island mode parallel generators

[wilans]

we have two generators in parallel but not connected to grid. after synchronize one generator is running on low load 4MW and the other generator running on 20 MW. why is the pf on smaller load higher that is 0.95 while the one with 20MW is 0.8. and cannot adjust the pf manually for each individual generator. can anyone explain briefly, thanks in advance

 

[jbt260]

Are the generators the same size? Are the conductors to the paralleling switchgear the same size and length?

 

[charlie b]

Whatever method this system is using to control the generators is not set up properly. When you have two generators in parallel, you can intentionally shift the KW load from one to the other, by varying the speed controls of the two machines. Taking the governor of one machine to "raise" will cause that machine to increase its share of the KW load. Taking the governor of one machine to "lower" will cause that machine to decrease its share of the KW load.

By the same token, you can shift reactive load, and therefore adjust the power factor of each machine, by adjusting the excitation currents for their respective generator fields. Taking the excitation control of one machine to "raise" will cause that machine to increase its share of the KVAR load. Taking the excitation control of one machine to "lower" will cause that machine to decrease its share of the KVAR load.

Back to my first statement: This type of control adjustments should be handled automatically by the synchronizing system. So I would suggest looking into the settings of the various control devices. Perhaps something is inadvertently set to "manual," instead of "automatic."

 

[wilans]

yes, they are the same. does that situation because of the cross current compensation. is it true that the lower MW Gen will have higher pf and when its MW near 0MW it will have pf near unity?

 

[IslandSparky]

kW and kVAR sharing depends largely on how the sharing controls are set. You can adjust it so one unit handles all the kVAR if you really want to (not recommended). The alternator pitch can have some impact as well if the units aren't matched.

I wouldn't describe the load sharing to be automatic as Charles stated; I would call it more autoMagical; the technician usually has to tweak everything and it usually is optimized for a single load point. As load varies, the kVAR sharing is difficult to maintain.

It sounds like your cross-current compensation doesn't optimize kW and kVAR like a modern generator though; you could be relying solely on impedance to share load.

 

[LarryFine]

When you have two generators in parallel, you can intentionally shift the KW load from one to the other, by varying the speed controls of the two machines. Taking the governor of one machine to "raise" will cause that machine to increase its share of the KW load. Taking the governor of one machine to "lower" will cause that machine to decrease its share of the KW load.

That's how power companies buy and sell power, from what I understand.

 

[iceworm]

--- can anyone explain briefly, ---

Everybody gfave you pretty good responses, but the specific answer is, "No - you did not give us enough information."

yes, they (the generators) are the same. ---

What we know: you have 2 generators, 20MW (or larger) operating in parallel. they can not be adjusted manually

What we don't know:
With two generators that size, there is a room full of equipment - or maybe even two rooms. Depending on the age and sophistication, there could automatic voltage regulators tied to var (or var share) controls, governors tied to kw (or kw share) controls. Governors can operate in droop, isoc, or base load. Although generally only one in isoc, the other in droop or base load.

If this is a new installation, then there are issues with commissioning, programming, wiring verification.

If an existing installation, then my first question is; Have the generators ever played well together before? If so, something broke.

--- cannot adjust the pf manually for each individual generator. ---

The simplist soltuion is one is in Isoc and the other is in droop. The paralleling contoller is not programmed to share. I'm guessing there is no front panel Manual/Auto switch or you would have seen it. If you wish to manually adjust - you will need to find the paralleling controller software switch.

--- why is the pf on smaller load higher that is 0.95 while the one with 20MW is 0.8. and cannot adjust the pf manually for each individual generator. ---

--- does that situation because of the cross current compensation. is it true that the lower MW Gen will have higher pf and when its MW near 0MW it will have pf near unity?

Two 20(+)MW generators operating in parallel is non-trivial. You really need to get the wiring diagrams, one-lines, tech manuals - and if you are living right there will be design documents explaining the operation.

worm

 

[millelec]

I agree that the load sharing circuitry needs to be looked at. You ideally want both in 'auto' mode and the voltage droop and speed droop set to the same percentages, say, 2.5% on each unit.
Example: Unit #1 is a 600 KW unit and Unit #2 is a 1 MW unit. Both can be running in parallel and sharing load according to their full load ratings. 800 KW total load should break down to 300 KW on the 600 KW unit and 500 KW on the 1MW unit if the speed regulator droop settings are the same. (50% Rated load on each machine)
Assuming 4160 supply voltages, Unit #1 at 300 KW should have 52 amps/phase, and Unit #2 at 500 KW should have 87 amps/phase, and they should maintain that same RATIO of amperage as total load rises or falls. This also only applies if both voltage regulators droop are set to the same percentage.

 

[wilans]

thanks all for the response, for additional information : both units are typically the same. rated about 35 MW each. what is confusing me is when i set the load equally ( 12 MW each ) then the power factor are the same automatically ( 0.85 each ) . when i change the load as i previously mentioned G1 to 4 MW and G2 to 20 MW then the pf also change automatically become G1 pf 0.95 and G2 pf about 0.8. frankly speaking i am junior in generator control. As far as i know there is fix VAR control or fix pf control. i would appreciate for all response