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SEL-351 Negative Sequence Overcurrent Testing Problems

I am attempting to test the Negative Sequence Overcurrent on a 351 and 351S with an Omicron test set and my test plans just don’t seem to be working very well. I have managed to get a one shot to work but not an automated plan any suggestions?

gcerniglia
Negative Sequence Formulas

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About the Author

Chris is an Electrical Engineering Technologist, a Journeyman Power System Electrician, and a Professional Engineer. He is also the Author of The Relay Testing Handbook series and founder of Valence Electrical Training Services. You can find out more about Chris here.

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  1. Going to have to take a stab in the dark at this one as your info is a little limited. As long as your curve is correct and you have set up your object and hardware properly there should be very little issue to using the omicron overcurrent module to test this feature of the relay. If your pickup is correct but your timing is useless after the first shot the likely problem is that you have the electromechanical reset feature turned on in the relay, turn this off and the timing woes should go away.

    The biggest thing to remember between SEL and Omicron is SEL sets their pickup in secondary amps and Omicron sets its pickup in per unit amps, if this relationship is screwed up at the start nothing will seem to work for you.

    Best of luck.

  2. SEL Negative sequence is also usually described as 3I2 which is three times the negative sequence current, or don’t divide by three in the negative sequence formula.

  3. I ran into this problem myself with an SEL-751 and an Omicron CMC356 / protection test library test plan. The problem seems to be that the Omicron is misinterpreting the SEL setpoint as I2 when it is really 3I2, leading to the Omicron putting out 3x the necessary current. There didn’t seem to be a setting to correct this, or any other way to manually change the overcurrent graph, so I disabled the built-in overcurrent module and tested pickup and timing via ramping and state sequencer modules. I kept the built-in overcurrent module in the test plan for determining the test points and times.

    1. Thanks for the comment. SEL using 3I2 instead of I2 is a very confusing application for many and is why I recommend you always contact the design engineer and ask them for a simulation that should only trip the I2 protection to ensure everyone is comparing apples to apples. If they program negative-sequence overcurrent thinking that I2 = I2 instead of 3xI2, then you will not be able to find it by testing the relay to the settings. The relay will always do what it is programmed to do.

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