# Electrical Systems questions (wind turbines)

1. Aug 5, 2009

### davidcowling

Hi, im currently working on a year placement between my 2nd and 3rd years at university.

I'm working at RWENpower in swindon, UK and goto Coventry University in the Midlands.

I have a couple of questions about the industry and certain technical issues.

1. Im currently looking at windfarm single line diagrams and confused about voltages from wind turbines. Do the voltages add up along the array? if so, then do tap changers on the primary coil of the transformer make up for the lack of voltage (say, 10 turbines output 500V each = 5kV, on a 33kv/132kV transfomer, does the 33kV derate itself or a combination of both tap changers on both sides of the transformer)

2. When the wind blows at different speeds, surely a variable frequency is output from the wind turbine, how is this changed to grid frequency standards.

3. Lastly, if each wind turbine on one particular array is outputting a slightly different frequency, then how do they interact with each other and how is the electrical signal cleaned up?

2. Aug 5, 2009

### Staff: Mentor

Welcome to the PF.

I don't know about the series addition question, but I asked a similar question about the frequency aspects myself a couple years back (I asked a general technical help line at my local newspaper -- this was before my PF days started). The windmills are locked to the grid because of the very low impedance of the grid. As long as the wind is strong enough, they transfer energy to the grid. If the wind goes light and the windmill stays connected, it is driven like a motor by the grid, and consumes power. So the windmill will generally only be connected to the grid when the wind is strong enough, and when it gets connected, it snaps the final couple of degrees into synch with the grid.

3. Aug 5, 2009

### mgb_phys

I wouldn't have thought you would wire windmills in series - it would make working on them a safety hazard if both sides of the generator where at 5KV for the last one in the string.

Not an expert on power electronics (I generally stay away from circuits where a dropped screwdriver can turn me into a hamburger) but i thought it was difficult to make efficient variable speed AC motors/generators. It would seem easier to have the generators DC and then use converters to generate grid power at a stable voltage/frequency/phase.

Some large installations have the turbine head driving an air or hydraulic pump and then generate the electricity from a generator set on the ground driven by the flow from the head. This would make it easier to combine the power from multiple turbines.

4. Aug 5, 2009

### Staff: Mentor

At least for the windmill farms around here, they are AC. You can tell because the whole farm is spinning in synch. Pretty cool to see.

5. Aug 5, 2009

### mgb_phys

I suppose the blades are only usable once they reach a minimum wind speed so at higher speeds it might be easier to use the blade pitch to govern the speed and 'waste' some wind energy rather than try and run at the wind speed.

6. Aug 5, 2009

### Staff: Mentor

I think the deal is that the windmill generator couldn't run faster, no matter how hard the wind is pushing. It would have to drive the whole grid faster to do that.

7. Aug 5, 2009

### mgb_phys

Thats my point, once you reach the RPM that produces 50/60Hz output you have to feather back the prop even if the current wind speed could drive it at 2xRPM and generate more power.

8. Aug 5, 2009

### Staff: Mentor

For the AC connected systems, I don't think they do any feathering. The grid controls the rotational speed. The wind speed determines how much current you feed to the grid.

BTW, wikipedia has a pretty big set of articles on wind power and related stuff:

http://en.wikipedia.org/wiki/Wind_power

They do mention some hybrid generation systems, where the turbine is not connected directly to the grid (like the DC generator option you mentioned). For those systems, there probably is an optimum rotational speed that depends on the wind velocity.

9. Aug 7, 2009

### Mike_In_Plano

Mike here,

I looked into the wind turbines and found that most, but not all, use synchronous generators. Assuming there's enough wind, the blades will turn and pull the generator in sync with the grid. Excitation current to the rotor is used as a control parameter allowing the generator to approximate the grid voltage as the blades bring the rotor up to grid frequency and phase. When the generator is in phase and approximately at the correct output voltage, the generator is tied to the grid.
From then on, the pitch of the blades and rotor excitation current are used together to derive the maximum usable power (load torque) while keeping the power factor near unity (current in the same phase as the line voltage)

As an aside, induction motors have been used as generators. They cannot be used as stand alone generators, but once tied to a power line, one need only drive the shaft with positive torque such that the motor turns above it's synchronous speed.

- Mike