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Published by Megger
Aug 2010 |
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ELECTRICAL
TESTER |
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In this issue |
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The SMRT way to test relays |
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Earth leakage tests on Portable Appliances |
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Q&A: Oil Testing |
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Megger makes beautiful music |
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The SMRT way to test relays |
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| Stan Thompson |
| Product Manager |
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| With the challenges of testing the smart grid just around the corner, and IEC 61850 networking starting to transform the way substations operate, is now really a good time to be thinking about buying a protective relay test set? Surprisingly perhaps, the answer is yes – provided that it’s a smart test set that has been designed with future developments in mind. |
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| The reasons for investing now rather than waiting are simple: there’s a new generation of test sets that makes testing easier, faster and more convenient. And, in today’s high pressure world where time and downtime are everything, which protection engineer doesn’t need these benefits now rather than later? |
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| Let’s take a look at what makes these revolutionary new test sets so attractive. |
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| First of all, they’re light and small. To put this in perspective, it’s not so very long ago that a complete three-phase test set with modest output power weighed in at around 150 kg, and even today, most of the instruments on sale tip the scale at around 25 kg. The new test sets halve this to 12 kg and they are also smaller than their predecessors, making them much easier and more convenient to transport and handle. |
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| Size isn’t everything, however, although it’s undeniably important! Protection engineers also need versatility. For testing three phase schemes, three current outputs plus three voltage outputs are the minimum requirement, but if numerical current differential relays are to be tested, six current channels are needed. The most convenient and economical way to provide these is to arrange for the three “voltage” channels to be convertible, so that they can be used either for current or voltage, as required. |
| With their requirements for size and versatility addressed, protection engineers will undoubtedly turn their thoughts to power. For convenient testing, current amplifiers with a constant power output are highly desirable, and it needs to be a high constant power to allow the testing of protection schemes that use electromechanical relays – we may be looking to the future, but legacy equipment is still going to be with us for a very long time. |
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| Fortunately, despite their small size and weight, the new generation test sets have no problems in the power department. They can offer a full 200 VA up to 30 A, with a compliance voltage of 50 V at up to 4 A. They even make provision for current outputs to be series connected to double the compliance voltage to 100 V and provide a constant 400 VA output power. |
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| Of course, power is also important for the voltage outputs and the designers of the new generation test sets have taken this into account. From 30 V to 150 V, the voltage amplifiers can deliver a constant 150 VA, thereby providing high current at “difficult” low test voltages. |
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| Continuous power rating is one thing, but protective relay test sets also need generous short time ratings. |
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| The secret of testing the SMART GRID revealed! |
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| The benchmark of performance for an amplifier with a 30 A continuous rating is a short-time rating of 60 A, plus the ability to supply up to 180 A at high power for instantaneous overcurrent test applications. |
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| So much for the brawn, but what about the brain? After all, as every protection engineer knows, power is nothing without control. Let’s start with the user interface, as this is what makes the difference between a test set that’s a pleasure to use, and one that’s viewed with dread and foreboding. |
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| The gold standard in this area is set by the latest touch-screen interfaces that provide a simple way of testing even the most complex relays. They allow users to perform manual, steady-state and dynamic tests quickly and easily, and they have built-in preset test routines for popular relays. Naturally, the new generation test sets also make provision for automatic testing, using powerful yet intuitive software running on a PC. |
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| It’s clear that the new generation relay test sets will handle all of today’s demands with ease and convenience, but what of the future? Relay test sets are no trivial investment, and users rightly expect them to retain their value and usefulness for many years. |
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| The best of the new test sets fully address this requirement. Their logic systems feature high-power processors to take care of future requirements, and their functionality can be readily enhanced to meet changing requirements by means of firmware upgrades. |
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| But what of IEC 61850? As might be expected, new generation test sets are IEC 61850 ready. That is, they can be supplied now without integrated IEC 61850 functionality, to eliminate the need for users to pay for features they don’t currently require, but they can be upgraded easily and economically to provide full IEC 61850 support as soon as the user has a need for it. |
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| Hopefully, by now these new protection relay test sets are starting to sound like a very attractive investment, but how can you get hold of one? The answer is simple – contact Megger. All of the characteristics and benefits discussed in this article are embodied in the new SMRT 36 test sets and, if you would like one, they’re ready for delivery right now! |
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Earth leakage tests on Portable Appliances |
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| Throughout the UK and Europe, appliance testing is becoming more common to ensure electrical equipment is safe to use. Tests that are usually performed include a visual inspection, earth continuity or earth bond test and an insulation resistance test. Also an operations test and an earth leakage test are often performed to gain further information. |
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| Earth leakage measurements are performed when the equipment is running in its normal operating mode. For appliances that have different settings, e.g. hair dryers, the appliance should be set to its highest setting and be switched on. Earth leakage tests are often performed in place of an insulation test if there is doubt an insulation test could damage the equipment under test. |
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| There are various earth leakage tests that can be performed: |
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| Differential Earth Leakage Tests (IDIFF) |
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| This measures the difference in current between the live and neutral conductors. The difference is displayed as the leakage current. The measured value is adjusted to reflect the worst leakage current at the upper operating voltage limit. |
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| Touch Current Test (IF) |
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| Where no earth return path exists, (Class II) one has to be provided to simulate the equipment being held in the hand. The measured value is adjusted to reflect the worst leakage current at the upper operating voltage limit. |
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| Substitute Leakage Test (IPE) |
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| This measures the leakage current in the earth conductor using a low AC voltage (typically 40 Vac). |
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| This reduces the risk of electric shock and prevents the equipment from running during the test, where this would otherwise be considered dangerous. The test socket is optional since this test is independent of the supply voltage. The measured value is adjusted to reflect the worst leakage current at the upper operating voltage limit. |
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| Megger manufactures a range of PAT testers that have the ability to test earth leakage, along with earth bond, insulation, operation and flash tests. Visit www.megger.com for more information. |
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Q&A: Oil Testing |
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| In this issue, experts from the Megger Technical Support Group supply answers to the questions they most frequently receive about the testing of insulating oils. |
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| Q: Sometimes when I’m carrying out breakdown tests on a sample of oil, the results I obtain are inconsistent. Why is this and what should I do about it? |
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| A: Some of the standards for testing insulating oils recommend that if the standard deviation divided by the mean for the test results is greater than 0.1, the test should be repeated and the mean taken of all the results. There are, however, steps that can be taken to help reduce the problem of inconsistent results. Variable results are often caused by particles in the oil lining up between the electrodes. To minimise this effect, the oil should be left to settle for a few minutes before it is tested. Another problem occurs when an oil sample is tested several times – carbon particles may be formed by the successive breakdowns, and these will lead to lower test results. Megger OTS oil test sets have been designed to limit energy dissipated in the breakdown spark to reduce this from happening. Finally, if the oil sample is left in the test chamber for too long, it is likely that it will become warmer and drier, leading to higher test results. |
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| Q: Why is it that, even with new oil of known quality, the test results I obtain are often suspiciously low? |
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A: Problems of this kind usually relate to sampling technique and/or to the cleanliness of the equipment. It is very important to ensure that the sample bottles and all of the other items of equipment that come into contact with the oil are kept dry and scrupulously clean. In particular, it is recommended that the following procedures should be employed for cleaning, storing and using test vessels:
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 When not in use, vessels should be store full of clean, dry oil. Vessels should be rinsed with the oil to be tested before they are filled with the test sample |
| If the oil sample is found to have a lower than expected dielectric strength, the vessel should be cleaned with a suitable solvent |
| In exceptional circumstances, it may be necessary to heat the empty vessel to re move all moisture |
| Always use clean lint-free room wipes to clean the vessel |
| DO NOT use paper towels – they leave behind particles of paper that soak up moisture causing breakdown values to be dramatically lowered. |
| In addition, it’s important to avoid touching the electrodes or the inside of the vessel. Electrodes should also be checked regularly for pitting or scratches that may cause oil breakdown values to be reduced. Remember that the electrode gap is also critical to accuracy. |
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| Q: Why are the results I get often not comparable when I use various different instruments to carry out tests in line with the ASTM D877 standard? |
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| A: ASTM D877 specifies that testing should be carried out using electrodes that have sharp (right angled) edges and the radius of the electrode edge has a big effect on breakdown value. Some oil test set manufacturers supply, as standard, electrodes that have radiused edges, and these give results that are not comparable with results produced by a test set with sharp-edged electrodes. Most test set manufacturers can, however, supply sharp-edged electrodes on request, as an aid to comparative testing. |
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| If you have any questions that you would like to ask Megger’s TSG, please give them a call on +44 (0) 1304 502102 or email
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Megger makes beautiful music |
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| Megger is certainly well known for instruments, but it’s instruments of a rather different kind that come to the fore when MELON is in town! That’s because MELON is the Megger Electric Light Orchestra – OK, the N doesn’t actually stand for anything, but it does provide the group with a catchy name and a convenient logo! |
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| Based at the Megger site in Taby, Sweden, MELON is a group of Megger employees with a shared interest in music, and is in great demand to perform at sales conferences and other special events. Specialising mostly in popular rock music, though occasionally writing and performing its own songs, MELON has now been in the vanguard of the test gear music scene for over 20 years. |
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| “There is a lot of interest in music at the Taby site,” said MELON lead vocalist Stefan Bornholm, “and, in fact, there are several professional musicians working there. It’s only natural that some of us should get together to form our own group to entertain our colleagues and customers. We toil on test equipment by day, but by night we make beautiful music together!” |
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| The present day line up is Matz Öhlen, Magnus Thieldgard, Stefan Bornholm, Peter Fagerström, Klas Petterson and Conny Edholm. Of these, Peter Fagerström and Conny |
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| Edholm are founder members who still retain their original enthusiasm for Megger music making. |
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| Megger may not yet be quite as well known for its musical output as it is for its test equipment but, given the enthusiasm of the members of MELON, it is perhaps, worth remembering that the world is still waiting for Sweden to produce a worthy successor to ABBA … |
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News 