Stub Bus Protection

[KenAlmon]

We have a 500 kV switchyard with a ring bus. The 500 MW generator feeds through a GSU to a 500 kV disconnect into the ring. We have a CT on the primary of the GSU and one each at the breaker.
My understanding is that the differential being used is not protecting when the disconnect is open.
The protection here is referred to as stub bus protection.
Can any one give me some good references to how this scheme works and why it does not work when the disconnect is open?
Thanks

 

[alehman]

The differential should work so long is there is not an open switch/breaker within the protection zone.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[ElectralJohn]

KenAlmon,
Where I hail from this sort of protection is usually called 'interzone' protection. This is if we assume that by 'primary' of the GSU you mean the HV bushings as opposed to the LV bushings. If the GSU CT is on the LV side then the scheme would be transformer differential protection. Either scheme works but I assume you have an interzone scheme.

The scheme works regardless of the status of the 500 kV disconnect switch. There is no good reason why opening the disconnect should disable the scheme because you will need it to do both of the following:-
1. Protect the stub
2. Protect the small zone between the GSU HV bushings and the open disconnect.

A possible trap is that while you have the disconnect open and the GSU out of service, if you inject the GSU HV CTs for some reason this will operate your differetnial protection and cause loss of one corner of your mesh. As this sort of injection testing is not likely after you have commissioned your switchyard you probably need not worry about it.