Interactive monitoring of pitch systems
"A wind turbine is a very complex motion control challenge, because the primary control on the turbine is a pitch system that plays a dual role in both the power output and the key safety function of the turbine," Dennis Webster, director of business development for Moog Inc., told us.
The pitch system is a collection of commonly understood motion control and automation components. This includes an onboard PLC that monitors and controls the system, plus the power electronics, servo drives, and motors that control the motion of the turbine blades.
BLADEcontrol technology from Bosch Rexroth analyzes the natural oscillations of the wind turbine blade. The frequencies of any blade are a unique pattern, like a fingerprint, in the range up to approximately 350Hz.
Webster said the trends in automation are to provide more interactive capabilities to monitor the performance, operation, and efficiencies of these subcomponents. The goal is not to provide a collection of components, but integrated functionality that raises performance and changes how the customer monitors and controls the system.
"With customers in the wind turbine industry, reliability has become a paramount concern. In order to achieve the return on investment necessary, the wind turbine needs to be operating whenever the wind is available," he said. "If you do a Pareto analysis on the reliability of the turbine, the pitch system is often flagged as causing downtime, because it functions as both a control device and a safety device" by serving as the brake for the turbine.
Given this dual role, the information available from the turbine manufacturer can often be incomplete or inadequate for the operator to understand the detailed operating conditions within the turbine. This can impact reliability and the ability to get the system back up and running after any downtime event.
One solution is a single terminal interface for the operation of the motion control components that offers more granularity and insight into what is happening with the pitch system. This information can be used proactively (in terms of preventive maintenance or modifications to the operating parameter set) or to enable advanced troubleshooting. "What we have tried to provide is a window and full visibility into the pitch system, whether the concern is the motion control algorithm or synchronization of the motors and drives," Webster said.
If 20 years is the current focus, what was the product lifetime rating in the past? I suspect much lower.
With the Siemens 6MW turbine, the current world's largest (dec 2012), I would want a 20 year life at the very least. The 8MW Vestas, set for 2015, will be even larger. I say bump the life to 30 years, then we'll have something.
A few points. In the future there will be no gearboxes cutting that expensive part and problem. They will be replaced by larger diameter generators with many more poles instead. GE already is doing it on their new units.
You can't look at a composite part and know if it's ok as delaminations and other problems have little to no visual effect until near fairlure. But a simple microphone can in real time at low cost. To actually find the problem Xray, Ultrasound or other tech is needed though tapping with a hammer can by someone who knows how.
The biggest problem are these huge WT's are really investment vehicles generating loans, commissions, units profits with generating power as a nessasary byproduct.
The real future in WT's are small home, build size units that make/save retail electric cost instead of wholesale electricity thus 2-3x's more cost effective.
WT's scale well into the .5kw size units though studying it a 2kw/16' dia size for most homes can supply their needs in many places.
Another is smaller local wind farms close to the demand as transmission lines for lrager, distant ones can cost as much as the wind farm does!!
They have always said 20 yr life but that has not proven anywhere near tue mostly because they keep increasing size thus don't have time to optimise designs before they are an 'obsolete size'.
Yet many small units from the 30's are still going strong!! A decent WT should have a 30-50 yr life simply to cut maintaince costs.
- rate life isn't the primary metric... it's return on investment (ROI). Life span has an impact, but so long as it is not different that expected, it remains just one of several variables impacting ROI.
- already (6 years ago) designed in processing power for monitoring the vibration with active pitch control for minimizing wear on the minimal transmission involved. (4 generators around one large gear - balanced torque on frame, made easier generator swap out inside nacelle - no crane or helicopters involved). Amazing to "listen" to difference in the system with high freq adjustments being made vs without any active control. The basic controls already have the maintenance monitoring , data logging, high level access across networks, etc. to minimize costs.
- Pitch is constantly changing based on location of blade in a single rotation. Why? because on really large diam systems, the speed of wind is significantly different at the top of the arc vs the bottom of the arc.
- smaller systems? Great, but they will always have some significant disadvantages to big systems. a- closer to the ground (slower wind speeds and near birds of prey food source) b- higher blade speeds (noise/bird strike) c- distributed maintenance over larger area (higher maintenance per watt) d- majority of population does not have a reasonable location for installation (limited urban options) e- most people will not want to take responsibility for their own power source (most don't want to be responsible for their own plumbing!) .
In many locations it can make sense... but for majority of population in US and Europe, it isn't an option. Centralized power (and all of it's weaknesses) is likely to be with us for a long while. And yes, this will waste power in distribution and be a major cost in maintenance of transmission lines.
I wish it wasn't so.,,. (in general, I prefer de-centralized systems)
Regardless, the real changes in this industry will come when all the energy sources operation on a "level" playing field, not because of 50 year product lives. It is amazing to see the gov incentives still being paid out to oil/gas .. while many alternative energy incentives have dried up.
There does exist an alternative for putting the gearbox and the generator up in the nacele where they are very hard to get to and expensive to service. The solution is to have the turbine directly driving a large variable displacement hydraulic pump, and then use a variable displacement hydraulic motor to turn the generator down on the ground. This wold avoid using a gearbox and also allow generation of power at lower wind speeds, with the added advantage of being able to run the generator at whatever speed was desired. The generator and associated support and control equipment would be at ground level, making them cheaper to install, maintain, and repair, and the power would already be at ground level, making the grid connectionssimpler. Moving the weight down to the ground would reduce the required strength for the base and support, so that would add to the savings. One more potential advantage is that hydraulics does offer a way to store energy in an accumulator, which could potentially assist in a method of longer term energy storage.
What I don't understand is why this approach has not been used very much so far.
The challenge with low speed alternators and generators is that they need a whole lot of poles to be "low speed". Each time the number of poles doubles the speed iscut in half, and to get to a 600 RPM synchronous speed one is already up to 16 poles. That winds up being a large device. Putting the generator on the ground allows for whatever speed and number of poles is convenient. Plus, the added advantage of being able to adjust the ratio almost instantly is a handy side benefit.
Using ground hydraulics is an interesting approach and I believe the reason it hasn't been tried would be the losses in moving all that fluid such a great distance. Up top you would still need something to control the blade pitch and positioning, so you may as well have the transmission up there too.
The trasmission is a high-wear and quite heavy device, and presently it is the one item that has a definite lifetime. In addition, it takes up a fair amount of space and the connection to both prop and generator must be quite precisely aligned. REplacing the gearbox in the upper assembly is a very big deal task. The generator, generator controls, gearbox, and gearbox cooling hardware comprise more than half the weight of the upper end, and so moving all of them to ground level would produce quite a savings in space and weight. An added advantage of the hydraulic approach is that all of the cooling could be located on the ground, since cooled oil would also cool the topside pump assembly. The piping losses can be minimized using techniques that have been well known in the hydraulics industry for many years. The somewhat reduced efficiency of using hydraulics would certainly be offset by the increased reliability and the reduced servicing costs associated with having much of the system at ground level.
I haven't studied wind turbine design in any depth, but the idea of a hydraulic transmission sounds brilliant to me. Sort of analagous to the automatic transmissions in cars instead of manual, I suppose. Efficiency should be good, but the maintenance, nacelle design, etc. should all be greatly simplified. It would be interesting to have someone in the industry comment on why this path hasn't been taken.
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