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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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    vici:


    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?




    As Alastair says "over distance" doesn't mean without cables: the direct trip circuit itself can be tens of m long, or for longer distances can be taken through remote I/O modules via a fibre-optic cable which is common at MV over tens of km, or in the extreme via power line carrier on EHV lines across continents (probably a bit of overkill for an MV motor!).

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  • 0

    vici:


    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?




    As Alastair says "over distance" doesn't mean without cables: the direct trip circuit itself can be tens of m long, or for longer distances can be taken through remote I/O modules via a fibre-optic cable which is common at MV over tens of km, or in the extreme via power line carrier on EHV lines across continents (probably a bit of overkill for an MV motor!).

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