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January 2017
Q and A: Protective relay testing

Q and A: Protective relay testing

01 January 2017

Testing the protection relays and protection schemes used in power   distribution systems is a specialist field where, for the most part, well-trained and experienced engineers have a full understanding of the test techniques they’re using. This probably explains why many of the questions we receive about our relay test products relate to the test sets themselves, rather than how they are used. Here are a few of the questions we’re most frequently asked.

Q: I work for a contracting company that gets involved with projects all over the world, which means that we encounter many different types of mains supplies. Therefore, when we buy test equipment we need to be sure that we can power it from the local supply whatever its voltage and frequency. Are there any relay test sets that will give us this flexibility?

A: This is no problem at all for Megger protection relay test sets. Instruments in both of our main ranges – the SVERKER900 series and the SMRT series – will operate without limitations from supplies anywhere between 100 and 240 V, with frequencies of 50 or 60 Hz. This caters to just about any public supply you’re likely to find anywhere in the world!

Q: From time to time, my company carries out work for shipyards and, in some cases, we come across electrical distribution systems that operate at 400 Hz. Can the protection relays used in these systems be tested successfully with a standard relay test set?

A: As you say, 400 Hz power systems are sometimes encountered in marine applications, and far more frequently, in fact, in the aeronautical industry, simply because transformers and other components operating at this frequency are smaller and lighter than their 50/60 Hz counterparts. As to whether it is possible to test 400 Hz protection relays with a standard test set, the answer depends on what you mean by a “standard test set”. All of the current Megger relay test sets offer this option, but that doesn’t necessarily mean that test sets from other manufacturers will also offer this functionality, so it pays to check carefully. Just for the record, test sets in the Megger SVERKER900 range can produce voltages and currents from DC to 600 Hz, while those in the SMRT range go even further, with the capability of producing voltages and current from DC to 1,000 Hz.

Q: Choosing the best relay test set to buy is a really confusing task! Why are there so many different options on offer?

A: The real answer is so that users can buy a test set that’s a good match for their requirements and budget. Relay test sets represent a substantial investment, so there’s little point in spending more than you need on buying one that has many facilities you’ll never use. On the other hand, buying a test set that seems inexpensive but can’t readily meet all of your requirements is false economy. You’ll either end up replacing it or having to use time consuming and uncertain workarounds to carry out your tests. That’s why Megger offers such a wide range of options. Consider three-phase testing, for example. Most applications require three voltages and three currents, and this is enough to simulate the majority of three-phase systems. But sometimes a fourth voltage is needed to test, for example, the synchro-check function in complex relays. And for testing transformer differential protection, six currents are needed! Megger offers test sets to cover all these applications, including the versatile SMRT46, which the user can configure to supply four voltages and three currents, or alternatively six currents.

Tags: answer, distribution, power, Protective, question, relay., system, test, testing