eklhad@ihnet.UUCP (K. A. Dahlke) (08/26/85)
A recent lunchtime discussion centered around the energy consumption of Fermi Labs (a nearby facility). Recall that AT&T typically hires many physicists, as government funding waxes and wanes. Well anyways, they were "harking" (I love that word) back to the days preceding superconducting magnets, when energy consumption was enormous. Once these magnets were installed, a general question was raised. Can we put power back onto the grid, and get a fair rebate. The ruling (after all, Com Ed is a regulated utility) was "yes". It probably isn't dollar for dollar, since Com Ed must pay the I*I*R losses, but you *can* get credit for putting energy back. Anybody know how much? Since I have only had a few basic EE courses, I don't understand *how* you put power onto the grid. What is the mechanism? If I drive a generator (e.g. wind powered), I would have to be damn careful, if I planned to simply plug the thing into the wall, wouldn't I? Voltage sources in parallel have to be exactly equal, in phase and amplitude, else you produce a virtual short circuit. How does a homeowner do it? How would they measure it, a separate meter? This seems necessary if they buy and sell at different rates. How does Fermi Labs put their I*I*L energy back onto the grid? You have to keep the current flowing in those magnets, from start to finish, while absorbing and releasing the energy. Sometimes the commercials talk about Com Ed's 6 generators, all running concurrently, to provide reliability. They *must* be (essentially) in parallel. How do you match phase and amplitude in all of them. When one fails, do circuit breakers take it out of the system? Now that I have demonstrated my complete ignorance concerning commercial power distribution systems ... any responses welcome. -- This .signature file intentionally left blank. Karl Dahlke ihnp4!ihnet!eklhad
karn@petrus.UUCP (Phil R. Karn) (08/27/85)
To feed power into the utility grid, your generator must be synchronous with the grid's frequency, but leading slightly in phase. You control how much power you put into the network by changing the phase difference. If you match it exactly, no power will flow at all, and if you lag the grid's phase, you will draw power from it instead. Here's a mechanical analog that might help to visualize this. Imagine your AC wall outlet as a steel shaft rotating at 3600 RPM (60 revolutions/sec). No matter what you connect to this shaft, you cannot change the speed at which it turns (unless you exceed its breaking strength, at which it stops!) but you can still take power out or put it in. If you connect a gasoline engine to the wall shaft through a stiff rubber coupling, it will turn even without fuel except that you'll be drawing power from the outside to do this, and the rubber coupling will twist such that the engine shaft phase lags the "line" phase. Now feed some gas into the engine and open the throttle slightly. If you give it exactly enough gas to overcome engine friction while operating at 3600 RPM, the rubber coupling will not be twisted in either direction and no power will flow across it. If you now open the throttle further, the engine will attempt to drive the outside shaft. The engine will now lead the drive shaft's phase, twisting the rubber coupling in the other direction. The amount of this phase difference is proportional to the amount of power being fed into the outside network. Notice that during all of this the engine has been running at constant speed; only its phase relationship with the outside has changed. This model works well as long as your generator is tiny in comparison with the others feeding the grid (valid for home windmills and the like). However, I suspect that things get tricky when dealing with the output of a GW nuclear plant, and I understand that sudden and large power flows can occur when things like blackouts and plant trips occur. One of the advantages of DC power transmission is supposed to be greatly increased control over power flows. Phil
henry@utzoo.UUCP (Henry Spencer) (08/27/85)
> ...I don't understand *how* you put power onto the grid. > What is the mechanism? If I drive a generator (e.g. wind powered), > I would have to be damn careful, if I planned to simply plug the thing into > the wall, wouldn't I? Voltage sources in parallel have to be exactly equal, > in phase and amplitude, else you produce a virtual short circuit. > How does a homeowner do it? Remember that most generators will also run as motors. If your generator is out of phase with the grid, the grid will drive your generator back into sync. Think of it as pushing on a crank attached to a huge, very massive spinning flywheel -- you will add energy, but you will not change the rotation rate noticeably, and either you start out in phase with it or it will drag you into phase. Of course, if you're too far off to begin with, it may break your arms doing this, so it's advisable to get things pretty close first. The power company often also wants to install some sort of special gadget at the interface, partly because when they take a power line down for maintenance they want to make sure it is really 100% dead. > How would they measure it, a separate meter? > This seems necessary if they buy and sell at different rates. Yup. Although it may be possible to build a meter that would run at different rates forward and backward. > Sometimes the commercials talk about Com Ed's 6 generators, > all running concurrently, to provide reliability. > They *must* be (essentially) in parallel. > How do you match phase and amplitude in all of them. A combination of careful synchronization plus the generator-as-motor effect. > When one fails, do circuit breakers take it out of the system? Yup. -- Henry Spencer @ U of Toronto Zoology {allegra,ihnp4,linus,decvax}!utzoo!henry
schwrtze@csd2.UUCP (Eric Schwartz group) (08/28/85)
As I understand it: Brookhaven uses the collapsing magnetic fields in magnets to drive a flywhell that stores the energy till the next pulse. You then have no hassles with 'lilco' save for not running during the summer and turning off during peak power periods. I wonder how efficient this method is. Hedley Rainnie.
control@almsa-1 (William Martin) (08/29/85)
The PBS TV show "This Old House" has several times shown the innards of solar-power home installations that sell the excess power back to the utility. They have a panel of circuitry interfacing the two systems, and usually two meters, one reading what was bought from the utility, and the other showing what was sold back. There is a "black box" (actually in grey electrical cabinetry :-) that does the phase matching between the two. (The solar systems produce DC, of course, and it is converted to AC before feeding into this load-balancing and distribution network.) Will
jb@rti-sel.UUCP (Jeff Bartlett) (08/30/85)
> As I understand it: Brookhaven uses the collapsing magnetic fields in magnets > to drive a flywhell that stores the energy till the next pulse. You then > have no hassles with 'lilco' save for not running during the summer and > turning off during peak power periods. I wonder how efficient this method > is. > Hedley Rainnie. I remember from somewhere that the water valves on a large hydro project (>10ft) would affect the utilities such that they had to buffer the energy. Motor/generator/flywheel sets would slowly be brought up to speed then the energy was dumped into the vane control motors. This would distribute the power demand spike over time. Jeff Bartlett Research Triangle Institute mcnc!rti-sel!jb
roy@phri.UUCP (Roy Smith) (09/04/85)
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 <allegra!phri!roy> System Administrator, Public Health Research Institute 455 First Avenue, New York, NY 10016
smh@mit-eddie.UUCP (Steven M. Haflich) (09/05/85)
In article <442@phri.UUCP> roy@phri.UUCP (Roy Smith) writes: > 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). About 25 years ago I was fortunate enough to witness the local electric power company prepare for the evening load by bringing online to the power grid an additional generator -- a steam-driven armature the size of a small building, spinning at 60Hz. (Generators are not normally spun down when offline because it takes days for their bearings to reach operating temperature, and to cool during careful spindown.) When offline, the generator spins free at approximately 60Hz, and bringing it online requires synchronizing its rotation with the grid. The monitoring devices was a simple lightbulb connected between the grid and the generator, along with a nifty rotating arrow to indicate phase angle and looking for all the world like something from a 1930's Frankenstein set. (Remember, this particular generator was an already considered aged when I saw it sometime in the 1950's.) The single operator carefully adjusted a servo controlling the steam valve for the turbine, finally getting the generator within about .5Hz, with the arrow rotating and the light varying along with the phase error. The generator was then `cut in' with remote control switches which connect it's output to the grid. His art consisted of compensating for the delay of the large mechanical switch mechanism -- a significant fraction of a second -- so that connection would be made at nearly zero phase difference. He explained that the generator must be riunning at nearly the correct speed, or else the mechanical impulse of the sudden speed change could destroy it machanically. The generator must also be cut in at approximately zero phase, or else the sudden electrical surge from the phase angle difference could destroy it (and/or the swtches) electrically. Remember, large generators are measured in megawatts and tens on tons. On the whole, I'm glad I have a job where I can recompile after a careless mistake :-).
mueller@hpisla.UUCP (Joe Mueller) (09/19/85)
This is all vaguely reminiscent of an oil well I saw in OK. Once. The thing was driven off of an electric motor. As the shaft was slowly raised by the motor, you could see the dial on the power meter spinning away. Then as the shaft was lowered, the dial took a brief step backwards! I suppose this is indicative of insufficient counter-weighting, but a rather amusing phenomenon. Joe Mueller Hewlett-Packard Instrument Systems Lab. Loveland CO.