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Thread: 87 Differential Relay

  1. #1
    Join Date
    Aug 2006

    87 Differential Relay


    I am working on a project where an 87 Differential Relay is being used to protect a substation transformer. From looking at the single-line, it seems that CT's are being used to monitor the current on the primary and secondary side of the transformer, and feeding this information into the Differential Relay.

    This is the first time I have dealt with such a relay, so I was wondering if anybody out there could tell me how a differential relay works or what its purpose is. If someone could point me to any additional information on the web concerning this topic, that would be great.

    Thanks a lot for the help


  2. #2
    Join Date
    Feb 2003
    New York, 40.7514,-73.9925
    Current going in equals current going out. If the difference between the two is more than the setting, then trip the upstream protective device.

  3. #3
    Join Date
    Mar 2005
    South East PA

    Schwietzer is one of the companies that make protective relays. As you look into it you will find that there are many protective element setings available, not just 87.

  4. #4
    Diff relays work to protect the zone that is in between the two sources of current that the relay see. Most schemes that we have go from the high side breaker bushing c.t.s to the low side bushing c.t.s, so the diff protects everything in the yard. It takes our high side currents, 69kv or 161kv, and compares them to the low side, 13kv, and if they don't total out the diff takes the high side breaker out thus telling you you have a fault in the station. There are many different schemes dependant on the equipment you have in the station. We have some that go from transformer c.t.s to low side breaker c.t.s and even in a few stations have some bus mounted c.t.s that feed the diff.

    Schwietzer is one of the best on the market, they make a relay, comm module, or control for just about any imaginable piece of equipment you can come up with. Matter of fact I am going tomorrow to a SEL training class on disturbance analysis that is going to be a very good learning experience for me.

  5. #5
    Join Date
    Aug 2004
    Plainfield, IN
    On a side note, the 87 is part of an ANSI list of industry standard numbers to identify diffferent types of protection, i.e phase relays, ground fault relays, differential relay, etc.

    I have mostly seen differential relays used for substation transformer protection; connected between the high side and the low side.

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