Ways to reduce the generated power in one wind generator 4

[petronila]

Dear All

Some customer is speaking about reduce the power in one wind generator,the history is the wind generator is equiped with squirrel cage induction motor that works like generator. the think is this people only need generate 100 KW and the wind generator can generate 300 KW. Is not possible to do mechanical modifications.

I know could be possible install some resistance in order to "burn" power but really this issue is new for me the other idea is replace the motor by one of 100 kw but with exacly sizes (frame ) and same shaft diameter.

How to do it??

All inputs are wellcomme


Thanks and regards


Carlos

 

[MikeHalloran]

No, I'm not _that_ Mike, but this may sound like one of his:

If you aren't allowed to change anything, the only remaining sensible course of action is to stay the hell away from the damn thing on windy days, let it blow itself up, and replace it with a properly designed machine with properly designed controls.





Mike Halloran
Pembroke Pines, FL, USA

 

[waross]

Can't we all be friends?? I appreciate the heads up from Scotty.
I am trying to compose a more tactful message than Mike's but I do agree.
If there is no chance to modify an unsuitable piece of equipment there may be no chance of success.
Wind turbines without some sort of energy storage are not a good choice for islanded applications.
I would like to help but even with modifications this application may fail as an islanded unit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LiteYear]

Nervously I offer another point of view, based on actual experience designing a new squirrel cage induction motor small wind turbine from generator to electrical load.

other idea is replace the motor by one of 100 kw but with exacly sizes (frame ) and same shaft diameter

Wont work unless the blades are redesigned. The power comes from the blades, which if designed to extract 300kW will just overpower a 100kW motor.

of course if you have a terrible wind you could have an overspeed, but usually the wind farms are calculated by previus study that take account the wind conditions

It's not true that the wind study prevents overspeed. Instead the wind study determines average wind speeds so the majority operation of the turbine can be optimised and yield can be estimated. The only constant with wind is that it changes. There will always be a higher wind event, and all turbines need a high wind shutdown function.

If you aren't allowed to change anything, the only remaining sensible course of action is to stay the hell away from the damn thing on windy days

No one ever said you couldn't change anything, just that mechanical modifications aren't available. Working within those parameters, I suggest the following:

First determine whether it is a fixed speed or variable speed turbine. You suggest there's no inverter so it's more likely to be fixed (ie. must provide a fixed electrical frequency), but that's not a given.

If fixed, options are probably limited to bleeding off the additional power. Because there is only a small operating speed range, there is very little scope for limiting the power generated by the blades. Therefore, to maintain the same turbine availability (uptime) you must be prepared to consume 300kW. If you only have 100kW of guaranteed load, you need to dedicate 200kW more. Plenty of options as already suggested (heat water, pump uphill, etc.) but the best solution really depends on a heap of details we don't have.

Alternatively, if lower availability can be tolerated (relying on turbine shutdown function) less dedicated load can be used. Note however that the trade off is steep. Roughly speaking, even if you have 250kW load available for a 300kW turbine, expect it to be shutting down regularly on windy days. The wear on the shutdown device must of course be considered.

If the turbine is variable, there is another method available. By regulating the speed of the blades the turbine can be pulled off its maximum power point curve by lowering the tip speed ratio (TSR) early in the power curve. This has to be done very conservatively to ensure there is enough load to stall the blades, so there is a large hit on overall yield, but at least it allows operation in higher wind speeds.

 

[Sparweb]

Carlos,
The problem as you state it is a tricky one! I have built many wind turbines, but I've never had a problem like yours! The scope of the problem goes well beyond the electrical generator, though.
All of my projects have been off-grid, battery powered systems, so you can view my advice in that context.
I did, however spend years modifying and testing the turbines I built, in order to properly match up the rotor power taken from the wind with the power curve of the generator. It's been a difficult but interesting task, done a lot by experimentation.

I would like to know what type of wind turbine you have. I have to rule out Nordex, Lagerwey, Enercon or even a very small Vestas, which have pitch-controlled blades. Wind turbines are all very different inside, so the advice you are being given may be relevant or irrelevant, but we can't be sure. You mentioned a gearbox, so that also eliminates the Enercon. I'm surprised that the blades have fixed pitch, because most turbines greater than 40-ft (15m) diameter rely on blade pitch control to regulate their speed in some way, and yours must be much bigger than that to have a 300kW rating. We also haven't figured out yet what the central problem is... we can try guessing but it depends on the electrical grid on your island. Also wondering what the rated maximum power output of the turbine is supposed to be, rather than what peaks you are actually experiencing. Perhaps there is already a problem in the speed regulation system?

If I were to guess, the rotor is about 100 feet (30m) in diameter. Almost all turbines of that size have pitchable blades or blade-tips for speed control, despite what you say. Trying to re-tune a blade pitch control system is unlikely to have an adequate effect, and more likely to render the machine unsafe if done in the field by trial-and-error.

Something else to bear in mind: You report 300kW peak output power. That is electrical power output at maximum load, from the generator. Depending on the generator, it could be anywhere from 70% to 90% efficient, so actually, the input power to the generator shaft is closer to 400kW. Ignoring gearbox efficiency, then there is 400kW of mechanical power arriving at the shaft of the rotor blades. This is exactly in line with what I'd expect from a 100-foot diameter rotor in a 25 mph (13m/sec) wind. If you were to de-rate the generator to 100kW, but you don't do anything to the blades, then when the next storm comes, the rotor blades will deliver 400kW to their shaft, the gearbox will dutifully deliver 400kW to the generator shaft, and the generator will absorb about 150kW of it...

Leaving the rest of the power, 250kW, to be converted into heat and acceleration. Either the brakes will melt or the blades will fly off when they exceed 200 RPM.

You can't just chop the tips off of the blades, especially if they are fitted with tips that change pitch for over-speed control. If it is true that the blades are of fixed pitch, then maybe the notion of shortening each blade by 30 feet is plausible, but hard to take seriously.

So I advise against "de-rating" your wind turbine, without being prepared for major modifications and a considerable engineering effort. It is not a simple generator switch-out job. If you change one thing, the consequences upon other parts could be catastrophic if you don't change them, too. By the time you get the whole thing sorted out you could have purchased a 100kW wind turbine!!

Often, there are governments, social agencies, and investors who contribute to pay for the cost of a wind turbine installation. They expect a return on their investment. Cutting the output of the turbine by 67% will make the time for them to recover their investment triple. They will not be happy with you.

It might be cheaper, easier, and safer, to sell the 300kW turbine, and buy a 100kW unit if that's what you really need.




STF

 

[petronila]

Hello Spar Web,

Thanks for your inputs, is good have an aerospace point of view and more comming from one designer.

Trick, obcourse is a trick, we can´t control the blades, we know the best option is one 100 kW wind turbine but unfortunatelly the costumers buys somme of this wind turbines and due to country laws the wind geneartors can´t generate more than 100 kW and power output most be limited.

Our company is not specialized in wind turbine designs, we are electro-mechanical company, mainly electric motor repair shop, the costumer is interested in one solution for the problem and we think in one cheap solution, that one is burn the amount of power.


Thanks and a Purple Satr for the inputs.

Carlos

 

[Sparweb]

There are a number of strategies for using up the excess power from a supply. Rather than wasting it as heat, though, you may find a way to do useful work with the excess power, rather than directing it to the grid. Pumping water comes to mind. If a nearby town uses electricity to pump water to a reservoir, maybe you could make a deal with them, to allow your excess energy to be used for that purpose.

What seems to escaped this entire discussion is that Carlos's customer has an excess of something valuable! A win-win arrangement can be found by following economic forces, rather than technological ones.


STF

 

[racookpe1978]

What you need to understand is that wind generators usually have a service factor (average actual delivered power/nameplate power rating) of 17-21%, with today's very large units able to 21% - 23% since they are taller and get better winds higher off the ground. So more than 80% of the time they cannot even generate their rated output -=> and for many, many hours of time when they DO generate some measurable power, it is not nameplate rated power but only 50 to 70% of that rated power.

Therefore, the fact that you "might, sometimes, for short periods of times" have to dump a a portion of your power to stay under the delivery limit of 100 KW is really a very small problem: Based on all wind turbines world wide, perhaps much less than 1/10 of the time is your wind turbine going to even approach nameplate power!

Therefore, you will need the ground-based electric dump resister bank, and will find it cheapest to build, requiring almost no maintenance cost, and you will find in use that it actually comes into play "wasting" power very very seldom. Because, even if it only happens 20% of the time, that extra power MUST be dumped before it hits the local grid. Now, what you could do as an (expensive!) option is feed a AC-DC converter from that resister bank and charge large DC batteries when the grid is overloaded. But you will face large and long-term battery and service unit maintenance and weekly charging procedures to avoid damaging - to the point of fire, flooding (of acid) and failure - the batteries.

 

[waross]

My wind turbine research, for a couple of projects that were abandoned when reality set in, and one expensive failure that I was not involved with but that I was aware of was about 10 or more years ago. At that time the accepted figure for average power developed as a percentage of nameplate rating was 15%. The newer, very large machines access more stable wind patterns but for a 400 kW induction motor turbine, I expect that the average power is closer to 15%. Other than that I agree with you racookpe1978

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LiteYear]

Therefore, the fact that you "might, sometimes, for short periods of times" have to dump a a portion of your power to stay under the delivery limit of 100 KW is really a very small problem: Based on all wind turbines world wide, perhaps much less than 1/10 of the time is your wind turbine going to even approach nameplate power!

This is a common misconception for those that haven't worked on wind turbines. If it really were a very small problem, manufacturers would lower the rating of the turbine, dump power for short periods, and have a much higher service factor. In fact, the low service factor is not by choice or dumb design - wind is just a hugely variable power source. There's a cubic relation of power to wind speed, which means you have to design the turbine for much higher power levels than its normal output.

The reason that large turbines have a higher service factor has less to do with their height and more to do with their inertia. The larger inertia smooths out the huge variations in input power so the peaks are not so much a problem. At the smaller end of the scale (most of my experience is with a very light 5kW turbine) you don't get the benefits of inertia and you hit peak power regularly, if fleetingly. At 100kW you'll be somewhere in between.

If a (fixed speed) turbine is rated at 300kW, you need to be able to load 300kW on the generator. Please put some faith in SparWeb's experience. All is not as it first seems with wind turbines!

 

[racookpe1978]

True, true. You need to be able to load that 300 KW on to the generator.

But the Original Poster's problem is that he cannot send that (potential) 300 KW to the local grid. it will get generated, but he cannot send it on and MUST get rid of it.

Therefore, he must dump the electric load to a assured, guaranteed reliable "dump" EVERY time it begins to exceed 100 KW.

my point was (is) that the feared loss of power is not going to happen very often because few windmills ever get to their rated capacity very often, when they get to near their rated capacity it is not for very long periods of time, and what power is generated at the rated capacity can be readily "lost" without a significant economic penalty. Like an oil refinery's flare - which has to be kept "uselessly" burning energy so it is immediately available to flame off dumps in the event of an accident - there are simple economic penalties that sometimes you just have to accept.

(Then again, one could (should) argue that windmills for ANY location that can be connected to the grid, are not economically competitive and they are being built now strictly for their national tax base rebates to the builder, erector, and fund politician's interest groups.

 

[LionelHutz]

Before worrying about loading the wind turbine the problem of this wind turbine with an induction generator seemingly being the only source or the main source on this "grid" needs to be addressed. There is no point planning on load dump equipment when the whole thing is going to fail.

 

[waross]

I'm with Lionel. I understand that this is an islanded application. A wind generator without storage is a bad choice. An islanded wind generator with an induction generator is a really bad choice. An islanded wind generator with an induction generator and no mechanical changes allowed ----- my vocabulary fails me.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[racookpe1978]

I don't know enough about static inverters operating in a isolated environment to tell, so I will ask it openly hoping for criticism: If he had two (conventional) generators, load-sharing between the two becomes "balanced" depending on the frequency curves of the two and the amount of load, right? The "grid" in our regular world forces the frequency, but here - with only two generators - that assumption would not be true, correct?

So, if he is on an island, running a single static generator, the electronics of the static generator control the frequency (just like they do in my house when I am on my battery backup system during a blackout) because there is nothing else to hook up to. Could he reset a static inverter do do the following? His windmill is generating AC power now, so he buys a AC-to-DC two-way static converter, and hooks the DC side to a battery. In normal running, the windmill's AC generator and the battery's dc-to-AC inverter would share the load. His battery is charged when possible, but is usually kept near a float discharge.

In event of an overcharge (when he needs to dump power) the DC side continues to charge the battery, but at a higher rate than a trickle charge. In the event of a power failure, or loss of wind ! power during a calm or during over-speed of the wind, the DC battery drains down and the inverter carries the whole load.

It would be more expensive, but doesn't that give him a more reliable total system? Now, in very high winds, he loses all power - since the windmill can't be kept turning in excess winds. in high but less-than-rated capacity winds, he loses power to a resister. In no winds, he loses power because nothing can be generated from the windmill.

 

[LiteYear]

Yeah that'd be fine. Typically called a hybrid system with AC backbone. But if the batteries are full and the loads are still only 100kW then you still have the problem of dumping the other 300kW.

 

[OperaHouse]

This is just another example of my theory that, "The customer is always wrong." When given any problem one should assume at least one third of the facts given are incorrect. If a customer could actually gather the correct information, they could solve their own problems.

Now we have a second issue, "Jumping to conclusions." A "consultant" trying to formulate a solution before gathering information. Nothing is really known at this point. Perhaps I missed the usual first step, "contact the manufacturer." We are lead to believe this mill has existed for a while and hasn't destroyed itself. And just how does the customer know they have excess power? Does it seem reasonable that someone makes a 300KW turbine that can't protect itself?. Is the customer off by three zeros? Did they get this off ebay with no instructions? I think someone is pulling your leg. All this bandwith would be better spent on an imaginary internet girlfriend.