25.2.3.2 Inverters for Stand-Alone PV Systems

Inverters convert power from DC to AC, while rectifiers convert it from AC to DC. Many inverters are bidirectional, that is, they are able to operate in both inverting and rectifying modes. In many stand-alone PV installations, alternating current is needed to operate 230 V (or 110 V), 50 Hz (or 60 Hz) appliances. Generally, stand-alone inverters operate at 12, 24, 48, 96, 120, or 240 V DC depending upon the power level. Ideally, an inverter for a stand-alone PV system should have the following features:

• Sinusoidal output voltage.

• Voltage and frequency within the allowable limits.

• Cable to handle large variation in input voltage.

• Output voltage regulation.

• High efficiency at light loads.

• Less harmonic generation by the inverter to avoid damage to electronic appliances like television, additional losses, and heating of appliances.

• Photovoltaic inverters must be able to withstand overloading for short term to take care of higher starting currents from pumps, refrigerators, etc.

• Adequate protection arrangement for over-/undervoltage and frequency, short circuit, etc.

• Surge capacity.

• Low idling and no load losses.

• Low battery voltage disconnect.

• Low audio and radio frequency (RF) noise.

Several different semiconductor devices such as metal-oxide-semiconductor field-effect transistor (MOSFETs) and insulated-gate bipolar transistors (IGBTs) are used in the power stage of inverters. Typically, MOSFETs are used in units up to 5 kVA and 96 V DC. They have the advantage of low switching losses at higher frequencies. Because the on-state voltage drop is 2 V DC, IGBTs are generally used only above 96 V DC systems.

Voltage-source inverters are usually used in stand-alone applications. They can be single phase or three phase. There are three switching techniques commonly used: square-wave, quasi-square-wave, and pulse-width modulation. Square-wave or modified square-wave inverters can supply power tools, resistive heaters, or incandescent lights, which do not require a high-quality sine wave for reliable and efficient operation. However, many household appliances require low distortion sinusoidal waveforms. The use of true sine-wave inverters is recommended for remote-area power systems. Pulse-width modulated (PWM) switching is generally used for obtaining sinusoidal output from the inverters.

A general layout of a single-phase system, both half bridge and full bridge, is shown in Fig. 25.18. In Fig. 25.18A, single-phase half bridge is with two switches, S₁ and S₂; the capacitors C₁ and C₂ are connected in series across the DC source. The junction between the capacitors is at the midpotential. Voltage across each capacitor is V_dc/2. Switches S₁ and S₂ can be switched on/off periodically to produce AC voltage. Filter (L_f and C_f) is used to reduce high-switch-frequency components and to produce sinusoidal output from the inverter. The output of inverter is connected to load through a transformer. Fig. 25.18B shows the similar arrangement for full-bridge configuration with four switches. For the same input source voltage, the full-bridge output is twice, and the switches carry less current for the same load power.

The power circuit of a three-phase four-wire inverter is shown in Fig. 25.19. The output of the inverter is connected to load via three-phase transformer (delta/Y). The star point of the transformer secondary gives the neutral connection. Three phase or single phase can be connected to this system. Alternatively, a center tap DC source can be used to supply the converter, and the midpoint can be used as the neutral.

Fig. 25.20 shows the inverter efficiency for a typical inverter used in remote-area power systems. It is important to consider that the system load is typically well below the nominal inverter capacity P_nom, which results in low conversion efficiencies at loads below 10% of the rated inverter output power. Optimum overall system operation is achieved if the total energy dissipated in the inverter is minimized. The high conversion efficiency at low power levels of recently developed inverters for grid-connected PV systems shows that there is a significant potential for further improvements in efficiency.

Bidirectional inverters convert DC power to AC power (inverter) or AC power to DC power (rectifier) and are becoming very popular in remote-area power systems [3,4]. The principle of a stand-alone single-phase bidirectional inverter used in a PV/battery/diesel hybrid system can be explained by referring Fig. 25.21. A charge controller is used to interface the PV array and the battery. The inverter has a full-bridge configuration realized using four power electronic switches (MOSFET or IGBTs) S₁–S₄. In this scheme, the diagonally opposite switches (S₁ and S₄) and (S₂ and S₃) are switched using a sinusoidally PWM gate pulses. The inverter produces sinusoidal output voltage. The inductors X₁, X₂, and the AC output capacitor C₂ filter out the high-switch-frequency components from the output waveform. Most inverter topologies use a low-frequency (50 or 60 Hz) transformer to step up the inverter output voltage. In this scheme, the diesel generator and the converter are connected in parallel to supply the load. The voltage sources, diesel and inverter, are separated by the link inductor X_m. The bidirectional power flow between inverter and the diesel generator can be established.

The power flow through the link inductor, X_m, is

$S_m=V_m{I}_m^{⁎}$

$P_m=\left(V_m{V}_{c}sin\delta \right)/X_m$

$Q_m=\left(V_m/X_m\right)\left(V_m-V_{c}cos\delta \right)$

$\delta ={\displaystyle {sin}^{-1}}\left[\left(X_m{P}_m\right)/\left(V_m{V}_c\right)\right]$

where δ is the phase angle between the two voltages. From Eq. (25.4), it can be seen that the power supplied by the inverter from the batteries (inverter mode) or supplied to the batteries (charging mode) can be controlled by controlling the phase angle δ. The PWM pulses separately control the amplitude of the converter voltage, V_c, while the phase angle with respect to the diesel voltage is varied for power flow.

25.2.3.3 Solar Water Pumping

In many remote and rural areas, hand pumps or diesel-driven pumps are used for water supply. Diesel pumps consume fossil fuel, affect environment, need more maintenance, and are less reliable. Photovoltaic powered water pumps have received considerable attention recently due to major developments in the field of solar cell materials and power electronic systems technology.

Types of pumps

Two types of pumps are commonly used for the water pumping applications: positive and centrifugal displacement. Both centrifugal and positive displacement pumps can be further classified into those with motors that are (a) surface mounted and those that are (b) submerged into the water (“submersible”).

Displacement pumps have water output directly proportional to the speed of the pump but almost independent of the head. These pumps are used for solar water pumping from deep wells or bores. They may be piston-type pumps or use diaphragm driven by a cam and rotary screw-type pumps or use progressive cavity system. The pumping rate of these pumps is directly related to the speed, and hence, constant torque is desired.

Centrifugal pumps are used for low-head applications especially if they are directly interfaced with the solar panels. Centrifugal pumps are designed for fixed-head applications, and the pressure difference generated increases in relation to the speed of the pump. These pumps are rotating impeller type, which throws the water radially against a casing, so shaped that the momentum of the water is converted into useful pressure for lifting [3]. The centrifugal pumps have relatively high efficiency, but it reduces at lower speeds, which can be a problem for the solar water pumping system at the time of low light levels. The single-stage centrifugal pump has just one impeller, whereas most borehole pumps are multistage types where the outlet from one impeller goes into the center of another and each one keeps increasing the pressure difference.

From Fig. 25.22, it is quite obvious that the load line is located relatively faraway from P_max line. It has been reported that the daily utilization efficiency for a DC motor drive is 87% for a centrifugal pump compared with 57% for a constant torque characteristic load. Hence, centrifugal pumps are more compatible with PV arrays. The system operating point is determined by the intersection of the I-V characteristics of the PV array and the motor as shown in Fig. 25.22. The torque-speed slope is normally large due to the armature resistance being small. At the instant of starting, the speed and the back emf are zero. Hence, the motor starting current is approximately the short-circuit current of the PV array. By matching the load to the PV source through MPPT, the starting torque increases.

The matching of a DC motor depends upon the type of load being used. For instance, a centrifugal pump is characterized by having the load torque proportional to the square of speed. The operating characteristics of the system (i.e., PV source, permanent magnet (PM) DC motor, and load) are at the intersection of the motor and load characteristics as shown in Fig. 25.23 (i.e., points a, b, c, d, e, and f for centrifugal pump). From Fig. 25.23, the system utilizing the centrifugal pump as its load tends to start at low solar irradiation (point a) level. However, for the systems with an almost constant torque characteristics in Fig. 25.22, the start is at almost 50% of one sum (full insolation) that results in short period of operation.

Power conditioning units for PV water pumping

Most PV pump manufacturers include power-conditioning units (PCU) that are used for operating the PV panels close to their MPP over a range of load conditions and varying insolation levels and also for power conversion. DC or AC motor-pump units can be used for PV water pumping. In its simplest form, a solar water pumping system is composed of PV array, PCU, and DC water-pump unit as shown in Fig. 25.24.

In case of lower light levels, high currents can be generated through power conditioning to help in starting the motor-pump units especially for reciprocating positive displacement type pumps with constant torque characteristics, requiring constant current throughout the operating region. In positive displacement type pumps, the torque generated by the pumps depends on the pumping head, friction, pipe diameter, etc. and needs certain level of current to produce the necessary torque. Some systems use electronic controllers to assist starting and operation of the motor under low solar radiation. This is particularly important when using positive displacement pumps. The solar panels generate DC voltage and current. The solar water pumping systems usually have DC or AC pumps. For DC pumps, the PV output can be directly connected to the pump through MPPT, or a DC-DC converter can also be used for interfacing for controlled DC output from PV panels. To feed the AC motors, a suitable interfacing is required for the power conditioning. These PV inverters for the stand-alone applications are very expensive. The aim of power-conditioning equipment is to supply the controlled voltage/current output from the converters/inverters to the motor-pump unit.

These power-conditioning units are also used for operating the PV panels close to their maximum efficiency for fluctuating solar conditions. The speed of the pump is governed by the available driving voltage. Current lower than the acceptable limit will stop the pumping. When the light level increases, the operating point will shift from the MPP leading to the reduction of efficiency. For centrifugal pumps, there is an increase in current at increased speed, and the matching of I-V characteristics is closer for wide range of light intensity levels. For centrifugal pumps, the torque is proportional to the square of speed, and the torque produced by the motors is proportional to the current. Due to decrease in PV current output, the torque from the motor and consequently the speed of the pump are reduced, resulting in decrease in back emf and the required voltage of the motor. Maximum power point tracker can be used for controlling the voltage/current outputs from the PV inverters to operate the PV close to maximum operating point for the smooth operation of motor-pump units. The DC-DC converter can be used for keeping the PV panel output voltage constant and help in operating the solar arrays close to MPP. In the beginning, high starting current is required to produce high starting torque. The PV panels cannot supply this high starting current without adequate power-conditioning equipment like DC-DC converter or by using a starting capacitor. The DC-DC converter can generate the high starting currents by regulating the excess PV array voltage. DC-DC converter can be boost or buck converter.

Brushless DC motor (BDCM) and helical rotor pumps can also be used for PV water pumping [35]. Brushless DC motors are a self-synchronous type of motor characteristics by trapezoidal waveforms for back emf and air flux density. They can operate off a low-voltage DC supply that is switched through an inverter to create a rotating stator field. The current generations of BDCMs use rare-earth magnets on the rotor to give high air gap flux densities and are well suited to solar application. The block diagram of such an arrangement is shown in Fig. 25.25 that consists of PV panels, DC-DC converter, MPPT, and BDCM.

The PV inverters are used to convert the DC output of the solar arrays to the AC quantity so as to run the AC motor-driven pumps. These PV inverters can be variable frequency type, which can be controlled to operate the motors over wide range of loads. The PV inverters may involve impedance matching to match the electric characteristics of the load and array. The motor-pump unit and PV panels operate at their maximum efficiencies. Maximum power point tracker is also used in the power conditioning. To keep the voltage stable for the inverters, the DC-DC converter can be used. The inverter/converter has a capability of injecting high-switch-frequency components, which can lead to the overheating and the losses. So care shall be taken for this. The PV arrays are usually connected in series, parallel, or a combination of series-parallel configurations. The function of power electronic interface, as mentioned before, is to convert the DC power from the array to the required voltage and frequency to drive the AC motors. The motor-pump system load should be such that the array operates close to its MPP at all solar insolation levels. There are mainly three types of solar-powered water pumping systems as shown in Fig. 25.26.

The first system shown in Fig. 25.26A is an imported commercially available unit, which uses a specially wound low-voltage induction-motor-driven submersible pump. Such a low-voltage motor permits the PV array voltage to be converted to AC without using a step-up transformer. The second system, shown in Fig. 25.26B, makes use of a conventional “off-the-shelf” 415 V, 50 Hz induction motor [5]. This scheme needs a step-up transformer to raise inverter output voltage to high voltage. The third scheme as shown in Fig. 25.26C is composed of a DC-DC converter, an inverter that switches at a high frequency, and a mains voltage motor-driven pump. To get the optimum discharge (Q), at a given insolation level, the efficiency of the DC-DC converter and the inverter should be high. So the purpose should be to optimize the output from PV array, motor, and the pump. The principle used here is to vary the duty cycle of a DC-DC converter so that the output voltage is maximum. The DC-DC converter is used to boost the solar array voltage to eliminate the need for a step-up transformer and operate the array at the MPP. The three-phase inverter used in the interface is designed to operate in a variable frequency mode over the range of 20–50 Hz, which is the practical limit for most 50 Hz induction motor applications. Block diagram for frequency control is given in Fig. 25.27.

This inverter would be suitable for driving permanent magnet motors by incorporating additional circuitry for position sensing of the motor's shaft. Also, the inverter could be modified, if required, to produce higher output frequencies for high-speed permanent magnet motors. The inverter has a three-phase full-bridge configuration implemented by MOSFET power transistors.

25.2.4.1 Inverters for Grid-Connected Applications

Power conditioner is the key link between the PV array and mains in the grid-connected PV system. It acts as an interface that converts DC current produced by the solar cells into utility grade AC current. The PV system behavior relies heavily on the power-conditioning unit. The inverters shall produce good quality sine-wave output. The inverter must follow the frequency and voltage of the grid, the inverter has to extract maximum power from the solar cells with the help of MPPT, and the inverter input stage varies the input voltage until the MPP on the I-V curve is found. The inverter shall monitor all the phases of the grid. The inverter output shall be controlled in terms of voltage and frequency variation. A typical grid-connected inverter may use a PWM scheme and operates in the range of 2–20 kHz.

25.2.4.2 Inverter Classifications

The inverters used for the grid interfacing are broadly classified as follows:

• Voltage-source inverters (VSI)

• Current-source inverters (CSI)

The inverters based on the control schemes can be classified as follows:

• Current controlled (CC)

• Voltage controlled (VC)

The source is not necessarily characterized by the energy source for the system. It is a characteristic of the topology of the inverter. It is possible to change from one source type to another source type by the addition of passive components. In the voltage-source inverter (VSI), the DC side is made to appear to the inverter as a voltage source. The VSIs have a capacitor in parallel across the input, whereas the CSIs have an inductor in series with the DC input. In the CSI, the DC source appears as a current source to the inverter. Solar arrays are fairly good approximation to a current source. Most PV inverters are voltage source even though the PV is a current source. Current-source inverters are generally used for large motor drives though there have been some PV inverters built using a current-source topology. The VSI is more popular with the PWM VSI dominating the sine-wave inverter topologies.

Fig. 25.28A shows a single-phase full-bridge bidirectional VSI with (A) voltage control and phase-shift (δ) control—voltage-controlled voltage-source inverter (VCVSI). The active power transfer from the PV panels is accomplished by controlling the phase angle δ between the converter voltage and the grid voltage. The converter voltage follows the grid voltage. Fig. 25.28B shows the same VSI operated as (B) current controlled (CCVSI). The objective of this scheme is to control active and reactive components of the current fed into the grid using PWM techniques.

25.2.4.3 Inverter Types

Different types are being in use for the grid-connected PV applications such as the following:

• Line-commutated inverter

• Self-commutated inverter

• Inverter with high-frequency transformer

Line-commutated inverter

The line-commutated inverters are generally used for the electric motor applications. The power stage is equipped with thyristors. The maximum power tracking control is required in the control algorithm for solar application. The basic diagram for a single-phase line-commutated inverter is shown in Fig. 25.29 [2].

The driver circuit has to be changed to shift the firing angle from the rectifier operation ($0<\varphi <90$) to inverter operation ($90<\varphi <180$). Six-pulse or twelve-pulse inverters are used for the grid interfacing, but twelve-pulse inverters produce less harmonics. The thysistor-type inverters require a low-impedance grid interface connection for commutation purpose. If the maximum power available from the grid connection is less than twice the rated PV inverter power, then the line-commutated inverter should not be used [2]. The line-commutated inverters are cheaper but inhibits poor power quality. The harmonics injected into the grid can be large unless taken care of by employing adequate filters. These line-commutated inverters also have poor power factor, poor power quality, and need additional control to improve the power factor. Transformer can be used to provide the electric isolation. To suppress the harmonics generated by these inverters, tuned filters are employed, and reactive power compensation is required to improve the lagging power factor.

Self-commutated inverter

A switch-mode inverter using pulse-width modulated (PWM) switching control can be used for the grid connection of PV systems. The basic block diagram for this type of inverter is shown in Fig. 25.30. The inverter bridges may consist of bipolar transistors, MOSFET transistors, IGBTs, or gate turn-off thyristors (GTOs), depending upon the type of application. GTOs are used for the higher-power applications, whereas IGBTs can be switched at higher frequencies, that is, 16 kHz, and are generally used for many grid-connected PV applications. Most of the present day inverters are self-commutated sine-wave inverters.

Based on the switching control, the voltage-source inverters can be further classified based on the switching control as follows:

• PWM (pulse-width modulated) inverters

• Square-wave inverters

• Single-phase inverters with voltage cancellations

• Programmed harmonic elimination switching

• Current-controlled modulation

Inverter with high-frequency transformer

The 50 Hz transformer for a standard PV inverter with PWM switching scheme can be very heavy and costly. While using frequencies more than 20 kHz, a ferrite core transformer can be a better option [2]. A circuit diagram of a grid-connected PV system using high-frequency transformer is shown in Fig. 25.31.

The capacitor on the input side of high-frequency inverter acts as the filter. The high-frequency inverter with PWM is used to produce a high-frequency AC across the primary winding of the high-frequency transformer. The secondary voltage of this transformer is rectified using high-frequency rectifier. The DC voltage is interfaced with a thyristor inverter through a low-pass inductor filter and hence connected to the grid. The line current is required to be sinusoidal and in phase with the line voltage. To achieve this, the line voltage (V₁) is measured to establish the reference waveform for the line current I_L⁎. This reference current I_L⁎ multiplied by the transformer ratio gives the reference current at the output of a high-frequency inverter. The inverter output can be controlled using current control technique [36]. These inverters can be with low-frequency transformer isolation or high-frequency transformer isolation. The low-frequency (50–60 Hz) transformer of a standard inverter with PWM is a very heavy and bulky component. For residential grid-interactive rooftop inverters below 3 kW rating, high-frequency transformer isolation is often preferred.

Other PV inverter topologies

In this section, some of the inverter topologies discussed in various research papers have been discussed.

Multilevel converters

Multilevel converters can be used with large PV systems where multiple PV panels can be configured to create voltage steps. These multilevel voltage-source converters can synthesize the AC output terminal voltage from different level of DC voltages and can produce staircase waveforms. This scheme involves less complexity and needs less filtering. One of the schemes (half-bridge diode-clamped three-level inverter [37]) is given in Fig. 25.32. There is no transformer in this topology. Multilevel converters can be beneficial for large systems in terms of cost and efficiency. Problems associated with shading and malfunction of PV units need to be addressed.

The utilization of cascaded multilevel inverter (CMLI) in high-voltage and high-power PV energy conversion has been recently increased [38–40]. CMLI consists of a number of inverter units that are fed by isolated PV array [38]. Each unit utilizes low-voltage components, and the final output voltage is synthesized by combining all units in series. This topology has modular characteristic, since all the units are similar and have the same control strategy. This modularity feature makes it very easy to replace any faulty unit. Without discontinuing the load, it is also possible to bypass the faulty module by adopting a proper control technique [41].

Fig. 25.33 shows a five-level cascaded H-bridge inverter. Each inverter arm consists of a number of H-bridge units whose outputs are connected in series. Each unit comprises four switches (S₁–S₄) and one DC input supply (V_dc). The basic H-bridge unit shown in Fig. 25.34 is able to produce three voltage levels (−V_dc, 0, and V_dc). The output voltage and switching logic of this H-bridge unit is shown in Table 25.2.

Table 25.2

Switching logic of an H-bridge unit

Vout	Turn on switches
Vdc	S1, S4
0	S1, S3
	S2, S4
−Vdc	S2, S3

In Fig. 25.34, each phase arm contains two series-connected units. The output line voltages comprises five levels (−2 V_dc, −V_dc, 0, V_dc, and 2 V_dc). If the number of units in each phase arm is “u,” then the number of levels in the line voltage is (l=2u+1). All possible switching combinations for a five-level H-bridge inverter are presented in Table 25.3.

Table 25.3

All possible switching combination for various output levels of a 5-level CHB inverter

	U1	U2	U1				U2
	Vo,U1	Vo,U2	S1	S2	S3	S4	S1	S2	S3	S4
2Vdc	Vdc	Vdc	1	0	0	1	1	0	0	1
Vdc	Vdc	0	1	0	0	1	1	0	1	0
	Vdc	0	1	0	0	1	0	1	0	1
	0	Vdc	1	0	1	0	1	0	0	1
	0	Vdc	0	1	0	1	1	0	0	1
0	Vdc	−Vdc	1	0	0	1	0	1	1	0
	0	0	1	0	1	0	1	0	1	0
	0	0	1	0	1	0	0	1	0	1
	0	0	0	1	0	1	1	0	1	0
	0	0	0	1	0	1	0	1	0	1
	−Vdc	Vdc	0	1	1	0	1	0	0	1
−Vdc	0	−Vdc	1	0	1	0	0	1	1	0
	0	−Vdc	0	1	0	1	0	1	1	0
	−Vdc	0	0	1	1	0	1	0	1	0
	−Vdc	0	0	1	1	0	0	1	0	1
−2Vdc	−Vdc	−Vdc	0	1	1	0	0	1	1	0

A three-phase “l”-level CHB MLI needs 6(l−1) switching devices. No additional diodes are needed, except the antiparallel diodes in the switches. The current and voltage ratings of all switching devices are similar in case of symmetrical condition. In case of PV energy conversion, the input DC supplies of the H-bridge units are replaced by the PV arrays [42–45].

Non-insulated voltage source

In this scheme [46], string of low-voltage PV panels or one high-voltage unit can be coupled with the grid through DC-to-DC converter and voltage-source inverter. This topology is shown in Fig. 25.35. PWM switching scheme can be used to generate AC output. Filter has been used to reject the switching components.

Non-insulated current source

This type of configuration is shown in Fig. 25.36. Noninsulated current-source inverters [46] can be used to interface the PV panels with the grid. This topology involves low cost, which can provide better efficiency. Appropriate controller can be used to reduce current harmonics.

Buck converter with half-bridge transformer link

PV panels are connected to grid via buck converter and half bridge as shown in Fig. 25.37. In this, high-frequency PWM switching has been used at the low-voltage PV side to generate an attenuated rectified 100 Hz sine-wave current waveform [47]. Half-wave bridge is utilized to convert this output to 50 Hz signal suitable for grid interconnection. To step up the voltage, the transformer has also been connected before the grid connection point.

Flyback converter

This converter topology steps up the PV voltage to DC bus voltage. Pulse-width-modulation-operated converter has been used for grid connection of PV system (Fig. 25.38). This scheme is less complex and has less number of switches. Flyback converters can be beneficial for remote areas due to less complex power-conditioning components.

Interface using paralleled PV panels

Low-voltage AC bus scheme [48] can be comparatively efficient and cheaper option. One of the schemes is shown in Fig. 25.39. A number of smaller PV units can be paralleled together and then connected to combine single low-frequency transformer. In this scheme, the PV panels are connected in parallel rather than series to avoid problems associated with shading or malfunction of one of the panels in series connection.

25.2.4.4 Power Control through PV Inverters

The system shown in Fig. 25.40 shows control of power flow onto the grid [49]. This control can be an analog or a microprocessor system. This control system generates the waveforms and regulates the waveform amplitude and phase to control the power flow between the inverter and the grid. The grid-interfaced PV inverters, voltage-controlled VSI (VCVSI), and current-controlled VSI (CCVSI) have the potential of bidirectional power flow. They not only can feed the local load but also can export the excess active and reactive power to the utility grid. An appropriate controller is required in order to avoid any error in power export due to errors in synchronization, which can overload the inverter.

There are advantages and limitations associated with each control mechanism. For instance, VCVSIs provide voltage support to the load (here, the VSI operates as a voltage source), while CCVSIs provide current support (here, the VSI operates as a current source). The CCVSI is fast in response compared with the VCVSI, as its power flow is controlled by the switching instant, whereas in the VCVSI the power flow is controlled by adjusting the voltage across the decoupling inductor. Active and reactive powers are controlled independently in the CCVSI but are coupled in the VCVSI. Generally, the advantages of one type of VSI are considered as a limitation of the other type [50].

Fig. 25.41 shows the simplified/equivalent schematic diagram of a VCVSI. For the following analysis, it is assumed that the output low-pass filters (L_f and C_f) of VSIs will filter out high-order harmonics generated by PWMs. The decoupling inductor (X_m) is an essential part of any VCVSI as it makes the power flow control possible. In a VCVSI, the power flow of the distributed generation system (DGS) is controlled by adjusting the amplitude and phase (power angle (δ)) of the inverter output voltage with respect to the grid voltage. Hence, it is important to consider the proper sizing of the decoupling inductor and the maximum power angle to provide the required power flow when designing VCVSIs. The phasor diagram of a simple grid-inverter interface with a first-order filter is shown in Fig. 25.42.

Referring to Fig. 25.41, the fundamental grid current (I_g) can be expressed by Eq. (25.7):

$I_g=\frac{V_g<0-V_c<\delta }{j{X}_m}=-\frac{V_{c}sin\delta }{X_m}-j\frac{V_g-V_{c}cos\delta }{X_m}$

where V_g and V_c are, respectively, the grid and the VCVSI's fundamental voltages and X_m is the decoupling inductor impedance. Using per-unit values ($S_{\mathit{base}}=V_{\mathit{base}}^2/Z_{\mathit{base}}$, $V_{\mathit{base}}=V_c$, and $Z_{\mathit{base}}=X_m$) where V_base, Z_base, and S_base are the base voltage, impedance, and complex power values, respectively, the grid apparent power can be expressed as Eq. (25.8):

$S_{gpu}=-V_{gpu}sin\delta +j\left(V_{gpu}^2-V_{gpu}cos\delta \right)$

Using per-unit values, the complex power of the VCVSI and decoupling inductor are

$S_{cpu}=-V_{gpu}sin\delta +j\left(V_{gpu}cos\delta -1\right)$

$S_{xpu}=j\left(V_{gpu}^2-2V_{gpu}cos\delta +1\right)$

where S_gpu, S_cpu, and S_xpu are per-unit values of the grid, VCVSI, and decoupling inductor apparent power, respectively, and V_gpu is the per-unit value of the grid voltage.

Fig. 25.43 shows the equivalent schematic diagram of a CCVSI. As a CCVSI controls the current flow using the VSI switching instants, it can be modeled as a current source, and there is no need for a decoupling inductor (Fig. 25.43). As the current generated from the CCVSI can be controlled independently from the AC voltage, the active and reactive power controls are decoupled. Hence, unity power factor operation is possible for the whole range of the load. This is one of the main advantages of CCVSIs.

As the CCVSI is connected in parallel to the DGS, it follows the grid voltage. Fig. 25.44 shows the phasor diagram of a CCVSI-based DGS in the presence of an inductive load (considering the same assumption as VCVSI section). Fig. 25.44 shows that when the grid voltage increases, the load's active power consumption, which is supplied by the grid, increases and the CCVSI compensates the increase in the load reactive power demand. In this case, the CCVSI maintains grid supply at unity power factor, keeping the current phase delay with respect to the grid voltage at a fixed value (θ). Therefore, the CCVSI cannot maintain the load voltage in the presence of a DGS without utilizing extra hardware and control mechanisms. This limitation on load voltage stabilization is one of the main drawbacks of CCVSI-based DGS.

Assuming the load active current demand is supplied by the grid (reactive power support function), the required grid current can be rewritten as follows:

$I_g^{⁎}={\displaystyle \mathrm{R}\mathrm{e}}\left[I_L\right]={\displaystyle \mathrm{R}\mathrm{e}}\left[\frac{S_L}{V_g}\right]$

where S_L is the demanded load apparent power. For grid power conditioning, it is preferred that the load operate at unity power factor. Therefore, the CCVSI must provide the remainder of the required current in Eq. (25.12):

$I_c=I_L-I_g^{⁎}$

For demand side management (DSM), it is desirable to supply the active power by the RES, where excess energy from the RES is injected into the DGS. The remaining load reactive power will be supplied by the CCVSI. Hence, Eq. (25.12) can be rewritten as Eq. (25.13):

$I_g^{⁎}={\displaystyle \mathrm{R}\mathrm{e}}\left[I_L\right]-{\displaystyle \mathrm{R}\mathrm{e}}\left[I_c\right]={\displaystyle \mathrm{R}\mathrm{e}}\left[\frac{S_L-P_{RES}}{V_g}\right]$

When using a voltage controller for grid-connected PV inverter, it has been observed that a slight error in the phase of synchronizing waveform can grossly overload the inverter whereas a current controller is much less susceptible to voltage phase shifts [49]. Due to this reason, the current controllers are better suited for the control of power export from the PV inverters to the utility grid since they are less sensitive to errors in synchronizing sinusoidal voltage waveforms.

A prototype current-controlled-type power-conditioning system has been developed by the first author and tested on a weak rural feeder line at Kalbarri in Western Australia [51]. The choice may be between additional conventional generating capacities at a centralized location and adding smaller distributed generating capacities using RES like PV. The latter option can have a number of advantages like as follows:

• The additional capacity is added wherever it is required without adding additional power distribution infrastructure. This is a critical consideration where the power lines and transformers are already at or close to their maximum ratings.

• The power-conditioning system can be designed to provide much more than just a source of real power, for minimal extra cost. A converter providing real power needs only a slight increase in ratings to handle significant amounts of reactive or even harmonic power. The same converter that converts DC PV power to AC power can simultaneously provide the reactive power support to the week utility grid.

The block diagram of the power-conditioning system used in the Kalbarri project has been shown in Fig. 25.45. This CCVSI operates with a relatively narrow switching frequency band near 10 kHz. The control diagram indicates the basic operation of the power-conditioning system. The two outer control loops operate to independently control the real and reactive power flow from the PV inverter. The real power is controlled by an outer MPPT algorithm with an inner DC link voltage control loop providing the real current magnitude request I_p⁎, and hence, the real power export through PV converter is controlled through the DC link voltage regulation. The DC link voltage is maintained at a reference value by a PI control loop, which gives the real current reference magnitude as its output. At regular intervals, the DC link voltage is scanned over the entire voltage range to check that the algorithm is operating on the absolute MPP and is not stuck around a local MPP. During the night, the converter can still be used to regulate reactive power of the grid-connected system although it cannot provide active power. During this time, the PI controller maintains a minimum DC link voltage to allow the power-conditioning system to continue to operate, providing the necessary reactive power.

The AC line voltage regulation is provided by a separate reactive power control, which provides the reactive current magnitude reference I_Q⁎. The control system has a simple transfer function, which varies the reactive power command in response to the AC voltage fluctuations. Common to the outer real and reactive power control loops is an inner higher bandwidth zero average current error (ZACE) current control loop. I_p⁎ is in phase with the line voltages, and I_Q⁎ is at 90° to the line voltage. These are added together to give one (per phase) sinusoidal converter current reference waveform (I_ac⁎). The CCVSI control consists of analog and digital circuitry that acts as a ZACE transconductance amplifier in converting I_ac⁎ into AC power currents [52].

25.2.4.5 System Configurations

The utility compatible inverters are used for power conditioning and synchronization of PV output with the utility power. In general, four types of batteryless grid-connected PV system configurations have been identified:

• Central plant inverter

• Multiple string DC/DC converter with single-output inverter

• Multiple string inverter

• Module-integrated inverter

Central plant inverter

In the central plant inverter, usually a large inverter is used to convert DC power output of PV arrays to AC power. In this system, the PV modules are serially stringed to form a panel (or string), and several such panels are connected in parallel to a single DC bus. The block diagram of such a scheme is shown in Fig. 25.46.

Multiple string DC/DC converter

In multiple string DC/DC converter, as shown in Fig. 25.47, each string will have a boost DC/DC converter with transformer isolation. There will be a common DC link, which feeds a transformerless inverter.

Multiple string inverters

Fig. 25.48 shows the block diagram of multiple string inverter system. In this scheme, several modules are connected in series on the DC side to form a string. The output from each string is converted to AC through a smaller individual inverter. Many such inverters are connected in parallel on the AC side. This arrangement is not badly affected by the shading of the panels. It is also not seriously affected by inverter failure.

Module integrated inverter

In the module-integrated inverter system (Fig. 25.49), each module (typically 50–300 W) will have a small inverter. No cabling is required. It is expected that high volume of small inverters will bring down the cost.

25.2.4.6 Grid-Compatible Inverters Characteristics

The characteristics of the grid-compatible inverters are as follows:

• Response time

• Power factor

• Frequency control

• Harmonic output

• Synchronization

• Fault current contribution

• DC current injection

• Protection

The response time of the inverters shall be extremely fast and governed by the bandwidth of the control system. The absence of rotating mass and use of semiconductor switches allow inverters to respond in millisecond time frame. The power factor of the inverters is traditionally poor due to displacement power factor and the harmonics. But with the latest development in the inverter technology, it is possible to maintain the power factor close to unity. The converters/inverters have the capability of creating large voltage fluctuation by drawing reactive power from the utility rather than supplying [53]. With proper control, inverters can provide voltage support by importing/exporting reactive power to push/pull toward a desired set point. This function would be of more use to the utilities as it can assist in the regulation of the grid system at the domestic consumer level.

Frequency of the inverter output waveshape is locked to the grid. Frequency bias is where the inverter frequency is deliberately made to run at 53 Hz. When the grid is present, this will be pulled down to the nominal 50 Hz. If the grid fails, it will drift upward toward 53 Hz and trip on overfrequency. This can help in preventing islanding.

Harmonics output from the inverters have been very poor traditionally. Old thyristor-based inverters are operated with slow switching speeds and could not be pulse-width modulated. This resulted in inverters known as 6-pulse or 12-pulse inverters. The harmonics so produced from the inverters can be injected into the grid, resulting in losses, heating of appliances, tripping of protection equipments, and poor power quality. The number of pulses is being the number of steps in a sine-wave cycle. With the present advent in the power electronics technology, the inverter controls can be made very well. Pulse-width-modulated inverters produce high-quality sine waves. The harmonic levels are very low and can be lower than the common domestic appliances. If the harmonics are present in the grid voltage waveform, harmonic currents can be induced in the inverter. These harmonic currents, particularly those generated by a voltage-controlled inverter, will in fact help in supporting the grid. These are good harmonic currents. This is the reason that the harmonic current output of inverters must be measured onto a clean grid source so that the only harmonics being produced by the inverters are measured.

Synchronization of inverter with the grid is performed automatically and typically uses zero-crossing detection on the voltage waveform. An inverter has no rotating mass and hence has no inertia. Synchronization does not involve the acceleration of a rotating machine. Consequently, the reference waveforms in the inverter can be jumped to any point required within a sampling period. If phase-locked loops are used, it could take up a few seconds. Phase-locked loops are used to increase the immunity to noise. This allows the synchronization to be based on several cycles of zero-crossing information. The response time for this type of locking will be slower.

Photovoltaic panels produce a current that is proportional to the amount of light falling on them. The panels are normally rated to produce 1000 W/m² at 25°C. Under these conditions, the short-circuit current possible from these panels is typically only 20% higher than the nominal current, whereas it is extremely variable for wind. If the solar radiation is low, then the maximum current possible under short circuit is going to be less than the nominal full-load current. Consequently, PV systems cannot provide the short-circuit capacity to the grid. If a battery is present, the fault current contribution is limited by the inverter. With the battery storage, it is possible for the battery to provide the energy. However, inverters are typically limited between 100% and 200% of nominal rating under current limit conditions. The inverter needs to protect itself against the short circuits because the power electronic components will typically be destroyed before a protection device like circuit breaker trips.

In case of inverter malfunction, inverters have the capability to inject the DC components into the grid. Most utilities have guidelines for this purpose. A transformer shall be installed at the point of connection on the AC side to prevent DC from entering into the utility network. The transformer can be omitted when a DC detection device is installed at the point of connection on the AC side in the inverter. The DC injection is essentially caused by the reference or power electronic device producing a positive half cycle that is different from the negative half cycle resulting in the DC component in the output. If the DC component can be measured, it can then be added into the feedback path to eliminate the DC quantity.

25.3.1.1 Types of Wind Turbines

There are two types of wind turbines available Fig. 25.53:

• Horizontal-axis wind turbines (HAWTs)

• Vertical-axis wind turbines (VAWTs)

Vertical-axis wind turbines (VAWTs) have an axis of rotation that is vertical, and so, unlike the horizontal wind turbines, they can capture winds from any direction without the need to reposition the rotor when the wind direction changes (without a special yaw mechanism). Vertical-axis wind turbines were also used in some applications as they have the advantage that they do not depend on the direction of the wind. It is possible to extract power relatively easier. But there are some disadvantages such as no self-starting system, smaller power coefficient than obtained in the horizontal-axis wind turbines, strong discontinuation of rotations due to periodic changes in the lift force, and the regulation of power that is not yet satisfactory.

The horizontal-axis wind turbines are generally used. Horizontal-axis wind turbines are, by far, the most common design. There are a large number of designs commercially available ranging from 50 W to 4.5 MW. The number of blades ranges from one to many in the familiar agriculture windmill. The best compromise for electricity generation, where high rotational speed allows the use of a smaller and cheaper electric generator, is two or three blades. The mechanical and aerodynamic balance is better for three-bladed rotor. In small wind turbines, three blades are common. Multiblade wind turbines are used for water pumping on farms.

Based on the pitch control mechanisms, the wind turbines can also be classified as follows:

• Fixed-pitch wind turbines

• Variable-pitch wind turbines

Different manufacturers offer fixed-pitch and variable-pitch blades. Variable pitch is desirable on large machines because the aerodynamic loads on the blades can be reduced and when used in fixed-speed operation they can extract more energy. But necessary mechanisms require maintenance, and for small machines, installed in remote areas, fixed pitch seems more desirable and economical. In some machines, power output regulation involves yawing blades so that they no longer point into the wind. One such system designed in Western Australia has a tail that progressively tilts the blades in a vertical plane so that they present a small surface to the wind at high speeds.

The active power of a wind turbine can be regulated either by designing the blades to go into an aerodynamic stall beyond the designated wind speed or by feathering the blades out of the wind, which results in reducing excess power using a mechanical and electric mechanism. Recently, an active stall has been used to improve the stability of wind farms. This stall mechanism can prevent power deviation from gusty winds to pass through the drivetrain [56].

Horizontal-axis wind turbines can be further classified into fixed speed (FS) or variable speed (VS). The FS wind turbine (FSWT) generator is designed to operate at maximum efficiency while operating at a rated wind speed. In this case, the optimum tip-speed ratio is obtained for the rotor airfoil at a rated wind speed. For a VS wind turbine (VSWT) generator, it is possible to obtain optimum wind speed at different wind speeds. Hence, this enables the VS wind turbine to increase its energy capture. The general advantages of a VSWT are summarized as follows:

• VSWTs are more efficient than the FSWTs.

• At low wind speeds, the wind turbines can still capture the maximum available power at the rotor, hence increasing the possibility of providing the rated power for wide speed range.

25.3.1.2 Types of Wind Generators

Schemes based on permanent magnet synchronous generators (PMSG) and induction generators are receiving close attention in wind power applications because of their qualities such as ruggedness, low cost, manufacturing simplicity, and low-maintenance requirements. Despite many positive features over the conventional synchronous generators, the PMSG was not being used widely [22]. However, with the recent advent in power electronics, it is now possible to control the variable voltage, variable frequency output of PMSG. The permanent magnet machine is generally favored for developing new designs, because of higher efficiency and the possibility of a rather smaller diameter. These PMSG machines are now being used with variable-speed wind machines.

In large power system networks, synchronous generators are generally used with fixed-speed wind turbines. The synchronous generators can supply both the active and reactive power, and their reactive power flow can be controlled. The synchronous generators can operate at any power factor. For the induction generator, driven by a wind turbine, it is a well-known fact that it can deliver only active power while consuming reactive power.

Synchronous generators with high power rating are significantly more expensive than induction generators of similar size. Moreover, direct-connected synchronous generators have the limitation of rotational speed being fixed by the grid frequency. Hence, fluctuation in the rotor speed due to wind gusts leads to higher torque in high power output fluctuations and the derived train. Therefore, in grid-connected application, synchronous generators are interfaced via power converters to the grid. This also allows the synchronous generators to operate wind turbines in VS, which makes gearless operation of the VSWT possible.

The squirrel-cage induction generators are widely used with the fixed-speed wind turbines. In some applications, wound rotor induction generators have also been used with adequate control scheme for regulating speed by external rotor resistance. This allows the shape of the torque-slip curve to be controlled to improve the dynamics of the drivetrain. In case of PMSG, the converter/inverter can be used to control the variable voltage, variable frequency signal of the wind generator at varying wind speed. The converter converts this varying signal to the DC signal, and the output of converter is converted to AC signal of desired amplitude and frequency.

The induction generators are not locked to the frequency of the network. The cyclic torque fluctuations at the wind turbine can be absorbed by very small change in the slip speed. In case of the capacitor-excited induction generators, they obtain the magnetizing current from capacitors connected across its output terminals [55,57,58].

To take advantage of VSWTs, it is necessary to decouple the rotor speed and the grid frequency. There are different approaches to operate the VSWT within a certain operational range (cut-in and cut-out wind speed). One of the approaches is dynamic slip control, where the slip is allowed to vary up to 10% [59]. In these cases, doubly-fed induction generators (DFIG) are used (Fig. 25.54). One limitation is that DFIG require reactive power to operate. As it is not desired that the grid supply this reactive power, these generators are usually equipped with capacitors. A gearbox forms an essential component of the wind turbine generator (WTG) using induction generators. This results in the following limitations:

• Frequent maintenance

• Additional cost

• Additional losses

With the emergence of large wind power generation, increased attention is being directed toward wound rotor induction generators (WRIG) controlled from the rotor side for variable-speed constant-frequency (VSCF) applications. A wound rotor induction generator has a rotor containing a three-phase winding. These windings are made accessible to the outside via slip rings. The main advantages of a wound rotor induction generator for VSCF applications are as follows:

• Easier generator torque control using rotor current control.

• Smaller generator capacity as the generated power can be accessed from the stator and from the rotor. Usually, the rotor power is proportional to the slip speed (shaft speed-synchronous speed). Consequently, smaller rotor power converters are required. The frequency converter in the rotor (inverter) directly controls the current in the rotor winding, which enables the control of the whole generator output. The power electronic converters generally used are rated at 20%–30% of the nominal generator power.

• Fewer harmonics exist because control is in the rotor while the stator is directly connected to the grid.

If the rotor is short-circuited (making it the equivalent of a cage rotor induction machine), the speed is primarily determined by the supply frequency, and the nominal slip is within 5%. The mechanical power input (P_TURBINE) is converted into stator electric power output (P_STATOR) and is fed to the AC supply. The rotor power loss, being proportional to the slip speed, is commonly referred to as the slip power (P_ROTOR). The possibility of accessing the rotor in a doubly fed induction generator makes a number of configurations possible. These include slip power recovery using a cycloconverter, which converts the ac voltage of one frequency to another without an intermediate DC link [60–62], or back-to-back inverter configurations [63,64].

Using voltage-source inverters (VSIs) in the rotor circuit, the rotor currents can be controlled at the desired phase, frequency, and magnitude. This enables reversible flow of active power in the rotor, and the system can operate in subsynchronous and supersynchronous speeds, both in motoring and generating modes. The DC link capacitor acts as a source of reactive power, and it is possible to supply the magnetizing current, partially or fully, from the rotor side. Therefore, the stator-side power factor can also be controlled. Using vector control techniques, the active and reactive powers can be controlled independently, and hence, fast dynamic performance can also be achieved.

The converter used at the grid interface is termed as the line-side converter or the front-end converter (FEC). Unlike the rotor-side converter, this operates at the grid frequency. Flow of active and reactive powers is controlled by adjusting the phase and amplitude of the inverter terminal voltage with respect to the grid voltage. Active power can flow either to the grid or to the rotor circuit depending on the mode of operation. By controlling the flow of active power, the DC bus voltage is regulated within a small band. Control of reactive power enables unity power factor operation at the grid interface. In fact, the FEC can be operated at a leading power factor, if it is so desired. It should be noted that, since the slip range is limited, the DC bus voltage is less in this case when compared with the stator side control. A transformer is therefore necessary to match the voltage levels between the grid and the DC side of the FEC. With a PWM converter in the rotor circuit, the rotor currents can be controlled at the desired phase, frequency, and magnitude. This enables reversible flow of active power in the rotor, and the system can operate in subsynchronous and supersynchronous speeds, both in motoring and generating modes (Fig. 25.55).

25.3.3.1 Battery Charging with Stand-alone Wind Energy System

The basic elements of a stand-alone wind energy conversion system are as follows:

• Wind generator

• Tower

• Charge control system

• Battery storage

• Distribution network

In remote-area power supply, an inverter and a diesel generator are more reliable and sophisticated systems. Most small isolated wind energy systems use batteries as a storage device to level out the mismatch between the availability of the wind and the load requirement. Batteries are a major cost component in an isolated power system.

25.3.3.2 Wind Turbine Charge Controller

The basic block diagram of a stand-alone wind generator and battery charging system is shown in Fig. 25.56.

The function of charge controller is to feed the power from the wind generator to the battery bank in a controlled manner. In the commonly used permanent magnet generators, this is usually done by using the controlled rectifiers [65]. The controller should be designed to limit the maximum current into the battery, reduce charging current for high battery SOC, and maintain a trickle charge during full SOC periods.

25.3.5.1 Soft Starters for Induction Generators

When an induction generator is connected to a load, a large inrush current flows. This is something similar to the direct online starting problem of induction machines. It has been observed that the initial time constants of the induction machines are higher when it tries to stabilize initially at the normal operating conditions. There is a need to use some type of soft-starting equipment to start the large induction generators. A simple scheme to achieve this is shown in Fig. 25.58.

Two thyristors are connected in each phase, back to back. Initially, when the induction generator is connected, the thyristors are used to control the voltage applied to the stator and to limit the large inrush current. As soon as the generator is fully connected, the bypass switch is used to bypass the soft-starter unit.

25.3.6.1 Fixed Speed Wind Turbines

In case of a fixed-speed wind turbine, synchronous or squirrel-cage induction generators are employed and are characterized by the stiff power train dynamics. The rotational speed of the wind turbine generator in this case is fixed by the grid frequency. The generator is locked to the grid, thereby permitting only small deviations of the rotor shaft speed from the nominal value. The speed is very responsive to wind speed fluctuations. The normal method to smooth the surges caused by the wind is to change the turbine aerodynamic characteristics, either passively by stall regulation or actively by blade pitch regulation. The wind turbines often subjected to very low (below cut-in speed) or high wind speed (above rated value). Sometimes, they generate below rated power. No pitch regulation is applied when the wind turbine is operating below rated speed, but pitch control is required when the machine is operating above rated wind speed to minimize the stress. Fig. 25.59 shows the effect of blade pitch angle on the torque-speed curve at a given wind speed.

Blade pitch control is a very effective way of controlling wind turbine speed at high wind speeds, hence limiting the power and torque output of the wind machine. An alternative but cruder control technique is based on airfoil stall [54]. A synchronous link maintaining fixed turbine speed in combination with an appropriate airfoil can be designed so that, at higher than rated wind speeds, the torque reduces due to airfoil stall. This method does not require external intervention or complicated hardware, but it captures less energy and has greater blade fatigue.

The aims of variable-pitch control of medium- and large-scale wind turbines were to help in start-up and shutdown operation, to protect against overspeed and to limit the load on the wind turbine [72]. The turbine is normally operated between a lower and an upper limit of wind speed (typically 4.5–26 m/s). When the wind speed is too low or too high, the wind turbine is stopped to reduce wear and damage. The wind turbine must be capable of being started and run up to speed in a safe and controlled manner. The aerodynamic characteristics of some turbines are such that they are not self-starting. The required starting torque may be provided by motoring or changing the pitch angle of the blade. In case of grid-connected wind turbine system, the rotational speed of the generator is locked to the frequency of the grid. When the generator is directly run by the rotor, the grid acts like an infinite load. When the grid fails, the load rapidly decreases to zero resulting in the turbine rotor to accelerate quickly. Overspeed protection must be provided by rapid braking of the turbine. A simple mechanism of one of blade pitch control techniques is shown in Fig. 25.60.

In this system, the permanent magnet synchronous generator (PMSG) has been used without any gearbox. Direct connection of generator to the wind turbine requires the generator to have a large number of poles. Both induction generators and wound-filed synchronous generators of high pole number require a large diameter for efficient operation. Permanent magnet synchronous generators allow a small pole pitch to be used [73]. The power output, P_mech, of any turbine depends mainly upon the wind speed, which dictates the rotational speed of the wind turbine rotor. Depending upon the wind speed and rotational speed of turbine, tip-speed ratio λ is determined. Based on computed λ, the power coefficient C_p is inferred. In the control strategy above, the torque output, T_actual, of the generator is monitored for a given wind speed and compared with the desired torque, T_actual, depending upon the load requirement. The generator output torque is passed through the measurement filter. The pitch controller then infers the modified pitch angle based on the torque error. This modified pitch angle demand and computed λ decides the new C_p, resulting in the modified wind generator power and torque output. The controller will keep adjusting the blade pitch angle till the desired power and torque output are achieved.

Some of the wind turbine generator includes the gearbox for interfacing the turbine rotor and the generator. The general drivetrain model [72] for such a system is shown in Fig. 25.61. This system also contains the blade pitch angle control provision.

The drivetrain converts the input aerodynamic torque on the rotor into the torque on the low-speed shaft. This torque on the low-speed shaft is converted to high-speed shaft torque using the gearbox and fluid coupling. The speed of the wind turbine here is low, and the gearbox is required to increase the speed so as to drive the generator at rated rpm, for example, 1500 rpm. The fluid coupling works as a velocity-in-torque-out device and transfers the torque [72]. The actuator regulates the tip angle based on the control system applied. The control system here is based on a pitch regulation scheme where the blade pitch angle is adjusted to obtain the desired output power.

25.3.6.2 Variable Speed Wind Turbines

The variable-speed constant-frequency turbine drivetrains are not directly coupled to the grid. The power-conditioning device is used to interface the wind generator to the grid. The output of the wind generator can be variable voltage and variable frequency, which is not suitable for grid integration, and appropriate interfacing is required. The wind turbine rotor in this case is permitted to rotate at any wind speed by power-generating unit.

A number of schemes have been proposed in the past that allow wind turbines to operate with variable rotor speed while feeding the power to a constant-frequency grid. Some of the benefits that have been claimed for variable-speed constant-frequency wind turbine configuration are as follow [69]:

• The variable-speed operation results in increased energy capture by maintaining the blade tip speed to wind-speed ratio near the optimum value.

• By allowing the wind turbine generator to run at variable speed, the torque can be fixed, but the shaft power allowed to increase. This means that the rated power of the machine can be increased with no structural changes.

• A variable-speed turbine is capable of absorbing energy in wind gusts as it speeds up and gives back this energy to the system as it slows down. This reduces turbulence-induced stresses and allows capture of a large percentage of the turbulent energy in the wind.

• More efficient operation can be achieved by avoiding aerodynamic stall over most of operating range.

• Better grid quality is due to the support of grid voltage.

Progress in the power electronic conversion system has given a major boost to implementing the concept of variable-speed operation. The research studies have shown that the most significant potential advancement for wind turbine technology was in the area of power electronic controlled variable-speed operation. There is much research underway in the United States and Europe on developing variable-speed wind turbine as cost-effective as possible. In the United States, the NASA MOD-0 and MOD-5B were operated as variable-speed wind turbines [69]. Companies in the United States and Enercon (Germany) made machines incorporate a variable-speed feature. Enercon variable-speed wind machine is already in operation in Denham, Western Australia.

The ability to operate at varying rotor speed effectively adds compliance to the power train dynamics of the wind turbine. Although many approaches have been suggested for variable-speed wind turbines, they can be grouped into two main classes: (a) discretely variable speed and (b) continuously variable speed [69,74].

25.3.6.3 Discretely Variable Speed Systems

The discretely variable-speed category includes electric system where multiple generators are used, either with different number of poles or with different ratio gearing connected to the wind rotor. It also includes those generators, which can use different number of poles in the stator or can approximate the effect by appropriate switching. Some of the generators in this category are those with consequent poles, dual winding, or pole amplitude modulation. A brief summary of some of these concepts is presented below.

25.3.6.4 Continuously Variable Speed Systems

The second main class of systems for variable-speed operation is those that allow the speed to be varied continuously. For the continuously variable-speed wind turbine, there may be more than one control, depending upon the desired control action [76–80]:

• Mechanical control

• Combination of electric/mechanical control

• Electric control

• Electric/power electronics control

The mechanical methods include hydraulic and variable ratio transmissions. An example of an electric/mechanical system is one in which the stator of the generator is allowed to rotate. All the electric category includes high-slip induction generators and the tandem generator. The power electronic category contains a number of possible options. One option is to use a synchronous generator or a wound rotor induction generator, although a conventional induction generator may also be used. The power electronics is used to condition some or all the power to form an appropriate to the grid. The power electronics may also be used to rectify some or all the power from the generator, to control the rotational speed of the generator or to supply reactive power. These systems are discussed below.

Electrical/power electronics

The general configuration is shown in Fig. 25.62. It consists of the following components:

• Wind generator

• Rectifier

• Inverter

The generator may be DC, synchronous (wound rotor or permanent magnet type), squirrel-cage wound rotor, or brushless doubly fed induction generator. The rectifier is used to convert the variable voltage variable frequency input to a DC voltage. This DC voltage is converted into AC of constant voltage and frequency of desired amplitude. The inverter will also be used to control the active/reactive power flow from the inverter. In case of DC generator, the converter may not be required or when a cycloconverter is used to convert the AC directly from one frequency to another.

25.3.6.5 Types of Generator Options for Variable Speed Wind Turbines Using Power Electronics

Power electronics may be applied to four types of generators to facilitate variable-speed operation:

• Synchronous generators

• Permanent magnet synchronous generators

• Squirrel-cage induction generators

• Wound rotor induction generators

Permanent magnet synchronous generators

The permanent magnet synchronous generator (PMSG) has several significant advantageous properties. The construction is simple and does not required external magnetization, which is important especially in stand-alone wind power applications and also in remote areas where the grid cannot easily supply the reactive power required to magnetize the induction generator. Similar to the previous externally supplied field current synchronous generator, the most common type of power conversion uses a bridge rectifier (controlled/uncontrolled), a DC link, and inverter as shown in Fig. 25.63 [82–84].

Fig. 25.64 shows a wind energy system where a PMSG is connected to a three-phase rectifier followed by a boost converter. In this case, the boost converter controls the electromagnetic torque, and the supply side converter regulates the DC link voltage and controls the input power factor. One drawback of this configuration is the use of diode rectifier that increases the current amplitude and distortion of the PMSG. As a result, this configuration has been considered for small size wind energy conversion systems (smaller than 50 kW).

The advantage of the system in Fig. 25.63 with regardant to the system showed in Fig. 25.64 is it allows the generator to operate near its optimal working point in order to minimize the losses in the generator and power electronic circuit. However, the performance is dependent on the good knowledge of the generator parameter that varies with temperature and frequency. The main drawbacks, in the use of PMSG, are the cost of permanent magnet that increase the price of machine, demagnetization of the permanent magnet material, and it is not possible to control the power factor of the machine.

To extract maximum power at unity power factor from a PMSG and feed this power (also at unity power factor) to the grid, the use of back-to-back connected PWM voltage-source converters are proposed [85]. Moreover, to reduce the overall cost, reduced switch PWM voltage-source converters (four switches) instead of conventional (six-switch) converters for variable-speed drive systems can be used. It is shown that by using both rectifier and inverter current control or flux-based control, it is possible to obtain unity power factor operation both at the WTG and the grid. Other mechanisms can also be included to maximize power extraction from the VSWT (i.e., MPPT techniques) or sensorless approaches to further reduce cost and increase reliability and performance of the systems.

Wound rotor induction generator

A wound rotor induction rotor has three-phase winding on the rotor, accessible to the outside via slip rings. The possibility of accessing the rotor can have the following configurations:

• Slip power recovery

• Use of cycloconverter

• Rotor resistance chopper control

Slip power recovery (static Kramer system)

The slip power recovery configuration behaves similarly to a conventional induction generator with very large slip, but in addition, energy is recovered from the rotor. The rotor power is first carried out through slip rings and then rectified and passed through a DC link to a line-commutated inverter and into the grid. The rest of the power comes directly from the stator as it normally does. A disadvantage with this system is that it can only allow supersynchronous variable-speed operation. Its possible use in the wind power was reported by Smith and Nigim [86].

In this scheme shown in Fig. 25.65, the stator is directly connected to the grid. Power converter has been connected to the rotor of wound rotor induction generator to obtain the optimum power from variable-speed wind turbine. The main advantage of this scheme is that the power-conditioning unit has to handle only a fraction of the total power so as to obtain full control of the generator. This is very important when the wind turbine sizes are increasing for the grid-connected applications for higher penetration of wind energy and the smaller size of converter can be used in this scheme.

Cycloconverter (static Scherbius system)

A cycloconverter is a converter that converts AC voltage of one frequency to another frequency without an intermediate DC link. When a cycloconverter is connected to the rotor circuit, sub- and supersynchronous operation variable-speed operation is possible. In supersynchronous operation, this configuration is similar to the slip power recovery. In addition, energy may be fed into the rotor, thus allowing the machine to generate at subsynchronous speeds. For that reason, the generator is said to be doubly fed [87]. This system has a limited ability to control reactive power at the terminals of the generator, although as a whole it is a net consumer of reactive power. On the other hand, if coupled with capacitor excitation, this capability could be useful from the utility point of view. Because of its ability to rapidly adjust phase angle and magnitude of the terminal voltage, the generator can be resynchronized after a major electric disturbance without going through a complete stop/start sequence. With some wind turbines, this could be a useful feature.

Rotor resistance chopper control

A fairly simple scheme of extracting rotor power as in the form of heat has been proposed in [48].

25.3.6.6 Isolated Grid Supply System with Multiple Wind Turbines

The isolated grid supply system with a wind park is shown in Fig. 25.66. Two or more wind turbines can be connected to this system. A diesel generator can be connected in parallel. The converters, connected with wind generators, will work in parallel, and the supervisory control block will control the output of these wind generators in conjunction with the diesel generator. This type of decentralized generation can be a better option where high penetration of wind generation is sought. The individual converter will control the voltage and frequency of the system. The supervisory control system will play an important part in coordination between multiple power generation systems in a remote-area power supply having weak grid.

25.3.6.7 Power Electronics Technology Development

To meet the needs of future power generation systems, power electronics technology will need to evolve on all levels, from devices to systems. The development needs are as follows:

• There is a need for modular power converters with plug-and-play controls. This is particularly important for high-power utility systems, such as wind power. The power electronic equipment used today is based on industrial motor drive technology. Having dedicated, high power density, modular systems will provide flexibility and efficiency in dealing with different energy sources and large variation of generation systems architectures.

• There is a need for new packaging and cooling technologies, and integration with PV and fuel cell will have to be addressed. The thermal issues in integrated systems are complex, and new technologies such as direct fluid cooling or microchannel cooling may find application in future systems. There is large potential for advancement in this area.

• There is a need for new switching devices with higher temperature capability, higher switching speed, and higher current density/voltage capability. The growth in alternative energy markets will provide a stronger pull for further development of these technologies.