So, thought I would throw a question in here if you don't mind.

I work at a gas turbine generation plant. Machine would normally synchronise at approx 1 - 3 Mvars incoming.

Over the past 6 months it had experienced incoming VARS to 17 Mvars.

Machine has also had increased vibration issues over this period.

It is a twinpac gas station running FT4 engines.

Thought I would throw this one in for people to analyse and give thoughts. I have trended this for some time and seem to have drawn a conclusion.

Can anyone give any advice?

 

I'm presuming the generator being discussed is a synchronous generator.

When a generator is synchronized, one of things that's usually done is to make the frequency of the generator slightly higher than the frequency of the bus being synchronized to. This is done so that when the generator breaker closes the extra fuel that was making the generator frequency higher than bus frequency would be converted into "positive" amps by the generator and positive watts would be registered on the wattmeter. (The higher the frequency differential, the higher the watts will be on synchronization.)

Another of the things that's usually done when synchronizing a generator is to make the generator terminal voltage a little higher, or at least equal to, the bus voltage. This is done so that when the generator breaker closes the VAr meter will read a few "positive", lagging VArs and the power factor meter will be "positive", indicating a lagging power factor.

If the generator terminal voltage is equal to the bus voltage at the time of synchronization, then the VAr meter will read zero, and the power factor meter will 1.0.

If the generator terminal voltage is less than the bus voltage at the time of synchronization, then the VAr meter will read "negative" (leading), and the power factor meter will less than 1.0, leading.

Watts are a function of fuel. VArs are a function of generator terminal voltage with respect to bus voltage. Generator terminal voltage is a function of excitation. Bus voltage, which can change as grids change and loads change and generation changes and switchyards are added and modified, is not a constant at all times, of the year or even of the day.

Many auto synchronizers (while you haven't told us, it's most likely the unit is automatically synchronized) don't even check generator terminal voltage other than to see it's above a minimum level. Some actually adjust excitation to make generator terminal voltage equal to or slightly greater than bus voltage during synchronizing, but those are the more expensive variety.

Let's say the auto synch module at your site only checks for a minimum generator terminal voltage. What was likely done during commissioning was to set the "off-line" excitation to some value that was adequate <b>at that time</b> to make the generator terminal voltage equal to or slightly greater than bus voltage <b>at that time</b> so that there would be a positive VAr and power factor indication on breaker closure.

Two things can happen after that. The settings of the exciter regulator (the "AVR") can drift, and the conditions of the grid can change. The drift of the exciter regulator can cause the generator terminal voltage to increase or decrease, but if there is drift and there is no appreciable change in bus voltage then the VAr indication will change on synchronization.

If the system voltage changes, which it can even during a normal day depending on system load and many other factors, then even if the exciter regulator hasn't drifted the VAr and power factor indication will change on synchronization.

If both happen, then there's no telling what the effects can be.

When the excitation of the generator is such that the generator terminal voltage is equal to the bus voltage, there is zero VAr indication and the power factor meter reads 1.0 (unity). When the excitation of the generator is such that the generator terminal voltage is less than bus voltage then the VAr meter will read "negative", leading, and so will the power factor meter. When the excitation is such that the generator terminal voltage is higher than bus voltage then the VAr meter will indicate "positive", lagging, and so will the power factor meter.

You need to understand how the generator terminal voltage is set or adjusted (if it's even adjusted by the auto synchronizer) when the unit is synchronized.

It may even be that there's something wrong with the exciter regulator that's cauing its output to decrease when load is applied. (I'm going to be very sorry for ever saying this, but exciter regulators have droop control settings, too.)

From your trends, can you see what the generator terminal voltages and bus voltages were at the time of synchronization when the VArs were lower and when they were higher?

How long has it been since the exciter regulator was "tuned up"?

Is the auto synchronizer working properly?

You didn't say if the vibrations went away when the VArs were brought back to more normal value. You also didn't say if the generator or the turbine was experiencing the higher vibrations.

There's a few things to get you started.

(And, no. I'm not touching the exciter regulator droop topic. If you need to understand how your exciter regulator's droop function works and is programmed, then RTFM (Read The Fine Manual) or get a knowledgeable person to site to help with the problem.)

 

If MVARs demands is increasing than the permissible VARs limit of the generator, what could be implication for the generator?