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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
[email protected], [email protected],
[email protected]
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
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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
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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,
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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
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simultaneous generation of optimized green power moreover
improving power quality.
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