Modern boiler firing rate and feed water controls could, if grid frequency is an input. ST governor valves could be positioned by limiter and load varies with steam pressure.
in argentina the primary frequency regulation (PFR) with gt's is achieved by keeping the gt in speed control 5% below the temperature control (rated load). BUT, and a big BUT you cannot use pre-selected load. the unit must be in DROOP. i.e. "floating" with the frequency.
this has the following effect: if the grid frequency drops below 50Hz the units doing PFR are able to load 5% to temperature control.
if the grid frequency increases above 50Hz... the units will shed load for some time (usually 2 min) and then resume running 5% below baseload.
the algorithm to do this with gt's is proprietory.
Yes, in the system the grid frequency is given as input. In The ST inlet valve positioning together with changing the firing rate takes place thus the fuel consumption rises. Are there any other mechanisms of PFC which does not effect fuel consump. rate?Does it help to damp steam extraction lines to heaters or any other mechanisms?
The method of PFC I am familair with envolves matching the power produced to the power comsumed. Thus if grid comsumes more than is produced, frequency droops, ST governor opens CVs, boiler firing increased, ST's generator makes more power to matched comsumed power.
I think you are asking. "can the boiler power be held constaint and the turbine power to its generator be varied to match grid comsumption?" or in other words "vary the turbine efficency to change load."
If the unit has an extraction process than can be varied, then it could be used to shed or increase turbine power to its generator.
It becomes very interesting question if the extractions are only to feed water heaters and some one more expertise in thermodymatics of the boiler-feedwater cycle will need to jump in.
But my thinking would be that the base load cycle should be at its most efficeint arrangement. Responding to to a frequency increase the cycle efficiency can be lowered by dumping some steam to condensor (then the $ paid for fuel will be going to heat the enviroment) Responding to a frequency decrease, limiting extraction to a heater to get an increase in flow through the turbine to make more MWs would result in a loss of boiler efficiency due to the cooler feed water and the MW inrease would decay away, plus a little more. It would think the temperary MW increase would only be for a few seconds.
It looks like a PFC which does not effect fuel consumption rate, would effect the plant's heat rate ($/MW).
Since one of my primary jobs is the sequencing of CVs to achieve linear load responds (and optimun efficency)I think this question is very interesting.
While the ST's governor will position the CVs to provide PFC. It also is to limit the turbine overspeed to less than OS trip following a load rejection.
This is one of my concerns with retrofit turbine controls and primary concern with maintenance of OEMs.
I am not too familiar with STs but work in GTs. We sometimes use variable nozzle power turbines to maintain the frequency. Why can’t we use such variable geometry in STs for PFC. I am sure it will be lot more efficient that throttle valves.
Turbine speed and load is controlled by the governor valve as this item is the fastest to cause a change in the turbine speed or power (according to the operation mode). Changing the boiler fuel feeding or steam pressure is a secondary loop which will change the power but much slower then the govrnor valve. You must use good and fast control valve and a relaible governor controller (Woodward or equivalent)
Cheer up - ilan
