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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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    Alasdair,


    thank you for your suggestion and apologies if i did't select the right section of the forum. I will maybe post it in "Ask the Community" later then. My question was meant more for applications in the UK. Regarding differentail protections (87), generally and at a very high level, these are used for internal faults. For example an internal transformer fault can be detected with the differential protection. The same concept applies for internal faults to motors or even to sections of power lines. The differential protection will exculde the faulty machine or the faulty section from the rest of the system.


    Thank you again,


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  • 0
    Alasdair,


    thank you for your suggestion and apologies if i did't select the right section of the forum. I will maybe post it in "Ask the Community" later then. My question was meant more for applications in the UK. Regarding differentail protections (87), generally and at a very high level, these are used for internal faults. For example an internal transformer fault can be detected with the differential protection. The same concept applies for internal faults to motors or even to sections of power lines. The differential protection will exculde the faulty machine or the faulty section from the rest of the system.


    Thank you again,


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