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April 2017
Q and A: Protective relay testing

Q and A: Protective relay testing

18 April 2017

Testing protection relays and protection schemes used in power distribution systems continues to be a popular theme for questions received by our technical support team. We make no apology therefore, for presenting some more questions and answers on this theme to supplement those that appeared in the January issue of Electrical Tester.

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.

Q:    I need to test under-voltage protection on a relay with two different set levels: Stage 1 at 87% of Un with a delay of 1.5 seconds, and Stage 2 at 80% of Un with a delay of 0.5 seconds. The trip is marshalled to the same relay output and it is not permissible to modify the set values or the timing of either of the stages. Bearing this in mind, can you recommend a way of accurately determining the pickup of the second stage?

A:    As your question implies, testing Stage 1 is indeed straightforward, as pickup and timing tests for this stage will not initiate Stage 2. But verifying the 80% pickup of stage use using a continuous voltage ramp will take longer than 1.5 seconds, so unwanted tripping of Stage 1 will occur, making the test result invalid. This problem can be avoided by using the pulse ramp feature offered by the RTMS software. Set the software so that pulses are applied for 0.7 seconds with the voltage returning to its nominal value for 1 second between each pulse (or for whatever period is long enough to reset the Stage 1 under-voltage protection.) In this way, you can accurately determine the Stage 2 pickup voltage without having to change any of the relay settings.

Tags: Protective, relay