In renewable power generation the wind energy has been noted as the most rapidly growing technology; it attracts interest as one of the most cost-effective ways to generate electricity from renewable sources.
Because of some challenges such as conventional energy sources consumption, pollution, global climate change and security of energy supply, significant efforts have been made to develop renewable energy sources such as wind energy. Wind power growth with a 20% annual rate has experienced the fastest growth among all renewable energy sources since five years ago. It is predicted that by 2020 up to 12% of the world’s electricity will have been supplied by wind power.
In terms of wind power generation technology as a result of numerous technical benefits (higher energy yield, reducing power fluctuations and improving var supply) the modern MW-size wind turbines always use variable speed operation which is achieved by electrical
converters .These converters are typically associated with individual generators and they contribute significantly to the costs of wind turbines. The variable speed wind turbine generators such as doubly fed induction generators (DFIGs) and permanent magnet synchronous generators (PMSGs) with primary converters are emerging as the preferred technologies.
As a result of large-scale wind power generation, interconnecting large wind farms to power grids and the relevant influences on the host grids need to be carefully investigated. Wind farms are now required to comply with stringent connection requirements including reactive power support, transient recovery, system stability and voltage/frequency regulation. Further to increase the maximum power extraction the variable speed generators are employed. These variable speed generators necessitate a AC-DC-AC conversion systems. The generator side converter controls the electromagnetic torque and therefore the extracted power while the grid side converter controls both the DC link voltage and the power factor. Moreover when designing the control strategy it seems that the generator-side converter must control the extracted power as it is located closer to the incoming power. Hence the grid-side converter would control the DC voltage.
Fulfilling the new grid codes constitutes one of the main challenges for the wind power industry. There are ride through requirements. Enhancing the operation of wind turbines in front of the grid faults is mandatory requirement. The wind turbines must stay connected to the grid during grid disturbances. They should continuously feed the reactive power in addition to limited active power. In modern wind turbines the increasing integration of power electronics enables to control the behavior of wind generation system under faulty scenarios.
The function of an electrical generator is providing a means or energy conversion between the mechanical torque from the wind rotor turbine as the prime mover and the local load or the electric grid. Different types of generators are being used with wind turbines. Small wind turbines are equipped with DC generators of up to a few kilowatts in capacity. Modern wind turbine systems use three phase AC generators.
The common types of AC generator that are possible candidates in modern wind turbine systems are as follows:
Squirrel-Cage (SC) rotor Induction Generator (IG);
Wound-Rotor (WR) Induction Generator;
Doubly-Fed Induction Generator (DFIG);
Synchronous Generator (With external field excitation); and
Permanent Magnet (PM) Synchronous Generator