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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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  • 0
    you refer to the American Naional Standards Institute relay numbering convention and to 'Medium Voltage'

    Both of these suggest you are after a US-centric answer, and this may not be the best forum, as our (UK) centric rules are rather different in the detail - the physics is the same, but if you open the box it is sometimes hard to believe ?.

    Certainly we do have systems that can trip off the primary side of transformers at 11kV or whatever if there is a load side fault on the 400v LV side, and the telemetry circuits that do so  need to be both fail-to safe, and testable. However there are other differences that may or may not matter - our HV circuits never carry neutral for example, and signalling may be voltage or current.

    I am probably not helping, but may explain the lack of responses.
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  • 0
    you refer to the American Naional Standards Institute relay numbering convention and to 'Medium Voltage'

    Both of these suggest you are after a US-centric answer, and this may not be the best forum, as our (UK) centric rules are rather different in the detail - the physics is the same, but if you open the box it is sometimes hard to believe ?.

    Certainly we do have systems that can trip off the primary side of transformers at 11kV or whatever if there is a load side fault on the 400v LV side, and the telemetry circuits that do so  need to be both fail-to safe, and testable. However there are other differences that may or may not matter - our HV circuits never carry neutral for example, and signalling may be voltage or current.

    I am probably not helping, but may explain the lack of responses.
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