Transformer Diff Protection Problem during the TF Energization 1

[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 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 relay settings and trip record (disturbance record); i will share.
Any help regarding to the above cases will be highly appreciated

 

[davidbeach]

What do the relay event reports show? The event reports should provide all the answers; start with looking at the phase relationships between sets of CTs.

 

[dpc]

Sounds consistent with incorrect wiring on CT inputs as David is suggesting. With diff protection disabled, look at the CT currents coming into the relay. Depending on the setting of the differential element, it may not see enough "difference" current to trip at low load, even though CTs are not wired correctly.

 

[ZeroSeq]

1. Check the CT compensation
2. Check the restraint elements

 

[prc]

Please check the second harmonic content in the inrush current and relay setting for second harmonics. In large GTS of recent make,the second harmonic content during inrush has come down drastically ( below 10 % compared to > 20 % earlier days) This happened due to change in CRGO grades used in modern transformers plus some changes in constructional details. Normally harmonic restraint setting of relay is 10-15 % and if actual harmonic content is <10 % the relay will take it as a fault current and actuate the relay. Some different restraining features are being introduced in modern relays to overcome this situation during inrush current.

 

[ubairkazi]

Thanks All; Kindly reply me your emails, so i can send you the setting and disturbance recorder files.

 

[ZapSib]

I suggest you put your settings/DR etc .. on e.g. DROPBOX ( or equivalent) and share here the link.

 

[ubairkazi]

please find the below Hotmail OneDrive link for the COMTRADE and setting files...

https://1drv.ms/f/s!AsmTiLrMNRoZpjoyYI3MX9oFyKZn

or
COMTRADE & Setting FIles

 

[ZapSib]

Thanks for sharing the files. I also replied to you on linkedin, please check.

 

[stevenal]

I'm seeing nothing but noise on channels 2 through 5. Check your CTs for shorts.
Channel 1 looks bad too, with A and B in phase with each other and C 180 degrees out from the other two.
The schematic indicates every CT secondary neutral is grounded twice, one ground through a switch. Unless the switch is open, you may find that stray ground currents have an improper path through a relay coil.

 

[Mbrooke]

How can I open the disturbance/event data files?

 

[stevenal]

I used the AcSelerator Quickset Event Viewer application, a free download (after registration) at selinc.com. I suspect most any Comtrade viewer will work.

 

[rcw retired EE]

Check the CT polarity & connections on the 6kv input. Trips occur when there is high current due to inrush or heavy load, indicating a CT wiring or setting issue. Since no trip occurs under normal generation output, either the 400kV and 15 kV generator CT's are correct or the generator output current has been less than the pickup.

If this is a typical gas turbine system, the heaviest 6kV load occurs during turbine starting. Maybe during normal operation, the 6 kV current is less than pickup so no tripping. Or it could be a setting issue or the multiple grounds on CT's.