Hello every one; now a days i am having a big issue related to the transformer differential relay.
Let me tell you the schemes and parameters:
2 generators (each 125MW) connected with the 1 stepup Transformer(350MVA/15kV-400kV); between generator & TF there is 52G and also two Auxiliary Transformer (15kV to 6.6kV). Aux TF is feeding the MV Switchgear & also stepdown TF (6.6kV to 440V) for LV switchgear.
Differential Protection taking the feedback (through CTs) from HV(400kV), tertiary(15kV) and MV (6.6kV) side. 87B also have REF, 50/51N, Thrmal OL, Overfluxing, Voltage & Frequency Protection…
1-when we energized the transformer (close Line Breaker); suddenly 87B operate (current differential) and trip the Line breaker.
2-By disable the diff protection; transformer energized and then again we enable it, transformer still energized in healthy condition but when we start 6.6kV induction motor (for cranking the turbine), this relay again trip (87) the line breaker.
If we disable the 87 protection and energize the TF and crank the turbine, after synch the generator we enable 87 then it works properly but during the above cases it trip the line breaker.
Can any body please explain why this is happening?
I think the relay settings for the 87 is not correct; if you want me to send you the relay settings and trip record (disturbance record); i will.
Any help regarding to the above cases will be highly appreciated.