POWER ELECTRONICS FOR CONTROL AND GRID INTEGRATION OF WIND TURBINE GENERATORS

The voltage magnitude and frequency of the AC electrical power generated by a wind turbine generator are usually variable due to the variation of the wind sources. Therefore, power electronic converters are commonly employed to convert the electrical power from the form generated by the wind turbine generator into the form required by the power grid or load. Depending on the generator and the power electronics system, wind turbine generators can be divided into four types, including1)

1.  A fixed speed wind turbine with a squirrel-cage induction generator (SCIG); 
2. A partial variable-speed wind turbine with a wound-rotor induction generator (WRIG), which has adjustable rotor resistances; 
3. A variable-speed wind turbine with a doubly fed induction generator (DFIG); and 
4. A variable-speed wind turbine generator with full-scale power electronic converters.

POWER ELECTRONICS FOR WIND TURBINE TYPE 1

Wind turbine type 1 represents one of the oldest wind power conversion technologies. It consists of an SCIG connected to the turbine rotor blades through a gearbox, as shown in Figure 28.2. The SCIG can only operate in a narrow speed range slightly higher (e.g., 0%–1% higher) than the synchronous speed. Consequently, wind turbine type 1 is commonly called a fixed-speed wind turbine. Many type 1 wind turbines use dual-speed induction generators where two sets of windings are used within the same stator frame. The first set is designed to operate in a low rotational speed (corresponding to low-wind speed operation); while the second set is designed to operate in a high rotational speed (corresponding to high wind speed operation). The reactive power necessary to energize the magnetic circuits of the SCIG must be supplied from the power grid or a switched capacitor bank in parallel with each phase of the SCIG’s stator windings. The mechanical power generated by the wind turbine can be limited aerodynamically by stall control, active stall, or pitch control.

FIGURE 28.2 Configuration of wind turbine type 1.

Connecting an SCIG to a power grid produces transients that are short duration with high inrush currents, which cause severe voltage disturbances to the grid and high torque spikes in the drive train of the wind turbine. To mitigate such adverse effects, many type 1 wind turbines employ a phase-controlled soft starter to limit the RMS values of the inrush currents to a level below two times of the SCIG rated current. The soft starter, as shown in Figure 28.2, consists of two antiparallel-connected thyristors in series with each phase of the SCIG to allow the phase current to follow in both directions. The firing angles of the thyristors are properly controlled to limit the stator currents of the SCIG by building up the magnetic flux slowly in the SCIG during the transient start-up period. The soft starter operates until the voltages at both sides of the soft starter are the same. Since the soft starter has a limited thermal capacity, at this moment, a contactor that electrically connects the wind turbine generator and the low-voltage terminals of the power transformer is energized, thus bypassing the soft starter. The contactor carries the full-load current when the connection to the grid has been completed. Finally, the capacitor banks are connected for reactive power compensation. To facilitate the excitation of the SCIG, it is desirable to connect the capacitor banks during the start-up period. However, the soft starter produces harmonic currents that can damage the capacitors, and, therefore, the connection of the capacitor banks will not be initialized until the grid connection process of the SCIG has finished.

ADVANTAGES AND DISADVANTAGES OF TYPE 1 WIND TURBINE:

The advantages of type 1 wind turbine are

1. The simple and cheap construction
2. No need of synchronization device.

Some disadvantages include

1. The wind turbine usually does not operate at optimal power points to generate the maximum power due to the fixed rotating speed;
2. The wind turbine often suffers from high mechanical stresses, since wind gusts may cause torque pulsations on the drive train; and
3. It requires a stiff power grid to enable stable operation, because the SCIG consumes reactive power during operation.

POWER ELECTRONICS FOR WIND TURBINE TYPE 2

Wind turbine type 2 consists of a WRIG with an adjustable external rotor resistance connected to the turbine rotor blades through a gearbox, as shown in Figure 28.3. The adjustable external rotor resistance is implemented by a combination of external three-phase resistors connected in parallel with a power electronic circuit, which consists of a B6 diode bridge and an insulated gate bipolar transistor (IGBT) module. Both the resistors and the power electronic circuit are connected to the rotor windings via brushes and slip rings. The duty ratio of the IGBT module is controlled to dynamically adjust the effective value of the external rotor resistance of the WRIG. It is well known that the electrical power, Pe, generated by a WRIG depends on the rotor resistance and slip as follows:

Where

n_p and V₁ are the number of phases and per-phase terminal voltage of the stator windings, respectively I₂ and R₂ are the per-phase rotor current and resistance referring to the stator side respectively, s is the slip.

FIGURE 28.3 Configuration of wind turbine type 2.

The wind turbine generator starts to generate electrical power when the rotor speed is above the synchronous speed. As the wind speed increases, the input aerodynamic power increases, the rotor slip increases; as a consequence, the electrical output power increases. If the electrical output power is lower than the rated value, the external rotor resistors are short circuited by setting the duty ratio of the IGBT module to be unity. Once the electrical output power reaches its rated value, the external rotor resistance is adjusted to keep the output of the wind turbine generator constant. This is done by keeping the ratio of the total rotor resistance to the slip to be constant as follows:

Where

S _rated is the rated slip when the rotor resistance is R₂.

R_{2 total}  is the sum of R₂ and the effective external rotor resistance.

Therefore, by adjusting the external rotor resistance using the power electronic circuit in Figure 28.3, the wind turbine generator can be operated in a wider speed range. Vestas uses this concept to achieve a speed variation of 0%–10% above the synchronous speed for their so-called Opti Slip wind turbine generators. To limit the rotor speed to its maximum value and to reduce the mechanical loads on the blades and the turbine structures, the aerodynamic power is also controlled by controlling the pitch angle of the blades in the high wind speed regions. Figure 28.4 illustrates the detailed topology and control for the power electronic circuit to adjust the external rotor resistance of a WRIG. A surge arrester may be connected in parallel with the IGBT module to protect it against overvoltage that is created in the DC circuit due to current pulsing. The control system regulates the rotor currents flowing through the external rotor resistors. Therefore, the effect of varying rotor resistance on the rotor terminal of the WRIG is created. The control system consists of an inner-loop current controller and an outer-loop power controller. The inputs for the current controller are the measured rotor current and the rotor current reference value, which is received from the power controller. The output of the current controller is the duty ratio for switching the IGBT. When the wind turbine is connected to the grid, the power controller is activated. The inputs for the power controller are the measured electrical output power and the power reference obtained from the power-slip relationship of the WRIG. If the wind speed is not high enough to produce enough torque on the turbine for running on the rated power, the power is controlled to increase with the generator slip up to 2%. If the wind speed rises to a point where the rated power can be produced, the wind turbine will be controlled to output a constant rated power; while the slip will be controlled up to 4% by using the pitch control. Short time speed changes at rated power output are controlled by possible slip changes between the rated slip which is approximate 0.5% with no external resistance connected and the maximum allowable slip of 10%.

FIGURE 28.4 Typology and control of the power electronic circuit to adjust the external rotor resistance of a WRIG.

As illustrated in Figure 28.3, type 2 wind turbine generators still need a soft starter to limit the inrush currents during the start-up process and switched capacitor banks for reactive power compensation.

ADVANTAGES OF WIND TURBINE TYPE 2 OVER WIND TURBINE TYPE 1:

1. It provides a partial variable-speed operation with a small power electronic converter, and,
2. Therefore, energy capture efficiency is increased;
3. The mechanical loads to the turbine structures at high wind speeds are reduced;
4. The flicker is mitigated and the quality of output power is improved;
5. The action frequency of pitch control system is reduced;
6. The noise emission is reduced in weak wind conditions because the turbine is rotating with lower speed;
7. The reliability of the wind turbine is improved and its life is extended.

DISADVANTAGES OF WIND TURBINE TYPE 2 OVER WIND TURBINE TYPE 1:

However, the connection of the external rotor resistances to the rotor terminal is usually done with brushes and slip rings, which is a drawback in comparison with SCIG due to the need of additional parts and increased maintenance requirements.

POWER ELECTRONICS FOR WIND TURBINE TYPE 3

Figure 28.5 illustrates the basic configuration of a type 3 wind turbine generator. It consists of a low speed wind turbine driving a high-speed WRIG through a gearbox. The WRIG is connected to a power grid at both stator and rotor terminals. The stator is directly connected to the power grid, while the rotor is connected to the grid by a variable-frequency AC–DC–AC power electronic converter through slip rings. As a consequence, the generator in this configuration is commonly called a DFIG. In order to produce electrical power at constant voltage and frequency to the power grid over a wide operating range from sub-synchronous to super-synchronous speeds, the power flow between the rotor circuit and the typically consists of two four-quadrant AC–DC voltage sources converters (VSCs), that is, a generator-side converter and a grid-side converter, connected back-to-back by a common DC link. The generator-side converter and grid-side converter usually have a rating of a fraction (typically 30%) of the generator nominal power to carry the slip power. As a consequence, the wind turbine generator can operate with the rotational speed in a range of ±30% around the synchronous speed. Below the synchronous speed, the rotor power flows from the grid to the rotor winding; above the synchronous speed, the rotor power flows from the rotor winding to the grid. In this configuration, the active and reactive power of the generator and power converter can be controlled independently. By controlling the active power of the converter, the rotational speed of the generator, and thus the speed of the rotor of the wind turbine, can be regulated. The acoustical noise from type 3 wind turbines can be effectively reduced, since the system can operate at a lower speed when the wind becomes weak. The dynamic response and controllability are excellent in comparison with type 1 and type 2 wind turbine systems. This type of wind turbines needs neither a soft-starter nor a reactive power compensator. They are typically equipped with a blade pitch control to limit the aerodynamic power during conditions of high wind speeds.

FIGURE 28.5 Configuration of wind turbine type 3.

Power electronic converters are constructed by power semiconductor devices, inductors, and capacitors with driving, protection, and control circuits to perform voltage magnitude and frequency conversion and control. There are two different types of power electronic converters: naturally commutated and forced-commutated converters. The naturally commutated converters are mainly thyristor converters, which use the line voltages of the power grid present at one side of the converters to facilitate the turn-off of the power semiconductor devices. A thyristor converter consumes inductive reactive power, and it is not able to control the reactive power. Thyristor converters are mainly used for high voltage and high power applications, such as conventional high-voltage direct-current (HVDC) systems and some flexible AC transmission system (FACTS) devices.

The forced-commutated converters are constructed by controllable power semiconductor devices, such as IGBTs, metal–oxide–semiconductor field-effect transistors (MOSFETs), integrated gate commutated thyristors (IGCTs), MOS-gate thyristors, and silicon carbide FETs, which are turned on and off at frequencies that are higher than the line frequency. Forced-commutated converters, such as IGBT-based pulse width modulated (PWM) VSCs, are normally used in type 3 wind turbine generators. As shown in Figure 28.6, the DFIG normally uses two bidirectional back-to-back PWM-VSCs sharing a common DC link. This type of converters has the ability to control both the active and reactive power delivered to the grid. The reactive power to the grid from the DFIG and converter can be controlled as zero or to a value required by the grid operator within the converter’s rating limit. These features offer potential for optimizing the grid integration with respect to active and reactive power control, power quality, and voltage and angular stability. The high-frequency switching of a PWM-VSC may produce harmonics and inter harmonics, which are generally in the range of a few kHz. Due to the high switching frequencies, the harmonics are relatively easy to be removed by small-size filters.

FIGURE 28.6 Topology of back-to-back PWM-VSCs used in wind power systems.

In order to reduce the cost per megawatt and increase the efficiency of wind energy conversion, the nominal power of wind turbines has been continuously growing in the past years. As a consequence, there is an increasing interest in multilevel power converters especially for medium to high-power, high voltage wind turbine applications. The increase of voltage rating allows for connection of the converters of the wind turbine systems directly to the wind plant distribution grid, avoiding the use of a bulky transformer. The general idea behind the multilevel converter technology is to create a sinusoidal voltage from stepped voltage waveforms, typically obtained from an array of power semiconductors and capacitor voltage sources. The commonly used multilevel converter topologies can be classified in three categories, which are diode-clamped multilevel converters, capacitor-clamped multilevel converters, and cascaded multilevel converters. Figure 28.7 illustrates commonly used three-level converters using these topologies.

FIGURE 28.7 Multilevel converter topologies: (a) one leg of a diode-clamped three-level converter; (b) one leg of a capacitor-clamped three-level converter; and (c) one leg of an H-bridge cascaded three-level converter.

Initially, the motivation of using multilevel converters was to achieve a higher voltage and power capability. As the ratings of the components increase and the switching and conducting properties improve, other advantages of multilevel converters become more and more attractive. Multilevel converters can generate output voltages with lower distortion and lower dv/dt. Consequently, the size of the output filters is reduced. For the same harmonic performance, multilevel converters can be operated with a lower switching frequency when compared with two-level converters. Therefore, the switching losses of multilevel converters are reduced.

The most commonly reported disadvantage of multilevel converters with split DC link is the voltage imbalance between the DC-link capacitors. Never-the-less, for a three-level converter, this problem is not serious, and the problem in the three-level converter is mainly caused by differences in the real capacitance of each capacitor as well as the inaccuracy in the dead time implementation or an unbalanced load. By a proper modulation control of the switches, the imbalance problem can be solved.

The three-level diode-clamped multilevel converter and the three-level capacitor-clamped multilevel converter exhibit an unequal current stress on the semiconductors. It appears that the upper and lower switches in a converter leg might be derated compared to the switches in the middle. For an appropriate design of the converter, different devices are required. The unequal current stress and the unequal voltage stress might constitute a design problem for the multilevel converter with bidirectional switch interconnection.

The cascaded H-bridge multilevel converter is heavy, bulky, and complex. Moreover, connecting separated DC sources between two converters in a back-to-back fashion is difficult, because a short circuit will occur when two back-to-back converters are not switching synchronously.

Another type of circuit configuration is the matrix converter, as shown in Figure 28.8. A matrix converter is a one-stage AC–AC converter that is composed of an array of nine bidirectional semiconductor switches, connecting each phase of the input to each phase of the output. The basic idea behind the matrix converter is that a desired input current, a desired output voltage, and a desired output frequency can be obtained by properly operating the switches that connect the output terminals of the converter to its input terminals. In order to protect the converter, the following two control rules must be complied with. First, only one switch in an output leg is allowed to be on at any instant of time. Second, all of the three output phases must be connected to an input phase at any instant of time. The actual combination of the switches depends on the modulation strategy.

FIGURE 28.8 Topology of a matrix converter.

Grid faults, even far away from the location of a wind turbine, can cause voltage sags at the connection point of the wind turbine. Such voltage sags result in an imbalance between the turbine input power and the generator output power, which initiates the machine stator and rotor current transients, the converter current transient, the DC-link voltage fluctuations, and a change in speed. One of the major problems of the type 3 wind turbines operating during grid faults is that the voltage sags may cause overvoltage in the DC link and overcurrent in the DFIG rotor circuit and the generator-side converter, which in turn may destroy the generator-side converter. To protect the generator-side converter from overvoltage or overcurrent during grid faults, a crowbar circuit is usually connected between the rotor circuit of the DFIG and the generator-side converter to short-circuit the rotor windings. During this time, the generator-side converter is blocked from switching, and its controllability is naturally lost. Consequently, there is no longer the independent control of active and reactive power in the DFIG. The DFIG becomes a conventional SCIG. It produces an amount of active power and starts to absorb an amount of reactive power. The grid-side converter can be operated to regulate the reactive power exchanged with the grid.

The crowbar circuit is connected between the rotor of the DFIG and the generator-side converter. The crowbar circuit may have various topologies. Figure 28.9a shows a passive crowbar consisting of a diode bridge that rectifies the rotor phase currents and a single thyristor in series with a resistor. The thyristor is turned on when the DC-link voltage reaches its limit value or the rotor current reaches its limit value. Simultaneously, the rotor circuit of the DFIG is disconnected from the generator side converter and connected to the crowbar. When the grid fault is cleared, the generator-side converter is restarted, and after synchronization, the rotor circuit of the DFIG is connected back to the generator side converter.

Figure 28.9b shows an active crowbar topology, which replaces the thyristor in the passive crowbar with a fully controllable semiconductor switch, such as an IGBT. This type of crowbar may be able to cut the short-circuit rotor current at any time. If either the rotor current or the DC-link voltage exceeds the limit values, the IGBTs of the generator-side converter are blocked and the active crowbar is turned on. The crowbar resistor voltage and DC-link voltage are monitored during the operation of the crowbar. When both voltages reduce below certain values, the crowbar is turned off. After a short delay for the decay of the rotor currents, the generator-side converter is restarted and connected back to the rotor circuit of the DFIG. In both topologies, the value of the crowbar resistance has significant effects on the dynamic performance of the DFIG, such as the maximum short-circuit current of the DFIG and reactive power control capability.

FIGURE 28.9 Topologies of crowbar circuits: (a) passive crowbar; (b) active crowbar.

POWER ELECTRONICS FOR WIND TURBINE TYPE 4

Wind turbine type 4 may have a variety of configurations, as illustrated in Figure 28.10. It could use an SCIG (Figure 28.10a) or a wound-rotor synchronous generator (SG) (Figure 28.10b) connected to the turbine shaft through a gearbox. It could use a wound-rotor SG (Figure 28.10c) or a permanent magnet SG (PMSG) (Figure 28.10d) connected directly to the turbine shaft without gearbox. The wound-rotor SGs in Figure 28.10b and c need an extra small AC–DC power converter, which feeds the excitation winding for field excitation. The generator is connected to the power grid through an AC–DC–AC power electronic converter, whose rating is the same as that of the electric generator used. Since the generator is decoupled from the grid, the generator can operate in a wide variable frequency range for optimal operation. The grid-side PWM converter can be used to control the active and reactive power delivered to the grid independently and to provide grid support features, such as power factor or voltage regulation. Therefore, compared to other types of wind turbines, the dynamic response of type 4 wind turbines is improved.

FIGURE 28.10 Configurations of wind turbine type 4 equipped with: (a) SCIG and gearbox; (b) wound-rotor SG and gearbox; 

FIGURE 28.10 (c) wound-rotor SG with a high number of poles but no gearbox; (d) PMSG but no gearbox.

The AC–DC–AC converters in Figure 28.10 can be implemented by using the bidirectional back-to-back PWM-VSCs as shown in Figure 28.6 to achieve full control of the active and reactive power for the generator. A wound-rotor SG or a PMSG requires only a simple diode bridge rectifier for the generator side converter, as shown in Figure 28.11. For a three-phase system, the diode rectifier consists of six diodes. The diode rectifier is simple and has a low cost. However, it can only be used in one quadrant. Therefore, it is not possible to control the active or reactive power of the generator by controlling the diode rectifier. In order to achieve variable-speed operation, the wind turbine equipped with a SG will require a boost DC–DC converter inserted between the diode rectifier and the DC-link, as shown in Figure 28.11.

FIGURE 28.11 Topology of diode rectifier and boost DC–DC converter used in type 4 wind turbine systems.

The type 4 wind turbines with a PMSG are the most popular configuration of small wind turbines for residential and other nonutility applications, in which the grid-side converter may be a single-phase full bridge instead of a three-phase PWM inverter. In this case, the AC terminals of the inverter may be connected between line and line or line and neutral of the power grid. Moreover, it is possible to use a matrix converter shown in Figure 28.8 to replace the AC–DC–AC converter for type 4 wind turbine systems.