The asynchronous generator is the most common method of power generation in a wind power plant. A four-pole machine is normally used which has a synchronous speed of 1500 rpm at 50 Hz.
The asynchronous generator or the induction generator is an induction motor working the reverse way. As a motor, it rotates at speed near to the synchronous speed, the difference in speed, is the “slip”. On the application of an external torque to this motor, so that its speed is more than the slip speed, the machine acts as a generator producing power. The slip is around 1 to 2% of the synchronous speed.
In a wind power plant, the machine first rotates as a motor using grid power to reach the near synchronous speed. After this, the wind turbine torque rotates the machine at a higher speed to generate power. As the wind power increases, the power output from the generator also increases. The speed slightly changes with the load. The output power has the frequency of the grid.
A fast response control is required to control generator output when there is a sudden gust of wind or a sudden change in the electrical load. Slip control is the most common method used to achieve this in an asynchronous generator.
Slip is a function of the resistance of the rotor. This resistance is varied to change the speed when there is sudden gust of wind or load change.
Different manufacturers offer different methods of slip control. Slip control involves an additional stationary resistance circuit connected to the rotating rotor. This means a slip ring is used to connect the external resistor circuit to the rotating rotor.
Some manufacturers overcome the mechanical issues of a slip ring by using an optical method. An optical signal adds or removes resistors, which are attached to the rotating rotor.
The slip control circuit cuts off when the wind or load conditions stabilize.
Another method of power generation is the variable frequency method.
In this method, the normal synchronous generator is used. The generator runs at a speed following the wind turbine. The result is a variable frequency AC power. Since this cannot be connected to the grid, it has to be conditioned. This variable power AC power is converted by a power electronic circuitry (diodes, triacs, and thyristors) to DC power. The DC is then converted by inverters to AC power at the required frequency.
Cost, efficiency, reliability, and maintenance issues hindered this becoming a popular choice. However, recent developments and reliability improvements in power electronics are now making this system the popular choice.
Various other generation methods are also used or being developed to better utilize wind power generation. Prominent are:
- Pole changing machines that can run at different synchronous speeds to match the wide range of wind velocities.
- Dual generators one at high power and another at low power connected to the same wind turbine are also used
- Doubly fed asynchronous generators that have separate grid connection for the rotor and the stator circuits are in use for large capacity wind turbines.