A 11kV, 5000kW motor is provided wi 1

[RRaghunath]

A 11kV, 5000kW motor is provided with Differential protection. This motor has PF correction capacitors of 1.9MVAR coming within the zone of differential protection (due to the specific location of CTs).

This will mean that the PFCC current is not accounted in the differential scheme and thus requires the differential protection to be set less sensitive so as not to have tripping on the PFCC current that is going to appear as spill in the relay.

Any experiences / suggestions.

Thanks in anticipation.

 

[DougMSOE]

Having been in the exact position that you describe, I would strongly suggest that the PFCC be moved such that the motor only is Diff. protected. Use other protection system to protect the PFCC.

 

[busbar]

IEEE C37.96 [and C37.99 and C37.103] do not seem to address the situation. Given the typical 6-CT 3-relay installation, if the capacitor-tap location cannot be changed to outside of the stator-differential zone, the only solution may be to experiment with another set of CTs having as-close-as-possible electrical characteristics to the existing CTs—paralleled with the existing set at the differential-relay restraint-coil terminals.

A non-mainstream {more risky} approach, if the capacitors are ungrounded-wye-connected, may be to install something like a PT and 59G relay configured to supervise or inhibit 87M operation.

 

[swgrmfg]

Simply add CT's on the PFCC tap and connect secondaries at motor side accordingly!

GH