Parallel Transformer Coordination Problem

[JBlackEE]

I am fairly new to realying and have a project that has posed a new problem for me. This project is at a 115-69kV Step-Down Station. To increase capacity of the station, a second transformer was added in parallel with the existing one. There is one high side Circuit Breaker and one low side Circuit Breaker. There is an SEL-351 (overcurrent relay) on the hig side breaker and a SEL-387 (differential relay) on each transformer.

The problem comes in with coordination. I would like for the 351 to opperate first because it will reclose in the appropriate circumstances. However, the 351 must be set at a threshold to allow 89MVA since it sees the load on both transfromers, where the 387's can't allow more than 35 or 54MVA since they are protecting each transformer.

I had a coordination worked out that kept everything under the damage curve of the transformer, but it turns out the time dial was too high for the SEL relay.

Help please! Am I maybe thinking about something the wrong way in regards to coordination? Has anyone faced a similar situation?


Thanks!
-JBlack

 

[davidbeach]

Move the 351 to the low side. Set it to coordinate with the outgoing feeders. Use the 387s principally as differential relays, the best protection that they can provide for the transformers. Set the high side and low side overcurrents of the 387s to protect each transformer while staying above half the setting of the 351. Consider using different settings groups for single transformer vs. dual transformer.

Even better would be to separate things out a bit with two more breakers, and throw in bus diff. Then you have the 387s protecting the transformers, the bus diff protecting the bus, and the feeder relays protecting the outgoing lines. The 351 would need a mate for one per transformer, or better yet double up the 387s and no need for the 351.

 

[JBlackEE]

david, thank you. Unfortunately, I realize having a breaker for each transformer would be optimal, but is not a possibility in this project. That would solve all of my problems, but just isn't feasable.

As for what you say in the first paragraph, if the 387's are set at half of the level of the 351, won't they still trip before the 351? Or can I set them to only trip on differential?

To be honest, there is a 351 on the high side breaker and another on the low side breaker already.

 

[davidbeach]

Yes, the 387 can be set to trip only on differential. A coordination study will show tripping times for different faults.

 

[bacon4life]

Is it the high side or low side 351 you are using to reclose? Hopefully you are not planning on reclosing for a transformer or bus fault.

You can also program two different trips in the 387, for example having the differential element lockout the substation and the low side overcurrent element only tripping the low side breaker.

Then, the 351 would monitor one the 387 output contacts to determine if reclosing was permissable. We routinely do something similar to this on transmission lines because we use two different types of relays for tripping, but only one relay has the reclosing functions enabled.