Power DC Voltage limiter with dump load for wind turbine 3

[SparkyJoe]

Hi,

We've a problem on a wind turbine installation and suspect this may appear on other installations.

The setup:
a. 6kW turbine outputing 3-phase AC - voltage and frequency varying with windspeed, connected to
b. Rectifying controller - which basically rectifies 3-phase AC to DC. Output from rectifier fed into
c. Grid tie inverter, which takes varying DC and generates 1-ph mains ac, synchronised with voltage and frequency of mains supply.

The Problem:
In strong winds the turbine is generating more than its rated 6kW. As the inverter can't process all the power, the DC voltage starts to rise. It's rising above the 600Vdc level the inverter allows, so it automatically cuts out. However, the now free-spinning/unloaded turbine causes the DC voltage level to rise even further and damages the triacs on the inverter input (inverter has no in-built protection)

My thoughts are to produce a voltage limiter that caps the excess voltage (and power) from the rectifier, perhaps dumping via a small heat load - whilst maintaining the 6kW power delivery to the inverter. Problem is, we're control engineers with little power electronic experience (although general electronics experience).

Any thoughts or suggestions gratefully received.
Joe.

 

[itsmoked]

This is indeed a problem common to wind.

6kW.. You could use a contactor and a voltage sensing relay with hysteresis to drop a large R on the DC whenever it gets too high.

This is the simplest method however with the simplicity comes problems. The main one would be what happens if your generator is trying to continuously output a little more than 6kW? The relay my 'short cycle' until it's toast.

It would be better to use electronic switches, (FETs, BJT, IGBT), that don't care about how often they are switched. Of course this is more expensive and you'll need to do some design.

Also if you have a 6kW generator and the wind tries to drive it at 8kW for half an hour is your generator going to toast?

Why can't you force the turbine to point off the wind some percentage to keep the output right at 6kW?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[SparkyJoe]

Thanks itsmoked,

Mechanically adjusting the turbine so its power output is reduced to the 6kW it should be is also being tried as a first approach - but isn't quite as straightforward as it might be.

I just want to also examine electrical control options.

Joe

 

[itsmoked]

What I don't understand is what happens when whatever electronics/electrical you have fails, and you aren't steering the turbine somehow, why don't you start pitching the blades as deadly spears?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MikeHalloran]

The blades must be overedesigned if they aren't breaking from overspeed.

I think the traditional low-tech means for limiting this condition is a passive trim tab on the rudder that steers the turbine off the wind axis as the wind gets stronger.

It should certainly be possible to extend that thinking a bit, sense an overvoltage limit, and drive the rudder to put the turbine sideways to the wind, sort of like a "de-thermalizer" on a free flying model airplane trips and kills the lift.

For a little more complexity, it should be possible to close the loop, steer the rudder toward one side as the power level goes up, and point the turbine far enough off-axis to keep the power level at the maximum rating.

Actually, I thought that last was sort of standard; are you guys new to wind turbines?



Mike Halloran
Pembroke Pines, FL, USA

 

[davidbeach]

I agree, this is entirely a mechanical issue and the wind turbine design needs to be such that the shaft output of the turbine never exceeds the capacity of the electrical system. PERIOD (FULL STOP for those on the other side of the pond) no ifs ands or buts.

 

[SparkyJoe]

Thanks guys,

For clarity - we don't make the turbine, we install them and it's a long established and pretty successful model.

The turbine is a downwind design and so doesn't furl in the traditional sense. Speed governance is controlled via the blades pitching and 'folding' back to reduce surface area as windspeed rises.

As I said earlier, the various mechanical adjustments that can be made are being examined as a first and preferred approach; but I see benefit in additionally having some form of electrical protection between the controller and the inverter - in order to protect the inverter. Rest assured, the turbine's max rpm is mechanically limited.

 

[LionelHutz]

Proven Turbine and Windyboy inverter?

You either need a way to automatically govern the turbine or you need a dump load. Since governing the turbine is mostly up to the manufacturer to design-in you're basically stuck with a dump load.

The dump load needs to handle full output power in case the inverter shuts down for any reason. Your grid tied inverter won't operate when the power goes out on a stormy windy night.

I'd look at VFD braking modules. This is a module that connects to the DC buss and turns-on a resistor to keep the buss voltage below a certain level. A module for a VFD using a line voltage of 380V-260V would probably work perfectly. Offhand, I know Bonitron makes some but I'm sure there are other companies making them too. Maybe check with US Drives too?

 

[itsmoked]

Never heard of a "VFD braking 'modules'".

Thanks Mr. Hutz

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

It appears that you are satisfied with the mechanical overspeed protection that is part of the wind turbine and are looking for an electrical solution.
The biggest concern is not the selection of a device to solve your problem, but the fact that when you allow a 6 KW generator to output more than 6 KW, you are overloading it.
You can add complexity and cost to your system to waste power.
Or, you can just use a larger inverter and return all the energy to the mains. I suspect that a suitably oversized inverter will add less cost than any load dump system will, and instead of wasting the excess power, it will be generating revenue.
You can use a simple contactor to disconnect the alternator in the event you lose the mains.
respectfully

 

[Snoball]

Why not connect to a big battery for a brake and charge it up?

 

[LionelHutz]

Keith;

They're made to use with drives that are made cheap by not including a braking or buss chopper transistor.

Yaskawa has them too. They call it a CDBR Braking Module.

SparkyJoe hasn't responded but it sure sounds like he's describing the Proven and Windboy combo. From what I've read, the Provens are pretty rugged. Still, if a turbine is catching wind it should be loaded.

 

[SparkyJoe]

Thanks again Guys,

Yes, it's a Proven with WindyBoy. The problem is that the WindyBoy doesn't allow much leeway if you start going over its rating - so a 6kW Proven feeding a 6kW WindyBoy will start to have problems if the blades don't regulate the output at just about 6kW. The Proven is very rugged and we do have the option to adjust blades and spring tension etc - all of which we're doing. But getting this just right isn't easy.

I do appreciate the braking module suggestions and have been investigating these since your posts. My plan is to have Proven alter the controller design to incorporate some electrical fail-safes that will help soak up the margins.

SparkyJoe

 

[LionelHutz]

Going back to Proven and getting them to do something better is a good idea. Does the Windyboy inverter have a dump-load back-up for when the lines fail? Last time I looked I couldn't find much technical info about them. It would seem silly of Proven to sell a package that can unload the turbine in high wind situations when a turbine should always be loaded.

Yes, saying the Proven is "pretty rugged" is a big understatement. Almost indestructable but they do have a small swept area and require a location with decent wind.