electrical tester

1 www.megger.com Megger ELECTRICAL TESTER July 2013

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ELECTRICALTESTER The industrys recognised information tool

Published by Megger July 2013

Its another world!Nick HilditchGroup marketing services manager

There can be no doubt that Megger employs some very talented engineers but its easy to forget that they dont 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 thats a question best left unanswered but, in the case of Mark Hadley, who is new product research manager at the companys Dover site, its already too late! Thanks to recent press reports, Marks spare

time activities are now very much in the public spotlight.

Because, when most of us have our feet up watching television, Mark is hunting new planets.

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

Limiting catastrophessee page 5

Insulationtesterfor sub-stationssee page 3

Cable faultssee page 7

Phase evaluation in power networks

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. Todays employees are expected to cover a wide range of activities, and simply dont 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 main-tenance of electrical power systems, thereby ensuring that high network reliability and economic efficiency are achieved.

PRACTICAL PHASE MEASUREMENTTo 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

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 isestablished 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 CORRECTIONIf the base station is not connected to L1 as the reference phase, the appropriate correction angle of either +120 or -120 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 EXAMPLESBecause 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

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

Mode 2: NO NETWhen 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.

Dr Frank Petzold and Alexander Stanischa Megger 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 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. Post-synchronisation can be carried out any time up to ten days after the recording of the measurement 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 with the base station is needed.

ConclusionThe 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.

around seven million light years from our solar system.

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 Universitys Zooniverse project.

continued on page 8

2 Megger ELECTRICAL TESTER July 2013 www.megger.com

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 and would like your input. If you have any questions or stories that you think we could use, then please email [email protected]

A printed newsletter is not as interactive as its email equivalent so to help you find items quickly on www.megger.com, we have underlined key search words in blue.

The industrys recognised information toolELECTRICAL

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The rights of the individuals attributed in Electrical Tester to be identified as authors of their respective articles has been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.

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

Its another world! ............................. 1Nick Hilditch, group marketing services manager

Dont take a chance on your CAT! ..... 2Simon Wood, UK wholesale and distribution sales manager

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

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

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

Interoperability and IEC61850 Goose ................................................. 4Andrea Bonetti, technical specialist in protection and relay test

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

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

Safeguard those services! .................. 6Mr Jrg Schubert, manager, line locating and inspection 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

Its another world - continued from page 1 ...................... 8

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

When testing electrical systems of any kind, its essential to make sure that the test equipment being used is suitable for the task in hand. If its 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 its 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 theres 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 doesnt 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, lets examine what happens if youre 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, theres 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 circuits protective device, whether its 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.

Its clearly important, therefore, to choose an instrument that has been designed to with-stand the level of transients its likely to encounter in use. But how can you tell? The answer is to look at the instruments 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

address the issue of transients in the testing of low-voltage installations. To understand how the ratings work, its 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.

Thats 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 single-phase systems up to 300 V and three-phase systems up to 520 V, making them suitable for the vast majority of low-voltage applications.

Theres one final point to mention. It would be easy to think that insulation testers and other instruments designed for use on dead circuits didnt 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, there-fore, still relevant for these types of instruments.

Once the significance of the CAT rating system is understood, its not difficult to choose an instrument thats 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.

Dont take a chance on your CAT! Simon WoodUK wholesale and distribution

sales manager

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 lets bring it up to date.

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 werent interested, as they develop their own programmers in-house.

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 in-house, 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 Tby, 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.

History is never static!

Stina Flogell OstlundhGeneral manager, Megger Sweden



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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 switch-yards can be challenging, not least because of the high levels of electrical noise that are present.

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 trans-former, or to use test leads that are screened.

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.

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 switch-yards, the levels of electrical noise are often much higher than those laid down in this standard.

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.

In fact, an instrument with 3 mA noise immunity will often be a good choice for general applications, but in EHV substations and switchyards its a very different story as noise levels are frequently much higher. The right choice here is the new S1Series of products from Megger that have been purpose designed and built for use in these very challenging environments.

The best of these instruments offer 8 mA noise immunity, which is an exceptionally high figure, and thats 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 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.

While their exceptionally high noise immunity is undoubtedly the key characteristic of the new S1-Series, these leading models have many other desirable features. Theyre 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.

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 inter-face, which again can help to enhance operator safety when carrying out tests in difficult environments.

Insulation tester for substations

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. Down-loading is performed via a USB or Bluetooth interfaces.

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

Clive PinkProduct manager

challenging environments in which to carry out electrical testing. As weve 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 thats been designed for the job: attempting to get by with a general-purpose instrument is all too likely to lead to frustration 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 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.

Josef HollweckSales engineer, Megger Germany

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 isnt even necessary to unroll the cable, which saves a lot of time and effort.

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

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 doesnt happen.

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 thats 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. Whats needed is an easy-to-use instrument that has good

resolution to ensure that the length measure-ments 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, dont reach for a tape measure and spend ages struggling with tangles as you unreel the cable, reach instead for your trusty TDR and, with youll have the answer in seconds, effortlessly!



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Interoperability and IEC 61850 GOOSE

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 GOOSEWith 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:

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. The remaining part of this article looks at some of the most common sources of interoperability problems with IEC 61850 GOOSE.

GOOSE messages modified by other IEDs in the network

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

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 ismissing.

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.

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.

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). 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.

Different interpretation of default values

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 theGOOSE message, when information is missing in the SCL file describing it. This interoper-ability problem has been seen in multi-vendor applications.

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 under-stand 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 net-work (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.

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 IEC 61850 GOOSE stack of one of the IEDs, this problem almost always occurs when using non-standard ASCII characters like or 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.

If the problem is in the receiving IED, the consistency check method doesnt help becausethe 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

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, every-thing 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 inter-operability problem, not only related to different configuration revisions.

IntroductionIn 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, with-out 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 inter-operability problems in multi- and single-vendor systems. However, a comprehensive list would be unmanageably long, especially if cases found in the early days of using GOOSE messages were included.

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.

Interoperability beforeInteroperability 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.

Even this simple connection can produce interoperability problems. Consider, for example:

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



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Gary Wright ([email protected]) 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 sub-stations, metering and relaying, and is now retired.

Gary Wright, Consultant

By properly maintaining switches and connections, technicians can avoid costly and time-consuming failures and outages.

When you work inside a substation, many problems will sneak in without your know-ledge and some of them may be catastrophic. As utility professionals know, a catastrophic substation failure can bring a lot of attention the kind you dont 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 werent set correctly. But one of the most common 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 halted because switches wont open, wont close, are raining down sparks when asked to carry load or are flashing over when asked to interrupt load.

Scanning with infraredWhen 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. Its beneficial to conduct infrared scanning of all substations and some transmission lines every year. Also, when possible, its best to schedule the work in times of heavier loads.

When doing the infrared scanning, its 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 preparationsTo 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, youll need some basic tools and maintenance parts for this testing. First, and most important, youll need a good micro-ohm

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, its due to someone improperly making them up, putting copper against aluminum. These can be redone installing the bimetal pads between the dissimilar metals. Its a good idea to upgrade the bolt system for the connections. Normally, its 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 /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 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. Theres no reason to use a gritted inhibitor because the parts are de-energized and can be completely cleaned. If the Scotch Bright cant 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 equipmentAfter cleaning and preparing the surfaces, its time to test with a micrometer. Technicians should put the clamp around the entire switch assembly, including all connections associated with the switch. Its 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. Its often effective to run the maximum current of 200 A, provided the components are rated for it. Its 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 doesnt take long to develop a feel for what to expect. You must look for things that stand out. If youre testing a different kind of switch and youre 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 equipmentChecking 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

The secret to limiting substation catastrophes

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.

If the switch happens to be a load break type, its 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 dont 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.

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 inter-ruption will occur either using a vacuum bottle or an expulsion-type snuffer device. In either case, youll 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, theres 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 maintenanceGary 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. Its usually beneficial to hire an infrared contractor to scan transmission lines and the most important distribution feeders.

Nuts: Its important to use a silicon bronze for the nut, because if you use a stainlesssteel nut, the nut can gall and stick. This means you cant 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.

A technician runs a diagnostic test using a micro-ohmmeter



TESTER

Safeguard those services!

It only takes a short walk or drive through any town centre to confirm that excavations holes in the road are common. In fact, theyre 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.

Mr. Jrg Schubert Manager, line locating and inspection Megger Baunach

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.

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 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 thats 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 receivers audible output to be monitored in noisy environments.

Line location systems of the type described here are readily portable, easy to use and modestly priced. All in all, theyre 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.

Multiple current injection Marius PitzerSales manager, Megger South Africa

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 isnt 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 worlds 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.

Ingenious engineers came up with a thought-provoking 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 arrange-ments 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.

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 soft-ware 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 manufacturers engineers agreed that this novel approach offered major

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 tech-niques, 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!

Figure 1: Two SMRT36s and two SMRT1s

interconnected together Figure 2: Three SMRT36s 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



TESTER

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:

n Best Relay Presentation: Utilising Ground Fault Resistance to Accurately Test Distance Element by Jason Buneo, Applications Engineer

n Best Reliability Presentation: Detecting Common Power Quality Issues by Andrew Sagl, Product Manager

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.

Explaining the art of testingElsa CantuMarketing communication manager, Megger Dallas

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 testingFault 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 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 cablesThe 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 (ethylene-propylene 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 under-standing the terminology used in the later sections of this guide to cable fault location.

ConductorThe conductor is the part of the cable that transmits current, and is usually soft electro-lytic copper or pure aluminium. The conductor can be round or sector-shaped, and made of single wire or multi-stranded.

InsulationThe purpose of the insulation is to prevent the flow of current between the conductors in the cable, and from the conductors to the cables 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

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

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. Semi-conducting 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.

Metallic sheathThe 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.

ArmourThe 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 sheathThe plastic sheath provides outer protection for the cable, and usually consists of either PVC or polyethylene.

Cable faultsWhen diagnosing and locating cable faults, the procedure depends on the type of cable fault. Cable faults are generally divided into the types listed here.

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).

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.

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.

Earth faults and sheath faultsThese 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 faultsOn 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.

CONCLUSIONA 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.

Sheath/jacket

Shield/screenSemi conductor

Insulation/dielectric

Inner semi conductor

Core/conductor

Cable faultPeter HerpertzProduct manager, power


Q&A 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 industrys recognised information tool

ELECTRICALTESTER

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

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 C. This is usually not a significant consideration when comparing

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 measure-ments or previous field measurements, small consistent changes should be expected.

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.

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.

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 measure-ments 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.

continued from page 1

Planethunters.org supplies volunteers with data sets acquired by NASAs 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.

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 doesnt guarantee that all potential extra-solar planets are detected. Thats where the Planethunter volunteers come in, because experience has

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

Its another world - in Hawaii. The probability of confirmation is, however, very high well over 90%.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.

Marks 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.

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 this contribution to the

Planethunter.org project, and Im just as delighted, when Im asked if Ive done any-thing interesting recently, to be able to say, very nonchalantly, that I discovered a planet.

Of course, Im 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!

Mark Hadley

Actual transit for APH41111337 - Candidate planet

Its 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? Youll find it at http://www.youtube.com/watch?v=iQBDN5s8IB0.

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.

For Evershed and Vignoles, the Ducter was a logical progression from the Megger insulation tester as it used the same type of meter move-ment, 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-

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-affected by either the test current or the applied voltage. A simplified schematic for a Ducter is shown in Figure 1.

The current source for the Ducter was one or more rechargeable nickel-iron (NiFe) cells, depending on the model. These cells, the fore-runners of todays 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 to 1 , uses a single 220 Ah NiFe cell and, on its lowest range, employs a test current of 100 A.

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:

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.

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.

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

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 Currentcontact

Potentialcontact

Resistance under test

Permanent magnet

Cut-out

Ligaments

Fixed centre iron

Control coil

Battery

Deflecting coil

electrical tester

Documents