other market forces mean that power
utilities increasingly have to allow third-party
access to their networks. At the same time,
staffing levels have in many cases been
reduced dramatically. This means that the
days when individual employees were
specialists in a particular area of work are
gone. Today’s employees are expected to
cover a wide range of activities, and simply
don’t have the time to develop in-depth
expertise.
Of course no compromises can ever be made
in matters of safety, even when human
resources are scarce.
Achieving and maintaining the highest levels
of safety is made even more complicated for
network operators by the restructuring of
networks that is now carried out almost
continuously to optimise operating efficiency.
In particular, the constant changes make it
difficult to ensure that documentation is up to
date and correct.
This means that it is now more important
than ever to be able to easily and reliably
determine the absolute phase of busbars in
switching equipment, transformer feeders and
substations, crossing points of overhead cables,
cable end closures and various parts of the
low-voltage network.
To meet this requirement, new phase evaluation
test instruments have been developed which
take advantage of modern technologies such
as the GSM mobile phone network and the
GPS satellite system that is most commonly
used for satellite navigation in vehicles.
Correctly used, these instruments will prevent
the occurrence of costly and often dangerous
errors during the commissioning and maintenance of electrical power systems, thereby
ensuring that high network reliability and
economic efficiency are achieved.
PRACTICAL PHASE MEASUREMENT
To explore the way that the new instruments
operate and how they are used, it is easiest
to consider a specific product, in this case
the new SebaKMT PVS100, but the functionality
described can be taken as a guide to what
users should expect from any modern phase
evaluation system.
The absolute phasing at any point in a
transmission or distribution network can
only be determined when the measurement
is made with respect to a known reference
phase. This means that for making phase
measurements in the field, a mobile unit that
is capable of being used with a reference
device (base station) is needed in order to
It’s another
world!
Nick Hilditch
Group marketing services manager
www.megger.com
edition
perform the necessary synchronisation. In the
PVS100, this requirement has been met by
designing the instrument as two identical units,
one of which acts as the base station while the
other is used as the mobile unit in the field.
A precise time base for synchronisation is
established using signals from the GPS satellites,
while for transmission of synchronisation data,
each unit incorporates a GSM module. This
can operate in the normal CSD data transmission mode or alternatively in GPRS mode.
When the voltage of the phase to be measured
is less than 400 V, it is connected directly to
the mobile unit. Direct connection can also be
used for measurements at capacitive test points
of switching equipment or angle plugs (elbow
connectors).
For higher voltages, a high-voltage measurement
sensor is used, and this communicates with
the base unit via an 866 MHz wireless link.
The sensor, which is attached to an insulating
rod approved for use at the appropriate voltage,
incorporates a high-intensity LED, visible even
in direct sunlight, which signals that the
measurement has been completed and the
phase identified successfully.
This arrangement means that the operator can
give their full attention to the positioning of
the sensor while the measurement is being
made, without being distracted by having to
look at the mobile unit. The mobile unit stores
the measurement data for later downloading
and analysis.
PHASE CORRECTION
If the base station is not connected to L1 as
the reference phase, the appropriate correction
angle of either +120º or -120º must be entered.
Depending on the application, there can be
transformers with the same or different vector
groups between the base station and the
mobile device. Each of the vector groups leads
to a specific resultant phase shift, which must
be entered on the mobile unit to obtain the
correct absolute phase indication. If the
correction values are not entered, only the
phase angle relative to the reference phase
can be determined.
Correction values are also needed for
measurements taken at capacitive voltage
test points. These values are restored in the
instrument for the most common types of
capacitive sensor, and there is provision for
entering the correction values for an almost
unlimited number of additional sensor types.
MEASURING MODES AND APPLICATION
EXAMPLES
Because conditions in the field vary in terms
of access to mains power and the availability
of GPS and GSM signals, the best phase
evaluation test sets offer multiple measuring
There can be no doubt that Megger employs
some very talented engineers but it’s easy to
forget that they don’t leave their talents behind
them when, at the end of a long working day,
they leave their desks and head homeward.
So what do these gifted engineers get up to in
their spare time?
Some might say that’s a question best left
unanswered but, in the case of Mark Hadley,
who is new product research manager at the
company’s Dover site, it’s already too late!
Thanks to recent press reports, Mark’s spare
Overhead power lines being tested using a PVS100
modes to enable the best results possible to
be obtained under all conditions. The PVS100
offers four modes: NET, NO NET, NO NET/
NO GSM and LOCAL.
Mode 1: NET
If a low-voltage mains supply is available in
the location where the measurement is being
made, the mobile device is simply connected
to any convenient mains socket and a onetime synchronisation process is carried out
with the base station. The mobile device
determines the phasing of the mains socket
and uses this as the local reference for all
measurements at this location. The mobile
device must remain connected to the mains
socket throughout the entire measurement
process. The advantages of this mode are that
GPS and GSM reception are only required for
a short time, during the one-time synchronisation process, and that results are obtained
very quickly when making measurements
with respect to the local reference.
Mode 3: NO NET/NO GSM
This mode is used when there is no lowvoltage mains supply available, and also no
GSM coverage. In this mode, only the GPS
time signals and the voltage zero crossings are
stored in the device while the measurement is
being made. Subsequently, when the unit is
moved to a location where GSM coverage is
available, post-synchronisation is performed:
the absolute phase identifiers are determined
and stored in a measurement file. Postsynchronisation can be carried out any time
up to ten days after the recording of the
measurement data.
Mode 4: LOCAL
In this mode, only the mobile device is used.
It is connected to a known reference phase,
such as a mains socket, and all measurements
are made with respect to this local reference.
No synchronisation to or communication with
the base station is needed.
Mode 2: NO NET
When measurements are being made on
overhead lines or in substations, there is often
no convenient low-voltage supply available.
In these cases, the mobile device operates
from a built-in rechargeable battery. For direct
phase display during the measurement under
these conditions, there must be continuous
synchronisation with the base unit via GSM,
and GPS reception must also be available.
Conclusion
The latest phase evaluation test equipment
allows safe, fast and reliable phase identification
at all voltage levels. The units are well suited
for use in the field, and are easy to operate.
Use of this equipment prevents errors that
may have serious safety implications and also
ensures that phase identification information is
correctly documented. It therefore contributes
significantly to improving overall network
reliability and efficiency.
time activities are now very much in the public
spotlight.
around seven million light years from our
solar system.
Because, when most of us have our feet up
watching television, Mark is hunting new
planets.
Mark made the discovery not, as might have
been expected, by shivering at a telescope on
cold and starry nights, but by analysing data
sets on his laptop in the warmth and comfort
of his living room. This is because Mark is a
volunteer for the Planethunters.org website,
which is led by Yale University as part of
Oxford University’s Zooniverse project.
And his efforts have recently paid off, when
his name was added to the very short list of
those who have made significant contributions
to this work. To achieve this accolade, Mark
has identified a new candidate planet about
the size of Jupiter, orbiting a sun-like star
Megger ELECTRICAL TESTER July 2013
continued on page 8
1
The industry’s recognised information tool
Contents
Phase evaluation in power
networks............................................. 1
Dr Frank Petzold, Alexander Stanischa,
Banuanch Germany
Don’t take a chance
on your CAT!
Simon Wood
UK wholesale and distribution
sales manager
ELECTRICAL
TESTER
History is
never static!
HV Supply
It’s another world!.............................. 1
Nick Hilditch, group marketing services manager
Stina Flogell Ostlundh
General manager,
Megger Sweden
Don’t take a chance on your CAT!...... 2
Simon Wood, UK wholesale and distribution sales
manager
History is never static!........................ 2
From time to time, Electrical Tester has included
brief histories of some of the well-known
companies that now form part of the Megger
group. Several years ago, we wrote in this
vein about the Swedish company, Programma.
History is never static and the story we told
then is now rather behind the times, so let’s
bring it up to date.
Stina Flogell Ostlundh, general manager,
Megger Sweden
Insulation tester for substations........ 3
Clive Pink, product manager
Measuring on a roll!........................... 3
Josef Hollweck, sales engineer, Megger Germany
Interoperability and IEC61850
Goose.................................................. 4
Andrea Bonetti, technical specialist in protection
and relay test
The secret to limiting substation
catastrophes....................................... 5
Gary Wright, consultant
Multiple current injection................... 6
Marius Pitzer, sales manager, Megger South Africa
Safeguard those services!................... 6
Mr Jörg Schubert, manager, line locating and
inspection department, Banuanch Germany
Explaining the art of testing.............. 7
Elsa Cantu, marketing communications manager,
Megger Dallas
Cable fault.......................................... 7
Peter Herpertz, product manager, power
My resistance is low!.......................... 8
Keith Wilson, electrical engineer
It’s another world continued from page 1....................... 8
Q&A.................................................... 8
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be identified as authors of their respective articles has been
asserted by them in accordance with the Copyright, Designs
and Patents Act 1988.
© Copyright Megger. All rights reserved. No part of Electrical
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Note from the Editor
Time for your say.
We have introduced a ‘Questions and Answers’ section and
would like your input. If you have any questions or stories
that you think we could use, then please email
electricaltester@megger.com
‘Views expressed in Electrical Tester are not necessarily the
views of Megger.’
The word ‘Megger’ is a registered trademark
Editor Nick Hilditch.
T +44 (0)1304 502232
E nick.hilditch@megger.com
www.megger.com
Megger Limited
Archcliffe Road Dover Kent CT17 9EN
T +44 (0)1304 502100
E electricaltester@megger.com
www.megger.com
2
EN 61010-1 categories
When testing electrical systems of any kind,
it’s essential to make sure that the test
equipment being used is suitable for the task
in hand. If it’s not, there is a significant risk
not only of damage to the test equipment
and the installation, but also of injury to the
user. That probably seems so obvious that it’s
hardly worth mentioning. After all, how many
technicians or engineers would use unsuitable
equipment for testing? The answer is that few
would do so knowingly, but many may be
doing so every day without even realising
that there’s a problem. And that problem
relates to transients. All electrical installations
experience transients, which are voltage
spikes that are super-imposed on the normal
supply. Although these spikes are usually of
very short duration – typically they last just
a few microseconds – their amplitude can
be thousands of volts. These transients come
from a variety of sources, but one source that
is surprisingly common even in temperate
climes is lightning strikes. Note that a direct hit
on the installation doesn’t have to be involved,
nor even a hit on the power lines supplying it;
a nearby strike is often enough to produce a
large transient.
But what have transients got to do with
testing and safety? To answer this question,
let’s examine what happens if you’re carrying
out a test – which could be something as
simple and routine as checking the voltage
of an LV supply – when it experiences a
transient. If the instrument in use has not been
specifically chosen to be suitable for the type
of work being carried out, there’s a very real
risk that the transient will cause a flash over
inside the instrument and set up an arc.
Because its duration is very short, the transient
itself is unlikely to have enough energy to do
a lot of damage. Unfortunately though, once
it is established, the arc provides a low
impedance path for current from the mains
supply. That current flow releases a lot of
energy inside the instrument. Of course, the
circuit’s protective device, whether it’s a fuse
or circuit breaker, will quickly disconnect the
supply and interrupt the fault current.
Before this has time to happen, however,
the energy released within the instrument is
enough to cause real problems. In the worst
cases, the instrument may explode, injuring or
even killing the person who is using it. Even
in less severe cases there is a definite risk of
fire and damage to the equipment under test
as well as to the instrument itself.
It’s clearly important, therefore, to choose an
instrument that has been designed to withstand the level of transients it’s likely to
encounter in use. But how can you tell? The
answer is to look at the instrument’s category
rating, which is more commonly called its CAT
rating.
CAT ratings are defined in the IEC 61010-1
standard, and are specifically intended to
Megger ELECTRICAL TESTER July 2013
address the issue of transients in the testing of
low-voltage installations. To understand how
the ratings work, it’s necessary to look at what
happens to transients as they pass through a
typical electrical installation.
Outside the building and at the point where
the mains supply enters the building, the
transients have their highest amplitude. For
testing in these locations, only instruments
with a CAT IV rating are suitable.
Transients are, however, quickly attenuated by
the wiring and equipment in an electrical
installation. Once the supply has passed
through the main switchboard, therefore, the
amplitude of the transients is much lower, and
instruments with a CAT III rating (or higher)
can be safely used. At the final circuit outlets,
the transient levels are lower still, and CAT
II or higher instruments can be used without
problems.
What about CAT I instruments? These are
for use within appliances such as VDUs and
photocopiers. In practice, major suppliers of
instruments designed for professional use are
unlikely to offer CAT I or CAT II instruments,
as their area of safe usage is so limited.
That’s not quite the whole story, as CAT
ratings must always include a voltage – for
example, CAT IV 300 V. This voltage is the
maximum RMS phase-to-earth voltage of the
system on which the instrument is suitable for
use. This means, for example, that instruments
with a 300 V rating can be used on singlephase systems up to 300 V and three-phase
systems up to 520 V, making them suitable for
the vast majority of low-voltage applications.
There’s one final point to mention. It would
be easy to think that insulation testers and
other instruments designed for use on dead
circuits didn’t need a CAT rating. Remember,
however, that these instruments could be
accidentally connected to a live supply, and
also that many of them incorporate facilities
for some live circuit tests, such as measuring
the supply voltage. The CAT rating is, therefore, still relevant for these types of
instruments.
Once the significance of the CAT rating system
is understood, it’s not difficult to choose an
instrument that’s appropriate for the type of
work being undertaken. As a general rule of
thumb, a CAT III 300V rating is likely to be
the minimum acceptable for general use.
It is, however, well worth considering
investing in CAT IV instruments, as these can
be used without restrictions anywhere within
a normal installation. Many utility companies
and other major purchasers of instruments are,
in fact, now specifying CAT IV instruments as
standard, since they deliver an extra level of
safety in return for a very modest additional
investment.
Programma was founded in 1976 by two
friends who saw designing and manufacturing
electronic products as an attractive business
opportunity. Their first idea was to produce
an electronic programmer for washing
machines, which explains the choice of
company name. Unfortunately, the washing
machine manufacturers weren’t interested,
as they develop their own programmers inhouse.
Fortunately, the brother of one of the friends
had been working as a protection relay test
engineer and had developed a small portable
relay tester for his own use. He suggested that
this could be commercialised, and estimated
that there would be a market for perhaps 20
of these instruments. In fact, including the
successors to the original design, more than
20,000 have been sold!
Programma was so successful that it became
a takeover target. It was purchased by GE
Energy and in an attempt to reduce
manufacturing costs, manufacturing was
transferred to China. By 2007, however,
GE Energy had decided to rationalise its
operations by divesting itself of non-core
businesses, and Programma found itself up
for sale.
Knowing that Programma had an excellent
reputation for quality, expertise and innovation,
as well as a product range that complemented
its own, this was an opportunity too good for
Megger to miss and in June 2007, it brought
the company into the group. One of its first
actions was to bring manufacturing back inhouse, as this restored the close control over
product quality and performance that can
only be achieved when the manufacturing
site is close to the design and development
facility.
Just a year later, in 2008, Megger acquired
another Swedish company, PAX Diagnostics,
a specialist in power transformer test and
diagnostics with industry-leading expertise
in sweep frequency response analysis and
dielectric frequency response analysis. Before
long, the PAX operations were moved to
share the Programma site in Taby, where they
benefitted from access to a much wider range
of resources.
Now operating under the Megger name, both
the Programma and PAX operations in
Sweden have continued to flourish, and are
producing a wide range of innovative power
test instruments that are sold all over the
world. In fact, the Swedish operations have
forged ahead so strongly that the latest update
in this story is another change of location.
After 30 years in Täby, all of the Swedish
operations moved to a much bigger and more
modern premises in Danderyd, Stockholm in
April 2013. The new premises provide a greatly
enhanced environment for the development
and manufacturing teams, as well as the
facilities needed to ensure that as its business
continues to grow, the company will be able
to maintain and enhance its already renowned
level of customer service well into the future.
www.megger.com
ELECTRICAL
TESTER
The industry’s recognised information tool
Clive Pink
Product manager
In distribution and transmission substations
and switchyards – as in almost every other
kind of electrical power installation – dc
insulation resistance testing (IRT) is an
invaluable tool for assessing the condition
of equipment and for diagnosing faults.
Unfortunately, however, obtaining dependable
insulation resistance measurements in Extra
High Voltage (EHV) substations and switchyards can be challenging, not least because
of the high levels of electrical noise that are
present.
Insulation tester for substations
In fact, an instrument with 3 mA noise
immunity will often be a good choice for
general applications, but in EHV substations
and switchyards it’s a very different story
as noise levels are frequently much higher.
The right choice here is the new S1–Series of
products from Megger that have been purpose
designed and built for use in these very
challenging environments.
A very effective solution would be to arrange
for all nearby equipment to be de-energised
while tests are carried out so as to minimise
noise levels, but in the real world this is rarely
possible. A more practical approach to tackling
the noise problem is to use the shortest
possible test leads and to route these near
earthed objects such as the casing of a transformer, or to use test leads that are screened.
The best of these instruments offer 8 mA
noise immunity, which is an exceptionally
high figure, and that’s not all. They also
incorporate powerful software-base filtering
that further reduces the effect of electrical
noise on measurements. The level of filtering
is user selectable as the highest levels extend
the time needed to perform a test, although
they do make it possible to obtain dependable
results in situations where measurements
would have previously been impossible.
These measures are effective in reducing high
frequency noise pick up on the test leads, and
this may sometimes be enough for dependable
measurements to be made, but they can do
nothing about noise picked up by the test
object itself or from noise currents flowing in
the ground. The only way to tackle this is to
use an insulation tester that offers high noise
immunity.
These new instruments have been tested in
the field and proved their worth in field trials.
Tests carried out with a Megger S1-1068 10 kV
test set in 765 kV substations in India yielded
accurate and repeatable results without even
needing to use the highest level of filtering.
This is a particularly notable achievement as
no other insulation test set had ever been able
to operate successfully in these locations.
Of course, all manufacturers of insulation testers
claim that their products offer high noise
immunity and, indeed, all test sets sold in the
EU must meet the EMC requirements of IEC
61326-1. Experience has shown, however, that
in environments like substations and switchyards, the levels of electrical noise are often
much higher than those laid down in this
standard.
While their exceptionally high noise immunity
is undoubtedly the key characteristic of the
new S1-Series, these leading models have
many other desirable features. They’re robust
yet lightweight, easy to transport and, because
low-voltage power is not always conveniently
available in substations and switchyards, they
incorporate rapid-charge Li-ion batteries that
allow hours of testing to be carried out even
when a mains an AC supply is not available.
It is, therefore, necessary to go beyond simple
claims of high noise immunity or IEC 61326
compliance and to look at quantitative data
about the noise immunity of an instrument.
This is usually specified in mA, and a typical
specification might be that a particular
instrument has an immunity of 3 mA at 50/
60 Hz. In simple terms, this means that if
the noise current induced in the test circuit
at power frequency is 3 mA or less, the
instrument will give reliable results.
These test sets also deliver a high short-circuit
current – typically up to 6 mA – to allow rapid
charging of items under test, and they have
a CAT IV 600 V safety rating up to 3000 m in
line with IEC 61010, to help ensure operator
safety. A further important feature is provision
for remote operation via a fully isolated interface, which again can help to enhance
operator safety when carrying out tests in
difficult environments.
USB beacon which enables remote operation from a PC
As would be expected, these instruments have
internal storage for date- and time-stamped
test results, which can be recalled to the
display or downloaded to external devices for
inclusion in reports or later analysis. Downloading is performed via a USB or Bluetooth®
interfaces.
There is no doubt that high voltage and EHV
substations and switchyards will always be
challenging environments in which to carry
out electrical testing. As we’ve seen, when it
comes to insulation resistance testing, the
challenges have now been very effectively
addressed. For successful results it is essential
to use a test set that’s been designed for the
job: attempting to get by with a generalpurpose instrument is all too likely to lead to
frustration and wasted time.
Measuring on a roll
Josef Hollweck
Sales engineer, Megger Germany
There are many cases when using a TDR for
cable length measurement is not only convenient,
but also an excellent way of guarding against
problems.
Construction companies, for example, often
hire submersible pumps to remove water
from deep excavations and these pumps are
usually delivered with power cables on drums.
Although the required cable length will have
been specified, mistakes happen and a short
cable can make it impossible to install the
pump at the required depth, which is likely to
delay the project and incur unnecessary costs.
A quick check on the cable length with a TDR
will ensure that this doesn’t happen.
TDR1000/3 being used to measuring the length of cable left on a drum
Most engineers and technicians who regularly
work with power cables will at some time,
have used a time domain reflectometer (TDR)
– one of those handy little instruments that
feeds an electrical pulse into a cable, then
measures the time it takes for reflections of
that pulse to return. Since the pulse is reflected
not only by the end of the cable, but also by
many kinds of cable fault, the TDR is an
invaluable tool for determining fault locations.
www.megger.com
For those who think creatively, this is not the
only application of these useful instruments.
For example, instead of tediously measuring
cable lengths by hand, why not use a TDR? In
fact, why not use a TDR to measure the length
of a cable coiled on a drum or a cable reel?
With this technique, it isn’t even necessary to
unroll the cable, which saves a lot of time and
effort.
For companies that stock and sell power
cable, using a TDR to measured cable lengths
on the drum is a particularly attractive option.
To check stocks, it is no longer necessary to
unroll the cable and measure it by hand – all
that’s needed is access to one end of the cable
and the measurement can be made in seconds,
with very little effort.
Of course, not every TDR is well suited to
this type of application. What’s needed is
an easy-to-use instrument that has good
resolution to ensure that the length measurements are accurate. Fortunately, convenient
and cost-effective handheld instruments that
meet these requirements are now readily
available.
The best incorporate an auto set-up feature
that instantly recognises the type of cable,
ensuring reliable and accurate results. In
addition, the pulse they send into the cable
is very short – around 2 ns – and so they can
measure cable lengths with an accuracy of
around 100 mm.
Finally, they have high-resolution screens
and a trace hold feature that make it easy
to interpret the results, and they feature robust
construction to handle the rough-and-tumble
of on-site use. These instruments are modestly
priced considering the benefits they offer and,
if you really want to, you can even use them
for cable fault location!
So, next time you want to know how much
power cable is on a reel or drum, don’t reach
for a tape measure and spend ages struggling
with tangles as you unreel the cable, reach
instead for your trusty TDR and, with you’ll
have the answer in seconds, effortlessly!
Megger ELECTRICAL TESTER July 2013
3
ELECTRICAL
TESTER
The industry’s recognised information tool
Interoperability
and IEC 61850
GOOSE
Andrea Bonetti
Technical specialist in protection relay test
Introduction
In general terms, interoperability is the ability
of diverse systems to work together effectively
and efficiently. Interoperability is a property
of a product or system whose interfaces are
completely understood to work with other
products or systems, present or future, without restrictions on access or implementation.
Interoperability helps to decrease complexity
and makes it easier to manage heterogeneous
environments while enhancing choice and
innovation in the market. Importantly, the
interoperability requirement of the IEC
61850 standard has beneficially increased the
“interoperability among different engineers”
working for companies that are nominally in
competition. This increased communication
among different vendors has contributed to
the fact that GOOSE messaging can today be
considered a working technology, even if
problems still arise, as they do in any
technology.
With more than six years of field experience
with IEC 61850 GOOSE communication in
protection and control applications, it is now
possible to list the main reasons for interoperability problems in multi- and singlevendor systems. However, a comprehensive
list would be unmanageably long, especially if
cases found in the early days of using GOOSE
messages were included.
case, the receiving relay may fail to receive the
signal. This is a frequent situation during the
commissioning of substations, and the usual
solution is to replace the binary input card of
the receiving relay. Finding this problem and
identifying its cause are time-consuming jobs
because the test engineer usually believes that
the problem is located in other parts of the
system and the real cause is identified only
after other “more probable causes” have been
eliminated.
Interoperability with IEC 61850 GOOSE
With IEC 61850 GOOSE technology, the
situation is very similar. The problem is
identified after a time-consuming investigation
concludes that the signal is not being correctly
received by the receiving IED. Relay engineers
usually describe interoperability failures by
saying something like:
A typical example is illustrated where,
depending on its own VLAN settings, the
switch (or switches) removes the VLAN tag of
the GOOSE message.
As the VLAN tag is a mandatory part of the
GOOSE message, an IED “has the right” to
refuse the GOOSE message if the tag is
missing.
One IEC 61850 TISSUE (nr.290, VLAN ID) has
been dedicated to this problem and the
decision taken – in essence – is that the IEDs
are allowed to receive GOOSE messages with
or without VLAN tag.
A sending relay has its binary output
polarized by the battery voltage at 110 V dc,
and the receiving relay has a binary input
card with nominal voltage of 220 Vdc. In this
Megger ELECTRICAL TESTER July 2013
From what has been seen in the field to date,
unless there is a design fault (bug) in the IEC
61850 GOOSE stack of one of the IEDs, this
problem almost always occurs when using
non-standard ASCII characters like ä or ö in
the SCL description of the GOOSE message.
The use of space characters has also created
problems. Not all engineering tools are very
robust when checking that only valid
characters have been used, and the definition
of “valid character” has to be found in the
XML file specification, as SCL files are XML
files. This interoperability problem has been
identified in multi-vendor applications.
Experience has shown that the best way of
avoiding these problems is to always use basic
ASCII characters and never use spaces when
defining GOOSE messages in the engineering
tools.
This problem has usually been found in the
sender IED, and if this is the case, the
consistency check method against the SCL file
detects the difference.
This type of interoperability problem is mainly
due to the different interpretation by individual
vendors of the default values that must be
assigned to the various attributes of the
GOOSE message, when information is missing
in the SCL file describing it. This interoperability problem has been seen in multi-vendor
applications.
Even this simple connection can produce
“interoperability” problems. Consider, for
example:
Different interpretation of “default
values”
GOOSE messages modified by other IEDs
in the network
Interoperability “before”
Interoperability is a word that commonly
refers to numerical technology or numerical
relays. Interoperability problems did and do
exist even within the so-called conventional
technology, where communication between
different protection relays is based on Boolean
signals expressed in terms of dc voltage level.
In other words, the binary output (contact
or similar) from one relay is connected to a
binary input of another relay. The connecting
medium is a couple of wires.
4
Depending on the switches used, they may
have problems in handling the VLAN 0, but
they should always be able to handle any
VLAN other than zero. If all GOOSE messages
have the same VLAN (001 for instance), it is
always possible to set all the ports of all the
switches to handle VLAN 1, with consequence
that the VLAN tags of the messages should
neither be removed nor modified.
The remaining part of this article looks at
some of the most common sources of
interoperability problems with IEC 61850
GOOSE.
This interoperability problem can occur in
both single- and multi-vendor applications.
Different interpretation of SCL (xml)
information (file importing/exporting)
It is also recommended to always use the
VLAN tag, even if in the horizontal
communication different VLANs are not used,
to make sure that all GOOSE messages are on
the same VLAN (for instance VLAN 1).
“The GOOSE message appears on the network.
It can be seen with any network analyzer or
dedicated GOOSE visualizer … But the IED
does not receive it.”
In order to commission substations with the
new IEC 61850 technology, there is a need to
use new tools and methods. The key to these
tools and methods is – paradoxically –
implicitly available in the IEC 61850 standard
itself.
What is interoperability in IEC 61850
communication?
The IEC 61850 standard clearly aims for
communication interoperability among IEDs
from different manufacturers and defines the
interoperability as “… the ability to operate
on the same network or communication path
sharing information and commands...”. When
data sent by Device A is not fully understood
or received by Device B, an interoperability
failure occurs. This situation was common
before the IEC 61850 standard, as most
numerical relays from different vendors had
their own proprietary communication
protocols. When the communication was
not required to perform real time tasks (like
handling protection signals for protection
schemes), it was possible to solve this
problem by using protocol converters.
This means that, depending on whether the
firmware of the IED was issued before or
after the TISSUE had been approved, some
IEDs may receive the message with an altered
VLAN tag, and others may refuse it. The
simplest solution to this problem is to set the
substation switches in such a way that the
VLAN tags are neither removed nor modified.
If the problem is in the receiving IED, the
consistency check method doesn’t help because
the GOOSE message on the network is the
same as the message in the SCL file. But in
this case, everything points to the receiving
IED and the manufacturer should be contacted
to help in the investigation.
Problems created by the IEC 61850
engineering process
Even where the standard is quite clear on the
default values, this type of interoperability
problem has often appeared; the solution is
usually a new firmware release for the IED.
The problem could be in the sender IED
(which sends the wrong default value) or in
the receiving IED that is not able to understand that the default value received on the
network is correct, even if its description on
the SCL file for that value is empty.
This non-interoperability can be detected by
comparing the SCL GOOSE information with
the GOOSE information available on the network (consistency check method).
The best way of avoiding this problem is to
always set all the possible attributes when
defining the GOOSE message with the IEC
61850 engineering tool, and to not leave any
fields empty.
Typically, this type of interoperability problem
is the result of a difference in the configuration
revision of the GOOSE message. For example,
in the SCL file there is Configuration Revision
3, but the published GOOSE has Configuration
Revision 2.
This means that the IEC 61850 horizontal
communication has been modified at SCL file
level, but maybe for that particular GOOSE
message nothing has been changed. The
engineering tool has nevertheless incremented
the configuration revision, but the sender IED
has not been updated with the new SCL file
and continues to work with the previous one.
This interoperability problem can occur in
single- and multi-vendor applications, but in
single-vendor applications the IEC 61850
engineering process is usually simplified by
the vendor tool, and the risk is minor. With
this problem, engineers typically say, “everything was working fine previously”. This is a
good indication of where the problem lies.
The use of several SCL files (for example,
several CID files for different IEDs rather
than a single SCD file) also increases the
probability of generating this type of interoperability problem, not only related to
different configuration revisions.
www.megger.com
ELECTRICAL
TESTER
The industry’s recognised information tool
The secret to limiting
substation catastrophes
By properly maintaining switches and
connections, technicians can avoid costly
and time-consuming failures and outages.
Gary Wright, Consultant
Gary Wright (gwright@forestgrove-or.
gov) consults for McMinnville Water &
Light and Forest Grove Light & Power
in Oregon. He started out in the power
industry in 1977, and recently worked for
Clark Public Utilities in charge of substations, metering and relaying, and is
now retired.
When you work inside a substation, many
problems will sneak in without your knowledge and some of them may be catastrophic.
As utility professionals know, a catastrophic
substation failure can bring a lot of attention
— the kind you don’t want.
Unfortunately, no silver-bullet solutions are
available to prevent substation failure. These
failures can be caused by a variety of factors,
including power transformers, batteries,
breakers or protection schemes that fail or
weren’t set correctly. But one of the most
common issues revolves around problems
with disconnect switches and bus connections.
attention to the condition of the contacts in
regards to pitting or loss of silver. Make sure
you have good contact pressure and then
apply a thin coat of Dow Corning® 1292 white
grease or similar. One benefit of this grease is
that it will stay soft, so when you are back in
three to five years for routine maintenance, it
will wipe off with a rag.
These types of problems can take down substations and inflict major damage. In addition,
they can require switching plans to be halted
because switches won’t open, won’t close, are
raining down sparks when asked to carry load
or are flashing over when asked to interrupt
load.
If the switch happens to be a load break type,
it’s important to test the interrupter. Load
break switches work by making a parallel
between the switch contacts and the interrupter
unit as the switch is starting to open. Normally,
most switch manufacturers don’t want any
current in the interrupter unit when the switch
is closed. If this is the case, test for the proper
clearances or the interrupter can burn up
under normal load.
Scanning with infrared
When evaluating the state of utility substations,
switches and connections should top the list.
In-house crews can perform switch maintenance
with just a little training. And, once the switches
and connections are operating properly, this
will eliminate one large opportunity for a
catastrophic failure and also make all future
switching go smoothly and predictably.
Infrared scanning helps technicians spot
problems. It’s beneficial to conduct infrared
scanning of all substations and some transmission lines every year. Also, when possible,
it’s best to schedule the work in times of
heavier loads.
When doing the infrared scanning, it’s
important to note that infrared is crucial but
not perfect. For example, infrared is not
effective for switches that sit open normally or
for switches feeding ‘out of service’ loads on
the day you scan. Wires may still burn down
even though they passed an infrared scan,
even if they were carrying load during the
scan.
If a connection does fail shortly after passing
an infrared scan, a utility could be looking at
a connection failure cycle. In this situation,
the connection can get so hot carrying load
that it will melt and then weld together. This
weld makes a good connection, at least for the
amount of current at the time, and the infrared
scan of the weld may show nothing. Then,
at a later date, the current is raised beyond
the capabilities of the weld area and the wire
burns in two.
Making preparations
To successfully guard against untimely outages,
substation technicians should not rely on
infrared technology alone. Instead, they should
also consider adding resistance-based tests in
the substation. These tests also can be applied
to transmission and distribution switches or
selected line connections.
Resistance-based testing involves using a
micro-ohm meter, which measures tiny
amounts of resistance (such as what would be
the resistance of a few feet of bus). After this
test is applied to all switches and connections
in a station, the tester can be sure the tested
items will not heat up under rated loads
because every switch is included, even the
ones that are normally open.
To begin, you’ll need some basic tools and
maintenance parts for this testing. First, and
most important, you’ll need a good micro-ohm
www.megger.com
A technician runs a diagnostic test using a micro-ohmmeter
meter such as the Megger DLRO200-115. This
is a meter that puts out 200 A of filtered dc
current. The advantage of filtered DC current
is that you can avoid a false trip. If you have
a differential scheme and the relays are still
connected to the current transformer (CT) of
the device you are testing, you can run this
filtered dc through the CT and the dc will not
be sensed in the differential circuit.
In addition, you should have upgraded bolts
for the connection pads and bimetal pads in
all three sizes (two-hole, 3 inch four-hole, and
4 inch four-hole). The reason for the bimetal
pads is that sometimes when connections fail,
it’s due to someone improperly making them
up, putting copper against aluminum. These
can be redone installing the bimetal pads
between the dissimilar metals. It’s a good
idea to upgrade the bolt system for the
connections. Normally, it’s only necessary
to change the bolts if a connection fails the
micro-ohm test.
The preferred connection system for bolted
pad type connections consists of a stainless
steel bolt (long enough so at least two threads
protrude through the nut), two stainless steel
³/16-inch-thick flat washers, one on each side
of the connection, one 3,500 lb stainless
steel Belleville washer on the nut side and a
silicon bronze nut. The bolts are ½ inch and
should be tightened to a torque of 45 ft. This
will compress the 3,500 lb Belleville washer
to about 60% compression, which will mean
the connection can expand and contract with
heating and cooling cycles and not stretch the
bolt. And the connection will maintain
constant tension through the years.
The last trick is to clean up the mating
surfaces with Scotch Bright® or whatever
cleaner is needed to get them clean. Then coat
the surfaces with an oxidation inhibitor like
DE-OX, a non-gritted ‘green’ inhibitor from
Ilsco®. There’s no reason to use a gritted
inhibitor because the parts are de-energized
and can be completely cleaned. If the Scotch
Bright can’t get the old dried inhibitor off, try
a scraper. As a general rule, you should avoid
sanding or filing because if the terminals are
tin-plated you could go through the tin very
easily.
Testing the equipment
After cleaning and preparing the surfaces, it’s
time to test with a micrometer. Technicians
should put the clamp around the entire switch
assembly, including all connections associated
with the switch. It’s important to clean the bus
to bare metal for this connection using a wire
brush and some light sanding.
When talking about micro-ohms, you cannot
have any resistance. It’s often effective to run
the maximum current of 200 A, provided the
components are rated for it. It’s important that
at least one side of the switch — or whatever
part you are testing — is not grounded.
One way to do it is to take a reading on the
entire assembly, and then if it passes, you are
done. The values you get will vary with switch
quality and materials, but a typical 1,200 A
copper switch (along with the switch pads at
both ends and the bus to terminal connections
at both ends) might be in the 350 micro-ohm
range. It doesn’t take long to develop a feel
for what to expect. You must look for things
that stand out. If you’re testing a different kind
of switch and you’re not sure what to expect,
if all three poles match, chances are you are
in good shape. If one or two poles stand out,
you probably should work on the higher poles
to see if you can get them to match the lower
poles.
If you run into a problem pole using this
connection across everything, then leave the
200-A current connections across the whole
switch but move your voltage leads across the
individual connections. For example, you can
measure the resistance of just one end of the
switch contacts, the bolted pads, the clamp to
the bus or the hinged end of the switch. This
will allow you to isolate your problem quickly
so you know what needs attention. All of this
is done by just moving along with the voltage
probes, and the 200 A current source stays
connected across the entire switch.
Checking equipment
Checking through all the switches with the
micro-ohm meter should be done in
conjunction with switch maintenance. The
contacts should be cleaned, being careful to
not scratch through the silver plating on the
contacts. You must check for proper alignment
of poles and contacts, and pay particular
As the switch opens, load current is paralleled
between the main contacts and the interrupter.
This parallel has to be maintained as you
continue to open the switch until a sufficient
gap exists between the main contacts to avoid
re-strike as the load is interrupted. The interruption will occur either using a vacuum
bottle or an expulsion-type snuffer device. In
either case, you’ll need to test that the closed
interrupter has low resistance. In most cases,
anything less than an ohm can be used for a
go/no-go.
After you have proven you can pass current
through the closed interrupter, the next step is
to prove the interrupter can interrupt the
current at full voltage. If the interrupter is of
the snuffer type, there’s no real test you can
do. If the interrupter is of the vacuum type,
verify with the manufacturer that the vacuum
bottle can be tested with a hi-pot. Applying
voltage to the open vacuum bottle is the only
way to prove the vacuum exists and the
interrupter will interrupt load at line voltage.
If all these procedures are followed on all
connections and switches in the station during
a three- to five-year cycle, you should have
eliminated at least one looming source for
serious catastrophic substation problems.
Insider Tips for maintenance
Gary Wright, has been responsible for the
maintenance of more than 100 sub-stations
over the last 35 years.
Infrared: Often some of the transmission
lines can be scanned as you drive to each
station. It’s usually beneficial to hire an
infrared contractor to scan transmission lines
and the most important distribution feeders.
Nuts: It’s important to use a silicon bronze
for the nut, because if you use a stainless
steel nut, the nut can gall and stick. This
means you can’t get the connection tight.
Unfortunately, it will appear tight, and you
might not be able to get the nut back off.
Grounding: There can only be one ground
connected anywhere on the conductor
you are testing. If there is a second ground
installed anywhere, it will become a parallel
current path and make your reading useless.
Megger ELECTRICAL TESTER July 2013
5
ELECTRICAL
TESTER
The industry’s recognised information tool
Multiple current injection
Figure 1: Two SMRT36’s and two SMRT1’s
interconnected together
Figure 3: Sixteen bay bus-zone stable condition while
injecting sixteen currents
The number of current outputs that users
require from protection relay test sets
seems to be constantly increasing and of
course, test sets are evolving to meet these
requirements. Some of the latest models
are capable of injecting ten test currents
simultaneously from a single test set.
Sometimes even this isn’t quite enough, and,
in certain applications, more are needed.
This application-based need was most
definitely apparent when a customer in South
Africa wanted to test a new bus-zone
protection panel, manufactured by one of the
world’s best-known relay manufacturers. The
panel was to be installed as an upgrade in one
of the South African 400 kV substations. This
particular bus-zone panel had 16 protection
relays, and, to test the bus-zone scheme
effectively, the most convenient solution was
to inject current into all of the relays
simultaneously. Unfortunately, there is no
single relay test set available that supports 16
current channels.
Safeguard
those services!
Mr. Jörg Schubert
Manager, line locating and inspection
Megger Baunach
It only takes a short walk or drive through
any town centre to confirm that excavations
– holes in the road – are common. In fact,
they’re very common. And, with every one
of those excavations goes a very real concern:
will the digging lead to costly and disruptive
accidental damage to underground services?
Of course in an ideal world, the routes of
buried cables and pipes should be properly
documented, which would make avoiding
them relatively straightforward. In the real
world however, plans are often missing or
just plain wrong.
6
Ingenious engineers came up with a thoughtprovoking alternative. Instead of using a
single test set, why not interconnect multiple
relay test sets to provide the required number
of current channels? The engineers from the
customer and the relay manufacturer found
this suggestion interesting, and so arrangements were made for it to be evaluated.
Largely because of equipment availability,
the test sets chosen for the exercise were two
SMRT36 three-phase units and two SMRT1
single-phase configured as shown in Figure 1.
The three-phase unit had three current
channels and three voltage channels, but
the voltage channels could be converted to
current channels to give six currents at one
time out of one test set. The single-phase unit
had one current channel and one voltage
channel and the voltage channel could be
converted to a current channel to give two
current channels.
Fortunately, this no longer needs to be a
problem, as convenient and dependable line
location systems for determining the position of
underground services are now available, like
the EasyLoc.
Comprising of two separate components – a
transmitter and a handheld receiver – which
are capable of tracing the routes of energised
and dead power cables and of metallic pipes.
When tracing energised cables, the receiver is
used on its own and looks for power frequency
signals radiated by the cable. The user simply
scans the likely route of the cable with the
receiver until a clear signal is indicated, and
then determines the cable location even more
accurately by carefully adjusting the position
of the receiver until the highest signal strength
is achieved.
Typically, the signal is indicated audibly via
a built-in loudspeaker, and also visually on a
backlit display panel. With the best instruments,
once the location of the cable has been
determined, its depth can be measured and
displayed simply by pressing a button on the
receiver.
Megger ELECTRICAL TESTER July 2013
Marius Pitzer
Sales manager, Megger South Africa
Figure 2: Three SMRT36’s interconnected together
Figure 4: Sixteen bay bus-zone Zone 1 fault while
injecting sixteen currents
The four test sets were interconnected via
Ethernet cable (RJ45) so that they could be
operated as if they were one single test set,
and overall control was provided for some of
the time with a touch-screen interface unit,
and on other occasions with a dedicated software package running on a PC.
The test equipment performed exactly as
required, and the simulation of different
zone faults went smoothly when a simple
pre-fault/fault test was run. In the pre-fault
stage, a stable bus-zone condition was
simulated as shown in Figure 3. After a
predetermined time period, a fault was
injected and the time taken to clear the fault
was measured. Figure 4 shows a Zone 1 fault
that tripped in 12.5 ms. After the zone timing
tests were completed successfully, breaker fail
testing was performed using seven stages and
once again this proved to be problem free.
At the end of the testing, both the customer
and the relay manufacturer’s engineers
agreed that this novel approach offered major
The procedure for tracing pipes and unenergised cables is very similar, but in this
instance the transmitter is used to inject a
unique test signal, usually at a frequency of
33 kHz, into the pipe or cable. This can be
done by direct connection of the transmitter,
or by using induction to couple the test signal.
With some instruments, the transmitter has an
output that’s protected against mains voltages,
allowing it to be used with energised cables
to improve accuracy in difficult operating
conditions.
The latest line location systems incorporate
automatic sensitivity control, which makes
them particularly easy to operate; together
with a self-calibration check routine that saves
on maintenance costs. Some also offer accessories
to further increase their usefulness. These
accessories may, for example, include pipeline
transmitters for locating non-metallic pipes,
transmitter clamps for coupling the transmitter
signal into energised cables without the need
for direct connections, house connection sets
for connecting the transmitter signal via a
standard socket outlet, and headphones to
enable the receiver’s audible output to be
monitored in noisy environments.
benefits, not the least of which was that it had
allowed the testing of the new bus-zone panel
to be completed in less than a day when
similar panels had in the past typically taken
three days to test using traditional test
techniques. Using the traditional test techniques, the customer would sectionalize the
bus-zone and test the panel using six test
currents at a time and would therefore, have
to go through a prolonged process to simulate
all possible faults in the bus-zone.
Since the initial test, the customer has bought
another SMRT36 three-phase test set and is
now using three of these to inject up to 18
currents, as shown in Figure 2. No end-user
is ever satisfied for very long, however, and
it is already being suggested that the next
challenge will be to test a protection system
that requires 32 currents to be injected
simultaneously!
Line location systems of the type described
here are readily portable, easy to use and
modestly priced. All in all, they’re an excellent
investment – they safeguard services and
thereby provide peace of mind by eliminating
one of the biggest concerns associated with
excavations of every kind.
When the location of the cable has been
determined, its depth can be measured and
displayed simply by pressing a button on the
receiver
www.megger.com
ELECTRICAL
TESTER
The industry’s recognised information tool
Explaining
the art of
testing
Elsa Cantu
Marketing communication manager,
Megger Dallas
Cable fault
Peter Herpertz
Product manager, power
In the first of a series of articles on cable
fault location, Peter Herpetz looks at the
construction of modern power cables and
examines the most common types of fault
that affect them.
The importance of cable testing
Fault location on power cables is a very
special area of electrical technology, and the
results obtained depend very much on good
logistics and knowledge. Accurate prelocation
is the foundation for fast and reliable fault
location, because it means that pinpointing
procedures only need to be carried out on a
short section of cable.
The importance of cable testing, cable fault
diagnosis and partial discharge analysis are
certain to become increasingly important in
the future, as condition-based maintenance
of cable networks more and more displaces
event-oriented maintenance.
A prime requirement for testing electrical
power systems is without doubt to have access
to the right equipment – the latest test sets
deliver levels of convenience and performance
that simply cannot be achieved with their
predecessors. But the right test equipment on
its own does not offer a complete solution;
adoption of the correct test techniques is also
essential if the most accurate and reliable
results are to be obtained.
Test techniques do not however, stand
still. On the contrary, they are continuously
developing, which is why NETA, the
InterNational Electrical Testing Association,
regularly invites papers on developments in
test techniques and associated subjects for
presentation at its annual PowerTest Electrical
Maintenance and Safety Conference.
The papers are carefully selected to highlight
significant advances and, at the end of the
conference, the best of them, which are
chosen through a searching judging and
evaluation process carried out by NETA,
receive awards. These coveted and prestigious
awards recognise not only the expertise of
the papers’ authors, but also the valuable
contribution that their work has made to the
power test community. Among the awards
made at PowerTest 2013, which was held in
New Orleans in February, were:
A good, detailed knowledge about the cable
network, cable types and cable accessories
greatly simplifies the evaluation of test results
and, in many cases, such knowledge is an
essential prerequisite for making correct
decisions. Among the most important things
that technicians need to know are the types of
cable faults and the steps needed to carry out
cable fault location and diagnosis.
Construction of power cables
The function of power cables is the
distribution of electrical energy, and they
must carry out this function reliably and safely
for very long periods. Depending on the
application, the external environment and
local conditions, such as the presence of
ground water and the type of ground voltages,
different types of cable are used. Cables with
impregnated insulation, such as PILC (paper
insulated lead covered) types were widely
used until the late 1960s and are still in
service in some areas. These cables have,
however, mostly been replaced by cables
with PVC (polyvinylchloride), EPR (ethylenepropylene rubber), PR, or XLPE (cross-linked
polyethylene) insulation. As a result of these
changes in the type of insulation used, cable
faults and cable testing techniques have also
changed considerably.
The following sections cannot cover all of the
possible types of cables, insulating materials
and cable construction, so they focus on the
most important variants. In many cases, details
are explained primarily as an aid to understanding the terminology used in the later
sections of this guide to cable fault location.
Best Relay Presentation: ‘Utilising Ground
Fault Resistance to Accurately Test Distance Element’ by Jason Buneo, Applications Engineer
n
n
Best Circuit Breaker Presentation: ‘Testing
and Troubleshooting of Low to Medium
Voltage Circuit Breakers’ by Bret Hammond
and Robert Foster, Technical Support Engineers
n
Best Safety Paper: ‘Electrical Safety Through
Design, Installation and Maintenance’ by Dennis Neitzel
All the authors of these award-winning papers,
with the exception of Dennis Neitzel are
Megger employees. Dennis Neitzel is Director
Emeritus of AVO Training Institute Inc., which
is a Megger subsidiary. Megger also won two
marketing awards at PowerTest 2013, for Best
in Show Trade Show Marketing, and Most
Entertaining Hospitality Night.
www.megger.com
As well as these typical materials, there are
many other types of insulation.
Semiconducting layers (at nominal
voltages above 6 kV)
The purpose of semiconducting layers is to
reduce the strength of electric fields within the
cable, and to eliminate partial discharge. Semiconducting layers reduce the electric field that
develops around the conductors, and thereby
eliminate the potentially damaging discharges
associated with high electric field strengths.
On modern cables, another type of semiconducting layer is sometimes integrated with
the outer insulating sheath/jacket. The purpose
of this type of layer is to aid the location of
sheath faults on cables that are installed in
ducts, where there is no return path through
the earth for fault currents.
Sheath/jacket
Shield/screen
Semi conductor
Flashing fault (parallel fault)
This is a very high resistance fault, and can
be present when the cable is charged.
Typically, the flashover occurs at several kV,
and is very often located in cable joints. The
cable behaves in the same way as an arc gap,
where the distance between the electrodes
determines the breakdown voltage. The
resistance of this type of fault is typically
infinity up to the breakdown voltage.
Open-circuit fault (series fault)
Faults of this type can be very high resistance,
up to infinity if the conductor is completely
severed. Very often, this type of fault is a
combination of series and parallel resistances.
The reason for this is that if the conductor is
completely cut or pulled out of a joint, this not
only produces a complete open circuit, but
also allows all possible variations of flashover.
If the conductor is partially burned, this type
of fault is called a longitudinal fault.
Insulation/dielectric
Inner semi conductor
Core/conductor
Metallic sheath
The metallic sheath performs multiple
functions. It seals the cable against the entry
of humidity, it provides a conductive path for
leakage and earth-fault currents, it provides
potential equalisation and it can be used as
either an earth conductor or a concentric
neutral conductor. For cables used in critical
or subsea applications, the metallic sheath
can be designed to provide robust mechanical
protection.
Shield (for MV and HV cables)
The shield provides electric field control, and
also offers a conductive path for leakage and
earth fault currents.
Armour
The armour provides mechanical protection.
It may consist of steel bands, flat steel wire,
round steel wires, etc. In some cases, the
armour may be made up of several different
layers.
Plastic sheath
The plastic sheath provides outer protection
for the cable, and usually consists of either
PVC or polyethylene.
Cable faults
When diagnosing and locating cable faults, the
procedure depends on the type of cable fault.
Cable faults are generally divided into the
types listed here.
Earth faults and sheath faults
These are faults between the metallic shield
and the surrounding soil for plastic-insulated
cables, or between the conductor and the
surrounding soil for LV and plastic-insulated
cables. Great care must be taken when using
high voltages to test for or locate this type of
fault, as the voltage discharges directly into the
earth, creating shock hazards for people and
animals.
Humid/wet faults
On multicore cables, all conductors are often
affected by this type of fault, but the flashovers
do not always occur at the point where the
water entered the cable. Impedance changes
occur at the fault position. Depending on the
cable construction (for example, the type of
longitudinal water sealing), these faults can
be confined to a single point or widespread
throughout the cable. Humidity/wet faults are
the most difficult faults to locate. They have
a tendency to change during the fault location
procedure, often very considerably. Particularly
in joints, this means that the fault becomes
highly resistive after one or two discharges, as
the water is blown out of the joint and dries
up. When this happens, the fault can no
longer be localised. Underwater faults are
another form of wet fault. With these, the
water pressure prevents effective ignition of
the fault when high voltage is applied. These
faults can be very difficult to localise.
Conductor-to-conductor fault (parallel
fault)
Unwanted connection between two or more
conductors. The resistance of the fault may be
anywhere between zero ohms (low resistance)
and several megohms (high resistance).
n
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propylene rubber (EPR)
n above 60 kV: paper with oil or gas, cross-
linked polyethylene (XLPE)
Conductor
The conductor is the part of the cable that
transmits current, and is usually soft electrolytic copper or pure aluminium. The conductor
can be round or sector-shaped, and made of
single wire or multi-stranded.
Insulation
The purpose of the insulation is to prevent the
flow of current between the conductors in the
cable, and from the conductors to the cable’s
metallic outer covering, which may be armour
or a lead sheath. Typical insulating materials
are:
n 1 to 10 kV: mass impregnated paper (PILC), polyvinylchloride (PVC)
n 1 to 30 kV: mass impregnated paper (PILC), cross-linked polyethylene (XLPE), ethylene Conductor-to-shield fault (parallel fault)
Connection between a conductor and the
shield or between multiple conductors and
the shield. The resistance of the fault may be
anywhere between zero ohms (low resistance)
and several megohms (high resistance).
Experience shows that the majority of faults
fall into this category.
CONCLUSION
A clear distinction must be made between
short-circuit, resistive and high-resistance
faults, because this distinction has a significant
influence on the procedures that should be
used for fault location. These procedures will
be described in future articles in this series.
Megger ELECTRICAL TESTER July 2013
7
ELECTRICAL
TESTER
The industry’s recognised information tool
Q&A
Q: How should transformer winding resistance
test results be evaluated?
A: Evaluation can be carried out by comparing
the test results with original factory measurements
or with previous measurements that have been
made in the field. Alternatively, the results can
be evaluated by comparing the phases with
each other. In most cases, phase-to-phase
comparisons are sufficient.
Q: How much difference between measurements
is acceptable?
A: The industry standard for factory tests
permits a maximum difference of 0.5% from
the average resistance of the three phase
windings. Measurements made in the field
may vary slightly more than this because there
are more variables, but if the measurements
are within 1% of each other, they can be
considered acceptable. Note that comparing
absolute resistance values measured in the
field with factory values can be difficult,
principally because of the difficulty of estimating
This time we turn our attention to questions that are frequently asked about interpreting the
results of transformer winding resistance measurements, and about sources of confusion that
can give rise to results that appear to be problematic even though, in reality, no problems
exist.
the winding temperature accurately. Values
within 5% are normally acceptable.
Q: If larger differences are found, what sorts of
problems might this indicate?
A: Variations from one phase to another or
inconsistent measurements can be indicative
of many different problems, including shorted
turns, open turns, poor brazed or mechanical
connections, defective ratio adjusting (RA)
switches or defective load tap changers
(LTCs).
Q: Why do winding resistance measurements
sometimes appear to show problems when, in
fact, none are present?
A: There are several factors that can result in
misleading measurements. The most common
are temperature changes, contact oxidation
and measuring errors.
Q: How do temperature changes influence
measurements?
A: The dc resistance of a winding varies as its
temperature changes. For copper windings,
the variation is 0.93% per ºC. This is usually
not a significant consideration when comparing
My resistance is low!
Keith Wilson
Electrical Engineer
It’s a fair bet that when the lovely Jane Russell
sang about her resistance being low in the
1952 movie ‘The Las Vegas Story’, there were
few things further from her mind than electrical
testing. To decide for yourself, why not take
a few minutes out to watch the clip?
You’ll find it at http://www.youtube.com/
watch?v=iQBDN5s8IB0.
For Evershed and Vignoles, the Ducter was a
logical progression from the Megger insulation
tester as it used the same type of meter movement, with two coils rigidly fixed together in a
magnetic field. In the Ducter, one coil carries
a current (I) proportional to the current flowing
in the object under test, while, the other
carries a current proportional to the voltage
drop (IR) across the object. The deflection de-
8
The current source for the Ducter was one or
more rechargeable nickel-iron (NiFe) cells,
depending on the model. These cells, the forerunners of today’s NiCd and NiMH cells, have
a low internal resistance and can supply very
high currents for short periods without risk of
damage. The popular model 37002 five-range
Ducter, which could measure from 1 µΩ to 1
Ω, uses a single 220 Ah NiFe cell and, on its
lowest range, employs a test current of 100 A.
London Passenger Transport: “The Ducter
is used in the electrical test room at Charlton
works for testing the resistance of trolleybus
traction motor field and interpole coils, traction
armatures and low-resistance blow-out coils.”
Using a Ducter to measure the contact
resistance of an AEI 132 kV 2,500 MVA oil circuit
breaker
pends on the ratio of these two currents (IR/I)
and it will be seen that this ratio is un-affected
by either the test current or the applied voltage.
A simplified schematic for a Ducter is shown
in Figure 1.
Foster Transformers: “Our test room uses
the Ducter ohmmeter to measure the
resistance of transformer windings to
determine their total copper losses, and their
temperature change after a heat run”.
Stewart and Lloyds Steel: “The testing of
graphite furnace electrodes for specific
resistance is now being made into a routine
check using a Ducter ohmmeter, which allows
in Hawaii. The probability of confirmation is,
however, very high – well over 90%.
continued from page 1
The Kepler satellite has already generated data
sets for hundreds of thousands of stars, and
is generating more all the time. All data sets
are analysed by computer, but this doesn’t
guarantee that all potential extra-solar planets
are detected. That’s where the Planethunter
volunteers come in, because experience has
Another issue that can lead to temperature
changes is the use of too high a test current.
When measuring the dc resistance of smaller
transformers, care should be taken to ensure
that the test current does not cause heating of
the windings. For this reason, the test current
should not exceed 10% of the winding rating.
British Railways: “A Ducter ohmmeter has
been in regular use in the Electric Carriage
Repair Shop in Wimbledon since February
1917. It is still in use for routine tests on
traction motor armatures”.
It’s another world Planethunters.org supplies volunteers with
data sets acquired by NASA’s Kepler satellite,
which is one of the most powerful tools in
the hunt for extra-solar planets. Each data set
shows how the observed brightness of a star
varies over time, and the volunteers look for
the characteristic drop in brightness that
occurs when a planet passes in front of the
star.
In addition to loading, temperature (and
therefore resistance) variations can be due to
cooling or warming of the transformer during
the test, particularly on large transformers with
an LTC where the time between the first and
last measurement is often an hour or more.
Note that the temperature of a transformer
that has been on load is likely to change
significantly during the first few hours off
load.
Ducters, just like the digital low resistance
ohmmeters that are their present-day successors,
were used in an amazingly wide range of
applications, as can be seen from these
fascinating testimonials, which are just a small
sample of those included in a user guide from
the late 1950s:
Whatever your conclusion, the electrical
engineers of that era needed, as they do
today, a convenient method for accurately
measuring low resistances. Evershed &
Vignoles, one of the forerunner companies
Megger, provided the answer. Alongside its
famous range of Megger insulation testers, the
company had also developed instruments for
low resistance testing, which it sold under the
trademark ‘Ducter’.
This trademark is, in fact, still registered but is
no longer applied to current products. In its
heyday, however, the phrase ‘Ducter testing’
was widely used as a generic term for low
resistance testing.
phases in a power transformer, as the load on
power transformers is usually well balanced,
which means that the winding temperatures
should be very similar. However, when
making comparisons with factory measurements or previous field measurements, small
consistent changes should be expected.
Despite the vast number of data sets being
analysed, finding a new candidate planet is
still a rare event. Planethunters.org revealed
in January 2013 that its volunteers had found
just 15 candidate planets to date, with two
confirmed and the remaining 13 awaiting
investigation.
Actual transit for APH41111337 - Candidate planet
shown that humans are better at spotting the
subtle variations that suggest the presence of a
planet than even the best computer algorithms.
When volunteers report a data set that shows
these variations, it is noted as a ‘candidate
planet’. It is only officially confirmed as a
planet when independent observations have
been made, typically by the Keck Telescope
Megger ELECTRICAL TESTER July 2013
Mark’s discovery is considered particularly
significant because his candidate planet is in
orbit around a star that is just 1.2 times the
size of our own sun and its orbit is at a
distance from the star that is compatible with
life. Even though the planet is almost certainly
a gas giant unable to support life, there is a
high probability that it has moons similar to
those of Jupiter in our own solar system, and
these might well be hospitable to life.
Q: How does contact oxidation influence
measurements?
A: Dissolved gases in transformer oils act the
contact surfaces of RA switches and LTCs.
Usually, higher resistance measurements will
be noted on taps that not used or are used
infrequently. This apparent problem can be
rectified simply by operating the switch a
few times, as the design of most LTC and RA
switch contacts provides a wiping action that
removes surface oxidation.
Q: What are the most common measuring
errors?
A: There are many possibilities, including
incorrect or poor connections, use of a
defective instrument or one requiring
calibration, incorrect operation of the
instrument, mistakes in recording results and
ambiguous or poorly defined test data. Note
also that there is often more than one way
of measuring the resistance of a transformer
winding. Typically, field measurements
are taken from external bushing terminals,
whereas factory measurements are not
limited to these terminals. Additionally, in
the workshop or factory, internal winding
connections can be opened, making measurements possible that are not practical in the
field. Unfortunately, details of test set ups and
connections are often omitted in test reports,
which can lead to confusion when comparing
test data.
the test to be carried out with ease and
rapidity, and without damaging the electrode”.
Dorman Smith: “Our Ducter ohmmeter is
treated as a sub-standard instrument. That is, it
is used wherever it is required to determine a
low resistance with accuracy”.
Hoover: “The Ducter ohmmeter is used for
measuring the resistance of bi-metal strips
used in thermal cutouts and to check the
circuit resistance of the complete welded
assembly”.
Yorkshire Switchgear and Engineering:
“For ten years we have standardised upon a
Ducter test as the basis of our acceptance or
rejection of all switchgear contacts and bolted
joints. In our opinion, this method of testing
is equally as accurate as the accepted millivolt
drop method, and is far more readily applied”.
Shunt
Ligaments
Fixed
centre
iron
Control
coil
Battery
Current
contact
Potential
contact
Resistance
under test
Permanent
magnet
Cut-out
Deflecting
coil
Fig 1. Simplified schematic diagram of a
Ducter
“Making the discovery was
rather a shock,” said Mark.
“It was a bit like looking for
a needle in a haystack and
actually finding the needle!
I am, however, delighted
to have been able to make
Mark Hadley
this contribution to the
Planethunter.org project, and I’m just as
delighted, when I’m asked if I’ve done anything interesting recently, to be able to say,
very nonchalantly, that I discovered a planet.”
“Of course, I’m not satisfied with being one of
the few who have discovered a planet, what
I really want is to be one of the even fewer
who have discovered two or even three, so
my evenings and my laptop are going to be
very busy for some time to come!”
www.megger.com