Hi everyone.. I am working in a power plant which has both gas turbines and diesel engines are synchronised internally not to the outside grid. When GT running in parallel /kw control mode and DG running in Isle/speed(directly connected to load) control mode, how the power factors of GT and DG are inversely proportional to each other.
If I raise GT power factor then reduces the DG power factor. how??

Jagadeeswaran,

You are describing exactly the situation when you have an isolated system with two generators feeding the same load, with AVRs that do not have their cross current or reactive droop compensation set properly.

As you raise the excitation level on one generator its terminal voltage rises, at the same time the other generator's AVR sees the voltage rising and tries to lower it to its setpoint by lowering the excitation. It's pure logic, your load only requires a certain amount of VARs and it doesn't care which machine provides them. The net sendout is always the same, but one machine will be absorbing the excess the other one delivers to the bus.

Alan

Very well explained by Mr, Alan Maltz.

I agree with Mr. Maltz

Thanks for your valuable comments Mr.Alan.

I have checked GT generator parameters and noticed that with the decrease in power factor followed by increase in reactive power and apparent power but not much change in terminal voltage.

Jagadeeswaran,

That "...not much change..." is all that it takes to cause the above scenario. AVRs are high gain negative feedback controllers that operate at electronic speeds, they will make changes in excitation levels much faster than the electrical speeds that limit how fast you can change the current in that very large inductor (known as the field winding) could ever respond to a step change, and that doesn't include the stator windings. For more info Google on AC machine time constants, in particular the rotor and stator time constants.

Alan

Power factor is a function of Load. The DG or GT are supplying the power as demanded by the load at whatever PF the load is. If you wish to improve the PF , you may have to improve it by adding capacitors with the load. Let me know what do you exactly want to do. Rotating machines normally generate power on 0.8 PF so let it be that, if the PF is going lower due to load characteristic; to bring it back to 0.8PF, you can do it by adding capacitor on load.
Let me clarify that if you are wanting to better the PF by changing the Excitation it is only possible when the system is running in parallel with the Grid. Also please note in a parallel running system the voltage increase thru change in Excitation voltage will only change the sharing of the KVAR.

for further automation or modification of your system you can get in touch with cre-technology in France for support. they are into automatic synchronous solutions. thank you