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Tricky wind turbine question
5

Tricky wind turbine question

Tricky wind turbine question

(OP)
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!
  

RE: Tricky wind turbine question

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

RE: Tricky wind turbine question

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 ??

RE: Tricky wind turbine question

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.  

RE: Tricky wind turbine question

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.

RE: Tricky wind turbine question

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

Regards,

Mike

 

RE: Tricky wind turbine question

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

RE: Tricky wind turbine question

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

RE: Tricky wind turbine question

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

RE: Tricky wind turbine question

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

RE: Tricky wind turbine question

Scattershields?

RE: Tricky wind turbine question

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

RE: Tricky wind turbine question

(OP)
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.

 

RE: Tricky wind turbine question

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".   

RE: Tricky wind turbine question

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?
 

RE: Tricky wind turbine question

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?


 

RE: Tricky wind turbine question

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

 

RE: Tricky wind turbine question

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

RE: Tricky wind turbine question

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?)

RE: Tricky wind turbine question

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

RE: Tricky wind turbine question

a different angle ... it sounds like you've experienced over-speeds already.  how do you verify that the structure is still sound ?

RE: Tricky wind turbine question

Brake that is actuated by a spring loaded mechanism, employed when centrifugal force exceeds a certain value (speed exceeds a rated value). Think the opposite of a non-reverse ratchet on an electric motor.

Retracts automatically as speed is reduced, no controls required. Comments above indicated a disc brake could already exist, although the concept might be more straight forward to apply to a drum brake.

RE: Tricky wind turbine question

Can you swivel the blades around 180 degrees so they are now pointed opposite the wind??

That would stop them - but then they would also start spinning in opposite direction.

Some of these hubs are 200-300' in the air.  Not too many water cannons and such can reach such a height.

RE: Tricky wind turbine question

these ones happen to be on 60' towers, per the post.

RE: Tricky wind turbine question

Turn it off of the wind. If you have active yaw control, you should have redundant wind sensing and yaw control.

If it pivots freely, Bergey, I think uses a steering vane that deflects in an overspeed situation, causing the rotor to turn off of the wind.

 

RE: Tricky wind turbine question

You could try mounting a drag parachute in each blade tip that would deploy at a set centrifugal force.

RE: Tricky wind turbine question

DC injection brake? Possible, if the generator design is favourable.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Tricky wind turbine question

Windar (Mechanical)
There used to be a system called "reefing" a wind mill, AKA wind turbine, where the yaw control turned the rotor  90 degrees to the wind.
  Is your system not capable of doing this?

B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them.  Old professor

RE: Tricky wind turbine question

I think the challenge presented assumes that _all_ of the controls have "gone off to see God", and are therefore unresponsive to external stimuli, and not behaving as they should.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Tricky wind turbine question

(OP)
MikeHalloran (Mechanical)    
26 Sep 11 9:51
I think the challenge presented assumes that _all_ of the controls have "gone off to see God", and are therefore unresponsive to external stimuli, and not behaving as they should.

This is correct.  Significant retrofits to the turbine could obviously address the issue, but if this is not possible, what about when the worst case occurs?  

What about a large net propulsion system? (the blades are fairly easily replaced if they were to fail).
 

RE: Tricky wind turbine question

i don't think any system is designed that way ... not aircraft, not missiles, not nuclear power stations.  

if all your controls are assumed to have failed then the machine becomes uncontrollable.

rational design works to levels of probability of failure, with redundant systems of control.

RE: Tricky wind turbine question

I think when the first blade is lost at high rpm the fact that blades are easy to replace may not be all that reassuring.

RE: Tricky wind turbine question

unbolt the tower base, install new tower, install new blades ...
oh, and pick the pieces of broken wind turbine scattered about.

RE: Tricky wind turbine question

...and call the insurance company to handle claims from homeowners 1 km away who have a blade slice through the roof.

see http://www.caithnesswindfarms.co.uk/accidents.pdf

"Pieces of blade are documented as travelling up to 1300 meters. In Germany, blade pieces have gone through the roofs and walls of nearby buildings."

Seriously, you'd need to run some kind of testing to verify that the various blade-braking schemes suggested would not just become blade-breaking schemes.  The blades aren't really designed to take any impacts more significant than a frozen chicken when at operating speed.  Even the reefing ideas would have issues with slew rate and gyroscopic forces if not controlled precisely.

A more reasonable method might be a means of erecting a series of poles/masts or a more-or-less solid "wall" upstream of the troubled rotor (I'm thinking of a D-8 cat as an anchor, and a couple of big cargo parachutes), in an effort to block all or most of the wind, allowing the rotor to spin down to a lower, safer speed before attempting some method of blade breaking/braking.  At least the lower speed will reduce the ballistic distance of the blades...

RE: Tricky wind turbine question

"and what is the flight speed of a frozen chicken?"

actually they use thawed chickens, since frozen are too solid.

and i wouldn't've thought bird strike was much of a design criteria.  a bird, even a 12ld-er, impacting at 15ft/sec (10mph), should be pretty easy to absorb ... at the root the blade is very rigid, at the tip very flexible.

maybe a design requirement is to safely depower if a blade is shed

RE: Tricky wind turbine question

The tip speed can be much greater than 10mph, and they don't make a lot of noise, which is why the birds don't always steer clear of the blade disc.

;--

The problem I see with physically snagging the blades with nets, ropes or parachutes, is twofold.

If the rope wraps around the hub, there may be enough inertia and mechanical advantage to lift and maybe even swing a Cat D8 as if launched from a trebuchet.  Modern ropes can do amazing things like that.

If the rope managed to snap off a blade, I'd expect the resulting imbalance to take out the tower pretty quickly.  Maybe the towers are designed for a missing blade imbalance at above design speed; they don't look like it to me.

;--

Deploying multiple parachutes or a tall screen upwind might slow the rotor a bit, but you don't want to accidentally snag the spinning rotor.  You still face the problem of stopping the rotor, but intervention becomes a little less dangerous.

;--

IF the nacelle is free to swing during control failure (not revealed so far), then pre-deploying a weighted rope from its upwind end might allow one to yaw the rotor away from the wind, with the Cat D8.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Tricky wind turbine question

tip speed sure, but bird strike is governed by forward speed (hence the need to accelerate the bird in a cannon, to get it up the speed of the airplane ... in real life, the plane flies into the bird).

i was thinking of a bird flying into the plane of the turbine, bending about the weak axis of the blade.  thinking more, the bird could be unlucky enough to be in the prop-plane at the wrong time whihc would resemble a more typical bird strike on the leading edge of the wing.  still the tip speed would be much less than an airplane's forward speed, the L/E radius of the balde would be much smaller (for the blade) ... factors which decrease the impact loads, but the section is much smaller than a wing which would increase the effects.

RE: Tricky wind turbine question

Mechanical tachometer that triggers an emergency brake? And I'm still partial to the idea of a brake actuated by centrifugal force.

I can't tell if some of these suggestions are jokes, or if I missed part of the original post about keeping the machine in one piece not being a requirement.

That's not entirely true, the "strongly worded note on the drawing" was obviously a joke. But some of the others still have me confused.

RE: Tricky wind turbine question

Windar:

I think what you should do is hire a couple, out of work, Samurai with large swords and long step ladders.  When things get out of hand, they would set up right next to the blade path and start wacking away, at the same elevation on all three blades, thus maintaining some balance in the rotating system.  After they've wacked off the first six or eight feet of the blade tips, they move up a few rungs and repeat.  If they do this on the down stroke side of the blade path, they should be able to pile up the blade parts like chord wood at the foot of their ladders, thus making clean-up easier.  Soon enough, you will have little airfoil left to over speed.

Alternatively, you could do about the same thing in 6' blade lengths with a machine gun.  They call this rotary blade nipping or trimming.  Although, given the systems that you seem to be operating, where all fail safe systems do not work, these final solutions will probably fail in your neighborhood too.

RE: Tricky wind turbine question

Paul Bunyon and a lasso ...

Explosive bolts at the root of the blades ...

 

RE: Tricky wind turbine question

You might want to ask yourself why you can't leave the runaway turbine to its own devices until some day when there's no wind.

If the answer to that is that you think it will be too dangerous and runaway turbines are already a known problem, then you probably ought to be looking at ways of immobilising the turbines today, and keeping them all immobile until somebody has brought the design to an acceptable state.

A.   

RE: Tricky wind turbine question

I saw a clip of one these self-destructing - probably on YouTube.  Pretty impressive and s&&t flew everywhere...

RE: Tricky wind turbine question

1gibson, you are closer to the right answer, yes, than many of us that are posting cheeky responses.  The right answer is that you should never have a full runaway, blade-shedding, building-destroying scenario develop; the blades I'd think would have a design that makes them auto-feather in the event of overspeed and/or loss of control signals, etc.

Rb,

A Canada goose can weigh much more than a chicken (tastier too, imo), and fly up to 50 mph.  If flying downwind in 40 mph storm, what is the effective impact speed?  We know these flying foreign terrorists can take down a jetliner...

...and if it's migrating in December, in Alberta, it might just be pretty well frozen...

I don't think the big 3-bladed rotors can survive a loss of one blade at operating speeds, at least not from the videos I've seen.  Lose 30% of the rotating mass, and something's gonna wobble, somewhere.

RE: Tricky wind turbine question

Didn't the OP mention that these were also mounted on lattice towers.
 I seem to remember reading somewhere, that these can come apart when things start wiggling violently.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them.  Old professor

RE: Tricky wind turbine question

i'd reckon that if a goose was somewhat below it's normal operating temperature so that it's tissues were more rigid than normal, then it wouldn't be flying, plummeting maybe.

also doubt if a goose can make headway against a 40mph gale.

at the OP ... all the smart a$$ suggestions are telling you that your scenario (all controls going "tits up") is unreasonable.

here's another one ... invent an inertial field (like ST's sub-space inertial dampening field) to control the uncontrollable, or possibly a time machine (for obvious applications).  then market these inventions and forget about making money from the wind.

RE: Tricky wind turbine question

Flying downwind, not upwind.

40mph wind speed + 50 mph goose speed (relative to the air) = 90 mph giant-long-necked-chicken-projectile

RE: Tricky wind turbine question

I'm not in the industry but it sounds like a very hazardous situation to deal with.  Obviously the designed-in solutions would be best, but from the way the question is posed, it sounds like you are in the middle of a hurricane and don't have time to go back to the drawing board.

Would there be a way to freeze up the bearings?  Maybe by applying heat/cold or injecting a sand/powder mixture?  Of course the issue is getting up there to do it in the middle of the storm.  Interesting topic.

<tg>

RE: Tricky wind turbine question

Put a big tank under the turbine filled with a suitably viscous liquid.

Gradually raise the tank (or the water level in the tank) so that the tips of the blades drag through it.

Are the blades metallic or at least conductive?  Might be able to do something with big magnets/electro magnetic if they are.

Of course, both options will likely damage the turbine.

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RE: Tricky wind turbine question

kenat, check post 23 sept 8:06 ...

RE: Tricky wind turbine question

Kenat, you can only brake one blade at a time that way, which is going to put an unbalanced load on the blade and rotor.  Not saying it couldn't work, but I'd definitely want to be standing on the upwind side, out of the plane of rotation, before opening the hydrant.

Look at some YouTube videos - search term "wind turbine failure".

RE: Tricky wind turbine question

sensibly, i wonder if you can use the centripedal load on the blade to unload the blade ?  or tailor the blade construction (assuming graphite plies) so that the blade will flex under load to depower (sort of like how you can design a forward swept wing to deflect in a stable manner). ??

RE: Tricky wind turbine question

fair point rb1957, I read the thread originally a few days back and didn't re-read the same thing.  

btrue, just 'cause Zdas said I might be management material hopefully doesn't mean I'm a complete moron.  I was hoping "Of course, both options will likely damage the turbine." might cover it.

Also when I said suitable viscous liquid, I was actually wondering if fairly low viscosity might be the way to go, rather than thick mud or similar.  Minimize the load on the blade.

Oh, and both suggestions are fairly tongue in cheek.

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RE: Tricky wind turbine question

careful you don't bite your tongue ...

RE: Tricky wind turbine question

Mine wasn't tongue in cheek.  And I said fire hose, not water cannon, so as to minimize the additional stress.  All that's needed is enough drag to decelerate the rotor assembly, however slowly.  If it hasn't exploded already, nobody will care if it takes an hour or two or six to bring the rotor to a stop.

Further, complete control failure is not a remote possibility; I get the impression that it already happened.  ... and I have yet to see fully redundant or even fail-safe controls featured in wind turbine fluffery.  The competitive nature of the market means that nobody is going to put extra money in controls until they are forced to do so.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Tricky wind turbine question

i note that Windar hasn't marked the thread and he hasn't chimed in for awhile ... maybe he's given up on us smartie pants ??

RE: Tricky wind turbine question

...or maybe...

RE: Tricky wind turbine question

Dunno, Kenat, you sound managerial from time to time.

No, you may be right, as I said if you read the whole post.  I guess I'm hung up (i.e not convinced) that losing a part of a blade won't unbalance those flimsy contraptions I see in the hills, and result in a complete blade-shedding, tower-crumpling fiasco, complete with ambulance-chasing lawyers and TV news crews.  I.e. more than just damaging the turbine, but a complete write-off of the whole installation plus collateral damage.  The link I posted suggests that it's been happening, at least over in Europe.

But maybe that would be the point, design the dang thing to be damage tolerant enough so that it could survive throwing one of 3 blades.

I like the centripetal balance to auto-feather and/or blade washout by means of tailored layers of fiber composites.  I think some of the bigger units use these techniques already...

RE: Tricky wind turbine question

Try adding a hydraulic motor to the slewing ring, and attach it to the tower.  Leave the supply/return tubing that you ran to the ground open.  With a runaway, you drive up with a truckmounted hydraulic powerpacl and couple up.  Then slew it around 90°, to the "reefed" position that berkshire brought up.  Fairly cheap 'fix'.

RE: Tricky wind turbine question

Perhaps Windar hasn't responded because he has been montioring the turbine with a fire hose nearby.

RE: Tricky wind turbine question

Couple the output shaft by a clutch to a rotor w/ the opposite pitch.

RE: Tricky wind turbine question

by the look of it only one turbine "exploded" so i guess it was an individual issue, not a systemic design problem.

the way it exploded looks like it uses a mechanical brake (that broke ?)

RE: Tricky wind turbine question

Use a failsafe vane to point it off the wind. The actively controlled position of the vane shall point it at the wind. If any control fails, a spring returns it to a position that steers it off the wind.  

RE: Tricky wind turbine question

I'm blown away.

RE: Tricky wind turbine question

The water, if pointed at the hub rather than at the blade tips will run outwards along the blades, possibly slowing them down uniformly.  I wonder if there could be a hose running up the tower to just behind the blades that could be turned on when needed.   

RE: Tricky wind turbine question

Incorporate into the blade design a small plate that is automatically (mechanically) deployed from the over speed that acts like the air brakes on aircraft wings.  Positioned at the tip.

RE: Tricky wind turbine question

Try adding a hydraulic motor to the slewing ring, and attach it to the tower.  Leave the supply/return tubing that you ran to the ground open.  With a runaway, you drive up with a truckmounted hydraulic powerpac and couple up.  Then slew it around 90°, to the "reefed" position that berkshire brought up.  Fairly cheap 'fix'.

But you've got some physical problems responding, and your response team (the truck and its hydraulic pump and tank and crew) would be parked directly under the over-speeding wind turbine in the line of fire of the breaking blades, underneath the (probable) burning nacelle and generator, and underneath the falling parts and debris.   Not a place I'd send a crew!

Response time?  These things are widespread apart (by miles down winding dirt and gravel roads) always in the boondocks (off of even paved roads, much less convenient high-speed highway exits.  So you'd have to detect the overspeed turbine, get your crew going, get to the highway exit, get to the county roads, get to the dirt roads, get down the dirt road, get to the right turbine, then hook up the hydraulics and start pumping - before the overspeeding turbine got fast enough (past the trip detection point of the overspeed sensor) to start destroying the turbine and drive gears.    

RE: Tricky wind turbine question

Windar,

I am familiar with your predicament.

What could work in this situation is to grab a bow and arrow.

Attach a very strong light weight cord to the arrow tail (about 200ft long).

Tie the small cord to a larger rope that is a bit longer.

Tie that rope to larger rope and so on until you have some large line like a barge halyard at the end.

Shoot the arrow above the top of the rotor and the rope will start to be pulled in...

STAND BACK!

Eventually the rope/line should create enough drag to slow down the rotor...

Good luck!

RE: Tricky wind turbine question

I think you mean 'hawser', not 'halyard', but it's a workable idea.
 

Mike Halloran
Pembroke Pines, FL, USA

RE: Tricky wind turbine question

i think that by the time the blade is whirling around out of control that the horse has bolted.  any attempt to 'restle the sucker back under control is frought with peril, such that the outcome is obvious and unavoidable (fun though it has been to dicuss).

IMHO, either the control system is designed so that complete failure is very improbable (and systems can be designed for this, think nukes, aircraft, spacecraft, railways, ...) or if failure is possible (it might be too expensive to design a fail-proof control system) then either you adopt Ford's Pinto economics (and we know how well that worked for Ford) or you design the blade so that it will depower itself through aero-elastic response, not though blowing itself into a million shards.  

RE: Tricky wind turbine question

It strikes me that if you have a mechanism of the type that  total failure is possible. It should be sited far enough away from people and other mechanisms, that it should be allowed to self destruct without creating other problems.
 Just stand back and let it do its thing.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them.  Old professor

RE: Tricky wind turbine question

Can you design it in a way that there are counter-balancers in the root of the hub that can be pushed out in the case of high inertia to shorten the turbines?

Or just have some fluid that will fill the internal passages of the blades if they get out of control?

Or, how about it pivots itself on the other axis and points up?

Explosive bolts that just blow measured bits off at a time? like 5 feet?

 
 

RE: Tricky wind turbine question

All you are trying to do in this situation is slow the rotor to keep it from self destructing.  A self destructing turbine is also a safety issue for personnel and the public obviously.

If you can keep it from destructing then it is possible to the reuse the blades, tower and gear case (usually new gears and bearings are needed).

Turbines in this power range (<100kW) are built as simply as possible...  The tip-brakes at the blade tips are the over speed control on these usually.

However, they sometimes don't deploy as designed...  overspeed results...

 

RE: Tricky wind turbine question

The designers/engineers need to apply a tether on the opposite side of the hub and connect it to a slip-ring so it can be quickly accessed.  Then it could be attached to a vehicle (or winch w/block) so the blades can be pulled out of the wind.  IMHO!

RE: Tricky wind turbine question

//The designers/engineers need to apply a tether on the opposite side of the hub and connect it to a slip-ring so it can be quickly accessed.  Then it could be attached to a vehicle (or winch w/block) so the blades can be pulled out of the wind.  IMHO! //

In a runaway or potential runaway,  you wouldn't want to be anywhere near the turbine.

RE: Tricky wind turbine question

Hence the beauty of the tether.  You don't have to be close.  Agreed, you would not want to be close enough to be hit with flying pieces.   

RE: Tricky wind turbine question

I'm going to expand on something gadkinsj began to think of ... But am going to reverse his thought:


gadkinsj (Mechanical)     
16 Dec 11 15:07
Can you design it in a way that there are counter-balancers in the root of the hub that can be pushed out in the case of high inertia to shorten the turbines?

Or just have some fluid that will fill the internal passages of the blades if they get out of control?   
  

Rules:
you must have something completely automatic (NO manual involvement because of time to detect and get to the wind turbine, access on site to the over-speeding turbine, height of the blades, climbing time, danger when near the turbine on the ground, etc.)
Whatever slows the whole turbine down MUST trip or deploy on all of the blades at the same time from the same trip setting: otherwise, one blade gets slowed (or gets a force thrown on it and it breaks, or the others get unbalanced forces on them and they get torn off, the root bearing breaks because of unbalanvced forces as it tries to slow down.
The mechanism must be re-setable or re-useable without destroying the turbine or its blades.

Therefore, I recommedn that each blade tip be "covered" with an "overspeed scoop tip" closely fitting to the current blade end contour.    The overspeed scoop tip is pivoted at one side so that, when open, the scoop catches the wind and slows the blade.  At slower speeds - safe speeds less than rotor maximum - the tip is held in the closed position by a latch to resist centrifugal forces.

The "overspeed scoop" is held in its closed position on the tip, and thus the scoop is held  firmly onto the blade tip with almost no air resistance, by hydraulic forces routed inside the blade via an internal "hose" (or thin diameter pipe) to the latch holding a scoop in the closed position.   This hydraulic pressure is retained by a set of two (or three) relief valves, and pumped up by a hydraulic pump fed by the turbine blade when the blade is rotating.  

(Zero rotation speed, hydraulic latch pressure = bearing oil pressure.  At higher turbine velocities => more hydraulic pressure => hydraulic pressure comes closer to the trip points of the relief valves.  At the required overspeed velocity, first (or second) relief valve opens => dumps latch pressure to all blades at the same time => opens latches => overspeed tips open on all blades => blades can't go faster => turbine is safe.)  

When problem is overcome (whatever caused the original overspeed fault is corrected) the safety tips are pushed back to closed position, the latches are reset and turbine can restart when the vented hydraulic fluid is reloaded.    

RE: Tricky wind turbine question

Ok guys.
Now seriously I think this one has not been mentioned. Since the most feasible and easy solution is the breaking mechanism on the gearbox sid of things. Why not rely 100% on mechanical design (as it is less likely to fail) and design a centripetal clutching break system. Such that when it reaches a certain rpm it automatically starts to engage, with that engagement becoming more intense if the rpm further increases.
Done deal. Agree?

peace
Fe (IronX32)

RE: Tricky wind turbine question

Waaaay too much energy to run through a mechanical brake - it would melt the metal portions, might melt ceranics.

RE: Tricky wind turbine question

Good point. I guess it would depend on the size of the break and energy transfered (size of turbine). I'm not saying it would still work, I just think it's something to put on the possible solutions board. smile
What about a liquid cooled centripetal break? (I know the complexity just increased)  

peace
Fe (IronX32)

RE: Tricky wind turbine question

racookpe1978
Expanding on your thoughts but at the same time not re-inventing the wheel.
 During the manufacture of the blade, install Schempp Hirth dive brakes in it activated by a weight, counterbalanced by a spring.
Set the spring to keep the brakes closed up to just over the safe operating speed of the blade. Above that speed centrifugal force would pull the weight agaist the spring and open the dive brakes, closing them when RPMs had dropped to safe limits.
 You do not need hydraulics for this and the technology is well established.
 The major question would be whether or not the manufactures would install such a safety device or consider it too costly?
B.E.  

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them.  Old professor

RE: Tricky wind turbine question

Interesting idea B.E.

peace
Fe (IronX32)

RE: Tricky wind turbine question

sorry to disagree but if you "trip" something as it detects an over-speed (ie the blade is already whirling around at a high rate of knots) and you deploy something to brake the blades, i reckon that's exactly what you'll do ... break the blades.

btw, as we're focusing on the blades, what's happening inside the motor as the drive speed increases ?  from the pix above it looks like the motor gives out first, before the blades distribute themselves over a wide area.

RE: Tricky wind turbine question

I've thought of using a water brake.  Essentially a low-efficiency hydraulic pump mounted on the main rotor shaft with no fluid outlet.  The only outlet for the pressurized fluid is the pump clearances.  Under normal running circumstances the water brake is dry; no seals, no losses.  If the turbine runs away, water is released from an overhead tank into the brake...  Braking force can be controlled by varying the water flow.  With the amount of energy available, most of the water would be vaporized, and so should be easily ducted overboard.  Vaporization of the water should also serve to control the pump component temperatures.

RE: Tricky wind turbine question

The speed limiting dive brake has been used on composite aircraft wings for 50 years now.
A wind turbine is nothing but several aircraft wings stuck on a common hub.
 The idea would be to deploy the things in an overspeed situation before structural limits are reached.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them.  Old professor

RE: Tricky wind turbine question

The OP described a "run away" turbine condition.  If you consider the basic wind turbine drivetrain consisting of a rotor driving against the generator load, a "run away" condition would imply that there is a failure somewhere between the rotor hub and generator.  Thus any proposed solution should be capable of directly stopping rotation of the rotor hub itself.

While the ideas proposed using absorbers could slow the rotor, unless they include some form of locking function they would not be capable of continuously absorbing/dissipating the rotor power.  The only practical solution to slowing the rotor without damaging the blades would be some form of mechanical fuse in the blade pitch control.  Once the blades are free to feather, they should naturally assume a neutral lift position, and the rotor rotation should cease.

Here's what can happen with even very small turbine blades that run away.

http://www.dailymail.co.uk/news/article-2083149/Wind-turbines-cope-UK-weather-3-blown-pieces.html

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