Optimization of a Direct-Drive Wind Power

Generation System by using PMSG Zinat Tasneem,

1,* Shafiul Hasan Rafi,

2 and M.R.I. Sheikh

3

1EEE Department, Pabna University of Science & Technology

Pabna-6600, Bangladesh. 2EEE Department, Varendra University

Rajshahi, Bangladrsh 3EEE Department, Rajshahi University of Engineering & Technology

Rajshahi-6204, Bangladesh. *tasneemzinat@gmail.com

AbstractThis paper investigates the optimized model and

the behavior of a variable speed wind energy conversion system

(WECS) based on permanent magnet synchronous generator

(PMSG). Models and equations that describe different

components of the WECS are addressed and their

implementations into PSCAD/EMTDC are described. There are

different types of synchronous generators, but the PMSG is

chosen because the elimination of gearbox and introduction of

variable speed control would even further increase the

availability of the system. The proposed model has a set of IGBT

converter-inverter connected through a DC bus associated with

the maximum power point tracking (MPPT) system. Three

models for the converter-inverter set have been used and their

performances have been compared. Simulation results show that

the controllers can extract maximum power and regulate the

voltage and frequency under varying wind conditions. The controllers show very good dynamic and transient performances.

Keywordswind turbine; PMSG; control strategy; converter; inverter

I. INTRODUCTION

Wind is a promising source of renewable energy. Worldwide there are now over two hundred thousand wind turbines

operating, with a total nameplate capacity of 282,482 MW as

of end 2012 [1]. With development of wind power

technologies and rapid growth of wind power capacity

installed worldwide, various concepts have been developed

and different wind generators have been built in the last two

decades. Over recent years, a new scheme for wind generators has become popular which includes a variable speed wind

turbine with permanent magnet synchronous generator

(VSWT-PMSG). Direct-drive PMSG raises great interest

because of its high efficiency and elimination of the gearbox [2]. PMSG has received much attention in wind energy

application because of their property of self excitation, which

allows an operation at a high power factor and high efficiency.

Usually, a PMSG-based wind energy conversion system

is composed of mechanical, electrical and control subsystems

whose time constants vary from microseconds to minutes or

even more. PMSG is directly driven by a wind turbine without

gear-box and it is connected to the AC power grid through

power converters. In the recent years, some works have been

done on PMSG based wind farm. Many control strategies for

wind generators have already been developed so as to

efficiently utilize the wind power, which is variable in nature [3-4]. In order to eliminate the drawbacks of induction

generator, PMSG is being used in WECS [5-6]. Recently

many power converter topologies have come up for

integrating wind generators with the grid [7-9].

This paper deals with the stability of WECS based on

PMSG. The system needs full-scale power converter and high

quality power devices which have high voltage endurance

level and can be modulated by PWM method. Due to the

recent development in power electronics, control system has

made it possible to regulate the grid side voltage, real power

and reactive power of PMSG in many different ways. In order

to solve the problem, three types of converter-inverter

topologies have been used. A comparison of their performances has been shown. In the first case, two-level six

IGBT have been used for the converter-inverter. In the second

case, three-level twelve IGBT have been used and in the final

case, three-level flying capacitor converter with carrier phase

shifted PWM technique has been used.

II. SYSTEM LAYOUT

The main parts of the gearless WECS are the wind-turbine,

the permanent magnet synchronous generator, the back to

back converters with their control, and the pitch controller. A

3 MW direct drive wind turbine unit is considered for this

analysis.

A. Wind Turbine Model

According to the blade element theory [10], modelling of

blade and shaft needs complicated and lengthy computations.

Moreover, it also needs detailed and accurate behaviour of the

system, a simplified method of modelling of the wind turbine

blade and shaft is normally used [11]. The mathematical

relation for mechanical power extraction from the wind can be

expressed as follows [10].

=1

22

3 , (1)

Where, is the mechanical power, is the air density ( Kg m3 ), R is the blade radius (m), is the power

coefficient, is the tip speed ratio, is the blade pitch angle (deg). and CP are expressed as[12]:

= R/VW (2) Where, is the wind turbine angular speed (rad/s), VW is the wind speed (m/s). The power coefficient, CP is, [12]

CP = 1/2 (-0.0222-5.6)0.17 (3)

Since, CP is expressed in feet and mile, is corrected as,

=(R/)*(3600/1609) (4) The torque coefficient, CT, is given by,

CT = CP()/ (5) The wind turbine torque is expressed as,

Tm=(1/2)R3VW

2CT() (6)

B. PMSG Model

The dynamic model of the PMSG is derived from the two-

phase synchronous reference frame, in which the q-axis is 90

degree ahead of the d-axis with respect to the direction of

rotation. The synchronization between the d-q rotating reference frame and the abc-three phase frame is maintained

by utilizing a phase locked loop [3]. The park model of PMSG

is given in the Fig. 1.

Fig. 1. PARK model of PMSG.

The schematic diagram of PMSG is shown in Fig. 2. The generator is connected through a back-to-back converter to the

grid. This provides maximum flexibility, full real and reactive

power control and fault ride through capability during voltage

dips.

Fig. 2. Schematic diagram of PMSG.

III. CONTROL STRATEGY OF PMSG

The control strategy of VSWT-PMSG is divided into two

parts, the generator side control system and grid side control

system. The three phase AC output of the generator is

rectified through a converter and the rectified DC output is fed

to an IGBT based grid side inverter whose output is then fed

to the step-up transformer and then to the grid.

A. Generator Side Frequency Converter Control

The structure of the control strategy of the frequency

converter is shown in the Fig. 3. The control signal applied to

the IGBTs gate switching is three phase sinusoidal voltage which is derived from the d-q axis signal.

B. Grid Side Frequency Inverter Control

The grid side inverter controls the DC link capacitor voltage at

the set value, so that the active power can be exchanged

efficiently from PMSG to the grid. It also controls the reactive

power output to the grid in order to control the grid side

voltage. In this control strategy, d-axis of the reference frame

is oriented along the grid voltage. Therefore active and

reactive power can be expressed as [12]:

=

= ( )

The proposed control structure of the grid side frequency inverter is shown in the Fig. 4.

Fig. 4. Control structure for grid side frequency inverter.

IV. CONVERTER-INVERTER SET

Three types of converter-inverter sets have been used in

this paper. They are explained in the following sub-sections.

A. Two-level six IGBTs based converter-inverter set

As shown in the Fig 5, a VSWT-PMSG system is

modelled with a fully controlled frequency converter. The

frequency converter consists of a generator side AC/DC

converter, a DC link capacitor and a grid side DC/AC inverter.

Each of the converter/inverter is a standard three phase two-

level unit, composed of six insulated gate bipolar junction

transistor (IGBTs) and anti parallel diodes. The full-rating

power converter is made up of two back- to-back IGBT

bridges (the generator side converter and grid side inverter)

linked by a DC bus. The power converters control the power

flow to the grid.

B. Three-level twelve IGBTs based converter-inverter set

The converter/inverter set shown in Fig. 6 are standard

three phase three-level unit composed of twelve IGBTs and

Fig. 3. Control structure for generator side frequency converter.

MPPT

Pref

abc/dq

IdmIqm

r r

PI-1

Pm

+

-

+-

PI-2

PWM

dq/abc

Vabc,m

Vd Vq

PI-3Qm

-

+PI-4

Iam,Ibm,IcmConverter

Firing Pulse

PLL

Va, Vb, Vc

abc/dq

PI-5Edc ref

Edc

+

- PI-6

Dq/abc

PWMFiring Pulse

Inverter

PI-9PI-8

PI-7Vgrid ref

Vgrid

+

-

Qgrid

+

-

Iq

-

+

Vq

Id+

-

Vd

PLL

Ia, Ib, Ic

PMSG

+

-Edc

Grid

Converter Inverter

anti parallel diodes. Here also the full-rating power converters

(generator side converter and grid side inverter) are linked by

a DC bus. Three-level (3L) neutral point clamped (NPC)

topology is considered for both converter and inverter. It 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 [13].

Fig. 5. Two-level IGBT converter-inverter set.

Fig. 6. Three-level IGBT converter-inverter set.

C. Flying capacitor multilevel converter

Three-level flying capacitor converter with carrier phase shifted SPWM technique has been used as shown in Fig. 7. Its advantages are that real and reactive power flow can be controlled, due to the presence of large number of capacitors. It has short duration fault ride through capability and good performance is attained even with the switching frequency of only 1 KHz [14]. The disadvantages of this topology are pre-charging of the capacitors to the same voltage level is required, large number of capacitors makes the system bulky and expensive, balancing control of flying capacitor voltages is required.

V. SIMULATION RESULTS

Simulations were run in power systems computer aided design/electromagnetic transient including DC (PSCAD/EMTDC) [15], for 350 sec with natural wind speed data. In this study the dynamic wind speed data shown in Fig.9 is used to make the proposed modified system more practical. For determining transient stability, three line-to- ground fault (3LG) has been considered. Time to apply fault is at 30th sec and the duration of the fault is .05sec.

Fig. 7. Flying capacitor three-level converter-inverter set.

Fig. 10 shows the rotor speed variation of PMSG under randomly varying wind speed. Fig. 11 and Fig. 12 show the voltage and real power response at the grid respectively during varying wind conditions. Fig. 13 shows the frequency fluctuation at the grid side in Hz. In all these figures, it can be seen that flying capacitor model can provide a better dynamic stability than the other two models. It provides a stable voltage and very small frequency fluctuation which is in the acceptable limit. It has a very good steady state characteristics under the randomly varying wind speed shown in Fig. 9, which has been used for simulation.

Fig. 9. Wind speed.

Fig. 10. Response of rotor speed (pu).

+

-

Edc

Wind

PMSGWr

Generator Side

Converter

Grid Side

Inverter

MPPT

Pref

theta

Frequency

Converter

Control

Pref

theta

Iabc Pg Qg

Frequency

Converter

Control

Edcref

beta

Iabc P Q

PLL

beta

Wind

PMSG

Wr

Frequency

Converter

Control

Frequency

Inverter

Control

Generator Side

ConverterGrid Side

Inverter

Wind

PMSG

Wr

Frequency

Converter

Control

Frequency

Inverter

Control

Generator Side

ConverterGrid Side

Inverter

0 50 100 150 200 250 300 3507

8

9

10

11

12

13

14

Win

d Sp

eed

(m/s

)

Time (sec)

0 50 100 150 200 250 300 3500.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

Rot

or S

peed

(pu

)

Time (sec)

2-level

3-level

Flying capacitor

Fig. 11. Grid side voltage (pu).

Fig. 14, 15, 16, and 17 show the transient response of grid

side voltage, real power, reactive power and frequency

respectively. The effectiveness of the flying capacitor model

can be understood from the transient response. The transient

voltage response follows the grid code, less fluctuation occurs

in the real power response which indicated good reactive

power compensation and a frequency fluctuation within the

acceptable range. Thus it is seen that the response of the farm containing flying capacitor converter-inverter set is the best

among these three, though the other responses are quite well

and can be accepted.

Fig. 12. Real power at the grid (pu).

Fig. 13. Frequency response at grid side (Hz).

Fig. 14. Grid side voltage during fault (pu).

Fig. 15. Real power during fault (pu).

Fig. 16. Reactive power during fault (pu).

Fig. 17. Frequency response during fault (Hz).

0 50 100 150 200 250 300 3500.98

0.99

1.00

1.01

1.02

Gri

d si

de v

olta

ge (

pu)

Time (sec)

2-level

3-level

Flying capacitor

0 50 100 150 200 250 300 3500.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Rea

l Pow

er (

pu)

Time (sec)

2-level

3-level

Flying capacitor

29.8 29.9 30.0 30.1 30.2 30.3 30.4

0.6

0.7

0.8

0.9

1.0

Volt

age

(pu)

Time (sec)

2-level

3-level

Flying Capacitor

29.50 29.75 30.00 30.25 30.50 30.75 31.000.0

0.5

1.0

1.5

2.0

2.5

3.0

Rea

l P

ow

er (

pu)

Time (sec)

2-level

3-level

Flying capacitor

29.50 29.75 30.00 30.25 30.50 30.75 31.00-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Rea

ctiv

e P

ow

er (

pu)

Time (sec)

2-level

3-level

Flying capacitor

29.5 30.0 30.5 31.0 31.5 32.0 32.5 33.0 33.5-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

0.020

Fre

quen

cy (

Hz)

Time (sec)

2-level

3-level

Flying capacitor

0 50 100 150 200 250 300 350-0.02

-0.01

0.00

0.01

0.02

Fre

qu

en

cy

Flu

ctu

ati

on

(H

z)

Time (sec)

2-level

3-level

Flying apacitor

VI. CONCLUSION

In this study, the modelling and control design of the

variable speed wind turbine (VSWT) driven by a permanent

magnetic synchronous generator (PMSG) is presented. The

modelling of the wind turbine is also described. The

modelling and control strategy for three types of converter-

inverter set is presented. Control strategies are suitable for

improving dynamic as well as transient stability. It is seen that the flying capacitor model has better dynamic as well as

transient response. It has a good fault ride through capability due to the presence of the capacitors, but it is a bit expensive.

So, this model can be used where precision of the wind farm

is needed.

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