Control of a Wind Turbine

JonAW

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Sep 2013
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Hi Guys,

This one might interest some of you.

The last year or so I've been working on a 500kW variable pitch wind turbine (restricted through settings to 250kW). This is a little unusual in that the turbine is designed to be grid connected but I have made a system to allow it to self excite and therefore run as an island generator. To date the system has been running off and on.

My PLC is a AB micrologix 1400. I am using a PID control loop to monitor the RPM of the generator(4 pole 50hz 1500rpm asynchronous motor) and adjust the load applied to stabilise the speed. The operating range of the turbine means I only have a tolerance of +- 100rpm on the generator otherwise the turbine's standard control system will fault on over-speed or under-speed.

In relatively constant wind I have the system fairly stable and it will operate well without stopping. The turbine will run at approx +- 40rpm. In gusty winds the system isn't too bad taking a very strong gust to overcome the control system. In higher winds (above 13m/s) the system can't run as the turbine is trying to restrict it's output by pitching the blades which causes a slow oscillation in my control system.

The parameters I have in my control loop are:
Kc = 3.6
Ki = 0.03
Kd = 0.00

Loop update 0.01.

Is it possible to tune a PID to be reactive in massively varying conditions? As you can imagine the rate of change of wind speed varies considerably and sometimes dramatically.

I'm quite new to PID control and have got the setting by trial and error. I find that if I apply a Kd it sends the system out of control but I'm really struggling to get a stable rpm with just Kc and Ki. Can anyone offer any help / advice on this or even tuning PID loops in general?

Thanks

Jonny
 
Is it possible to tune a PID to be reactive in massively varying conditions? As you can imagine the rate of change of wind speed varies considerably and sometimes dramatically.
Yes, I don't know if a ML1400 can change gains on-the-fly without having the control output jump but I know other controllers can do it. You really need to have a model of the system under varying conditions. You may need a variable transmission between the wind turbine and the generator. Some designs use a hydrostatic transmission. Another advantage is that the wind turbine pump can fill and accumulator during bursts. The accumulator is like a capacitor.

Other systems vary the pitch of the blades to absorb more or less energy.

This isn't a matter of just applying a PID. Some design work is required.
 
Hi,

The turbine does vary it's pitch to limit output which is quite successful in that it will peak on output to 300kw before dropping back to 250kw but this upsets my tuning. I have kept the turbine on it's standard control system as it already has all the necessary protections built in. This is a bespoke controller designed specifically for this turbine and would be well out of my skill levels to try to decode it.

I'm currently trying out a system without PID. Initial trials seems to have a much smoother load control but doesn't control the speed quite as well, but it is early days on that one.
 
If I get what you are saying correctly is that I could have a variable gain based on amount of overspeed or underspeed?

The PID in the ML1400 from memory will not jump with small changes, I'm not sure about large changes.
 
the rpm is controlled by the pitch and the load, so you will need some settings and just change the PID settings on the fly.
you could also have a look at a fuzzy (expert) system as this will give you always the best settings.
 
If I get what you are saying correctly is that I could have a variable gain based on amount of overspeed or underspeed?
Yes. This is not a trivial application to do right. Actually all the controls should be done in one controller. It is necessary to coordinate blade pitch with wind speed. There will need to be some way of buffering/storing or smoothing out energy surges. When using motors the energy should be use to charge batteries. If using hydraulics the excess energy should go to charging an accumulator or many accumulators. Basically this is the hydraulic equivalent of a capacitor.

When you say you can adjust the load do you mean control how much power you are generating for the outside power grid?

The PID in the ML1400 from memory will not jump with small changes, I'm not sure about large changes.
The ML1400 PID is simplistic so don't make big changes in gains.
 
Hi Guys,

Thanks for replying.

It is necessary to coordinate blade pitch with wind speed.

From monitoring the turbine I believe this particular model varies blade pitch according to load. The turbine is designed to maximise it's output on a mains grid which can absorb sudden bursts of power short term. It may use the wind speed and direction to assist with this though.

This application is for a drying floor to dry waste or timber chippings so there is no requirement for batteries. The turbine is being harnessed by a large bank of heating elements into a water tank which is then blown out with fans under the drying floor. It is a strange application.

It is necessary to coordinate blade pitch with wind speed.

In an ideal world putting this system into one controller would be the best all round. In this case we don't have plans to remove the standard controller as the turbine designers would have spent years developing the software to the point it is at now. It would take me even longer to get the turbine to where it is now.

I believe the pitching of the turbine is upsetting my control so what I plan to do is change the PID settings on the pitch control to soften it a little and allow my system to take the wind gusts which I believe it can handle. The softer blade pitching control should help maintain a steadier rpm and therefore a more stable system.
 

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