Line differential relays - phase shift

[bspace123]

Hello

We have two line differential relays on two parallel cables supplying a 6.6kv switchboard. The diff relays are linked by fiber communication. The cable run lengths are 100m and 200m.

The switchboard supplies a bunch of DOL started 1Mw motors.

Whenever we start one of the motors, the differential relays trip on diff current. When we observe the waveform capture and events file from the relay sets, we notice a 60deg phase shift between phases (i.e a-a b-b c-c) on one cable and 30deg on the other. Although shifted, the phases remain 120deg apart and magnitudes are the same.

If we disable the diff prot, start the motors then re-enable they do not trip.

Seems to me like the phase shift is due to inductance in the cable, but 60deg is much more than I’ve ever seen on such cable run lengths. Cables are 2x3c 400mm2 CU in parallel.

Anyone have any other thoughts on why the phase shift is occurring ?

 

[davidbeach]

Miswired CTs. When the motors are running you’re below the minimum trip point but the starting current pushes you above. Capture an event while the motors are running and see how that looks.

 

[bspace123]

Thanks for the response

CT’s are both wired in star and appear correct on the schematics. If they were transposed wouldn’t we see either 120 or 180 deg phase shift? I’m not sure how you would wire them differently to get 60 and 30deg.

I guess the next step is to physically verify them by opening up the boards

 

[davidbeach]

No, the next step is triggering an event capture with running current. Properly connected the currents from the two ends should be 180 degrees apart.

 

[newelecteng]

hi bspace123

we face slightly different problem when we install new 3.3kv 1MW motor. When we start this motor it cause the gas turbine differential relay to trip. But when start the motor again it start normally without any problem to relay.
we think the cause of this trip is the high DC component in the inrush current. this for first time trip . this cause the motor charge any virtual capacitor. So, when we start it again it start properly.

 

[jwatty]

David's right. Upload some records if you have them. You'll get 60 degrees if you have both phasing and polarity backwards. 30 degrees is going to require a delta connection somewhere.