Power Factor on Generator Set Parallel with Utility


Thread Starter


Hi guys,

Is it possible for a genset that is parallel with utility power factor to decrease for .94 to .60 with causing a rise in the system voltage? The power plant consist of diesel genset operating in share (isochronous) and base load. The genset in share load is in voltage control an genset in baseload is in Pf control. The utility total load is 25 MW.

say Gen A & Gen B are operating with power factor 0.8 with overall system power factor 0.8, voltage 11Kv & load 12MW + 12MW.
Now if, load changes say it becomes 25MW and accordingly, gen B reacts and take the extra load i.e. 13MW. With this extra load system voltage will also decrease say 10.97KV and gen B has to increase its field current.

Instead, gen A increase its field current to make up the system voltage, power factor of the two gen will be unmatched, say 0.7 & 0.9.

Step by step:
primary position
G-1 12MW, 11KV, 0.8 pf
G-2= 12MW, 11KV, 0.8pf

transient position, before voltage increase but after load increase, for understanding only

G-1= 12MW, 10.97KV, 0.771 pf
G-2= 13MW, 10.97KV, 0.829 pf

final position case-1, if Gen-A is in voltage control and increase its voltage to make up system voltage

G-1= 12MW, 11KV, 0.7 pf
G-2= 13MW, 11KV, 0.89 pf

final position case-2, if gen-B is in voltage control and increase its voltage to make up the system voltage

G-1= 12MW, 11KV, 0.8 pf
G-2= 13MW, 11Kv, 0.8 pf

Note overall pf is not in our control, as it is a load characteristic.

I think this same thing is happening in your case.
please check and revert.
I do not understand the scenario as mentioned in the OP like one DG set in isochronous and the other in baseload when operating in parallel with utility grid. Can it be confirmed?
According to OP there are two DG sets, one in isochronous control and the other in base load control operating in parallel with the utility grid. And their AVRs are one in voltage control mode and the other is PF contol mode. Is it really possible to operate like this? Can any one explain this?

Not normally--BUT, there are sites that operate with special controls that use Isochronous control which is "over-ridden" by another controller. It's a very strange set of circumstances described by the OP (Original Poster), and one of the reasons people haven't likely responded is that it's not clear, it seems odd, and when asked for information the OP hasn't responded.

IF the units were synchronized with some kind of grid (with other generators and their prime movers all synchronized together supplying a load that is much larger than any single unit could supply) then in theory--and in general practice--only one unit can be in Isochronous. Maybe that unit is the one the OP is referring to, but in reality on most grids of any size (large or small), there is NOT usually any unit operating in Isochronous (except for small isolated "islands" like some refineries or process plants). All the unit are operating in Droop speed control, and someone somewhere is raising and lowering the load(s) of one or more Droop units to control frequency.

So, the OP hasn't really provided enough information to explain the circumstances so that anyone could really understand the issue.

But, one unit could be in PF control and the other in VAr control, IF the units were, in fact, synchronized with a large grid.
Thanks for your reply CSA. As you mentioned, only OP can give more clarity on the plant operating scenario.