Wind Energy Conversion
The development in wind turbine systems has been steady for the last 25 years and four to five generations of wind turbines exist. The main components of a wind turbine system, including the turbine rotor, gearbox, generator, transformer, and possible power electronics.
The turbine rotor converts the fluctuating wind energy into mechanical energy, which is converted into electrical power through the generator, and then transferred into the grid through a transformer and transmission lines.
Wind turbines capture the power from the wind by means of aerodynamically designed blades and convert it to rotating mechanical power. The number of blades is normally three and the rotational speed decreases as the radius of the blade increases.
For meagwatt range wind turbines the rotational speed will be 1015 rpm. The weightefficient way to convert the low-speed, high torque power to electrical power is to use a gearbox and a generator with standard speed.
The gearbox adapts the low speed of the turbine rotor to the high speed of the generator. The gearbox may be not necessary for multipole generator systems.
The generator converts the mechanical power into electrical energy, which is fed into a grid through possibly a power electronic converter, and a transformer with circuit breakers and electricity meters.
The connection of wind turbines to the grid is possible at low voltage, medium voltage, high voltage, and even at the extra high voltage system since the transmittable power of an electricity system usually increases with increasing the voltage level.
While most of the turbines are nowadays connected to the medium voltage system, large offshore wind farms are connected to the high and extra high voltage level.
The electrical losses include the losses due to the generation of power, and the losses occur independently of the power production of wind turbines and also the energy used for lights and heating.
The losses due to the power generation of the wind turbines are mainly losses in the cables and the transformer.
The low-voltage cable should be short so as to avoid high losses. For modern wind turbine system, each turbine has its own transformer to raise voltage from the voltage level of the wind turbines (400 or 690 V) to the medium voltage.
The transformer is normally located close to the wind turbines to avoid long low-voltage cables. Only small wind turbines are connected directly to the low-voltage line without a transformer or some of small wind turbines are connected to one transformer in a wind farm with small wind turbines.
Because of the high losses in low-voltage lines, large wind farms may have a separate substation to increase the voltage from a medium voltage system to a high voltage system. The medium voltage system could be connected as a radial feeder or as a ring feeder.
At the point of common coupling (PCC) between the single wind turbines or the wind farm and the grid, there is a circuit breaker for the disconnection of the whole wind farm or of the wind turbines.
Also the electricity meters are installed usually with their own voltage and current transformers. The electrical protective system of a wind turbine system needs to protect the wind turbine and as well as secure the safe operation of the network under all circumstances.
For the wind turbine protection, the short circuits, overvoltage, and overproduction will be limited to avoid the possibly dangerous damage to the wind turbine system. Also the system should follow the grid requirements to decide whether the wind turbine should be kept in connection or disconnected from the system. Depending on the wind turbine operation requirement, a special relaymay be needed to detect if the wind turbine operates in a grid connection mode or as an autonomous unit in an isolated part of the network due to the operation of protection devices. The conversion of wind power to mechanical power is done aerodynamically as aforementioned.
It is important to control and limit the converted mechanical power atnhigher wind speed, as the power in the wind is a cube of the wind speed.
The power limitation may be done by stall control (the blade position is fixed but stall of the wind appears along the blade at higher wind speed), active stall control (the blade angle is adjusted in order to create stall along the blades), or pitch control (the blades are turned out of the wind at higher wind speed).
It can be seen that the power may be smoothly limited by rotating the blades either by pitch or by active stall control while the power limited by the stall control shows a small overshoot, and this overshoot depends on the aerodynamic design.
The possible technical solutions of the electrical system are many It involves solutions with and without gearbox as well as solutions with or without power electronic conversion.
In the following chapters, main wind turbine configurations will be presented and explained.
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