CTv140S862012001.pdf

TAP CHi KHOA HQC I& CONG NGHE CAC TRU'ONG hAHIQC KY TEIUAT * .St^ 86-2012

SYNCHRONIZATION AND POWER FLOW CONTROL OF DFIG FOR WIND ENERGY SYSTEM DIEU KHIEN HOA DONG BQ VA DONG CONG SUAT CUA DFIG

TRONG HE THONG N A N G H 'ONCi GIO

Phan Quoc Dzung, i\guyen Bao Anh, Le Minit Phuong, Le Dinh Khoa Ha Chi Minh University ofTevlinology

Received October 31 "•'. lU 11 TOM TAT

Bai bao trinh bay cac thuat toan d4 diSu khi4n bo bi§n doi phia rotor (RSC) cua he thong may phat cam t/ng ngudn k^p (DFIG) bao gdm giai thu$t dieu khiSn tru-c ti4p cdng suat (DPC) de di&u khiSn dong cong suit va giai thu$t dtiu khien (rue tiep moment do (DVTC) de hda ddng bd DFIG vao twoi. Trong ci hai giai thuat tren, cdc khdi diSu chinh ti le • tich phan d6u khong dux?c siy dung Ban chat cua hai thuat toan la chon Iwa vector dien ap thich hap clio bd bi4n doi phia rotor K4t qua thiK nghiem tren mdy 1,5 kW duxyc trinh bay, su dijng phan mdm MATLAB/SiMUUNK va vi di6u khiSn DSpace 1103.

ABSTRACT

This paper presents algorithms for controlling rotor-side converter (RSC) of a double-fed induction generator (DFIG) system including direct power control (DPC) strategy for controlling power flow and direct virtual torque control (DVTC) strategy for synchronizing DFIG with grid. There is no proportional-integral (PI) regulator in both of strategies. The essence of two strategies is selection of appropnate voltage vectors on the rotor-side converter. The experimental results on a 1 5'kW machine are explained using MA TLAB/SIMULINK together DSpace 1103.

Keyword Direct Power Control (DPC), Direct Virtual Torque Control (DVTC), Direct Torque Control (DTC), Artificial Neural Network (ANN), Doubly-Fed Induction Generator (DFIG), Rotor-side converter (RSC), Gnd-side converter (GSC)

Nomenclature vg (grid voltage), is (stator current), ir (rotor current), 9r (rotor angle) H'g 4's (grid and stator flux), 4^9 Tv (rotor flux and virtual torque), Ps Qs (active and reactive power), Lm (mutual inductance) Ls Lr (stator and rotor inductance), Rs Rr (stator and rotor resistance), p (pole pairs), w (frame speed), wr (rotor speed), tos (synchronous speed)

I. INTRODUCTION turbines have many advantages when compared . , _ , . , , to wind turbines using fixed speed induct''^"

In recent \cars. fossil fuel is exploited and used more and more, this leads to the

generatoib, which are variable ;.peed operation.

, four-quadrant active and reactive power exhaustion of fossd fuel and the a.r pol ution by ^apabiliues, lower converter costs and louer greenhouse gas. So, human are researching and |^^^^^ [,j ^ ,<,|,„^^,i, ji3„,,„, „f , developing new technologies to use the types of DpiQ.based wind enerav a^ncrauon" system h renewable energy. Among the types of shown in Figure 1. "' "

renewable energy, wind energy is applied in large power plans such as large wind farms.

Wind turbine technology can be divided (icarx>\ (. (,.|^

into fixed speed category (generally with

squirrel-cage induction generator) and variable WiiuliuibiiK

speed one (with doubly-fed induction (n.'iicraiiir MICIIIIK-MVIC d ,^\-.\\.k generator). For many wind farms, wind turbines annciicr i.iiii\jrii.'r based on doubly-fed induction generator

(DFIG) with converters rated at 25% - 30% of l^ig /- Schematic diagram of a DFlG-hase.

the generator rating are used. DFIC-based wind windener^generation

TAB'CHi KHOA HQC & c 6 N G N G H t : CAC T K U ' O N G D A H I Q C K V T H U ^ T * S 6 86-2012 A DFIG has a wounded rotor which is

connected to grid through a back-to-back converter, and the stator is connected to grid directly. Controlling DFIG is implemented through controlling back-to-back converter, thai is rotor-side converter (RSC) and grid-side converter (GSC). Objectives are good responses of current, voltage and power.

n. MATHEMATICAL MODEL OF DFIG Equivalent circuit of DFIG is shown in Figure 2

i / y)\\lJ l.n. Lr.r i(V)-i<),)\\l,' •.•

R. Ltn R,

Fig 2. Equivalent circuit of DFIG In a frame which rotates at a speed of to, voltage equation of stator and rotor are

' dt dy/^

~dt~

Flux equation of stator and rotor are V J = ^asK + ^n,i'' + 'r ) = k's + AnC

And mechanical equations of DFIG are L=-P¥sX'.

(1) (2)

(3) (4)

(5) p dt (6)

III. DVTC ALGORITHM

When the stator is connected to grid which has a constant magnitude and frequency of voltage, voltage vectors are applied to the RSC, the algorithm is called DTC algorithm.

According to (5), the expression depicts the electromagnetic torque is shown In equation (7), torque is a function of stator and rotor flux and the angle between them. So the torque can be controlled through the control of magnitude and angle y of rotor flux and stator flux [4]

':^yM-

' 2' alL ⁽⁷⁾

When the stator is about lo connect to grid, there is no current on stator. According (5), electromagnetic torque is equal zero and the stator flux vector is collinear to the rotor flux.

So, a virtual torque is defined as given in equation (8) [3] and this algorithm is called DVTC algorithm.

'V,nj(//^ sin5 (8)

The variation of Ihc rotor flux vector is achieved through the application of the appropriate rotor voltage vector as (9)

A4'; (9)

The rotor flux vector rotates in the same direction as the rotor voltage and with a rotation speed proportional to the rotor voltage magnitude as shown in Figure 3.

Fig. 3. Rotor flux vector and rotor \oltage vector

DVTC algorithm is implemented as a look-up table as shown in Table 1 that generates the switching states of the power semiconductors corresponding to the appropriate rotor voltage vector. The choice of rotor voltage vector depends on fogic outputs of rotor flux and torque hysteresis controllers and on the position of rotor flux vector in rotor plane divided into six sectors.

Tabic I. Optimal switching table for DVTC algorithm

N S,,= l

S,-0 S i - I S i - 0 S i - I L _ ? - ' 1

1 V,.

v»

V , i

v,<

2 3 V,] V,j Vrl V,, V r t V *

v„

v„

4

V r t

v„

V,.

V,.

5

V,6 V,4 V r l

6

V , i

V„ v,>

V,3 1 V^

T^P CHi KHOA HQC & CONG NGHE CAC TRU'dNG OAI HQC KY THUAT * S6 86-2012 Connection of DFIG to grid requires the

adjustment of stator voltage to be synchronized with the grid voltage. The objecdve of RSC control is meeting the synchronization conditions, these conditions are equality in phase, frequency and magnitude between tlie grid and stator voltage vector. [3]

The first two synchronization conditions can be met through bringing to collinearity grid flux and rotor flux vectors b> eliminating the angle 5 between them. Other-word, virtual torque will be controlled so that it is equal to zero.

The third synchronization condition is equality in magnitude between grid voltage and stator voltage. The amplitude of stator voltage is indirectly controlled through adjusting the rotor flux. An appropriate rotor flux reference as (10) is calculated and used to control rotor flux in order to guarantee the required stator voltage amplitude.

rr.

⁽¹⁰⁾

The overall control strategy scheme is shown in Figure 4.

Fig. 4. DVTC .scheme for DFIG grid connection IV. DPC ALGORITHM

In stator grid-connected mode, DFIG exchanges power with grid. If stator resistance can be neglected, the stator active and reactive power can be expressed as (11) and (12) [ 1 ]

3 co,L„

laL^L, 3 w, I ,\,L,

,(^ It// isin6

^ k | ( f | v . : i c o s ^ - | < | )

(11)

(12)

According to (10) and (II), active power and reactive power can be controlled by adjusting v',' sin^ and L H C O S ^ . They are components of rotor flux vector at the perpendicular and ihe same direction of the stator flux respectively as shown in Figure 5

Fig 5. Relation of .tlator and rotor flux in slalionary and rotor reference frame

Similarly DVTC algorithm, two above components of rotor flux can be controlled through selecting the appropriate rotor vector voltage.

States of active power and reactive power which are calculated by two 3-level hysteresis comparator and the position of stator flux in rotor reference fi^me are used to determine the appropnate rotor voltage vector. An optimal switching table is given in Table 2.

Tahie 2. Optirral switching table for DPC algorithm

N Sy=l

Sy=0

Sy=-I S,.= l Sp=0 S,.=-l Sp=l Sp=0 S | > - l Sp=l Sp=0 Sp=-I

1 Vre V,i V,-.

V,s Vrf.

Vrl V n Vri V,3

2 Vrl

v,^

Vr3 Vr^

Vr7 V , j

v^ v^

V,4 3 V , ; Vr-, V , j V,i Vrt, V n Vrl Vrf, Vr^

4 V,5 Vr4 V r i V r ' V,7 Vrt, Vrt Vrl

v„,

5 Vr4 V,s Vrt Vr-, Vrt, Vrl Vr, V,t V,i

6 V , . V.fr V,i Vr4 V,7 " V,-. V , , V,3 V , :

The overall control strategy scheme is shown in Figure 6.

:- P5

Fig 6. DPC scheme for DFIG grid connertion

T ^ P CHJ KHOA HQC & CONG NGHf. CAC TIUI'6rNC DA,\ HQC KY THUAT * S 6 86 - 2012

V. EXPERIMENTAL RESULTS An experimental model will verify two above algorithms. That model consists power components, processor and support circuits.

Algorithms will be implemented on DSpace 1103 which connect to computer through MATLAB/SIMULINK. software. The emulation of wind turbine is done by means of a 2 2 -kW/1500-rpm induction motor. The DFIG is a I 5-kW;i500-rpm wound rotor induction machine. The RSC are IGBT-bascd converter. Hall-effect sensors are used lo measure current (HX 20-P) and voltage ([,V25- P)-

DFIG and IM parameters are sliouii in Table 3.

Table 3 Parameters of DFIG and IM Rated power

Rated stator \ ollasc Rated rotor \ ollaue Rated stator frequency Nominal speed Rated stator current Poles pair

DFIU l.SkW 415 V 145 V 50 Hz 3.7 A 2

IM 2.2 kW 380 V 50llz 1435 rpm 5 A 2 The overall experimental model is shown in Figure 7

Fig. 7. Experimental model of DFIG ywlcm The rotor shaft operates at speed of 1350 rpm (10% slip), four cases are examined in ihis experimental model. They include:

1) Case .study I: synchronizing DFIG with grid

2) Case study 2. distributing aclixe power P = 1OOOW

3) Case study 3: distributing active power P = 500W and reactive power Q ^ 500 Var

The responses of three cases are shown in following figures

i . ' J :

!

Ji ....

(c)

Fig. S. Responses of .system in case I (a) Grid (red) and Slalor (green) line-to-line voltage

(b) Reference (red) and real rotor flux (green) (c) Virtual torque

\ . CONCLUSION

These algorithms used to operate the DFIG system have been proposed in this paper.

Both of algorithms have simple structure because there is no need of PI regulator, thus problems related lo parameter tuning and machine parameter dependence are eliminated.

For DPC algorithm, the only machine parameter required to control system is stator resistance which has small effect on responses of algorithm and can be neglected. The method selects appropnate \oltage vectors based on the stator flux position and active and reactive power errors, thus the difficulties associated with rotor flux estimation are removed.

T^P CHI KHOA HQC •& CONG NGHg CAC TRUING Oi^l HQC KY THU^T * S6 86-2012 The experimental model gives good

responses as shown in above figures. The results are similar to-tlie reference signal, and it

K,i

proves that both of algorithms are suitable for controlling DFIG system.

Jaf Stator voltage and current^ (aj^ Sfator voltage and current

0^

(b) Stator power Fig. 9. Responses of system in case 2

(b) Stator power Fig. 10. Responses of system in case 3 REFERENCES

1. Lie Xu, Phillip Cartwright, "Direct Active and Reactive Power Control of DFIG for Wind Energy Generation", IEEE Transactions on Energy' Conversion, Vol 21, No.3. September 2006 2. Rajib Datta, V.T. Ranganathan, "Direct Power Control of Grid-connected Wound Rotor Induction

Machine Without Rotor Position Sensors", IEEE Transactions on Power Electronics, Vol, 16, No.3, May 200!

3. Jihen Arbi, Manel Jebali-Ben Ghorbal, llhem Slama-Belkhodja, Lotfi Charaabi, "Direct Virtual Torque Control for Doubly Fed Induction Generator Grid Connection", IEEE Transactions on Industrial Electronics, Vol. 56, No 10, October 2009

4. Z. Mahi, C. Serban, H.Siguerdidjane, "Direct Torque Control of a Doubly Fed Induction Generator of a Variable Speed Wind Turbine Power Regulation", European Wind Energy Conference, Milan, May 2007

5. S. Muller, M. Deicke, Rik W. De Doncker, "Doubly Fed Induction Generator System for Wind Turbines", IEEE Industiy Application Magazine, May/June 2002

6. C. Belfedal, S.Moreau, G.Champenois, T..AIlaoui, M.Denai, "Comparison of PI and Direct Power Control with SVM of Doubly Fed Induction Generator", Journal of Electrical & Electronics Engineering. Istanbul University, 2008

7. Luna A, Lima F.K..A, Rodriguez P, Watanabe E.H, Teodorescu R, 'Comparison of Power Control Strategies for DFIG Wind Turbines", Proceedings of The 34th Annual Conference of the IEEE Industrial Electronics Society. lECON 200S. IEEE. 2008

8. F. Poitiers, M. Machmoum, R. Le Doeuff, M.E. Zaim, "Control of a Doubly Fed Induction Generator for Wind Energy Conversion Systems", Australian Universities Power Engineering Conference, 2001

9. D. Aouzellag, K. Ghedamsi, E.M. Berkouk, "Power Control of a Variable Speed Wind Turbine Driving a DFIG", International Conference on Renewable Energies and Power Quality. 2006 Author's address.'Phan Quoc Dzung - Tel: 0903657486 - Email: phan_quoc_dung@yahoo com

Faculty of Electrical- Electronics Engineering Ho Chi Minh University of Technology