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Smart Grid and Renewable Energy, 2010, 1, 119-131

doi:10.4236/sgre.2010.13017 Published Online November 2010 (/journal/sgre)

119

Wind Energy Conversion System from Electrical

Perspective

—A Survey

Hyong Sik Kim, Dylan Dah-Chuan Lu

School of Electrical and Information Engineering, University of Sydney,

Sydney, Australia

Email: hkim4210@

Received October 20

th

, 2010; revised November 14

th

, 2010; accepted November 20

th

, 2010

ABSTRACT

This paper focuses on the wind energy conversion system (WECS) with the three main electrical aspects: 1) wind tur-

bine generators (WTGs), 2) power electronics converters (PECs) and 3) grid-connection issues. The current state of

wind turbine generators are discussed and compared in some criteria along with the trends in the current WECS mar-

ket, which are ‘Variable Speed’, ‘Multi-MW’ and ‘Offshore’. In addition, the other crucial component in the WECS,

PECs will be discussed with its topologies available in the current WECS market along with their modulation strategies.

Moreover, three main issues of the WECS associating with the grid-connection, fault-ride through (FRT) capability,

harmonics/interharmonics emission and flicker, which are the power quality issues, will be discussed due to the in-

creasing responsibility of WECS as utility power station. Some key findings from the review such as the attractiveness

of BDFRG are presented in the conclusion of this paper.

Keywords: Wind Energy, Wind Turbine Generators, Power Electronic Converters, Grid-Connection, Brushless,

Reluctance, Pulse-Width Modulation, Fault Ride Through Capability, Voltage Dip, Harmonics, Flicker,

Power Quality, BDFRG

1. Introduction

Green house gas reduction has been one of the crucial

and inevitable global challenges, especially for the last

two decades as more evidences on global warming have

been reported. This has drawn increasing attention to

renewable energies including wind energy, which is re-

garded as a relatively mature technology [1]. It recorded

159 GW for the total wind energy capacities in 2009,

which is the highest capacity among the existing renew-

able energy sources with excluding large-scale hydro

power generators as shown in Figure 1 [2].

Also, its annual installation growth rate marked 31.7%

in 2009 with its growth rate having been increasing for

the last few years, which indicates that wind energy is

one of the fastest growing and attractive renewable en-

ergy sources [3]. The increasing price-competitiveness of

wind energy against other conventional fossil fuel energy

sources such as coal and natural gas is another positive

indication on wind energy [4]. Therefore, a vast amount

of researches on WECS have been and is being under-

taken intensively.

WECS consists of three major aspects; aerodynamic,

mechanical and electrical as shown in Figure 2.

The electrical aspect of WECS can further be divided

into three main components, which are wind turbine

generators (WTGs), power electronic converters (PECs)

and the utility grid.

There are many review papers on those electrical as-

pects available; however, there seem small amount of

investigation and discussion on some newer concepts of

Figure 1. World renewable energy capacities in 2009 (based

on [2]).

Copyright © 2010 SciRes. SGRE

120

Wind Energy Conversion System from Electrical Perspective—A Survey

Figure 2. Wind energy conversion system (based on [5,6]).

WGTs as well as PECs along with its modulation strate-

gies. The purpose of this paper is, therefore, to review

these three important electrical aspects of WECS with

some of the newer concepts for WTGs, PECs with their

modulation strategies, and some of the grid connection

issues that have risen as the penetration of wind energy

on the utility grid has been increasing rapidly in the last

few years [4].

The structure of this paper is as follows: wind turbine

generators are firstly discussed in Section 2, followed by

PECs and their modulation strategies in Section 3. Then,

grid-connection issues of WECS will be addressed in

Section 4. In Section 5, the discussion on these three com-

ponents is presented and followed by the conclusion in

Section 6.

2. Wind Turbine Generators

2.1. Wind Turbine Generators in the Current

Market

WTGs can be classified into three types according to its

operation speed and the size of the associated converters

as below:

 FSWT (Fixed Speed Wind Turbine)

 VSWT (Variable Speed Wind Turbine) with:

o PSFC (partial scale frequency converter)

o FSFC (full scale frequency converter)

FSWT including SCIG (Squirrel-Cage Induction Gen-

erator), led the market until 2003 when DFIG (Doubly

Fed Induction Generator), which is the main concept of

VSWT with PSFC, overtook and has been the leading

WTG concept with 85% of the market share reported in

2008 [4]. For VSWT with FSFC, WRSG (Wound Rotor

Synchronous Generator) has been the main concept;

however PMSG (Permanent Magnet Synchronous Gen-

erator) has been drawing more attention and increasing

its market share in the past recent years due to the bene-

fits of PMSG and drawbacks of WRSG [7].

Since there is much literature available on these WTG

concepts in the market such as [6-13], the following sec-

tion will only address the two newer concepts of WTGs,

which are BDFIG (Brushless Doubly Fed Induction

Generator) and BDFRG (Brushless Doubly Fed Reluc-

Copyright © 2010 SciRes.

tance Generator), followed by the discussion with the

comparison of them to the existing concepts.

2.2. Two Newer WTG Concepts

2.2.1. BDFIG

BDFIG is one of the most popular VSWT with PSFC

types in the current research area due to its inherited

characteristics of DFIG, which is the most popular WTG

type at the current market, along with its brushless aspect

that DFIG do not possess. As shown in Figure 3, BDFIG

consists of two cascaded induction machines; one is for

the generation and the other is for the control in order to

eliminate the use of sliprings and brushes, which are the

main drawback of DFIG.

This brushless aspect increases its reliability, which is

especially desirable in offshore application [14,15]. Other

advantages are reported in [6,16,17] including its capa-

bility with low operation speed. On the other hand,

BDFIG has relatively complex aspects in its design, as-

sembly and control, which are some of the main disad-

vantages of BDFIG [8].

2.2.2. BDFRG

There is also another brushless and two-cascaded-stator

concept of VSWT with PSFC type in the research area,

which is BDFRG. As shown in Figure 4, one distinct

design compared with BDFIG is its reluctance rotor,

which is usually an iron rotor without copper windings,

which has lower cost than wound rotor or PM (perma-

nent magnet) rotor.

This design offers some advantages on top of the ad-

vantages of BDFIG including higher efficiency, easier

construction and control including power factor control

capability as well as the cost reduction and higher reli-

ability including its “fail-safe” operating mode due to its

reluctance rotor [18-21]. Due to its very high reliability,

reluctance generators have also been of interest in air-

craft industry where design challenges such as harsh en-

vironment operation and stringent reliability exist [22,

23]. On the other hand, some of the drawbacks for BDFRG

exist such as complexity of rotor deign, its larger ma-

chine size due to a lower torque-volume ratio and so

forth [20,24,25].

SGRE

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