Electrical Tester July 2013

8/20/2019 Electrical Tester July 2013

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edition

ELECTRICALTESTER The industry’s recognised information tool

Published by MeggerJuly 2013

It’s another world!

Nick HilditchGroup marketing services manager

There can be no doubt that Megger employssome very talented engineers but it’s easy toforget that they don’t leave their talents behindthem 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 intheir spare time?

Some might say that’s a question best leftunanswered but, in the case of Mark Hadley,

who is new product research manager at thecompany’s Dover site, it’s already too late!Thanks to recent press reports, Mark’s spare

time activities are now very much in the publicspotlight.

Because, when most of us have our feet up

watching television, Mark is hunting newplanets.

And his efforts have recently paid off, whenhis name was added to the very short list ofthose who have made significant contributionsto this work. To achieve this accolade, Markhas identified a new candidate planet aboutthe size of Jupiter, orbiting a sun-like star

Limiting

catastrophes

see page 5

Insulation

tester

for sub-

stations

see page 3

Cable

faults

see page 7

Phase evaluation in power networks

The spread of green micro-generation systemsand other market forces mean that powerutilities increasingly have to allow third-partyaccess to their networks. At the same time,staffing levels have in many cases beenreduced dramatically. This means that thedays when individual employees werespecialists in a particular area of work aregone. Today’s employees are expected tocover a wide range of activities, and simplydon’t have the time to develop in-depthexpertise.

Of course no compromises can ever be madein matters of safety, even when humanresources are scarce.

Achieving and maintaining the highest levelsof safety is made even more complicated fornetwork operators by the restructuring ofnetworks that is now carried out almostcontinuously to optimise operating efficiency.In particular, the constant changes make itdifficult to ensure that documentation is up todate and correct.

This means that it is now more importantthan ever to be able to easily and reliablydetermine the absolute phase of busbars inswitching equipment, transformer feeders andsubstations, crossing points of overhead cables,cable end closures and various parts of thelow-voltage network.

To meet this requirement, new phase evaluationtest instruments have been developed whichtake advantage of modern technologies suchas the GSM mobile phone network and theGPS satellite system that is most commonlyused for satellite navigation in vehicles.Correctly used, these instruments will preventthe occurrence of costly and often dangerouserrors during the commissioning and main-tenance of electrical power systems, therebyensuring that high network reliability andeconomic efficiency are achieved.

PRACTICAL PHASE MEASUREMENT

To explore the way that the new instrumentsoperate and how they are used, it is easiestto consider a specific product, in this casethe new SebaKMT PVS100, but the functionalitydescribed can be taken as a guide to whatusers should expect from any modern phaseevaluation system.

The absolute phasing at any point in atransmission or distribution network canonly be determined when the measurementis made with respect to a known referencephase. This means that for making phasemeasurements in the field, a mobile unit thatis capable of being used with a referencedevice (base station) is needed in order to

perform the necessary synchronisation. In thePVS100, this requirement has been met bydesigning the instrument as two identical units,one of which acts as the base station while theother is used as the mobile unit in the field.

A precise time base for synchronisation isestablished using signals from the GPS satellites, while for transmission of synchronisation data,each unit incorporates a GSM module. Thiscan operate in the normal CSD data trans-mission mode or alternatively in GPRS mode.

When the voltage of the phase to be measuredis less than 400 V, it is connected directly tothe mobile unit. Direct connection can also beused for measurements at capacitive test pointsof switching equipment or angle plugs (elbowconnectors).

For higher voltages, a high-voltage measurementsensor is used, and this communicates withthe base unit via an 866 MHz wireless link.The sensor, which is attached to an insulatingrod approved for use at the appropriate voltage,incorporates a high-intensity LED, visible evenin direct sunlight, which signals that themeasurement has been completed and the

phase identified successfully.This arrangement means that the operator cangive their full attention to the positioning ofthe sensor while the measurement is beingmade, without being distracted by having tolook at the mobile unit. The mobile unit storesthe measurement data for later downloadingand analysis.

PHASE CORRECTIONIf the base station is not connected to L1 asthe reference phase, the appropriate correctionangle of either +120º or -120º must be entered.Depending on the application, there can betransformers with the same or different vectorgroups between the base station and themobile device. Each of the vector groups leadsto a specific resultant phase shift, which mustbe entered on the mobile unit to obtain thecorrect absolute phase indication. If thecorrection values are not entered, only the

phase angle relative to the reference phasecan be determined.

Correction values are also needed formeasurements taken at capacitive voltagetest points. These values are restored in theinstrument for the most common types ofcapacitive sensor, and there is provision forentering the correction values for an almostunlimited number of additional sensor types.

MEASURING MODES AND APPLICATIONEXAMPLESBecause conditions in the field vary in termsof access to mains power and the availabilityof GPS and GSM signals, the best phaseevaluation test sets offer multiple measuring

modes to enable the best results possible tobe obtained under all conditions. The PVS100offers four modes: NET, NO NET, NO NET/NO GSM and LOCAL.

Mode 1: NETIf a low-voltage mains supply is available inthe location where the measurement is beingmade, the mobile device is simply connectedto any convenient mains socket and a one-time synchronisation process is carried out

with the base station. The mobile devicedetermines the phasing of the mains socketand uses this as the local reference for allmeasurements at this location. The mobiledevice must remain connected to the mainssocket throughout the entire measurementprocess. The advantages of this mode are thatGPS and GSM reception are only required for

a short time, during the one-time synchro-nisation process, and that results are obtained very quickly when making measurements with respect to the local reference.

Mode 2: NO NET When measurements are being made onoverhead lines or in substations, there is oftenno convenient low-voltage supply available.In these cases, the mobile device operatesfrom a built-in rechargeable battery. For directphase display during the measurement underthese conditions, there must be continuoussynchronisation with the base unit via GSM,and GPS reception must also be available.

Dr Frank Petzold and Alexander StanischaMegger Baunach

Overhead power lines being tested using a PVS100

Mode 3: NO NET/NO GSMThis mode is used when there is no low-

voltage mains supply available, and also noGSM coverage. In this mode, only the GPStime signals and the voltage zero crossings arestored in the device while the measurement isbeing made. Subsequently, when the unit ismoved to a location where GSM coverage isavailable, post-synchronisation is performed:the absolute phase identifiers are determinedand stored in a measurement file. Post-synchronisation can be carried out any timeup to ten days after the recording of themeasurement data.

Mode 4: LOCALIn 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 withthe base station is needed.

ConclusionThe latest phase evaluation test equipmentallows safe, fast and reliable phase identificationat all voltage levels. The units are well suitedfor use in the field, and are easy to operate.Use of this equipment prevents errors thatmay have serious safety implications and alsoensures that phase identification information iscorrectly documented. It therefore contributessignificantly to improving overall networkreliability and efficiency.

around seven million light years from oursolar system.

Mark made the discovery not, as might have

been expected, by shivering at a telescope oncold and starry nights, but by analysing datasets on his laptop in the warmth and comfortof his living room. This is because Mark is a

volunteer for the Planethunters.org website, which is led by Yale University as part ofOxford University’s Zooniverse project.

continued on page 8


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HV Supply

EN 61010-1 categories

Contents

Editor Nick Hilditch. T +44 (0)1304 502232E [email protected] www.megger.com

Megger LimitedArchcliffe Road Dover Kent CT17 9ENT +44 (0)1304 502100E [email protected] www.megger.com

‘Views expressed in Electrical Tester are not necessarily the

views of Megger.’

The word ‘Megger’ is a registered trademark

Note from the Editor

Time for your say.We have introduced a ‘Questions and Answers’ section andwould like your input. If you have any questions or storiesthat you think we could use, then please [email protected]

A printed newsletter is not as interactive as its emailequivalent so to help you find items quickly onwww.megger.com, we have underlined key search words inblue.

The industry’s recognised information tool

ELECTRICALTESTER

The rights of the individuals attributed in Electrical Tester tobe identified as authors of their respective articles has beenasserted by them in accordance with the Copyright, Designsand Patents Act 1988.

© Copyright Megger. All rights reserved. No part of ElectricalTester may be reproduced in a retrieval system, or transmittedin any form or by any means, electronic, mechanical, photo-copying, recording or otherwise without the prior writtenpermission of Megger.

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

please email [email protected], with a briefoutline of the reasons for your request.

All trademarks used herein are the property of their respectiveowners. The use of any trademark in this text does not im plytrademark ownership rights in such trademarks, nor does useof such trademarks imply any affiliation with or endorsementof Electrical Tester by such owners.

Phase evaluation in powernetworks ............................................ 1Dr Frank Petzold, Alexander Stanischa,Banuanch Germany

It’s another world! ............................. 1Nick Hilditch, group marketing services manager

Don’t take a chance on your CAT! ..... 2Simon Wood, UK wholesale and distribution salesmanager

History is never static!....................... 2Stina Flogell Ostlundh, general manager,Megger Sweden

Insulation tester for substations ....... 3Clive Pink, product manager

Measuring on a roll! .......................... 3 Josef Hollweck, sales engineer, Megger Germany

Interoperability and IEC61850Goose ................................................. 4

Andrea Bonetti, technical specialist in protectionand relay test

The secret to limiting substationcatastrophes ...................................... 5Gary Wright, consultant

Multiple current injection .................. 6Marius Pitzer, sales manager, Megger South Africa

Safeguard those services! .................. 6Mr Jörg Schubert, manager, line locating andinspection department, Banuanch Germany

Explaining the art of testing ............. 7Elsa Cantu, marketing communications manager,Megger Dallas

Cable fault ......................................... 7Peter Herpertz, product manager, power

My resistance is low! ......................... 8Keith Wilson, electrical engineer

It’s another world -continued from page 1 ...................... 8

Q&A ................................................... 8

When testing electrical systems of any kind,it’s essential to make sure that the testequipment being used is suitable for the taskin hand. If it’s not, there is a significant risknot only of damage to the test equipmentand the installation, but also of injury to theuser. That probably seems so obvious that it’shardly worth mentioning. After all, how manytechnicians or engineers would use unsuitableequipment for testing? The answer is that few

would do so knowingly, but many may bedoing so every day without even realisingthat there’s a problem. And that problemrelates to transients. All electrical installationsexperience transients, which are voltagespikes that are super-imposed on the normalsupply. Although these spikes are usually of

very short duration – typically they last justa few microseconds – their amplitude canbe thousands of volts. These transients comefrom a variety of sources, but one source thatis surprisingly common even in temperateclimes is lightning strikes. Note that a direct hiton 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 alarge transient.

But what have transients got to do withtesting and safety? To answer this question,let’s examine what happens if you’re carryingout a test – which could be something assimple and routine as checking the voltageof an LV supply – when it experiences atransient. If the instrument in use has not beenspecifically chosen to be suitable for the typeof work being carried out, there’s a very realrisk that the transient will cause a flash overinside the instrument and set up an arc.

Because its duration is very short, the transientitself is unlikely to have enough energy to doa lot of damage. Unfortunately though, onceit is established, the arc provides a lowimpedance path for current from the mainssupply. That current flow releases a lot ofenergy inside the instrument. Of course, thecircuit’s protective device, whether it’s a fuseor circuit breaker, will quickly disconnect thesupply and interrupt the fault current.

Before this has time to happen, however,the energy released within the instrument isenough to cause real problems. In the worstcases, the instrument may explode, injuring oreven killing the person who is using it. Evenin less severe cases there is a definite risk offire and damage to the equipment under testas well as to the instrument itself.

It’s clearly important, therefore, to choose aninstrument that has been designed to with-stand the level of transients it’s likely toencounter in use. But how can you tell? Theanswer is to look at the instrument’s categoryrating, which is more commonly called its CATrating.

CAT ratings are defined in the IEC 61010-1standard, and are specifically intended to

address the issue of transients in the testing oflow-voltage installations. To understand howthe ratings work, it’s necessary to look at whathappens to transients as they pass through atypical electrical installation.

Outside the building and at the point wherethe mains supply enters the building, thetransients have their highest amplitude. Fortesting in these locations, only instruments

with a CAT IV rating are suitable.

Transients are, however, quickly attenuated bythe wiring and equipment in an electricalinstallation. Once the supply has passedthrough the main switchboard, therefore, theamplitude of the transients is much lower, andinstruments with a CAT III rating (or higher)can be safely used. At the final circuit outlets,the transient levels are lower still, and CATII or higher instruments can be used withoutproblems.

What about CAT I instruments? These arefor use within appliances such as VDUs andphotocopiers. In practice, major suppliers ofinstruments designed for professional use areunlikely 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 CATratings must always include a voltage – forexample, CAT IV 300 V. This voltage is themaximum RMS phase-to-earth voltage of thesystem on which the instrument is suitable foruse. This means, for example, that instruments

with a 300 V rating can be used on single-phase systems up to 300 V and three-phasesystems up to 520 V, making them suitable forthe vast majority of low-voltage applications.

There’s one final point to mention. It wouldbe easy to think that insulation testers andother instruments designed for use on deadcircuits didn’t need a CAT rating. Remember,however, that these instruments could beaccidentally connected to a live supply, andalso that many of them incorporate facilitiesfor some live circuit tests, such as measuringthe supply voltage. The CAT rating is, there-fore, still relevant for these types ofinstruments.

Once the significance of the CAT rating systemis understood, it’s not difficult to choose aninstrument that’s appropriate for the type of

work being undertaken. As a general rule ofthumb, a CAT III 300V rating is likely to bethe minimum acceptable for general use.

It is, however, well worth consideringinvesting in CAT IV instruments, as these canbe used without restrictions anywhere withina normal installation. Many utility companiesand other major purchasers of instruments are,in fact, now specifying CAT IV instruments asstandard, since they deliver an extra level ofsafety in return for a very modest additionalinvestment.

Don’t take a chanceon your CAT! Simon Wood

UK wholesale and distributionsales manager

From time to time, Electrical Tester has includedbrief histories of some of the well-knowncompanies that now form part of the Meggergroup. Several years ago, we wrote in this

vein about the Swedish company, Programma.History is never static and the story we toldthen is now rather behind the times, so let’sbring it up to date.

Programma was founded in 1976 by twofriends who saw designing and manufacturingelectronic products as an attractive businessopportunity. Their first idea was to producean electronic programmer for washingmachines, which explains the choice of

company name. Unfortunately, the washingmachine manufacturers weren’t interested,as they develop their own programmers in-house.

Fortunately, the brother of one of the friendshad been working as a protection relay testengineer and had developed a small portablerelay tester for his own use. He suggested thatthis could be commercialised, and estimatedthat there would be a market for perhaps 20of these instruments. In fact, including thesuccessors to the original design, more than20,000 have been sold!

Programma was so successful that it becamea takeover target. It was purchased by GEEnergy and in an attempt to reducemanufacturing costs, manufacturing wastransferred to China. By 2007, however,

GE Energy had decided to rationalise itsoperations by divesting itself of non-corebusinesses, and Programma found itself upfor sale.

Knowing that Programma had an excellentreputation for quality, expertise and innovation,as well as a product range that complementedits own, this was an opportunity too good forMegger to miss and in June 2007, it broughtthe company into the group. One of its firstactions was to bring manufacturing back in-house, as this restored the close control overproduct quality and performance that canonly be achieved when the manufacturingsite is close to the design and developmentfacility.

Just a year later, in 2008, Megger acquiredanother Swedish company, PAX Diagnostics,

a specialist in power transformer test anddiagnostics with industry-leading expertisein sweep frequency response analysis anddielectric frequency response analysis. Beforelong, the PAX operations were moved toshare the Programma site in Taby, where theybenefitted from access to a much wider rangeof resources.

Now operating under the Megger name, boththe Programma and PAX operations inSweden have continued to flourish, and areproducing a wide range of innovative powertest instruments that are sold all over the

world. In fact, the Swedish operations haveforged ahead so strongly that the latest updatein this story is another change of location.

After 30 years in Täby, all of the Swedishoperations moved to a much bigger and more

modern premises in Danderyd, Stockholm in April 2013. The new premises provide a greatlyenhanced environment for the developmentand manufacturing teams, as well as thefacilities needed to ensure that as its businesscontinues to grow, the company will be ableto maintain and enhance its already renownedlevel of customer service well into the future.

History isnever static!

Stina Flogell OstlundhGeneral manager,Megger Sweden




ELECTRICALTESTER

In distribution and transmission substationsand switchyards – as in almost every otherkind of electrical power installation – dcinsulation resistance testing (IRT) is aninvaluable tool for assessing the conditionof equipment and for diagnosing faults.

Unfortunately, however, obtaining dependableinsulation resistance measurements in ExtraHigh Voltage (EHV) substations and switch-

yards can be challenging, not least becauseof the high levels of electrical noise that arepresent.

A very effective solution would be to arrangefor all nearby equipment to be de-energised

while tests are carried out so as to minimisenoise levels, but in the real world this is rarelypossible. A more practical approach to tacklingthe noise problem is to use the shortestpossible test leads and to route these nearearthed objects such as the casing of a trans-former, or to use test leads that are screened.

These measures are effective in reducing highfrequency noise pick up on the test leads, andthis may sometimes be enough for dependable

measurements to be made, but they can donothing about noise picked up by the testobject itself or from noise currents flowing inthe ground. The only way to tackle this is touse an insulation tester that offers high noiseimmunity.

Of course, all manufacturers of insulation testersclaim that their products offer high noiseimmunity and, indeed, all test sets sold in theEU must meet the EMC requirements of IEC61326-1. Experience has shown, however, thatin environments like substations and switch-

yards, the levels of electrical noise are oftenmuch higher than those laid down in thisstandard.

It is, therefore, necessary to go beyond simpleclaims of high noise immunity or IEC 61326compliance and to look at quantitative data

about the noise immunity of an instrument.This is usually specified in mA, and a typicalspecification might be that a particularinstrument has an immunity of 3 mA at 50/60 Hz. In simple terms, this means that ifthe noise current induced in the test circuitat power frequency is 3 mA or less, theinstrument will give reliable results.

In fact, an instrument with 3 mA noiseimmunity will often be a good choice forgeneral applications, but in EHV substationsand switchyards it’s a very different storyas noise levels are frequently much higher.

The right choice here is the new S1–Series ofproducts from Megger that have been purposedesigned and built for use in these verychallenging environments.

The best of these instruments offer 8 mAnoise immunity, which is an exceptionallyhigh figure, and that’s not all. They alsoincorporate powerful software-base filteringthat further reduces the effect of electricalnoise on measurements. The level of filteringis user selectable as the highest levels extendthe time needed to perform a test, althoughthey do make it possible to obtain dependableresults in situations where measurements

would have previously been impossible.

These new instruments have been tested inthe 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 yieldedaccurate and repeatable results without evenneeding to use the highest level of filtering.This is a particularly notable achievement asno other insulation test set had ever been ableto operate successfully in these locations.

While their exceptionally high noise immunityis undoubtedly the key characteristic of thenew S1-Series, these leading models havemany other desirable features. They’re robust

yet lightweight, easy to transport and, becauselow-voltage power is not always convenientlyavailable in substations and switchyards, theyincorporate rapid-charge Li-ion batteries thatallow hours of testing to be carried out even

when a mains an AC supply is not available.

These test sets also deliver a high short-circuitcurrent – typically up to 6 mA – to allow rapid

charging of items under test, and they havea CAT IV 600 V safety rating up to 3000 m inline with IEC 61010, to help ensure operatorsafety. A further important feature is provisionfor remote operation via a fully isolated inter-face, which again can help to enhanceoperator safety when carrying out tests indifficult environments.

Insulation tester for substations

As would be expected, these instruments haveinternal storage for date- and time-stamped

test results, which can be recalled to thedisplay or downloaded to external devices forinclusion in reports or later analysis. Down-loading is performed via a USB or Bluetooth®interfaces.

There is no doubt that high voltage and EHVsubstations and switchyards will always be

Clive PinkProduct manager

challenging environments in which to carryout electrical testing. As we’ve seen, when it

comes to insulation resistance testing, thechallenges have now been very effectivelyaddressed. For successful results it is essentialto use a test set that’s been designed for thejob: attempting to get by with a general-purpose instrument is all too likely to lead tofrustration and wasted time.

USB beacon which enables remote operation from a PC

Measuring on a roll

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 thatfeeds an electrical pulse into a cable, thenmeasures the time it takes for reflections ofthat pulse to return. Since the pulse is reflectednot only by the end of the cable, but also bymany kinds of cable fault, the TDR is aninvaluable tool for determining fault locations.

Josef HollweckSales engineer, Megger Germany

For those who think creatively, this is not theonly application of these useful instruments.For example, instead of tediously measuringcable lengths by hand, why not use a TDR? Infact, why not use a TDR to measure the lengthof a cable coiled on a drum or a cable reel?

With this technique, it isn’t even necessary tounroll the cable, which saves a lot of time andeffort.

TDR1000/3 being used to measuring the length of cable left on a drum

There are many cases when using a TDR forcable length measurement is not only convenient,but also an excellent way of guarding againstproblems.

Construction companies, for example, oftenhire submersible pumps to remove waterfrom deep excavations and these pumps areusually delivered with power cables on drums.

Although the required cable length will havebeen specified, mistakes happen and a shortcable can make it impossible to install thepump at the required depth, which is likely todelay the project and incur unnecessary costs.

A quick check on the cable length with a TDR will ensure that this doesn’t happen.

For companies that stock and sell powercable, using a TDR to measured cable lengthson the drum is a particularly attractive option.

To check stocks, it is no longer necessary tounroll the cable and measure it by hand – allthat’s needed is access to one end of the cableand the measurement can be made in seconds,

with very little effort.

Of course, not every TDR is well suited tothis type of application. What’s needed isan easy-to-use instrument that has good

resolution to ensure that the length measure-

ments are accurate. Fortunately, convenientand cost-effective handheld instruments thatmeet these requirements are now readilyavailable.

The best incorporate an auto set-up featurethat instantly recognises the type of cable,ensuring reliable and accurate results. Inaddition, the pulse they send into the cableis very short – around 2 ns – and so they canmeasure cable lengths with an accuracy ofaround 100 mm.

Finally, they have high-resolution screensand a trace hold feature that make it easyto interpret the results, and they feature robustconstruction to handle the rough-and-tumbleof on-site use. These instruments are modestlypriced 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 muchpower cable is on a reel or drum, don’t reachfor a tape measure and spend ages struggling

with tangles as you unreel the cable, reachinstead for your trusty TDR and, with you’llhave the answer in seconds, effortlessly!




ELECTRICALTESTER

Interoperability

and IEC 61850

GOOSE case, the receiving relay may fail to rece ive the signal. This is a frequent situation during thecommissioning of substations, and the usual solution is to replace the binary input card ofthe receiving relay. Finding this problem andidentifying its cause are time-consuming jobsbecause the test engineer usually believes thatthe problem is located in other parts of the system and the real cause is identified onlyafter other “more probable causes” have beeneliminated.

Interoperability with IEC 61850 GOOSE With IEC 61850 GOOSE technology, thesituation is very similar. The problem isidentified after a time-consuming investigationconcludes that the signal is not being correctlyreceived by the receiving IED. Relay engineersusually describe interoperability failures bysaying something like:

“The GOOSE message appears on the network. It can be seen with any network analyzer ordedicated GOOSE visualizer … But the IEDdoes not receive it.” The remaining part of this article looks atsome of the most common sources ofinteroperability problems with IEC 61850GOOSE.

GOOSE messages modified by other IEDsin the network

This interoperability problem can occur inboth single- and multi-vendor applications.

A typical example is illustrated where,depending on its own VLAN settings, theswitch (or switches) removes the VLAN tag ofthe GOOSE message.

As the VLAN tag is a mandatory part of theGOOSE message, an IED “has the right” torefuse the GOOSE message if the tag ismissing.

One IEC 61850 TISSUE (nr.290, VLAN ID) hasbeen dedicated to this problem and thedecision taken – in essence – is that the IEDsare allowed to receive GOOSE messages withor without VLAN tag.

This means that, depending on whether thefirmware of the IED was issued before orafter the TISSUE had been approved, someIEDs may receive the message with an altered

VLAN tag, and others may refuse it. Thesimplest solution to this problem is to set thesubstation switches in such a way that the

VLAN tags are neither removed nor modified.

It is also recommended to always use the VLAN tag, even if in the horizontalcommunication different VLANs are not used,to make sure that all GOOSE messages are onthe same VLAN (for instance VLAN 1).

Depending on the switches used, they mayhave problems in handling the VLAN 0, butthey should always be able to handle any

VLAN other than zero. If all GOOSE messageshave the same VLAN (001 for instance), it isalways possible to set all the ports of all theswitches to handle VLAN 1, with consequencethat the VLAN tags of the messages shouldneither be removed nor modified.

Different interpretation of “default values”

This type of interoperability problem is mainlydue to the different interpretation by individual

vendors of the default values that must beassigned to the various attributes of theGOOSE message, when information is missingin the SCL file describing it. This interoper-ability problem has been seen in multi-vendorapplications.

Even where the standard is quite clear on thedefault values, this type of interoperabilityproblem has often appeared; the solution isusually a new firmware release for the IED.The problem could be in the sender IED(which sends the wrong default value) or inthe receiving IED that is not able to under-stand that the default value received on thenetwork is correct, even if its description onthe SCL file for that value is empty.

This non-interoperability can be detected bycomparing the SCL GOOSE information withthe GOOSE information available on the net-

work (consistency check method).

The best way of avoiding this problem is toalways set all the possible attributes whendefining the GOOSE message with the IEC61850 engineering tool, and to not leave anyfields empty.

Andrea BonettiTechnical specialist in protection relay test

Different interpretation of SCL (XML)information (file importing/exporting)

From what has been seen in the field to date,unless there is a design fault (bug) in the IEC61850 GOOSE stack of one of the IEDs, thisproblem almost always occurs when usingnon-standard ASCII characters like ä or ö inthe SCL description of the GOOSE message.The use of space characters has also createdproblems. Not all engineering tools are veryrobust when checking that only validcharacters have been used, and the definitionof “valid character” has to be found in the

XML file specification, as SCL files are XMLfiles. This interoperability problem has beenidentified in multi-vendor applications.Experience has shown that the best way ofavoiding these problems is to always use basic

ASCII characters and never use spaces whendefining GOOSE messages in the engineeringtools.

This problem has usually been found in thesender IED, and if this is the case, theconsistency check method against the SCL filedetects the difference.

If the problem is in the receiving IED, theconsistency check method doesn’t help becausethe GOOSE message on the network is thesame as the message in the SCL file. But inthis case, everything points to the receivingIED and the manufacturer should be contactedto help in the investigation.

Problems created by the IEC 61850engineering process

Typically, this type of interoperability problemis the result of a difference in the configuration

revision of the GOOSE message. For example,in the SCL file there is Configuration Revision3, but the published GOOSE has ConfigurationRevision 2.

This means that the IEC 61850 horizontalcommunication has been modified at SCL filelevel, but maybe for that particular GOOSEmessage nothing has been changed. Theengineering tool has nevertheless incrementedthe configuration revision, but the sender IEDhas not been updated with the new SCL fileand continues to work with the previous one.

This interoperability problem can occur insingle- and multi-vendor applications, but insingle-vendor applications the IEC 61850engineering process is usually simplified bythe vendor tool, and the risk is minor. Withthis problem, engineers typically say, “every-

thing was working fine previously”. This is agood indication of where the problem lies.

The use of several SCL files (for example,several CID files for different IEDs ratherthan a single SCD file) also increases theprobability of generating this type of inter-operability problem, not only related todifferent configuration revisions.

IntroductionIn general terms, interoperability is the abilityof diverse systems to work together effectivelyand efficiently. Interoperability is a propertyof a product or system whose interfaces arecompletely understood to work with otherproducts or systems, present or future, with-out restrictions on access or implementation.

Interoperability helps to decrease complexityand makes it easier to manage heterogeneousenvironments while enhancing choice andinnovation in the market. Importantly, theinteroperability requirement of the IEC61850 standard has beneficially increased the“interoperability among d ifferent engineers”

working for companies that are nominally incompetition. This increased communicationamong different vendors has contributed tothe fact that GOOSE messaging can today beconsidered a working technology, even ifproblems still arise, as they do in anytechnology.

With more than six years of field experience with IEC 61850 GOOSE communication inprotection and control applications, it is nowpossible to list the main reasons for inter-operability problems in multi- and single-

vendor systems. However, a comprehensivelist would be unmanageably long, especially ifcases found in the early days of using GOOSEmessages were included.

In order to commission substations with thenew IEC 61850 technology, there is a need touse new tools and methods. The key to thesetools and methods is – paradoxically –implicitly available in the IEC 61850 standarditself.

What is interoperability in IEC 61850communication?The IEC 61850 standard clearly aims forcommunication interoperability among IEDsfrom different manufacturers and defines theinteroperability as “… the ability to operateon the same network or communication pathsharing information and commands...”. Whendata sent by Device A is not fully understoodor received by Device B, an interoperabilityfailure occurs. This situation was commonbefore the IEC 61850 standard, as mostnumerical relays from different vendors hadtheir own proprietary communicationprotocols. When the communication wasnot required to perform real time tasks (likehandling protection signals for protectionschemes), it was possible to solve thisproblem by using protocol converters.

Interoperability “before”Interoperability is a word that commonlyrefers to numerical technology or numericalrelays. Interoperability problems did and doexist even within the so-called conventionaltechnology, where communication betweendifferent protection relays is based on Booleansignals expressed in terms of dc voltage level.In other words, the binary output (contactor similar) from one relay is connected to abinary input of another relay. The connectingmedium is a couple of wires.

Even this simple connection can produce“interoperability” problems. Consider, forexample:

A sending relay has its binary output polarized by the battery voltage at 110 V dc,and the receiving relay has a binary inputcard with nominal voltage of 220 Vdc. In this




ELECTRICALTESTER

Gary Wright ([email protected]) consults for McMinnville Water &Light and Forest Grove Light & Powerin Oregon. He started out in the powerindustry in 1977, and recently worked forClark Public Utilities in charge of sub-stations, metering and relaying, and isnow retired.

Gary Wright, Consultant

By properly maintaining switches andconnections, technicians can avoid costlyand time-consuming failures and outages.

When you work inside a substation, manyproblems will sneak in without your know-ledge and some of them may be catastrophic.

As utility professionals know, a catastrophicsubstation failure can bring a lot of attention

— the kind you don’t want.

Unfortunately, no silver-bullet solutions areavailable to prevent substation failure. Thesefailures 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 mostcommon issues revolves around problems

with disconnect switches and bus connections.

These types of problems can take down sub-stations and inflict major damage. In addition,they can require switching plans to be haltedbecause switches won’t open, won’t close, areraining down sparks when asked to carry loador are flashing over when asked to interruptload.

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 switchesand connections are operating properly, this

will eliminate one large opportunity for acatastrophic failure and also make all futureswitching go smoothly and predictably.

Infrared scanning helps technicians spot

problems. It’s beneficial to conduct infraredscanning of all substations and some trans-mission lines every year. Also, when possible,it’s best to schedule the work in times ofheavier loads.

When doing the infrared scanning, it’simportant to note that infrared is crucial butnot perfect. For example, infrared is noteffective for switches that sit open normally orfor switches feeding ‘out of service’ loads onthe day you scan. Wires may still burn downeven though they passed an infrared scan,even if they were carrying load during thescan.

If a connection does fail shortly after passingan infrared scan, a utility could be looking ata 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 theamount of current at the time, and the infraredscan of the weld may show nothing. Then,at a later date, the current is raised beyondthe capabilities of the weld area and the wireburns in two.

Making preparationsTo successfully guard against untimely outages,substation technicians should not rely oninfrared technology alone. Instead, they shouldalso consider adding resistance-based tests inthe substation. These tests also can be appliedto transmission and distribution switches orselected line connections.

Resistance-based testing involves using amicro-ohm meter, which measures tinyamounts of resistance (such as what would be

the resistance of a few feet of bus). After thistest is applied to all switches and connectionsin a station, the tester can be sure the testeditems will not heat up under rated loadsbecause every switch is included, even theones that are normally open.

To begin, you’ll need some basic tools andmaintenance parts for this testing. First, andmost important, you’ll need a good micro-ohm

meter such as the Megger DLRO200-115. Thisis a meter that puts out 200 A of filtered dccurrent. The advantage of filtered DC currentis that you can avoid a false trip. If you havea differential scheme and the relays are stillconnected to the current transformer (CT) ofthe device you are testing, you can run thisfiltered dc through the CT and the dc will notbe sensed in the differential circuit.

In addition, you should have upgraded boltsfor the connection pads and bimetal pads inall three sizes (two-hole, 3 inch four-hole, and4 inch four-hole). The reason for the bimetalpads is that sometimes when connections fail,it’s due to someone improperly making themup, putting copper against aluminum. Thesecan be redone installing the bimetal padsbetween the dissimilar metals. It’s a goodidea to upgrade the bolt system for theconnections. Normally, it’s only necessaryto change the bolts if a connection fails themicro-ohm test.

The preferred connection system for boltedpad type connections consists of a stainlesssteel bolt (long enough so at least two threadsprotrude through the nut), two stainless steel³/16-inch-thick flat washers, one on each sideof the connection, one 3,500 lb stainlesssteel Belleville washer on the nut side and asilicon bronze nut. The bolts are ½ inch andshould be tightened to a torque of 45 ft. This

will compress the 3,500 lb Belleville washerto about 60% compression, which will meanthe connection can expand and contract withheating and cooling cycles and not stretch thebolt. And the connection will maintainconstant tension through the years.

The last trick is to clean up the matingsurfaces with Scotch Bright® or whatevercleaner is needed to get them clean. Then coatthe surfaces with an oxidation inhibitor likeDE-OX, a non-gritted ‘green’ inhibitor fromIlsco®. There’s no reason to use a grittedinhibitor because the parts are de-energizedand can be completely cleaned. If the ScotchBright can’t get the old dried inhibitor off, trya scraper. As a general rule, you should avoidsanding or filing because if the terminals aretin-plated you could go through the tin veryeasily.

Testing the equipment After cleaning and preparing the surfaces, it’stime to test with a micrometer. Techniciansshould put the clamp around the entire switchassembly, including all connections associated

with the switch. It’s important to clean the busto bare metal for this connection using a wirebrush and some light sanding.

When talking about micro-ohms, you cannothave any resistance. It’s often effective to runthe maximum current of 200 A, provided thecomponents are rated for it. It’s important thatat least one side of the switch — or whateverpart you are testing — is not grounded.

One way to do it is to take a reading on theentire assembly, and then if it passes, you aredone. The values you get will vary with switchquality and materials, but a typical 1,200 Acopper switch (along with the switch pads atboth ends and the bus to terminal connectionsat both ends) might be in the 350 micro-ohmrange. It doesn’t take long to develop a feelfor what to expect. You must look for thingsthat stand out. If you’re testing a different kindof switch and you’re not sure what to expect,if all three poles match, chances are you arein good shape. If one or two poles stand out,

you probably should work on the higher polesto see if you can get them to match the lowerpoles.

If you run into a problem pole using thisconnection across everything, then leave the200-A current connections across the wholeswitch but move your voltage leads across theindividual connections. For example, you canmeasure the resistance of just one end of theswitch contacts, the bolted pads, the clamp tothe bus or the hinged end of the switch. This

will allow you to isolate your problem quicklyso you know what needs attention. All of thisis done by just moving along with the voltageprobes, and the 200 A current source staysconnected across the entire switch.

Checking equipmentChecking through all the switches with themicro-ohm meter should be done inconjunction with switch maintenance. Thecontacts should be cleaned, being careful tonot scratch through the silver plating on thecontacts. You must check for proper alignmentof poles and contacts, and pay particular

The secret to limitingsubstation catastrophes

attention to the condition of the contacts inregards to pitting or loss of silver. Make sure

you have good contact pressure and thenapply a thin coat of Dow Corning® 1292 whitegrease or similar. One benefit of this grease isthat it will stay soft, so when you are back inthree to five years for routine maintenance, it

will wipe off with a rag.

If the switch happens to be a load break type,it’s important to test the interrupter. Loadbreak switches work by making a parallelbetween the switch contacts and the interrupterunit as the switch is starting to open. Normally,most switch manufacturers don’t want anycurrent in the interrupter unit when the switchis closed. If this is the case, test for the properclearances or the interrupter can burn upunder normal load.

As the switch opens, load current is paralleledbetween the main contacts and the interrupter.This parallel has to be maintained as youcontinue to open the switch until a sufficientgap exists between the main contacts to avoidre-strike as the load is interrupted. The inter-ruption will occur either using a vacuumbottle or an expulsion-type snuffer device. In

either case, you’ll need to test that the closedinterrupter has low resistance. In most cases,anything less than an ohm can be used for ago/no-go.

After you have proven you can pass currentthrough the closed interrupter, the next step isto prove the interrupter can interrupt thecurrent at full voltage. If the interrupter is ofthe snuffer type, there’s no real test you cando. If the interrupter is of the vacuum type,

verify with the manufacturer that the vacuumbottle can be tested with a hi-pot. Applying

voltage to the open vacuum bottle is the only way to prove the vacuum exists and theinterrupter will interrupt load at line voltage.

If all these procedures are followed on allconnections and switches in the station duringa three- to five-year cycle, you should have

eliminated at least one looming source forserious catastrophic substation problems.

Insider Tips for maintenanceGary Wright, has been responsible for themaintenance of more than 100 sub-stationsover the last 35 years.

Infrared: Often some of the transmissionlines can be scanned as you drive to eachstation. It’s usually beneficial to hire aninfrared contractor to scan transmission linesand the most important distribution feeders.

Nuts: It’s important to use a silicon bronzefor the nut, because if you use a stainlesssteel nut, the nut can gall and stick. Thismeans you can’t get the connection tight.

Unfortunately, it will appear tight, and youmight not be able to get the nut back off.

Grounding: There can only be one groundconnected anywhere on the conductor

you are testing. If there is a second groundinstalled anywhere, it will become a parallelcurrent path and make your reading useless.

A technician runs a diagnostic test using a micro-ohmmeter




ELECTRICALTESTER

Safeguard those services!

It only takes a short walk or drive throughany town centre to confirm that excavations

– holes in the road – are common. In fact,they’re very common. And, with every oneof those excavations goes a very real concern:

will the digging lead to costly and disruptiveaccidental damage to underground services?

Of course in an ideal world, the routes ofburied cables and pipes should be properlydocumented, which would make avoidingthem relatively straightforward. In the real

world however, plans are often missing orjust plain wrong.

Mr. Jörg SchubertManager, line locating and inspectionMegger Baunach

Fortunately, this no longer needs to be aproblem, as convenient and dependable linelocation systems for determining the position ofunderground services are now available, likethe EasyLoc.

Comprising of two separate components – atransmitter and a handheld receiver – whichare capable of tracing the routes of energisedand dead power cables and of metallic pipes.

When tracing energised cables, the receiver isused on its own and looks for power frequencysignals radiated by the cable. The user simplyscans the likely route of the cable with thereceiver until a clear signal is indicated, andthen determines the cable location even moreaccurately by carefully adjusting the positionof the receiver until the highest signal strengthis achieved.

Typically, the signal is indicated audibly viaa built-in loudspeaker, and also visually on abacklit display panel. With the best instruments,once the location of the cable has beendetermined, its depth can be measured anddisplayed simply by pressing a button on thereceiver.

When the location of the cable has been

determined, its depth can be measured and

displayed simply by pressing a button on the

receiver

The procedure for tracing pipes and un-energised cables is very similar, but in thisinstance the transmitter is used to inject aunique test signal, usually at a frequency of33 kHz, into the pipe or cable. This can bedone by direct connection of the transmitter,or by using induction to couple the test signal.

With some instruments, the transmitter has anoutput that’s protected against mains voltages,allowing it to be used with energised cablesto improve accuracy in difficult operatingconditions.

The latest line location systems incorporateautomatic sensitivity control, which makesthem particularly easy to operate; together

with a self-calibration check routine that saveson maintenance costs. Some also offer accessoriesto further increase their usefulness. Theseaccessories may, for example, include pipelinetransmitters for locating non-metallic pipes,transmitter clamps for coupling the transmittersignal into energised cables without the needfor direct connections, house connection setsfor connecting the transmitter signal via astandard socket outlet, and headphones toenable the receiver’s audible output to bemonitored in noisy environments.

Line location systems of the type describedhere are readily portable, easy to use andmodestly priced. All in all, they’re an excellentinvestment – they safeguard services andthereby provide peace of mind by eliminatingone of the biggest concerns associated withexcavations of every kind.

Multiple current injectionMarius PitzerSales manager, Megger South Africa

The number of current outputs that users

require from protection relay test setsseems to be constantly increasing and ofcourse, test sets are evolving to meet theserequirements. Some of the latest modelsare capable of injecting ten test currentssimultaneously from a single test set.Sometimes even this isn’t quite enough, and,in certain applications, more are needed.

This application-based need was mostdefinitely apparent when a customer in South

Africa wanted to test a new bus-zoneprotection panel, manufactured by one of the

world’s best-known relay manufacturers. Thepanel was to be installed as an upgrade in oneof the South African 400 kV substations. Thisparticular bus-zone panel had 16 protectionrelays, and, to test the bus-zone schemeeffectively, the most convenient solution wasto inject current into all of the relays

simultaneously. Unfortunately, there is nosingle relay test set available that supports 16current channels.

Ingenious engineers came up with a thought-

provoking alternative. Instead of using asingle test set, why not interconnect multiplerelay test sets to provide the required numberof current channels? The engineers from thecustomer and the relay manufacturer foundthis suggestion interesting, and so arrange-ments were made for it to be evaluated.

Largely because of equipment availability,the test sets chosen for the exercise were twoSMRT36 three-phase units and two SMRT1 single-phase configured as shown in Figure 1.The three-phase unit had three currentchannels and three voltage channels, butthe voltage channels could be converted tocurrent channels to give six currents at onetime out of one test set. The single-phase unithad one current channel and one voltagechannel and the voltage channel could beconverted to a current channel to give two

current channels.

The four test sets were interconnected via

Ethernet cable (RJ45) so that they could beoperated as if they were one single test set,and overall control was provided for some ofthe time with a touch-screen interface unit,and on other occasions with a dedicated soft-

ware package running on a PC.

The test equipment performed exactly asrequired, and the simulation of differentzone faults went smoothly when a simplepre-fault/fault test was run. In the pre-faultstage, a stable bus-zone condition wassimulated as shown in Figure 3. After apredetermined time period, a fault wasinjected and the time taken to clear the fault

was measured. Figure 4 shows a Zone 1 faultthat tripped in 12.5 ms. After the zone timingtests were completed successfully, breaker failtesting was performed using seven stages andonce again this proved to be problem free.

At the end of the testing, both the customerand the relay manufacturer’s engineersagreed that this novel approach offered major

benefits, not the least of which was that it had

allowed the testing of the new bus-zone panelto be completed in less than a day whensimilar panels had in the past typically takenthree days to test using traditional testtechniques. Using the traditional test tech-niques, the customer would sectionalize thebus-zone and test the panel using six testcurrents at a time and would therefore, haveto go through a prolonged process to simulateall possible faults in the bus-zone.

Since the initial test, the customer has boughtanother SMRT36 three-phase test set and isnow using three of these to inject up to 18currents, as shown in Figure 2. No end-useris ever satisfied for very long, however, andit is already being suggested that the nextchallenge will be to test a protection systemthat requires 32 currents to be injectedsimultaneously!

Figure 1: Two SMRT36’s and two SMRT1’s

interconnected together Figure 2: Three SMRT36’s interconnected together

Figure 3: Sixteen bay bus-zone stable condition while

injecting sixteen currents

Figure 4: Sixteen bay bus-zone Zone 1 fault while

injecting sixteen currents




ELECTRICALTESTER

A prime requirement for testing electricalpower systems is without doubt to have accessto the right equipment – the latest test setsdeliver levels of convenience and performancethat simply cannot be achieved with theirpredecessors. But the right test equipment onits own does not offer a complete solution;adoption of the correct test techniques is alsoessential if the most accurate and reliableresults are to be obtained.

Test techniques do not however, standstill. On the contrary, they are continuouslydeveloping, which is why NETA, theInterNational Electrical Testing Association,regularly invites papers on developments intest techniques and associated subjects forpresentation at its annual PowerTest ElectricalMaintenance and Safety Conference.

The papers are carefully selected to highlightsignificant advances and, at the end of theconference, the best of them, which arechosen through a searching judging andevaluation process carried out by NETA,receive awards. These coveted and prestigiousawards recognise not only the expertise ofthe papers’ authors, but also the valuablecontribution that their work has made to thepower test community. Among the awardsmade at PowerTest 2013, which was held inNew Orleans in February, were:

n Best Relay Presentation: ‘Utilising GroundFault Resistance to Accurately Test DistanceElement’ by Jason Buneo, ApplicationsEngineer

n Best Reliability Presentation: ‘DetectingCommon Power Quality Issues’ by

Andrew Sagl, Product Manager

n Best Circuit Breaker Presentation: ‘Testingand Troubleshooting of Low to Medium

Voltage Circuit Breakers’ by Bret Hammondand Robert Foster, Technical SupportEngineers

n Best Safety Paper: ‘Electrical Safety ThroughDesign, Installation and Maintenance’ byDennis Neitzel

All the authors of these award-winning papers, with the exception of Dennis Neitzel areMegger employees. Dennis Neitzel is DirectorEmeritus of AVO Training Institute Inc., whichis a Megger subsidiary. Megger also won twomarketing awards at PowerTest 2013, for Bestin Show Trade Show Marketing, and MostEntertaining Hospitality Night.

Explaining the art of testing

Elsa CantuMarketing communication manager,

Megger Dallas In the first of a series of articles on cable fault location, Peter Herpetz looks at theconstruction of modern power cables andexamines the most common types of fault

that affect them.

The importance of cable testingFault location on power cables is a veryspecial area of electrical technology, and theresults obtained depend very much on goodlogistics and knowledge. Accurate prelocationis the foundation for fast and reliable faultlocation, because it means that pinpointingprocedures only need to be carried out on ashort section of cable.

The importance of cable testing, cable faultdiagnosis and partial discharge analysis arecertain to become increasingly important inthe future, as condition-based maintenance

of cable networks more and more displacesevent-oriented maintenance.

A good, detailed knowledge about the cablenetwork, cable types and cable accessoriesgreatly simplifies the evaluation of test resultsand, in many cases, such knowledge is anessential prerequisite for making correctdecisions. Among the most important thingsthat technicians need to know are the types ofcable faults and the steps needed to carry outcable fault location and diagnosis.

Construction of power cablesThe function of power cables is thedistribution of electrical energy, and theymust carry out this function reliably and safelyfor very long periods. Depending on theapplication, the external environment andlocal conditions, such as the presence of

ground water and the type of ground voltages,different types of cable are used. Cables withimpregnated insulation, such as PILC (paperinsulated lead covered) types were widelyused until the late 1960s and are still inservice in some areas. These cables have,however, mostly been replaced by cables

with PVC (polyvinylchloride), EPR (ethylene-propylene rubber), PR, or XLPE (cross-linkedpolyethylene) insulation. As a result of thesechanges in the type of insulation used, cablefaults and cable testing techniques have alsochanged considerably.

The following sections cannot cover all of thepossible types of cables, insulating materialsand cable construction, so they focus on themost important variants. In many cases, detailsare explained primarily as an aid to under-standing the terminology used in the later

sections of this guide to cable fault location.

Conductor The conductor is the part of the cable thattransmits current, and is usually soft electro-lytic copper or pure aluminium. The conductorcan be round or sector-shaped, and made ofsingle wire or multi-stranded.

InsulationThe purpose of the insulation is to prevent theflow of current between the conductors in thecable, and from the conductors to the cable’s

metallic outer covering, which may be armouror a lead sheath. Typical insulating materialsare: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

propylene rubber (EPR)n above 60 kV: paper with oil or gas, cross-

linked polyethylene (XLPE)

As well as these typical materials, there aremany 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 thecable, and to eliminate partial discharge. Semi-conducting layers reduce the electric field thatdevelops around the conductors, and therebyeliminate the potentially damaging dischargesassociated with high electric field strengths.

On modern cables, another type of semi-conducting layer is sometimes integrated withthe outer insulating sheath/jacket. The purposeof this type of layer is to aid the location ofsheath faults on cables that are installed inducts, where there is no return path throughthe earth for fault currents.

Metallic sheathThe metallic sheath performs multiplefunctions. It seals the cable against the entryof humidity, it provides a conductive path forleakage and earth-fault currents, it providespotential equalisation and it can be used aseither an earth conductor or a concentricneutral conductor. For cables used in criticalor subsea applications, the metallic sheathcan be designed to provide robust mechanicalprotection.

Shield (for MV and HV cables)

The shield provides electric field control, andalso offers a conductive path for leakage andearth fault currents.

Armour The armour provides mechanical protection.It may consist of steel bands, flat steel wire,round steel wires, etc. In some cases, thearmour may be made up of several differentlayers.

Plastic sheathThe plastic sheath provides outer protectionfor the cable, and usually consists of eitherPVC or polyethylene.

Cable faults When diagnosing and locating cable faults, theprocedure depends on the type of cable fault.Cable faults are generally divided into the

types listed here.

Conductor-to-conductor fault (parallelfault)Unwanted connection between two or moreconductors. The resistance of the fault may beanywhere between zero ohms (low resistance)and several megohms (high resistance).

Conductor-to-shield fault (parallel fault)Connection between a conductor and theshield or between multiple conductors andthe shield. The resistance of the fault may beanywhere between zero ohms (low resistance)

and several megohms (high resistance).Experience shows that the majority of faultsfall into this category.

Flashing fault (parallel fault)This is a very high resistance fault, and canbe present when the cable is charged.Typically, the flashover occurs at several kV,and is very often located in cable joints. Thecable behaves in the same way as an arc gap,

where the distance between the electrodesdetermines the breakdown voltage. Theresistance of this type of fault is typicallyinfinity 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 completelysevered. Very often, this type of fault is acombination of series and parallel resistances.The reason for this is that if the conductor iscompletely cut or pulled out of a joint, this notonly produces a complete open circuit, butalso allows all possible variations of flashover.If the conductor is partially burned, this typeof fault is called a longitudinal fault.

Earth faults and sheath faultsThese are faults between the metallic shieldand the surrounding soil for plastic-insulatedcables, or between the conductor and thesurrounding soil for LV and plastic-insulatedcables. Great care must be taken when usinghigh voltages to test for or locate this type offault, as the voltage discharges directly into theearth, creating shock hazards for people andanimals.

Humid/wet faultsOn multicore cables, all conductors are oftenaffected by this type of fault, but the flashoversdo not always occur at the point where the

water entered the cable. Impedance changesoccur at the fault position. Depending on thecable construction (for example, the type oflongitudinal water sealing), these faults canbe confined to a single point or widespreadthroughout the cable. Humidity/wet faults arethe most difficult faults to locate. They havea tendency to change during the fault locationprocedure, often very considerably. Particularlyin joints, this means that the fault becomeshighly resistive after one or two discharges, asthe water is blown out of the joint and driesup. When this happens, the fault can nolonger be localised. Underwater faults areanother form of wet fault. With these, the

water pressure prevents effective ignition of

the fault when high voltage is applied. Thesefaults can be very difficult to localise.

CONCLUSION A clear distinction must be made betweenshort-circuit, resistive and high-resistancefaults, because this distinction has a significantinfluence on the procedures that should beused for fault location. These procedures willbe described in future articles in this series.

Sheath/jacket

Shield/screen

Semi conductor

Insulation/dielectric

Inner semi conductor

Core/conductor

Cable fault Peter HerpertzProduct manager, power


http://slidepdf.com/reader/full/electrical-tester-july-2013 8/88 Megger ELECTRICAL TESTER July 2013 www megger com

Q&A This time we turn our attention to questions that are frequently asked about interpreting theresults of transformer winding resistance measurements, and about sources of confusion thatcan give rise to results that appear to be problematic even though, in reality, no problemsexist.


ELECTRICALTESTER

Q: How should transformer winding resistancetest results be evaluated?

A: Evaluation can be carried out by comparingthe test results with original factory measurementsor with previous measurements that have beenmade in the field. Alternatively, the results canbe evaluated by comparing the phases witheach other. In most cases, phase-to-phasecomparisons are sufficient.

Q: How much difference between measurements

is acceptable? A: The industry standard for factory testspermits a maximum difference of 0.5% fromthe average resistance of the three phase

windings. Measurements made in the fieldmay vary slightly more than this because thereare more variables, but if the measurementsare within 1% of each other, they can beconsidered acceptable. Note that comparingabsolute resistance values measured in thefield with factory values can be difficult,principally because of the difficulty of estimating

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 orinconsistent measurements can be indicativeof many different problems, including shortedturns, open turns, poor brazed or mechanicalconnections, 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 inmisleading measurements. The most commonare temperature changes, contact oxidationand measuring errors.

Q: How do temperature changes influence

measurements? A: The dc resistance of a winding varies as itstemperature changes. For copper windings,the variation is 0.93% per ºC. This is usuallynot a significant consideration when comparing

phases in a power transformer, as the load onpower transformers is usually well balanced,

which means that the winding temperatures

should be very similar. However, whenmaking comparisons with factory measure-ments or previous field measurements, smallconsistent changes should be expected.

In addition to loading, temperature (andtherefore resistance) variations can be due tocooling or warming of the transformer duringthe test, particularly on large transformers withan LTC where the time between the first andlast measurement is often an hour or more.Note that the temperature of a transformerthat has been on load is likely to changesignificantly during the first few hours offload.

Another issue that can lead to temperaturechanges is the use of too high a test current.

When measuring the dc resistance of smallertransformers, care should be taken to ensure

that the test current does not cause heating ofthe windings. For this reason, the test currentshould not exceed 10% of the winding rating.

Q: How does contact oxidation influence

measurements? A: Dissolved gases in transformer oils act thecontact surfaces of RA switches and LTCs.Usually, higher resistance measurements willbe noted on taps that not used or are usedinfrequently. This apparent problem can berectified simply by operating the switch afew times, as the design of most LTC and RAswitch contacts provides a wiping action that

removes surface oxidation.

Q: What are the most common measuring

errors?

A: There are many possibilities, includingincorrect or poor connections, use of adefective instrument or one requiringcalibration, incorrect operation of theinstrument, mistakes in recording results andambiguous or poorly defined test data. Notealso that there is often more than one wayof measuring the resistance of a transformer

winding. Typically, field measurementsare taken from external bushing terminals,

whereas factory measurements are notlimited to these terminals. Additionally, inthe workshop or factory, internal windingconnections can be opened, making measure-ments possible that are not practical in the

field. Unfortunately, details of test set ups andconnections are often omitted in test reports, which can lead to confusion when comparingtest data.

continued from page 1

Planethunters.org supplies volunteers withdata sets acquired by NASA’s Kepler satellite,

which is one of the most powerful tools inthe hunt for extra-solar planets. Each data setshows how the observed brightness of a star

varies over time, and the volunteers look forthe characteristic drop in brightness that

occurs when a planet passes in front of thestar.

The Kepler satellite has already generated datasets for hundreds of thousands of stars, andis generating more all the time. All data setsare analysed by computer, but this doesn’tguarantee that all potential extra-solar planetsare detected. That’s where the Planethunter

volunteers come in, because experience has

shown that humans are better at spotting thesubtle variations that suggest the presence of aplanet than even the best computer algorithms.

When volunteers report a data set that showsthese variations, it is noted as a ‘candidateplanet’. It is only officially confirmed as aplanet when independent observations havebeen made, typically by the Keck Telescope

It’s another world - in Hawaii. The probability of confirmation is,however, very high – well over 90%.

Despite the vast number of data sets beinganalysed, finding a new candidate planet isstill a rare event. Planethunters.org revealedin January 2013 that its volunteers had foundjust 15 candidate planets to date, with twoconfirmed and the remaining 13 awaitinginvestigation.

Mark’s discovery is considered particularlysignificant because his candidate planet is inorbit around a star that is just 1.2 times thesize of our own sun and its orbit is at adistance from the star that is compatible withlife. Even though the planet is almost certainlya gas giant unable to support life, there is ahigh probability that it has moons similar tothose of Jupiter in our own solar system, andthese might well be hospitable to life.

“Making the discovery wasrather a shock,” said Mark.“It was a bit like looking fora needle in a haystack andactually finding the needle!I am, however, delightedto have been able to makethis contribution to the

Planethunter.org project, and I’m just asdelighted, when I’m asked if I’ve done any-thing interesting recently, to be able to say,

very nonchalantly, that I discovered a planet.”

“Of course, I’m not satisfied with being one ofthe few who have discovered a planet, whatI really want is to be one of the even fewer

who have discovered two or even three, somy evenings and my laptop are going to be

very busy for some time to come!”

Mark Hadley

Actual transit for APH41111337 - Candidate planet

It’s a fair bet that when the lovely Jane Russellsang about her resistance being low in the1952 movie ‘The Las Vegas Story’, there werefew things further from her mind than electricaltesting. To decide for yourself, why not takea few minutes out to watch the clip?

You’ll find it at http://www.youtube.com/ watch?v=iQBDN5s8IB0.

Whatever your conclusion, the electricalengineers of that era needed, as they dotoday, a convenient method for accuratelymeasuring low resistances. Evershed &

Vignoles, one of the forerunner companiesMegger, provided the answer. Alongside itsfamous range of Megger insulation testers, thecompany had also developed instruments forlow resistance testing, which it sold under thetrademark ‘Ducter’.

This trademark is, in fact, still registered but isno longer applied to current products. In itsheyday, however, the phrase ‘Ducter testing’

was widely used as a generic term for lowresistance testing.

For Evershed and Vignoles, the Ducter was alogical progression from the Megger insulationtester as it used the same type of meter move-ment, with two coils rigidly fixed together in amagnetic field. In the Ducter, one coil carriesa current (I) proportional to the current flowingin the object under test, while, the othercarries a current proportional to the voltagedrop (IR) across the object. The deflection de-

My resistance is low!Keith WilsonElectrical Engineer

Fig 1. Simplified schematic diagram of a

Ducter

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-affectedby either the test current or the applied voltage.

A simplified schematic for a Ducter is shownin Figure 1.

The current source for the Ducter was one ormore rechargeable nickel-iron (NiFe) cells,depending on the model. These cells, the fore-runners of today’s NiCd and NiMH cells, havea low internal resistance and can supply veryhigh currents for short periods without risk ofdamage. The popular model 37002 five-rangeDucter, which could measure from 1 µΩ to 1Ω, uses a single 220 Ah NiFe cell and, on itslowest range, employs a test current of 100 A.

Ducters, just like the digital low resistanceohmmeters that are their present-day successors,

were used in an amazingly wide range ofapplications, as can be seen from thesefascinating testimonials, which are just a smallsample of those included in a user guide fromthe late 1950s:

British Railways: “A Ducter ohmmeter hasbeen in regular use in the Electric CarriageRepair Shop in Wimbledon since February1917. It is still in use for routine tests ontraction motor armatures”.

London Passenger Transport: “The Ducteris used in the electrical test room at Charlton

works for testing the resistance of trolleybustraction motor field and interpole coils, tractionarmatures and low-resistance blow-out coils.”

Foster Transformers: “Our test room usesthe Ducter ohmmeter to measure theresistance of transformer windings todetermine their total copper losses, and theirtemperature change after a heat run”.

Stewart and Lloyds Steel: “The testing ofgraphite furnace electrodes for specificresistance is now being made into a routinecheck using a Ducter ohmmeter, which allows

the test to be carried out with ease andrapidity, and without damaging the electrode”.

Dorman Smith: “Our Ducter ohmmeter istreated as a sub-standard instrument. That is, itis used wherever it is required to determine alow resistance with accuracy”.

Hoover: “The Ducter ohmmeter is used formeasuring the resistance of bi-metal stripsused in thermal cutouts and to check thecircuit resistance of the complete weldedassembly”.

Yorkshire Switchgear and Engineering: “For ten years we have standardised upon aDucter test as the basis of our acceptance orrejection of all switchgear contacts and boltedjoints. In our opinion, this method of testingis equally as accurate as the accepted millivoltdrop method, and is far more readily applied”.

Shunt Currentcontact

Potentialcontact

Resistanceunder test

Permanentmagnet

Cut-out

Ligaments

Fixedcentreiron

Controlcoil

Battery

Deflectingcoil

Documents

Electrical Tester July 2013