We are having generator synchronizing issues with 3 generators in parallel using a cross current (reactive differential) loop circuit. We have gen A, gen B, and, gen C in that order. We are using AB CGCM for the load share controls.

The problem we are having is that generator B will not synch nicely with other generators A or C. Generator B will try to take all the load and the kVARS are exceeding high compared to the other units. There is no issue with synching generators A and C together though.
Also noticed the voltage setpoint for generator B is 4153 with generator voltage at 4158 (4160 system). Voltage set point for generator C is 4200 with voltage around 4160, and the bus voltage is 4157.

Could this be a cross current CT wired backwards with generator B?

 

bhol,

Did this problem with the second generator just start, or has it been going on since commissioning?

If it just started, did it start after some kind of maintenance outage?

Why do you specifically suspect a reversed CT? Was that CT recently replaced, or was the wiring to that CT disturbed for some purpose?

Do the schematic diagrams show polarity markings for the CT--and has anyone verified the polarity to be correct, including "flashing" the CT as well as visually checking the CT polarity marking/physical orientation?

 

This has been happening since commissioning. We haven't verified wiring just yet, but I was doing research and though maybe this could be a symptom of reverse polarity. Looking in the junction box where the CT's terminate to CGCM and branch off to other units, it looks like somebody was marked these wires with and terminal blocks + and - with a sharpie like somebody was unsure. There are no markings on the other 2 generators CT wiring.

 

It has been ongoing since commissioning about 7 years ago... I believe we have had some issues with this generator tripping offline during syncing years ago. Now they can sync it, but they have to manually adjust the voltage set points drastically on this generator and the generator they are syncing to.

Thought maybe a reverse polarity issue, the wiring and terminal block for cross current CT is marked with a sharpie for generator B like maybe somebody was unsure or messing with it sometime? There is no markings for generator A or generator C.

 

bhol,

I have seen an improperly wired CT cause the reactive currents to swing wildly when the cross current compensation circuit was "cut in" on two generators which were connected on the same bus ahead of a step-up transformer--but NOT the loads. Only the reactive currents; the power factor for one machine went to zero leading and the other machines went to zero lagging. Of course both machines tripped off the line pretty quickly, and there was a great consternation about that.

I'm referring to your original post which said the B unit would take all the load. That, to me, sounds like the governor of the gen-set's prime mover isn't adjusted correctly, or, maybe it's in Isoch when it should be in Droop. (We don't know of these three units are supplying a load independent of a larger grid, or are synchronized to a larger grid. Also, we don't know if there's any kind of PMS (Power Management System, or load/VAr management system) being used on these machines.)

Please write back to let us know what you find!