Phase and frequency

[davsoft]

I've been trying to understand a few things regarding power generation.

Firstly, how do they syncronise the phase of the AC from all the power generators in the grid? Take a simple example. I have a single phase alternator producing 240 RMS at 50hz. If I add another alternator to the circuit producing the same 240RMS at 50hz, if they're not set up correctly I'd expect the AC waveform to not be the correct sine wave, you'd have additive and subtractive interference. But then if they were in the same phase the voltage would all be adding up. Maybe I just don't understand the concepts but how is this done in the power grid out there?

Secondly, if I have a wind turbine driving and alternator, the frequency will be directly proportional to the RPM of the alternator. When they have turbines hooked up to the grid how do they get it at the right frequency etc?

Just some stuff I've been wondering about for ages and haven't been able to find answers for. Thanks,

David

[pvale]

First of all, if you have one generator running, and try to connect a second generator to it without synchronizing them, they are going to buck each other hard, possibly to the point of burnout of one or both of them. This has even been known to shear generator shafts. Google generator sychronizing and you'll get tons of info. I've gotten 2 small portable gensets into synchronization, but it is tough to get them to share load equally. You'll need lots of metering and patience.

The permanent magnet alternators as otherpower builds them produce wild frequency AC and cannot be hooked to the grid as they are. The best way to get them onto the grid is to rectify the output, and feed a wind capable invertor.

There were some early small wind generators that used an induction motor. They had something to sense the wind speed, and connect the motor to the grid. Once connected, spinning the motor at it's rated RPm + slip would generate low power factor power into the grid. You can do this with any 1725 rpm electric motor. Connect it to power, then apply a driving force to speed it up to 1850-1875 rpm and you will be sending power back into the grid. A 1725 rpm motor's synchronous speed is actually 1800 RPM. They have about 75 rpm of "slip". So, 1800 + 75 rpm is synchronous speed plus slip. Those early wind generators back in the 70's had many problems, and most of them are out of service now. They had proprietary controls, and when the manufacturer died, parts were not available to fix them.

Now, the big 1-2MW generators work this way. It's an induction motor up in the nacelle doing the work. Like I said earlier, induction motors playing generator put out watts but not many vars, and the other power plants on the grid must make up vars for them.

Some of the newer wind fields are being required to place capacitor banks at the aggregation point where they all come together and feed onto the grid.

Probably too much information for you, but generators etc. is a fascinating study. At least for me, but then I'm a power engineer anyway.

[davsoft]

Ok thanks, what's var?

With the induction motor method I take it that the generator is actually a motor and the grid is supplying energy to give it the extra push required to match the frequency of the grid?

David

[pvale]

Vars is Volt-amperes reactive. The simplest way is to describe it as part of the power that has to be there to excite such reactive things as motors, but does no real work.

When synchronizing a generator to the grid, the big, old stiff grid will hold the generator in synchronization, and you can manipulate the throttle of the genset to control the power fed to the grid. 2 small gensets are going to do some hunting in trying to share load. I always wanted to experiment with feeding them both into a paralleling reactor to force equalization, but never had the time, and don't have 2 gensets now to play with.

[Flux]

"With the induction motor method I take it that the generator is actually a motor and the grid is supplying energy to give it the extra push required to match the frequency of the grid?"

Induction machines don't synchronise and have no defined frequency.

If you run an induction motor off a 3 phase supply it will run a bit below synchronous speed. Take 4 pole and 60 Hz then the synchronous speed will be 1800 rpm. At that speed the rotor is going the same speed as 3 phase rotating field and no current is induced in the rotor and it produces no torque.

It will slow down to the point where there is enough current induced in the rotor winding ( often a cast aluminium cage) to balance the torque. On no load it will run very close to sync speed and on full load the slip may be perhaps 5% depending on the design.

If you drive it above sync speed then it will become a generator and will transfer power from the driving source to the supply. The supply will determine the frequency at all times ( and it will also supply the magnetising current but don't worry about that now). At full generated power the slip will be typically 5% as for a motor but the speed will be above the 1800rpm.

It it this ability to generate at fixed frequency ( supply determined) that lends itself to use with wind power where torque and speed is constantly changing. It is almost impossible to maintain synchronism with a synchronous machine powered by a wind turbine without some form of soft coupling to absorb the changes in inertia. The old Smith-Putnam machine used a hydraulic coupling but modern machines use a dc link to a special inverter and the synchronous generator is not held at constant speed. ( same is true of small grid tie machines).

That comment may cause more confusion if I don't explain. A synchronous generator is one in which the frequency is directly dependent on its rotational speed. It can run at any speed as long as you can accept the frequency produced. It is only tied to a fixed speed if synchronised to something else.

Wind is much more tricky than a controlled prime mover such as a turbine or engine used in normal installations where alternators are synchronised.

Flux

[davsoft]

Ok thanks, I understand that now. Still don't understand how an induction motor works yet, but will read up on that now. Thanks,

David

[davsoft]

Just another stupid question. I have a 1MW generator online, stuff hooked up to it is using 900KW, what happens to the other 100KW? I heard that it's not actually producing all that power when it's not being used, but I don't really understand why. Anyone give me a bit of a clue?

David

[Flux]

You don't generate it.

If the alternator was on no load ( forget losses) you would produce volts but no power, the turbine would have no load on it. If you load to 900kW then you will generate 900kW and the power into the alternator from the turbine ( neglecting losses) will be 900kW.  

All the 1Mw means is that this is the maximum rating of the alternator. It will exceed that but not for long without over heating. It's like a car engine it may do 60hp flat out but on part throttle it will do whatever the car demands. You won't go flat out on tickover just because you have a 60hp engine.

Flux