Subject: Ancillary Services and OCGT
Date: 2010-10-03 (at 01:37:20)
I thought "frequency reserve" ancillary services would include some other stuff - especially for coping with sudden loss of a large generator (or corresponding transmission links).
That infrequent but regular occurrence seems to me far more important for "spinning reserve" than fluctuations in demand which are much smaller over short intervals.
eg a) as soon as frequency drops some large interruptible loads drop off automatically and stay off for say 30 minutes until other generators have time to ramp up. These are paid compensation as though they were generators. (eg cold stores, aluminium smelters).
b) lots of small diesels used for emergency power in hospitals, water pumping etc start up automatically to give slower ramping generators time to take over. Again these only run for say 30 minutes or so, but take longer to come on than the interruptible loads do to drop off.
c) hydro spinning reserve ramps up fast at say 30MW/minute to take over.
d) CCGT plant already despatched at less than 100% to provide spinning reserve ramps up as fast as it can to take over from the more expensive (or limited) hydro. CCGT is MORE maneuverable than simple cycle or OCGT. The latter is used for peaking because its capital costs are much lower so being unused most of the time matters less despite being more inefficient, ie more costly to actually run and less maneuverable. CCGT is cheaper to actually run. OCGT cheaper to provide "spare" capacity. This "spinning reserve" doesn't wait for the next 5 minute dispatch but responds automatically through droop frequency control to counter the frequency drop. The OCGT and CCGT that isn't dispatched as "spinning reserve" is already at optimum 100% capacity so can't help.
e) above "instant" reaction merges into the 5 minute dispatch of "warm standby" CCGT and where necessary, more expensive OCGT to take over and allow the interruptible loads a) to come back on and emergency diesels b) to drop off when the sudden loss has been compensated with extra dispatch of gas. Likewise the hydro can drop off.
f) at the same time dispatch puts additional CCGT into spinning reserve on less than 100% capacity to cope with any further sudden losses, replacing the spinning reserve that is now in use. Also puts more CCGT and OCGT that wasn't in warm standby into warm standby.
I'm not sure about this stuff, but its worth checking.