Variation in Excitation Current of Synchronous Generator

"...for one Generator the corresponding transformer ratio is 409.5/15.75 KV..."

Find out why this one is different, if no good reason is found then move its tap to be identical to the other ones, your "problem" should go away.

Thanks for your kind review of the problem.

Sir, The Generator transformer with different Ratio is of Different Make and its present Tap setting is nearest to rest of the Generator transformers in the station.

May please review the same in above context.

Reducing excitation of the machine in question should help bring down MVAR equal to other manhines.

The difference in sharing in MVAR is due to difference in Z% of transformer which you have to bear with.

"...Moreover during grid voltage variation and frequency variation this rotor current also changes with large spread from Maximum to minimum value as compared to other machine in the plant...."

Yes, and that will continue to be the case if you simply reduce the excitation of the subject machine. The problem is that you may reduce the excitation to the point that the machine is operating near its stability limit; while this will become obvious under steady-state conditions, during a system transient event that particular machine will be the first to slip poles, making the overall system even more unstable.

It's your choice, Band Aid or proper engineering solution.

Like for small injury band aid is better solution instead of full blown open heart surgery.

This problem can be solved to extent without much of engineering.

Yes, and taking aspirin for chest pain often leads to open heart surgery which might have been prevented through proper early intervention. Do it right, now, or explain why your inaction was the root cause of the system failure later.

To me, it looks that the trafo turns ratio / %Z could not be causing the machine share higher MVAR than the neighbouring ones.

How sure are you that the 'Voltage reference' to the AVR of subject machine is correct!! That is, can there be a problem in the VT ratio or length of the VT secondary leads to the AVR! How about load current signal to the AVR! Can there be some issues with the CT ratio, accuracy of the CT core, CT lead lengths etc.!!

Lots of things to check, including AVR performance / calibration.

GA

Also, check gain settings on the AVR.

Thanx Raghun!

The Voltage reference and actual values are displayed in % in Microteminal of DVR and are matching with the output of PT secondary. The CT secondary is also checked for correctness. The action of DVR in terms of field current,field voltage, Gain setting is checked before synchronisation of machine with the Grid and is found OK.

The problem is for same reference value setting of Voltage, as the frequency and GRID Voltage at 400 KV side varies , The actual value deviates and the DVR than tries to modulate the field current to match with the reference value.

kindly review.

DVR...... MICROTERMINCAL......

Is it BHEL make?

The technology and generation is similar to what u have mentioned.

Therefore, RamConsult is correct. You live with it or change the transformer to one with the same specs as the others. The error is ~ 2% which is almost the difference with the Z% of this transformer. The compensation looks normal.