Generator?Utility Drifting & Instability

[jscaufi]

Our utility has a customer that co-generates to the grid with a 5 MVA coal powered steam generator. It is synchronized to the grid. Our substation (5/6.25 MVA xfmr) cannot supply the entire factory, nor can the generator, unless they shed some load.

Recently, this customer called to inform us that their terminal voltage on the generator (4160 V) was dropping and they couldn't get the output they desired. Over the next 24 hours the terminal voltage of the generator dropped to less than 3900 volts. All the while, the vars delivered by our Xfmr steadily dropped, while our watts steadily rose.

We heard they might have installed digital controls in the recent past but can't be sure as they don't inform us of everything. I read in an article from a web search that digital generator controls interacting with a nearby Xfmr LTC can cause a condition they called "Who's in Charge," between the controls of the generator and the LTC. The LTC reacts so slow that the generator controls take over. I also read that utilities usually like only the utility to be in control of the voltage, and the generator control system was on voltage control.

At any rate, we could not find anything wrong with our LTC. We did notice on our SCADA Data for the substation that the voltage became more "jagged" looking during this episode, but only to the extent of a 1.5 volt range on a 120 volt base at the LTC. They finally separated from our grid after shedding enough load so both the generator and the utility could handle their separate loads on each side of the bus tie switch that separates the generator from the utility.

My question is: Has anyone heard or experienced anything like this and if so what did you find to be the problem? Otherwise, what theories do you have?

Thanks so much.

jack

 

[DanDel]

For whatever reason, if your(I'm assuming that you are the utility) customer's generator voltage dropped to 3900V, it's output would decrease, and your output would increase to make up the difference.
If your(the utility) voltage remained at the nominal level, then it would appear that the problem is with the customer's generator controls.
Did you have any unusual activity with your LTC to verify this?

 

[jscaufi]

We had no unusual activity concerning the LTC controls, other than the voltage strip charts we retrieved from SCADA showed the typical variations in voltage prior to this event, then the xfmr voltage started looking like the blades of a hack saw; very jagged, varying by about 1.5 volts (124.5-123 volts at the LTC). However, with a line technician at the sub, there was little LTC activity. We assume there were problems with the customer's controls but we can't just go in pointing fingers, but we'd surely like to learn more in the event it happens again.

Thanks

Jack

 

[DanDel]

Would the 1.25% voltage changes you mention(1.5V at 120V nominal) be due to your LTC changing taps or not? What were the time intervals for these voltage changes?

 

[GerH]

Looks very like a problem with the CoGen generator excitation. Possibly that their AVR was switching between Voltage Control and PF/VAR control mode i.e. between Island and Parallel mode. If the sawtooth was regular period it is probably a control problem. Can you see you VAR flow related to voltage. If VAR flow toward the customer increases as the voltage drops the problem is originating at their side.

 

[jbartos]

Suggestion: More information about the Customer power generation and distribution is required. There may be various causes involved. E.g. if the Customer installs capacitive loads, e.g. power factor correction capacitors. Then, Watts from the Utility may increase while the inductive VARs will decrease.