There are very few applications of DC wind turbines. The most common models are constant-speed and constant-frequency squirrel-cage induction generators, variable-speed and constant-frequency doubly-fed induction generators, and variable-speed and variable-frequency direct-drive permanent magnet synchronous generators.
1. Constant speed and constant frequency squirrel cage induction generator
Constant-speed constant frequency (CSCF) wind turbine is a generator that operates at approximately constant speed and produces approximately constant AC frequency within the effective wind speed range. In general, the generator sets in this type of wind power generation system mostly use squirrel cage induction generator sets.
The constant-speed and constant-frequency squirrel-cage induction generator achieves a constant speed through the constant pitch stall control, and then outputs an alternating current with a frequency equal to the grid frequency. Since the rotor speed of this type of wind turbine must be kept in an approximately constant range, and the rotor speed can only be kept approximately constant accordingly, it cannot guarantee that the tip speed ratio remains optimal, and its wind energy utilization rate is generally low.
2. Variable speed constant frequency doubly-fed induction generator
The main feature of the variable speed constant frequency (VSCF) wind power generation system is that within the effective wind speed range, the operating speed of the generator set is allowed to vary, and the frequency of the AC power generated by the generator set stator is constant. Generally, the generator set in this type of wind power generation system is a double-fed induction asynchronous generator set. The double-fed induction generator combines the characteristics of a synchronous generator and an asynchronous generator. Both the stator and the rotor can exchange power with the grid, which is also the origin of its “double-fed” name.
Doubly-fed induction generators generally use speed-up gearboxes to increase the speed of the wind wheel several times, and the rotor speed transmitted to the generator is significantly increased. Therefore, high-speed motors can be used, which are small in size and light in weight. The capacity of the doubly-fed converter is only related to the slip capacity of the motor, with high efficiency and low price. The disadvantage of this scheme is that the speed-up gearbox is expensive, noisy, and easy to fatigue and damage.
3. Variable speed variable frequency direct drive permanent magnet synchronous generator
Figure 1 shows the structure diagram of the synchronous permanent magnet generator in the variable speed variable frequency (VSVF) permanent magnet wind power generation system. The stator of the permanent magnet synchronous generator is the same as the stator of the asynchronous generator, consisting of a stator core and a three-phase stator winding, as shown in Figure 2. The rotor has permanent magnets to generate a magnetic field. The stator windings are generally made of multi-phase (three, four, five phases, etc.), and the most common is three-phase windings. The three-phase windings are symmetrically distributed along the stator core, with a mutual difference of 120° in space. When three-phase alternating current is passed in, the winding generates a rotating magnetic field. The rotor uses permanent magnets, and the most commonly used permanent magnet material is neodymium iron boron. The use of permanent magnets not only simplifies the structure of the wind generator, but also improves its reliability, and there is no rotor copper loss, thereby improving the efficiency of the generator.