Published by Megger April 2013 ELECTRICAL TESTER · Published by Megger April 2013 To mark the...

1 www.megger.com Megger ELECTRICAL TESTER April 2013

ELECTRICALTESTER The industry’s recognised information tool

Published by Megger April 2013

To mark the launch of the new purpose designed training centre at its Dover site, Megger is offering the first public course to be held in the centre at the discounted introductory price of £400 per person, instead of the normal £650 per person.

The two-day course, which will provide comprehensive coverage of Power Transformer Testing Techniques, is being held on Tuesday 16th and Wednesday 17th April 2013. The course has been formulated

by and will be presented by testing experts from Megger who have wide experience in transformer working. It is suitable for all those who work with HV/MV and LV power transformers, the only prerequisite for those attending being a working knowledge of industrial electrical safe systems of work.

In order to provide the delegates with the greatest possible insight into transformer testing and also give them the confidence to perform tests in the field, the course includes

Special offer on transformertraining!Tony WillsTechnical Support Group

José MancheñoSales Manager, Megger Spain

In a field that’s as fast moving as electrical technology, it’s essential for those involved professionally to ensure that their knowledge is kept up to date. By doing so, they not only ensure that they are always in a position to apply the latest best practice in their work, they also significantly improve their CVs and therefore, their career and salary prospects.

For these reasons, professional institutions encourage or even require that their members follow a structured programme of continuing professional development (CPD) throughout their working lives. There is, however, a catch – the costs associated with a CPD programme can quickly mount up. This is a particular concern for those who have to fund their CPD programme from their own pockets, but even when an employer may foot the bill, cost is still an important issue in this era of tight budgets and spending cuts.

Much of the cost of CPD is associated with attending conferences and symposia. These events are undoubtedly valuable and it would be wrong to suggest that anyone should try to eliminate them completely from their CPD programme. Nevertheless, there are less costly – and sometimes even free – alternatives that if carefully selected, can significantly reduce the need to spend money on formal large-scale events.

Excellent examples of these alternatives are the conferences, often free to attend, that run alongside many exhibitions. In some cases, the presentations at these conferences are made by manufacturers and suppliers who will of course, want to promote the merits of their own technologies and products. Nevertheless, a lot of useful information can be gained.

It is not unknown for participants to present a paper that is the same as, or at least similar to a paper that they’ve recently presented at an expensive paid-for event. It takes a lot of time and effort to produce a good paper, so it’s very understandable that those who have done so will want to extract maximum benefit from it!

It’s also worth bearing in mind that the more insightful suppliers have long ago learned that exhibition-related conferences are not

the place for the hard sell, and that they can generate much more interest in their products and technologies by providing information and advice that is genuinely useful.

The same goes for other manufacturer sponsored events, such as roadshows andin-house product introduction sessions, so these are all worth considering, and it can be very useful to put your name on key manufacturers’ email mailing lists to ensure that you always know what’s on offer. Of course, it might be thought that this cost cutting approach to CPD would be unacceptable to professional bodies, but at least in the case of the Institution of Engineering and Technology (IET) that’s simply not true. Here’s an extract from what that well respected organisation has to say about CPD on its website: “Any activity that contributes to your learning, to developing skills or to forming a professional attitude can be considered CPD. You may keep your knowledge up-to-date by reading and researching, keep current with the latest ideas by networking, develop new skills through secondments, going on training courses or even taking on a new activity outside of work, such as volunteering”.

In short, your CPD input doesn’t have to come from formal seminars and conferences, although they do have a part to play. Inexpensive and free sources are equally valid.

Megger very much supports this viewpoint and takes pride in providing a wide range informative events at venues around the world, such as the Verification of Electrical Installations seminar which ran alongside the Matelec Exhibition in Madrid last Autumn and generated a high level of interest.

It’s also worth noting that the best of today’s supplier-sponsored events are no longer just about presentations explaining theory and new developments, they include a whole range of educational tools and activities. A recent Megger event in Cadiz, for example, attracted more than 60 engineers in a single day to its two-and-a-half hour sessions. These sessions included not only live presentations delivered by experts, but also video segments to show test equipment in use in real-life situations, and even opportunities for those attending to gain practical hands-on experience by using the latest instruments in a simulated test environment.

CUTTING THE COST OF CPD

All of this shows that CPD doesn’t have to be costly. There are many inexpensive or even free opportunities for engineers to learn about recent developments and to keep their skills up to date. All that’s needed is to be aware

As well as live presentations, delegates had the chance to use the latest test equipment on display

that these opportunities exist, to look out for them, and to take full advantage of them. Those who do so will without doubt find that it really is possible to learn a lot while saving a lot!

Condition monitoring considered in Bali

see page 2

Protecting people working near overhead services

see page 3

The future at Teen Tech

see page 8

a high proportion of hands-on work using the training centre’s transformers and an extensive array of latest transformer test instruments.

Megger’s power transformer training course will discuss the most important methods used for measuring key transformer parameters, examine practical aspects of the operation and testing of distribution transformers, and explain how to interpret various test results obtained.

Continued on page 8

2 Megger ELECTRICAL TESTER April 2013 www.megger.com

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 industry’s recognised information toolELECTRICAL

TESTER

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.

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

To request a licence to use an article in Electrical Tester, please email [email protected], with a brief outline of the reasons for your request.

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Cutting the cost of CPD ..................... 1José Mancheño, Sales Manager, Megger Spain

Special offer on transformer training! ............................................. 1Tony Wills, Technical Support Group

Condition monitoring condsidered in Bali ..................................................2Grace Tsang, Marketing Manager, Megger Hong Kong

Putting testers to the test .................. 2Freddie Chin, Project Manager

Protecting people working near overhead services ................................3Graeme Thomson, VP America’s Distribution and Telco

The measurement of transformer winding resistance...............................4Oleh W Iwanusiw P.Eng, (Retired)

A towering achievement ......................5Dr. Li Huang

The first line of defence for MV substations ..........................................5Hein Putter, Production Manager, Testing and Diagnostics, SebaKMT, Baunach Germany

An online database for the evaluation of PD measurement on MV cable systems ................................6Hein Putter, Daniel Götz, Frank Petzold and Steffan Böttcher

Take the lead in design .......................7Paul Swinerd, Product Portfolio Manager

The future at Teen Tech .......................8Graham Heritage, Technical Director

Special transformer training continued .............................................8

A partnership to be proud of ............ 8Peg Houck, Marketing Communications Manager

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

Freddie ChinProject Manager

Businesses committed to innovation are constantly seeking way of applying the latest technological developments to deliver genuine and significant benefits for their customers. In the field of test equipment, as in most other areas of electronics, this has led to the introduction of products that incorporate increasingly densely populated printed circuit boards.

Manufacturing such boards successfully and consistently certainly creates challenges, but thanks to the exceptional performance and versatility offered by modern component placement and flow soldering machines, these manufacturing challenges have, for the most part, been successfully addressed. But even when the board has been completed, one very significant challenge remains – how to test it.

In former times, the answer was relatively straightforward. The circuit board would feature designed-in test points, and connections would be made to these via a so-called “bed-of-nails” test jig. This is essentially a set of spring-loaded probes, which is pressed against the board so that each probe makes contact with one of the test points.

With this arrangement, multiple test connections are made quickly in a single operation, allowing the board to be comprehensively tested by an automatic test system. The problem for many of today’s innovators however, is that their densely packed double-sided circuit boards simply have no room to accommodate the necessary test points. This was the situation recently confronted by engineers at Megger during the development of a new portable appliance

tester that, in its final form, was destined to set new standards for testing productivity.

For help in finding a solution, the engineers approached JTAG Technologies, a specialist in boundary-scan test and programming for solving the physical access problems involved in testing today’s printed circuit boards. JTAG examined the board in question to assess test coverage using the boundary-scan technique. This type of testing was made possible because the main processor was designed to be boundary-scan ready. In simple terms, this means that the chip incorporates additional logic to facilitate testing, even when connectivity options are limited.

By exploiting the capabilities of this device it proved possible to devise an efficient and effective test solution in the form of a dedicated test card that mates with the 200-pin edge

connector of the processor board used in the new instrument. This allows all of the signal lines on the edge connector to be exercised.

Damaged pins and tracks, and shorts between pins and solder pads are quickly and reliably located. The test system also allows the correct fitting of the memory devices used on the board to be verified, along with identities of these devices. JTAG runtime and visualiser tools are used to pinpoint any manufacturing issues, which can then be readily corrected.

The new test system was a major aid during the product development stage, and has also made it possible to refine manufacturing processes now that the instrument is in production. As a result, yields are now close to 100%, ensuring that Megger’s new PAT tester will be reliable.

PUTTING TESTERS TO THE TEST

Testing the ‘dedicated’ test card prior to assembly using boundary -scan technology

With ever-increasing pressures for electrical utilities to trim budgets while minimising service interruptions, condition monitoring and diagnosis of existing plant – especially those assets that are approaching or have already exceeded their design live – are fast becoming vital concerns.

With this in mind, the IEEE Dielectrics and Electrical Insulation Society, working in conjunction with the Department of Electrical Engineering at Udayana University in Indonesia and the School of Electrical Engineering and Informatics at Bandung Institute of Technology, recently organised a conference to consider the latest developments in condition monitoring and diagnostic technology.

Held in Bali in September 2012 and attended by over 300 participants from all over the world, the conference provided a globally important forum for the exchange of ideas, discussion and dissemination of research results and technologies in the field of condition monitoring and diagnostics for power equipment and systems.

Its wide-ranging sessions considered failure phenomena related to electrical, mechanical, chemical and thermal effects; dielectric materials and their aging mechanisms; degradation assessment; modern maintenance tools and environmental issues, as well as strategic management and planning for condition monitoring and analysis.

The conference recognised that among the key assets of every electrical transmission and distribution system are transformers, since transformer failures can create huge problems in the smooth functioning of power systems, resulting in major service interruptions and significant loss of revenue. Matz Öhlen, Megger’s Director of Transformer Test Systems presented a thesis and practical examples of

Condition monitoring considered in Bali

Grace TsangMarketing Manager, Megger Hong Kong

current best practice for dielectric frequency response measurements and analysis of trans-formers in real-world substation environments.

The presentation highlighted modern technology and developments in signal acquisition and analysis techniques, which have provided new tools for effective transformer and bushing diagnostics. The material presented was based on the use of dielectric response measurements for investigating the properties of oil-paper insulation systems.

It was explained that dielectric frequency response (DFR), which is also known as Frequency Domain Spectroscopy (FDS) is a well-established technique, having been introduced more than 20 years ago and that DFR data, in combination with mathematical modelling of oil-paper insulation has proved

itself to be an excellent tool for evaluating oil and moisture conductivity in power transformers.

The presentation included an investigation into the influence of electromagnetic interference in terms of ac hum currents, induced dc currents and low frequency interference in ac and HVDC substations. Solutions for addressing these various types and levels of interference were proposed, and examples of measurements made with these solutions in use were presented.

Also covered was DFR response analysis using the XY model. Numerical analysis using COMSOL had been performed, and this was compared with the simplified analytical XY model to illustrate how non-ideal conditions influence geometry parameters and results.

Matz Öhlen, Megger’s Director of Transformer Test Systems



TESTER

Protecting people working near overhead services

Graeme ThomsonVP America’s Distribution and Telco

One of the most attractive of the English counties, Worcestershire also has the unique distinction of being the basis for The Shire, part of J R R Tolkien’s Middle Earth, as described in The Hobbit and The Lord of the Rings. Despite these strong fictional connections, Worcestershire is today a very real and progressive part of England where keeping pace with developments often means that Worcestershire County Council employees find themselves working beneath power cables and other overhead service lines.

To help ensure the safety of its workers in these circumstances, the council has introduced strict guidelines. Overhead power lines in particular can pose a serious hazard with life threatening consequences. Striking an overhead power line with a vehicle or other equipment can kill or seriously injure personnel and damage or even destroy the equipment. Short of these dramatic outcomes, it can cause a disruption of electrical service for residents and businesses in the county.

Steve Wallis, Health and Safety Advisor, reports that Worcestershire Highways has adopted a hierarchical approach to working beneath overhead services, with the following steps required before work commences:

Avoidance: First a check is made to see if the work can be done at a different location that is free of overhead services.

Diversion: If avoidance is not feasible, the possibility of diverting the overhead service away from the work location is examined. This step requires the co-operation of the electrical and/or communication service providers.

Isolation: If the services can’t be diverted, the possibility of isolating them to remove the electrical hazard is examined. Again, this step requires the co-operation of the service provider.

Manage the risk: This step involves all personnel adhering to a safe system of work around the hazard.

To manage risk effectively, the service must first be identified. In the case of the electrical lines, this includes determining the supply voltage. The distance from overhead services must then be accurately measured. All over-head services are assumed to be live unless the service provider has confirmed otherwise. For this reason, Worcestershire County Council has mandated that cable height must not be measured physically with a tape measure or height pole. Instead, the measurement must be made with a Cable Height Meter (CHM). The CHM is placed on the ground directly underneath any cables in the vicinity and ultrasonically measures the height of the cables.

Central Networks, the main supplier of electrical power in Worcestershire, and the county council have developed a table of safe distances that shows the minimum permissible clearance distances to the overhead line based on supply voltage. These distances cover the minimum height, minimum passing clearances and minimum working clearances. The council also requires that if conditions dramatically change on the day of the actual work, heights should be re-measured.

Minimum height of overhead power lines:

Low and high voltage (240 V up to and including 66,000 V)

Line conductor at any point not over a highway surface

5.2 metres

Line conductor over a highway

5.8 metres

Line conductor over high load route

6.9 metres

Line conductor above boundary walls and hedgerows

4.0 metres

Line conductor between domestic properties (inaccessible to vehicles); low voltage only

3.5 metres

Extra high voltage (132,000 V)

Line conductor at any point not over a highway surface

6.7 metres

Line conductor over a highway

6.7 metres

Line conductor over high load route

7.8 metres

Nationally accepted passing distances (not working distances):

Low voltage (240/415 V) 0.8 metres

11,000 V 1.1 metres

33,000 V 1.1 metres

66,000 V 1.1 metres

132,000 V 1.4 metres

Clearances for working safely:

Low voltage (240/415 V) 2.0 metres

High voltage (11,000, 33,000 and 66,000 V)

3.0 metres

132,000 volt tower lines 6.0 metres

Ensuring safe working conditions As already mentioned, Cable Height Meters (CHMs) are used to ensure that personnel and vehicles are working a safe distance from overhead power lines. A CHM sends an ultra-sonic wave from the instrument to the target (overhead cable or cables) and measures the time for the echo to return to the instrument. The CHM then calculates the distance to the cable based on the speed of sound. It auto-matically corrects for ambient temperature.

The CHM is placed on the ground directly beneath the cable or cables to be measured. When the measure key is pressed, the CHM will first measure the height to the lowest conductor, and then the spacing in between

any additional conductors stacked above the first one, provided they are all within the ultra-sonic beam.

With a typical instrument, such as the Megger CHM, height measurements are shown sequentially by pressing the read key. Each time the key is pressed, the height of a wire is displayed, with the wire number also shown.

An infinity reading indicates either that that number wire is not present or is outside the range of the meter. The measurements are held in the unit until the unit powers off.

In this application, the reading of interest is the height of the lowest cable. The clearance tables provide the minimum safe distances for overall cable height, working clearance and ‘passing underneath’ (in a vehicle) clearance. The key measurement to ensuring safe conditions is the difference between the measured height and the minimum safe distances for that specific cable (see below

If, for example, work is to be carried out beneath an 11,000 volt cable, those carrying out the work would have to ensure that they were always at least 3.0 metres from the cable (based on the guidelines). The following graphic shows the safety clearance they would have on a cable that measured 9.7 metres above the ground.

If the work involved passing a vehicle under that same 11,000 volt cable, the workers would have to ensure that the vehicle was at least 1.1 metres from the cable (based on the guidelines). The following graphic shows the safety clearance they would have for their vehicles on that cable.

Ultimately, ensuring safe working conditions is a discipline. Avoidance, diversion or isolation are preferred to actually working beneath high voltage cables. If the area cannot be avoided, or the electrical cables cannot be diverted or isolated, proper safety procedures must be followed. As Worcestershire County Council has discovered, the Megger CHM cable height meter greatly simplifies the implementation of these procedures.

Operational photographs courtesy of Worcestershire

County Council.



TESTER

Based on the original paper by: Oleh. W. Iwanusiw, P.Eng, (retired)

Introduction:The resistance of windings of power transformers is measured in order that copper losses can be calculated and also the measurement of average temperature rise within the windings can be determined after a heat run.

When measuring the resistance for winding temperature determination, there is a need to measure it quickly, otherwise the winding will cool down and an incorrect temperature will be determined. A typical time quoted in some specifications is four minutes - that is the time allowed from the time the heat run is shut down, to the time the first reading is taken.

Transformer windings are also measured for maintenance application in order to confirm that the winding is in good condition, namely that there are no shorted turns or that some of the parallel strands have not been broken.

Such measurements can also show any bad contacts in on-line or off-line tap changers.

Therefore, winding resistance measurement can be divided into a factory application, where accurate measurements are to be taken quickly and there is no particular stress on having the test equipment light and portable. The second application, that for maintenance applications, where the equipment needs to be portable and there is not such a need to take the readings quickly.

It may be appropriate to point out at the outset that there are basically two important conditions that must be met if a transformer winding resistance is to be measurements. One of these is transformer core saturation – namely that the current will not flow in the winding until the core is saturated. The other condition is that the test current must be maintained at a fixed level.

This presentation is intended to be a discussion of the above two conditions as they apply to different winding connections of single-phase and three-phase power transformers. This is primarily applicable to the measurements of windings of large power transformers. The measurements of smaller distribution trans-formers do not present so much of a problem.

Core saturation:The first problem encountered when measuring winding resistance is that of saturating the transformer core in order to set up a current in the winding. To saturate a transformer core requires the application of ‘volt-seconds’ - a measure proportional to magnetic flux in the core. The required volt-seconds (vs) depends on the rated voltage of the winding to be tested. A winding rated at 110 kV @ 50 Hz will require the application of approximately 600 volt seconds in order to saturate the core. This number (600 vs) can vary, as it depends on any residual magnetism in the core and the polarity of the applied current. As the residual magnetism may be as high as 75%, the actual volt-seconds required for saturation may be from around 150 to about 1000.

The above indicates that to speed up the magnetization process, a large voltage is desirable. A typical instrument providing an output of around 30 volts, will take 5 to 30 seconds to saturate the above transformer. Some saturation times for higher voltage windings would be 10 to 60 seconds when testing one phase of a 400 kV transformer, or 20 to 120 seconds when testing two phases on a 400 kV winding simultaneously.

Current Stabilization:It should be pointed out at the outset that typical ‘electronic current regulation’ will typically not work for this application. The reason for this is that the load (transformer winding) is inductive and this causes instability and oscillations in the circuit. A method used to overcome this problem is to add ballasting resistance to the circuit, another – is to increase the test current, thus reducing the

inductance and increasing the losses of the winding. This reduces the quality factor ‘Q’ of the winding being measured, thus stabilizing the regulator. Regardless of the method used on single-phase transformers, or a three-phase transformers connected Y-Y, once the core is saturated and current established, it will take some additional time for the current to stabilize so that accurate resistance readings can be taken

The situation is quite different for three-phase transformers connected Y-D. The difficulty with such transformers is that a current is typically set up within the delta connected windings during the ‘saturation process’ and this current must be allowed to decay to very close to zero, before stable and accurate readings can be taken. The decay of this cur-rent depends on the time-constant (L/R) of the delta windings. As the inductance of the winding of a large transformers can be very high. If the resistance is very small, the time constant will likewise be relatively large. Time constants of minutes and waiting periods in excess of 15 minutes (T = ~ 5 TCs) have been experienced when testing large Y-D transformers.

A variety of techniques have been developed in order to overcome the problem of dealing with large time-constants and long stabilization periods. These are discussed below.

Testing on single-phase and y-y connected transformers:As indicated above, the measurement of winding resistance of single phase transformers typically presents little problem. The same applies to Y-Y connected windings, as each phase behaves as a single-phase transformer. Once the transformer core is saturated, current can be quickly stabilized and resistance measured. There is, however, a technique available that speed up the process. This same techniques allows accurate resistance measurements to be taken even under some-what unstable current source conditions. The technique is called the ‘Rate-of-change of current compensation’.

As the voltage drop across a winding can be written as ‘V=I*R + L*di/dt’, we can measure the winding resistance under variable current condition provided we can determine and correct for the ‘L*di/dt term’. As a quantity proportional to ‘L*di/dt’ can be measured on a second winding on the transformer, the equa-tion can be solved and resistance measured.

The above technique allows the winding resistance to be measured very quickly, as soon as the core is saturated and current flows. There is hardly any time required for current stabilization when using this method. Another advantage is that it does not require a current source with precise current regulation.

FIG 1. Measurement of winding resistance us-ing the ‘Rate-of-change of current’ compensation method.

Testing on y-d connected transformers:The measurement of Y-D connected transformers presents a problem due to circulating current that can be set up within the delta connection during the saturation period. The magnitude

of the circulating current depends on the winding or windings that are excited as well as on any residual magnetism in the core. Once set-up, this circulating current then de-cays according to the L/R time constant of the delta connection, where L is the inductance and R is the resistance of the delta winding connection.

As the excitation of certain windings acts symmetrically on the delta connection, these excitations typically do not induce a circulat-ing current within the delta. It is therefore highly recommended that these connections be used for winding resistance measurements, as they will provide a resistance reading in the fastest possible time. These connections are described below.

The measurement of other windings, those whose excitation induces a circulating current, must be carried out with the typical delay, but steps can be taken to reduce the long waiting period by reducing the time constant. One method of reducing the time constant of the delta connected windings is to reduce the inductance of the winding. This can be readily done by increasing the test current or by passing the test current through other windings available on the same core. This connection applies additional ampere-turns on the core, substantially reducing the inductance of the winding and reducing the time constant.

The measurement on y-connected windings in y-d transformers:Connections of the Y winding, usually the HV winding of the transformer, that induce a minimal circulating current within the Delta winding are possible. These connections allow the resistance reading to be taken in the fastest possible time.

For transformers with a neutral connection on the winding: 1. Excite and measure the ‘B’ phase winding (located on the center section of the core). This winding is symmetrically located on the core structure, thus induces only a minimal circulating current within the Delta.2. Excite and measure the ‘A’ and ‘C’ phases (located on the two outer core sections). Similarly to (#1), this connection also induces only a minimal circulating current within the Delta. For transformers without an available neutral connection on the Y winding:3. Excite and measure the ‘A’ and ‘C’ phases (located on the two outer core sections). This connection uses the ‘B’ phase to access the potential between phases ‘A’ and ‘C’.4. Excite and measure the resistance of the ‘B’ phase in series with phases ‘A’ and ‘C’ connected in parallel. The resistance of the ‘B’ phase has to be calculated by subtracting the parallel equivalent of phases ‘A’ and ‘C’ (#3) from the reading obtained here. Any other connection will result in a sizeable circulating current within the Delta connection, extend the current stabilizing period and causes an apparent drift in the measured value.

Testing on delta connected windings in y-d transformers:The measurement of the delta windings, usually the low voltage windings on a power transformer, presents the largest difficulty as exciting these windings always induces a circulating current within the delta connection.

Exciting and measuring the ‘B’ phase winding resistance (located on the center section of core) will result in a modest circulating current within the delta, while exciting and measuring the ‘A’ or ‘C’ windings will result in a much larger circulating current. The problems of this circulating current can not be avoided when measuring the delta connection, such as was possible with the Y windings, only the magnitude of the problem can be reduced.

In order to reduce the time constant, it is desirable to reduce the inductance (L) of the transformer windings. Increasing the test current will result in the lowering of the inductance (L) and therefore a shorter time constant (TC). Thus, we can show that the time to stabilize a reading will take approxi-mately half as long with a 10 ampere test current as it will when using a 5 ampere test current. Thus the transformer core can be saturated and the undesirable inductance reduced by applying current to other windings. This is typically done by applying the test current to other windings, such as exciting the Y winding in series with the Delta winding when measuring the Delta. Some transformers have their HV windings connected star, and the LV windings connected Delta, the application of the test current to the Star (Y) winding dramatically increases core saturation, thus dramatically reducing the residual inductance. The stabilization time constant is proportionally reduced.

As an example, take a 400 kV/11 kV, Star - Delta transformer (turns ratio ~21/1). When testing the Delta winding it will have about 6.6 amperes flowing in it. Now, with the current source connected to one of the phases of the Y winding, the core saturation will be increased by the ratio of the transformer windings, namely about 21 fold. This saturation level would be equivalent to testing the delta connection with a current of ~210 amperes. The inductance (L) of the delta winding will be drastically reduced, and so will be the stabilization time constant (TC).

Thus, the fastest way of measuring the resistance of Delta windings is to connect phase B of the Y winding in series, with the winding to be tested. This connection substantially increases the saturation time of the transformer core due to the higher voltage rating of the winding, but this drastically reduces the stabilization period due to the reduced inductance of the Delta winding.

Summary:1. Single-phase, or Y-Y connected three-phase transformers, should be tested with the HV winding connected in series with the LV winding. This connection saturates the transformer core more completely, reducing the stabilization time required for a stable resistance measurement.2. Y connected windings of Y-D transformers should be tested in two phases - first the winding on the middle core-portion, then the two windings on the outer core-portions. 3. Delta connected windings of Y - Delta transformers should be tested while they are connected in series with the middle phase of the Y winding.

Note - caution on magnitude of the test current:

Transformer manufacturers caution against using HIGH test currents; that is currents >10 A when measuring winding resistance. The danger is that large DC test current produces very large forces on the core and may cause damage.

A test current that is approximately equal to the peak magnetizing current of the winding is typically recommended.

Having the above in mind, we can consider a 150 MVA, 230/11 kV transformer, and assuming the magnetizing current to be about 1% of the load current: A recommended test current of about 4.5 A is calculated.

Similarly, the recommended test current for the HV winding of a 450 MVA, 400/11 kV transformer, can be determined to be approxi-mately 9 amperes.

Background to paper:Oleh. W. Iwanusiw, was for many years the Chief Engineer at Megger Limited in Canada.He developed many advanced, and highly accurate test instruments during that time, many involved with transformer testing. He presented conference papers in numerous countries and has patents on many unique circuits.

The measurement of transformer winding resistance



TESTERHein PutterProduct Manager, Testing and Diagnostics, SebaKMT, Baunach Germany

MV substations play a crucial role in the power distribution network, which means that substation failures are almost always costly and disruptive. There is a good case, therefore, for regular testing to find problems before they develop into failures, but taking substation equipment out of service for testing is often difficult. Fortunately, on-line partial discharge testing offers an effective solution to this dilemma.

Partial discharge (PD) is an electrical discharge that occurs across a section of insulation between two conducting electrodes, but does not completely bridge the gap between the electrodes. PD can for example, occur in voids in solid paper or polymer insulation, in gas bubbles in liquid insulation and, as corona, around an electrode in a gas. PD sources radiate both electromagnetic energy and acoustic energy, with the peak intensity of the acoustic radiation usually falling in the ultra-sonic region.

Research and experience have shown that many types of PD are extremely damaging to the health of insulation systems. In MV sub-stations this is an issue of considerable importance, particularly as IEEE statistics indicate that up to 90% of failures in some types of high voltage equipment are the result of the deterioration of electrical insulation because of age or stressing.

Various techniques have been developed for detecting and measuring PD, many of which require the equipment under test to be de-energised during testing. In appropriate applications, such as pre-commissioning testing of new cables, these offline techniques are invaluable, as they can provide accurate and detailed information about PD magnitude and location.

Offline PD testing is, however, not necessarily the most suitable nor the most convenient choice for routine ‘first-line’ testing in MV sub-stations. For this application, on-line PD field measurement can be a better option as it does not require equipment to be taken out of service for testing and it provides a true indication of the PD performance of the equipment with its normal operating voltage and load.

On-line PD field measurements are non-destructive and, as no over-voltages are used, the equipment under test is not exposed to stresses greater than those it experiences under normal operating conditions, which means that existing problems will not be worsened. In addition, results from tests performed at different times are directly comparable, so it is possible to trend them as a further aid to detecting insulation deterioration and to help gain an understanding of the effects of changes in environmental conditions (temperature, humidity, etc.) and service conditions.

In short, PD field measurements are ideal as a first line of defence against insulation deterioration in MV substations. The measurements can be performed easily and quickly – often in a matter of seconds – without disruption or interruption of service, and they provide results that clearly reveal where more detailed investigations are needed to guard against potential failures.

Having established the value of on-line PD field measurements, now let’s turn to the very practical aspect of how such measurements are made. Unlike a lot of test equipment for power installations, on-line PD surveying tools are typically small, light handheld instruments but in spite of their small size they offer multiple methods for detecting PD. Today’s most useful and popular instruments offer four options – a transient earth voltage (TEV) sensor, airborne acoustic (AA) sensor, an AA

sensor with parabolic reflector and locator, and a high frequency current transformer (HFCT) sensor.

Each of the sensors covers a specific range of applications and each is used is a slightly different way. TEV sensors detect electro-magnetic radiation from PD sites, which is induced into the metal enclosure of the equipment under test. The sensor is attached to the metal enclosure close to vents, seams and gaskets, and to cable terminations. This arrangement, in effect, makes it possible for the PD survey instrument to see PD sources through the metal enclosure.

AA sensors detect ultrasonic sound that is transmitted through the air from corona and surface discharges in air-insulated plant such as switchgear. AA sensors are placed over vents, gaps or seams in the housings of MV plant, and should be positioned so that there is a line of sight in air to the PD source.

AA sensors with parabolic reflectors are essentially the same as AA sensors but the addition of a parabolic reflector increases their sensing range, typically to 15 m or more. This type of sensor is particularly useful for testing outdoor overhead plant, including power transmission lines and insulators.

The last type of sensor – the HFCT – is the most suitable for detecting current impulses from PD in cables, cable terminations and the plant or switchgear in which the cables terminate. HFCT sensors are attached to the MV cable earth strap/drain wire or to the power cable with the earth strap/drain wire routed back through the sensor.

It can be seen that, with the range of sensors mentioned, first-line testing can be carried out quickly and easily on almost any type of substation equipment. The best on-line PD surveying instruments also deliver results that are easy to interpret.

The first line of defence for MV substations

The SebaKMT PDS Air has a display that incorporates seven LEDs, colour coded green, yellow, orange and red, as well as a digital PD intensity readout for use with TEV sensors.

The LED display works with all types of sensor and can be interpreted at a glance.

Essentially, if only the green LED is lit, the plant is healthy and in most cases will not need retesting for twelve months. If either or both of the yellow LEDs are lit, a moderate level of PD is present and more

frequent retesting is recommended.

When one or both of the orange LEDs are lit, a moderate to high level of PD has

been detected, and further investigations should be carried out as soon as possible to determine its source. Finally, if one or both of the red LEDs are lit, a high level of PD is present and further tests should be

carried out immediately to determine and locate the cause, and to decide whether plant should be taken out of service or have access restricted.

These recommended actions are, of course, guidelines only and, in specific instances, there may be special factors that have to be taken into account when determining the meaning and implications of the on-line PD measurements. Nevertheless, the recommendations are based on long experience of testing MV plant, and are relevant in almost all cases.

On-line PD testing is of course, not the complete solution for fault finding in MV substations, but used carefully and regularly it can provide invaluable warnings of developing problems at an early enough stage to allow remedial action to be taken before the problem progresses to become a major failure. In short, it’s an excellent, convenient and cost-effective first line of defence.

There can be no doubt therefore, that investing in on-line PD test equipment and in regular on-line PD surveys is a very good use of money, as the potential for making savings by reducing the incidence of costly disruptive faults is enormous.

Outdoor insulation testing with the optional acoustic parabolic receiver

Dr. Li HuangSebaKMT, China

Transmission tower enthusiasts – and there are many – who live in the UK and Europe are being seriously short changed compared with their fellow enthusiasts in other parts of the world and, in particular, those who live or work in the Far East. That’s because even on the 400 kV transmission network the typical height of a UK transmission tower is less than 50 m, and the UK’s tallest towers, which are part of the 400 kV overhead line crossing of the River Thames at Greenhithe Marshes in Kent, are just 190 m tall.

That may sound tall enough but it’s a very long way from what the rest of the world has to offer. In fact, the world’s tallest towers are currently those that form part of the Zhoushan Island Overhead Powerline, a 220 kV three-phase interconnection between the transmission system on Zhoushan Island and that on the Chinese mainland. Two of the towers on this link are an impressive 370 m tall – only a few metres short of twice the height of the tallest towers in the UK.

The span between the two towers is equally impressive at 2.7 km. That’s certainly a lot of cable to worry about when it starts swaying in a high wind and, just to make things a bit more interesting, the Zhoushan Islands are in an area where typhoons are by no means unknown.

Even the gargantuan Zhoushan transmission towers may not retain their status as the world’s tallest for very much longer. In September 2010, the Jakarta Post reported plans to construct towers 376 m high as part of a US $240 million scheme to link the electricity systems of Java and Bali.

It’s probably worth mentioning that the Jakarta Post article included a list of some other notable transmission towers and, as a service to enthusiasts everywhere, here it is:

n Jianggyin Crossing, China – 346 metres

n Nanjing Crossing, China – 257 metres

n Orinoco Crossing, Venezuela – 240 metres

n Zhujiang Crossing, China – 235 metres

n Elbe Crossing, Germany – 227 metres

n Chusi Crossing, Japan – 226 metres

n Daqi Crossing, Japan – 223 metres

n Suez Canal Crossing, Egypt – 221 metres

n Lingbei Crossing, Japan – 214 metres

Be warned that the accuracy and completeness of this list have not been checked, but it is nevertheless a great selection of possible holiday destinations for transmission tower enthusiasts!

A towering achievement



TESTER

AbstractIn almost every power distribution system, the medium voltage (MV) cable network is a key asset. Reliable operation of the cable network is essential and, from an economic point of view, it is also necessary to extend its life as much as possible. However as they age, cables degrade and become more likely to develop faults that compromise the reliability of the network.

Important symptoms of degradation are discharges at local insulation imperfections or defects in particular sections of cable or in cable accessories, such as joints and terminations. Around 50% of failures in MV power cable networks are the result of insulation problems.

The use of on-site partial discharge (PD) measuring systems is becoming increasingly common to detect and locate incipient faults as an aid to preventing failures. While performing PD measurements is straight-forward, interpreting the results often raises additional questions that make it difficult to decide whether to replace or to repair.

The increasing number of joint failures over the last 15 years provided the impetus to set up a VDE working group to collect and share experiences of on-site PD measurements. More than 30 German power utilities and several manufacturers of PD test equipment and of cable accessories are members of this group. The outcome of their work is an online database to collect, and present in a clear way, the key PD parameters from field measurements together with the results of visual inspections of the sources of PD faults.

This paper describes the database and explains that the evaluation of PD measure-ments and estimation of the severity of PD faults must take into account the type of accessory, where applicable, and the nature of the defect. Because the ignition of PD at a defect depends on the type of excitation voltage, the database is structured accordingly.

It is intended that use of the database should not be limited to utilities and industrial network operators in Germany, but that it should be open for worldwide use as an aid to interpreting on-site PD measurements.

IntroductionThis paper is published on behalf of a VDE working group that has given detailed consideration to the content and structure of the PD database. The intention of the paper is to inform international experts about the database, and encourage them to use it as a source of information to assist in the evaluation of their own PD measurements.

The physics and causes of PD defects in XLPE and PILC cable systems are for the most part, well understood. From the perspective of the network owner, the first priority is to know whether or not the cable system is operating with permanent PD activity under normal service conditions. The second priority is to be able to predict the behaviour of the insulating system when it is subjected to overvoltages produced by earth faults or switching operations.

In networks with resonance grounding, for example, an overvoltage of 1.7 U

0 could

persist for as long as two hours. If a cable system has continuous PD during normal operation at U

0, this raises questions about the

risks posed by the PD.

Basically, three PD parameters are important for judging the PD behaviour of a cable system:

PD inception voltage PDIVThe PD inception voltage is determined by applying to the test object a voltage that increases continuously or in steps. PDIV is the voltage where measurable PD starts. Note that the sensitivity of the measuring system and the background noise during the measure-ment influence the value of inception voltage recorded.

PD extinction voltage PDEVPD sources often show hysteresis in relation to inception and extinction voltages. That is, it is often necessary to reduce the test voltage to a level below the inception voltage in order to extinguish PD that has been ignited in a particular location. This means that knowledge of the extinction voltage is also important for judging the level of risk. In practice, PDEV is normally 10% to 35% below PDIV.

PD levelNormally, the average impulse charge at U0 is used as the assessment criterion. There is some global experience in evaluating the risk factor for operational reliability in relation to the location of the PD (cable, joint or termination), the type of cable insulation and the design of the accessories.

For typical PD sources, a phase-resolved display of PD allows comparisons to be made with so-called “fingerprints”. For GIS systems, these fingerprints are already relatively well defined, but for cable systems, the fingerprints are influenced by many factors, including the type of excitation voltage and the nature of the defect. This means that, at present, accurate correlation is only possibly in a limited number of cases, but comparisons can, nevertheless, provide useful additional information.

For network operators, the following factors are important when assessing cable systems:n The cable system should be free from PD at the rated voltage U

0.

n In networks with resonance earthed star point, there should ideally be no PD up to 1.7 U

0, but if PD is present, it must

extinguish at a voltage greater than U0.

n For PD diagnosis, the test voltage wave- form used should produce PD parameters (PDIV and PDEV) that are comparable to those produced at the 50 Hz or 60 Hz service voltage.n The voltage stress during PD diagnosis must ignite existing PD faults in order to detect them, determine their intensity and locate their position.n The PD diagnosis must be non-destructive; that is, no electrical trees should be initiated at new fault locations.n When using power frequency or similar voltage waveforms, the gradual increase in voltage during testing should be limited to a maximum of 1.7 U

0 so that the risk of

damage to the insulation is minimised.n When using distinctly different voltage waveforms (0.1 Hz sinewave, for example), there should be an established method of relating the test results to 50/60 Hz service conditions.

For all the topics mentioned so far, the new database at www.vde-kabeldatenbank.de provides information about PD parameters and the nature of related PD-defects. Only

data from PD measurements that have been validated by “post mortem” visual inspection of the associated faults is included. The nature of the PD defect strongly influences the risk of a complete breakdown in service and, therefore, the reliability of the cable system. Documentation of PD parameters, typical PD patterns and pictures of dissected faulty terminations, joints or cable segments (PILC) will help all users of PD measurement equipment to evaluate their own measure-ments and to make informed decisions about maintenance or replacement actions.

General structure of the databasePD diagnosis is equally well suited for quality control of newly installed cables and for assessing the condition of service-aged cable systems. A typical application for PD diagnosis is the investigation of serious failures in accessories after a period of operation.

For onsite PD measurements on cable systems, these types of excitation voltage are commonly used:n 50/60 Hz resonance technique (ACR)n 50 to 500 Hz damped AC voltage (DAC)n 0.1 Hz sinusoidal voltage (VLF)

Because of the physics of PD ignition, the results obtained using different types of test voltage, especially ACR/DAC and VLF, are not directly comparable. The results in the data-base are, therefore, sorted according to the type of test voltage used. As shown in Figure 1, the overview page displays the following key information at a glance:n type of cablen type of test voltagen type of joint or insulation failuren nominal voltage U

0

n partial discharge inception voltage PDIVn PD level at U

0

Figure 1. Overview of VDE database

Selecting an individual data set shows the detailed results of the PD measurements along with the related PD pattern. Pictures of the post-mortem analysis can also be examined, as shown in Figures 2 and 3. Because PD defects in cable systems occur mainly in accessories, the database is arranged according to accessory type. The types currently included are:

n Joint types for XLPE cable n heat shrink n cold shrink n slip on joint

n Joint types for PILC and mixed cable n oil filled taped joint n cast resin n transition joint (TJ) cast resin n TJ heat shrink (belted cable/radial field cable) n TJ cold shrink (belted cable/radial field cable) n TJ slip on joint (radial field cable)

n Termination types n heat shrink n cold shrink n slip on termination n elbow type n oil filled

Figure 2. Detailed view of one data set

Figure 3. Image viewer – pattern and pictures of

visual inspection

Results relating to PD in the insulation of PILC cable or in other types of accessory can also be entered by creating new default descriptions.

Participation and use of the databaseBoth active participants who contribute data and passive users who do not may use the database free of charge. Registration is necessary only for new users who wish to contribute their own results to enhance the knowledge base available to all users. It is not necessary to register or log in to browse or search the database. Data can be modified or deleted only by the user who entered it.

The key benefit of this database is that it makes it possible to better estimate the level of risk associated with different types of PD fault. Most PD faults in accessories are the results of workmanship failures. The nature of these defects is often related to the specific design details of the accessory. The potential impact of these defects strongly depends on the position and character of the fault.

For example, a void between the field stress tube and the insulation tube of a heat shrink joint, which is a common workmanship-related fault, can “survive” for more than ten years with a PD level at U

0 of several thou-

sand pC. A much higher risk is posed by an interface defect beneath the field stress element, which can fail after a few months of operation at 200 - 500 pC. These typical PD defects can be identified by comparing PDIV, PD level at U

0, pattern, and visual

presentation in the database.

To assess the results of an actual PD measure-ment, the type of cable as well as the location and type of accessories must be known. In case of a single PD source, the PD parameters can be directly compared with results in the database. Multiple PD faults in the test object must be differentiated by considering the local PDIV and, when possible, the local PD pattern of known fault locations.

For this reason, the evaluation software in the PD measuring equipment should be able to verify not only the locations and PD values in the PD mapping, but also the related inception voltage levels.

Initial results from the on-line data-basePartly as a result of knowledge gained from the on-line database, the VDE working group responsible for setting up the database has published new recommendations for the commissioning testing of MV cables. These recommendations include strategies for both newly installed and service-aged cables, as shown in Table 1.

AN ONLINE DATABASE FOR THE EVALUATION OF PD MEASUREMENT ON MV CABLE SYSTEMSHein Putter, Daniel Götz, Frank Petzold and Steffen Böttcher



TESTER

Table 1: Recommendation for testing of MV cables [10]

In addition to the normal sheath and VLF tests, the recommendations suggest that a PD test should be performed during commissioning and during condition analysis. In addition, for commissioning testing, it clearly defines PD limits, as shown in Table 2. Basically the cable and its accessories need to be PD free up to at least 1.7 U

0 . Field

experience has however shown that joints are sometimes not completely PD free immediately after installation. One of the reasons is that the joint body still needs to settle. It is exactly this experience that has led the new recommendation that, when this effect is noted, the cable should be retested after it has been in operation for between four and eight weeks. If the second test shows it is still not PD free, the joint should be replaced.

Table 2: Evaluation and recommendation of

measurement results [10]

Other conclusions that can be derived from the database are for example, that if the PD levels in joints on newly installed XLPE cables are very high (>2500pC), the most likely problem is incomplete shrinking of a heat-shrink joint. This type of defect is not a major threat to the operation of the cable, which can remain in service for some time, allowing replacement to be planned over a longer time span. This is especially useful if the cable is installed in densely populated areas where permission for roadworks and diversions is not easy to get.

On the other hand, if the PD levels in newly installed XLPE cables are very low, the problem in the PD affected joint is more critical as it will often be another type of workmanship failure – for example, a cut in the insulation or left over semicon. These types of defect are a greater threat to the reliability of the network and should be dealt with quickly.

Finally, with the aid of the information published in the online database, new criteria can be defined for PD measuring systems, relating to local PD patterns where PD is present at multiple locations.

Conclusions and recommendationsn The database can already be used for practical comparison of PD results from similar test objects, as an aid to estimating the nature and severity of a particular PD defect.n Contributions to the database from a large number of participants will in the medium term build up a comprehensive knowledge base. n Some important conclusions have already been derived from the database, and these have influenced the latest recommendations for testing MV cables.n When sufficient data is available, it will be possible to develop statistical approaches to the creation of generic values for the risk assessment of specific PD faults in various types of accessories. Interested universities and scientific institutions are invited to support this work.

The design of HV insulation test lead sets is intended to facilitate connection to a variety of de-energized systems for the purpose of making insulation resistance measurements.In all cases it is the responsibility of the user to employ safe working practices and verify that the system is safe before connection. Even electrically isolated systems may exhibit significant capacitance which will become highly charged during the application of the insulation test. This charge can be lethal and connections, including the leads and clips, should never be touched during the test. The system must be safely discharged before touching connections.

Test leads are a key component of any precision instrument. Safety, long life, and the ability to provide reliable connections to the wide variety of test pieces found in real applications are of utmost importance.

Careful design ensures repeatable connections, which are practical and safe to use. Only the best materials and most appropriate materials should be used to provide the essential blend of performance and safety. As an example the careful specification of the cable ensures it remains flexible in all conditions and has extremely good insulation properties which will not affect the measurements made.

Using a double-insulated silicon cable will ensure reliable and safe measurements. Testing with poor or electrically leaky leads can provide misleading measurements and may result in perfectly good insulation being diagnosed faulty, wasting both time and money on unnecessary repairs. This is especially so when using long test leads.

Significant safety enhancementsThe international standard IEC 61010-031 details the safety requirements for hand-held probe assemblies for electrical measurement and test. A number of amendments were made to the standard, in particular: prevention of hazard from arc flash and short circuits.Two hazards are considered: (1) the dangers of a probe tip or crocodile clip temporarily bridging two high energy conductors, and (2) the dangers of a contact being broken while current is flowing.

These hazards are particularly applicable to many of the environments in which 5 kV and 10 kV insulation resistance testers are used. Should a probe or clip momentarily short out two high energy conductors during connection, an extremely high current will flow heating the metal and melting insulation. This itself may cause serious burns to the operator or bystander near the clip or probe. Additionally, should the contact be broken while current is flowing, arcing may occur leading to anextremely serious situation known as arc-flash.The standard describes the danger of arcing as follows:‘The arcing will ionize the air in the vicinity of the arc, permitting continued current flow in the vicinity of the probe tip or crocodile clip. If there is sufficient available energy, then the ionization of the air will continue to spread and the flow of current through the air continues to increase. The result is an arc flash, which is similar to an explosion, and can cause injury or death to an operator or a bystander’.

IEC 61010-031:2008 requires probe tips and crocodile clips to be constructed to mitigate the risk of arc flash and short circuits, and this requirement applies to all crocodile clips or clamps that are rated to Installation Category III or IV (CATIII or CATIV). The outer surfaces of crocodile clips must not be conductive and no metal parts should be accessible (as defined by the standard) with the clip closed.

During design phase, detailed measurement and test procedures are used to assess the electrical creepage and clearance paths, to assure compliance with the standard. Accessibility of conductive metalwork is assessed using an IEC standard test finger.

Things to consider for safe operationIn electrical test environments, safe working practices are essential to ensure the safety of operators. Insulation testing in high-voltage, high-energy environments poses a number of unique hazards listed below:1. Maintaining practicality with a fully insulated clipIf a clip’s added insulation impedes the operation and ability to make reliable connection to the wide variety of bus bars, wires and terminals that are needed, the design is useless and the operator may be tempted to remove the additional insulation to make connection.2. Protection from charged capacitance of long cablesLocked high-voltage plugs at the instrument end reduce the likelihood of a plug losing connection or pulling out which could result in the load inadvertently remaining lethally charged at the end of a test and the instrument to incorrectly report that no voltage was present. The lock facility is simple to use and prevents “plug end” disconnection and helps ensure the integrity of load discharge aftera test.3. Protection from high voltage in CATIV 600 V environmentAs a connection is made to more upstream supply systems, (overvoltage Category IV relates to incoming supplies of industrial premises), increased protection is required from overvoltages. These are transients that naturally occur on the supply, which are typically caused by switching actions or distant lightning strikes and present the connected equipment, test leads, clips etc with impulses of many thousands of volts. Such equipment must provide protection to the operator during the process of connection. A clip rated for use on a 600 V supply in overvoltage category CATIV must be able to withstand such impulses up to 8 kV.

Clips that are molded from a high dielectric strength insulating polymer with carefully defined dimensions ensure electrical creepage and clearance distances are maintained even under adverse conditions.4. Protection from instrument output (5 kV or 10 kV)Many people fear the electrical output from their insulation tester may be 5 or 10 kV. However, in reality the current available from the instrument is generally limited to a few milliamperes and in itself presents a relatively low hazard.

The danger here is not so much the output of the instrument but more the working environment. If the connected load is capacitive, this can provide very significant energy when charged to high voltage by the instrument, and can be lethal if touched. Additionally, when testing insulation in many HV environments, it is not uncommon to have to climb ladders

to reach connections on equipment such as transformers, with associated risks of working at height. In such situations, an otherwiseharmless electrical impulse may cause the user to react automatically, with a potentially serious injury from a fall. Fully insulated clips help minimize the risk.

PRACTICAL INSULATION DESIGN

Moving jaw fingers maintain the clips touch proof safety when the clip is closed but flex back to allow the metal teeth of the clip to contact the test piece unimpeded when in use.

Megger clip being tested with IEC standard test finger for creepage and clearance.

PRACTICAL JAW DESIGN

Curved jaws allow reliable connection around test pieces and flat jaw tips provide excellent connection and gripping of individual wires.

More detailed information can be found on the 5 kV and 10 kV insulation tester lead sets application note. This document can be downloaded from: www.megger.com

Paul SwinerdProduct Portfolio Manager Take the lead in design

The latest 10 kV test leads - desigbed by Megger


Q&A In this edition of Electrical Tester we’re looking at some of the questions we are most frequently asked about performing on-site partial discharge (PD) diagnostic testing on power cables using damped ac (DAC) voltage techniques. The questions included here offer an introduction to DAC PD testing and, in future issues, we plan to look at questions that explore this useful technology in more detail.


TESTER

An educational game devised by young engineers played an important role at the exciting Teen Tech event recently held in Folkestone, Kent. The event was one of many currently being held around the UK to create interest among schoolchildren about careers in engineering, science and technology, and to dispel the negative impressions that children – and adults – often associate with these professions. Teen Tech events are the brainchild of Maggie Philbin, a former presenter of Tomorrow’s World, a popular UK TV science programme that ran for many years.

The events are attended by as many as 300 twelve-year-old children from up to 30 schools and, to help maximise interest and excitement among the participants, the events always include technology-based competitive challenges. For the Folkestone event, a group of four young engineers who are training and working at Megger, devised and constructed a challenge that involved answering questions about the transmission and distribution of electricity.

A team from each of the schools attending the event was invited to enter the challenge and, for each question they got right, members of the team could make a link on a game board representing the national grid, to connect

Graham HeritageTechnical Director, Megger Instruments Limited

The future at Teen Tech

It is frequently said that the best form of supplier relationship is a partnership, and many companies claim that they have adopted a strategy of putting effort and investment into building partnerships. Most of us will, how-ever, know of instances when these claims ring rather a little hollow, as sometimes the idea of partnerships remains just that – an idea rather than reality.

Of course, it can be difficult for companies to judge for themselves whether they are genuinely perceived as true business partners, so it’s exceptionally gratifying when they receive unequivocal confirmation of a successful partner relationship. And that’s exactly what happened to Megger recently when it received the “Award of Excellence for Outstanding Strategic Partner Performance in Fiscal 2012” from Transcat, one of the company’s national distributors in the USA.In presenting the award, Transcat noted that

A partnership to be proud of

its relationship with Megger spanned more than 25 years and, over that time, new elements had consistently been introduced to the partnership between the two companies, supporting the continuing growth and success of both. Recently, for example, the partnership has allowed Transcat to reach the wind energy market, which opens up a new niche for the company.

Awards from Transcat are presented on the basis of vendor scorecard which features input from all departments, including sales, operations, materials management, marketing and logistics. The elements of the scorecard include account management, customer service, quality of products, training and resources, sales and marketing support, and financial considerations. Transcat reports that Megger attained perfect scores in nearly every category.

Peg HouckMarketing Communications Manager, Megger Valley Forge USA

A proud day for the employees at Megger, Valley Forge USA

a 10,000 volt supply to the last “generating station” on the board. When the final link was made, it set off a spectacular electrical discharge, which was a big hit with the competitors.

Completion times were recorded and a prize, generously donated by Eurotunnel, was awarded to the school that achieved the fastest time. The winning team was from the Harvey Grammar School in Folkestone.

The prize is a behind-the-scenes visit to Eurotunnel. The winning team will be able to choose between three experiences: the driver training simulator, where they will take a turn at “driving” a train and learn all about shuttle locomotives; the truck x-ray system, where they will see how x-rays help to keep the Channel Tunnel safe; and the Channel Tunnel ventilation system, where they will see the ventilation and cooling plant in operation.

From left to right - Graham Heritage, Technical Director of Megger Instruments Limited with Chris Waller, Dave Pitkin (Test Gear dept) and Anna Mastrogiovanni (PCB test) who devised and constructed the exhibits

Pupils from local schools taking part in the quiz

One of the exhibits - National Grid

Q: How is DAC PD testing different from any other form of PD testing?A: The essential difference is type of test voltage used to energise the cable under test. Conventionally, continuous ac sources have been used, but these are big, heavy and expensive. With DAC testing, an inductor is connected in series with the cable and then the combination is charged from dc source. When the cable is charged, a high-speed switch connects the inductor in parallel with the capacitance of the cable to form a resonant circuit. As a result, damped oscillations at approximately power frequency are set up in the cable, and these oscillations provide the PD test voltage. DAC test sets are much smaller, lighter and less costly than their continuous ac equivalents.Q: Is DAC PD testing a new method of on- site cable testing?A: No. DAC testing at a frequency in the range 20 Hz to 400 Hz with a cable/ inductor system exhibiting low damping (<30%) was first proposed more than 20 years ago. As long ago as 1989, it was investigated by CIGRE as a method for on-site testing of power cables to complement, and as an alternative to, ac testing of the cables at the time of manufacture.Q: Is DAC testing accepted in recognised cable testing standards?A: Yes. DAC voltages are recommended for on-site cable testing and partial discharge diagnostics in many standards, including:• IEC 60060-3 High Voltage test technique – Part 3: Definitions and requirements for on-site testing; Chapter 10

• IEEE 400 Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems; Chapter 10 as oscillating wave test system• IEEE 400.3 Guide for PD Testing of Shielded Power Cable Systems in a Field Environment; Clause 6.2Q: Is DAC really a valid alternative to continuous ac?A: Yes. Because the frequency of the DAC voltages is close to supply frequency, the DAC voltages produce a PD response in the cable that is very close to that produced by continuous ac test voltages at 50/60 Hz. This means that DAC is an excellent alternative to continuous ac. The similarity in PD response covers PD pattern, PD inception voltage and PD level, and ha been confirmed in several published papers.Q: Is the electrical stress generated in the cable by DAC similar to dc stress?A: No. In line with the fundamentals of ac and dc field theory, applying damped ac voltages to cable insulation produces only ac voltage stresses. In addition, as the cable is charged with a continuously increasing HV voltage, no steady state dc conditions occur during charging. Q: Is DAC potentially more harmful to the cable than continuous ac?A: A CIGRE investigation (TF 21.09/02) carried out in 1990 showed that the average breakdown field strength for DAC voltage stresses is always slightly higher than that for continuous ac voltage stresses. In other words, DAC is less potentially harmful than continuous ac. It has also been shown that DAC voltage stressing is harmless to healthy insulation.

Transformer training continued from page 1

Topics covered in detail during the course include transformer operation and the theory of testing, safety requirements, ratio testing on voltage and current transformers, winding resistance testing, magnetisation tests, Class X current transformers and the importance of spill current testing, also the application of insulation resistance and high potential testing, and working with software to maximise the amount of useful information that can be extracted from the test results.

The course includes comprehensive documentation, including a certificate of completion, and refreshments and lunches on both days.

Overnight accommodation is not included, but those attending will receive an information pack from Megger, which includes details of hotels and other local accommodation.

To ensure that all delegates have adequate access to test equipment and ample time for questions and discussions, the number of places on the course is strictly limited.

Demand is expected to be high, so if you are interested in attending you should request a booking form as soon as possible by calling Megger on 01304 502 101 or by sending an email to [email protected].

Bookings will be dealt with strictly on a first-come, first-served basis.

Alan Purton, course leader on transformer training.

Alan has over 20 years of training experience with Megger and other training providers.

Book your place by email [email protected]

Published by Megger April 2013 ELECTRICAL TESTER · Published by Megger April 2013 To mark the...

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