The dominant technology for utility-scale applications is the horizontal axis wind turbine. Typical ratings range from 500 kW to 5 MW. A wide variety of wind turbine technologies are in use today. Typical wind power plants consist of hundreds of turbines, usually all employing the same technology. These technologies vary in cost, complexity, efficiency of wind power extraction, and equipment used. A typical wind turbine employs a blade and hub rotor assembly to extract power from the wind, a gear-train to step up the shaft speed at the slowly-spinning rotor to the higher speeds needed to drive the generator, and an induction generator as an electromechanical energy conversion device. Induction machines are popular as generating units due to their asynchronous nature, since maintaining a constant synchronous speed in order to use a synchronous generator is difficult due to variable nature of wind speed. Power electronic converters may be used to regulate the real and reactive power output of the turbine.
- 1 Background
- 2 Technology Trends
- 3 References
Almost all of the wind turbines deployed in large wind generation facilities in the U.S. over the past decades can be generally described by one of the configurations listed below
Direct-Connected Induction (Asynchronous) Generator (Type I)
Sometimes referred to as fixed-speed wind turbines employ stall-regulated (fixed-pitch) blades connected to a hub, which is coupled via a gearbox to a conventional squirrel-cage induction generator. The generator is directly connected to the line, and may have automatically switched shunt capacitors for reactive power compensation and possibly a soft-start mechanism which is bypassed after the machine has been energized. The speed range of the turbine is fixed by the torque vs. speed characteristics of the induction generator. Some of these turbines do not have blade-pitching capability.
Although relatively robust and reliable, there are significant disadvantages of this technology, namely that energy capture from the wind is sub-optimal and reactive power compensation is required.
Wound-Rotor Induction Generator with External Resistance Control (Type II)
Sometimes referred to as variable-slip wind turbines employ a wound rotor induction generator with a mechanism for controlling the magnitude of the rotor current through adjustable external rotor circuit resistors, and pitch regulation of the turbine blades to assist in controlling speed. The speed range of the turbine is widened because of the external resistors.
Doubly-fed Asynchronous Generator – DFAG (Type III)
Sometimes referred to as doubly-fed induction generator (DFIG) wind turbines employ a wound rotor induction generator where the rotor circuit is coupled to the line terminals through a four-quadrant power converter. The converter provides for vector (magnitude and phase angle) control of the rotor circuit current, even under dynamic conditions, and substantially widens the operating speed range of the turbine. Flux-vector control of rotor currents allows decoupled real and reactive power output, as well as maximized wind power extraction and lowering of mechanical stresses. Since the converter is only handling the power in the rotor circuit, it does not need to be rated at the machine’s full output. Turbine speed is primarily controlled by actively adjusting the pitch of the turbine blades.
Variable Speed Turbine with Full-Rated Power Converter (Type IV)
Sometimes referred to as full-converter wind turbines employ a variable-speed wind turbine with a full-rated power converter between the electrical generator and the grid. The power converter provides substantial decoupling of the electrical generator dynamics from the grid, such that the portion of the converter connected directly to the electrical system defines most of the characteristics and behavior important for power system studies. These turbines may employ synchronous or induction generators and offer independent real and reactive power control.
The value of variable speed technology for large wind turbines has been proven in the marketplace over the past decade, and will be the predominate technology going forward. Variable speed operation has benefits in terms of managing mechanical loads on the turbine blades, drive train, and structure. The grid-side benefits are also significant, and include dynamic reactive power control, increased dynamic control over electric power generation, and opportunities for further enhancement of grid-integration features of the turbine.
Wind turbine vendors are now well aware of the need for improving turbine electric robustness, especially in terms of the ability to ride-through faults on the transmission system. Enhanced low-voltage ride through is already an option for several commercial turbines, and will likely be a standard feature in the coming few years. Farther down the road, it is expected that wind turbines will be no more sensitive in terms of tripping for transmission system faults than conventional generators, and will provide flexibility with respect to “programming” their shutdown modes for grid events.
Real Power Control
At present, commercial wind turbines generally operate to maximize energy production. When winds are at or above the rated speed, electrical output is “capped” at the nameplate rating. In light to moderate winds, however, the turbine is operated to capture as much energy as possible, such that the output will fluctuate when wind speed fluctuates. These fluctuations are not optimal from the perspective of the grid, as they can lead to voltage variations and potentially increase the regulation burden at the control area level. In future generations of wind turbines, it will be possible to “smooth” these fluctuations to a greater degree than is achieved now with mechanical inertia alone. More sophisticated pitch regulation schemes, improved blade aerodynamic designs, and wider operating speed ranges will provide means for limiting the short-term changes in turbine output while at the same time minimizing the loss of production. Such a feature could be enabled only where and when it has economic value in excess of the lost production. Extending this type of control would allow wind turbines to participate in Automatic Generation Control (AGC). In this mode, the turbine would have to operate at a level somewhat below the maximum available from the wind to provide room for “ramping up” in response to EMS commands. Again, the value of providing this service would have to be evaluated against the cost in terms of lower production as well as the cost of procuring this service from a different source. Technically, though, such operation is possible even with some of the present commercial wind turbine and wind plant technology.
The dynamic characteristics of the more advanced commercial turbine technologies are complicated functions of the overall turbine design and control schemes. Little consideration has been given thus far to what would constitute desirable dynamic behavior from the perspective of the power system. Much of the attention to date in this area has been focused on the ride-through question. Once that matter is resolved, there may be opportunities to fine-tune the dynamic response of the turbine to transmission network faults so that it provides maximum support for system recovery and enhances overall stability. Given the sophistication inherent in the topology and control schemes of future wind turbines, it should be possible to program the response to a degree to achieve such stability benefits. Such a feature would allow a wind turbine / wind plant to participate in a wide-area Remedial Action Scheme (RAS) or Special Protective System (SPS) as is sometimes done now with HVDC converter terminals and emerging FACTS devices.
- Documentation, User Support, and Verification of Wind Turbine and Plant Models (DE-EE0001378), September 2012, [Online]. Available: http://www.osti.gov/bridge/servlets/purl/1051403/1051403.pdf. [Accessed May 2013].
- NREL, Dynamic Models for Wind Turbines and Wind Power Plants (NREL/SR-5500-52780), October 2011, [Online]. Available: http://www.nrel.gov/docs/fy12osti/52780.pdf. [Accessed February 2013].