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.
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
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