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23rd Sep 2019 Login  
Wind Power and Grid Regulation
by Tony Smith at 2008-11-05 18:56:55 (Forum::Public::Wind)
Just wondered how large-scale wind generators interact with the national grid, from a regulation and stability point of view.
Hi,

Back in "my day" the grid was mostly (exclusively?) served by rotating generators where the rotation was directly linked to the frequency and phase of the grid. For a reason that escapes me at the moment, the grid frequency was the primary indication of supply vs demand - and the machines were regulated by speed so that if the frequency were to drop then they would increase their output, or if the frequency rose then their output would reduce.

As I understand it now, even big wind turbines use electronic invertors to generate the AC and sync with the grid. How do these interact with the grid?

For example in a situation where a lot of wind power was poured into the grid resulting in an oversupply, would the wind turbine cut their output or would they be relying on conventional power stations making the correction?

--
aesmith

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Wind Power and Grid Regulation Tony Smith - 2008-11-05 18:56:55
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Re: Wind Power and Grid Regulation Website Administrator - 2009-09-07 15:17:35
Right, that's enough spam from you.
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Website Administrator
Re: Wind Power and Grid Regulation Bryan Rendall - 2009-09-24 21:02:18
An old question, never answered and bumped to the top again by indirect means!

Will have a go at answering it for nothing other than a bit of debate stimulation!

Having run generation plant driven by diesel, gas turbines and now wind turbines, I hope to be at least half in a position to pitch in something of worth!

As more and more wind generation penetrates the UK grid, then the control systems implemented become more and more sophisticated.

An "old" induction generator wind turbine really had no control - It produced whatever the wind was fit to drive it at. If system frequency rose or fell, the turbine would run slightly faster, or slower, accordingly, without any change in power output. A simple system of power factor correction, allowed simple changes to power factor, but always leading.

Most modern wind turbines now use dual fed induction generators, with the stator directly grid connected, and the rotor, inverter fed. This permits the generator to run at variable speed, whilst still "hard wired" to the 50 Hz grid. It also allows a broad range of control functions, including running at leading or lagging power factors or running in voltage support / suppression modes, amongst others. Coupled with Statcoms (or Static Var Compensators) which are more commonly being applied on remote windfarms where the grid is weak, and pitch regulated turbines, which can reduce or increase their output on demand from a remote telemetry system, then the modern wind turbine is very versatile!

How they are controlled depends entirely on how the grid connection is managed. Some windfarms now have very tight limits to operate within, and this can be varied on an hour by hour basis, even remotely, depending on grid requirements - In all, not vastly different from the controllability of a conventional power station!

New and innovative methods of operating grid systems, such as the Registered Power Zone in Orkney, seek to explore ways of increasing utilisation of embedded generation, including wind turbines, and this is releasing additional capacity in a previously "full" grid, by implementing innovative diversified control systems that ensure the embedded generators are as much a key part of controlling the system as the conventional, centralised generation.

Hope this helps answer the original question without too much waffle!

Cheers

Bryan
--
Bryan Rendall

Re: Wind Power and Grid Regulation Tony Smith - 2009-09-25 08:50:19
Thanks.

The "dual fed" generator is new to me (no surprise there), at first glance it doesn't sound possible for a variable speed generator to have any of its windings directly grid connected. My reading of the fairly sketchy information on Enercon's web site suggested that they rectified the generator output then used an invertor to handle synchronisation with the grid.

More reading needed I suspect, as my knowledge of power generation dates back to the middle '80s and was only second hand at that time in any case.

What I was really getting at, is what happens to Wind generation if there's an over-supply to the grid? Do they back off in the way that traditional rotating machinery would, or do they continue to generate and expect other generators to back off?

There used to be a sort of analogy with Hydros since they could change output quickly without thermal problems, so my understanding was that their controls were set more sensitive to grid frequency.

Re: Wind Power and Grid Regulation Bryan Rendall - 2009-09-28 18:00:55
Hi Tony,

Dual fed simply means that the rotor is connected, via sliprings to a variable frequency 4 quadrant inverter, so what it can do, with the aid of a shaft position encoder, is vary the frequency and angular phasing of the rotor current so that the rotating magnetic field remains "synchronous". Well, not exactly synchronous, as it is still an induction generator, but "at or about synchronous speed" The reality is that the rotor can run at variable speeds, but the rotating magnetic field remains at ~1500 rpm

Enercon are different, in that they use a low speed generator without a gearbox. This requires all the power to be converted by a frequency converter at the bottom of the tower.

If there is an oversupply, a wind turbine will not back off simply because the frequency has increased, unless there is a specific control function required to that at that particular windfarm. All the ones we have worked on, where there is the ability to load manage, use external control signals, usually over leased lines direct from the electricity substations.

So its still very much a case by case basis, but I don;t think I have ever seen a windfarm that controls power by frequency.

Cheers

--
Bryan Rendall

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