We have a power plant with six diesel-fuel generators. The AVRs are UNITROL 1000 from ABB. The grid has an excess of reactive power produced by the underground cables. The AVRs are in voltage regulation mode. During the night the load is quite small and the voltage goes up. The AVRs reduce the generation of reactive power and sometimes start taking it in.
I'm worried to have the reactive power being supplied by the grid.
If there is a fault on the grid that makes the voltage decay significantly, are the AVRs capable to change immediately from taking in to generating reactive power?
What about the ancillary services of the power plant? Will the motors and transformers, supplied by the grid reactive power, stop working because of a voltage collapse?
I need your help to know if I should be concerned of a power plant taking in reactive power from the grid.
>If there is a fault on the grid that makes the voltage decay
>significantly, are the AVRs capable to change immediately
>from taking in to generating reactive power?
Only a proper power system study (which takes a long time and a lot of money) can answer this question. ABB might be able to provide some response time data which could be helpful in the interim, but faults are unpredictable and it's very difficult to plan for any contingency. Worse, as transmission and distribution systems are enlarged and modified over time conditions can and do change, making periodic review of the power system and possible fault conditions necessary (not often done, but they should be).
>What about the ancillary services of the power plant? Will
>the motors and transformers, supplied by the grid reactive
>power, stop working because of a voltage collapse?
This is entirely dependent on the auxiliary power supply situation and what the source(s) of auxiliary power are. Some power plants have fast-acting switches that can switch the auxiliary power supply from one source to another in the event of some kind of fault or interruption. We can't know how your plant is configured (we don't have the drawings for your plant).
It's not unusual for some plants, when designed for the service, to be capable of "absorbing" reactive power (VArs) to help with grid issues. But, again--the plant has to be capable of operating in this manner, and most generators are not designed for such operation. (I believe some transformers with load tap changers can be useful in these kinds of situations, but I don't personally have experience with them or with operating them.)
Each generator should have been provided with a reactive capability curve, sometimes called a "D-curve" because of the shape of the curve. This curve defines the limits of operation of the generator in three conditions: over-excited (positive reactive power; lagging VArs); under-excited (negative reactive power; leading VArs); and maximum real power (watts; kW, MW). There are usually two or more concentric (I think that's the right word) sets of "curves", for different generator cooling air conditions (I presume the generators are air-cooled). These should always be present and available to the operators in the control room at all times, and should always be referred to for guidance when questions arise about operating limits.
The proper answer to your questions, however, really involves a power system study to understand and analyze the current operating conditions for your plant and the nearby transmission and distribution system. You may get some help from the grid owner/operator as they are likely relying on your plant to help ride through most disturbances and so should be willing to help make the plant as reliable as possible. HOWEVER, my experience with most grid owner/operators is that they don't share information very easily (there ARE exceptions), as they are afraid of liability issues. Somewhat understandable, I guess.
Best of luck!