20th Iranian Conference on Electrical Engineering, (ICEE2012), May 15-17, Tehran, Iran

A Novel Direct Power Control Strategy for Integrated DFIG/Active

Filter System

Mohannnad lafar Zandzadeh, Abolfazl Vahedi, Alireza Zohoori Centre of Excellence for Power System Automation and Operation

Iran University of Science and Technology, Tehran, Iran mjzandzadeh@elec.iust.ac.ir, alirezazohouri@iust.ac.ir, avahedi@iust.ac.ir

Abstract: In this paper a novel method for controlling DFJG based wind turbine is proposed in order to compensate the most prominent harmonics of the utility grid in addition with providing required network active and reactive power. This harmonics elimination provide better power quality and power capturing of wind energy fluctuating wind speed concurrently. Direct power control (DPC) is applied to control DFIG by employing voltage space vectors of rotor side converter (RSC) using optimum switching table based on stator flux position and active and reactive power states. Distorted active and reactive power of the nonlinear load is measured. Then they are added to active and reactive power reference of DFIG respectively for harmonics compensation. Simulation results on a 2 MW DFIG demonstrate robust, precise, and fast dynamic behaviour of the machine.

Keywords: DFIG, DPC, Harmonic mitigation, Active Filter.

1. Introduction

Recently variable speed wind turbines using DFIG have been prompting more interest than constant speed systems Due to their improved dynamic behaviour [1] . The most advantage of these turbines is rating of the converter which is around 25-35% of the turbine rated power. Moreover cost, size and weight collaborated with a small converter are lower and losses are smaller than systems in which converter is connected to the stator [2] . In this system stator is directly connected to grid as it is shown in Fig l. One of the most common methods in controlling DFIG is vector control in which rotor currents are decoupled into stator active power (or torque) and reactive power (or flux). Control of these two currents take place in the reference frame fixed to stator flux (or voltage) [3] -[6] . This method needs the exact value of machine parameters such as resistances and inductances. The nonlinear operation of converter for regulating current controllers is not considered. So performance of vector control method is affected by changing machine parameters and operation condition.

978-1-4673-1148-9112/$3l.00 2012 IEEE 564

Fig. I: A figure fitted in a column

Direct torque control (DTC) of induction machine drives was developed in the mid 1980s [1] . DTC is based on decoupled torque and flux control that have very fast and accurate dynamic without using inner control loop. In [8-9] DTC is used to control DFIG where the rotor flux is estimated and an optimal switching table is used based on rotor flux position .. In [1] , [7] , [10] direct power control (DPC) is developed in order to control the DFIG. The drawback of this method is that the variation of slip frequency affects on rotor resistance value and subsequently on estimation of rotor flux [7] . Hence in this paper a DPC strategy based on stator flux position detection is employed as a solution of the mentioned drawbacks. The existence of nonlinear loads decreases power quality in power systems hence installing active filter for power quality improvement seems necessary which impose an additional cost. While the DGs can be used to enhance the power quality of the system and significant reduction in the additional cost. Reference [11] proposed a control strategy for grid connected DC-AC converters with load power factor correction. Macken et al. studied the compensation of distorted currents through multiple converter-interfaced renewable generation units [12] . Many researches have been performed based on employing additional control plan in DFIG control systems in order to achieve active filtering. A sensorless

field oriented control of an integrated electric alternator capable of controlling the amount of harmonic compensation is presented in [13] , [14] . In [15] vector control is used for harmonic compensation using the rotor current in d-q excitation reference frame. To the latest knowledge of the author all of the researches that have been carried out on DFIG are based on vector control method. In this paper a new DPe control strategy is employed which has the following advantages compared with the ye. The employed DPe strategy can provide simultaneous generation of optimized green power moreover improving power quality by mitigation of the most prominent and annoying harmonics of the utility lines.

2. Direct power control of the DFIG

2. 1 DFIG dynamic model in rotor reference model DFIG equations in the rotor reference frame can be reached as follow:

r _ r dlf/;' . r Vs - R,ls +Tt+ JOVf/, (I)

r _ R r dlf/; (2) vr - rlr + dt If< = LJ + LlJJ (3)

If/; = LJ; + Lmi; (4) According to (3) and (4) stator current, i can be calculated from:

i' = If/;' _ Lmlf/;' (5) s (}L, (}L,Lr where (} = 1- L / LsLr is the leakage inductance. The

stator input active power from network and stator output reactive power can be written as [7] .

P 3 r r .. =-V . . . I .. . , 2 " .,

Q 3 r .r ,. =--V,X/,. . 2 "

(6)

(7)

Since 11f/: 1=1 '1< 1= e te , stator flux in rotor reference frame can be expressed as: IIfr = Ifr" -jOJ,J =I"r" 1 JOJJ -)OJ,I =I lffr 1 ej(OJ,-OJ,)1 'f's 'f's e 'f's e e 'f's Hence the dlf/; term in (1) can be obtained by:

dt

(8)

dlf/; C ) r (9) dt=J ms-mr If/, .

Substituting (l), (5) and (9) into (6) and (7) and neglecting stator windings resistance result in:

P _ 3m, I r II r I

. (10) s - --- If/, If/r sm r 2(}L,

Q _ 3m, I r I Lm I r I I r I) (11) s --- If/s (- If/r cos Y- If/, 2(}L, Lr where y is the angle between the rotor and the stator flux space vectors.

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2.2 Effect of converter voltage vectors on stator active and reactive powers Equations (10) and (11) reveal that tuning the amplitude of stator and rotor flux space vectors along with angle between them can control the stator active and reactive powers. A derivative from (10) and (11) yields: dP,=_3ms I rl

dCIIf/;'lsinr) (12) dt 2 L If/s dt lhs

dQ, = 3m, I r I dCIIf/;'lcosr) (13) dt 2(}L, If/, dt The above equation depicts that changing 11f/; I sin rand, 11f/; I cos r affect on stator active and reactive power. As it is shown in Fig 2, 11f/;' I cos rand 11f/;' I sin r are rotor flux components in same and vertical direction of stator flux vector respectively. Rotor converter output voltage space vectors in rotor reference frame for a two level converter is depicted in Fig. 3. It can be divided in zero voltage vectors (Va and V7) and active voltage vectors (Vi -V6)' The effect of six active vectors on rotor flux and consequently on stator input active and output reactive power in each sector based on aforesaid analysis can be summarized as Table 1. As shown each voltage vector increase or decrease active and reactive power and two of them in first half sector increase and in second half sector decrease active and reactive power or vice versa (for example V and Vi in first sector).

1 Isinr

Fig,2. Stator and rotor flux vectors in rotor reference frame

--,/ I ..........

/ 3 I 2"'" I "\ 1. V3(010) I V,(110) ,."

I ................ I

,... \ ............. I ".,// \

I 4 V,(OlD" I ,." V1(100) 1 VO(OOO).., __ -; ,. I "" I \ V7 (111 ,. ,. I " " " I \-,." V5(OOl) : Vo(lOl)"" i "\ 5 1 6 /

"'" I ,/ .......... I _ Fig.3. Rotor converter output voltage space vectors for a two level converter

TABLE I: Eflect Of Voltage Space Vectors On Stator Input Active Power And Output Reactive Power In K'h Sector.

Voltage Vk Vk.l Vk+l V"-2 Vk Vk+2 Vk+J vector P, It t t t H t H Q, t t t t t t t

2.3 Proposed Active Filtering Analysis Assuming the grid voltage sinusoidal the PCC voltage and current of the nonlinear load can be obtained in stationary reference frame as: vLp = cosOJJ (14)

vLp = cos OJJ (15)

iux = 11.1 sin(wJ + fPl) + L=/Ih sin(hwJ + fPh) (16)

iLp = ILl cos(wJ + fPl) + L=2 I Lh cos(hwJ + fPh) (17) Where V; and OJs are amplitude and frequency of the network voltage respectively. I Ll and I Lh are amplitude of fundamental component and hth is the harmonic of nonlinear load current. Active and reactive power of the nonlinear load can be calculated as follows:

3 PL = -(v LaiLa + vLfJiLfJ) 2 3 . . Q, = 2(v/fJl,a -v,al/fJ)

Substituting (14-17) in (IS) and (19) yields:

P, = % VI I,.] cos rp] + % 2.::=2 V] I,.h cos((h -I)m,t + rph)

(IS)

(19)

(20)

(21 )

So nonlinear load active and reactive power can be divided into a constant term (PLl and QL1 ) and oscillating term (Prh and Qlh) : PL =PLl +PLh Qr =Qn + Qr.h Where:

PLI = 'i V; ILl cos rpl 2 (IS)

(19)

(20)

(21 )

Therefore for harmonic compensation of the nonlinear load current, DFIG must provide the oscillating terms of active and reactive power in addition to required power for delivering to the grid. It should be mentioned that for power factor improvement it is essential to compensate the constant term of the reactive power generated by the nonlinear load, hence the reference power of DFIG is achieved by:

Q.: = Qrequired + QI, (22)

(23)

Where Prequired and Qreqllired are required active and reactive power must be injected to the grid. Prequired is

566

determined according to maximum power tracking of the wind. [3]

2.4 Control Strategy As specified in last section with stator flux orientation and selecting appropriate rotor voltage vector, instantaneous stator active and reactive powers can be changed in desired direction so it is needed to detect stator flux position and instantaneous active and reactive power values. For this propose, power mismatch is crossed to two three level hysteresis comparators to generate active and reactive power states (el' and eq) as shown in Fig 4. An optimum switching table based on table I and what mentioned in past section is used which is shown in Table 2. In the case of el'=eq=O zero voltage vectors are arranged alternatively for changing only one leg of converter to reduce switching frequency.

3. Simulation results

The simulated system parameters are shown in table 3. The simulation is performed by Matlab/Simulink software. As it can be seen from Fig.1 the simulated system include a local load with l.S and 0.5 MYA active and reactive power respectively, in addition to a nonlinear load. It is assumed that DFIG generates 1.5 MW active powers along with 0.4 MY AR reactive power while the grid provide remaining demand active power.

At First, the acts of filtering is not provided by DFIG then at t=.05 s FLh and QJ.h are added to the reference power for harmonic mitigation. It can be concluded from Fig.5 that at t=0.5s when the harmonic mitigation begins, the grid current become more sinusoidal that approves the effectiveness ofDPC method.

Nonlinear Load * Power Calculator Fig.4. block diagram of appJied DPe strategy for DFIG

Q,

TABLE-2 Optimal Switching Table For Direct Power Control Of DFIG eq -I 0 I ep -I 0 I -I 0 I -I 0 I

I VJ V. Vs V1 Vo Vs V, VI V6 2 V. Vs V6 V. V7 V6 V3 V, VI

Stator 3 Vs Vo VI Vs Vo VI V. V1 V, flux

position 4 V6 VI V, Vo V7 V2 Vs V. VJ 5 VI V2 VJ VI Vo V1 V6 Vs V.

6 V2 VJ V. V2 V7 V. VI V6 Vs

T ABLE-3 Parameters Of Simulated System Rated power 2 MW Stator voltage 690 V

Statorlrotor tum ratio OJ R, 0.0108 pu

Generator R, 0.0121 pu Lm 3J62 pu Lis 0.102 pu L" 0.11 pu H 0.5 s

Number of pole pairs 2 DC link voltage 1200 V

Converter DC link capacitor 16 mF Grid side inductance O.4 mH

Fig.6 illustrates that after t=0.5 DFIG injects the distorted power into the grid; consequently the grid power distortion will be eliminated. This is a significant proof of the appropriate performance of the proposed DPC method. Rotor current is shown in Fig.7. As DFTG compensates nonlinear load distorted power, some low distortions are produced in the rotor current that can be neglected. The behavior of the system during nonlinear load change is investigated in Fig.S and Fig.9. It can be understood from Fig. 9 that after a very short transient status active power injected by the DFIG remain l.5 MW. The extra active power needed for load is supplied by grid and only the harmonic term of the generated active power ofDFTG is increased for current harmonic restoration. As it can be seen the DFTG shows suitable, fast and precise dynamic performance during nonlinear load change.

g 0.45 0.46 0.47 0.48 0.49 0.5 0.51 0.52 0.53 0.54 0.55 2000

D'200: 0.45 0.46 0.47 0.48 0.49 0.5 0.51 0.52 0.53 0.54 0.55

c::: 0.45 0.46 0.47 0.48 0.49 0.5 0.51 0.52 0.53 0.54 0.55 time (5)

Fig.5 a. nonlinear load current, b. stator current of DFIG, c. grid current

567

'Ee o, - . ====:1 0.4 60.42 0.44 0.46 0.48 0.5 0.52 0.54 0.56 0.58 0.6

8 ; e,," : 0.4 0.42 0.44 0.46 0.48 O.S 0.S2 0.S4 0.S6 0.58 0.6 time (s)

Fig.6 a. active and reactive power of DFIG, b active and reactive power of grid

1000

SOD

j SO: 1 OOL.4 -0 .4CC 2-0 .4:-C 4-0 .4-:- 6 ----:o .4c:- 8 -0.S=----=O. SC:- 2 ----::0 .S4=---c0 .S6=---::-0.S8:----='0.6

Fig.7 rotor current of DFIG

500

.50: 0.55 0.6 0.65

2000

g .200:

]&x2Zx%&>&X2z;j' 0.55 0.6 0.65

time (5) Fig.8 a. nonlinear load current, b. stator current of DFIG, c. grid current During nonlinear load change

X10C o rrmmm:""""m'rrr""'.rrlY.rrlY''''' .. '''rrlY'rr'''''''''l

1 L------::-":-::-----c:_':_::__-=__-___=_c=_- 0.5 c 0.55 0.6 0.65 0.7 0.75 0.8 'f : ': : '" :: : 0.5 0.55 0.6 0.65 0.7 0.75 0.8

Time Fig.9 a. active and reactive power generated by DFIG during nonlinear load changeb. active and reactive power of the grid during nonlinear load change

4. Conclusion The proposed DPC method mitigates harmonic components of the network current with a high accuracy. Furthermore based on the simulation results, it can be concluded that applied DPC strategy for controlling the

DFTG has fast, precise response to any changes. Tn addition stator and rotor currents have a low THD by mitigation of the most prominent and annoymg harmonics of the utility lines. Because of low volume calculation that is applied in this method, implementation of this strategy is much easier that others such as vector control. Therefore the proposed method can provide

simultaneous generation of optimized green power moreover improving power quality.

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