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Electrical Tester Magazine - March 2020
Explore our first issue of ET magazine, online. Inside you’ll find our feature article that discusses the coordinated efforts that will be required to successfully merge the electrified transportation sector with the electric utility sector, sharing the latest thinking, research and challenges ahead. Transformer testers/engineers will appreciate learning how several variables may unknowingly affect your TTR test results. A bird’s-eye view of the rise of transformer electrical, dielectric testing provides an overview of the tests necessary to fully assess a transformer’s dielectric health. As a necessary test to detect inter-turn insulation breakdown – one of the most common early initiators of motor faults, Motor insulation test voltages: How high is too high? dispels myths surrounding surge testing. The missing link in motor condition monitoring makes the case for electrical tests as vital requirements for achieving plant reliability – not just tests that detect mechanical problems. Andrea Bonetti explains ROCOF protection and how to test it. He also fields some of the most frequent questions he gets asked about IEC61850 in ET’s Q&A section. DC testing of vacuum integrity in MV switchgear explains the advantage of using DC over AC to test vacuum and the importance of the ‘DC’ your test instrument delivers. Marius Averitai discusses the choice of using primary or secondary injection to test a relay protection system; why, in many cases, primary injection is better; and important features in a primary injection test set. Read about a more effective approach for detecting ground/earth faults in TT distribution systems and more…
Published: 17 October 2020
Seven Tips for Insulation Resistance Testing
Seven tips for insulation resistance testingPaul Swinerd - Product portfolio manager - Power Insulation testing at voltages above 1 kV can be a quick and convenient way of gathering a lot of useful information about the condition of electrical equipment. To stay safe and to get the best results, however, it is important that the testing is carried out correctly. These tips should help, but remember that it’s always essential to follow the manufacturer’s instructions for the test set that’s being used, to abide by the relevant standards and to follow good working practices. 1. Use the right test leads.Manufacturers of insulation resistance testers go to a lot of trouble to produce test lead sets that will help make their instruments safe and convenient to use. Always use a lead set that’s designed for the instrument, appropriate for the test voltage you’re planning to use and suitable for the test object you’re working on. If the connections can’t be made securely, the test lead may become accidentally disconnected, leaving the test object charged to a dangerously high voltage. Never ever use test leads that show any sign of damage and never attempt to repair damaged or worn leads – replacing them is the only safe option. 2. Choose the best test voltage. Test sets are now available that will allow testing to be carried out at voltages up to 15 kV. Testing at higher voltages can yield additional and more useful information about the condition of the test object’s insulation, but using a voltage that’s too high for a particular test object to withstand can seriously damage it. Always refer to the supplier’s data for the object under test and follow the guidance it contains on testing. If this isn’t possible, seek help from the manufacturer of your insulation tester. 3. Choose the right test. A quick one-off insulation resistance measurement can sometimes provide useful data, but modern insulation resistance test sets have much more to offer. Typically, they offer facilities for polarisation index (PI), dielectric absorption ratio (DAR), dielectric displacement (DD), step voltage (SV) and ramp tests. Full information on these tests and how to carry them out should be in your instrument’s handbook – if it isn’t consult the manufacturer. Some of these more advanced tests take a little longer to perform but, with many kinds of test object, they can provide much more reliable information about insulation condition. 4. Use an instrument with a high measuring range.If your instrument shows all results above, say, 1 TΩ as infinity, you’ve got no way of knowing that the insulation resistance of your test object has fallen from 30 TΩ to 2 TΩ since the last time you tested it. This latest result might still fall within the range that’s considered acceptable for the test object, but a big fall in resistance value like this is often a valuable early warning that a problem is developing. An instrument with a high measuring range will alert you to this situation. 5. Complete the test before disconnecting the test set. Test objects can hold a lot of charge and, particularly when they’re being tested at high voltages, the stored charge can be lethal. Modern testers guard against this problem by safely discharging the test object when the test has run to completion or when it is terminated by the user. If the test leads are disconnected prematurely, however, the discharge function can’t operate, and the test object will remain charged – and dangerous. 6. Use the guard terminal.Surface leakage over test objects like bushings can greatly reduce their apparent insulation resistance and, as a result, there have been many cases of insulators being scrapped when all that was really necessary was to clean them. Using the test set’s guard terminal – which is usually connected to a bare wire wrapped around the surface of the object under test – eliminates or at least greatly reduces the effect of surface leakage on the test results. And don’t forget that making two measurements, one with the guard terminal connected and one without, can provide a very good indication of whether or not the insulator needs cleaning. 7. Record and trend your results.A single insulation resistance measurement can give you a quick indication of insulation condition, but a series of measurements over a period of time, with the results recorded and trended, will tell you a lot more. If the insulation resistance of your test object is declining over time, for example, it’s probably a good idea to find out why, well before it declines to the point of failure. Accurate records will also quickly show up any sudden deviation from the usual insulation resistance values, which is always a strong indication that further investigation is necessary. For additional information that will help you to get the best from the time and money you invest in high-voltage insulation testing, download the free application guide to insulation testing above 1 kV.
Published: 2 April 2020
Electrical Tester Magazine - October 2020
Check out our latest ET magazine online. Inside you’ll find an introduction to energy storage and 3 articles on partial discharge (PD) topics including: an interview of Dr Detlev Gross and Markus Soeller, two acknowledged experts in this field; excitation voltages for PD testing of cables, and PD testing for transformers. Asset managers will want to read On-line testing of circuit breakers, which illustrates a pragmatic approach to efficiently surveying a circuit breaker fleet using first-trip and VDS testing. If protection schemes are among your areas of interest, you will appreciate the article on end-to-end testing. And those who know how challenging GIS testing can be will benefit from Boel Ekberg’s answers to typical questions she is asked. Transformer enthusiasts will find The high stakes of ignoring the mechanical health integrity of a transformer very informative and an article about dynamic testing of on-load tap changers useful in understanding the several variants of this test. You will also find inside an article about instantaneous torque in motors, which includes three case studies that will convince you of the value of this test. Another great case study is presented in A toolbox approach to fault location in LV power cables and those of you who work in industry won’t want to miss Maximising security of supply at Höchst Industrial Park. Did you know that HVDC transmission was around in 1904!? Open, enjoy and don’t forget to submit your answer to Megger’s third ‘Geek Challenge’ for a chance to win a prize.
Published: 18 March 2020