Levitating magnets The transformer Stakeless earth testing threat See page 6 See page 3 See page 5 ELECTRICAL TESTER Published by Megger April 2011 The industry’s recognised information tool Little known facts about tan delta/power factor testing Dinesh Chhajer Technical Support Group With an increasing failure rate of substation electrical equipment, utilities and heavy industry must focus on preventive and predictive maintenance to ensure power system integrity and reliability. Electrical insulation is a common reason for electrical equipment failures and tan delta/dissipation/power factor (PF) testing is a popular way of diagnosing and estimating the condition of insulation as it ages. There are, however, a number of issues relating to PF testing that are not nearly as widely understood as they should be. PF testing is widely used on electrical equipment such as power transformers, circuit breakers, generators and cables. PF values, trended over time, can help in detecting problems like contamination, high moisture content and the presence of voids in insulation. Excitation current tests, along with PF tests, performed on power transformers, can also help in detecting turn-to-turn insulation failure. Dissipation factor tests are usually performed at 10 kV or the readings are converted to 10 kV equivalent. The best voltage for PF tests is a frequently debated topic as instruments are now available that allow the tests to be performed at voltages from 27 V to 12 kV. What test voltage is “good enough” for accurate and reliable measurements? The answer depends on the type of test specimen and the test conditions. Most power transformers have oil-paper type insulating systems that exhibit a flat response when PF is measured as a function of test voltage. However, motors and generators typically have dry or solid insulation whose PF values may vary with test voltage. Values increase with increasing test voltage due to the voids that are almost invariably present in solid insulation. The increase in PF value as a function of voltage corresponds to increasing ionization in the voids. One reason that industry has standardized on a 10 kV test voltage is for immunity against electrostatic interference; power transformers operating in HV substations are subject to a lot of electrical noise and interference. An HV test signal provides better signal to noise ratio, giving more accurate measurements. Test instruments with very high noise suppression capability are required for measurements in HV substations as the test current is very low in insulation tests and noise levels can be as high as 20 times the test current. Battery testing means cruise passengers are never in the dark! www.megger.com DELTA4000 performs automated power factor tests at up to 12 kV along with that single capacitance, the results For perfect insulation, the PF should be zero. look strange. For example, when performing In practice, any value close to zero is considered tests on bushings, three-winding transformers to indicate a good insulation system. PF test or inter-phase insulation of rotating machinery, sets always try to measure a single capacitor, but the PF values will sometimes be negative. if the test object has some phantom circuits Tony Wills Applications Engineer Battery impedance testing is making life a lot easier and more comfortable for Rey B Crisostomo, an engineer who works on ships operated by Royal Caribbean International, the largest global cruise brand. Among many other duties, Rey is responsible for ensuring that the enjoyment of Royal Caribbean customers is never interrupted by a power failure while they are on board the company’s vessels. ‘Before I became aware of battery impedance PF is a measure of watts loss in the insulation. testing’, reports Rey Crisostomo, ‘I had some bad experiences with batteries, usually finding out when it was too late that my back-up power source wasn’t able to supply power when it was really needed. The result was always the same – lots of time was wasted and a lot of money was lost, impacting our profits. And, since my salary comes out of those profits, I had a very personal interest! In addition, it was never a pleasant experience explaining to my supervisor why the incident had occurred. Negative PF therefore corresponds to watts generation. Obviously, insulation cannot generate power, which shows that negative PF values are not real but instead tell us that the insulation does not behave as an expected capacitance. continued on page 2 Figure 1: Specimen in UST mode with surface However, with a little help and support from Megger, I discovered the BITE2P battery impedance test set and learned how to use it to check the condition of my batteries, and to predict reliably when they need replacing. Now I can be confident about relying on the battery back up systems if there should be a problem with our primary shipboard power sources. What I like most about battery impedance testing is that I can use it to test the battery continued on page 2 Megger ELECTRICAL TESTER April 2011 1 ELECTRICAL TESTER The industry’s recognised information tool Contents negative values after taking these precautions could point toward contamination or a bad insulation system. A Little known facts about tan deltal/ power factor testing........................... 1 Dinesh Chhajer, Technical Support Group Battery testing means cruise passengers are never in the dark!...... 1 Tony Wills, Applications Engineer Putting electric vehicles to the test.... 3 Dave Moore, Product Manager Mumbai technical conference goes with a swing....................................... 3 Testing to address the transformer threat.................................................. 3 Online testing of first trip analysis.... 4 Simanand Gandhi, Applications Engineer Magnetic shielding............................. 5 Dr Stan Zurek, Magnetics Technical Specialist Swiss army knife of testers!............... 5 loss components Negative values also appear with some specimens that have high surface leakage. As shown in Fig 1, phantom circuits introduce a current (Is) which changes the phase angle of the measured test current (IT). The surface loss current (Is) is predominantly resistive (Rs) and has a very small phase angle with respect to the applied voltage. Capacitive coupling (Cc) may be present as a result of this parallel path of Rs to main insulation under test. Excitation current testing is commonly performed along with PF testing. It is a voltage dependent test and is always performed in UST mode. Like PF tests, the excitation current readings are normalized to 10 kV equivalent values, using a linear approximation. When dealing with specimens that are highly inductive, such as power transformers, the relationship between voltage and current is, however, not linear. Assuming a linear relation to determine 10 kV equivalent excitation gives only approximate values. It is, therefore, important to perform tests at the same voltage if excitation current historical data needs to be trended. Tests performed at different voltages and then corrected to 10 kV may not be comparable. This is important as trending data is critical when evaluating problems with turnto-turn insulation. Dave Moore, Product Manager Earth testing without the stake Part 2.................................................. 6 Paul Swinerd, Product Manager Hannover 2011................................... 7 Georg Halfar, Marketing Communications Manager, Germany Figure 3: Excitation current measurement on a Delta winding with third leg grounded Q&A.................................................... 8 When performing excitation current measurements on delta windings, it is important to ground the third leg of the delta configuration as shown in Fig 3. Since excitation current is a UST test, grounding the third leg eliminates the current flowing in the other two windings from the measurement circuit. Depending on the inductance and resistance of each winding, if third leg is not grounded the results would be approximately 30% to 50% higher than true readings. Safety in testing - Part 3..................... 8 Jeff Jowett, Applications Engineer The past of PowerDB......................... 8 Mark Meyer, Director of Marketing Pylons in dresses!............................... 8 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 ElectricalTester@Megger.com, with a brief outline of the reasons for your request. All trademarks used herein are the property of their respective owners. The use of any trademark in this text does not imply trademark ownership rights in such trademarks, nor does use of such trademarks imply any affiliation with or endorsement of Electrical Tester by such owners. 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. Note from the Editor Time for your say. We have introduced a ‘Questions and Answers’ section and would like your input. If you have any questions or stories that you think we could use, then please email electricaltester@megger.com ‘Views expressed in Electrical Tester are not necessarily the views of Megger.’ The word ‘Megger’ is a registered trademark Editor Nick Hilditch. T +44 (0)1304 502232 E nick.hilditch@megger.com www.megger.com Megger Limited Archcliffe Road Dover Kent CT17 9EN T +44 (0)1304 502100 E electricaltester@megger.com www.megger.com 2 Fig 2 Vector diagrams with different Is phase angles and different magnitude of I NET Smaller phase angles for surface loss current (Is) can lead to negative PF values. Measured test current (IT) is the vector difference of total current (INET) and surface loss current (Is). In UST or GST configurations, this surface loss makes the measured test current (IT) phase angle greater than 90º, and this results in negative PF values. It is important to understand where negative PF values come from. For some specimens it is just a result of design – for example, the presence of electrostatic grounded shield between the inter-windings of a transformer. In other cases, where negative values are encountered users should consider eliminating all external effects by following best testing practices such as verifying proper grounding circuits, cleaning external bushing surfaces, avoiding unfavorable weather conditions and using guard circuits effectively. Repeated continued from page 1 system without disturbing the normal operation of the equipment it serves – I don’t need to take the system off line, yet I can still test each of the batteries in the strings. Impedance testing also provides me with the data I need to make an informed decision about whether or not a battery needs to be replaced. This saves a lot of money, as we used to replace the batteries automatically after a certain period of service, and our experience with impedance testing has shown that we were often replacing batteries that were still serviceable. Megger ELECTRICAL TESTER April 2011 A transformer with magnetized core can exhibit higher excitation current measurements than normal. IEEE 62-1995 section 6.1.3.4 states, “If a significant change in the test results is observed, the only reliable method of excluding the effect of residual magnetism is to demagnetize the transformer core.” The factors discussed here that affect excitation current measurements should be borne in mind before performing the test. The dissipation factor values are highly dependent on temperature. IEEE C57.12.90 section 10.10.4 Note 3 (b) states that “Experience has shown that the variation in power factor with temperature is substantial and erratic so that no single correction curve will fit all cases.” Nevertheless, correction factor tables have traditionally been used to bring all data to a common base of 20 °C. It is imperative only to compare a specimen’s PF values that are taken at a similar temperature or corrected to the same temperature accurately. For different specimens, changes in temperature affect PF values in different ways. And even the same specimen will become more temperature dependent as it ages. Temperature On board an ocean-going vessel it’s important to know that all of the machinery will remain operational in all circumstances, and battery power is almost always the best alternative power source to ensure this. At Royal Caribbean, we have always worked hard to make sure our customers have the best vacation on earth. Now, thanks to battery impedance testing, we can do even more by testing our batteries regularly so that there will never be any worries about a ship suddenly being blacked out in the middle of the ocean!’ Megger BITE2P battery impedance test sets of the type used by Rey Crisostomo are rugged correction factors are highly dependent on insulating material, its structure, ageing, presence of moisture or contamination and other influences. However, temperature correction data is based upon the average values. Since each test object is unique, using these average corrections introduces errors. New transformers have relatively weak temperature dependence and the use of standard tables overcompensates. As the object ages, same average correction factors would under compensate and error predominates in the other direction. Trending of PF values becomes more critical in the second half of the life cycle. In this second half, correction factors should be larger because of the increased effect of temperature on the insulation. Using average factors can lead to incorrect trending and inaccurate estimation of the remaining healthy life of the object. IEEE Standard 62-1995 states, “Testing at temperatures below freezing should be avoided, since this could significantly affect the measurement. Among the primary reasons for performing this test is the capability of detecting moisture in insulation. The electrical characteristics of ice and water are quite different and it is much more difficult to detect the presence of ice than it is to detect water; sometimes it is impossible. ”Measuring PF at too high or too low a temperature can introduce errors, and the IEEE recommends performing PF tests at or near 20 °C. However, it’s not always practical to cool down or heat up the test specimen to 20 °C. Fortunately, new technology makes it possible to accurately correct PF values to 20 °C without resorting to correction factor tables based on averages. Using dielectric frequency response (DFR), the unique temperature correction factor of each test object can be determined. This is possible because a PF measurement at a certain temperature and frequency corresponds to a PF measurement made at different temperature and frequency. Therefore by measuring PF at different frequencies, it is possible to determine the temperature dependence of the specific test object. With this technique, PF can be measured at any insulation temperature [5 °C-50 °C] and then corrected to 20 °C accurately and precisely. Electric apparatus has failed and will continue to fail because of insulation deterioration. A proactive approach is the key to monitoring the integrity of the insulation system and preventing or at least anticipating such failures. Tan delta/power factor diagnostic testing is an important tool in determining the quality of the insulation and estimating its remaining healthy life. Dissipation factor readings are dependent on various factors and it is important to be aware of these. Test voltage, electrostatic interference, temperature, humidity, surface losses and other parameters can greatly influence PF measurements. A better understanding of the impact of these parameters will help in obtaining accurate measurements that can be relied upon in the decision making process. self-contained units designed to determine the condition of lead-acid and nickel-cadmium cells up to 7000 Ah. They work by applying a test current across the battery string while on-line, then measuring the total current (ac ripple + test current) and the voltage drop of each cell. They then calculate the impedance, which has been shown to be an excellent indicator of battery condition. The test sets also measure dc voltage and interconnection (strap) resistance to help determine the overall condition of the entire battery string’s electrical path from terminal plate to terminal plate. www.megger.com ELECTRICAL TESTER The industry’s recognised information tool Putting electric vehicles to the test Photo: Bosch Dave Moore Product Manager As concern continues to grow over the environmental impact of vehicles with conventional petrol engines, the popularity of electric and hybrid vehicles is increasing rapidly. In fact, it has been reported that vehicles with hybrid drives already mak e up 2% of new registrations worldwide, and that this figure is expected to reach at least 7% in the next five years. Much more appropriate are handheld insulation testers that work at 1 kV. What is important, however, is that the instrument should have a high maximum reading. There’s a lot of difference between a tester that shows infinity for everything over, say, 1 GΩ, and one that gives dependable readings up to 200 GΩ, because the latter provides much more information about faults that may be developing, but have not yet reached a critical stage. Mumbai technical conference goes with a swing! Also important is support for diagnostic insulation tests, such as Polarisation Index (PI) and Dielectric Absorption Ratio (DAR). At present, these tests are not widely used in connection with electric vehicles but, when performed regularly and the results logged over time, they can provide useful additional information about the life expectancy of wound components like motors. The information these tests provide is particularly useful in applications where there is a risk of moisture ingress and carbon deposits, as there invariably is with the drive trains of electric vehicles. Internal storage of test results and facilities for downloading these results for inclusion in reports are other highly desirable features, since not only do they save time, they also eliminate the risk of errors that is always present when results are recorded manually. While result down-loading via a USB cable or similar is satisfactory, there is a lot to be said for instruments that offer the added convenience of Bluetooth wireless data transfer. ® This is good news for the environment, but it does create challenges for the garages and service agents that will be called upon to maintain these new vehicles. In particular, they will have to deal with electrical systems that work at much higher voltages than those found on traditional vehicles. One important consequence is that, in order to ensure safety and reliability, insulation testing will be a prime requirement, in much the same way that it currently is for electrical installations in buildings. In the fullness of time, standards will undoubtedly be developed for insulation testing on vehicles but, for the present, there are none. That doesn’t mean, however, that there are no instruments available. On the contrary, a huge range of insulation testers is currently available, so the problem is more likely to be choosing the best option for vehicle applications. With this in mind, it’s worth looking at the characteristics that are desirable in insulation testers for use on electric vehicles. High voltage types – instruments that test at 5 kV or above – are not necessary. While they have advantages in some applications, for example with large railway traction motors, they are not needed for the current generation of road vehicles. The physical characteristics of the instrument are also important. For regular use it needs to be small and light, and it should ideally allow one-handed operation. It should have a large clear display, and the provision of an analogue arc alongside the usual digital readout is a distinct advantage, as the behaviour of the analogue display can provide useful extra information when performing insulation tests. Finally, the ideal insulation tester for electric vehicle applications needs to be robust, and to have a good ingress protection rating – say IP54 – as garages and vehicle service centres can be tough operating environments. Instruments that meet all of these criteria are available today – products in Megger’s MIT400 range being good examples – and it is worth bearing in mind that they will not only fulfil the requirements of those called upon to test electric and hybrid vehicles, but will also find many applications in testing the charging stations associated with these vehicles. “Minakshi” and her dance troupe perform a traditional Rajasthani folk dance Delegates from Asian and Middle East countries recently attended a Megger technical conference in Mumbai, India. The aim of the conference was to provide expertise and training to engineers working to ensure electrical supply in the fast growing economies of the countries in these regions. India is a country of colourful and proud regional traditions, and dancers from the different regions of India showed some amazing energy and versatility in their performances – not accidental, since these were two of the values that the conference organisers wanted to demonstrate to delegates! The conference saw the first public demonstration in Asia of the new SMRT test set. Experts from Megger’s relay test set manufacturing sites in Dallas, Texas and Taby, Sweden flew in specially for the event, and provided some hands-on training. SMRT is physically much smaller and lighter than comparable three phase instruments and has a higher output. It’s very convenient to have a test instrument that weighs only 12 kg, with a power box that is specifically designed to test protective relays that are used in conjunction with CTs having 1 A and 5 A secondaries – a common need not only in India, but throughout Asia. SMRT has a redesigned TVI (Touch View Interface) and works with the world renowned AVTS relay test software, so users get up and running in very fast time. Delegates reported that there is substantial programme to bring new assets on line, but it takes time to establish the new assets, there are inevitable teething problems and the new test set was seen as a great help. As with much of the rest of the world, India has a vast resource of legacy plant which needs to be maintained to meet the demands of the country’s rapidly growing economy. Bruce Buxkemper Delegates received demonstrates the new MPRT1 theoretical and hands-on smart touch view interface. training. Questions and Answers sessions are always popular, and some robust discussions were the result. Testing to address the transformer threat If the reliability of the public electricity supply is to be maintained in the future, routine diagnostic testing of transformers by the power utilities is no longer optional – it’s necessity, says Megger, one of the world’s leading developers and suppliers of transformer test equipment. Megger’s opinion is reinforced by an article recently published by Lloyds of London, the world’s largest insurance market, which identifies transformer failures as one of the biggest risks to the security of electrical supplies to homes and businesses. The article was published at www.lloyds.com/News-andInsight Many of the transformers in the electrical supply network have now reached or even passed the service lives for which they were www.megger.com designed, which means that the possibility of failure is increased. Replacing defective transformers is, however, becoming increasingly problematic. Limited worldwide manufacturing capacity, coupled with strong demand from economically buoyant countries like India and China, means that prices are rising fast and delivery times for the largest types are now measured in years. Failure of a major transformer in the supply network can, therefore, give rise to long-term problems that are very difficult and expensive to address. While routine testing cannot completely prevent transformer failures, it is a very effective means of identifying those units that are particularly at risk, thus allowing palliative measures to be put in place. In some cases, for example, it may be possible to take action to improve the condition of the transformer, while in other cases an at-risk unit may be moved to a less arduous duty to extend its useful life. Despite the benefits, there has in the past been some reluctance to put in place programs of routine transformer testing on the basis of the cost and disruption involved. These arguments are no longer valid. In present day conditions, if a testing program prevents one single transformer failure, it will have paid for itself many times over. In addition, modern transformer testing techniques yield accurate results rapidly and with a minimum of disruption. Those utilities that have not already done so have every reason, therefore, to implement routine transformer testing without delay and reap the benefits of doing so. Such a program is an affordable measure that can play a big role in ensuring the reliability and continued resilience of the supply network. To assist with routine testing, Megger offers an extensive range of transformer test equipment. This includes dielectric frequency response analyzers for determining the moisture content in transformer insulation, and sweep frequency response analyzers that can detect electromechanical changes inside transformers. Also offered by Megger are turns ratio test sets, transformer oil analyzers, and transformer ohmmeters. Megger ELECTRICAL TESTER April 2011 3 ELECTRICAL TESTER The industry’s recognised information tool Online testing with first trip analysis Simanand Gandhi Applications Engineer This is testing done on high voltage circuit breakers to evaluate their condition without taking them out of service. Two options are available for online testing: n Capture the first trip test values (explained later) before removing the breaker from service for maintenance works. n Perform a Reliability Centred Maintenance (RCM) task by closing the breaker immediately (allowing for ionisation time) after the trip and make a close and first trip recording using a circuit breaker analyser. Figure 3 Online first trip analysis of HV circuit breaker – typical OC recording Based on the results obtained, an informed decision can be made about whether the breaker can remain in service or whether it should be taken out of service for maintenance. I C: 1 A/div A circuit breaker in service may be in the closed position for months or even years without tripping. In such cases, a circuit breaker analyser is hooked up to the system in live conditions, as shown in the figure. A trip is initiated and the very first trip of the circuit breaker is recorded. This trip recording obtained provides vital information that the classic offline tests done later will not provide. I B: 1 A/div The following online tests can be carried out on site with Megger circuit breaker analysers. Customers can select which of these they need to perform and can then connect and test accordingly. n n n n n n n n Coil current graph recording (three current coils simultaneously) Coil voltage graph recording (three voltage coils simultaneously) Three-phase load current graph recording from CT secondary Motion trace recording of the mechanism movement (for breakers pre-mounted with motion transducers) Auxiliary contact timing Vibration recording Spring charge motor current Ambient temperature measurement The circuit breaker analyser is taken to a live circuit breaker in a switchyard or an indoor location. With extreme care, a fully authorised person connects the test leads as shown in the figure. The circuit breaker analyser can be triggered remotely and this function is used so that the operator can safely trip the breaker from control room. The analyser will start recording with the same trigger. The recorded results should be compared with the previous reference recordings made in the healthy condition. Deviations from the reference values will provide the information about the condition of the circuit breaker. I A: 1 A/div I CIR: 5 A/div V TCMD1: 10 A/div 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0 60.0 65.0 70.0 75.0 Figure 4 Online first-trip analysis of HV circuit breaker – typical first-trip recording n n To save time and resources To obtain critical information from the circuit breaker The mechanism of a circuit breaker that is in continuous service for months is likely to be stiff because of the dried out lubricants sticking to the operating coil plunger mechanisms and levers. Many years of experience has proved that because of this the results of a first-trip test carried out on a circuit breaker that has been closed for a long period are often different from the results provided by subsequent tests on the same breaker. Even though this may not be the case for all circuit breakers in service, the first-trip results still provide valuable and critical information that may be missed by classic off-line preliminary test methods. The lubrication practice and environmental conditions also need to be considered. The results from vibration analysis and coil-current graphs will almost always pinpoint any problems in a circuit breaker. Another big benefit of on-line first-trip testing is the minimisation of outage time. The disturbance caused on the power system because of circuit breaker maintenance work is considerably reduced and hence the cost of testing is also substantially reduced. Whether the breaker can subsequently remain in service or whether further analysis is needed can be decided on the basis of the test results. Some of the problems that can be revealed by online first-trip analysis are: n n n n Figure 1 Online first-trip analysis of HV circuit breaker – typical connection scheme n n n n Sticky trip latch components in the mechanism (revealed by trip coil current graph comparison) Loose connections in the control wiring (revealed by trip/close coil current graph comparison) Delay in trip or close initiation (revealed by auxiliary contact timing measurement) Battery charger/battery/long distance cabling from control room to CB issues (revealed by coil voltage graph) Sources of delay in the tripping or closing processes (pinpointed by vibration testing DTW (Dynamic Time Warping) analysis) Sluggishness in spring or hydraulic or pneumatic operating mechanism (revealed by speed measurement from motion graph) Sticky charging mechanism (first closed revealed by first motor current) Difference in readings over a period revealed by ambient temperature This is the most critical part of first-trip analysis and the part where many power engineers will be puzzled. What is the next step if the values deviate from the last test value and the operation of the breaker seems suspect during the first-trip analysis? Further analysis, such as dynamic resistance measurement, multipoint vibration testing, along with timing, motion, and coil current graph tests, should be done off line in different sequences. These tests will pinpoint the trouble before any maintenance activity is initiated. The circuit-breaker analyser should be capable of performing all these tests as well as first-trip analysis. Figure 2 Online first-trip analysis of HV circuit breaker – wiring scheme 4 Megger ELECTRICAL TESTER April 2011 More than 4,500 Megger circuit breaker analysers are successfully performing circuit breaker maintenance jobs all over the world. They can perform a complete circuit breaker analysis, whereas many products claiming to be circuit breaker analysers provide only some of the tests needed, which often means that more than one instrument has to be used. In spite of their versatility, however, Megger circuit breaker analysers can be configured to suit specific requirements, ensuring that users never pay for features they don’t need. Whether it’s a 6.6 kV breaker, a 132 kV breaker or even a 765 kV breaker, the flexible modular design of the instruments makes it simple to configure them according to the need. www.megger.com ELECTRICAL TESTER The industry’s recognised information tool Magnetic shielding Dr Stan Zurek Magnetics Technical Specialist Shielding plays an important role in many aspects of engineering and technology. It can contribute to measurement accuracy, electromagnetic compatibility (EMC), security of signal transfer or even human safety in high power devices. Of course shielding can be applied to many different forms of energy, but in this article we will focus on a just one – low frequency magnetic shielding as a special case of electromagnetic radiation. The electromagnetic field can be analysed by splitting it roughly into so-called “far field” and “near field”. In the far field region the electromagnetic radiation behaves just like any other wave so similar concepts can be applied there. The wave can be reflected, absorbed, or passed through a medium (Fig. 1). If the “shield” is made out of non-magnetic material (e.g. plastic) then the magnetic field penetrates it fully and the sensitive element “B” is strongly affected by the field (Fig. 2a). However, if the shield is made out of appropriate magnetic material, it creates a low reluctance path shunting the field away from the sensitive area (Fig. 2b). However, we can see that the number of field lines did not change. The field was not destroyed or absorbed – it could only be diverted from one location to another. a) million times lower than the Earth’s magnetic field, which makes the measurements very difficult. Special high performance shielding is therefore required to reduce the ambient magnetic noise. This usually involves multiple layers of thick high permeability material. An example of shield capable of lowering the Earth’s field by around 1000 times is shown in Fig. 3. The magnetically “quietest” place in the world is a unique shielding room in Physikalisch-Technische Bundesanstalt in Berlin, Germany. However, even inside that room, which is used for research in biomagnetism, the residual field is still higher than the magnetic field produced by the human brain. b) Fig. 3. A high performance magnetic shield made from five layers of 10 mm thick mumetal sheets, the drum is around 1 m in diameter Fig. 1. Definition of shielding for far fields However, this sort of analysis fails when applied to magnetic field emanating from 50/60 Hz frequency equipment, and of course from DC currents and sources like the Earth’s magnetic field. Due to low frequency the wavelength is hundreds of kilometres, and for instance the reflection could happen only if the structures were similar in size. Hence, the “near field” effects will be dominating and different approach is required. In the near field the purely inductive effects will dominate. The reflection and absorption can be neglected and the effect of “shielding” has to be achieved in a completely different way. Fortunately, the “near field” effect decays very quickly with the distance, so this type of shielding is required in fewer situations than the far field. The low frequency magnetic shielding is in some way similar to the lightning protection. In the latter the voltages are so high that it is impossible to electrically insulate them. So the only solution is to introduce a low resistance path and re-direct the dangerous bolt of current away from sensitive areas. Magnetic shielding uses an analogous approach. The magnetic field cannot be eliminated or pushed away – it can only be directed away with passive shielding. This is achieved by inserting a suitable low reluctance path (an analogy of low resistance in our example of lightning above), through which the magnetic field is shunted away from the sensitive area or element. The concept is visually demonstrated in Fig. 2. There are two elements: “A” is a wire with current which generates some magnetic field around it, and “B” is the sensitive element we want to protect by putting a shield around it. www.megger.com Fig. 2. Illustration of magnetic shielding: a) element “B” being affected by the magnetic field emanated from “A”, b) high permeability shield directs the field away from the element “B” By doing this, we achieved the desired result – the sensitive element is no longer exposed to the high field. (Actually the field still penetrates the shield, but it is weakened inside, proportionally to magnetic performance of the shield.) However, it is evident that the re-directed energy has some impact on the shield. Some heat will be dissipated in the shield, because it can be magnetised to quite high levels (see the intense colours). If the configuration is such that eddy currents can be induced then this might cause for the shield to reach very high temperatures, which will have an adverse effect on the efficiency of the whole device. For instance, the oil tank of a power transformer is usually made of steel sheet. This offers the required mechanical strength and also some additional magnetic shielding. However, the losses dissipated in the oil tank due to the shielding effect must be taken during the design process in order to avoid overheating. In very sensitive equipment, current transformers (CTs) are sometimes heavily shielded to improve their electromagnetic compatibility or reduce measurement noise. Ideally, the whole CT would need to be shielded, but this cannot be done as it would make a single shorted turn and would render the device useless, and probably also lead to a small explosion of molten metal! The shield must be simply designed to have a controlled gap so as to avoid it acting as a shorted turn. The most sensitive magnetic sensors can detect activity of the human heart or even the brain. These magnetic fields are around In some cases superconductors are used as magnetic shields. Under their “normal” operating conditions (at the temperatures of liquid helium or nitrogen) they behave like perfect diamagnets – the magnetic field cannot penetrate a superconductor. However, this is actually occurring due to the surface currents induced in the superconductors. These currents can flow without losses for infinite length of time and their orientation is such as to oppose any external field. This leads to such interesting effects as magnetic levitation, in which the surface currents are strong enough to suspend a substantial metal body (Fig. 4). So if something is placed “inside” a superconductor then it will be quite well protected from the external fields – this is therefore magnetic shielding in its true meaning. Swiss army knife of testers! Dave Moore Product Manager Electrical engineers and technicians working in the industrial and utility sectors often see multifunction installation testers as instruments that are only of interest to contractors. That’s unfortunate, as the best multifunction testers are not only sufficiently versatile to meet a whole range of day-to-day testing requirements, they are also easy to use, convenient as they combine several testers into one small instrument, and they offer excellent value for money in both initial outlay and reduced cost of ownership in calibration. All modern multifunction testers offer facilities for insulation testing up to 1 kV, continuity testing at 200 mA, earth loop impedance testing and fault current measurement across phases, and also 3-phase RCD testing. Most types also incorporate a convenient voltmeter function. The best instruments, however, offer even more, such as frequency measurement, phase rotation, and the ability to measure the resistance of earth electrodes - functions that are definitely going to be of interest and value to those working in industry and the utilities. The latest multifunction testers are also designed to be small and light, so easy to carry around, while fast and easy to use. They incorporate dual displays – a digital readout that is convenient for results that have to be recorded in test reports, and an analogue arc that allows many types of test to be carried out more quickly and conveniently. The top instruments also have a secondary display that shows additional useful information, such as the actual test voltage when an insulation test is being performed. RCD testing, a common requirement in the industrial sector, is easy to carry out with a multifunction tester, and the latest instruments can be expected to offer facilities for testing type A, AC and S devices. Some now also offer a pass/fail ramp test, which is a big time saver in applications where it is only necessary to show that the RCD trips between pre-set minimum and maximum limits. Safety is, of course, always a vital concern and it is important to remember that transient levels in industrial systems may well be higher than in ordinary domestic and commercial installations. When choosing a multifunction tester for use in industry it is, therefore, essential to ensure that it has a CAT IV 300 V rating in line with IEC 61010. Fig. 4. A magnet levitates above a superconductor (cooled to -200 ºC) However, for our everyday measurement, testing and production equipment the costs of superconductors are prohibitive, so we need to keep using the boring old high-permeability materials like iron and nickel... Finally, it is worth noting that multifunction testers are now available with internal memory for storing results, so that these can be subsequently recalled to the display or downloaded to certification software. This eliminates the need for results to be recorded manually, which not only saves time but also eliminates the all-too-common problem of transcription errors. Clearly, the appeal of multifunction testers is not limited to electrical contractors; they also have much to offer for engineers and technicians working in other sectors. It is, therefore, well worth taking a little time to check out the latest models, such as those in the new MFT1700 range from Megger. Megger ELECTRICAL TESTER April 2011 5 ELECTRICAL TESTER The industry’s recognised information tool The previous part of this article provided an introduction to the stakeless measurement of earth resistance and looked at some applications where this method is particularly suitable. This second and concluding part looks at further applications, potential sources of error, and the benefits of stakeless testing. If you would like to see the first part, please email us at ElectricalTester@megger.com Telephone pedestal electrodes can also be tested using the stakeless method. Cable sheaths are all connected to an earth bar, which in turn is connected to earth electrode. The instrument clamp can be placed around the cable connecting the ground bar to the electrode to perform a test. If access is difficult a temporary extension cable can be fitted to accommodate the clamp. Earth testing without the stake – Part 2 Paul Swinerd Product Manager The stakeless measuring technique is well suited to testing earth electrodes installed within primary cross-connection points, which are sometime called street cabinets or flexibility points (Figure12). These electrodes typically need to have a resistance below 25 Ω. In this application there may only be two parallel earth paths in series with the electrode. However, provided that the stakeless method gives a result below 25 Ω, then the resistance of the electrode itself must certainly be below 25 Ω. Figure 15 - Telephone pedestal Figure 12 – Street cabinet Potential sources of error When applied correctly and a good quality instrument is used, the stakeless method gives very reliable measurements. Nevertheless, it is well to be aware of factors that may introduce errors. Among these are: A very similar application is cable TV street cabinets Figure 13 shows the stakeless technique being used at a remote switching site. In this application, the object of the test is not to measure the earth resistance but to verify earth connections. By recording the test results and trending these over time, it is possible to identify the onset of problems such as corrosion. Figure 13 – remote switching site Cellular sites/microwave and radio towers are another good application for stakeless earth resistance testing. Figure 14 shows a typical four-leg tower. Each leg has been individually earthed and connected to a buried copper ring. As with the remote switching site, this test is used to verify an electrical connection, and is not a true measurement of earth resistance. Switchyard and substation earths are yet another good application for stakeless testing. The method is ideal for checking connections to earth mats, but caution needs to be exercised over possible interference from induced ground currents. Substation and switchyard metal fencing connections to earth mats can be easily checked for continuity using the stakeless method. Stakeless testing is very useful to transformer test engineers. Pad-mounted transformer earths can be readily verified. However sometimes there are a number of connections to the same electrode. In such cases, it may be necessary to clamp around the electrode itself, below the connections. Poor understanding of the circuit under test Remember the two rules of stakeless testing: n There must be a loop resistance to measure. n The earth path must be included in the circuit unless the test is being carried out solely to verify a connection. Dirt trapped in the clamp head n Dirt trapped between the closing faces of the clamp will modify the magnetic circuit. The result will be false low readings and, in some cases, this could result in a poor electrode being measured as good. n Many instruments use interlocking laminations (teeth). These trap dirt and are difficult to clean. They are also easily damaged. Damaged teeth will produce inaccurate measurements and may even render the instrument unusable. Noise currents affecting measurement n In noisy environments, high noise currents may be flowing in the electrode under test. This can cause readings to fluctuate making them difficult to interpret or, if the noise current is too high, it can make measurement impossible. To avoid these problems, clamp-type earth resistance testers with good noise immunity should be used. The benefits of stake-less earth resistance testing n Tests can be carried out without dis connecting the earth electrode from the system. This method is, therefore, safer and less time consuming. n Loop testing includes bonding and grounding connections. Because of this, it identifies poor continuity anywhere in circuit earth current. If an electrode is to be disconnected, the instrument can be used prior to disconnection to measure the current flowing in it, thereby confirming whether or not it is safe to proceed. It is worth noting, however, that the results from stakeless measurements will rarely be the same as those obtained with a three-pole instrument, as the stakeless test is technically a loop resistance measurement. In applications with only one or a small number of return earth paths the measurement may be higher than the expected electrode resistance limit. In this case the stakeless method is still a useful tool to identify changes over time. The benefits of Megger instruments The latest Megger digital clamp-type earth resistance testers, models DET14C and DET24C, offer a number of additional benefits. These include: n n n n n n Elliptical clamp with a slim profile, which facilitates access to earth straps and electrodes in pits. Large clamp capacity, allowing tapes up to 50 mm wide as well as electrodes and cables up to 39 mm in diameter to be easily accommodated. Low maintenance flat jaw faces, with no interlocking teeth that easily become bent and damaged. CATIV 600 V safety rating in line with IEC 61010. This is the highest safety rating currently available for an instrument of this type. Auto-current measurement safety feature that provides an instant warning if current exceeds a user-set limit. Automatic noise filter function that reduces the effect of noise current in electrically noisy environments such as substations. n Figure 14 - Cellular sites/microwave and radio tower 6 Figure 16 – Pad mounted transformers Megger ELECTRICAL TESTER April 2011 There is no need to drive auxiliary test spikes into the ground, so testing can be carried out easily in locations with hard ground or concrete surfaces. There are also timesavings as there is no need to run out test leads. n Clamp-type earth electrode resistance testers can also be used to measure The most common reason given by users for not being able to use the stakeless method is poor access. Often cable or tape sizes are too large for the clamp. Until now 50 mm wide earth tapes could not easily be tested. To circumvent this problem, some users resorted to cutting the tape and welding in a round cable to make their earth clamp testers usable. This time-consuming procedure is not required with the Megger DET14C and DET24C clamps as their elliptical heads can accommodate 50 mm tapes with ease. www.megger.com ELECTRICAL TESTER The industry’s recognised information tool Accuracy built-in manufacturers of the testers quote accuracy levels for their products. For example, a particular tester may be specified as providing Accuracy Level III or even Accuracy Level IV. These levels define the accepted maximum deviation from baseline measurements and therefore guarantee that the required level of accuracy is achieved for the test being performed. The acceptable accuracy limits vary by frequency. To give an example, Level III accuracy is specified by the standards for CAT 6 installations, whereas for CAT 7 installations, Level IV accuracy is required. Note that Level IV accuracy is superior to Level III, so instruments with Level IV accuracy can be used in all applications where Level III accuracy is specified. There is absolutely no need to buy a tester with Level III accuracy for use on CAT 6 systems and another with Level IV accuracy for use on CAT 7 systems, since the higher level of accuracy is always indicative of a closer match to baseline measurements. Bryan Phillips Market Development Manager Anyone who buys test equipment can reasonably expect it to give accurate results but, in this context, what exactly does accuracy mean? Purchasing decisions are often made without examining in detail the specification of an instrument, but it is always worth bearing in mind that the accuracy specified for an instrument has a big impact on its usefulness and, ultimately, its capabilities. When looking for LAN certification equipment, this issue is even more crucial, as the testing of networks is carried out to confirm that those networks will deliver certain specified performance levels. Fortunately, when the network test standards were formulated, much thought was given to accuracy and, as a result, accuracy requirements were incorporated into all published standards. When purchasing a LAN tester, therefore, it’s most definitely worth choosing an instrument that meets or, like the Megger SCT2000, exceeds the requirements for Level IV accuracy, as this guarantees maximum versatility and the best possible future proofing. If you choose such an instrument, you can be certain that, when you carry out network certification, your work has accuracy built in! In order to allow the users of LAN testers to relate the performance of their instruments to the requirements of the standards, the Visit Megger in Hall 12 Stand E37 Georg Halfar Market Communications Manager, Germany Training a la carte! Good training is essential if utility staff are to work safely and efficiently, but releasing staff for training costs money. And that money is not particularly well spent if the training they receive is only partly relevant to their work. If this sounds as if it’s an unlikely scenario, think again! Many training providers offer a regular programme of courses with predetermined The training can be provided either at the content. customer’s premises or at Megger’s comprehenThat sounds fine in principle, but the content sively equipped training centre in Dover. The workshops currently available have been careoften isn’t a very good match for the requirefully tailored to suit the utilities sector and are: ments of those attending. Some may be interested, for example, by the topic that’s • Diagnostic insulation testing covered in the morning but have very • Transformer testing little need to learn about the afternoon topic, Protection testing while for other attendees the situation may be • • Earth testing exactly vice versa. Either way, such a course Cable fault location is not an efficient use of the trainees’ valuable • • Power quality testing time, nor of hard-pressed training budgets. • Introduction to 17th testing • Portable appliance testing So what’s to be done? After extensive • Battery testing discussions with its customers, Megger has developed a new “à la carte” training service All of the workshops are delivered by to solve this problem. experienced engineers, and all have a high level of hands-on practical content. Megger Users of the service simply choose from a will also be pleased to discuss requirements menu of training workshops that can be for custom workshops on subjects not covered delivered in any combination; currently there are nine different workshops to choose from. in the list above. Further details of the training The time needed for each workshop is clearly workshops and how they help users to make the best use of staff time and training budgets shown on the menu, so it is easy to work can be obtained by calling the Megger support out whether the required training can all be delivered in half a day or whether a full day – team on 01304 502101, or by sending an email or even several days, consecutive or separate to tony.wills@megger.com – will be needed. www.megger.com Hannover Messe is the world’s biggest industrial tradeshow. This year, the organisers are expecting 6,000 trade stands and over 200,000 visitors. Hannover Messe acts as an umbrella for 13 international tradeshows, and while exhibitions in other parts of the world have declined, this one seems to grow during periods of recession and growth. Megger’s stand this year will be in hall 12 stand. You will be able to see these new products on the stand and talk with Megger’s technical experts about how they could be used in your business. MOM2 microohmeter DET14c and DET24c earth clamp MIT1020/2 10 kV Diagnostic insulation resistance tester DET3TD earth ground resistance testers BITE3 Battery Impedance test equipment OTS60/80PB Oil test set TT300 series Transformer turn ratio test equipment PFL40 Cable fault location system MCT1605 Multi-Tap Automatic Current Transformer test set FRAX 150 SFRA Frequency spectroscopy analysers We will be delighted to see you – and if you complete and bring along this coupon, you can collect a free Megger hard hat conforming to EN397 from the stand! Name ................................................................................................................................ Position................................................................. Company............................................... Address.............................................................................................................................. Country................................................................ Post Code.............................................. Telephone........................................................................................................................... Email ................................................................................................................................. Megger ELECTRICAL TESTER April 2011 7 ELECTRICAL TESTER The industry’s recognised information tool Q&A In this issue, Megger’s technical support group experts tackle some rather general but surprisingly common questions relating to installation testing in commercial and industrial buildings. Q: In addition to performing tests on power systems, my team and I are often asked to carry out routine tests on ordinary building electrical installations, so what’s the best type of instrument to use? A: Although some engineers who regular carry out installation testing prefer to use separate instruments for each of the main tests involved, the most popular solution is a multifunction tester. This provides, in a single instrument, facilities for all of the tests needed to certify an installation in line with current regulations. Modern multifunction testers are compact, dependable and easy to use. Q: Do these testers have any facilities for working on three-phase systems? A: Most of them don’t, but there are a few exceptions. Some of the latest, for example, Safety in Testing – Part 3 Jeff Jowett Applications Engineer Parts 1 of this article, which dealt with general considerations, CAT ratings and safety in insulation testing, and Part 2, which dealt with test leads and multifunction testers, appeared in previous issues of Electrical Tester. If you missed these parts, email us at ElectricalTester@megger. com. This third and final part considers safety in relation to ground testers, and the particular safety hazards associated with high-voltage testing. Ground testers don’t get a lot of attention in safety discussions because grounds tend to be out of sight, out of mind. But grounding electrodes can come on line and can also carry surprising levels of constant current. Older testers produced higher voltages and currents to allow accurate measurements to be made in widely varying soil types. Geophysical models still use high voltages and currents, but for normal tests on electrical grounding, microprocessor sensitivity has permitted test voltages and current to be brought down to much safer levels. Electrical systems can now also be tested while on line without interference. But danger to the operator can occur if the system being tested comes on line, as it might during a fault when the tester and its leads could become part of the clearance path. The best protection resides with safe working practices, such as the use of insulated boots, gloves and mats, as defined in standards such as those of the IEC. It is also a good idea to check for current before connecting to test. A clamp-on ammeter is a simple and effective way to do this, and modern ground testers sometimes come with built-in current clamps. The more an instrument eliminates opportunities for error, the better the safety program will perform. As an example, the greatest safety challenge comes in high-voltage maintenance, where hotline poles and voltage detectors must first be employed to determine that the test item is in fact de-energized. The detectors indicate voltage by visual and audible signals. They should be ready to operate at all times, can check phase rotation and have facilities for testing three-phase RCDs in installations where a neutral is not available. When working on three-phase systems or indeed on single-phase systems close to the point where the supply enters the building, the CAT rating of the instrument needs to be given careful attention. In general an instrument rated CAT IV 300 V or better should be chosen, as this is safe to use on all parts of low voltage installations. Q: Apart from the features routinely needed for installation testing, do multifunction instruments typically offer an “extras” that might be useful? A: Almost all will offer facilities for measuring voltages and some for checking supply frequency. The latest models, however, like Megger’s new MFT1730, can also be used to with no on/off switches that could be forgotten. Industry practice is to first test the tester, so that an inoperative unit won’t be mistaken for a lack of voltage on the circuit under test. A built-in test circuit facilitates this life-saving check. A green light should then remain on to indicate that the unit has been checked. This protects the operator from picking up the wrong unit after, say, putting the tested one down to put on gloves. The test is performed on the supposedly de-energized circuit, and then the tester is tested again. This prevents a unit that was damaged by the performance of the test from being returned to stores. This is the kind of redundancy that is necessary for assured safety. Safe working distances must also be maintained when approaching energized lines to prevent arcing to the worker, and the indicators may have collapsible poles with markings for safe distances from different voltages across the range that the indicator covers. These poles also must be regularly maintained against cracks, dirt and corrosive materials that could facilitate current flow along the pole. In conclusion it is important to note that no matter how good, the instrument’s safety features are only ever a second line of defence. A well-trained and alert operator is the most important part of electrical safety, with a well-designed instrument providing the necessary redundancy. Pylons in dresses! Before Elena’s ideas could be translated into reality, however, eighteen months of study were needed for, among other things, technical validation of the proposed methods for transforming the towers and the selection of suitable materials that would ensure the sustainability and resistance to degradation of the installations. 8 The past of PowerDB Mark Meyer Product Manager In our regular series of articles looking at the histories of the many companies that form the core of today’s Megger family, the focus this time falls on PowerDB. Although PowerDB is one of the youngest family members, its influence is having a big impact on a wide range of Megger products, particularly those aimed at users in the power sector. PowerDB has its roots in Optima Systems, a company started in 1994 by Bruce Buxkemper who had previously worked for Megger in Dallas as a software developer. Located in College Station, Texas and with strong connections to the Texas A&M University, Optima Systems initially focus ed on developing automation systems and software for industry. In the late 1990s, the company won a contract from Megger to develop the company’s AVTS software, a powerful yet easy to use package that complements Megger protection relay test sets. By the turn of the century, Optima Systems was also hard at work on an important project of its own – the development of PowerDB, a new software system that would provide a common user interface and uniform results handling facilities across a wide range of power instruments. This groundbreaking concept generated widespread interest and PowerDB was launched in the USA to considerable acclaim in 2002. For many engineers, the transmission towers that span the countryside are works of art in their own right, but for installation artist Elena Paroucheva, who lives and works in France, these towers were just the beginning. In her work, Elena searches for connections between art, energy and the environment, and she saw that transmission towers provided her with, in effect, a blank canvas that she could use to develop and display these ideas. Accordingly, she entered into discussions with RTE, the organisation responsible for the French electricity transmission network, and after two years of negotiation, she was granted permission to transform the appearance of four high-voltage transmission towers on the 225 kV line between the French cities of Amnéville and Montois. RTE also sponsored and supervised her work. measure earth electrode resistance. This is an increasingly common requirement in photovoltaic and other types of “green” energy systems and, indeed, in power installations. Another useful feature provided by some multifunction instruments is internal results storage, with facilities for Bluetooth wireless download to a laptop computer or even a smartphone. The best units of this type are complemented by software that automatically fills in the test certificates as the results are received by the laptop or smartphone. This saves a lot of time as well as eliminating the errors that are almost inevitable when the certificates are filled out by hand. Examples of Elena Paroucheva’s work Ultimately Elena Paroucheva started work on her metamorphosis of the transmission towers in August 2001 and completed it by December 2003. The project was no minor undertaking – in total it used 3,284 m of steel cable, 2,708 m of stays, 525 m of plastic canvas, 576 m of steel tube and 40 light projectors that are controlled via a satellite communications link. For this project, which has been much admired by the public in general and by the art world, Elena has received widespread recognition, including most recently the laureate award for Science and Technology in European Art in a competition organised by SUPELEC within the framework of the French Presidency of the European Union. The title of the project is “Source – A Monumental Artwork” and the themes for the four transmission towers are “Source – Light”, “Source – Water”, “Source – Energy” and “Source – Flame”. As the pictures accompanying this article show, the results she has achieved are both inspirational and impressive. Much more information about Elena Paroucheva’s unique “Source” project can be found on her website at www.art-elena.com, and there can certainly be no doubt that she has made her own rather special contribution to the art of power transmission! Megger ELECTRICAL TESTER April 2011 In 2005, Optima Systems was purchased by Megger, an arrangement that has made it possible for the PowerDB system to be sold and supported in markets worldwide, and also provided access to the resources needed for its continuing development. Following its purchase by Megger, Optima Systems became PowerDB Inc., a Megger subsidiary. From its early days as a one-man operation, PowerDB has now grown to employ many more fulltime staff and continues its partnership with Texas A&M University by regularly giving experience to interns. Its primary focus is on the PowerDB system, which is now available in a range of implementations including an on-board version designed for total integration with instruments. The company continues, however, to develop other innovative hardware and software solutions for industry. www.megger.com