Power Electronics

From Wind wiki

Power electronics is defined as the application of solid state electronics for the conversion and control of electric power. Before discussing the electronic aspects of wind turbines, it is imperative that a discussion on how wind turbines convert mechanical energy to electric energy. Generally, it is composed of three major conversions or transfer. It starts with the rotor converting wind energy into mechanical energy, the generator converts that mechanical energy into electrical power, and then the transformer transfers the electric power to the grid.

Wind turbines catch the power of the wind through their aerodynamic blades and transform it to rotating mechanical power. Normally, the blade number is three and the speed of rotation decreases as the radius of the blade increases. Under normal circumstances, the rotational speed for megawatt range turbines is 10 – 15 rpm. To convert this low-speed, high-torque power, there’s a need to use a standard-speed generator and a gearbox. The gearbox adjusts the low speed produced by the turbine rotor to the high speed of the generator.

By the use of a generator, mechanical energy is then converted into electrical energy fed into a grid. In this stage, a power electronic converter and a transformer with circuit breakers and electricity meters are needed. Wind turbines can be connected to the grid at low, medium, high, and extra high voltage systems since an electricity power system’s transmittable power is usually directly proportional to the voltage level. Turbines these days are mostly using a medium voltage system while large wind farms use high and extra high voltage settings.

Power electronic devices – Power electronic converters

Power electronic systems are used by many wind turbines as interfaces. Wind turbines function at variable rotational speed; thus the generator’s electric frequency varies and needs to be decoupled from the grid’s frequency. This action is possible if a power electronic converter system comes in handy.

The power converter is an interface found between the load/generator and the grid. Depending on the topology and the applications present in the system, power can flow into the direction of both the generator and the grid. In using converters, three important things must be considered: reliability, efficiency, and cost.

Converters are made by power electronic devices, and circuits for driving, protection and control. Two different types of converter systems are currently in use: grid commutated and self commutated converters. Grid commutated converters are thyristor converters containing 6 or 12 pulse, or even more, that can produce integer harmonics. This kind of converter does not control the reactive power and consume inductive reactive power.

The other type of converter, self-commutated converter systems, are pulse width modulated (PWM) converters that mainly use Insulated Gate Bipolar Transistor (IGBTs). In contrast to grid-commutated, self-commutated converters control both active and reactive powers. PWM-converters, therefore, have the capacity to provide for the demand on reactive power and a high frequency switching that make them produce high harmonics and interharmonics.