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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
Parents
  • 0
    Hi John,


    thank you for your reply, which is very helpful.


    For a transformer you mentioned that "the relay can be housed locally and the trip signal transmitted over distance to the breaker." I believe that over distance means withouth cables. Am I right?


    For a motor it seems to be that the circuit breaker is generally installed on a panel local to the motor itself. This means that, in practice, there might be 2 breakers in series: one from the main MV swirchboard protecting the cable downstreem and one protecting just the motor. I wondered if in practical exaples distances of only 25m or 50m could avoid duplicating MV breakers as explained above. But I believe you already gave a good answer which is related to the CT burden.


    Thank you again for your help.
Reply
  • 0
    Hi John,


    thank you for your reply, which is very helpful.


    For a transformer you mentioned that "the relay can be housed locally and the trip signal transmitted over distance to the breaker." I believe that over distance means withouth cables. Am I right?


    For a motor it seems to be that the circuit breaker is generally installed on a panel local to the motor itself. This means that, in practice, there might be 2 breakers in series: one from the main MV swirchboard protecting the cable downstreem and one protecting just the motor. I wondered if in practical exaples distances of only 25m or 50m could avoid duplicating MV breakers as explained above. But I believe you already gave a good answer which is related to the CT burden.


    Thank you again for your help.
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