Cause of TRIP Query

[ThePunisher]

Hi all,

One of distribution utility have a 115-69 kV three phase, wye (grounded) primary and wye (grounded) secondary step-down transformer with one 69 kV feeder line dedicated feeder connected to a nearby 69-13.8 kV delta primary-solid grounded secondary substation with further 13.8 kV feeder lines. This 69-13.8 kV transformer is protected with primary fuses and secondary power circuit breaker with 51/51G relays.

A ground fault occurred at 13.8 kV feeder line and the upstream 69 kV circuit breaker protection relay tripped on 2 phase and ground fault. I am wondering why the upstream relay ground fault protection may have picked up when the downstream transformer primary winding is DELTA.

Is there a way primary fuse will blow at the same time when line-to-ground fault occurs at the downstream transformer secondary?

Thank you for your comments and opinions as always.

 

[Kiribanda]

ThePunisher,
1)GF Relay operation:It is difficult to explain how the GF relay of 69kV DELTA side has operated for a
GF at its WYE side because there cannot be zero seq currents on the delta side to trigger a
GF relay connected in residual connection.Is this GF relay connected in residual mode
or does it have a ZCT as its GF input?If it is residual then I suspect its polarity/connection.
If a ZCT had been used,then I suspect the conductor positioning through the ZCT.
2)Fuse Operation at 69kV Side: The GF had happened at 13.8kV side where its neutral is solidly grounded.
That means GF currents are bit high than on a low resistance (LRG) system.Therefore, it is possible,
to blow the fuse by the higher reflected primary (69kV) currents due to a single line
to a GF on solidly grounded 13.8kV side.
3)You also observed that 2P has operated by the reflected line currents on 69kV side due to a
single line to GF at the 13.8kV side.

 

[waross]

A phase to neutral/ground fault on the secondary of a delta/wye transformer presents as a line to line fault on the primary delta.
Possible causes for the primary trip:
A faulty relay.
Poor coordination

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

What types of relays? If they're EM relays, do you know for sure that none of the targets are stale? How are they connected? Any chance that the 13.8 circuit that faulted is under the 69kV circuit on the same poles? Is the 69kV system radial or are there other sources into it?

 

[ThePunisher]

I have an update that the upstream 69 kV trip occurred when the downstream 13.8 kV phase C conductor was cut open circuited. This will have caused single phasing on the secondary side. I just don't know what is the ripple effect on the primary line currents that triggered the trip.

I will get it checked at the site the relay type and what alarm was actually triggered as a result of the trip.