Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site phri.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxt!houxm!vax135!timeinc!phri!roy From: roy@phri.UUCP (Roy Smith) Newsgroups: net.analog Subject: Re: Selling Energy to the Utilities Message-ID: <442@phri.UUCP> Date: Tue, 3-Sep-85 19:45:20 EDT Article-I.D.: phri.442 Posted: Tue Sep 3 19:45:20 1985 Date-Received: Thu, 5-Sep-85 02:47:52 EDT References: <482@petrus.UUCP> <2550001@csd2.UUCP> <387@rti-sel.UUCP> Organization: Public Health Research Inst. (NY, NY) Lines: 66 While I'm far from an expert in the field of power generation and transmission, I once took a course on it. The professor explained a method used to make sure a generator was in phase before bringing it on-line. First, get your generator running as close to line frequency as you can with mechanical speed control. Once you have it matched to within a few cycles per minute, you take a standard light bulb and attach one side to the power grid and the other side to your generator (after appropriate step-down, of course!). As the two sources drift in phase, the bulb gets brighter and dimmer (it sees the beat frequency of the two sources). When the bulb goes out, you throw the switch to connect your machine to the grid (and cross your fingers and/or hide someplace far away). Of course, he may have been spicing the story up a bit (I'm sure they use something fancier than a light bulb to measure phase difference), but the basic idea is right. Looking inside a synchronous generator (or a synchronous motor -- they are identical), you see a rotating magnetic field produced by the stator and another by the rotor. Call them B1 and B2. The fields are rotating at the same speed (at least when things are working right), but there is a phase difference, phi, between them. If you have two magnetic fields (rotating or stationary, it makes no difference) which are out of alignment by an angle phi, they exert a torque B1*B2*sin(phi) which tries to re-align them. If the machine is motoring, that is the torque exerted by the shaft which you can use to do mechanical work. If the machine is generating, that is the torque you are exerting on the shaft to turn it. Notice that this only works if the rotational speed of the two fields is the same; there can only exist a phase difference, but no overall frequency difference (I guess this is an electro-mechanical phase-locked loop). Also notice that the torque is 0 when phi is 0: when the fields are perfectly aligned you get no torque and hence no work. If phi is negative, you get negative (mechanical) work out of the machine (i.e. a generator). If phi is positive, you get positive (mechanical) work out (i.e. a motor). The mechanical work into the shaft plus the electrical work into the terminals sum to zero (modulo friction and other losses). Even more important is that the torque is at a maximum when phi is 90 degrees. Past that point, the torque decreases and you go out of sync (A Bad Thing). You keep driving your shaft harder and harder; the lag angle increases, and you pump more and more power into the power grid. If you keep increasing the torque with which you drive the shaft, eventually you will exceed the magic phi = 90 degree point and loose sync. If your machine is small enough, it will just sit there and vibrate. To see what I mean, take a small synchronous motor and run it with no load. Then (carefully) grab the shaft with a pliers. As you apply more pressure with the pliers, the shaft shouldn't slow down, but inside the motor, phi is increasing. At the magic point, the shaft will all of a sudden stop and just wiggle. If the motor is big enough, it will tear itself (and you) apart violently, so you probably don't want to do this with anything bigger than a phonograph motor. Most typical motors you find will probably be induction motors, but you can get sort of the same effect if you clamp the shaft before starting it up. Well, I hope I've given a bit of insight to the problem. I should add, however, that analyzing the transient behavior of synchronous machines under sudden changes in load is, as they say, non-trivial. -- Roy Smith System Administrator, Public Health Research Institute 455 First Avenue, New York, NY 10016