Generator tripping on high MVARs

[Photocell]

I work in a gas processing plant where electrical power is generated by three identical gas turbine driven turbo generators (A, B and C). Each generator is rated 11kV 20.38 MW at 0.8 PF 430C. During normal operation 2 out of 3 generators are run to feed the plant load (32 MW approx.) while the third generator remains standby. The excitation system is brushless excitation consisting of PMG as the main exciter. These three generators are supplied by Peebles electrical machines.

The major loads on the plant are four big compressor motors rated 11kV 6.2MW each. During startup, each motor takes 24MVARs approximately.

We are experiencing some strange problems in the excitation system. Since last one month, whenever we have tried to start any of the four compressor motors, generator A grasps all the MVARs while running in parallel with generators B and / or C. (Please note that the generators were running normal for two years before the last one month). This causes generator A to trip on high MVARs from its protection relay (GE SR 489). The generator A behaves abnormally only during the start of the motors while it remains stable during the normal operation and shares equal MVARs with the other generators. The AVR installed on generators is BASLER DECS 125-15. The parameter settings of AVR of generator A is identical to AVRs of generator B and C. We have tested the protection, metering and quadrature droop CTs (on the middle phase) which are found normal. The protection relay is working normal and the only abnormality observed is high MVAR grasping of generator A. Please explain what can be the possible cause of the generator A abnormal behavior and what should be our way forward to resolve this issue.

Thanks and Regards

 

[RRaghunath]

It is not clear whether the AVR of TG-A is really picking up more MVARs or is it the relay malfunction (that is causing the unit trip).

If the droop settings of all the AVRs are identical, I don't see any way how TG-A can pick-up more MVARs during transient phase.

May be you could throw more light on the subject based on your investigation of AVRs and the relays. You may also like to look at the limiters and the response of field forcing function in the AVRs.

 

[Alwaysexciter]

Your brushless exciter has diodes mounted internally that produce the DC current for the generator field. You should isloate these individually and test them. You may have one or two bad ones.

 

[Photocell]

rragunath

The GT-A is actually picking the MVARs and it is not relay malfunction.
The setting of all the AVRs is identical

Always excitor we have checked the diodes and did not observe any abnormality

 

[RRaghunath]

The field forcing feature in AVR helps whenever there is sudden large drawal of MVARs (under fault conditions or during motor starting) from generator.

This feature normally requires compounding CTs in the generator output terminals to be wired to the excitation panel and will cater to the extra power requirement of the excitation system under field forcing conditions. You may like to check whether the same is in order in GT-A.

The other one is the overexcitation or MVAR limiter in excitation system. This, if set lower in GT-B & C than that in GT-A could cause GT-A pick-up higher MVARs.

Trust you find the above helpful.