Pilot wire relays

[appunni]

Sir,
What is the purpose of giving pilot wire relays in all terminals? Is it not enough providing (summation)CTs in all terminals and providing pilot wire relay at one only and also providing intertrips to other terminals from the relay?
What is the purpose of providing resistance to pilot wire loops?
Please give answer.
with regards,
appunni

 

[bigamp]

With a differential scheme (one relay and CT's at each end) there is a practical limit as to how far apart the ends can be. The further apart they are, the higher the burden on the CT's at the end remote from the differential relay (due to the resistance of the conductors connecting the CT's to the relay) so the greater the CT VA and knee point voltage requirements.

Also, since the relay has to trip a circuit breaker at a remote end you find that the resistance ot the cables connecting the trip coil (or aux relay) at the remote end to the differential relay become more and more significant and you reach a point where the voltage drop in the cable would be such that there would be insufficient voltage available to energise the trip solenoid or aux relay at the remote end.

A pilot scheme overcomes all of these difficulties and the ends of the circuit being protected can be very distant.

A pilot scheme can tolerate only so much pilot resistance. Some are designed for a maximum pilot resistance. For shorter distances between ends (where the resistance of the pilots will be less), you add resistance to the pilots via adjustible padding resistors to bring the pilot resistance up to the design level.

 

[busbar]

If the relay instruction manual is unclear, you might refer to Pilot Protective Relaying book by Walt Elmore (retired) from ABB. Else, review applicable information of single-function electromechanical pilot-wire protection:

http://www.abbus.com/abbrelays/pdf/ILsIBs/Cs41/419714l.pdf

http://www.abbus.com/abbrelays/pdf/ILsIBs/Cs41/41-971.3M.pdf

http://www.abbus.com/abbrelays/pdf/ILsIBs/Cs41/419712h.pdf

http://www.abbus.com/abbrelays/pdf/400/41_471e.pdf

 

[appunni]

Thanks for your replies.
Sir, In an ordinary differential relay, the relay is placed in a location where the potential across the relay is neglibly small as the lead plus CT resistance to either side will be almost equal. But in pilot wire relays are on terminal ends. In a current circulation scheme, how can this be achieved? If it is not, will there not be spill current in the pilot wire relays which may make operation during external faults? I request for your expert and detailed comments.
With regards,
appunni

 

[gordonl]

Assuming the currents at both ends are equal and CT ratiosare equal, if you look at the two nodes where all three connections meet, (two CT wires & the relay wire) the current in must equal the current out. With equal current entering from one CT and exiting to the second CT the current in the relay must be zero.