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
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