cmcmanis@sun.uucp (Chuck McManis) (05/01/87)
In article <561@neoucom.UUCP>, wtm@neoucom.UUCP (Bill Mayhew) writes: > Roy's article got me wondering what your friendly local power > company does when they want to put an additional alternator on > line. It must be pretty tricky to match the rpm, and then the > phase angle of the output waveform too. > I've had a mental image of the operators getting the rotation of > the machine matched up as close as possible and then taking cover > as the machine lurches into phase with the rest of the power grid. > It seems like the multipole generators used in hydroelectric plants > would be especially tricky since they spin at relatively low rpm. > I suppose that in a hydroelectric plant, you could start out by > motoring the alternator and then letting the water add useful power > when the machine was up to speed. > --Bill When I went on a tour of one of PG&E's facilities for a class in Electromechanics we noticed a row of big diesal(sp) engines. When asked if they were backups for the steam turbines the answer was "No, they are for cold starting the plant. When bringing a new generator on line power from the grid is allowed to spin it up to speed using it as a motor. As it gets closer to the speed and phase of the power line it is stabilized. Then the steam vents are opened and it switches over from using power to producing power at the same phase as the voltage that started it." -- --Chuck McManis uucp: {anywhere}!sun!cmcmanis BIX: cmcmanis ARPAnet: cmcmanis@sun.com These views are my own and no one elses. They could be yours too, just call MrgCop() and then ReThinkDisplay()!
roy@phri.UUCP (Roy Smith) (05/05/87)
In article <17838@sun.uucp> cmcmanis@sun.uucp (Chuck McManis) writes: > "No, they [the diesels] are for cold starting the plant. When bringing a > new generator on line power from the grid is allowed to spin it up to > speed using it as a motor. As it gets closer to the speed and phase of > the power line it is stabilized. Then the steam vents are opened and it > switches over from using power to producing power at the same phase as > the voltage that started it." I can't quite parse the above, so maybe I'm just missing something. First you say the diesels are used to cold-start the plant, and then you say that grid power is used to spin up the generators. Can you clarify? I believe most commercial plants require power from the grid to start up. The large synchronous generators have smaller induction motors on the same shaft for spin-up purposes. (All synchronous motors need induction motors to start them because until you get up to speed, the synchronous motor can't produce any mechanical work.) The assumption is that the grid never goes down, just selected parts of it (people who have built network time deamons will be familiar with this idea). This is not always a good assumption. I believe there was a recent natural disaster in Hawaii which caused the entire Hawaian grid to go down, causing some consternation about how to start it all up again. Of course, The Blackout was a perfect example of this too. Earlier, somebody (I don't remember who) was wondering about problems with starting up hydro plants which run at low RPM's. I don't see any particular problem with synching a low-RPM generator, at least not any more than with a high-RPM generator. For those not in the know, steam driven plants (be they coal, oil, or nuclear fired) generally run at high shaft speed; typically 4-pole machines running at 1800 RPM. Hydro plants usually run higher-pole machines at lower shaft speed. The problem has to do with the speed of sound and Reynolds numbers in the varous driving fluids (high-pressure steam vs. water). If you tried to drive a water turbine at higher speeds, you would get turbulance, cavatation, and all sorts of other horrible things. Electrical engineers know better than to worry about the details of stuff like that and leave it to the ME's to grovel over. :-) If I remember, the generators at Robert Moses are 120 pole running at 60 RPM (1 rotation per second). The operative equation is f = 2F/N where f is the electrical frequency in Hertz (60 in North America, 50 in Europe), F is the mechanical shaft speed in revolutions per second, and N is the number of poles, always an even integer. Some texts will write it as f = F/N, where N is the number of pole-pairs. You rarely see 2-pole (3600 RPM) machines because they are harder to wind and don't run very A good question to ask is "with all these random (albeit synchronized) generators all over the place, what's to keep the grid frequency from drifting?" The answer is that somewhere on the grid, somebody is monitoring the line frequency, and comparing it to a standard (WWV, probably). Each day, they add up how many cycles the grid lost or gained compared to the standard and then try to make that up the next day by running everybody a little fast or a little slow. I understand that over the course of a week, the power grid is accurate to a fraction of a cycle per day. Considering that you have about 5*10^6 cycles per day, that's pretty amazing. -- Roy Smith, {allegra,cmcl2,philabs}!phri!roy System Administrator, Public Health Research Institute 455 First Avenue, New York, NY 10016 "you can't spell deoxyribonucleic without unix!"