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Medium voltage motors - differential protection (87)

vici
Hi everyone,


I was studying the concept of differential protection (87M) for motors suppplied at medium voltage (MV) and I  have a general question about it.

I undestand that current transformers (CTs) are installed at motor's terminals and that CT secondaries feed the relay which, in turn, will trip the MV breaker when and if needed.

Can the MV breaker tripping the motor be located in a MV switch board which is far away from the motor, e.g. 50m away? I believe that there might be limitations in terms of cable lengths form the secondary of the CTs to the relay. Does anyone know what kind of distances, at a high level, are generally acceptable between the breaker and the motor? I don't have specific data as I was mainly trying to understand factors to take into account and understand the concept of differential protection.


thank you
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    Firstly the differential relay doesn't have to be in the same place as the breaker.  This is often the case for transformer differential protection, where the circuit breaker can be tens of km away.  If the breaker is a long way away then the relay can be housed locally and the trip signal transmitted over distance to the breaker.  Transmitting a trip signal over a long distance is easier than extending CT leads over a long distance.


    But for a motor it is likely that the circuit breaker is fairly local to the motor.

    The CT lead to the relay is electrically just a load - a resistance with a current passing through it.  If you calculate what current you expect, and you know the wire resistance, then you know the VA that the CT has to provide.  Order a CT with an adequate rating.  If that becomes expensive or difficult to house then reduce the VA load (usually called the VA burden in the case of CTs, but it's just a load) by increasing the wire size to reduce the resistance.
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  • 0
    Firstly the differential relay doesn't have to be in the same place as the breaker.  This is often the case for transformer differential protection, where the circuit breaker can be tens of km away.  If the breaker is a long way away then the relay can be housed locally and the trip signal transmitted over distance to the breaker.  Transmitting a trip signal over a long distance is easier than extending CT leads over a long distance.


    But for a motor it is likely that the circuit breaker is fairly local to the motor.

    The CT lead to the relay is electrically just a load - a resistance with a current passing through it.  If you calculate what current you expect, and you know the wire resistance, then you know the VA that the CT has to provide.  Order a CT with an adequate rating.  If that becomes expensive or difficult to house then reduce the VA load (usually called the VA burden in the case of CTs, but it's just a load) by increasing the wire size to reduce the resistance.
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