Tom Van Baak wrote, "If it wouldn't be too much to ask, yes, I think several of us would be interested in how, and under what rules, they tweak the frequency."
It helps to have a mechanical understanding of the problem. By the time you get to the end of this, you should know why the frequency can't be continuously regulated. If you deliver steam to a turbine from a large boiler, the speed of the turbine is limited by the rate of steam flow and the pressure drop across the turbine. Flow times drop gives you the power from the boiler to the turbine, just like amps times volts. A throttling valve is put in the steam line to regulate the power to the turbine. The turbine will rotate at a speed determined by the power balance between the steam and the shaft load. Increase the load and the turbine slows down. The load is a synchronous generator. This doesn't mean much for an isolated turbine and generator combination. If you connect thousands of generators to a common distribution system, they are all locked together in speed, but not phase angle. As Dean said, the phase angle varies with the power flow. The frequency varies with the power balance. To emphasize the common speed aspect of the power system, if one generator is disconnected from the system (perhaps because its hydrogen cooling gas has developed a leak) then it must be re-connected very carefully. The steam throttle is adjusted to bring the speed of the generator (and its frequency) up to match the distribution network frequency. Then a phase indicator is used to detect the phase angle difference. The breaker connecting the generator to the power system can be closed when the phase angle is zero. If the angle is not zero, the power of thousands of generators tries to bring the one generator into phase alignment instantly. The mechanical forces can do great damage before the breaker trips. So here we have this network of thousands of turbines and generators all locked to the same frequency because they are synchronous machines. In effect, we are back to the simple case of the speed being determined by the power balance between steam and electrical load. But we really have a network, and the ability to measure power flows at various points in the network. Electric utilities have joint power dispatching centers. The dispatchers work to balance the power in the system by telling power generation stations how much power to generate. This works in large scale systems. You can't tell the users how much to consume, you just try to meet the demand. Small generating plants and large networks maintain a clock attached to the power line and another from a standard, like WWV before GPS. The station manager adjusts the fuel flow to the boilers to alter the power balance to change the turbine speed. The throttling valve can't be adjusted with sufficient precision to do frequency control. Large or small, the power line clock loses time when there isn't quite enough fuel and gains when there's a bit too much. Large networks barely stay ahead of demand with new power plants. There are "peaking" plants that only generate when the demand exceeds the base capability of the boilers. They use expensive fuel for rapid response (gas turbines) but they hold the power balance. If you measure the difference between power line time and GPS time over 24 hours, you will see the line clock lose time during the day, when loads are highest, and regain that lost time in the very early morning, when loads are low. The dispatchers tell generating stations to add a bit more fuel to bring the line clock back to match GPS or a little ahead, anticipating the day's demand. Why not use automatic frequency control? Because the controllers in the different stations would fight each other, because they are tied together by the synchronous network. In other words, it is not possible to regulate a stable speed/frequency with more than one integrating controller - and one controller can't regulate enough power to stabilize the network - unless the controller is a dispatcher able to tell many stations what to do. More than I intended to say, but I hope someone finds it useful. None of the above is guesswork. I've been interested in this since a summer job on an island in 1955. I've not talked to a dispatcher, but I have confirmed the details with a control engineer at a large power station. Regards, Bill Hawkins _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
