There is a detail we mostly pass over here in discussing generators and motors (which can be the same thing) used with AC.
A generator or motor can be synchronous or asynchronous. That is, its rotating part can have predefined poles (magnetic regions) or not. You can make more efficient big generators and big motors with predefiined poles, and adding one to the system is complicated because you have to make the speed and phase match first.
But an asynchronous generator develops its own poles because it “slips” or drifts away from the synchronous frequency, spinning a little faster. Its poles slowly migrate around the rotor. In fact their migration is what generates the current inside the rotor that causes there to be any poles at all.
Asynchronous motors are much more common than synchronous ones. If you buy a motor for your air compressor or attic fan, and connect it to the power lines, and try to spin it faster than it is already going, it acts as a generator. And it takes care of its frequency and phase. The faster you spin it, the more power it contributes to the system (and the hotter it gets).
For the sake of power plant designers this is an unimportant tangent, but if you have a windmill or a water turbine and want to sell power back to the utility, this is conceptually the simplest way to do it.
What is the voltage of electricity at the powerplant and as it enters the grid? 480v? And how is that power regulated…transformers? Here at my work I’ve got three phase power (480v), florescent lights with 230v ballasts, and 110v power from normal outlets. How are those voltages distributed like that? How is it “stepped down”, so to speak?
The exact numbers will vary from one system or region to another, but in Ireland you might have 11kV at the generator terminals, stepped up to (say) 220kV for transmission.
The station has a huge (hundreds of tonnes) transformer for stepping up from 11kV to 220 kV. It will also have one or more smaller transformers for stepping down to supply the needs of the station itself.
The transmission and distribution systems will also use transformers of various sizes to step down from 220 kV to 38 kV, 10 kV, 380 V, etc., for supply to customers. The final step to household voltage is achieved by splitting the three phase supply into single phases.
Here in the US depends on the area and age of power plant. The generator’s output voltage is contolled by varing the field strenght. The plant by design will product 3 phase AC power at design voltage. At the plant the voltage is steped up to tranmission voltage. Normally either 250,000 or 500,000 volts. When it gets near the end point it will be dropped down to 100KV, then distributed around the city the next drop down may be in the 11KV range. Then at the building the voltage is dropped down to 4160 or 480 volts.
The voltage out put of a transformer is determined by the input voltage and the ratio between the number of turns of windings on the high side to the low side.
250,000 to 500,000 volts for transmission? Wow. It’s no wonder those huge electric line towers have all those warning signs on them!
So am I correct in guessing that the voltage is ramped up that high for transmission across miles and miles of power lines because so much is lost getting it from one place to another (kinda like signal degradation)?
The loss from transmission is current squared times line resistance. If you double the voltage then the I2R losses are less by a square. If you raise it a 1000 fold then the losses becomoe very small.
During the early growth of the power grid, converting DC to higher voltage DC was difficult. AFAIK, motors turning generators were the only method used, and that wasn’t much. But today there are numerous high voltage DC transmission lines. The I2R loss is only one kind of loss, and another loss is energy radiated away at 60 Hz, essentially as very low radio frequency energy and inductive and dielectric losses heating things near the lines. DC transmission fixes this problem. There are now solid state converters that package the energy as DC for the hundreds or thousands of miles long haul and turn it back to AC for handling and distribution on a local basis.
What happens when transformers overload and explode? I know it isn’t just from lightning strikes…sometimes it just happens. Isn’t there some kind of internal circuitry that acts as a breaker to prevent this from happening to handle power surges and such?
BTW, this has been a very educational thread, and sorry to the OP if I come off as hijacking.
I have a degree in English and understand very little about such things. I just want it to work when I flip the switch!
While inductive and capacitive losses are an issue, one of the main benefits of high voltage DC power transmission is that the line voltage is always at peak.
For AC, you have a sine wave. It is constantly switching on and off and on and off in a sine wave pattern. This on and off sort of averages out to about the square root of 1/2 multiplied by the peak value of the sine wave (called the “root mean squared” value or “RMS” value - you can’t really say “average” because the average value of a sine wave is zero). The 120 volts you are used to in your house is 120 volts RMS. It’s actually about 170 volts peak to peak. You effectively get about the same amount of power as if you had 120 volts DC, but the wire and insulation has to be designed to handle 170 volts.
The same thing works on high voltage transmission lines. If you have a 220 kV AC transmission line, the insulating standoffs on the poles and all other equipment has to be designed for 312 kV. A 220 kV DC transmission line only has to be designed for 220 volts though. For the same size wire, same insulation, etc. you can pump more power through the line using DC.
AC to DC and DC to AC converters have come a long way since the days of motor generator sets, but they are still a lot more complicated than an AC transformer. An AC transformer is just two coils of wire around a hunk of iron. That’s hard to beat for simplicity. DC requires more expensive switch gear as well. If you open a switch on AC, any arc generated tends to extinguish itself because the voltage drops to zero twice during the AC cycle (it’s a sine wave). DC runs at a constant voltage, which means there’s nothing to naturally suppress the arc.
The extra cost of the switch gear, transformers, and inverters has to be weighed against the savings in cheaper wire for the same amount of power. Once the wires get long enough, DC is more efficient. Below a certain distance, AC is cheaper because the wire savings don’t beat out the switchgear costs. The break even distance is getting shorter and shorter as transformers, inverters and switchgear gets cheaper, but you still only see DC used for transmission from one area to another. We’re nowhere near the point of seeing DC being used for distribution around neighborhoods.
I know Snnipe 70E knows this, but for others in this thread, power is voltage times current. When you put transformers at either end of the line, the overall power stays the same, so if you double the voltage, you get half the current. That’s why he is saying if you double the voltage you get one fourth the resistive heating losses, since those losses are related to the square of the current. Triple the voltage and your I(squared)R losses go to 1/9th.
Transformers are just two coils of wire around an iron core. The voltage increase or decrease is proportional to the number of turns of wire in each coil.
The transformers you see on utility poles or in boxes in yards have their coils immersed in oil. The oil helps to keep the transformer cool, and also allows them to use a higher voltage on the wires without the electricity arcing over and shorting out the coils.
When a transformer overloads, the oil boils, pressure builds up inside the can, and the whole thing goes boom. You can also have oil leak out through a hole, in which case the coils of wire get exposed, the electricity starts to arc, this arcing heats up the remaining oil, and boom!
There is no circuitry or breaker inside the transformer. At various locations throughout the system, there are breakers which work kinda like the breakers in your house, only on a much larger scale. These breakers often have “automatic reclosers” in them. This is basically automated equipment that flips the breaker back on after a fault. When the breaker trips in your house, you have to walk downstairs and flip it back on. If a breaker trips on a power line, the power company has to send a guy out to wherever the breaker is located to flip it back on, which is usually a lot longer hike than just walking down to your basement. Since most power lines trip because of temporary faults (a tree blowing into a line during a storm and temporarily shorting out the line, for example), it would get really annoying to have to send guys out to flip breakers all the time. So, they invented automatic reclosers.
The recloser is programmable. The way they are usually set up is they try once or twice fairly quickly, say maybe once at 1 second and once at 5 seconds after the fault. Then they typically wait something like 2 minutes and try again. If that attempt fails, the recloser usually gives up and a lineman has to go out and figure out what the problem is.
If a power line is down, stay away from it. You may think it is dead, and it may be dead, but then you can get killed when the recloser kicks it back on at the 2 minute mark.
Actually, the breaker on the generator will probably trip long before any damage to the generator occurs.
If you get “close enough” but not so far away that the overcurrent caused by the phase difference makes the breaker trip, you can dim the lights and make generators jump on their mounts.
When I was an electrician on submarines, it was always fun when trainees would try to shut a breaker way out of phase, and the whole room went nuts screaming NOOOOOOOOOOO!!! at him.
Interesting, and I would never touch or even approach a downed power line, no matter the situation. Except maybe if it was ensnaring a loved one or something, but that’s so far beyond the realm of actual possibilities that it isn’t even worth considering.
Thanks for the transformer info. Quite educational.
There are six ways to directly create electricity. The two most common are magnetism(generators) and chemical reactions(batteries).
Less efficient, and therefore less common are heat (through the use of thermopiles and thermocouples), pressure(piezo crystals), light (photovoltaics), and friction(static electricity).
Fun bit of trivia from when I used to teach this, is that five of those can work in reverse – that is to say, electricity can perform five of those actions directly, without any intermediary machinery/steps. Friction is the odd man out.
Radiative losses also depend on the length of the wire. Roughly, they start becoming significant at about the point where the length of wire is comparable to the speed of light divided by the frequency-- For 60 Hz, this is a few thousand miles. For this reason, the US doesn’t have a single power grid, since a single grid spanning the entire country would be a very efficient antenna for just beaming power off into space. Instead, the country is divided into three grids, one each for the east, west, and Texas.
Sometimes what happens is in a car accident a power pole is compromised and the lines touch the vehicle or the ground nearby. In that case the ground itself may be energized. Many people have heard that you should keep both feet together and hop or shuffle away if you have to leave the vehicle but they don’t know why. If the power lines are energizing the ground there will be a voltage gradient like the ripples around a stone thrown in the water. If you walk normally you could have one foot at one potential and the other foot at a different potential causing a fatal current to pass through your body.
Thanks Laudenum for a most incredible thread. I must confess that I’ve only just gotten around to reading it. Shit, everyone knows the answer, water turns the generator and the generator puts out the juice. But having finally finished chasing Cecil’s ghost thru 57 pages of archives, I again find myself with idle time and curiousity. You should know that anyone who claims to know electricity just got a good game goin’.
Nonetheless, what you got in #2 thru #8 is a college level course that anyone with an interest in electricity can understand. True, the thread begins to wander somewhere after your last post #9. What can you really say after ‘thanks for everything’? The electrical engineers will tell you.
Well, true. But only if the resistance/length stays the same.
The real answer is that insulation is cheaper than aluminum.
This is an over-simplification, but if you are designing a power grid that serves millions of customers, and you want to transmit a given amount of power from Point A to Point B, then you have the following choices:
High current, low voltage.
Medium current, medium voltage.
Low current, high voltage.
(And everything between, of course. But I’m trying to keep this simple.)
Due to the high current, choice #1 would require conductors (wires) that have a large cross-sectional area, else the voltage drops and I[sup]2[/sup]R power losses would be excessive. This means you would have to use big, heavy, aluminum conductors. Aluminum is expensive. Plus it would be so damn heavy that you would need lots of supports. This solution sucks.
Choice #2 is a better. But #3 is the best… the current is low, which means you can use conductors with a small cross-sectional area, which in-turn means the conductors will be lighter and cheaper. Because of the high voltage, the only major disadvantage is that you’ll need to make sure you use good insulators where the conductors are anchored to the towers and supports. And you’ll need to use transformers that are designed to operate at this voltage. But this is a LOT cheaper than running miles and miles of big, heavy, aluminum conductors.
This is true for both AC and DC, BTW.
And again, my explanation is a bit of an over-simplification; I’m glossing over a bunch of other stuff in an effort to stay focused on the major issues.