**1. Introduction**

In response to the introduction of feed-in tariffs around the world, an increasing number of small wind turbines are being grid connected [1]. Variable speed wind turbines, though ini‐ tially more costly, have several advantages over fixed speed systems: (i) average power pro‐ duction is typically 10% higher since the turbine operates more frequently near its ideal tipspeed-ratio, (ii) the turbine and mechanical transmission operate with reduced stresses, (iii) turbine and generator torque pulsations are reduced, and (iv) noise is reduced [2-4]. Varia‐ ble speed operation in general has only become possible over the last twenty years or so be‐ cause of major developments in power electronics and associated cost reductions [5]. This reference indicates that power electronic devices are reducing in cost at about 1-5% per year. This chapter will discuss the impact of these developments on small turbine design and op‐ eration along with some important aerodynamics issues related to turbine starting.

The blades of most small horizontal-axis wind turbines (HAWTs) have no pitch adjustment. This reduces their cost but makes starting and low wind speed performance a major chal‐ lenge. In order to extract the maximum possible energy available from the wind, power ex‐ traction should begin at the smallest possible wind speed in the shortest possible time. Few researchers have examined wind turbine starting. Ebert and Wood [6] and Mayer et al. [7] measured starting sequences from separate 5 kW HAWTs. Kjellin and Bernhoff [8] devel‐ oped a scheme for starting a small vertical axis wind turbine (VAWT) using auxiliary gener‐ ator windings that are not used for power production. Hill et al. [9] discuss the aerodynamic starting of Darrieus VAWTs which have problems similar to, or worse than, those for HAWTs. The remainder of this chapter considers only HAWTs with fixed-pitch blades con‐ figured with an AC generator and power converter that delivers fixed frequency electrical power while allowing the turbine to operate at variable rotational speeds.

© 2012 Aner et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Aner et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The lack of pitch adjustment in small wind turbines (considered here to have a rated power output of 50 kW or less) means the blades experience high angles of attack during starting and, hence, low lift to drag ratios. Moreover, most small wind turbines employ permanent magnet synchronous generators (PMSGs) whose cogging torque1 can have a significant im‐ pact on starting. Wood [13] showed that the cogging torque must be less than 1% of the rat‐ ed torque to be unimportant. Partly because of high cogging torque, the blades of the 500 W HAWT studied by Wright & Wood [11] did not start for a wind speed below 5 m/s approxi‐ mately, whereas the conventionally-measured cut-in wind speed was 3.5 m/s. The difference was due to the blades' ability to keep rotating as the wind speed decreased below 5 m/s. In other words, the "stopping" wind speed of a HAWT is significantly lower than the aerody‐ namic "starting" speed and the cut-in speed is some average of the two. It is also noted that 5 m/s is a typical average wind speed for locations where a small turbine is likely to be em‐ ployed. Wood [10] described the multi-dimensional optimization of blade design including the minimization of starting time. Subsequent tests on 2.5 m long 5 kW blades designed for good aerodynamic starting are described below. From rest, they take about 13 s to reach the power-extracting angular velocity at a wind speed of 10 m/s and about 40 s at 3 m/s. These times can result in a significant loss of output energy compared to operation with much fast‐ er starting times.

As modern small wind turbine systems evolve, several issues are emerging that may be effectively and economically dealt with by the use of modern power electronics. While many small turbines operate with variable rotor speed, it is uncommon for their power electronics to allow power flow to and from the grid. In fact, many small turbines are sold with only a diode rectifier or battery charger. The diode rectifier configuration (either a four diode full-bridge for a single-phase generator or a six diode full-bridge for a threephase generator) is often used because of its simplicity, reliability and economy. A bat‐ tery charger may be added, in the form of a single-transistor based buck converter or boost converter. Both the diode rectifier and the battery charger allow power flow in one direc‐ tion only (from generator to a DC capacitor or battery), which to date has been satisfacto‐ ry, but as suggested below, bi-directional converters may be used in the near future to extract more energy from small wind turbines.

With the conventional diode rectifier based system, small wind turbines normally require a third party inverter for grid connection. This inverter often has to provide the maximum power point tracking (MPPT) for optimal power extraction. In turn, many such inverters are based on photovoltaic inverters for which the "perturb and observe" (P & O) strategy for MPPT is effective [12]. P & O does not work well for wind turbines because of the (usually) rapidly changing wind speed and most sophisticated small turbines with integral inverters base their MPPT on some form of look table for maximum power as a function of generator frequency. Thus the basic operation of small wind turbine inverters needs to be fundamen‐ tally different from those used for photovoltaics.

<sup>1</sup> By "cogging torque" we mean the maximum value of the phase-dependent cogging torque. The value is independent of the direction of rotation.

Another particular issue is the need to have high rectifier efficiency and high inverter effi‐ ciency over a very wide range of power levels. In principle, this is also a requirement for large turbines, but the lower average wind speeds seen by small turbines makes the low power performance especially important. Many commercial small inverters now have effi‐ ciencies of over 95% at rated power but drop off alarmingly at lower power levels due to losses in filter, transformer, and controller components (such losses are much more signifi‐ cant in small power converters than in high power converters). The development of modern semiconductor switching devices and advances in high frequency switching control contin‐ ue to be crucial to the extension of high efficiency operation in low wind speed (i.e. low power) regimes.

The lack of pitch adjustment in small wind turbines (considered here to have a rated power output of 50 kW or less) means the blades experience high angles of attack during starting and, hence, low lift to drag ratios. Moreover, most small wind turbines employ permanent magnet synchronous generators (PMSGs) whose cogging torque1 can have a significant im‐ pact on starting. Wood [13] showed that the cogging torque must be less than 1% of the rat‐ ed torque to be unimportant. Partly because of high cogging torque, the blades of the 500 W HAWT studied by Wright & Wood [11] did not start for a wind speed below 5 m/s approxi‐ mately, whereas the conventionally-measured cut-in wind speed was 3.5 m/s. The difference was due to the blades' ability to keep rotating as the wind speed decreased below 5 m/s. In other words, the "stopping" wind speed of a HAWT is significantly lower than the aerody‐ namic "starting" speed and the cut-in speed is some average of the two. It is also noted that 5 m/s is a typical average wind speed for locations where a small turbine is likely to be em‐ ployed. Wood [10] described the multi-dimensional optimization of blade design including the minimization of starting time. Subsequent tests on 2.5 m long 5 kW blades designed for good aerodynamic starting are described below. From rest, they take about 13 s to reach the power-extracting angular velocity at a wind speed of 10 m/s and about 40 s at 3 m/s. These times can result in a significant loss of output energy compared to operation with much fast‐

As modern small wind turbine systems evolve, several issues are emerging that may be effectively and economically dealt with by the use of modern power electronics. While many small turbines operate with variable rotor speed, it is uncommon for their power electronics to allow power flow to and from the grid. In fact, many small turbines are sold with only a diode rectifier or battery charger. The diode rectifier configuration (either a four diode full-bridge for a single-phase generator or a six diode full-bridge for a threephase generator) is often used because of its simplicity, reliability and economy. A bat‐ tery charger may be added, in the form of a single-transistor based buck converter or boost converter. Both the diode rectifier and the battery charger allow power flow in one direc‐ tion only (from generator to a DC capacitor or battery), which to date has been satisfacto‐ ry, but as suggested below, bi-directional converters may be used in the near future to

With the conventional diode rectifier based system, small wind turbines normally require a third party inverter for grid connection. This inverter often has to provide the maximum power point tracking (MPPT) for optimal power extraction. In turn, many such inverters are based on photovoltaic inverters for which the "perturb and observe" (P & O) strategy for MPPT is effective [12]. P & O does not work well for wind turbines because of the (usually) rapidly changing wind speed and most sophisticated small turbines with integral inverters base their MPPT on some form of look table for maximum power as a function of generator frequency. Thus the basic operation of small wind turbine inverters needs to be fundamen‐

1 By "cogging torque" we mean the maximum value of the phase-dependent cogging torque. The value is independent

er starting times.

154 Advances in Wind Power

extract more energy from small wind turbines.

tally different from those used for photovoltaics.

of the direction of rotation.

Another power electronics issue of rapidly increasing importance is that of AC power quali‐ ty. At very small power levels (below 500W), where economy is a driving market require‐ ment, the inverter operates with a simple controller that produces a modified square-wave AC output voltage (i.e. a square wave modified by the addition a zero voltage step between negative and positive voltage levels). Such inverter operation is reliable, with low switching losses (since the transistors are switched at the power frequency of 50Hz or 60Hz) and with‐ out any need for filter components. The total harmonic distortion (THD) is a measure of de‐ viation from a sinusoidal waveform; for a typical modified square wave the THD is about 30%. However with increasing use of renewable energy sources, it is very possible that in the near future national electronic equipment standards will require manufacturers of small inverters to produce AC voltage (or AC current in the case of grid-connection) having a THD on the order of 5% or even lower. Since about 1990, inverters with a power output above 2kW have increasingly been manufactured with pulse width modulation (PWM) that rapidly (in the range of 1kHz to 10kHz) switches transistors such that the transistor bridge output can be filtered with low cost inductor and/or capacitor components to produce AC output current with low THD for grid connection. It is expected that PWM will become more commonly used even at the very low power levels of a few hundred Watts.

One problem with PWM operation is the issue of increased cost and reduced reliability re‐ sulting from the use of the required filter components (without filtering the THD would be well above 100%). A trend that is in its infancy for high power inverters may one day be applied to lower power inverters is the multilevel inverter, which produces a stair-stepped approximation of a sinusoidal voltage or current and requires little filtering (or even no filter if PWM is not used at all and there are a sufficient number of levels). A significant disad‐ vantage of the multilevel inverter is an increase in the number semiconductor power transis‐ tors, and hence decreased reliability and increased cost. However with rapidly evolving power semiconductor switching technology, multilevel inverters may one day become eco‐ nomical even at lower power levels.

In this chapter, the focus is on the issue of bi-directional power flow in the power converter of a small wind turbine system. Bi-directionality of power flow is an example of what is now available (and quickly becoming more economical) using modern power electronics. This chapter presents one such system and explores its use in motor-starting a small wind tur‐ bine to reduce the starting time. The ideal case of turbine starting after the wind speed makes a step change from zero to a constant value, *U*, within the turbine operating range is considered, and any issues regarding the measuring of *U* is ignored. It is also assumed that power extraction takes over instantaneously from starting at a nominated rotor angular ve‐ locity. The justification is that if motor-starting for this case results in an energy gain, it is worth pursuing. If it does not, then it is to be discarded. We show that motor-starting is worth pursuing

The following sections present the electrical system in which motoring and generating modes are controlled by field oriented control (FOC) through a bi-directional very sparse matrix converter (VSMC) which is connected in a backward configuration (i.e. the "rectifier" portion of the VSMC is connected to the grid and the "inverter" portion is connected to the generator). This allows a sufficient DC-link voltage for grid connection at any wind speed while the system is in the generating mode, and also permits straightforward grid synchro‐ nization where power is regulated by current control, depending on grid voltage to avoid voltage flicker. In addition, by using a small filter, sinusoidal current can be injected into the grid at unity power factor (or slightly leading or lagging power factor as desired) to improve power quality [13]. An alternative to the VSMC is the well known back-to-back converter (where the rectifier is implemented with a conventional inverter structure). However, the back-to-back converter requires some form of DC-link storage such as a capacitor, which is not the case for the a VSMC.

One particular recent power electronics development of interest for matrix converters is the emergence of the Reverse Blocking IGBT (RB-IGBT) which has become commercially availa‐ ble and tested for matrix converter implementations. It has been shown [14] that the conduc‐ tion losses, switching losses, and conduction voltage of RB-IGBTs are lower than the current generation IGBTs which should lead to higher converter efficiencies. Another recent power electronics development is the commercialization of the Bi-directional Reverse Blocking IGBT (BRB-IGBT) for use in high power converters which may one day be used in lower power converters. A prime candidate for such switching technology is the matrix converter and its derivatives. Matrix converters (having the ability for extremely flexible bi-directional control of power in three-phase systems) in their conventional (non-sparse) form have long been criticized because of the high cost and high power losses associated with a high transis‐ tor and diode count (e.g. 18 transistors and 18 diodes in one particular form). The use of bidirectional reverse blocking devices would remove the need for external reverse voltage blocking diodes, and reduce the control requirements of a full matrix converter (only nine devices to be controlled), as well as reduce power losses in the converter. Thus, BRB-IGBT devices may lead to economical matrix-based or other converter configurations with lower power losses and simplified gate drive requirements [15, 16]. The study discussed below, uses an established implementation of bi-directional power flow in a power switch, namely, a conventional IGBT transistor within a four-diode bridge as shown in Fig.1 (the diodes pro‐ vide both reverse voltage blocking capability and bi-directional power flow control). This configuration has the advantage of a simplified switch drive circuit (only one IGBT needs to be controlled regardless of power flow) but the disadvantage of increased conduction losses in two conducting diodes. An alternative bi-directional power switch configuration consists of two anti-parallel IGBT (or other transistor) devices, each with a reverse blocking diode, which would reduce the losses (only one conducting diode) but require a more complicated transistor driver (two transistors need to be controlled for each bi-directional switch). Figure 2 shows the configuration of VSMC built with BRB-IGBT switching devices.

Section 3 describes a 5 kW wind turbine and its starting performance. An expression for the energy gain (i.e. harvested and delivered to the grid) is derived using the proposed motor‐ ing approach verses aerodynamic starting. Section 4 presents the model and operation of the PMSG and the VSMC in the backward configuration, including a space vector modulation approach. Then in Section 5, the simulation results for a 5.6 kW wind turbine are presented, including a plot of energy gain as a function of wind speed which compares well with the analytical result presented in Section 4. Conclusions are presented in Section 6.
