When paralleling 2 units together separate from the grid. i read that the voltages have to be the same so no circulating currents occur. So why is it that when i up/lower the voltage at a site the vars on that unit go up. is it taking more of the reactive load? or are there circulating currents. I was under the impression that when you up/lower the voltage nothing happens you just take more of the system vars.

like i know with the fuel fuel you raise fuel unit will take more watts.
does same thing happen with voltage and vars.

But where i'm getting mixed up is how can this jive with upping the voltage you would have 2 different voltages on the gensets and therefore have circulating currents.

 

Are you getting confused because in the first sentence you state you are asking about what happens when two machines (generators with their prime movers) are paralleled together, separate from the grid, and then you talk about being "...at a site..."? And is that unit (or are those unites) paralleled with a grid, or separate from a grid?

Because there is a difference. On a large grid there is both a real load and a reactive load which can be shared by many generators. When you want to increase the amount of real load (watts) you increase the fuel, which tends to increase the speed of the machine, but doesn't increase it appreciably at all if the grid is being operated properly. The extra torque being produced is converted to watts by the generator.

To increase the amount of the reactive load on the system one increases the amount of excitation being provided to the generator, which effectively tries to increase the generator terminal voltage (and it might to varying degrees, depending on many other factors). But the energy is converted to reactive current by the generator.

When only two generators are supplying a load, real and reactive, the load (real and reactive) is fixed by the devices that make up the load (motors, lights, power supplies, etc.). If you try to increase the load being produced by the prime movers you can't--you will only succeed in increasing the frequency (speed of the prime movers and generators). If you try to increase the reactive load of the generators you can't--it's fixed by the reactive component of the total load on the two generators.

If you consider watts and VArs as variables that can be produced and consumed, then you are correct in your statement that fuel is proportional to watts and excitation is proportional to VArs. This is a generalization, but it's basically what happens to units on a large grid being operated properly.

For smaller grids/loads of one or two generators being operated in parallel things are different.

Hopefully this helps!

 

While increasing the voltage of particular unit, if you observe power factor you will come to know how the reactive load is getting shared.
The reactor load sharing of the particular generator will go on increasing as you keep on increasing voltage (excitation)and the other generator sharing will come down and at one stage the other unit power factor become unity, means there is no circulating current but entire reactive load in the system has been taken by one generator.

If further increase the voltage than other unit power factor become leading (Over exited), that means there is circulating current.
Even with fuel (speed-frequency) if you increase the speed of one unit, first it will take more share of external load (active power), and if you keep on increasing the (fuel) speed, it will take entire load and finally other generator will act like a load (motor)(reverse power) which is dangerous.

There is speed droop setting in governor and power factor or voltage droop setting in AVR, which has to be set equal for both generators in initial commissioning stage to make sure that there will be equal load sharing (active and reactive).

Regards
Sunil Kumar

 

From Leonard;

The information below is very clear on my problem. However, how is the active load (kW) balanced when two gensets are running in parallel and using an automated load-sharing device? Await your reply

>While increasing the voltage of particular unit, if you
>observe power factor you will come to know how the reactive
>load is getting shared.
>The reactor load sharing of the particular generator will go
>on increasing as you keep on increasing voltage
>(excitation)and the other generator sharing will come down
>and at one stage the other unit power factor become unity,
>means there is no circulating current but entire reactive
>load in the system has been taken by one generator.
>
>If further increase the voltage than other unit power factor
>become leading (Over exited), that means there is
>circulating current.
>Even with fuel (speed-frequency) if you increase the speed
>of one unit, first it will take more share of external load
>(active power), and if you keep on increasing the (fuel)
>speed, it will take entire load and finally other generator
>will act like a load (motor)(reverse power) which is
>dangerous.
>
>There is speed droop setting in governor and power factor or
>voltage droop setting in AVR, which has to be set equal for
>both generators in initial commissioning stage to make sure
>that there will be equal load sharing (active and
>reactive).

Rgds,
Leonard