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Help with 27/47/59N Protection in an SEL-387E

A reader has requested some help for an upcoming project:

I will be reviewing some relay testing that needs to performed shortly and I am trying to understand how the 27/47/59N (within a SEL 387E) relay function is being applied. The 387E provides the 87T function for the 115KV to 35Kv transformer. In addition, the 387E receives line potential from CCVT’s on the 115KV primary side, for the 27/47/59N function. The 35KV side of the transformer has an LTC and a 35KV secondary side breaker. The 27/47/59N function trips the 35KV secondary breaker only, yet they describe it as 115KV “line protection”. Could it be it is used as “LTC runaway” protection, or protecting the 35KV side from 115KV UV, OV or Volt Unbalance, or even preventing closure of the 35KV secondary breaker into a backfed transformer if the 115KV outside of the acceptable 27/47/59N “window”??

I don’t see a 27/47/59N function in my SEL-387 bulletin, nor have I seen anyone use those elements in a transformer relay.

Can someone help?

About the Author Chris Werstiuk

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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