**2. Small-scale wind turbine system**

A small wind turbine generally consists of the following components: A rotor with a varia‐ ble number of blades for convert the power from wind to mechanical power, an electric gen‐ erator, control and protection mechanisms, and power electronic components for feeding electricity into a battery bank, the public grid or, occasionally, into a direct application such as a water-pump[1,13].

The generator is the main part of a small wind turbine. The generator converts the mechani‐ cal power into electrical power. The two common types of electrical machines used in small scale wind turbines are self excited induction generators (SEIG) and permanent magnet syn‐ chronous generators (PMSG). In these cases, the common way to convert the low-speed me‐ chanical power to electrical power is a utilizing a gearbox and a SEIG with standard speed. The gearbox adapts the low speed of the turbine rotor to the high speed of generators, though the gearbox may not be necessary for multiple-pole generator systems. In the selfexcited induction generators, the reactive power necessary to energize the magnetic circuits must be supplied from parallel capacitors bank at the machine terminal. In this case, the ter‐ minal voltage or reactive power may not be directly controlled, and the induction genera‐ tors may suffer from voltage instability problem. There is considerable interest in the application of the multiple-pole Permanent Magnet Synchronous Generators (PMSG) driven by a wind-turbine shaft without gearbox [1]. As described above, the electric generators of modern small wind turbines are generally designed to use permanent magnets and a direct coupling between rotor and generator. The following common topologies can be encoun‐ tered:

**1.** Axial flow air-cored generators

small-scale wind turbines enabling them to operate with optimum speed to extract maxi‐

For rural and remote areas, the small-scale stand-alone wind power system with a battery bank as the energy storage component is common and essential for providing stable and re‐ liable electricity [2,7-10]. For the stand-alone wind power system, the load is a battery that can be considered as an energy sink with almost constant voltage. The battery can absorb any level of power as long as the charging current does not exceed its limitation. Since the voltage remains almost constant, but the current flows through it can be varied, the battery

There is increasing market for a grid connected small wind generating system (without bat‐ tery storage) for home owners and small businesses in rural areas. In this case the excess en‐ ergy form the wind generator is fed to the utility grid. The AC grid can also be a diesel grid or a battery/diesel mini hybrid grid. A grid connected inverter structure which extracts ener‐ gy even at low wind speeds will assist in reducing capital cost and offer opportunities for interfacing small-scale wind generators with the AC grid. Conventional grid connected wind turbines use a charge controller to charge the batteries and a grid connected inverter to

This chapter presents a power electronic energy conversion system for small-scale standalone wind power system with a battery bank as the energy storage component and grid connected power electronic interface for interfacing variable speed small-scale wind genera‐ tors to a grid. Small-scale wind turbine consist of permanent magnet synchronous generator (PMSG), AC/DC converter, DC/DC converter as the maximum power point tracking control‐

A small wind turbine generally consists of the following components: A rotor with a varia‐ ble number of blades for convert the power from wind to mechanical power, an electric gen‐ erator, control and protection mechanisms, and power electronic components for feeding electricity into a battery bank, the public grid or, occasionally, into a direct application such

The generator is the main part of a small wind turbine. The generator converts the mechani‐ cal power into electrical power. The two common types of electrical machines used in small scale wind turbines are self excited induction generators (SEIG) and permanent magnet syn‐ chronous generators (PMSG). In these cases, the common way to convert the low-speed me‐ chanical power to electrical power is a utilizing a gearbox and a SEIG with standard speed. The gearbox adapts the low speed of the turbine rotor to the high speed of generators, though the gearbox may not be necessary for multiple-pole generator systems. In the selfexcited induction generators, the reactive power necessary to energize the magnetic circuits must be supplied from parallel capacitors bank at the machine terminal. In this case, the ter‐

can be also considered as a load with a various resistance [2,11,12].

process power from the battery to the utility grid [4].

**2. Small-scale wind turbine system**

mum power from wind [1,2,4].

306 Advances in Wind Power

ler, inverter and load.

as a water-pump[1,13].


In the topologies above the type of flow refers to the direction of the magnetic flow lines crossing the magnetic gap between the poles with respect to the rotating shaft of the genera‐ tor [13].

It is important to be able to control and limit the converted mechanical power during higher wind speeds. The power limitation during higher wind speeds in small scale wind turbines may be done by furling control or soft-stall control [1,14]. Furling is a passive mechanism used to limit the rotational frequency and the output power of small-scale wind turbine in strong winds. While other mechanisms, such as passive blade pitching or all-electronic con‐ trol based on load-induced stall can occasionally be encountered, furling is the most fre‐ quently used mechanism [13]. Many small wind turbines use an upwind rotor configuration with a tail vane for passive yaw control. Typically, the tail vane is hinged, allowing the rotor to furl (turn) in high winds, providing both power regulation and over-speed protection. At higher wind speeds, the generated power of the wind turbine can go above the limit of the generator or the wind turbine design. When this occurs, small wind turbines use mechanical control or furling to turn the rotor out of the wind resulting in shedding the aerodynamic power or a steep drop in the power curve [1,13-16]. The basic operating principle of furling system is shown in Figure 1.

Often, small turbine rotors furl abruptly at a wind speed only slightly above their rated wind speed, resulting in a very "peaky" power curve and poor energy capture at higher wind speeds. This energy loss is compounded by the furling hysteresis, in which the wind speed must drop considerably below the rated wind speed before the rotor will unfurl and resume efficient operation. One way to improve the performance of furling wind turbines is to design the rotor to furl progressively, causing the power output to remain at or near rated power as the wind speed increases beyond it's rated value. This approach has two draw‐ backs: wind turbine rotors operating at high furl angles tend to be very noisy and experi‐ ence high flap loads. Note that manufactured wind turbines use a damper to reduce the furling loop hysteresis. Damping is necessary to keep the wind turbine from cycling or chat‐ tering in and out of furling. The damping plus the gyroscopic effect of turning wind turbine blades add to the unproductive time of entering and leaving the furling condition creating a hysteresis during transition. All of these delays reduce the wind turbine energy production [1,14-16].

**Figure 1.** Overview of the operating principles of a furling system. (a) Aerodynamic forces. (b) Furling movement in strong winds. (c) Restitution of normal (aligned) operation upon reduction of the wind speed [13].

The soft-stall concept is to control the generator rotations per minute (rpm) and achieve op‐ timum operation over a wide range of rotor rpm. In order to control the generator rpm, the soft-stall concept regulates the stall mode of the wind turbine, thus furling can be delayed in normal operation. Furling is still used in the soft-stall concept during very high winds and emergency conditions. Potential advantages of soft-stall control are listed as follows:


The only difference between furling and soft-stall control is the addition of the DC-DC con‐ verter that allows the power to be controlled. With the DC-DC Converter between the recti‐ fier and load, the transmitted power to the load can be controlled according to prescribed power/rpm schedule.

A variable speed wind turbine configuration with power electronics conversion corresponds to the full variable speed controlled wind turbine, with the generator connected to the load or to the grid through a power converter as shown in Figure 2.

**Figure 2.** AC/DC/AC power electronic interface for a wind generator.

blades add to the unproductive time of entering and leaving the furling condition creating a hysteresis during transition. All of these delays reduce the wind turbine energy production

**Figure 1.** Overview of the operating principles of a furling system. (a) Aerodynamic forces. (b) Furling movement in

The soft-stall concept is to control the generator rotations per minute (rpm) and achieve op‐ timum operation over a wide range of rotor rpm. In order to control the generator rpm, the soft-stall concept regulates the stall mode of the wind turbine, thus furling can be delayed in normal operation. Furling is still used in the soft-stall concept during very high winds and

emergency conditions. Potential advantages of soft-stall control are listed as follows:

**•** Controls the wind turbine rotational speed to achieve the maximum power coefficient

**•** Operates the wind turbine at a low tip-speed ratio during high wind speeds to reduce

The only difference between furling and soft-stall control is the addition of the DC-DC con‐ verter that allows the power to be controlled. With the DC-DC Converter between the recti‐ fier and load, the transmitted power to the load can be controlled according to prescribed

A variable speed wind turbine configuration with power electronics conversion corresponds to the full variable speed controlled wind turbine, with the generator connected to the load

strong winds. (c) Restitution of normal (aligned) operation upon reduction of the wind speed [13].

**•** Delays furling as long as possible, which increases energy production

or to the grid through a power converter as shown in Figure 2.

noise and thrust loads [1,14-16].

power/rpm schedule.

[1,14-16].

308 Advances in Wind Power

The grid-connected inverters will inject the active power to the grid with minimum total harmonic distortion (THD) of output current and voltage. The grid voltage and inverter out‐ put voltage will be synchronized by zero-crossing circuit. The generator can be self-excited asynchronous generator (SEIG), or permanent magnet synchronous generator (PMSG). The stator windings are connected to the load or to the grid through a full-scale power convert‐ er. Some variable speed WTSs are gearless. In these cases, a direct driven multi-pole genera‐ tor is used.
