Switched capacitors at generating plant
Switched capacitors at generating plant
(OP)
I have a situation in which we have installed switched capacitors on the bus at a generating plant. There are five generator units and 4 cap banks. Obviously, there is a conflict between the caps and generators since they are controlling the voltage at the same bus. In addition, for stability reasons, it is desirable to minimize (or eliminate) reactive power absorption of the generators (i.e. Qmin=0). My question is what is the recommended method for controlling the capacitors? One alternative I thought of was to limit the exciters on the generators so that Qmin=0 and set caps as normal. This has one minor drawback since the exciter settings are manual and cannot easily be overridden. Does any one have other suggestions?
Thanks.
Thanks.






RE: Switched capacitors at generating plant
Maybe I’ve misunderstood the question, but isn’t the usual practice to install {shunt} capacitors closer to loads where the reactive power is needed and not "on top" of synchronous generators? It seems like excitation {and its very controllable dynamics} would be more than adequate for conditions at a generator site, and effectively without additional costs or equipment.
RE: Switched capacitors at generating plant
RE: Switched capacitors at generating plant
RE: Switched capacitors at generating plant
Michael Sidiropoulos
RE: Switched capacitors at generating plant
Static capacitors cannot be used for dynamic VARs by themselves, but they can free up dynamic VARs from generators that are outputing maximum VARs already. In my situation, the capacitors are needed because there is more load (both real and reactive) in the area than generation.
RE: Switched capacitors at generating plant
RE: Switched capacitors at generating plant
RE: Switched capacitors at generating plant
That is correct and brings me back to my original question: How to coordinate control of the capacitors and generators so that generators do not absorb VARS? I have a couple of ideas (one already mentioned) but am looking for other opinions.
RE: Switched capacitors at generating plant
One way of sensing absorbed machine vars may be a reverse-power [32] relay with the potential input connected in quadrature.
RE: Switched capacitors at generating plant
Seems to me that you may need to consider operating the generators at rated power factor, on PF control if available. As a minimum, you need to ensure that the minimum excitation limiter on the AVR is properly set up to avoid absorbing VARs. Possibly tie line PF monitoring to control the capacitor bank switching? (Sounds as though what you are really trying to achieve is minimizing VAR flow on the tie lines). Re-reading the original post, you may have to upgrade the AVRs to achieve this.
peebee -
If you examine a typical generator capability curve, the stability limit line is shown in the negative VAR region. Operation beyond this line will result in the machine falling out of step. Normal practice would be to operate with an adequate margin separating the operation point from the stability limit, usually resulting in operation well into the positive VAR quadrant. This ensures that the excitation is high enough to maintain stability when swings occur.
RE: Switched capacitors at generating plant
I was recently on a project where they parallelled (3) 760kW, 480v generators full time with the utility, the utility tx normally only saw 100kW of load, the gens ran near rating. Due to the tx being unloaded, they would experience overvoltage. But if the gens shut down, they'd go to sever undervoltage. Someone came up with the bright idea to import LOTS of vars from the utility in an attempt to drop the voltage. And that seemed to help somewhat with the voltage issues. Someone else came up with the warning that on a similar installation, someone turned the dial a little too far and it launched the tie breaker out of the switchboard.
"Instability" sounds like a reasonable diagnosis there. Any more explanation of what's going on there would be appreciated.
I've asked this question of our local [#1 gen mfgr] dealer, seems I already knew more about this problem than they did [they were the one's that came up with the idea to suck vars from the utility in the first place].
RE: Switched capacitors at generating plant
RE: Switched capacitors at generating plant
Alternately, the capacitors can be installed at the generator if the load is very inductive, high VARsind. They also cause problems, namely, heating problems.
RE: Switched capacitors at generating plant
RE: Switched capacitors at generating plant
If you know in which configuration of the network (ie which loads are connected) you really need to have the capacitors connected, why not proceed like this:
- keep the generators excitation as it is (for them to control the bus voltage).
- add some sequential devices (for exemple relays) to have the capacitors breaker closed when they are needed, and open when not.
Regards,
RE: Switched capacitors at generating plant
You may have to adjust levels and time delays in order to avoid excessive capacitor switching.
Similar controls are used to extend the reactive range of SVCs - operators/suppliers of these devices may be able to provide more insight.
RE: Switched capacitors at generating plant
RE: Switched capacitors at generating plant