Vector Control With Direct Power Control for Grid Side Converter of DFIG

7/29/2019 Vector Control With Direct Power Control for Grid Side Converter of DFIG

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Comparative study of vector control with direct

power control for grid side converter of DFIG

wind power generation system

Meihua Zhao, Yi Ruan, Yang ShenBingying Ye, Qinhong Zhong

School of Mechatronics Engineering and Automation

Shanghai UniversityShanghai, 200072, China

Zhao Meihua

Department of Mechanical and Electronic Engineering

Luoyang Institute of Science and Technology

Luoyang, 471023, [email protected]

AbstractDeveloping a high performance control strategy for

grid side PWM converter of DFIG wind power generation system

is a significant point. In this paper, first, deriving and analyzing

the mathematical model of grid side PWM converter, a vectorcontrol strategy is proposed. Second, the voltage vectors influence

on the active power change and the reactive power change was

analyzed, a direct power control based on a new switching table

was proposed. At last, comparative experiments were tested for

vector control and direct power control strategy. The feasibility

and correctness, advantage and disadvantage of the control

strategy are verified by experimental results.

Keywords- DFIG wind power generation; grid side converter;

space vector control; direct power control (DPC)

I. INTRODUCTION

In recent years, much attention has been paid on cleanrenewable energy sources like solar and wind .Wind energy isgaining momentum because of its relatively low cost [1-2].Doubly-fed induction generator (DFIG) wind powergeneration system is becoming increasing popular for its someadvantages such as maximum wind-energy capturing, four-quadrant active and reactive power regulation and lowconverter cost [3-4] .Schematic of DFIG wind powergeneration system is shown in Fig.1.

n

Fig.1 Schematic of DFIG wind power generation system

The function of rotor side PWM converter is to realizepower control of wind power generation system. The controlobjective of grid side PWM converter is to maintain thestability of the DC link voltage (Udc) and to adjust power factor.

For grid side PWM converter, different control strategies areproposed in recent years, the main control systems are space

vector control and direct power control (DPC) [5-6].Vectorcontrol has been widely applied for its fixed switchingfrequency, the sinusoidal output current, the low harmoniccomponent and high voltage utilization ratio. In recent years,DPC becomes a study hotspot for its simple control model, hasno use for the PWM modulation module and fast dynamicresponse [7-10].

In this paper, first, a space vector control strategy for gridside converter was proposed based on its dynamicmathematical model. Then the influence of the voltage vectoron active and reactive power variations were analyzed, theDPC strategy was proposed based on novel switch table [11-12]. Finally, the two control strategies were compared withexperiment. Experimental results verify that the two schemesare correct and feasible, comparing the advantages anddisadvantages of them, two kinks of feasible control strategiesare provided for the grid side converter of DFIG wind powergeneration system.

II. MATHEMATICAL MODEL OF GRID SIDE CONVERTER

a

bc

ai

bi

ci

ae

be

ce

N

1S

4S

2S

5S

3S

6S

n

Fig.2 Schematic of three-phase grid side converterof DFIG wind power generation system

Schematic of grid side converter of DFIG wind powergeneration system shown in Fig.2. Where Udc is DC link

voltage ,a

u ,b

u andc

u are the output voltages of the converter

ai , bi and ci are the output currents, R , L is respectively

the resistance and inductance of filter reactor,a

e ,b

e ,c

e is

respectively three-phase grid phase voltage. N n is

respectively lower bridge arm common point and the neutral

point of power supply.

Fund: This paper and its related research are supported by grants fromthe postgraduate innovation foundation of Shanghai University. Fund

Number: SHUCX112182

3398

978-1-4244-8165-1/11/$26.00 2011 IEEE


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According to KVL gets

1 0 0

0 1 0

0 0 1

a

a aa

b

b b b

cc cc

di

dtu eidi

u L R i edt

iu edi

dt

= + +

(1)

Coordinate transformation from three-phase stationary

ABC frame to two -phase stationary

frame is given

a

die L Ri

u dt

u die L Ri

dt

+ +

=

+

(2)

Transformation from the stationary to the synchronous

dq frame, which is rotating with the angular frequency can be obtained as follows

d d d d q

q q q q d

u i i e idL R L

u i i e idt

= + + +

(3)

Equation (3) is the main basis for proposing vector control

strategy.

III. VECTORCONTROL STRATEGY

By placing the d-axis of the rotating coordinates on the

grid voltages synthesis vectors

E ,the Space vector is shown in

Fig.3,wheresU is the output voltages synthesis vector of the

grid-connected inverters

I is the output current synthesis

vector of the grid-connected inverter LV is the inductance

voltage synthesis vector is Power factor Angle. is the

angle betweens

E and a -axis.

Fig.3 Diagram ofSpace vector

The output voltages in the synchronous dq frame areexpressed as

0

d s

q

e E

e

=-

=(4)

where,s

E is the amplitude of voltage synthesis vectors

E

Substituting (4) into (3) gets

d

d d s q

q

q q d

diL Ri u E Li

dt

diL Ri u Li

dt

-+ = +

+ =

(5)

Equation (5) contains the coupling of active currentd

i and

reactive currentq

i . In order to decoupled

i andq

i , assuming

*

1

*

1

d d s q

q q d

u u E Li

u u Li

- = +

= (6)

According to (5) and (6) can obtain

*1

*

1

dd d

q

q q

diL Ri udt

diL Ri u

dt

- + =

+ =

(7)

From (7) it can be seen that decoupling control of active

currentdi and reactive current

qi can be achieved by using the

coordinate transformation and vector orientation.

In order to make the output currents track the reference

current, the PI current controllers can be utilized [13-14] .In

this case, the output voltages of the PI current controllers are

as follows

* * *

1

* * *

1

( ) ( )

( ) ( )

d p d d d d i

q p q q q qi

u K i i K i i dt

u K i i K i i dt

- = +

= +

(8)

The d-axis reference voltage and q-axis reference voltage

are expressed as* *

1

* *

1

d s q d

q d q

u E Li u

u Li u

- = +

= +(9)

From7to9, the voltage, current double closed-

loop control strategy of the grid side converter can be got,

which is shown in Fig.4. The power factor can be controlled

via changing q-axis current. Coupling term are expressed as

dCOMP q s

qCOMP d

u Li E

u Li

= +-

=(10)

One main problem of vector control system is that its

performance depends highly on accurate machine parameters

such as resistance and inductances. Therefore, direct powercontrol (DPC) was proposed

dq abc , ,a b ci i i

,ab ace e

di*

di

qi*qi

g

g*1d

u

*

1qu

*

du

*

qu

dCOMPu

qCOMPu

*u

*

u

PLL

dcu

*

dcu

dcu

Fig.4 Vector control strategy of the grid side converter

IV. DIRECT POWERCONTROL(DPC)

The basic ideas of direct power control (DPC) is that

switching vectors were selected form the optimal switching

table based on instantaneous errors of active and reactive andthe angular position of converter terminal voltage vector.

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Active power and reactive power are controlled by hysteresis

control [15].The hysteresis control law is given as

1

0

P ref P

P ref P

S P P H

S P P H

= >-

= < (11)

1

0

Q ref Q

Q ref Q

S Q Q H

S Q Q H

= >-

= <

(12)

where,P

H Q

H is respectively the hysteresis bandwidth of

active power and reactive power. 1P

S = ( 1Q

S = ) represents

active (reactive) power needs to be increased, 0P

S = ( 0Q

S = )

represents active (reactive) power needs to be reduced,ref

P ,

refQ is respectively a given value for active power and reactive

power.

According to (2) gets

a

diL u e Ri

dtdi

L u e Ridt

-=

=

(13)

Assuming three-phase power grid voltage balance and the

sampling period iss

T .then (13) can be discredited as

( 1) ( ) [ ( ) ( ) ( )]

( 1) ( ) [ ( ) ( ) ( )]

sa

s

Ti i k i k u k e k Ri k

L

Ti i k i k u k e k Ri k

L

- = + =

= + =

(14)

The instantaneous active power and reactive power of thegrid-connected inverter in the stationary frame are

expressed as

P e i e i

Q e i e i

= +-

= (15)

Ignoring the variations of the power grid voltage within aPWM sampling period, the active and reactive power

variations within the two consecutive sampling periods can be

expressed as

( 1) ( )

[ ( 1) ( )] [ ( 1) ( )]

( 1) ( )

[ ( 1) ( )] [ ( 1) ( )]

P P k P k

e i k i k e i k i k

Q Q k Q k

e i k i k e i k i k

= + =-

+ + +

= + = + +

(16)Substituting (14) into (16) and neglecting the voltage drop

across resistance gets

2 2[ ( ) ( ) ( ) ( )] [ ( ) ( )]

[ ( ) ( ) ( ) ( )]

s s

s

T TP e k u k e k u k e k e k

L L

TQ e k u k e k u k

L

- = + +

=

(17)

The impact of different space voltage vector on active

power change and reactive power change can be expressed as

2 2

[ ( ) ( ) ( ) ( )]

[ ( ) ( )] 0,1,...,7.

[ ( ) ( ) ( ) ( )]

si i i

s

si i i

TP e k u k e k u k

L

Te k e k i

L

TQ e k u k e k u k L

- = +

+ =

=

18

where,i

P i

Q represents the variation of active

power(reactive power) when voltage vector iu functions. aiu

i

u represents component of the output voltage

of the grid-connected inverter in the reference frame

when voltage vectori

u functions.

Dividing the two sides of (18) by 2 22

( ) ( )3

s

dc

TU e k e k

L +

gets

2 2

cos sin( ) ( )

2 / 3 2 / 3

( ) ( )0,1,...,7.

2 / 3

cos sin( ) ( )

2 / 3 2 / 3

i i i

dc dc

dc

i i i

dc dc

P u k u kU U

e k e k i

U

Q u k u k U U

- = +

+=

= +

19

The waveforms ofi

P andi

Q Shown in Fig.5.

In order to optimize three-phase grid converter output

voltage vector, the output space is divided into 12 sectors,space voltage vectors and sectors are shown in Fig.6. Where

arctan( / )e e = .

(a) Influence on active power when voltage vectorused iP

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(b) Influence on reactive power when voltage vectorused

iQ

Fig.5 Influence on power when voltage vectorused

7u

0u

2(110)u

3(010)u

5(001)u

6(101)u

1

234

5

6

7

89 10 11

12

1

2

34

5

6

7

8

9 1011

121(100)u4

(011)u

Fig.6 Space voltage and sectors

According to Fig.5 and Fig.6, switching table for DPC can

be given as table.1

TABLE1SWITCHING TABLE FORDPC

PS

QS

1

2

3

4

5

6

10

2u 2u 3u 3u 4u 4u

11u 1u 2u 2u 3u 3u

0

04u 4u 5u 5u 6u 6u

15

u 5

u 6

u 6

u 1

u 1

u

PS

QS

7

8

9

10

11

12

1

05

u 5

u 6

u 6

u 1

u 1

u

14

u 4

u 5

u 5

u 6

u 6

u

00

1u

1u

2u

2u

3u

3u

12

u 2

u 3

u 3

u 4

u 4

u

Schematic ofDPC on the grid side converter is shown inFig.7

,ab ace e

,a bi i

Switching

Table

N

pS

qS

P

*P

Q

*Q

dcu

g

Power Caclculation

Grid angle Calculation

Fig 7 Schematic ofDPC on the grid side converter

V. EXPERIMENT

An experimental hardware platform is developed to

compare the performance of vector control with DPC.

Assuming grid is ideal. Experiment parameters are shown in

table 2.TABLE2 EXPERIMENT PARAMETERS

Parameters Value

Nominal power 3KWN

P =

DC link voltage 200Vdc

U = V

Grid voltage 380VU =

Inductance of AC side 3mHL =

Grid angular frequency 2 314 /f rad s = =

PWM switching period 100ss

T =

The experimentresults for vector control

(a) Power factor is 1 ( iq*=0A)

(b) Reactive power compensation ( iq*=-5A)Fig 8 steady experimental waveforms of

a phase voltage and current forvector control

Fig .9 Dynamic experimental waveforms of

a phase voltage and current for vector control

)The experimentresults for DPC

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(a) Power factor is 1(P*=1000W,Q*=0var)

(b) Reactive power compensation (P*=500W, Q*=750var)

Fig 10 steady experimental waveforms of

a phase voltage and current fordirect power control

Fig.11 Dynamic experimental waveforms fordirect power control

from (P*=1000W, Q*=0var) to (P*=500W, Q*=0var)

Comparing Fig 8 and Fig 10, Fig 9 and Fig 11 can be seen,

whether using vector control or direct power control strategy,

the System maintains perfect performance in steady and

transient conditions. From Fig.8 to Fig.9 show that voltage

vector control strategy has a good sinusoidal current, a small

harmonic component. Comparing Fig 9 with Fig 11,

obviously, DPC results in higher current harmonic distortion

than vector control. But transient performance of DPC is

better.

VI.CONCLUSION

In this paper, both vector control and DPC strategies for

grid side converter were proposed. Comparing Fig.8 withFig.10, Fig 9 with Fig 11, in the same conditions, the steady

state performance of the vector control is better than that of,

DPC. But dynamic response of DPC is better. All in all, both

the quality of the steady-state behavior and the transient

response of vector control and DPC are good, which

established a sound foundation for the next study of doubly

fed wind power generation system.

ACKNOWLEDGMENT

The author wishes to thank the IEEE for providing thistemplate and all colleagues who previously provided technicalsupport

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Vector Control With Direct Power Control for Grid Side Converter of DFIG

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