1

Research of Megawatt Level Variable Speed

Constant Frequency Wind Power Unit Based on
Doubly Fed Induction Generator
Guo Jin-dong

Abstract-- Based on the stator flux linkage oriented control ⎪⎧ud = us

theory of doubly fed induction generator (DFIG), the control ⎨ (2)
system of the megawatt level variable speed constant frequency ⎪⎩ uq = 0
(VSCF) wind turbine generator system, which adopts back-to- us is the grid voltage vector.
back PWM converters, is proposed. The Dual-PWM converter is
divided into two parts: grid side converter and rotor side ⎧⎪ Pg = ud id
⎨ (3)
converter. The grid side converter adopts the voltage and current ⎪⎩Qg = ud iq
dual close-loop control strategy. The rotor side converter adopts
rotor current open-loop control strategy and the active and So that the active and reactive decoupling is realized, Pg is
reactive current close-loop control strategy. The full-load controlled by id, Qg is controlled by iq.
experiment is conducted when DFIG is operating under sub- By the system power factor the reactive power given of the
synchronization or super-synchronization condition. The control total system Q* can be obtained, if the stator reactive power
system can also respectively regulate the active and reactive Qs* has been calculated, the rotor side of reactive power is
power output under variable wind speed condition. The given:
experiment results validate the feasibility of the control systems.
Index Terms-- wind power; variable speed constant frequency; Qg* = Q∗ − Qs* = ud iq (4)
megawatt; doubly fed induction generator; Thus given reactive current iq*can be obtained。
To ensure the active power of the rotor coordinated
I. INTRODUCTION exchange in the two converter there should be Pg= Pr, DC

I n order to improve the utilization of wind energy and reduce

wind power costs, the development of high-capacity and
high-performance megawatt-class wind turbine wind turbine is
voltage udc should be constant values, and the active power is
control by id, when udc change, the active power should be
regulated by id in order to keep the DC voltage stability, so the
an inevitable direction. The VSCF-DFIG units have also output of the closed-loop voltage regulation should be id*.
gradually become the mainstream models of megawatt-class According to the coordinate transformation, d, q-axis
wind power units. The research of VSCF doubly-fed wind component of AC current id, iq meet following formula:
power key technology is of great significance. This paper ⎧⎪ Lg did / dt = − Rg id + ω Lg iq + ud − U d
analysis the VSCF megawatt-class wind turbine control ⎨ (5)
⎪⎩ Lg diq / dt = − Rg iq − ω Lg id + uq − U q
system and divide the system into two parts: grid side
converter and rotor side converter. Ud, Uq are the final input voltage of the converter. From the
above equation, the diagram of the rotor side converter control
II. GRID SIDE CONVERTER CONTROL STRATEGY strategy which used dual voltage ant current closed-loop
In short, the grid side converter is an AC / DC converter strategy is shown in Fig. 1.
id* Ud
that can work either in the rectifier or in the inverter state [1-2]. udc* PI PI
Ud*
dq
Uα
αβ PWM
+ + − id* Uβ
In this system, grid side converter should solve the problem of − iq* − Uq* +
αβ abc Drive
IGBT
PI
the two-way flow of energy and meet the overall reactive udc + − Uq
− +
power of wind turbine units. θ Voltage
angle
The active power and reactive power of grid side converter + calculate L

is as follows: +
ωL g ua
⎧⎪ Pg = ud id + uq iq dq αβ ub
⎨ (1) αβ abc uc
⎪⎩Qg = ud iq + uq id +
ωL g
Grid voltage vector will be integrated on the d-axis id iα
αβ
ia
dq ib
orientation, and then iq
αβ
iβ
abc ic

Grid
Fig. 1 Grid converter control scheme
This work was supported in part by The National High Technology
Research and Development of China (863 Program) (2003AA512022-2).
Guo Jindong is with Institute of Electrical Engineering Chinese Academy
of Sciences, Beijing China (e-mail:jdguo@mail.iee.ac.cn).

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 2

III. ROTOR SIDE CONVERTER CONTROL STRATEGY ⎧uds = 0

⎪
A. Mathematical model of DFIG ⎪uqs = ω1ψ ds = U s
⎪
The mathematical model of DFIG has be introduced in the ⎨udr = Rr idr + Lr didr / dt (11)
literature [3-4], In order to facilitate the discussion, make the ⎪u = L ω i
following assumptions: ① overlook the stator and rotor of ⎪ qr r s dr

high-order harmonic currents and the stator and rotor magnetic ⎪⎩ψ ds = Lm idr
Momentum high harmonics; ② neglected Motor Core The stator voltage control objective is ensuring that the
hysteresis, and eddy current loss of magnetic saturation; ③ stator voltage and the grid voltage are all the same before
rotor parameters converted to the stator side; ④ variables are cutting-in. The stator voltage control diagram is shown in Fig.
in accordance with the direction of motor selection practices. 2.
uar*
Based on d-q coordinate system in synchronous rotation speed ψs* Voltage
udr*
dq
uα*r
αβ ubr* PWM
U/ω1 1/Lm uqr*
of rotation, the motor voltage and flux equation is: idr*
Calc
αβ u*
βr
abc ucr* Driver

⎧uds = Rs ids + Dψ ds − ω1ψ qs ωs θs

⎪
⎪uqs = Rs iqs + Dψ qs + ω1ψ ds
⎨ (6) − ωr
d/dt
⎪udr = Rr idr + Dψ dr − ω sψ qr +
− θr
⎪u = Rr iqr + Dψ qr + ω sψ dr
Encoder

⎩ qr ω1 θ1
+
θ ua
⎧ψ ds = Ls ids + Lm idr d/dt θ−90° Coordin
ate ub
⎪ U transfor uc
⎪ψ qs = Ls iqs + Lm iqr mation

⎨ (7) Grid
⎪ψ dr = Lr idr + Lm ids
Fig. 2 Stator voltage no load cutting-in control scheme
⎪ψ = Lr iqr + Lm iqs
⎩ qr C. Decoupling control of DFIG
Rs,Rr are the stator and rotor winding equivalent resistance; The literature [6-8] introduces the power, reactive power
Ls,Lr,Lm are the d, q-axis, self-inductance and mutual decoupling control strategy of DFIG. In this paper, in order to
inductance of the stator and rotor winding; ids,iqs,idr,iqr are the d, reduce the difficulty of design and realization, the active and
q-axis current of the stator and rotor; uds,uqs,udr,uqr are the d, q- reactive current dual closed-loop control strategy is adopted.
axis voltage of the stator and rotor; ψds,ψqs,ψdr,ψqr are the d, q- The active power and reactive power of the stator of DFIG are
axis flux of the stator and rotor; ω1, ωs are the synchronous as follows:
speed and slip angular velocity; D is the differential operator.
When the stator flux is orientated on the d-axis and ignores ⎧⎪ Ps = Teω1 = − n pω1iqr ( Lmψ s / Ls )
⎨ (12)
the stator windings voltage drop, the flux equation is given: ⎪⎩Qs = n pω1ψ ds ids = n pω1ψ ds (ψ ds − Lm idr ) / Ls
⎧⎪ψ ds = ψ s = Ls ids + Lm idr ⇒ ids = (ψ s − Lm idr ) / Ls
⎨ (8) By the formula (12) we can see that generator stator active
⎪⎩ψ qs = 0 = Ls iqs + Lm iqr ⇒ iqs = − Lm iqr / Ls power and reactive power has the linear relationship with the
And the voltage equation is simplified: rotor torque current component and excitation component. By
adjusting rotor current, the stator active power and reactive
⎧uds = 0 power can be independent controlled, that is the decoupling
⎪
⎪uqs = ωψ1 ds = Us control.
⎪⎪ Rotor-voltage equation can be achieved by formula (9):
L2m d idr L2 L dψ
⎨udr = Rr idr + (Lr − ) − (Lr − m )ωs iqr + m s (9 ⎧ L2m didr L2
⎪ Ls dt Ls Ls dt ⎪udr = Rr idr + (Lr − ) − (Lr − m )ωsiqr
⎪ ⎪ Ls dt Ls
L di
2
L2
Lψ ⎨ (13)
⎪uqr = Rr iqr + (Lr − m ) qr + (Lr − m )ωsidr + m s ωs
⎪u = R i + (L − Lm ) diqr + (L − Lm )ω i + Lmψ s ω
2 2
⎪⎩ Ls dt Ls Ls
⎪ qr r qr r
Ls dt
r
Ls
s dr
Ls
s
) ⎩
Torque equation: The reactive power control decoupling control diagram of
Te = −(n p Lm / Ls )ψ s iqr (10) VSCF-DFIG is shown in Fig. 3.

B. Cutting-in control strategy of DFIG

In this paper, rotor current open-loop control strategy is
adopted [5].
When the stator of DFIG is no-load, the d, q-axis stator
current is zero; so the voltage equation is simplified.

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 3

− idr
idr* + Voltage udr* uar*
current waveforms when DFIG is in full power operation
PI uα*r
iqr*
compen
sation
uqr* dq
u *
βr
αβ ubr* PWM (1.5MW).
PI strategy αβ abc ucr* Driver Table 1 1.5MW EXPERIMENTAL DATA TABLE
+
− iqr
θs idr Current
iar ωr* is Ps Cosφ ir Pr PT
iqr Measur ibr
ement
− ωr 1200 493 564 1 198 130 434
d/dt
ωs + 1400 666 753 1 248 55 698
− θr Encoder
1500 774 849 0.99 257 29 820
ω1 θ1 + ua
θ
d/dt θ−90° Coordi
ub
1600 815 942 1 292 53 995
nate
transfor uc 1700 950 1110 1 317 151 1260
mation
Grid 1800 1040 1237 1 360 240 1477

Fig. 3 Rotor converter decoupled control scheme 2000 952 1133 1 328 373 1506
ωr* is the reference speed (rpm); is is the stator current (A), ir
IV. EXPERIMENTAL RESULTS is the rotor current (A); Ps is the stator output power (kW), Pr
To verify the effectiveness of the control system, the is the rotor power (kW), PT is the total system power (kW);
functional experiment of the megawatt-class VSCF-DFIG Cosφ is stator power Factor. From the data in Table 1, we can
control system is completed. see that the active and reactive power decoupling control of
The parameters of DFIG are as follows: rated power rotor side converter has ideal performance.
1.5MW, rated frequency 50Hz, stator connectionΔ; stator
resistance 0.008Ω, stator inductor 15.86mH; rotor connection V. CONCLUSIONS
Y, rotor resistance 0.0188Ω; rotor inductor 16.2 mH, mutual For depth study of design and experience optimization of
inductance 15.66mH, the rotor parameters are all converted to MW-class VSCF-DFIG control technology, this paper put
the side of the stator; grid parameter is 690V, 50Hz. forwards the control system of MW VSCF wind power
generating units by use of dual-PWM converter structure, and
DC Voltage1 100V the full power experiment has been carried out. The
500V/grid,200A/grid

experiment results shows that the control system is sample,

and has clear principles, at the same time the experiment also
verifies the feasibility of the control system.

VI. REFERENCES
Phase cuttent 213A
line voltage 690V
[1] R.Wu, S.B.Dewan , G.R.Slemon. A PWM AC-to-DC converter with fixed
10ms/grid
switching frequency[C].IEEE Transactions on Industry Applications,,
Fig. 4 Rotor converter input voltage current
and DC bus voltage vol.26(5) ,pp:880- 885,1990.
Fig.4 shows grid side converter input voltage and current [2] L M Malesani, L Rossetto, P Tomasin. AC/DC/AC PWM converter with
and DC bus voltage waveforms when DFIG is in full power reduced energy storage in the DC link[J].IEEE Trans.Ind.Applicat.,
operation. vol.31(2),pp:287-292,1995.
[3] R Pena, J C Clare, G M Asher. Doubly fed induction generator using back-to-
GirdVoltage Stator Voltage
500V/grid,100A/grid

back PWM converters and its application to variable speed wind-energy

generation[J].IEE Proc., Electr. Power Appl.,vol.143 (3),pp:231-241,1996.
[4] J B Ekanayake, L Holdsworth, X G Wu, et al, Dynamic modeling of doubly
fed induction generator wind turbines[J].IEEE Trans.Power Syst.,
vol.18(2),pp:803-809, 2003.
[5] Zhao Dongli,Xu Honghua,Zhao Bin, et al.Study on the cut-in voltage control of
stator cuttent
a variable-speed constant-frequency doubly-fed induction generator[J].ACTA
Fig. 5 Grid voltage, stator voltage and stator current (cutting-in) Energiae Solaris Sinica, vol.25(5),pp:587-5912004 (in Chinese).
Stator Voltage [6] P Cartwright, L Holdsworth, J B Ekanayake,et al. Co-ordinated voltage control
500V/grid,500A/grid

strategy for a doublyfed induction generator (DFIG)-based wind farm[J].IEE

Proc.Gener.Transm.Distrib., vol.151(4),pp:495-502.,2004.
[7] Tang Y,Xu L.A flexible active and reactive power control strategy for a
variable speed constant frequency generating system[J].IEE Trans.Power
Electron.,vol.10 (4),pp:472-478, 1995.
stator cuttent
[8] Tang Y, Xu L.Stator field oriented control of doubly-excited induction
machine in wind power generating system[C].IEEE 35th Midwest
Fig. 6 stator current and voltage (1.5MW) Symp.Circuits and Systems, Washington DC, 1992.
Fig.5 shows the stator voltage and current waveforms
before and after cutting-in. Fig. 6 shows the stator voltage and

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VII. BIOGRAPHIES

Guo Jindong was born on June 2, 1981. He graduated from Graduate School
of the Chinese Academy of Sciences and received Ph.D degree.
He is employed in Institute of Electrical Engineering Chinese Academy of
Sciences now, and his research direction is the power electronics and electric
power transmission. His special fields of interest includes wind power
technologies and power electronics converters.

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