voltage variation in a generator

[eengrimrankhan]

Hi guys,

I work for a local refinery. I am looking after the operations and maitainance of diesel power plant. We have 4 1.2MW 1577kVA diesel gensets. We run three at a time and we are facing problem as one of our gensets after synchoronizing to main busbar produces voltage varaitions and produces powerfactor and kA swings keeping the kW constant. We have checked its exciter winding its resistance is 8ohm and meggar results also found ok. Changed AVR card many times but problem remains same. Sometimes it automatically remains calm and other times it is very distrubtive. Please need help.
Thanks

 

[waross]

Look for a loose wire in the quadrature circuit. It is common to find a CT on one phase which develops a voltage across a resistor. This voltage is used to bias the AVR so that parallel sets will share VARs properly. A loose connection or a failing switch will result in erratic VAR production. The resistor is often onboard the AVR so a failing resistor may be ruled out.
If the AVR has terminals for connection of the quadrature CT the resistor will have been changed with the AVR. If the resistor is external to the AVR it may be failing.

Bill
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"Why not the best?"
Jimmy Carter

 

[rasevskii]

In addition to what waross has said, also check everything in the PT circuits: loose crimps, bad HV fuses, or particularly the sliding contacts if you have drawer type PTs. Or it could be a bad voltage settinfg pot which may be external to the AVR card that you have changed out.

rasevskii

 

[eengrimrankhan]

thank you very much waroos and rasevskii,

@ waroos: do you mean droop ct by saying quadrature ct? and the avr i using is beslers aec63-7, and no external resistor is available, only we have to connect the wires to the avr card. is there a possibility that quadrature (droop) ct, got faulty? what do u say.

@rasevskii: boss, we are generating at 460V, no pt, taking voltage directly. only cts are available.

 

[waross]

The simplest Quadrature circuit has AVR voltage sensing From A phase to B phase. There is a CT on C phase. The CT current is fed through a resistor to develop a small voltage. This voltage is added to the sense voltage of the AVR.
When the power factor is unity this voltage is a right angles to the sense voltage and has little effect on the sensed voltage. However any reactive current will develop a voltage component that is in phase with the sense voltage and either add or subtract from the sensed voltage to adjust the excitation for proper VAR sharing between machines. Any intermittent faults in this circuit including the single/parallel switch will result in voltage instability.

Bill
--------------------
"Why not the best?"
Jimmy Carter