Generator Var compensation for voltage support

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mull982

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I was wondering how a generator was able to supply vars for voltage support on a downstream load?

I'll use an example system of a voltage source connected to an inductive impedance transmission line and a load.

If the load is an inductive load then it will draw inductive reactive power and this lagging inductive reactive power across the transmission line will result in a voltage drop at the downstram load/bus.

However if a large enough capacitor bank was installed at the downstream load and was enough to cause a leading current and leading capacitve reactance across the transmission line then there will be a voltage rise at the downstream load/bus.

I was wondering if rather than placing a capacitor bank at the downstream load if you could configure the generator to supply the leading kVARs and thus cause the same voltage rise at the downstream bus load? So rather than the downstream capacitors causing the leading kVARS, the generaotr would be supplying them? Is this possible? Is this similar to how generators can be configured to provide voltage support?
 
I believe you can achieve this by overexciting the generator. But if you do, then any other loads between the generator and the load that you are discussing will have an overvoltage condition. A better answer is to have the capacitor bank at the load include a control mechanism to switch in and out the amount of capacitance needed for the loading conditions.
 
charlie b said:
I believe you can achieve this by overexciting the generator. But if you do, then any other loads between the generator and the load that you are discussing will have an overvoltage condition. A better answer is to have the capacitor bank at the load include a control mechanism to switch in and out the amount of capacitance needed for the loading conditions.
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Thanks Charlie

Yes I understand that all loads in between may be subject to overvoltage as well. Just wanted to see if this was possible.

I've also heard other talk about inverters being used to supply vars for voltage support and I guess this is the same concept.
 
120330-1506 EDT

mull982:

If you have a single generator supplying a load, and the generator produces a sine wave, then at the generator all you can do is control the generator output voltage. You have no control of the current. The current will be what it will be based on the load. If the current load is too high for the generator, then the generator won't be able to maintain voltage.

If the load is a large grid system, then you have a totally different discussion.

.
 
gar said:
120330-1506 EDT

mull982:

If you have a single generator supplying a load, and the generator produces a sine wave, then at the generator all you can do is control the generator output voltage. You have no control of the current. The current will be what it will be based on the load. If the current load is too high for the generator, then the generator won't be able to maintain voltage.

If the load is a large grid system, then you have a totally different discussion.

.
Click to expand...

I was referring to a case of overexciting a generator as charlie mentioned. I think by doing this you can supply excess leading VARS to the system. I'm not sure if this would only apply in a situaion where a generator was connection to a grid or if it would work with a single load.

I believe it is a similar concept to running a synchronous motor unloaded to provide Vars?
 
generator was connection to a grid or if it would work with a single load.

I believe it is a similar concept to running a synchronous motor unloaded to provide Vars?[/QUOTE]

Raising excitation to a single load will only raise the voltage and not appreciably change the load power factor.

A synchronous motor is just a generator running at reverse power. Increase its excitation above what is needed for the motor field and the motor will supply vars back to the power system.


Watts flow to where they are needed at the load. Vars flow from high voltage to lower voltages. To push more vars out of a generator, increase excitation to raise the voltage. Or change the tap on the generator step up transformer to increase transmission voltage and var output to the transmission line.

The var output limit of a generator is its MVA limit (120 MVA = 100 MW @ 0.8 power factor) or its voltage limit, usually 105% of nominal voltage.

As you stated, capacitors can improve voltage by supplying vars closer to the loads. The capacitor eliminates the voltage drop of the vars flowing in the transmission line.

Transmission line and transformer voltage drops also affect the generator's ability to deliver vars. If the load voltage remains relatively stable, the transmission line voltage drop raises the sending end voltage at the generator to its limit as we try to push vars That's one reason why it is not practical to deliver vars a long distance.
 
mull -
Adding to the other posts, here is a method I've developed to understand generators: It is a bit simplistic, but I believe it is a good explanation of the physics.

The load is what it is, you can't change it. The voltage, current, pf are what they are. (Yes ,you can add caps or reactors, but that is just a new load to deal with) Once the voltage and frequency are set, the current and phase angle are fixed. The kw and kvars are fixed.

Every generator on the earth has the same two knobs, the throttle and the DC drive to the field.

If a generator is islanded, then the DC Drive to the field controls the terminal voltage. The throttle controls the frequency.

If the load were heavily inductive, the generator terminal voltage will be dragged down. It is necessary to turn up the DC drive knob to raise the voltage back to spec. Is it over excited? It is excited exactly where it needs to be to keep the terminal voltage at spec. Does adding caps at the load help out? Absolutely yes, If you don't two things happen. As you noted, the heavy inductive load sucks a lot of vars. If the vars are coming from the gen, then:
1. there is a lot of current over the distribution that is not doing useful work, but is still heating transformers and distribution wires - not generating revenue

2. If the var load is heavy enough, the DC drive to the field is turned way up. The rotor is getting hot, the high reactive current is getting the stator hot. The lagging side of the generator reactive power curve is generally cut back for a pf lower than 0.8

If one put caps out at the load, the the caps trade the reactive current with the inductive load. All that reactive current never has to go back to the generator. Makes everybody run cooler.

Now let's look at a generator connected to a grid. You still have the same two knobs. However, the frequency and voltage are stuck - the generator can't change them. Now the throttle controls KW sharing. The DC drive to the field controls VAR sharing.

The load still is what it is. KVARS and KW to the load are stuck.

Now if one opens the generator throttle, then the generator puts out more power. The load is not going to take any more. So the other generators on the grid back the throttles down and put out less power.

Say one turns up the DC drive to the field. The generator puts out more vars. The load won't take any more, so the other generators back down their DC drive to the field.

Again, adding caps at the load helps the entire distribution system.

However,If the caps are added at the generator, it helps the generator, but the voltage out at the load is still low - all that VD in the transmission line and transformers. If it is a single generator, one must crank up the DC drive to raise the voltage to get it right at the load. If it is a grid, then one turns up a transformer tap to get the load voltage up.

This is not a lot different than what you are saying - perhaps just a different way of looking at it.

ice
 
120331-1026 EDT

mull982:

Response to your post #6.

With a single AC generator (voltage source) as the voltage source, then there is nothing you can do to adjust the load power factor, assuming the load is linear (impedance does not change with voltage) by changing the AC generator excitation. Excitation to the generator only adjusts voltage.

If you adjust generator speed, then you change frequency and this will change the power factor if the load remains unchanged.

When you connect a small generator to a large grid, then grid voltage and frequency are essentially unchanged by what you do to your small generator. Now you have the ability to adjust to some extent the reactive and real components of power from the small generator by adjusting torque and field excitation to the generator. Note there is series impedance between your generator and a large grid.

.
 
mull982 said:
...I've also heard other talk about inverters being used to supply vars for voltage support and I guess this is the same concept.
Click to expand...
Active Front End (AFE) drives can be run in leading power factor to provide a limited amount of VARs into a system (limited by the overload capacity of the VFD). It would rarely make sense to bother with that on a small low voltage VFD, but it is quite common on large MV VFDs where a little bit of extra unused capacity on the VFD will provide a substantial amount of VARs to compensate for numerous small inductive loads, especially if those large MV VFDs don't charge extra for an AFE.
 
A slight variation on this is a practice known as "Spinning Reserve". This is done in the UK & I guess in the US to. It invloves running several (usually large) generators, dotted around the system, synchronised onto the grid but only exporting VARs & zero real power.

The idea is to help with the unpredicability of system load..... several large machines are ready to pick up real power load with a few seconds notice (by openning the steam valve & providing more torque) but meanwhile they provide a useful function by exporting reactive power (which any transmission system needs to function) and hence voltage support.
 
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