Asynchronous Generators
From Wind wiki
The most common generator used in wind turbines is the induction generator. It has several advantages, such as robustness and mechanical simplicity and, as it is produced in large series, it also has a low price. The major disadvantage is that the stator needs a reactive magnetizing current. The asynchronous generator does not contain permanent magnets and is not separately excited. Therefore, it has to receive its exciting current from another source and consumes reactive power. The reactive power may be supplied by the grid or by a power electronic system. The generator’s magnetic field is established only if it is connected to the grid.
In the case for AC excitation, the created magnetic field rotates at a speed determined jointly by the number of poles in the winding and the frequency of the current, the synchronous speed. Thus, if the rotor rotates at a speed that exceeds the synchronous speed, an electric field is induced between the rotor and the rotating stator field by a relative motion (slip), which causes a current in the rotor windings. The interaction of the associated magnetic field of the rotor with the stator field results in the torque acting on the rotor.
The rotor of an induction generator can be designed as a so-called short-circuit rotor (squirrel cage rotor) or as a wound rotor.
Squirrel Cage Induction Generator
The wind turbines with a squirrel cage generator are equipped with a soft starter mechanism and an installation for reactive power compensation as squirrel cage generators consume reactive power. The generator’s speed changes by only a few percent because of the generator slip caused by changes in wind speed. Therefore, this generator is used for constant-speed wind turbines. This generator and the wind turbine rotor are coupled through a gearbox, as the optimal rotor and generator speed ranges are different.
During normal operation and direct connection to a stiff AC grid, the SCIG is very robust and stable. The slip varies and increases with increasing load. The major problem is that, because of magnetizing current supplied from the grid to the stator winding, the full load power factor is relatively low. This has to be put in relation to the fact that most power distribution utilities penalize industrial customers that load with low power factors. Clearly, generation at a low power factor cannot be permitted here either. Too low a power factor is compensated by connecting capacitors in parallel to the generator.
In SCIGs there is a unique relation between active power, reactive power, terminal voltage and rotor speed. This means that in high winds the wind turbine can produce more active power only if the generator draws more reactive power. For a SCIG, the amount of consumed reactive power is uncontrollable because it varies with wind conditions. Without any electrical components to supply the reactive power, the reactive power for the generator must be taken directly from the grid. Reactive power supplied by the grid causes additional transmission losses and in certain situations, can make the grid unstable. Capacitor banks or modern power electronic converters can be used to reduce the reactive power consumption. The disadvantage of this is the electrical transients that occur during switching-in.
Wound Rotor Induction Generator
In the case of a WRIG, the electrical characteristics of the rotor can be controlled form the outside, and thereby a rotor voltage can be impressed. The windings of the wound rotor can be externally connected through slop rings and brushes or by means of power electronic equipment, which may or may not require slope rings and brushes. By using power electronics, the power can be extracted or impressed to the rotor circuit and the generator can be magnetized from either the stator circuit or the rotor circuit. It is thus also possible to recover slip energy from the rotor circuit and feed it into the output of the stator. The disadvantage of the WRIG is that it is more expensive and not as robust as the SCIG. The OptiSlip induction generator and the doubly fed induction generator are two types of wound rotor induction generators.
The OptiSlip generator was introduced by the Danish manufacturer Vestas in order to minimize the lad on the wind turbine during gusts. The OptiSlip feature allows the generator to have variable slip (narrow range) and to choose the optimum slip, resulting in smaller fluctuations in the drive train torque and in the power output. The variable slip is a very simple, reliable and cost-effective way to achieve load reductions compared with more complex solutions such as full variable-speed wind turbines using full-scale converters.
OSIGs are WRIGs with a variable external rotor resistance attached to the rotor winding. The slip of the generator is changed by modifying the total rotor resistance by means of a converter, mounted on the rotor shaft. The converter is optically controlled, which means that no slip rings are necessary. The stator of the generator is connected directly to the grid.
The advantages of this generator concept are a simple circuit topology, no need for slip rings and an improved operating speed range compared with the SCIG. To a certain extent, this concept can reduce the mechanical loads and power fluctuations caused by gusts. However, it still requires a reactive power compensation system. The disadvantages are: the speed range is typically limited to 0-10%, as it is dependent on the size of the variable rotor resistance; only poor control of active and reactive power is achieved; and the slip power is dissipated in the variable resistance as losses.
Double Fed induction generator
The DFIG consists of a WRIG with the stator widings directly connected to the constant-frequency three-phase grid and with the rotor windings mounted to a bidirectional back-to-back IGBT voltage source converter.
The term ‘doubly fed’ refers to the fact that the voltage on the stator is applied from the grid and the voltage on the rotor is induced by the power converter. This system allows a variable-speed operation over a large, but restricted, range. The converter compensates the difference between the mechanical and electrical frequency by injecting a rotor current with a variable frequency. Both during normal operation and faults the behavior of the generator is thus governed by the power converter and its controllers.
The power converter consists of two converters, the rotor-side converter and grid-side converter, which are controlled independently of each other. The rotor side converter controls the active and reactive power by controlling the rotor current components, while the line-side converter controls the DC link voltage and ensures a converter operation at unity power factor (zero reactive power).
Depending on the operating condition of the drive, power is fed into or out of the rotor: in an over-synchronous situation, it flows from the rotor via the converter to the grid, whereas it flows in the opposite direction in a sub-synchronous situation. In both cases the stator feeds energy into the grid. The DFIG has several advantages. It has the ability to control reactive power and to decouple active and reactive power control by independently controlling the rotor excitation current. The DFIG does not necessarily need to be magnetizing from the power grid; it can be magnetized from the rotor circuit, too. It is also capable of generating reactive power that can be delivered to the stator by the grid-side converter. However, the grid side converter normally operates at unity power factor and is not involved in the reactive power exchange between the turbine and the grid. In the case of a weak grid, where the voltage may fluctuate, the DFIG may be ordered to produce or absorb an amount of reactive power to or from the grid, with the purpose of voltage control. A drawback of the DFIG is the inevitable need for slip rings.