AckermannTh. (ed) Wind Power in Power Systems

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Generators and Power

Electronics for Wind Turbines

Anca D. Hansen

4.1 Introduction

Today, the wind turbines on the market mix and match a variety of innovative concepts

with proven technologies both for generators and for power electronics. This chapter

presents from an electrical point of view the current status of generators and power

electronics in wind turbine concepts. It describes classical and new concepts of gen-

erators and power electronics based on technical aspects and market trends.

4.2 State-of-the-art Technologies

This section will describe the current status regarding generators and power electronics

for wind turbines. In order to provide a complete picture, we will first briefly describe

the common power control topologies of wind turbines.

4.2.1 Overview of wind turbine topologies

Wind turbines can operate either with a fixed speed or a variable speed.

4.2.1.1 Fixed-speed wind turbines

In the early 1990s the standard installed win d turbines operated at fixed speed. That

means that regardless of the wind speed, the wind turbine’s rotor speed is fixed and

determined by the frequency of the supply grid, the gear ratio and the generator design.

Wind Power in Power Systems Edited by T. Ackermann

Ó 2005 John Wiley & Sons, Ltd ISBN: 0-470-85508-8 (HB)

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It is characteristic of fixed-speed wind turbines that they are equipped with an

induction generator (squirrel cage or wound rotor) that is directly connected to the

grid, with a soft-starter and a capacitor bank for reducing reactive power compensation.

They are designed to achieve maximum efficiency at one particular wind speed. In order

to increase power production, the generator of some fixed-speed wind turbines has two

winding sets: one is used at low wind speeds (typically 8 poles) and the other at medium

and high wind speeds (typically 4–6 poles).

The fixed-speed wind turbine has the advantage of being simple, robust and reliable

and well-proven. And the cost of its electrical parts is low. Its disadvantages are an

uncontrollable reactive power consumption, mechanical stress and limited power qual-

ity control. Owing to its fixed-speed operation, all fluctuations in the wind speed are

further transmitted as fluctuations in the mechanical torque and then as fluctuations in

the electrical power on the grid. In the case of weak grids, the power fluctuations can

also lead to large voltage fluctuations, which, in turn, will result in significant line losses

(Larsson, 2000).

4.2.1.2 Variable-speed wind turbines

During the past few years the v ariable-speed wind turbine has become the dominant

type among the installed wind turbines.

Variable-speed wind turbines are designed to achieve maximum aerodynamic

efficiency over a wide range of wind speeds. With a variable-speed operation it has

become possible continuously to adapt (accelerate or decelerate) the rotational speed

! of the wind turbine to the wind speed v. This way, the tip speed ratio  is kept

constant at a predefined value that corresponds to the maximum power coefficient.

(1)

Contrary to a fixed-speed system, a variable-speed system keeps the generator torque

fairly constant and the varia tions in wind are absorbed by changes in the generator

speed.

The electrical system of a variable-speed wind turbine is more complicated than that

of a fixed-speed wind turbine. It is typically equipped with an induction or synchronous

generator and connected to the grid through a power converter. The power converter

controls the generator speed; that is, the power fluctuations caused by wind variations

are absorbed mainly by changes in the rotor generator sp eed and consequently in the

wind turbine rotor speed.

The advantages of variable-speed wind turbines are an increased energy capture,

improved power quality and reduced mechanical stress on the wind turbine. The

disadvantages are losses in power electronics, the use of more components and the

increased cost of equipment because of the power electronics.

The introduction of variable-speed wind-turbine types increases the number of applic-

able generator types and also introduces several degrees of freedom in the combination

of generator type and power converter type.

(1)

Tip speed ratio, , is equal to !R/v, where R is the radius of the rotor.

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4.2.2 Overview of power control concepts

All wind turbines are designed with some sort of power control. There are different ways

to control aerodynamic forces on the turbine rotor and thus to limit the power in very

high winds in order to avoid damage to the wind turbine.

The simplest, most robust and cheapest control method is the stall control (passive

control), where the blades are bolted onto the hub at a fixed angle. The design of rotor

aerodynamics causes the rotor to stall (lose power) when the wind speed exceeds a

certain level. Thus, the aerodynamic power on the blades is limited. Such slow aero-

dynamic power regulation causes less power fluctuations than a fast-pitch power regu-

lation. Some drawbacks of the method are lower efficiency at low wind speeds, no

assisted startup and variations in the maximum steady-state power due to variations in

air density and grid frequencies (for an example, see also Chapter 15).

Another type of control is pitch control (active control), where the blades can be

turned out or into the wind as the power output becomes too high or too low,

respectively. Generally, the advantages of this type of control are good power control,

assisted startup and emergency stop. From an electrical point of view, good power

control means that at high wind speeds the mean value of the power output is kept close

to the rated power of the generator. Some disadvantages are the extra complexity arising

from the pitch mechanism and the higher power fluctuations at high wind speeds. The

instantaneous power will, because of gusts and the limited speed of the pitch mechan-

ism, fluctuate around the rated mean value of the power.

The third possible co ntrol strategy is the active stall control. As the name indicates, the

stall of the blade is actively controlled by pitching the blades. At low wind speeds the

blades are pitched similar to a pitch-controlled wind turbine, in order to achieve

maximum efficiency. At high wind speeds the blades go into a deeper stall by being

pitched slightly into the direction opposite to that of a pitch-controlled turbine. The

active stall wind turbine achieves a smoother limited power, without high power

fluctuations as in the case of pitch-controlled wind turbines. This control type has the

advantage of being able to compensate variations in air den sity. The combination with

the pitch mechanism makes it easier to carry out emergency stops and to start up the

wind turbine.

4.2.3 State-of-the-art generators

In the following, the most commonly applied wind turbine configurations are classified

both by their ability to control speed and by the type of power control they use.

Applying speed control as the criterion, there are four different dominating types of

wind turbines, as illustrated in Figure 4.1.

Wind turbine configurations can be further classified with respect to the type of power

(blade) control: stall, pitch, active stall. Table 4.1 indicates the different types of wind

turbine configurations, taking both criteria (speed control and power control) into

account. Each combination of these two criteria receives a label; for example, Type

A0 denotes the fixed-speed stall-controlled wind turbine. The grey zones in Table 4.1

indicate the combinations that a re not used in the wind turbine industry today (e.g.

Type B0).

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In this chapter, we will look mainly at the standard wind turbine types, depicted

in Figure 4.1 and Table 4.1. Other alternative, slightly different, wind turbine

designs will not be discus sed. Therefore, only the typical wind turbine configurations

and their advantages as well as disadvantages will be presented in the following

discussion.

SCIG

Gear

Type A

Gear

Grid

Soft-starter

Capacitor bank

Gear

Type B

Gear

GearGear

Gear

Grid

WRIG

Soft-starter

Capacitor bank

Variable resistance

Type D

Grid

Gear

PMSG/WRSG/WRIG

Full-scale

frequency converter

Type C

Grid

WRIG

Partial scale

frequency converter

Figure 4.1 Typical wind turbine configurations. Note: SCIG ¼squirrel cage induction generator;

WRIG ¼wound rotor induction generator; PMSG ¼permanent magnet synchronous generator;

WRSG ¼wound rotor synchronous generator. The broken line around the gearbox in the Type D

configuration indicates that there may or may not be a gearbox

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4.2.3.1 Type A: fixed speed

This configuration denotes the fixed-speed wind turbin e with an asynchronous squirrel

cage induction generator (SCIG) directly connected to the grid via a transformer (see

Figure 4.1). Since the SCIG always draws reactive power from the grid, this configur-

ation uses a capacitor bank for reactive power compensation. A smoother grid connec-

tion is achieved by using a soft-starter.

Regardless of the power control principle in a fixed-speed wind turbine, the wind

fluctuations are converted into mechanical fluctuations and consequently into electrical

power fluctuations. In the case of a weak grid, these can yield voltage fluctuations at the

point of connection. Because of these voltage fluctuations, the fixed-speed wind turbine

draws varying amounts of reactive power from the utility grid (unless there is a capacitor

bank), which increases both the voltage fluctuations and the line losses. Thus the main

drawbacks of this concept are that it does not support any speed control, it requires a

stiff grid and its mechanical construction must be able to tolerate high mechanical stress.

All three versions (Type A0, Type A1 and Type A2) of the fixed-speed wind turbine

Type A are used in the wind turbine industry, and they can be characterised as follows.

Type A0: stall control

This is the conventional concept applied by many Danish wind turbine manufacturers

during the 1980s and 1990s (i.e. an upwind stall-regulated three-bladed wind turbine

concept). It has been very popular because of its relatively low price, its simplicity and

its robustness. Stall-controlled wind turbines cannot carry out assisted startups, which

implies that the power of the turbine cannot be controlled during the connection sequence.

Type A1: pitch control

These have also been present on the market. The main advantages of a Type A1 turbine

are that it facilitates power controllability, controlled startup and emergency stopping.

Its major drawback is that, at high wind speeds, even small variations in wind speed

result in large variations in output power. The pitch mechanism is not fast enough to

avoid such power fluctuations. By pitching the blade, slow variations in the wind can be

compensated, but this is not possible in the case of gusts.

Table 4.1 Wind turbine concepts

Speed control Power control

Stall Pitch Active stall

Fixed speed

Type A Type A0 Type A1 Type A2

Variable speed Type B Type B0 Type B1 Type B2

Type C Type C0 Type C1 Type C2

Type D Type D0 Type D1 Type D2

Note: The grey zones indicate combinations that are not in use in the wind

turbine industry today.

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Type A2: active stall control

These have recently become popular. This configuration basically maintains all the power

quality characteristics of the stall-regulated system. The improvements lie in a better

utilisation of the overall system, as a result the use of active stall control. The flexible

coupling of the blades to the hub also facilitates emergency stopping and startups. One

drawback is the higher price arising from the pitching mechanism and its controller.

As illustrated in Figure 4.1 and Table 4.1, the variable speed concept is used by all

three configurations, Type B, Type C and Type D. Owing to power limitation con-

siderations, the variable speed concept is used in practice today only together with a

fast-pitch mechanism. Variable speed stall or variable speed active stall-controlled wind

turbines are not included here as potentially they lack the capability for a fast reduction

of power. If the wind turbine is runn ing at maximum speed and there is a strong gust,

the aerodynamic torque can get critically high and may result in a runaway situation.

Therefore, as illustrated in Table 4.1, Type B0, Type B2, Type C0, Type C2, Type D0

and Type D2 are not used in today’s wind turbine industry.

4.2.3.2 Type B: limited variable speed

This configuration corresponds to the limited variable speed wind turbine with variable

generator rotor resistance, known as OptiSlip

(2)

It uses a wound rotor induction

generator (WRIG) and has been used by the Danish manufacturer Vestas since the

mid-1990s. The generator is directly connected to the grid. A capacitor bank performs

the reactive power compensation. A smoother grid connection is achieved by using a

soft-starter. The unique feature of this concept is that it has a variable additional rotor

resistance, which can be changed by an optically controlled converter mounted on the

rotor shaft. Thus, the total rotor resistance is controllable. This optical coupling

eliminates the need for costly slip rings that need brushes and maintenance. The rotor

resistance can be changed and thus controls the slip. This way, the power output in the

system is controlled. The range of the dynamic speed control depends on the size of the

variable rotor resistance. Typically, the speed range is 0–10 % above synchronous speed.

The energy coming from the external power conversion unit is dumped as heat loss.

Wallace and Oliver (1998) describe an alternative concept using passive components

instead of a power electronic converter. This concept achieves a 10 % slip, but it does

not support a controllable slip.

4.2.3.3 Type C: variable speed with partial scale frequency converter

This configuration, known as the doubl y fed induction generator (DFIG) concept,

corresponds to the limited variable speed wind turbine with a wound rotor induction

generator (WRIG) and partial scale frequency converter (rated at approximately 30 %

of nominal generator power) on the rotor circuit (Plate 4, in Chapter 2 shows the nacelle

of a Type C turbine). The partial scale frequency converter performs the reactive power

(2)

OptiSlip is a registered trademark of Vestas Wind Systems A/S.

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compensation and the smoother grid connection. It has a wider range of dynamic speed

control compared with the OptiSlip

, depending on the size of the frequency converter.

Typically, the speed range comprises synchronous speed 40 % to þ30 %. The smaller

frequency converter makes this concept attractive from an economical point of view. Its

main drawbacks are the use of slip rings and protection in the case of grid faults.

4.2.3.4 Type D: variable speed with full-scale frequency converter

This configuration corresponds to the full variable speed wind turbine, with the gen-

erator connected to the grid through a full-scale frequency converter. The frequency

converter performs the reactive power compensation and the smoother grid connection.

The generator can be excited electrically [wound rotor synchronous generator (WRSG)

or WRIG) or by a permanent magnet [permanent magnet synchronous generator

(PMSG)].

Some full variable-speed wind turbine systems have no gearbox (see the dotted

gearbox in Figure 4.1). In these cases, a direct driven multipole generator with a

large diameter is used, see Plate 3, in Chapter 2 for instance. The wind turbine

companies Enercon, Made and Lagerwey are examples of manufacturers using this

configuration.

4.2.4 State-of-the-art power electronics

The variable-speed wind turbine concept requires a power electronic system that is

capable of ad justing the generator frequency and voltage to the grid. Before presenting

the current status regarding power electronics, it is important to understand why it is

attractive to use power electronics in future wind turbines: Table 4.2 illustrates the

Advantages Disadvantages

Controllable frequency

(important for the wind turbine)

Extra costs

Additional losses

Power plant characteristics

(important for the grid)

Controllable active and reactive power

Local reactive power source

Improved network (voltage) stability

Improved power quality

reduced flicker level

filtered out low harmonics

limited short circuit power

High harmonics

Table 4.2 Advantages and disadvantages of using power electronics in wind turbine systems

Power electronics properties

Energy optimal operation

Soft drive train

Load control

Gearless option

Reduced noise

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implications of using power electronics in wind turbines both for the wind turbine itself

and for the grid to which the wind turbine is connected.

Power electronics have two strong features:

Controllable frequency: power electro nics make it possible actually to apply the

variable-speed concept, and it is therefore important from a wind turbine point of

view. This feature results in the following direct benefits to wind turbines: (1) optimal

energy operation; (2) reduced loads on the gear and drive train, as wind speed

variations are absorbed by rotor speed changes; (3) load control, as life-consuming

loads can be avoided; (4) a practical solution for gearless wind turbines, as the power

converter acts as an electrical gearbox; and (5) reduced noise emission at low wind

speeds. Regarding the wind turbi ne, the disadvantages of power electronics are the

power losses and the increased costs for the additional equipment.

Power plant characteristics: power electronics provide the possibility for wind farms

to become active elements in the power system (Sørensen et al., 2000). Regarding the

grid, this property results in several advantages: (1) the active or reactive power flow

of a wind farm is controllable; (2) the power converter in a wind farm can be used as a

local reactive power source (e.g. in the case of weak grids); (3) the wind farm has a

positive influence on network stability; and (4) power converters improve the wind

farm’s power quality by reducing the flicker level as they filter out the low harmonics

and limit the short-circuit power. As far as the grid is concerned, power electronics

have the disadvantage of generating high harmonic currents on the grid.

Power electronics include devices such as soft-starters (and capacitor banks), rectifiers,

inverters and frequency converters. There is a whole variety of different design phil-

osophies for rectifiers, inverters and frequency converters (Novotny and Lipo, 1996).

The basic elements of power converters are diodes (uncontrollable valves) and elec-

tronic switches (controllable valves), such as conventional or switchable thyristors and

transistors. Diodes conduct current in one direction and will block current in the reverse

direction. Electronic switches allow the selection of the exact moment when the diodes

start conducting the current (Mohan, Undeland and Robbins, 1989). A conventional

thyristor can be switched on by its gate and will block only when there is a zero crossing

of the current (i.e. when the direction of the current is reversing), whereas switchable

thyristors and transistors can freely use the gate to interrupt the current. The most

widely known switcha ble thyristors and transistors are gate turn-off (GTO) thyristors,

integrated gate co mmutated thyristors (IGCTs), bipolar junction transistors (BJTs),

metal oxide semiconductor field effect transistors (MOSFETs) and insulated gate

bipolar transistors (IGBTs). Table 4.3 compares characteristics and ratings of five of

these switches. The values for voltage, current and output power are maximum output

ratings. The switching frequency defines the operational frequency range.

Conventional thyristors can control active power, while switchable thyristors

and transistors can control both active and reactive power (for detai ls, see Mohan,

Undeland and Robbins, 1989).

Today, variable-speed wind turbine generator systems can use many different types of

converters. They can be characterised as either grid-commutated or self-commutated

converters (Heier, 1998). The common type of grid- commutated converter is a thyristor.

60 Generators and Power Electronics