Wind farm

[Scott]

I visited the UK's largest on-shore wind farm this afternoon. Some of the turbines were running and others were not. I assume this was simply a factor of demand.

There was one turbine we watched (turbine 40). It was running as we arrived, then it stopped, then it restarted (all as others in the same section continued to run). Why would they stop and start one turbine? Do they need to be rested or tested or rebooted periodically?

How do they maintain the frequency if the rotation speed can vary with the wind? I know they can adjust the angle of the blades but it was clear the turbines were not all running at the same speed.

[Vir Campestris]

There's one near me with only a dozen turbines.

I reckon there's one off more often than not. They aren't very reliable.

Frequency - AIUI generate DC, and use some clever electronics to get it on the grid. Not quite clever enough though...

Andy

[Chris Green]

Presumably there are inverters actually feeding power onto the grid.

[T i m]

<snip>

I think we went to a small power station when I was at college doing my HND in Electrical Engineering and I think they synched the frequency of their output to that of the grid manually ... or hit the big connection switch manually suggesting if they got it wrong by more than a bit, it could rip the generators off their mounts?

Cheers, T i m

[John Rumm]

I would expect by using a grid tie inverter - i.e. they just slave themselves to the incoming mains.

So there is no requirement to sync a rotating mass with the grid, which makes it easier from their point of view (and presumably less use to the grid)

[Paul]

GENERAL

Nominal power 4.3 MW Rotor diameter 147 m IEC class IIA

Turbine concept Direct drive, variable speed, variable pitch, full power

Power control Electric pitch control

GENERATOR Lagerwey multi-pole synchronous generator Field generation Permanent magnet

Cooling Based on air flow around the outside of generator (cooling fins)

CONVERTER Type Full power AC-DC-AC Control system IGBT-Control Cooling Water-cooled

OPERATIONAL DATA

Cut in wind speed 2.5 m/s Cut out wind speed 25.0 m/s Power factor Regulated

SAFETY SYSTEM 3 independent pitch control systems with emergency power provision

OTHER

Service brake system Standard Lubrication system Automatically controlled bearing and gear lubrication

The AC-DC-AC part, the output AC side matches the grid frequency. The output side could be single-phase or three-phase, whatever the customer wants.

Paul

[Brian Gaff (Sofa]

One I remember had some clever kind of gearing that could be used to power the generator even in light winds with the blades whizzing around and in strong winds with them turning more slowly, but presumably giving more torque. I never bothered to find out. I'm sure there are many types with all sorts of clever gadgets to try to be as efficient as possible. One worry though is local climate effects of such devices. If you are taking power from the wind, then one assumes downwind of the turbines the air is moving slower and with more turbulence as you often get when high rise buildings are built in windy places. Brian

[Chris Hogg]

So, for the benefit of my understanding, they generate AC of variable frequency depending on wind speed, rate of rotation etc. This is then rectified to DC, which is then converted back to AC of frequency corresponding to the grid frequency and locked in to it. Is that a reasonable summary?

[Chris Hogg]

The down-wind effects of wind turbines can be significant.

[The Natural Philosopher]

Yes.

All rotating generators are AC, and DC is obtained from them either by mechanical rectification (commutators) or via silicon diodes.

Only constant speed turbines running at some sub-multiple of 50Hz (3000 rpm) can generate synchronous mains frequencies.

Wind turbines by their nature are not constant speed devices and gearboxes to push than from a few revs per minute up to synchronous speed would be lossy fragile and expensive. And locking their blades to a constant speed would probably destroy them.

So its AC=>DC=>mains freq AC via two pieces of power electronics

[The Natural Philosopher]

There are more scholarly articles out there. studies done to find best 'planting separations'

Obviously it depends on wind turbine cost versus land cost.

[Paul]

Yes, that's it.

Just don't underestimate the complexity of that converter though. It's got a lot to do, to prevent the tower from toppling over, the blades from flying off, and so on. Since the document above claims an ordinary "generator" with a permanent magnet rotor is being used, there are some dangerous conditions involved there if the generator is under-loaded. Part of what the converter has to do is "get rid of the energy with haste". Transferring it to the grid, helps keep down the terminals voltage on the generator part.

They could use a generator, an alternator, a magneto. Each has different properties. On something like this, maybe they're shooting for decent efficiency. I don't know if the alternator is the best choice for that. Your car uses an alternator, because it's "dead easy to control". But it also happens to run at a rather impressively high temperature. Even generators get warm. That's why the 97% number in that spec sheet is pretty amazing.

Paul

[Ian Jackson]

In message [e-mail], T i m [e-mail]> writes

I did a fair amount of 'heavy current' electrical engineering at university, and one of the lab experiments was to run up a large (and fairly ancient) three-phase generator (driven by a DC motor) and synchronise it with the mains.

Not only did you have to get the speed right, but (obviously) you had to ensure that the three phases were correct. This was achieved by having a lamp bulb across each of the contacts of the three-pole switch that connected the three generator armature outputs to the mains. [I believe the lamps were coloured red, blue and yellow.] As you approached synchronous speed the fluctuations in their brightness would slow to a stop - and when the brightness was at a minimum you then had to adjust the generator field current to get the generator output voltage to exactly match the mains voltage (when the lamps went out). Only then could you throw the switch - and if there wasn't a big bang and a grinding of metal you knew you had got the generator successfully locked to the mains.

You then did various measurements about (for example) what happened when you altered the drive power supplied by the DC motor and the amount of generator field current. Again you had to be careful, because if you varied things too far the system could unlock from the mains (with the aforementioned spectacular results).

All this was pretty scary stuff, and these days I expect that the operation of power stations is far more automated and foolproof than it used to be.