Tricky wind turbine question 5

[Windar]

Hello everyone,

I have a difficult question regarding wind turbines that I was hoping to get some feedback on.

Situation: There are a number of ~90 kW turbines mounted on 60-foot lattice towers. Due to the design of the machine, it occasionally happens that the turbine will go into what's called runaway mode. This means that there is no effective way to stop the turbine blades from spinning faster than they should be. We are looking at ways to bring a runaway turbine to a stop.

First of all, I realize the common answer is: you don't do anything. However, assuming that something HAD to be done, do people have suggestions for ways to bring an out of control wind turbine to a stop. The rotor diameter is about 17 meters, three-bladed, mounted on a 60' tower.

Any action taken would have to be done from the upwind side because these are downwind machines.

Thanks!

 

[Cockroach]

Variable pitch propeller design, feather back on the angle of attack on the blades during run away situations. Note you can also maximize the efficiency of output using variable pitch.

Second option, design the propeller shaft with a brake system that would be applied in situations of a run away.

Third option, design a transmission box that would increase the gearing ratio in favor of a stall. You could put a heavy viscous oil liquid in the gear transmission casing to act as a dashpot.

Hope this helps.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[rb1957]

sounds like a very undesirable occurrence, and maybe the control siftware needs to be updated so it doesn't.

how about having the pitch control dump so that the blades go into a "wind-mill" mode ?

could you apply an electrical load onto the generator, an electrical brake ??

 

[metengr]

Normally, large wind turbine blades have a pitch control to allow them to reduce airfoil exposure to the wind. You could see about retrofitting a multi-blade pitch control device that could handle all three blades at once.

 

[MiketheEngineer]

All good ideas - I like "feathering" because the prop pitch control also allows you to "tune" the pitch for max efficiency. You could even "reverse" pitch for a few moments to speed the slow down.

Or call the "Jolly Green Giant" and have him stick a finger in it.. That's how I used to stop my model airplane engines. Kind of painful - but worked.

 

[SnTMan]

Windar, would you care to reveal approximately where these runaways are located?

Regards,

Mike

 

[MikeHalloran]

Don't stick a finger in it.

Got water?
Aim a firehose at the advancing blades.

That's something you can do without climbing the tower and without adding anything, until you can get the controls straightened out.


Actually, I might be inclined to put a nozzle on the after end of a big old truck so I wouldn't have to be near the blade plane while adding the water impact stress to the overspeed stress.



Mike Halloran
Pembroke Pines, FL, USA

 

[zdas04]

If the blades have the variable pitch, I'm sure that the OP wouldn't have posted the question.

I'd look for a way to apply a heavy electrical load.

David

 

[flash3780]

I like David's idea. Maybe you could install some massive resistors somewhere near the towers (though not too near) to act as a brake if you were to flip an emergency switch.

//signed//
Christopher K. Hubley
Mechanical Engineer
Sunpower Incorporated
Athens, Ohio

 

[MikeHalloran]

ISTR that one likely reason for an overspeed is that something has already gone wrong with the electrics, and some protection mechanism has disconnected the generator from the grid, so connecting resistors to the generator may not be an option. ... but I am often wrong, so it should be investigated.

Also, couplings and gearboxes sometimes fail, again resulting in overspeed.

If I had the option of adding something, I'd consider a spring-set brake as close to the propeller hub as possible, triggered by an external paddle or panel that could be impacted by a rifle shot.



Mike Halloran
Pembroke Pines, FL, USA

 

[SnTMan]

Scattershields?

 

[dvd]

A strongly worded drawing note stating that "turbine shall stop upon overspeed."

 

[Windar]

Hello all,

Thanks for the fast responses and good ideas!

Someone got it right when they assumed we do already have variable pitch. Indeed there is a pitching brake mechanism in the wind turbines, but it is not 100% reliable, thus disconnection from the controllers in combination with the emergency feathering mechanism failure can sometimes result in a runaway.

My favorite idea so far was the water cannon pointed against the motion of the blades. This may be able to slow the turbine blades down enough to implement an electrical braking mechanism, and could possibly be done while avoiding the rotational plane of the blades. Stopping the machine is more important than saving the blades.

Thanks again for the feedback.

 

[tbuelna]

Windar,

Most larger wind turbines have friction brakes (usually a disc brake) located on the gearbox output shaft. The disc brake system would be sized to absorb and dissipate the inertia of this type of rotor overload/overspeed condition. This is probably the cheapest and most reliable method, since it can be made fail-safe.

If the rotor is already overspeeding, then using the generator to brake it is likely not an option. Most larger wind turbine generators use inverter/rectifier electronics to condition their power output, and these electronic devices do not have much excess thermal capacity to absorb drivetrain overloads.

Finally, I would recommend taking a second look at redesigning your pitch control system to make it fail safe and fault tolerant. Loss of pitch control on a single blade can produce extreme rotor cyclic hub moment loads, which can also quickly lead to catastrophic structural failures.

Loss of just part of a blade can create a dangerous out-of-balance condition in the rotor, which can quickly lead to catastrophic structural failures, so I'd also forget the "water cannon".

 

[ivymike]

to be clear, you're looking for a retrofit for a particular set of turbines, or an action that can be taken externally without modification to the original turbines?

 

[ivymike]

perhaps you could use a large air nozzle, in much the way that was proposed w/water cannon? picturing a jet engine with a nozzle on the outlet, but perhaps an engine+compressor+nozzle would be fine. break up the airflow in front of the blades so that the whole thing slows down?

 

[ivymike]

or maybe a confetti sprayer, shooting lots of long plastic ribbons? get enough plastic ribbon trailing the blades, and efficiency has got to drop.

 

[ivymike]

I guess that'd just slow it though, not likely stop it.

 

[rb1957]

a fan blowing the opposite way (to oppose the windspeed) ...

ok, from the upwind side, you'd need a "reverse" fan.

how about a great big trough of mud (or jelly, it'd be more fun) to deploy into the path of the blade tip ... something to great a ton of drag ? mind you you'll need to pick up the pieces of the tower, 'cause it'll probably disintergrate from the imbalance of losing a blade).

as ivymike has asked ... are you looking to retro-fit a design, developing something new, or do you have a design working that you have to make good.

i think you need to analyze the fault tree ... what is failing that creates the overspeed situation ? and address these rather than trying to recover from the overspeed situation.

what do you do with increasing wind speeds ? you use pitch control to tailor the power output and to depower as wind speed increases (so the blades travl slower and slower as wind speed increases ?? ('cause you'd want them to stop (or be stoppable) at some design speed, no?)

 

[MintJulep]

You take them off line until you fix the fundamental design flaw.