Winds of Change for Power and Control
Drives and controls aspect of wind power technology provides new engineering challenges and lessons as more multi-megawatt, utility-scale wind turbines come online. Most critical, costliest parts of the system are the turbine blades, which must be controlled and protected. Blade-pitch control regulates the amount of power and torque extracted from the wind by optimally orienting blades to the air flow, while yaw control keeps the nacelle pointing into the wind-or out of the wind in case of need.
At home on sweeping plains, arid deserts, remote mountain sides—and more recently moving offshore—growing numbers of multi-megawatt (MW) wind turbines are producing clean electric power. Wind turbine size is rising to take advantage of proportionately higher power generation-to-cost ratios obtainable. The American Wind Energy Association (AWEA) puts average size of turbines installed in 2008 at 1.67 MW. Meanwhile, the top end of rated capacity is 5-6 MW, intended for offshore sites. Still larger wind turbines are in planning.
Except in some European countries, wind power historically has been a small contributor to electricity production relative to long-established sources. However, winds of change are starting to blow. The U.S. took the lead in installed wind power capacity at the end of 2008 with 25.2 gigawatt (GW), followed by Germany (23.9 GW), Spain (16.8 GW), and China (12.2 GW), according to the Global Wind Energy Council (GWEC). China is expected to take the lead in the near future.
These giant wind turbines house electric, hydraulic, and mechanical drives, various subsystems, plus electronic controls in a nacelle atop a tower that stands as high as 135-m (450 ft). Rotor (blade) diameters can measure well over 100 m.
Installed equipment must withstand harsh ambient conditions, on/off cycling due to high or low winds, and nacelle vibrations without undue maintenance.
Pitch and yaw control
Arguably, the most critical, costliest parts of the system are the turbine blades, which must be controlled and protected. Blade-pitch control regulates the amount of power and torque extracted from the wind by optimally orienting blades to the air flow, while yaw control keeps the nacelle pointing into the wind—or out of the wind in case of need. “They’re generally independent in operation, though pitch control responds to yaw adjustments,” says Henrik Stiesdal, CTO of Siemens Wind Power. “Operation of yaw control affects the amount of available energy to a very marginal degree, and pitch control adjusts accordingly, so in that sense the two systems influence each other, but generally not in a purposely coordinated way.”
Dan Throne, sales and marketing manager at Bosch Rexroth Corp., Electric Drives & Controls, notes, “While pitch and yaw control serve different functions, they work together to maximize turbine energy output under changing wind speed and direction, as well as provide start-up operation and failsafe protection of the blades and tower in hazardous weather conditions.” Pitch control also slows the rotor beyond the rated power point.
Control of individual blades is needed for optimal power generation due to wind variations. In fact, different wind and ambient conditions can exist even between the top and bottom of the rotor’s swept area. Newer, large wind turbines provide pitch control at each blade root, using a pitch drive/motor. Yaw control usually includes several drives and motors to distribute gear loading. Pitch and yaw drives can be either electromechanical or hydraulic.
Electric power generation and distribution from wind energy is a multi-step process. ABB Inc. produces variable-frequency drives/converters, generators, transformers, and switchgear for wind turbine applications.
Wind energy that turns the rotor must be converted into useful electric power by a generator. A gearbox is often but not always used to increase rotor speed suitable for generator input. Then the converter modifies the generator’s variable voltage/frequency output into constant voltage and frequency electricity (60/50 Hz) for coupling to the grid—see “Power generation” diagram. The converter has two sections, which form an interface between the wind turbine and the ac grid. However, the separate generator-side and grid-side converter/inverter is housed in the same air- or liquid-cooled ac drive cabinet.
Wind turbines are designed to operate over a range of wind speeds. Typical characteristics, with some variation depending on turbine size, include:
Cut-in speed of 3 m/s (6.7 mph), where the generator is connected to the grid;
Rated speed of 12-13 m/s, where maximum output is reached; and
Cut-out speed of 25 m/s—which corresponds to a 56 mph gale-force wind.
At cut-out speed the generator is disconnected from the grid, rotor blades are pitched to feathered position (parallel to wind flow), and the braking system is actuated to stop and park the rotor. Brakes also help stop the turbine during a loss of load emergency and relieve yaw drive gear loads when that motion is inactive, according to Bosch Rexroth.
Anemometers and sensors atop the nacelle measure wind speed and other environmental conditions for transmission to the turbine control system. The miniature “weather station” also communicates with blade-pitch and yaw controls. Slip rings serve the vital function of transmitting control power and data between the rotor hub and nacelle. “Custom software and control system developed by the turbine OEM provide the decision to start, run, or stop the rotor,” says Cliff D. Cole, director, low voltage drive products at ABB Inc.
Multiple redundant controllers in the nacelle monitor weather conditions, turbine operation, pitch and yaw control, braking systems, remote condition reporting, and more, Bosch Rexroth’s Throne explains. A central turbine controller interfaces with the supervisory control (SCADA system) in charge of multiple turbines in a wind farm. The latter can be remotely accessed via wireless or Internet communication.
For example, GE Energy’s central turbine control, named WindControl system, claims the ability to operate wind power plants “more like conventional power plants,” through tight rotor torque and blade-pitch control of multiple turbines—even under variable wind and grid conditions. The company’s WindSCADA system reportedly offers a broad set of operator tools that range from turbine monitoring and control to generating power production reports.
GE Energy offers wind turbines in three basic sizes: 1.5, 2.5, and 3.6 MW rated power. The 1.5 and 3.6 MW machines feature double-fed asynchronous generator, while the 2.5 MW turbine comes with permanent magnet generator and full power converter (see below). GE claims title to the largest wind turbine supplier in the U.S. and second largest worldwide.
Generator, converter choices
Among several generator designs, asynchronous (induction) type—especially its double-fed induction (DFI) generator variant—has dominated wind turbine applications. Popularity of induction generators stems from wide availability and application experience, similar to that of induction motors. Basically, a generator is a motor made to run backward. More recently, permanent magnet synchronous (PMS) generators have seen increased usage. Each generator type has relative advantages and disadvantages.
A power curve charts instantaneous turbine output over a range of wind speeds derived from numerous meansurements. Significant wind speed points are shown for GE Energy's 2.5-MW turbine.
The main advantage of DFI generators is that they work with a smaller converter, sized to about 30% of full rated power, thus offering a lower cost solution, explains Teemu Ronkainen, product line manager for large hp system ac drives at ABB. “The converter needs to supply magnetizing flux only to the generator rotor, because the stator is directly connected to the grid in the double-fed design. It results in high efficiency at the nominal speed of the generator,” he says. Generators other than DFI type require a full power converter.
“Induction machines require high operating speeds, hence a gearbox. An induction generator is rugged, reliable, inexpensive, and field-proven over many years and thousands of applications,” says Siemens’ Stiesdal.
However, a major downside of DFI generators (not having a full power converter) is “limited ability to fulfill the latest and/or upcoming grid connection codes,” states Ronkainen. Also, DFI generator efficiency drops at speeds below nominal because the rotor draws active power. This limits useful speed range and becomes significant with recent interest to operate turbines at lower wind speeds. “Maintenance of slip rings used to deliver magnetizing flux to the DFI generator’s rotor is another issue,” Ronkainen notes.
“Permanent magnet machines can be made for high-speed operation, also requiring a gearbox, but unlike induction generators they may also be designed for direct-drive, eliminating the need for a gearbox,” continues Stiesdal. “PMS generators tend to be slightly more efficient than induction machines, but they’re also comparatively more expensive.”
Stiesdal notes that output voltage of PMS generators is not easily adjusted; they have less operating flexibility than variable-slip induction machines. Also, machine clearances, temperatures, and other parameters become even more important with permanent magnet materials. “Consequently, PM machines introduce a new set of operating requirements to the power conversion system,” he says.
Bosch Rexroth’s Throne likewise notes use of either induction or synchronous generators combined with a multi-stage gearbox to supply suitable input shaft speeds. He calls both generator technologies “well proven,” which in the PM synchronous case refers more to smaller turbine experience. “In the last few years, technology utilizing either direct-drive PM generators or hydrostatic drives (motors and pumps) have been in development,” Throne adds.
PM synchronous trend
While an induction generator with full power converter is an option, ABB notes a trend toward PM synchronous generators. Ronkainen cites three designs based on different operating speeds, all offered by ABB. A high-speed PM generator with multi-stage gearbox ranges up to 1,800 rpm nominal speed, depending on frequency and pole number. “Mechanically, this is the same as a DFI generator,” he says. Medium-speed PM generator with integrated single-stage gearing would run at upwards of 150 rpm, while the low-speed version (17-30 rpm, typical), is direct driven from the turbine rotor without intermediate gearing.
Full power converter (FPC) supports all three PM synchronous designs and adds benefits versus DFI generators. FPC also helps to meet increasingly stiff grid code requirements. “It isolates the generator from line transients, enables fast response to line faults, and has better ride-through/grid support during a fault,” Ronkainen adds. Moreover, full power converter promotes “global design” of wind turbines by the ability to fulfill different grid codes.
The Switch Controls & Converters Inc. is another company keen on PMS generators. In a presentation at AWEA’s WindPower 2009 conference, Anders Troedson, vice president of The Switch, referred to PMS generators with full power converter as the “new drive train standard” for wind turbines. Troedson cited annual energy production calculations that show PM generators (low-, medium-, and high-speed) offering overall system efficiency superior to DFI generators. The Switch makes all three generator types.
Assembly of nacelles for Siemens 2.3-MW wind turbines is shown at the company's nacelle production facility in Brande, Denmark. (Image courtesy of Siemens Energy.)
Eliminating the gearbox simplifies wind turbine design but requires a low-speed PM generator with much larger diameter to accommodate the high magnetic pole count (80 or more). Larger diameter also provides peripheral speed needed for flux generation. Cost and low production numbers have limited application of large capacity direct-drive generators to date. However, one notable implementer of this gearless design is Germany’s Enercon GmbH, manufacturer of the largest wind turbines.
In another presentation at WindPower 2009, Stiesdal commented on direct-drive (DD) turbine developments at Siemens. He explained that cost per torque output decreases with increasing power, with a breakeven point likely at 3.6-MW rating. Stiesdal’s preliminary conclusion states, “For large offshore turbines, the DD concept will become a commercial alternative; for mainstream onshore turbines the verdict is still open.”
Electricity produced from wind is growing. To take its appropriate share in the mix of energy sources, wind power must solve other issues, such as turbine siting, environmental concerns, and greater public acceptance. Eventually, wind power also must compete equally without relying on subsidy. With apologies to the Bard, “ 'tis a consummation devoutly to be wished.”
Many participants in wind power technology
Electric power generation and distribution from wind energy is a relatively young but growing industry. It has attracted a large community of manufacturers and developers whose scope of supply ranges from complete wind turbine systems to hardware and software components and auxiliary subsystems.
The following is a selected listing of supplier companies to the multi-megawatt (MW) wind turbine market. The list is far from being comprehensive-and does not include erection or maintenance services.
Suppliers of large wind turbines (>1 MW)
Suppliers of subsystems/components for large wind turbines
Frank J. Bartos, P.E., is Control Engineering consulting editor. Reach him at email@example.com.
Also read, from Control Engineering : Whitepaper: Windmill applications, signal conditioning tips .
Frank J. Bartos, P.E., is Control Engineering consulting editor. Reach him at firstname.lastname@example.org .