Urgent: question abou differential protection

If the station service Transformer is 2% capacity of the Main Transformer, is it necessory to add this branch to Main Transformer/Generator (Unit) Differential Protection?

Thanks, Tony

Yes. Generally, the sensing points would be the grounded side of the generator (assuming an external wye), the HV side of the main transformer, and the load side of station service (all with suitable CT ratios). The idea is to draw a 'circle' around the generator output and each component it is connected to. Stations that I've seen/worked on don't have circuit breakers between generator, step-up transformer, and station service. They just have some disconnect switches for maintenance. So the protective relaying treats the generator, main transformer, and station-service as one large protective unit. The boundaries of that unit are where the differential protection CT's are placed.

Think about what happens when you are bringing the unit on-line. Main generator load is still very low, and you transfer 'house loads' from reserve/off-site to station service transformer. At that point, station-service makes up much more than 2% of the total load on the generator. Depending the type of differential relay and the settings, you may get away without the station service tap being in the differential protection, but it could be 'dicey'.

All the differential protection hookups I've seen (both unit protection and bus/transformer protection) have *always* had all branches monitored. If the differential protection is around a large transformer, one of several schemes may be used to prevent false tripping from magnitizing in-rush (anything simple time-delays triggered from breaker status to harmonic restraint relays).

daestrom

Thanks very much, appreciate your comments.

T> > If the station service Transformer is 2% capacity of the Main