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News Article

03 March 2017

Q and A Protective relay Testing

Testing the protection relays & protection schemes used in power distribution systems is a specialist field. Find out some most frequently asked questions from Megger

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 the 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:    As a contractor who offers protection relay testing services, I have customers with a whole range of different test procedures and reporting requirements.     How easy is it for me to customise the standard test procedures programmed into Megger instruments so that I can meet these requirements?
A:    Real-time Test and Management Software – which is usually known by the acronym RTMS – is the software application at the core of Megger protection relay testing products. This software allows you to build customised test procedures easily and quickly. By using the generic test tools provided and adjusting the test report sequence you can set up tests and reports that will accurately match the needs of even your most demanding customers.

Q:    Testing very high set values of overcurrent protection can be challenging. How can I do this safely, without risk of damage to the relay?
A: 
   It’s true that performing a measurement of the pickup value by continuously injecting a high test current could thermally overload the relay, as most overcurrent relays must not have currents greater than ten times their nominal rating applied for longer than one second. With test sets in our SMRT family, however, you can overcome this test time limitation by using the pulse ramp feature. This produces a test current in the form of successive short pulses that increase in amplitude with each pulse, thereby keeping the relay within acceptable thermal duty cycle limits.

 

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