How can different generators all contribute to a grid? Do they have to somehow be in sync with other, so they’re not “pushing” or “pulling” against one another?
For those of us not steeped in electromagnetic theory, it will help if you use more than two words to explain that concept. 
This is what I meant to think. Forget the other bit I posted.
OK. A wave can travel faster than a particle, as it has no mass. The electrons don’t have to move much to transmit a wave. If they traveled as a particle, they would have to be speedy bits indeed to make the round trip from a power plant.
Light can be described as both a wave and a particle, as some phenomena can best be explained by one or the other. Waves on the ocean travel horizontally, and can travel fast (tsunami), but the actual water particles don’t move much except slightly up and down (until the tsunami reaches land).
Yes.
But when “periodically” is used in regular conversation, the connotation is that it changes slowly. When I read the OP, it seemed that he had the impression that the current flows one way, then sometime later, like maybe this afternoon, it will reverse. Leaffan was right to point out that it’s changing at 60 times per second.
When he said that the electrons are vibrating back-and-forth real fast, I think he meant that 60 times per second is real fast, not that their velocity is fast.
periodically used in regular conversation is often used for slow change, maybe also for irregular change (this corruption may have come about because of slowness then applies to all changes).
in physics the period of the wave is the time it takes for one wavelength to pass. all waves have it though it is generally only used for slow waves, faster waves are referred to with their frequency.
confusing language sometimes.
:smack: Dammit, I screwed this part up. It should read:
During the second part of the AC cycle, the alternator at the plant is producing large negative voltage, i.e. terminal #1 is at much lower voltage than terminal #2.
Sorry for any confusion this caused to anyone who took the trouble to read through my long-winded essay.
This is what my simple mind was yearning for. Thank you! It’s now as clear as I deem it should be.
So how does that work? The thing that synchronizes the generator’s output is powered by the grid so it matches? What would happen if it weren’t in sync? Does it happen?
http://www.omicron.at/fileadmin/user_upload/files/pdf/en/01pAccurSyncDevEnerLyzer.pdf
If the alternator’s output is not synchronized (i.e. in phase) with the grid voltage, then when the two are connected there will be large voltage differences across very-low-impedance connections, resulting in correspondingly large current flows. Expect lots of heat, light, sparks, and equipment damage (unless there are protective devices that immediately break the connection to avoid such damage).
The generators themselves naturally sync to each other. There isn’t anything external required to do it. It’s a natural property of motors and generators.
You start with one generator. Then you add a second one to it. You use a device like a synchroscope (see Machine Elf’s link) or synchronizing lights to make sure they are in phase, and as soon as they are, you throw the switch that connects them. As soon as they are connected, if you try to slow one down, the other one will push it along like a motor and make it spin at the same speed. With only two generators, the extra drag caused by slowing one down will slow the other one down somewhat, but you keep repeating this process and once you’ve got more than a handful of generators on the line then one individual generator can’t effectively slow down and drop the frequency of the entire line any more.
Now that you’ve got all of these generators running in lock step together, if you try to speed one up, the only way it will actually speed up is if it speeds up every other generator on the line. What happens in the real world is it just supplies more and more power to the line until it reaches its maximum output, which usually isn’t anywhere near enough power to speed up every generator, so the line frequency isn’t affected. Similarly, if you try to slow one generator down, the other generators spin it like a motor and keep it in sync. By trying to make the generator spin more, all you do is add power without increasing its speed. By trying to make it spin less, all you do is decrease its power (possibly to the point of it becoming a motor instead of a generator) and again, you don’t actually decrease its speed.
This makes it difficult for the power company to adjust the speed of the line. They used to be required to keep the long term frequency adjusted to 60 Hz so that clocks running off of synchronous motors would keep accurate time. The electrical load during the day would tend to slow things down, so the power company would speed the frequency up at night to compensate for it. Increasing the line frequency isn’t as simple as turning up a dial somewhere. It has to be a coordinated effort between all of the generators on the system, or it won’t work.
If you didn’t use a synchroscope and just randomly tried to connect a generator to the grid, the fact that the voltages would be out of sync would cause very large amounts of current to flow, resulting in very large electromagnetic forces that would try to almost instantaneously get the generator into sync with the grid. These currents and magnetic forces would generally far exceed what the generator was designed to handle, and you would very likely have a very spectacular failure of some sort (burned out or melted wires, smoke, and possibly a generator that mechanically self-destructs into very large pieces).
It’s been my experience that a lot of folks don’t understand the difference between these.
For those who don’t, power is generated at a plant somewhere, usually not in the middle of a residential neighborhood. The power is then transmitted (sometimes over a very large distance) to a substation. This is typically done at very high voltages (several tens of thousands up to even a few hundred thousand volts) which helps to reduce the losses. Generally the higher the voltage the better, since the resistive losses in the wire are proportional to the square of the current. When you step the voltage up or down, the current is reduced by the same factor, otherwise you start defying physics (the overall power has to be the same, neglecting minor losses in the transformer). In other words, if you step up the voltage by a factor of 10, you reduce the current by a factor of 10. So the higher the voltage, the better. You start running into practical limits though, since higher voltages can arc through air and short out. Higher voltages therefore require better insulation in the standoffs that hold the wire to the towers and everything else on the line as well.
Transmission lines, by the way, are generally not insulated. They are just bare wire.
After it goes through the substation, the power is distributed around your neighborhood. Again, higher is better with respect to voltage, but distribution voltages are typically a lot lower than transmission voltages. A typical distribution line will be somewhere between say three and ten thousand volts or thereabouts. From there, the voltage goes through another transformer which drops it down to a split (center tapped) 240 volt phase, which typically feeds three or four homes from a single transformer.
DC has two main advantages. First, it always runs at peak voltage. AC lines have to handle the peak voltage with respect to insulation and such, but effectively (for how much power you get out of it) run at the RMS value. Second, DC lines don’t suffer from inductive and capacitive losses, which in longer lines can be very significant. The disadvantage of DC is that a DC transformer is not a simple device, where by comparison an AC transformer is just a couple of coils of wire wrapped around a hunk of iron. AC also has an advantage over DC when it comes to switching. As you move switch contacts farther apart, you tend to draw an arc between them. AC voltages drop to zero twice during each AC cycle, which will tend to naturally extinguish this arc. Since DC remains constant, it will not suppress the arc. DC switchgear therefore has to have special arc suppression in it, making it more complex and expensive.
So now you have a tradeoff. DC is cheaper and better in the wire, but much more expensive at the transformers and switch equipment at either end of the line. In the early days of electricity, the equipment cost for DC so far exceeded the equipment cost for AC that DC systems weren’t practical. Over time, though, DC transformers and switch equipment have gotten cheaper, to the point where DC power transmission is actually cheaper in the long run than AC. Once you get pasta certain distance, the cost savings in the efficiency of the line outweighs the cost of the extra equipment involved, and this distance gets shorter and shorter as they keep coming up with newer and cheaper equipment.
That break-even distance though is still measured in miles, and for power distribution, DC is still so expensive and the cost savings across distribution distances are so small that DC will never be a practical alternative to AC at any time in the foreseeable future.
There has been a trend to use DC more and more for transmission in recent years, and I would expect that trend to continue. Don’t expect to see DC in distribution any time soon. Not gonna happen.
Very informative post – thanks for taking the time to write it.
Along those same lines, how does it work when you connect a solid state cheap-o suicide cord grid tie inverter to the mains? Is the phase simply matched exactly? Does a phase angle regulator fit in here somewhere?
At least for lower power levels, not really a disadvantage; most electronic equipment now uses switch-mode power supplies instead of AC transformers; while more complex, they are cheaper because of the cost of all of that iron and copper (a SMPS transformer may be 100-1000 times smaller and lighter). Of course, AC transformers are much more rugged, especially from surges, unless protection is added (additional circuitry is also needed to simulate a resistive load if the input is AC due to rectification).
Yes. Just the voltage, or just the direction of current isn’t enough, but with both you can tell.
In a two-wire transmission line, there will be a voltage between the two wires, which means there will be an electric field E pointing from one wire to the other. This will be strongest between the wires.
There will also be current flowing on the two wires, in one direction on one, and in the opposite direction on the other. The magnetic field H curls around a wire with current flowing in it, following the right-hand-rule: with the thumb of your right hand pointing in the direction of current flow, your fingers point in the direction of the magnetic field. The magnetic field from the two wires will add together between the wires.
So between the wires you’ve got an electric field pointing from one wire to the other, and a magnetic field pointing between the wires. The cross-product E X H, called the Poynting vector, points in the direction of power flow. That page doesn’t have a good picture for a two-wire line, but it does have one for a coaxial line, and the same thing applies there.
In the picture, if you’re not familiar with the convention, a circle with a dot in it means the vector is pointing out of the screen, and the circle with an x in it means the vector is pointing into the screen. Supposedly representing an arrow point coming at you, and the arrow feathers going away from you. So in that picture, power is flowing into the screen.
Haven’t read all the replies, so I apologize if this has already been covered.
People refer to the electricity available at your wall receptacle as “alternating current” (AC). In my opinion, however, it should never have been called AC. It should have been called “alternating voltage” (AV). Because that’s what it is; the polarity of the voltage is constantly alternating between the hot and neutral conductors. It is an alternating voltage regardless of what’s plug in to the receptacle (unless it’s a complete short, obviously). And it’s an alternating voltage even when nothing is plugged in to the receptacle.
The *current *at the receptacle is a different story. It might be periodically changing direction when something is plugged into the receptacle. And it might not be periodically changing direction when something is plugged into the receptacle.
Oh…
So, it’s a Mechanical wall? (spinny stoof)
Would more (faster) = better, or it doesn’t matter? (use the same at any rate despite frequency, 50-60 Hz vs 1-100,000 Hz) :dubious:
When I first started understanding wiring…There were Two hot wires and One ground.
Later on, there was Two hot wires and the ground was tied to the black wires at the box, meaning white = Hot.
Now, it’s the Black wire is the Hot one tied to the Ground at the box and white is the neutral.
Considering I played with electric for 20 years with the same conflicting knowledge.
Are they just messing with my head, or am I remembering wrong from the get go (taught wrong)? 
The guy that wired my house 20 years ago must have gotten his info from the same place that you did. Did Cracker Jack distribute a flip book on home wiring or something back in the day?
Chicago had a thing for yellow wires at one point, but I don’t recall any time in my lifetime where white was hot. Certainly nothing gets “tied to ground at the box” besides neutral.