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SEL-451 and Frequency Testing Issue

A customer was having problems timing a SEL-451 with a delay setting of 8 cycles. He was measuring a 12 cycle delay when transitioning from 59.5 to 58.5 Hz.

If you look at the spec sheet, the SEL-451 frequency element has a “Maximum Pickup/Dropout Time: 3.0 cycles”. When you add up all of the delays, you are looking at a possible 3.87 cycle delay when testing this relay. Remember to watch the specifications when performing high-speed tests!

The specification also has a “Slew Rate: 15 Hz/s maximum” which means the frequency calculation will only be accurate if the frequency changes less than 15 Hz in a second. An instantaneous transition from 59.5 – 58.5 exceeds that setting which probably caused some problems as well.

SEL-451 Underfrequency and OverFrequency Testing Specifications  SEL-451 Nominal Frequency Rating Specification for Frequency Testing

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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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