Good day,

Thanks for your reply to my query last August 26, 2009.

When the power plant nearby is on line our system voltage increases.
We are not the biggest power station operating in the area. The power
Plant in question is about 1km from our site.

Some other questions arise, please spare time to help:

We generate voltage at 11kv and steps up to 22kv. We have a step-up
Transformer. It has 3 tap change settings (22kv, 22.2kv and 22.4kv).
We are currently set at 22kv. Actually the grid system operates at 22.4kv To 22.8kv (varying) which is being controlled by a bigger power station Which has OLTC and capacitor banks.

My first question: Would it be advisable to set our transformer tap changer Setting to a higher one?

Second question:
Is there any adverse effect of running a generator at negative VAR?
This negative value actually is within the reactive capability curve.

Third question:
Can you please differentiate VAR control and Voltage control?

At what instance are we going to use Voltage control?

At what instance are we going to use VAR control?

Your reply is very much appreciated.

Tor

 

I think your tap-changer setting and the negative VAr question are related, and perhaps changing the tap would help with returning the VAr flow closer to zero, maybe even positive.

As long as you are operating the generator within the reactive capability curve of the generator you should be okay. However, it would seem that it might be possible given the conditions at your site that you might be operating with high excitation current just to try to keep the VArs inside the reactive capability curve, and doing so could be causing higher-than-expected rotor temperatures, but that's just a possible scenario, one which I've never encountered but have read about. So, you could be operating the generator within the reactive capability curve while overheating the generator rotor.

Voltage control and VAr control are related in that as you change generator terminal voltage (by changing excitation) you will change the VArs. All of this depends on the relationship to system voltage. If your generator terminal voltage is exactly equal to the grid voltage, the VAr reading will be zero, and the power factor reading will be 1.0 (unity). If your generator terminal voltage is higher than the grid voltage, then the VAr reading will be lagging (positive) and the power factor will be less than 1.0 in the lagging direction. If your generator terminal voltage is less than the grid voltage, then the VAr reading will be leading (negative) and the power factor will be less than 1.0 in the leading direction.

So, you can pick a voltage setpoint at one time during the day, and note the VAr reading at that time. You can return later to find the VAr reading quite a bit different because the system voltage has changed with respect to your generator terminal voltage. As you noted, when the larger power station that is close to you comes on line there is a change in the system and in your station's operating characteristics.

If you use voltage control, you are basically deciding to allow the VAr reading to be whatever it will be as the system voltage changes with respect to your generator terminal voltage.

If you use VAr control, then the exciter regulator (the "AVR") will adjust the generator terminal voltage to try to maintain the VAr setpoint you have chosen.

I like to use this <b>analogy</b> when talking about VArs. If we considered VArs as being something that is produced, like watts, then we can consider the excitation power being provided to the synchronous generator rotor to be the "force" that generators convert to VArs the way they convert torque to amps to produce watts.

If the amount of torque being transmitted to a synchronous generator is exactly equal to the amount of torque required to keep the generator spinning at the same speed (frequency) as the grid then there is no power being produced by the generator.

If the amount of torque being transmitted to an generator is greater than the amount of torque that is required to keep it spinning at the same speed (frequency) as the grid then watts are being produced by the generator. (In this scenario, if the same amount of torque was being provided to the generator rotor when it was not connected to the grid, the generator rotor would be spinning faster than synchronous speed or synchronous frequency.)

If the amount of torque being transmitted to a generator is less than the amount of torque that is required to keep it spinning at the same speed (frequency) as the grid then the generator will be consuming watts from the grid to maintain the frequency, and this is called "reverse power" or "motoring the generator" and can be very destructive to some prime movers. (In this scenario, if the same amount of torque was being provided to the generator rotor when it was not connected to the grid, the generator rotor would be spinning slower than synchronous speed or synchronous frequency.)

Now, if the amount of excitation that's being provided to a synchronous generator's rotor is exactly equal to the amount that is required to keep the generator's terminal voltage equal to the grid voltage then zero VArs are "flowing" and the power factor is 1.0 (unity).

If the amount of excitation that's being provided to a generator exceeds the amount that's required to keep it's terminal voltage equal to the grid voltage then positive (lagging) VArs are considered to be "flowing out" of the generator and the power factor is less than 1.0 in the lagging direction. (In this scenario, if the same amount of excitation was being provided to the generator rotor when it was not connected to the grid the terminal voltage would be above rated or normal.)

If the amount of excitation that's being provided to a generator is less than what's required to keep it's terminal voltage equal to the grid voltage then negative (leading) VArs are considered to be "flowing into" the generator and the power factor is less than 1.0 in the leading direction. (In this scecario, if the same amount of excitation was being provided to the generator rotor when it was not connected to the grid the terminal voltage would be below rated or normal.)

So, in this analogy VArs and watts are either "produced" by (considered to be "flowing out of") the synchronous generator or "consumed" by (considered to be "flowing in to") the synchronous generator. Producing watts means the generator is converting torque into amps at synchronous speed and those amps are being used to power motors and lights and computers and such. This is easy to understand.

Consuming watts means the generator has become a motor and is driving the prime mover (which is not generally a desirable condition for several reasons).

Since we don't have a similar tangible way to describe what VArs do, it might be helpful to think of it as above. We all know that an AC system is not a straight resistive load, but that it inductive and capacitive elements to it, and that in general systems are mostly inductive in nature. We also know that in an inductive (or capacitive) load the voltage and current sine waves shift their relationship to each other. If that shift is not "countered" to a certain extent then the system will be subject to "brown-outs" and eventually black-outs. So, it's necessary for some VArs to be "produced" to counter the effects of the inductive nature of the load to keep the voltage and current sine waves closer to being in phase with each other.

The above is just an analogy, the way some people attempt to equate water and electricity. Some people will say that VArs don't flow, and technically that's probably correct. But I'm just trying to find a way to explain how VArs (and power factor) are related to generator terminal voltage and system voltage and excitation current.

And excitation current is power (excitation volts multiplied by excitation amps equals excitation power), variable power that is put into the generator, in the same way that variable torque is put into a generator. And if we can think of VArs as being produced and consumed as watts are produced and consumed, then it helps to understand all the relationships. It's just an analogy and the terms and description are just to try to help understand.

The situation at your site is made a little more difficult to understand with the step-up transformer, tap-changer variability, and the changing system voltage conditions on the grid in your area.

It sounds like it might be prudent to have a power system study done with the addition of the new generating station on the grid in your area.

 

Thank you for this very cogent explanation. It has greatly aided my understanding of VAR flow in generation.

 

Good day,

Your enlightenment to my queries is very much appreciated. I am very grateful for your time to answer them. It is overwhelming that there are people like you who are more than ready to help.

Please keep up the good work. You are helping not only the specific person who ask the question but also others who can read all of those explanations.

I hope that future questions again can be given time.

Thanks once again,
Tor

 

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