87 Differential tripping main breaker??? 5

[rockman7892]

I have a problem where a SEL 387 differential is tripping our main 5kV breaker. The differential zone is between the secondary bus of our substation transformer and on the load side of our main breaker right after the breaker. The cable that is between the differential zone is 9 runs of 1000MCM for a distance of 1000ft.

The SEL 387 keeps tripping the main and indicating an 87 trip for phases B and C only. The only thing that we did different was added an additional small load in the plant to this breaker however the breaker was closed when we added this load and did not trip. It was not until we opened the main breaker ( unrelated reason) and then tried to close it again that it tripped on differential. After it tripped we tried to close this main again an it tripped on the B and C phase again.

I'm confused b/c nothing that we do on the load side of the breaker (in the plant) should effect the differential zone since what goes in must come out. I'm thinking that there is a problem with the cable run between the substation transformer, or something wrong within the main breaker.

Any ideas???

 

[davidbeach]

When looking at IAX from the remote end you need to look at IAL from the local end. IA is time shifted to match the timing for IAX to produce IAL. The relay uses IAL and IAX for the differential calculation, not IA and IAX.

Look at the phasor plots and you will see that IAX and IAL are in phase, as are IBL/IBX and ICL/ICX.