Settings for a ring main unit

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Thread Starter

Paul Wilkinson

I have a feeder in an 11kV switchboard protected by a Micom P127 relay. The CTs are 250 amp and the cable is a 185 square mm Cu, rated at 370 amps, conductor short circuit is 26.5kA for 1 sec and screen is 10.1kA for 1 sec.

The feeder supplies a 2 + 2 ring main unit which in turn supplies 2 off 2MVA 11/0.433V distribution transformers. The transformers are each protected by a vacuum circuit breaker with Woodward SEG relay. The transformer feed cables are 70 square mm Cu, rated at 215 amps, conductor short circuit is 10kA for 1 sec and screen is 10.1kA for 1 sec.

The combined network and transformer MVA, including network damping is anticipated to be 28MVA.

What over current and earth fault settings would you recommend for the P127 and the SEG relays?

At present I have chosen the following settings for the SEG relays protecting the transformers, OC = 125 amp, EI (IEC), TMS 0.8 (would prefer 0.5 but must be 0.8 due to grading issues with the circuit breaker on LV side). EF = 25 amp, definite time, 0.1 sec
 
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