Transformer Diff Protection Problem during the TF Energization

[ubairkazi]

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. 87 also have REF, 50/51N, Thrmal OL, Overfluxing, Voltage & Frequency Protection...
Problem Case:
1-when we energized the transformer (close Line Breaker); suddenly 87 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; 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 share.
Any help regarding to the above cases will be highly appreciated.

 

[Ingenieur]

Have you tried with differential only across the transformer (15:400 kv)?

why are you including the 6.6 aux power in the main differential scheme?

when you energize the xfmr you have current on the 15 but not the 6.6 and 400
when you start the 6.6 motor you have current on the 15 and 6.6 but not the 400

perhaps a time delay or increasing the diff setting may help

 

[Bugman1400]

Have you tried with differential only across the transformer (15:400 kv)?

why are you including the 6.6 aux power in the main differential scheme?

when you energize the xfmr you have current on the 15 but not the 6.6 and 400
when you start the 6.6 motor you have current on the 15 and 6.6 but not the 400

perhaps a time delay or increasing the diff setting may help

There could be numerous possibilities and issues. Has there been any wiring revisions involving the CT circuits to the relays? If the main GSU is tripping out upon energization with no load, it sounds like you don't have the harmonic blocking or restraint set correctly. What is the relay displaying for the Operate and Restraint currents when the unit(s) are loaded up? The settings, disturbance records, and onelines would be a big help.