Types of Wind Turbines

28.3.1.1 Types of Wind Turbines

There are two types of wind turbines available Fig. 28.55: rated power

aerodynamic power

• Horizontal axis wind turbines (HAWTs).

wind turbine power

• Vertical axis wind turbines (VAWTs). Vertical axis wind turbines (VAWTs) have an axis of rota-

tion 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 (with-

out a special yaw mechanism). Vertical axis wind turbines were FIGURE 28.52 Power curve of wind turbine as a function of wind

cut-in

cut-out wind speed

also used in some applications as they have the advantage that

752 C. V. Nayar et al.

Rotor diameter

Rotor blade

Rotor

Gearbox Generator

height Wind direction

Nacelle

tower

Wind direction

for an upwind

of a downwind

rotor

rotor

Fixed pitch

Hub

rotor blade

height

Equator height

Tower Rotor base Generator

Gearbox

Horizontal-axis wind turbine

Vertical-axis wind turbine

(HAWT)

(VAWT)

FIGURE 28.55 Typical diagram of HAWTs and VAWTs.

to extract power relatively easier. But there are some disadvan- regulation involves yawing blades so that they no longer point tages such as no self starting system, smaller power coefficient into the wind. One such system designed in Western Australia than obtained in the horizontal axis wind turbines, strong dis- has a tail that progressively tilts the blades in a vertical plane continuation of rotations due to periodic changes in the lift so that they present a small surface to the wind at high speeds. force, and the regulation of power is not yet satisfactory.

The active power of a wind turbine can be regulated by The horizontal axis wind turbines are generally used. Hori- either designing the blades to go into an aerodynamic stall zontal axis wind turbines are, by far, the most common design. beyond the designated wind speed, or by feathering the blades There are a large number of designs commercially available out of the wind, which results in reducing excess power using a ranging from 50 W to 4.5 MW. The number of blades ranges mechanical and electrical mechanism. Recently, an active stall from one to many in the familiar agriculture windmill. The has been used to improve the stability of wind farms. This stall best compromise for electricity generation, where high rota- mechanism can prevent power deviation from gusty winds to tional speed allows use of a smaller and cheaper electric pass through the drive train [52]. generator, is two or three blades. The mechanical and aero-

Horizontal axis wind turbines can be further classified into dynamic balance is better for three bladed rotor. In small wind fixed speed (FS) or variable speed (VS). The FS wind turbine turbines, three blades are common. Multiblade wind turbines generator (FSWT) is designed to operate at maximum effi- are used for water pumping on farms.

ciency while operating at a rated wind speed. In this case, the Based on the pitch control mechanisms, the wind turbines optimum tip-speed ratio is obtained for the rotor airfoil at a can also be classified as:

rated wind speed. For a VS wind turbine generator (VSWT), it is possible to obtain optimum wind speed at different wind

• Fixed pitch wind turbines. speeds. Hence this enables the VS wind turbine to increase • Variable pitch wind turbines. its energy capture. The general advantages of a VSWT are

Different manufacturers offer fixed pitch and variable pitch summarized as follows: blades. Variable pitch is desirable on large machines because the aerodynamic loads on the blades can be reduced and when

• VSWTs are more efficient than the FSWTs. used in fixed speed operation they can extract more energy.

• At low wind speeds the wind turbines can still capture But necessary mechanisms require maintenance and for small

the maximum available power at the rotor, hence increas- machines, installed in remote areas, fixed pitch seems more

ing the possibility of providing the rated power for wide desirable and economical. In some machines, power output

speed range.

28 Power Electronics for Renewable Energy Sources 753

synchronous generators are interfaced via power converters Schemes based on permanent magnet synchronous genera- to the grid. This also allows the synchronous generators to tors (PMSG) and induction generators are receiving close operate wind turbines in VS, which makes gear-less operation attention in wind power applications because of their qual- of the VSWT possible. ities such as ruggedness, low cost, manufacturing simplicity,

28.3.1.2 Types of Wind Generators

The squirrel-cage induction generators are widely used with and low maintenance requirements. Despite many positive the fixed-speed wind turbines. In some applications, wound features over the conventional synchronous generators, the rotor induction generators have also been used with adequate PMSG was not being used widely [23]. However, with the control scheme for regulating speed by external rotor resis- recent advent in power electronics, it is now possible to tance. This allows the shape of the torque-slip curve to be control the variable voltage, variable frequency output of controlled to improve the dynamics of the drive train. In PMSG. The permanent magnet machine is generally favored case of PMSG, the converter/inverter can be used to con- for developing new designs, because of higher efficiency and trol the variable voltage, variable frequency signal of the the possibility of a rather smaller diameter. These PMSG wind generator at varying wind speed. The converter con- machines are now being used with variable-speed wind verts this varying signal to the DC signal and the output of machines.

converter is converted to AC signal of desired amplitude and In large power system networks, synchronous generators frequency. are generally used with fixed-speed wind turbines. The syn-

The induction generators are not locked to the frequency of chronous generators can supply the active and reactive power the network. The cyclic torque fluctuations at the wind turbine both, and their reactive power flow can be controlled. The can be absorbed by very small change in the slip speed. In synchronous generators can operate at any power factor. For case of the capacitor excited induction generators, they obtain the induction generator, driven by a wind turbine, it is a the magnetizing current from capacitors connected across its well-known fact that it can deliver only active power, while output terminals [51, 53, 54]. consuming reactive power

To take advantage of VSWTs, it is necessary to decouple Synchronous generators with high power rating are signif- the rotor speed and the grid frequency. There are different icantly more expensive than induction generators of similar approaches to operate the VSWT within a certain operational size. Moreover, direct connected synchronous generators have range (cut-in and cut-out wind speed). One of the approaches the limitation of rotational speed being fixed by the grid fre- is dynamic slip control, where the slip is allowed to vary quency. Hence, fluctuation in the rotor speed due to wind gusts upto 10% [55]. In these cases, doubly-fed induction genera- lead to higher torque in high power output fluctuations and tors (DFIG) are used (Fig. 28.56). One limitation is that DFIG the derived train. Therefore in grid-connected application,

require reactive power to operate. As it is not desired that

i g1 i g1 i g2 i g2

Grid i g3 i g3

Speed Sensor

Induction Generator

Rotor Side

Front End

Digital Controller

FIGURE 28.56 Variable speed doubly-fed induction generator (VSDFIG) system.

754 C. V. Nayar et al. the grid supply this reactive power, these generators are usu- (P ROTOR ). The possibility of accessing the rotor in a doubly-fed

ally equipped with capacitors. A gear box forms an essential induction generator makes a number of configurations possi- component of the wind turbine generator (WTG) using ble. These include slip power recovery using a cycloconverter, induction generators. This results in the following limitations: which converts the ac voltage of one frequency to another with-

out an intermediate DC link [56–58], or back-to-back inverter • Frequent maintenance.

configurations [59, 60].

• Additional cost. Using voltage-source inverters (VSIs) in the rotor circuit, the • Additional losses. rotor currents can be controlled at the desired phase, frequency,

With the emergence of large wind power generation, and magnitude. This enables reversible flow of active power in increased attention is being directed towards wound rotor the rotor and the system can operate in sub-synchronous and induction generators (WRIG) controlled from the rotor side super-synchronous speeds, both in motoring and generating for variable speed constant frequency (VSCF) applications. A modes. The DC link capacitor acts as a source of reactive power wound rotor induction generator has a rotor containing a and it is possible to supply the magnetizing current, partially 3-phase winding. These windings are made accessible to the or fully, from the rotor side. Therefore, the stator side power outside via slip rings. The main advantages of a wound rotor factor can also be controlled. Using vector control techniques, induction generator for VSCF applications are:

the active and reactive powers can be controlled independently and hence fast dynamic performance can also be achieved.

• Easier generator torque control using rotor current The converter used at the grid interface is termed as the control. line-side converter or the front end converter (FEC). Unlike the • Smaller generator capacity as the generated power can be rotor side converter, this operates at the grid frequency. Flow of accessed from the stator as well as from the rotor. Usually active and reactive powers is controlled by adjusting the phase

the rotor power is proportional to the slip speed (shaft and amplitude of the inverter terminal voltage with respect speed–synchronous speed). Consequently smaller rotor to the grid voltage. Active power can flow either to the grid power converters are required. The frequency converter or to the rotor circuit depending on the mode of operation. in the rotor (inverter) directly controls the current in the By controlling the flow of active power, the DC bus voltage rotor winding, which enables the control of the whole is regulated within a small band. Control of reactive power generator output. The power electronic converters gener- enables unity power factor operation at the grid interface. In ally used are rated at 20–30% of the nominal generator fact, the FEC can be operated at a leading power factor, if it power. is so desired. It should be noted that, since the slip range is • Fewer harmonics exist because control is in the rotor limited, the DC bus voltage is less in this case when compared while the stator is directly connected to the grid. to the stator side control. A transformer is therefore necessary

If the rotor is short-circuited (making it the equivalent to match the voltage levels between the grid and the DC side of a cage rotor induction machine), the speed is primarily of the FEC. With a PWM converter in the rotor circuit, the determined by the supply frequency and the nominal slip is rotor currents can be controlled at the desired phase, frequency, within 5%. The mechanical power input (P TURBINE ) is con- and magnitude. This enables reversible flow of active power in verted into stator electrical power output (P STATOR ) and is fed the rotor and the system can operate in sub-synchronous and to the AC supply. The rotor power loss, being proportional super-synchronous speeds, both in motoring and generating to the slip speed, is commonly referred to as the slip power modes (Fig. 28.57).

P STATOR

P ROTOR

P STATOR

P ROTOR

P TURBINE

P TURBINE

(a)

(b)

FIGURE 28.57 Doubly-fed induction generator power flow in generating mode: (a) sub-synchronous and (b) super-synchronous.

28 Power Electronics for Renewable Energy Sources 755

28.3.2 Types of Wind Power Systems

the battery, reduce charging current for high battery SOC, and maintain a trickle charge during full SOC periods.

Wind power systems can be classified as: • Stand-alone.

28.3.4 Wind–diesel Hybrid Systems

• Hybrid. • Grid-connected.

The details of hybrid systems are already covered in Section 28.2.4. Diesel systems without batteries in remote area are characterized by poor efficiency, high maintenance, and

fuel costs. The diesel generators must be operated above a cer- Stand-alone wind power systems are being used for the tain minimum load level to reduce cylinder wear and tear due following purposes in remote area power systems:

28.3.3 Stand-alone Wind Power Systems

to incomplete combustion. It is a common practice to install dump loads to dissipate extra energy. More efficient systems

• Battery charging. can be devised by combining the diesel generator with a battery • Household power supply.

inverter subsystem and incorporating RES, such as wind/solar where appropriate. An integrated hybrid energy system incor- porating a diesel generator, wind generator, battery or flywheel

28.3.3.1 Battery Charging with Stand-alone Wind

storage, and inverter will be cost effective at many sites with an

average daily energy demand exceeding 25 kWh [62]. These The basic elements of a stand-alone wind energy conversion hybrid energy systems can serve as a mini grid as a part system are:

Energy System

of distributed generation rather than extending the grid to the remote rural areas. The heart of the hybrid system is a

• Wind generator. high quality sine-wave inverter, which can also be operated • Tower.

in reverse as battery charger. The system can cope with loads • Charge control system.

ranging from zero (inverter only operation) to approximately • Battery storage.

three times greater capacity (inverter and diesel operating in • Distribution network.

parallel).

In remote area power supply, an inverter and a diesel gen- Decentralized form of generation can be beneficial in remote erator are more reliable and sophisticated systems. Most small area power supply. Due to high cost of PV systems, prob-

isolated wind energy systems use batteries as a storage device to lems associated with storing electricity over longer periods level out the mismatch between the availability of the wind and (like maintenance difficulties and costs), wind turbines can the load requirement. Batteries are a major cost component in

be a viable alternative in hybrid systems. Systems with battery an isolated power system.

storage although provide better reliability. Wind power pene- tration can be high enough to make a significant impact on the operation of diesel generators.

High wind penetration also poses significant technical prob- The basic block diagram of a stand-alone wind generator and lems for the system designer in terms of control and transient battery charging system is shown in Fig. 28.58.

28.3.3.2 Wind Turbine Charge Controller

stability [30]. In earlier stages, wind diesel systems were The function of charge controller is to feed the power from installed without assessing the system behavior due to lack the wind generator to the battery bank in a controlled manner. of design tools/software. With the continual research in this In the commonly used permanent magnet generators, this is area, there are now software available to assist in this process. usually done by using the controlled rectifiers [61]. The con- Wind diesel technology has now matured due to research and troller should be designed to limit the maximum current into development in this area. Now there is a need to utilize this

DC load

Battery Permanent Magnet or Capacitor Excited

Wind turbine

FIGURE 28.58 Block diagram for a stand-alone wind generator and battery charging system.

756 C. V. Nayar et al. knowledge into cost effective and reliable hybrid systems [63].

In Western Australia, dynamic modeling of wind diesel hybrid system has been developed in Curtin/MUERI, supported by the Australian Cooperative Research Centre for Renewable Energy (ACRE) program 5.21.