Though each wind power plant is designed and optimized according to the conditions prevailing at its installation site, the plant needs some wind power plant control, and nonetheless fits into one of three main concepts as illustrated below.
These individual concepts are described in more detail next.
Constant-speed wind power plant
Asynchronous generators connected directly to the power supply system were common particularly in the early stages of electricity generation using wind power plants. In combination with stall-controlled, three-vane rotors on Danish wind power plants, asynchronous generators were the most widely used electrical concept, especially in the case of small facilities with capacities in the kilowatt range. The squirrel-cage, asynchronous generators forming part of such systems require little maintenance and are relatively economical. Furthermore, they do not require complex vane pitch control. This design is also known as the Danish concept.
Wind usually impinges at a variety of speeds on a wind power plant. In order to utilize the wind power associated with each speed as efficiently as possible, modern wind power plants are equipped with a power control system that can be considered to encompass the rotor and generator. Constant-speed and variable-speed power control systems are available.
In the case of a constant-speed system, the rotor vanes usually have a fixed pitch, though some constant-speed systems also have a variable vane pitch. Moreover, the (asynchronous) generator driven by the rotor is coupled directly with the power grid.
Power control is performed as described next. From a certain wind speed and, consequently, power (rated power) onward, the air flow impinging on the rotor vanes is disrupted, this effect being termed “stall”. This type of power limitation is therefore also termed stall control. This principle is described in detail on the page titled “Stall” in the chapter titled “Physical principles”.
The generator supplies an alternating current which needs to have the same frequency as the grid current, otherwise disruptions would occur in the power grid or wind power plant. The grid frequency in Europe is 50 Hz. Other regions (e.g. USA) employ a grid frequency of 60 Hz.
In the case of a constant-speed wind power plant, the frequency of the current supplied by the generator depends directly on the rotor speed. If adverse wind conditions prevent the wind power plant from maintaining this frequency, the network is decoupled. Once the rated frequency can be delivered again, the wind power plant is re-connected “softly” to the network, e.g. via a thyristor controller which acts like a dimmer and prevents undesired surges during circuit entry.
Variable-speed wind power plant
Dynamic loads can only be reduced by means of a variable speed range for the rotor vis-à-vis the grid frequency. Though
By contrast, an asynchronous generator only requires part of the generated electrical power to be converted by the frequency converter. The asynchronous generator’s slip is used for this purpose: In the case of an intentionally high slip value, lost energy (slip power) is fed back to the stator power flow via suitable converters. In this case, a squirrel-cage rotor is no longer suitable for the asynchronous generator,
Synchronous generator with full feed
Variable-speed operation by a wind power plant incorporating a synchronous generator can be achieved by means of a frequency converter with a DC link. The variable-frequency alternating current produced by the generator is rectified before being fed via an inverter to the power grid.
Asynchronous generator with double feed
Variable-speed operation of a wind power plant incorporating a
Wind Power Plant Control
Variable-speed systems have established themselves in modern wind turbines. Both under partial load and full load, the rotor blades’ angle can be adjusted by means of a special mechanism in accordance with wind speed and generator power, and thereby aligned nearly ideally into the wind. This kind of mechanism is referred to as pitch control.
The generators forming part of such wind turbines are coupled to the electricity grid not directly, but via an additional component:
This enables a positive utilization of wide fluctuations in wind speed.
The rotor’s magnetic field serves to couple the rotor to the stator which is connected to the grid. This coupling depends on the rotor currents. The slip control diverts a portion of these currents via resistors, thereby weakening the magnetic field and coupling, and increasing the machine’s slip. Wind power plants employ this mechanism to offset gusts of wind. If a gust impinges on the rotor, its torque increases sharply, thereby also tending to raise the system’s power very quickly.
To give the pitch control time to readjust the blade angle, the generator’s slip is increased to up to 10%. While outputting a constant power level to the grid, the system accelerates and part of its excess energy is stored as rotational energy by the rotor and drive train. As the wind speed decreases again, the slip is reduced and the drive train slows down as a result. In this process, the stored energy is fed into the grid too. This smoothens the plant’s power output characteristic. The increased slip reduces the generator’s efficiency and also causes the involved resistors to produce a lot of heat, thus necessitating effective cooling in this operating mode.
In a doubly-fed generator, the rotor’s speed can be varied by up to 30% of the rated speed. This raises power levels under changing wind conditions. It also minimizes undesirable fluctuations in the power grid and stresses exerted on the structure’s crucial components.
To achieve this, the rotor windings are routed out via slip rings and connected to the grid via special inverters. The generator is thereby connected to the stator as well as the rotor, hence the term dual (or double) feed. This permits the controller to directly influence the magnetic conditions inside the rotor. The inverters can rectify alternating current in both directions, and convert direct current into alternating current of any required frequency.
At low wind speeds, the drive train’s rotation is slower compared with the grid’s operation. In this mode, a rotary field is fed into the rotor and superimposed on its rotation frequency. In this manner, the machine magnetically attains its rated slip, even though the rotor’s mechanical operation is slower compared with the grid’s operation. In this process, energy is drawn from the grid in order to produce the rotor field. However, this amount of energy is significantly lower than the stator’s output energy. This enables a plant’s generator to cover a wide speed range.
When the wind speed increases, this rotary field’s frequency is lowered accordingly, thus keeping the magnetic slip constant. To offset gusts and high wind speeds, the rotor field’s direction of rotation is reversed. This makes it possible to raise the mechanical speed at a constant magnetic slip. To achieve this, the converters feed portions of the rotor currents to the grid, resulting in a flow of energy in this direction. About 10% of the plant’s power is thus generated in the rotor and fed via the converters to the grid.
Because the machine’s excitation takes place via the converters, reactive power from the grid is not needed. Instead, the control system makes it possible to provide capacitive and inductive reactive power in accordance with the grid operator’s specifications. The plant, therefore, contributes toward stabilizing the grid.
A First Course on Wind Power Plant Systems
We hope you’ve liked this article on various design features of wind power plant systems. This course on wind power plants, you’ll learn about the basic functioning of a wind turbine and how they convert wind energy into electric energy. There are other energy resources that have been discussed in detail. Continue learning this series on wind power plants to learn more.