Asynchronous (induction) generator
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 of 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 SCIG has been the prevalent choice because of its mechanical simplicity, high efficiency and low maintenance requirements. the SCIG of the configuration Type A is directly grid— coupled. The SCIG 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 (Type A). The generator and the wind turbine rotor are coupled through a gearbox, as the optimal rotor and generator speed ranges are different.
Wind turbines based on a SCIG are typically equipped with a soft-starter mechanism and an installation for reactive power compensation, as SCIGs consume reactive power . SCIGs have a steep torque speed characteristic and therefore fluctuations in wind power are transmitted directly to the grid. These transients are especially critical during the grid connection of the wind turbine, where the in-rush current can be up to 7–8 times the rated current. In a weak grid, this high in-rush current can cause severe voltage disturbances. Therefore, the connection of the SCIG to the grid should be made gradually in order to limit the in-rush current.
SCIGs can be used both in fixed-speed wind turbines (Type A) and in full variable speed wind turbines (Type D). In the latter case, the variable frequency power of the machine is converted to fixed-frequency power by using a bidirectional full-load back to- back power converter. The wind turbine rotor may speed up (slip increases), for instance, when a fault occurs, owing to the imbalance between the electrical and mechanical torque. Thus ,when the fault is cleared, SCIGs draw a large amount of reactive power from the grid, which leads to a further decrease in voltage.
· Wound rotor induction generator
In the case of a WRIG, the electrical characteristics of the rotor can be controlled from the outside, and thereby a rotor voltage can be impressed. The windings of the wound rotor can be externally connected through slip rings and brushes or by means of power electronic equipment, which may or may not require slip 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 wind turbine industry uses most commonly the following WRIG configurations: (1) the OptiSlip_ induction generator (OSIG), used in the Type B concept and (2) the doubly-fed induction generator (DFIG) concept, used in the Type C configuration. The OptiSlip_ feature was introduced by the Danish manufacturer Vestas in order to minimize the load on the wind turbine during gusts. The OptiSlip_ feature allows the generator to have a 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 windings .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 extend, 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: (1) the speed range is typically limited to 0–10 %, as it is dependent on the size of the variable rotor resistance; (2) only poor control of active and reactive power is achieved; and (3) the slip power is dissipated in the variable resistance as losses.
· Doubly-fed induction generator
The concept of the DFIG is an interesting option with a growing market. The DFIG consists of a WRIG with the stator windings 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 main idea is that 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 (i.e. zero reactive power).
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 has not necessarily to be magnetized 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. A drawback of the DFIG is the
inevitable need for slip rings
The synchronous generator
The synchronous generator is much more expensive and mechanically more complicated than an induction generator of a similar size. However, it has one clear advantage compared with the induction generator, namely, that it does not need a reactive magnetizing current. The magnetic field in the synchronous generator can be created by using permanent magnets or with a conventional field winding. If the synchronous generator has a suitable number of poles (a multi pole WRSG or a multi pole PMSG), it can be used for direct-drive applications without any gearbox.
As a synchronous machine, it is probably most suited for full power control as it is connected to the grid through a power electronic converter. The converter has two
(1) to act as an energy buffer for the power fluctuations caused by an
inherently gusting wind energy and for the transients coming from the net side, and (2) to control the magnetization and to avoid problems by remaining synchronous with the grid frequency. Applying such a generator allows a variable-speed operation of wind turbines.
Two classical types of synchronous generators have often been used in the wind turbine industry: (1) the wound rotor synchronous generator (WRSG) and (2) the permanent magnet synchronous generator (PMSG).
· Wound rotor synchronous generator
The stator windings of WRSGs are connected directly to the grid and hence the rotational speed is strictly fixed by the frequency of the supply grid. The rotor winding is excited with direct current using slip rings and brushes or with a brushless exciter with a rotating rectifier. Unlike the induction generator, the synchronous generator does not need any further reactive power compensation system. The rotor winding, through which direct current flows, generates the exciter field, which rotates with synchronous speed.
The speed of the synchronous generator is determined by the frequency of the
rotating field and by the number of pole pairs of the rotor. The wind turbine manufacturers Enercon and Lagerwey use the wind turbine concept Type D with a multi pole (low-speed) WRSG and no gearbox. It has the advantage that it does not need a gearbox. But the price that has to be paid for such a gearless design is a large and heavy generator and a full-scale power converter that has to handle the full power of the system. The wind turbine manufacturer Made also applies the wind turbine concept Type D, but with a four-pole (high-speed) WRSG and a gearbox .
· Permanent magnet synchronous generator
Many research articles have suggested the application of PMSGs in wind turbines because of their property of self-excitation, which allows an operation at a high power factor and a high efficiency. In the permanent magnet (PM) machine, the efficiency is higher than in the induction machine, as the excitation is provided without any energy supply. However, the materials used for producing permanent magnets are expensive, and they are difficult to work during manufacturing. Additionally, the use of PM excitation requires the use of a full scale power converter in order to adjust the voltage and frequency of generation to the voltage and the frequency of transmission, respectively. This is an added expense.
However, the benefit is that power can be generated at any speed so as to fit the current conditions. The stator of PMSGs is wound, and the rotor is provided with a permanent magnet pole system and may have salient poles or may be cylindrical. Salient poles are more common in slow-speed machines and may be the most useful version for an application for wind generators. Typical low-speed synchronous machines are of the salient-pole type and the type with many poles.