Back in the ancient times of 10 or 15 years ago, it wasn’t unusual for wind turbines to generate power in the range of tens or hundreds of kilowatts. Compared to the power levels found on the utility grid, this puny generating capacity almost made individual turbines an afterthought. Utilities didn’t have to worry much about grid operations being interrupted by problems at a wind farm.
But wind farms are getting bigger. The next generation of multi-megawatt- scale turbines start to look less like quaint curiosities and more like regular power plants. That also means the wind turbines now coming off the drawing boards must have the same kind of reliability and safeguards as conventional power plants. Their economics must be in the same ballpark as well.
The latter point is important because since about 2002, the cost of wind-powered electricity has not declined. To that end, wind-turbine designers are striving to come up with designs that are both more economical and reliable.
Enter Clipper Windpower and its Liberty turbine. The Liberty sports innovations that include a distributed generation drivetrain design (dubbed the Quantum Drive) and high-efficiency, permanent- magnet generators. These pair up with a sophisticated control system that adjusts both rotor blades and generator operation to squeeze power even out of light breezes.
The Liberty design also incorporates ideas aimed at simplifying the maintenance of wind farms. For example, many of its components are light enough to be winched up its tower rather than be manhandled by a mobile crane.
Checking out the Drivetrain
The meat and potatoes of a wind turbine are its rotor, gearbox, and generator. Liberty’s major drivetrain components illustrate some of the directions in which windturbine development is progressing today.
The gearbox cranks up the rotational speed that the wind rotor provides. The traditional way of doing this has been through a planetary gearbox with three or four stages of rpm step-up. The gearbox output shaft turns the shaft of a generator. The generator connects to the utility grid through conversion circuits that change the ac it generates to dc, and finally to ac synchronized with the grid frequency. A transformer then boosts the output to about 30 kV for insertion onto the grid.
Traditional wind turbines configured this way have drawbacks that become apparent in machines big enough to generate more than about a megawatt. Generators and gearboxes on this scale are large and heavy. Problems in either of these two mechanisms can take the wind turbine off-line, potentially for a long time. Moreover, windfarm operators often must bring in a crane to take heavy generators and gearboxes out of the wind-turbine nacelle for maintenance.
One way the Liberty wind turbine addresses these problems is by using its gearbox to drive four smaller generators rather than one big one. The generators are light enough to be raised and lowered with a hoist built into the turbine structure. And a problem with a single generator doesn’t take the whole wind turbine off the grid.
The gearbox driving the generators has a special patented design with features aimed at minimizing the need for maintenance. Unlike those in most wind turbines, it is not in a planetary configuration. Instead, the input shaft from the turbine blades couples to a pair of 65-in.-diameter bull gears. Four double-helical pinions engage the bull gears. On the other end of each pinion shaft is an intermediate gear.
The intermediate gears are configured so they engage four singlehelix pinions. Each pinion engages two adjacent intermediate gears.
Thus the gearbox has two stages. The pair of bull gears and double-helix pinions make up the first stage. The intermediate gears engaging the output pinions on the output shafts comprise the second.
Helical gears promote smooth meshing. Clipper uses the double-helix pinions in the first stage for their ability to cancel out internally generated thrust loads. But the helices on the pinions are set at two different angles. The exact rationale for using two different helix angles is proprietary. All Clipper will say is that due to the double helix, the net thrust on the intermediate shaft assembly is zero. Thus, there is no need for a thrust bearing on the intermediate shaft. The highspeed shaft has a net thrust so it is equipped with a thrust bearing.
Besides providing quiet meshing, use of double-helical teeth let the gear have a wide face while loosening up the manufacturing tolerances. They also have about half the deflection sensitivity of spur gearing thanks to the fact that each face of the doublehelical system distributes loads independently.
Also, the two-tooth engagement of the intermediate gears with output pinions lets the intermediate gear transmit twice the torque of a spur gear’s single-tooth engagement. The load sharing lets the gears be smaller than they would be otherwise. And tooth pressure is in one direction rather than reversing so the gears can handle higher loads than would be the case in planetary systems.
It is also interesting to note there are eight load paths in the first stage and the load is split twice in the second stage. This contrasts with planetary systems where torque typically gets transmitted in three stages to three planetary gears, each with two teeth simultaneously engaged, one with the sun gear, the other with the ring gear. Thus the load divides into onesixth the total at each gear mesh. But in planetary systems, loads alternate on opposite sides of the teeth. This effect, plus the need for a fudge factor to meet life targets, means the effective load division is about a factor of four.
Finally, the gearbox includes features designed to mitigate the vibrational energy that arises from the transmission error in gear meshes. Developers specified the number of teeth so the meshing on different power paths would be out of phase, to keep their vibrational energy from building up. Gear teeth were optimized for low noise rather than cost. All in all, these measures eliminated the need for rubber isolation mounts on the gearbox that eventually wear out.
The generators connected to the gearbox are different than usual designs. First consider the typical wind turbine that is able to produce power at variable wind speeds. The generator operates in what’s called doubly fed mode. According to Clipper Senior Vice President of Engineering Amir Mikhail, the idea was created to get around limitations of IGBT technology as it was in the 1990s. “The IGBTs of the time couldn’t handle as much power as generators could produce,” he says. Mikhail is one of the patent holders on the technique which is now owned by GE.
The doubly fed generator bleeds a small bit of ac power off the utility grid and converts it to a signal that creates the wound rotor’s magnetic field at low wind speeds, when the generator is spinning at below its synchronous speed. Fast electronics control the spinning rotor’s magnetic field such that the generator puts out a 60-Hz signal even when it is turning slowly. (Above synchronous speed, the rotor of doubly fed generators produce power to the grid.)
Over time, IGBT capacity has grown, and doubly fed generation schemes are no longer necessary. Present- day IGBT switching devices can handle the full output power of the four 670-kW generators in the Clipper Liberty. Though many of the generator’s construction details are proprietary, Clipper says it uses a permanent-magnet rotor with neodymium-iron-boron magnets for compactness. It is a three-phase synchronous design and is only about 3 ft in diameter.
Besides being compact, PM generators have the advantage of working at a power factor that is higher than that of induction generators, about 98% down to low values of rated power, says Clipper’s Mikhail. The frequency of the generator output varies with wind speed. So rather than being connected directly to the grid, the generator output gets rectified to dc. The dc then is converted to ac synchronized to the grid frequency.
The converter circuitry does more than just generate synchronized ac. It also implements any necessary power factor correction and handles what’s called fault ride through. This capability is mandatory for large generators on the grid. It essentially ensures that if there are faults on the utility grid that momentarily drop the grid voltage to zero, the wind turbine will still try to put out current at the right phase and frequency.
The technique uses the fact that grid frequency and phase information are both still detectable even when grid voltage is essentially zero. So the converter watches the frequency on one phase of the grid connection, then uses this information to try driving converter current onto the grid regardless of the grid voltage.
A control unit manages the generators and coordinates the servomechanism that controls the pitch of the turbine blades. Its overarching goal is to optimize the generator torque and blade pitch to capture the most amount of energy while minimizing the mechanical loads. To do so, it starts with a digitized map of the power the wind turbine should put out for a given wind speed. It then adjusts the actual power out of the converter to compensate for mechanical resonances arising because of compliance in the gearshafts, blade inertia, and other factors. Resonances manifest themselves as repetitive signals in the rectified dc from the generator.
One factor that comes in handy for damping out resonances is that the synchronous generators can double as tachometers. Generator speed is one of the inputs factored into a control scheme for minimizing resonance effects. Here the rectified dc from the generator gets passed through a filter tuned to the resonant frequency of the main shaft. The resulting signal is scaled and used as one of the inputs for controlling power switches in the inverter producing the ac for the grid.
Basically the controller looks at what the power output should be for the wind conditions, then adjusts it by controlling the on and off times of power switches in the inverter. The amount of power extracted from the generator affects the torque on the drivetrain components. Thus judiciously controlling the inverter this way can actively damp out the mechanical resonances.
Look for Liberty wind turbines going up in your area soon. The first prototype went live in 2005. Clipper says it has gotten orders for 1,530 MW of capacity (612 units) and joint development/contingent sale agreements for up to 4,000 MW. Current customers include BP Alternative Energy, Edison Mission Energy, Florida Power and Light, and UPC Wind.
Clipper Windpower, clipperwind.com
Danish Windpower tutorial on wind-turbine generators, tinyurl.com/2c87m4