# 15 kv bus differential relay



## EC2253 (Mar 7, 2008)

We do some medium voltage work but this is new to me.
What is a differential relay, what is involved in replacing one?


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## Lone Crapshooter (Nov 8, 2008)

They look at all of the power coming in and all of power leaving and as long as they are the same they don't trip.
If you are extremely sensitive to vibration so be extremely carefully when working around them.
There should be a bank of Fliex I test switches to isolate the 86 switch to keep 86 from tripping when working with BDR's.


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## Tonedeaf (Nov 26, 2012)

If its mechanical type i would get the same model exactly so it fits in the same case...I personally read all the manufactures directions, they are a tough read so if you don't have a Protective relay background it like reading greek.

If you are unfamiliar with Protective Relays Theory and Application, I would find a HV testing firm that does relay testing(some firms are just blow and go so you will need to find one that has a relay guy.)


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## Big John (May 23, 2010)

EC2253 said:


> ...What is a differential relay, what is involved in replacing one?


 It's a much more sophisticated version of a GFCI: It has two sets of inputs for current sensors, and the area between where the current sensors are located is the zone that the relay will protect. It looks for a delta between those two inputs and if the delta exceeds the set values then the relay knows that there is a fault shunting current along an unintended path, and it will trip which isolates the zone of protection.

As far as replacing them, if it's an old electromechanical then you can find a rebuilt version and just swap it into the existing case. It would take 2 minutes. If it's a multifunction or electronic relay it's gonna have to be unwired and rewired. The big thing there is making sure all your sensor polarities get connected properly or the relay will see the subtractive CT as differential. 

Either way, once the replacement is installed it needs to be commissioned by somebody who understands protective relaying, so you should plan on subbing that part out.


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## EC2253 (Mar 7, 2008)

Thanks for all the great answers!


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## McDaniel8402 (Jan 14, 2014)

I write the settings files for microprocessor based relays of all kinds, to include bus differential relays, transformer diff relays, overcurrent relays, etc etc etc. If current "in" doesn't equal current "out", "TRIP!!!!".


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