A Variable Speed Wind Turbine Control Strategy to Meet Wind Farm Grid Code Requirements

IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 25, NO. 1, FEBRUARY 2010 331

A Variable Speed Wind Turbine Control Strategyto Meet Wind Farm Grid Code Requirements

S. M. Muyeen, Member, IEEE, Rion Takahashi, Member, IEEE, Toshiaki Murata, andJunji Tamura, Senior Member, IEEE

Abstract—This paper presents a new operational strategy for asmall scale wind farm which is composed of both fixed and vari-able speed wind turbine generator systems (WTGS). Fixed speedwind generators suffer greatly from meeting the requirements ofnew wind farm grid code, because they are largely dependent onreactive power. Integration of flexible ac transmission systems(FACTS) devices is a solution to overcome that problem, thoughit definitely increases the overall cost. Therefore, in this paper,we focuses on a new wind farm topology, where series or parallelconnected fixed speed WTGSs are installed with variable speedwind turbine (VSWT) driven permanent magnet synchronousgenerators (PMSG). VSWT-PMSG uses a fully controlled fre-quency converter for grid interfacing and it has abilities to controlits reactive power as well as to provide maximum power to thegrid. Suitable control strategy is developed in this paper for themultilevel frequency converter of VSWT-PMSG. A real gridcode defined in the power system is considered to analyze thelow voltage ride through (LVRT) characteristic of both fixed andvariable speed WTGSs. Moreover, dynamic performance of thesystem is also evaluated using real wind speed data. Simulationresults clearly show that the proposed topology can be a costeffective solution to augment the LVRT requirement as well asto minimize voltage fluctuation of both fixed and variable speedWTGSs.

Index Terms—Frequency converter, low voltage ride through(LVRT), multilevel converter/inverter, permanent magnet syn-chronous generator (PMSG), voltage source converter (VSC),wind generator.

I. INTRODUCTION

WIND energy is playing a vital role in the world’s en-

ergy markets nowadays, considering its striking growth

rate in the last few years. GWEC is predicting the global wind

market to grow by over 155% from the current size reaching

240 GW of total installed capacity until 2012. This means a

146-GW increase in just five years [1]. Due to this huge pene-

tration of wind power to the grid, it is important to analyze both

power quality and low voltage ride through (LVRT) issues of

wind farms to meet the requirements of a new set of grid codes.

Presently, variable speed wind turbine generator system

(WTGS) is becoming more popular than that of fixed speed

[2]. Induction generators are used, in general, as fixed speed

Manuscript received January 13, 2009; revised March 11, 2009. First pub-lished September 25, 2009; current version published January 20, 2010. Thiswork was supported by the Grant-in-Aid for JSPS Fellows from Japan Societyfor the Promotion of Science (JSPS). Paper no. TPWRS-00992-2008.

The authors are with the Electrical and Electronic Engineering Department,Kitami Institute of Technology, Kitami, Hokkaido, 090-8507, Japan (e-mail:[email protected]).

Digital Object Identifier 10.1109/TPWRS.2009.2030421

wind generator due to their superior characteristics such as

brushless and rugged construction, low cost, maintenance free,

and operational simplicity. However, it requires large reactive

power to recover the air gap flux when a short circuit fault

occurs in the power system [3], unless otherwise the induc-

tion generator becomes unstable due to the large difference

between electromagnetic and mechanical torques, and then it

requires to be disconnected from the power system. A shut-

down of large wind farm may have a serious effect on the

power system operation. Therefore, a new set of grid codes

[4]–[7] have been defined recently, which includes the LVRT

requirements for WTGSs during the network disturbances to

avoid the shutdown phenomenon of large wind farms. For

example, according to the U.S. wind farm grid code, if the

voltage of wind farm remains at a level greater than 15% of

the nominal voltage for a period that does not exceed 0.625 s,

the plant must stay online [6].

Therefore, it is important to investigate a suitable method to

enhance the LVRT capability of fixed speed wind generators.

Voltage or current source inverter-based flexible ac transmission

system (FACTS) devices such as static var compensator (SVC),

static synchronous compensator (STATCOM), dynamic voltage

restorer (DVR), solid state transfer switch (SSTS), and unified

power flow controller (UPFC) have been used for flexible power

flow control, secure loading, and damping of power system os-

cillation [8]–[10]. FACTS/ESS, i.e., FACTS with energy storage

system (ESS), has recently emerged as more promising devices

for power system applications [11]. Some of those are even ap-

plicable to wind farm stabilization. STATCOM, SVC, super-

conducting magnetic energy storage system (SMES), and en-

ergy capacitor system (ECS) composed of electric double layer

capacitor, and power electronic devices have already been pro-

posed to enhance the LVRT capability of fixed speed wind farm

[12]–[18]. However, the installation of FACTS devices at a wind

farm composed of fixed speed wind generators increases the

system overall cost.

On the contrary, variable speed WTGS equipped with full or

partial rating power electronic converters has strong fault ride

through capability during the network disturbance [19]–[23].

It also has the ability to extract maximum power from the

wind due to its variable speed operation. These aforementioned

issues make the market share higher for variable speed WTGSs

in the recent years. In [23], double fed induction generator

(DFIG) is used for transient stability enhancement of squirrel

cage induction generator. However, two-mass drive train model

of WTGS is not considered therein, which has significant effect

on the transient stability analysis of fixed speed WTGS [24],

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332 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 25, NO. 1, FEBRUARY 2010

[25]. Moreover, underground cable is not considered in that

study for wind generator connection. In real system, a wind

farm generally employs underground cables with high capac-

itance to connect neighboring wind generators. Therefore, for

LVRT analysis, underground cables should be considered for

preciseness.

Permanent magnet machines are characterized as having

large air gaps, which reduce flux linkage even in machines

with multimagnetic poles [26], [27]. As a result, low rotational

speed generators can be manufactured with relatively small

sizes with respect to its power rating. Moreover, gearbox can be

omitted due to low rotational speed in PMSG wind generation

system, resulting in low cost. In a recent survey, gearbox is

found to be the most critical component, since its downtime per

failure is high in comparison to other components in a WTGS

[28]. Moreover, PMSG wind generation system can supply

more reactive power to the network than that of DFIG [29].

Therefore, in this paper, PMSG is considered as variable speed

wind generator to augment the LVRT capability of wind farm.

In this paper, a small scale new wind farm topology con-

trolled by a new strategy is presented to augment the LVRT

capability of fixed speed WTGS during network disturbances

by using variable speed WTGS installed in the same wind

farm. In the model system used in this paper, a VSWT driving

a PMSG is connected to the power system through a fully con-

trolled frequency converter, which consists of generator side

ac/dc converter, dc-link capacitor, and grid side dc/ac inverter.

The fixed speed WTGSs are connected to the terminal of vari-

able speed WTGS through short underground cables. Though

two-level converter/inverter topology is frequently used in

wind power application [20], [21], multilevel converter/inverter

topology is more suitable in the frequency converter consid-

ering the factors of better harmonic performance and capability

to withstand high phase currents [13]. Therefore, multilevel

topology for the frequency converter is considered in this study

for VSWT-PMSG operation. A suitable control strategy for the

multilevel frequency converter of VSWT-PMSG is developed,

which can provide necessary reactive power demand of induc-

tion generator during the network disturbance as well as can

extract maximum power from the wind. Two-mass drive train

model and underground cables are considered in this study for

preciseness of the analysis. Both series and parallel connected

fixed speed WTGSs are investigated. Extensive simulation

analyses have been carried out considering both symmetrical

and unsymmetrical faults to analyze the LVRT characteristics

of both fixed and variable speed WTGSs. Moreover, dynamic

performance of the system is also verified using the real wind

speed data measured in Hokkaido Island, Japan. Finally, it is

concluded that the proposed WTGS topology is a good and

cost effective solution to enhance the LVRT capability and

minimize voltage fluctuation of both fixed and variable speed

wind generators. This is because additional cost to integrate

the FACTS devices at wind farm terminal can be eliminated to

meet the wind farm grid code. Moreover, additional electrical

output can be supplied to the grid from VSWT-PMSG, which is

not possible in the case of using FACTS devices. This topology

might be suitable to new wind farm as well as to the existing

wind farm where wind farm expansion is considered.

Fig. 1. Model system. (a) Series connected fixed speed WTGSs. (b) Parallelsconnected fixed speed WTGSs.

II. MODEL SYSTEM

The model system used for the LVRT and dynamic stability

analyses are shown in Fig. 1. Here, one PMSG (5 MVA) is

connected to 11.4-kV distribution system through a frequency

converter, 1.25/11.4-kV step-up transformer, and underground

cable. Two induction generators rated at 2.5 MVA each are

connected in series and parallel to the high voltage side of

1.25/11.4-kV transformer through 0.69/11.4-kV transformer

and short underground cables as shown in Fig. 1(a) and (b),

respectively. A capacitor bank, , has been considered to

be connected to the terminal of each induction generator (IG)

for reactive power compensation at steady state. The value of

capacitor Cbank is chosen so that power factor of the wind

generator during the rated operation becomes unity [30]. Total

output of the wind farm are supplied to the utility grid though

a common step-up transformer (11.4 kV to 66 kV). PMSG

and IG parameters are shown in Tables I and II, respectively.

Underground cable parameters are given in the Appendix. The

system base is 10 MVA.




MUYEEN et al.: VARIABLE SPEED WIND TURBINE CONTROL STRATEGY 333

TABLE IPARAMETERS OF PMSG

TABLE IIPARAMETERS OF IG

Fig. 2. � � � curves for different pitch angles (used in FSWT).

III. WIND TURBINE MODELING

The mathematical relation for the mechanical power extrac-

tion from the wind can be expressed as follows [31]:

(1)

where is the extracted power from the wind, is the air

density , R is the blade radius [m], is the wind speed

[m/s] and is the power coefficient which is a function of both

tip speed ratio, , and blade pitch angle, [deg]. Both fixed and

variable speed wind turbine characteristics used in this study are

shown in Figs. 2 and 3, respectively [31]–[33].

In variable speed wind turbine, when the wind speed changes,

the rotational speed of wind turbine is controlled to follow the

maximum power point trajectory. Since the precise measure-

ment of the wind speed is difficult, it is better to calculate the

maximum power, , without the measurement of wind

speed as shown in the following:

(2)

Fig. 3. Turbine characteristic with maximum power point tracking (used inVSWT).

Fig. 4. Pitch controller used in variable speed WTGS.

Fig. 5. Conventional pitch controller used in fixed speed WTGS.

where and are optimum values of tip speed ratio

and power coefficient, respectively. From (2), it is clear that the

maximum generated power is proportional to the cube of rota-

tional speed.

The range of rotor speed variation is, in general, approxi-

mately 5 to 16 rpm. During the control of generator side ac/dc

converter, the maximum power, , is calculated based on the

MPPT, which becomes the reference power, , for the con-

verter. If this reference power is greater than the rated power

of PMSG, then the pitch controller shown in Fig. 4 is worked

to control the rotational speed. Therefore, the PMSG output

will not exceed the rated power. A conventional pitch controller

shown in Fig. 5 is also considered in fixed speed WTGS.

As VSWT-PMSG is decoupled from the grid through the fre-

quency converter, the drive train modeling has almost no in-

fluence on fault analysis [33]. Therefore, one-mass drive train

model of variable speed WTGS is considered in the simula-

tion for simplicity. However, in the case of fixed speed WTGS,

detailed drive train dynamics is needed to be considered, espe-

cially for flicker or transient stability analysis [24], [25]. There-

fore, two-mass drive train modeling is considered in this study

for fixed speed WTGSs.

IV. MODELING OF PWM FREQUENCY CONVERTER

The electrical scheme of VSWT-PMSG topology is shown

in Fig. 6. The PMSG model available in the package software





Fig. 6. Electrical scheme of VSWT-PMSG.

Fig. 7. Control block for the grid side 3L-NPC inverter.

PSCAD/EMTDC [34] is used. The pulse width modulation

(PWM) frequency converter consists of generator side ac/dc

converter, dc-link capacitor, and grid side dc/ac inverter.

Three-level (3L) neutral point clamped (NPC) topology is

considered for both converter and inverter as shown in Fig. 6.

The three-level converter has the advantages that the blocking

voltage of each switching device is one half of dc-link voltage

in contrast to the full dc-link voltage in case of the two-level

converter, and the harmonic contents of three-level converter

output voltage are much less than those of two-level one, at

the same switching frequency. The control strategies of the

3L-NPC converter and inverter are demonstrated below.

A. Grid Side Multilevel Inverter

The aim of the grid side converter control is to keep the dc link

voltage constant, thereby ensuring that the active power gener-

ated by the generator is fed to the network. Additionally, it is

possible to control the reactive power fed to the grid.

The well-known cascaded control scheme shown in Fig. 7 is

used as a control methodology for the grid side inverter. The

grid side converter is controlled in a synchronous reference

frame that rotates synchronously with the grid voltage. The

grid side voltage phasor is synchronized with the controller

reference frame by using the phase locked loop (PLL) as shown

in Fig. 8 [35]. The insulated gate bipolar transistor (IGBT)

switching table is shown in Table III. The pulse generation

TABLE IIISWITCHING TABLE

system of 3L-NPC inverter is shown in Fig. 9. For reference

frame transformation, the park’s transformation is considered

as follows:

(3)

The angle of the transformation is detected from the three

phase voltages at the high voltage side of the grid

side transformer. The dc voltage of the dc-link capacitor is con-

trolled constant by two PI controllers. The d-axis current can

control the dc-link voltage. On the other hand, the q-axis cur-

rent can control the reactive power of grid side inverter. The

reactive power reference is set in such a way that the terminal

voltage at high voltage side of the transformer remains constant.

Therefore, total three PI controllers are used to control the re-

active power of grid-side inverter. The additional PI controller

provides excellent transient characteristic during network dis-

turbance as shown later.

B. Generator Side Multilevel Converter

A generator side converter connected to the stator of the

PMSG effectively decouples the generator from the network.

Thus, the generator rotor and the wind turbine rotor can rotate

freely depending on the wind conditions.

The control block for generator side converter shown in

Fig. 10 is based on vector control in rotor reference frame.

Park’s transformation is used herein for reference frame trans-

formation. As the converter is directly connected to the PMSG,

its q-axis current is proportional to the active power. The active





Fig. 8. Block diagram for PLL [35].

Fig. 9. Pulse generation system.

Fig. 10. Control block for the generator side converter.

power reference, , is determined in such a way to provide

the maximum power to the grid as explained in Section III.

However, when a network disturbance occurs, the terminal

voltage, , of the high voltage side of the transformer

decreases considerably. Therefore, in such a situation, it would

be appropriate not to keep the active power reference setpoint

at the maximum extracted power level in order to avoid dc-link

overvoltage, which is further discussed in the following section.

Therefore, in this study, is varied according to the terminal

voltage during the time when voltage drops below 0.9 p.u.

It gives excellent transient performance as demonstrated in

Section V. This is one of the salient features of this work. On

the other hand, the d-axis stator current is proportional to the

reactive power. The reactive power reference is set to zero to

perform unity power factor operation. The switching strategy

of 3L-NPC converter is the same that is used in Section IV-A.

In both converter and inverter, the triangular carrier signal

is used as the carrier wave of PWM operation. The carrier

frequency is chosen 1000 Hz for converter and 1050 Hz for

inverter, respectively. The dc-link capacitor value of each

switching device is 15 000 [21]. The rated dc-link voltage

across the two capacitor legs is 2.3 kV.

C. Overvoltage and Undervoltage Protection Schemes

of DC-Link

During the severe network disturbance such as short circuit

fault, the front end inverter has to provide sufficient reactive cur-

rent to support the grid. Therefore, the grid side inverter cannot

supply the active power from the generator to the grid, which

results in overvoltage in the dc-link of the frequency converter

This may hamper normal operation of the frequency converter

unit. In this paper, as the frequency converter of VSWT-PMSG

is used to provide the necessary reactive power support for the

proposed topology which includes fixed speed wind generators,

any type of interruption of the frequency converter would result

in instability in the overall system. Therefore, overvoltage pro-

tection scheme (OVPS) is taken into consideration in this study

as shown in Fig. 6 along with the control technique for gener-

ator side converter explained in Section IV-B.

The braking chopper is modeled in the dc-link in order to pro-

tect the dc-link capacitor during fault situation. The chopper is

activated when the dc-link voltage increases over the predefined

limit and dissipates the active power into the resistance during

the voltage dip in the grid.

Besides the overvoltage protection, undervoltage protection

is also taken into consideration, in this paper. In the grid side

inverter shown in Fig. 7, the output of integrators used in PI

controllers are reset when the dc-link voltage goes below a pre-

defined value. In this way, the undervoltage tripping of the drive





Fig. 11. Low voltage ride-through standard set by FERC, U.S. [6].

can be avoided during the network disturbance when dc-link

voltage tends to decrease extremely.

V. SIMULATION RESULTS

The grid codes were originally decided with synchronous

generators in mind. But due to the recent addition of huge

amount of wind power to the grid, in many countries [4]–[7],

the new grid codes have been developed to ensure secure power

system operation. The wind farm grid codes are more or less

similar to each other. In this study, the simulation results are

described in light of the U.S. grid code, set by Federal Energy

Regulatory Commission (FERC). If the voltage does not fall

below the minimum voltage indicated by the solid line in

Fig. 11 and returns to 90% of the nominal voltage within 3 s

after the beginning of the voltage drop, the plant must stay

online [6].

In this study, a new wind farm topology is considered which

is composed of fixed and variable speed wind generators and

LVRT and dynamic characteristics are analyzed. The frequency

converter of the VSWT-PMSG is controlled in such a way to

maintain the grid voltage (voltage at the high voltage side of

transformer followed by the frequency converter) at desired ref-

erence level set by transmission system operator (TSO). In that

way, the necessary reactive power of the induction generator

can be supplied to restore the electromagnetic torque. To ob-

tain realistic responses, the two-mass shaft model of WTGS

is considered. All types of dampings are disregarded to ob-

tain the worse scenario [25]. Symmetrical three-line-to-ground

fault (3LG) and unsymmetrical double-line-to-ground (2LG),

line-to-line (2LS), and single-line-to-line (1LG) faults are con-

sidered as a network disturbance, which occurs at fault points

F in Fig. 1. The fault occurs at 0.1 s, the circuit breakers (CB)

on the faulted lines are opened at 0.2 s, and at 1.0 s, the cir-

cuit breakers are reclosed. In the simulation study, it is assumed

that wind speed is constant and equivalent to the rated speed for

both fixed and variable speed WTGSs. This is because it may be

considered that wind speed does not change dramatically during

the short time interval of the simulation. Time step and simula-

tion time are chosen 0.00002 s and 10 s, respectively. Simu-

TABLE IVCASE STUDY

lations were carried out by using PSCAD/EMTDC [34]. Two

cases shown in Table IV are considered in the simulation study

to show the effectiveness of the control strategy of the proposed

system to meet the wind farm grid code requirements.

A. LVRT Characteristic Analysis

To show the effectiveness of the control system of the fre-

quency converter under fault condition, pitch controllers con-

nected to the fixed speed WTGSs are not considered in both

Cases 1 and 2.

1) Using Model System (a): In this case, series connected

fixed speed WTGSs are connected to the terminal of VSWT-

PMSG at bus-K as shown in Fig. 1(a). The grid side inverter can

provide necessary reactive power during the severe symmetrical

3LG fault which is shown in Fig. 12. Therefore, the terminal

voltage shown in Fig. 13 can return to its pre-fault level for Case

1. As the necessary reactive power of the induction generators

are provided by the VSWT-PMSG, the electromagnetic torques

of IG can be restored quickly. Therefore, electromagnetic and

mechanical torques balance to each other and as a result the rotor

and turbine hub speeds of fixed speed WTGSs become stable for

Case 1 as shown in Figs. 14 and 15, respectively. From Fig. 13,

it is seen that Case 2 fails to achieve the LVRT requirement of

wind farm due to lack of reactive power supply during network

disturbance. The response of PMSG rotor speed is shown in

Fig. 16. When the fault occurs, the reference of the real power

reference for the generator side converter is varied with terminal

voltage as shown in Fig. 17. The real power responses of grid

side inverter (real power from the PMSG) and induction gen-

erators are shown together in Fig. 18. The dc-link voltage re-

sponse of the frequency converter is shown in Fig. 19. It is no-





Fig. 12. Reactive power of grid side inverter (3LG) (Case 1).

Fig. 13. Terminal voltage at bus-K (3LG).

Fig. 14. Rotor speed of induction generator 1 (3LG).

Fig. 15. Turbine hub speed of WTGS1 (3LG).

ticeable from the reactive power response shown in Fig. 12 that

almost rated level reactive power support is needed from the fre-

quency converter of VSWT-PMSG. Therefore, if the proposed

wind farm is required to meet the wind farm grid code during

Fig. 16. Rotor speed of the PMSG (3LG) (Case 1).

Fig. 17. Reference power of the generator side converter (3LG) (Case 1).

Fig. 18. Real powers of the PMSG and the IG (3LG).

Fig. 19. DC-Link voltage of the frequency converter (3LG) (Case 1).

the most severe 3LG fault, the power ratings of fixed and vari-

able speed wind generators need to be nearly equal.

The PLL circuit shown in Fig. 8 is suitable for unbalanced

faults as well as balanced 3LG fault to detect the phase angle






Fig. 21. Terminal voltage at bus-K (2LS).


used in the grid side inverter. The responses of the terminal volt-

ages at bus-K in the case of 2LG, 2LL, and 1LG faults are shown

in Figs. 20–22, respectively. From the simulation results, it is

seen that the LVRT requirement of wind farm can be met well

for the fault types described in this paper.

2) Using Model System (b): In this case, parallel connected

fixed speed WTGSs are connected to the terminal of VSWT-

PMSG as shown in Fig. 1(b).

The response of terminal voltage at bus-K is shown in Fig. 23

from which it is seen that Case 1 can achieve the LVRT require-

ment of wind farm. The responses of reactive power and dc-link

voltage of frequency converter are shown in Figs. 24 and 25, re-

spectively. Therefore, this model can also help wind farms meet

the new grid code requirements.

B. Dynamic Characteristic Analysis

To evaluate the dynamic performance of the proposed system,

the real wind speed data measured in Hokkaido Island, Japan,


Fig. 24. Reactive power of grid side inverter (3LG) (Case 1).

Fig. 25. DC-Link voltage of the frequency converter (3LG) (Case 1).

Fig. 26. Wind speed data for generators.

shown in Fig. 26, is used in the simulation. Model system (a)

is considered in this analysis. It is seen that the wind farm ter-

minal voltage at bus-K can be maintained constant in Case 1 also

during randomly fluctuating wind speed as shown in Fig. 27.

The reactive power response at bus-K is also shown in Fig. 28.





Fig. 27. Wind farm terminal voltage at bus-K.

Fig. 28. Reactive power at bus-K.

VI. CONCLUSIONS

Integration of FACTS devices to wind farm terminal can aug-

ment the LVRT capability of fixed speed WTGSs, which is re-

ported already in several literatures. However, it increases the

overall system cost. In this paper, a new type of wind farm

topology composed of fixed and variable speed wind turbine

generator systems has been presented. The variable speed wind

turbine driving a PMSG is connected to the grid through a fully

controlled three-level NPC frequency converter which has reac-

tive power control ability. Taking this advantage from VSWT-

PMSG, fixed speed wind generators are connected either series

or parallel to its terminal, so that they can receive the neces-

sary reactive power from VSWT-PMSG during network dis-

turbances. The LVRT characteristics of both fixed and variable

speed WTGSs are evaluated in light of U.S. grid code consid-

ering both symmetrical and unsymmetrical faults. Moreover, the

dynamic performance is verified using real wind speed data. Fi-

nally, it is concluded that the proposed system can be a cost-ef-

fective solution to achieve the requirements of new wind farm

grid code.

APPENDIX

The underground cable parameters are given below [36].

(a) Underground cables between transformers, 1.25/11.4 kV

and 0.69/11.4 kV (350 m): ,

, and .

(b) Underground cables between transformers, 1.25/11.4 kV

and 11.4/66 kV (700 m): ,

, and .

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S. M. Muyeen (M’08) was born in Khulna,Bangladesh, on September 8, 1975. He receivedthe B.Sc. Eng. degree from Rajshahi University ofEngineering and Technology (RUET), Rajshahi,Bangladesh, in 2000 and the M.Sc. Eng. and Dr.Eng. degrees from Kitami Institute of Technology,Kitami, Japan, in 2005 and 2008, respectively, all inelectrical and electronic engineering.

Presently, he is doing research works under theJapan Society for the Promotion of Science (JSPS)Postdoctoral Fellowship Program at the Kitami

Institute of Technology. His research interests are power system, electricalmachine, FACTS, energy storage system (ESS), wind generator stabilization,multi-mass drive train of wind turbine, and HVDC system.

Rion Takahashi (M’07) received the B.Sc. Eng.and Dr. Eng. degrees from Kitami Institute ofTechnology, Kitami, Japan, in 1998 and 2006,respectively, both in electrical and electronicengineering.

Now, he is working as a Research Assistant in theDepartment of Electrical and Electronic Engineering,Kitami Institute of Technology. His major researchinterests include analysis of power system transient,FACTS, and wind energy conversion system.

Toshiaki Murata received the Dr. Eng. degree fromHokkaido University, Japan, Sapporo, in 1991.

He completed his Electrical Engineering Cur-riculum at the Teacher Training School fromHokkaido University. Since 1969, he had been aResearch Assistant at the Kitami Institute of Tech-nology, Kitami, Japan. Presently, he is an AssociateProfessor at the Kitami Institute of Technology.

Junji Tamura (M’87–SM’92) received the B.Sc.Eng. degree from Muroran Institute of Technology,Muroran, Japan, in 1979 and the M.Sc. Eng. and Dr.Eng. degrees from Hokkaido University, Sapporo,Japan, in 1981 and 1984, respectively, all in electricalengineering.

In 1984, he became a Lecturer and in 1986, anAssociate Professor at the Kitami Institute of Tech-nology, Kitami, Japan. Currently, he is a Professor atthe Kitami Institute of Technology.




A Variable Speed Wind Turbine Control Strategy to Meet Wind Farm Grid Code Requirements

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