I know our power grid is AC. I know as a grid, we are all interconnected, such that if one plant goes down, other plants can feed into the affected area. (right so far???)
Does this mean that all of the power plants in the grid are synchronized such they are all alternating in unison at 60 hz. I’d think if a plant was totally out of phase with the rest of the plants, it would essentially be wasting power.
Also does that mean for those few individuals that produce their own power and sometimes feed back into the system (their meters run backwards)…their generating system has to be in sync with the rest?
Power plants have whats known as a governor that is linked to the grid frequency. When the frequency is dropping the governor increases the output of the plant and when the frequency is increasing it reduces the output.
If you were foolish enough to even try and connect a generator that was out of phase with the grid you’d basically end up with little pieces of generator all over the floor, and possibly embedded in nearby walls and people and such. The failure would probably be rather spectacular.
When you hook up a generator to the grid, you use a synchroscope to insure that the generator is exactly in sync with the grid. Then and only then do you try and connect the generator to the rest of the world.
Once the generator is locked in with the grid, if you try and spin the generator faster it will produce power to the grid. If you try and slow it down it will suck power from the grid and will effectively become a motor. It won’t change speed, though. It remains locked at the grid frequency.
So its kind of a feedback system that keeps all of the generators in the country spinning not only at the same frequency but also in the same phase. Is the whole continent in phase then?
Is there some gradual shift in phase comparing east coast and west coast? such that a operator in california can call his buddy in new york and see that his sine wave is just a few milliseconds behind? Did I phrase that clear enough?
Yes, each grid is synchronized. My solar panels go through a solid state system to ensure the 60Hz power generated is in synch with the rest of the grid and back in my Navy days as an electrician, we used to manually synch the 8 Generators to each other and most importantly we would first synch our generators to the shore supply before disconnecting them and shutting them down so we would have no power interruption. I had a switchboard watch screw up royally and bring his generator in 180° out of syn ch and blast back open the main breaker, causing so major damage to the contacts. It took a lot of work to replace and repair the breaker. My job as the watch was my trainee.
The Los Angeles Times columnist, the late Jack Smith, recounted a tale that a professor at UCLA told him about the professor’s days at Berkeley. It seems that PG&E was short of water during a drought so they slowed the frequency down in order to conserve water and all the electric clocks ran slow.
I wrote Smith and said that you just plain can’t slow down the frequency of the power grid quite that easily. That if you try to slow down a generator it begins to run as a motor and if you try to speed it up all that happens is that it delivers more current to the grid. All generators on the grid are essentially magnetically coupled together and would have to be simulateously slowed and that’s quite a trick.
Smith followed up by by calling the professor who insisted that the clocks were slow, which I don’t doubt. Utilities use load shedding and they might have turned the power to the University off at night. When asked what he was a proffessor of he he said, “Germanic literature.” Smith also called PG&E who said that your couldn’t do it.
Despite that many people wrote Smith to explain that PG&E and I simply didn’t understand the system.
Two ways to synchronize a generator before adding it to the grid:
Connect three light bulbs between the A, B and C phases of the generator and the like phases of the grid.
Start the generator and bring it up to speed.
As you approach the right speed, you will see the light bulbs winking on and off. The winking will race around the three like chaser lights. If you’re going too slow, they’ll race one way - too fast, the other way. Keep adjusting the speed until the chaser light effect comes to a stop.
Now adjust the phase - that is, temporarily speed up or slow down the generator and return it to the constant speed - until all the lights are in the off state.
Now you can throw the switch.
Another way to synchronize a generator:
Just use an ordinary induction motor as your generator. Say you get a 1750 RPM motor (a very common speed). It is rated that way because at 1800 RPM it would be turning in phase with the grid. Slipping relative to that speed generates a current in its rotor winding (which is just a shorting winding) which makes it spin. At 1800 there’s no current. At 1850 there’s a current again, but now you’re pumping power into the grid - in phase and everything.
Due to peak loading, it is normal for the entire grid to slow by a fraction of a Hz during the day. They compensate at night (actually counting individual cycles ) to keep the clocks about right on a daily bases, and dead nuts over the long term.
If you compare an old fashioned electric clock to one of the newfangled WWV enabled clocks you’ll see the second hands get out of sync, and then come back in over a day.
In the 80’s I worked breafly at a low budget radio station (yes there was a lot of truth in WKRP). Anyway, their satillite feed for network content (Paul Harvey, national news, etc.) was exactly to the minute.
By watching the sweep second hand they would introduce the shows pretty seamlessly: [DJ]And now, with “The Rest of The Story”, here’s Paul Harvey[/DJ][PH]Good Day!..[/PH]
They kept a yellow sticky note with an arrow on the studio analog clock face to show where the second hand would be at the top of the minute. They’d have to tweak it (the note, not the clock) every hour or so, and the DJ’s would come in a few minutes early to check it before they started thier shows. It moved by around +/- 5-8 seconds over the course of a day.
I’ve a very vague recollection from A Level physics, that the UK grid aims to gain or lose no more than one cycle per 24 hours. Might be spurious, I’d be interested to hear any other similar comments.
I saw the same thing when I had a digital clock that used the AC line to generate a 60 Hz square wave for the counter circuits. It would often slow down by a second or two during the day, but would catch up overnight. A friend, who used to work in a power plant, said that keeping the long-term frequency at exactly 60 Hz was part of their job. Think of all the wall clocks that used to run on AC synchronous motors. Many older computers also used the line frequency to drive their real-time clocks.
This doesn’t work so well when you are on an island or some other isolated place where your grid is composed of several diesel generators.
One thing to wonder about… Everyone knows that a transformer will change the voltage and frequency of AC, but it’s less widely known that they also change the phase by 90 degrees. Would that mean that you have to keep track of how many transformers any given part of the grid has gone through?
That does not sound right, are you sure of this? We did not have this concern on the Carrier’s Distribution system. I think it might depend on use of the Transformer involved. I was under the impression that on the electrical grid, the transformers on the pole just correct the voltage and that the large transformers further back are the only ones that Transform the voltage from 10,000V down to 450V. I do not recall any phase problems.
Neither I, nor any of the other electrical engineers whom I know well enough to discuss such things with (several), know any of that except for the voltage part.
Transformers change the frequency only to the extent that non-linearity introduces harmonic distortion of the input waveform, almost none for good designs. They will alter phase by either 0 or 180 degrees depending on how you choose to connect the windings.
It would be exceptionally convienient if a device as simple as a transformer could shift phase by 90 degrees over a wide range of load conditions. It would make motor starting, and converting single-phase power to three-phase trivial for example.
Current might well lag voltage in the primary of a lightly loaded transformer, but this has nothing to do with relitive phase of input and output voltages.
Of course there is. If you view NYC from Los Angeles it will be 15 milliseconds behind Los Angeles. If you view Los Angeles from NYC it will be 15 milliseconds behind NYC. If you view them from a neutral location equidistant from the two they will be in phase
The preceding semi-humorous reply was to illustrate that yes, there will always be some phase issues due to the maximum speed of propagation (which is less than c, of course.) How the network is built to deal with this, or if it even has to or if simply matching phase to the local phase as per previous posters will be sufficient, I have no clue.
Nope - A phase change doesn’t change the frequency - any generator hooked up to the grid downstream from one or more transformers would operate at the correct frequency (speed) and still be at the correct phase (angle).
Actually there’s another horrible way. We had a small town in Missouri which had a 1MW (small) generator that consisted of a diesel motor driving a generator via a large belt. The operator would just “get er up to speed” and then dump the connection switch. The belt would then slip and smoke until the motor and the generator “synched”
Actually they were protecting, not planning. By designing and operating the grid just within the state, they were trying to avoid federal oversight of their “local” commerce. Another factor was the prominance of Texas oil contribution to the US during the time that the grids were established. It was easy to meter outgoing oil products but they were sceptical of anything which would circumvent the the metering process by using oil to generate power and shipping the energy out via wire instead of drums. Times have changed and this is no longer an issue but the grids remain the same.
The Eastern and Western grids happened because the countries non-rural population grew from the East and West coasts respectively. By the time the electric systems approached each other near the Rocky mountains each grid was so large that a single AC tether between them could not handle even a subtle difference between the two (analogy a 1 inch rope might be able to secure two rowboats together but would snap if you tehtered two battleships together). Its possible to connect these grids via DC and theoretically they could be connected AC if you created many interconnections and switched them all at once. That’s not practical so the major grids use DC ties to transfer power.
Time correction is done frequently on the grid. Usually one of the larger area utilities is the central coordinator on time and they will notify the other utilities in the area to speed up or slow down for so many minutes or hours usually during early morning. The variance is very small 59.95 or 60.05. They can’t drop the frequency too low or take it too high because some protection equipment keys on low frequency (automatic load shedding) while other equipment keys on high frequency (generator protection)
