grid connection control of dfig based on pscad
TRANSCRIPT
Grid Connection Control of DFIG Wind Power Generation Based on PSCAD
Jie Zhou,Yongfeng Ren, Hanshan Li and Zhihe Wang College of Information Engineering
Inner Mongolia University of Technology Hohhot, China
Zhongquan An, Jinguo Liu, Hongbin Hu Inner Mongolia Electric Power Research Institute
Hohhot, China [email protected]
Abstract—The stator flux-oriented vector control theory is adopted for megawatt double-fed induction generator (DFIG). Rotor excitation current is regulated according to grid voltage and rotor speed to realize no-load grid connection of DFIG. The control strategy is switched after grid connection successfully to realize maximum power point tracking (MPPT) and power decoupled control. The control of the rotor-side converter adopts voltage outer loop and current inner loop before grid connection, power outer loop and current inner loop after grid connection. The variable speed constant frequency (VSCF) wind power generation system is developed based on PSCAD/EMTDC. The effectiveness of the proposed strategy is validated by simulation results.
Keywords-double-fed induction generator (DFIG); grid connection control; maximum power point tracking (MPPT); decoupled control; simulation
I. INTRODUCTION Wind energy is a kind of clean and renewable source
which has caught great attention from various countries in the world. The variable speed constant frequency (VSCF) double fed induction generator (DFIG) has many advantages such as variable speed generation, four-quadrant power flow control, improved power quality, handling a fraction (20–30%) of the total system power and it has come into wide use in wind power system [1-3].
Wind power generation connected to grid has become the mainstream in recent years. DFIG is AC-excited wind generation system. The dual PWM converters are connected between rotor and grid, which are able to regulate the frequency, amplitude and phase angle of rotor current according to gird and generator speed, to regulate stator voltage so as to meet grid connection condition. The stator is directly connected to grid via switch [7]. There is nearly no current shock and large voltage fluctuation is avoided in the process of grid connection. Maximum power point tracking (MPPT) is implemented via regulating the speed of DFIG after connection [8,9].
The accurate mathematical model of DFIG is derived and stator flux-oriented vector control strategy is adopted. The control strategy is different before and after grid connection, so it is switched after grid connection successfully. The full process is simulated based on PSCAD/EMTDC. Simulation
results confirm the correctness and effectiveness of the proposed control strategy.
II. VSCF DFIG OPERATION PRINCIPLE The schematic diagram of VSCF DFIG wind power
generation system is shown in Fig.1.
DFIG
C
RSC GSC
Filter
Lg
Figure 1. Schematic diagram of VSCF DFIG wind power generation system
According to the knowledge of electrical motor, the relationship of stator and rotor rotating magnetic field can be expressed as:
1 2 rn n n= + (1) The equation above can also be written as:
2 160rpn
f f+ = (2)
Where 1n is the rotating speed of stator magnetic field;
2n is the rotating speed of rotor magnetic field; rn is the rotating speed of rotor; p is the number of poles; 1f is the frequency of stator current; 2f is the frequency of rotor current.
As the rotor speed changes, the output frequency of stator will be constant by adjusting the exciting current frequency of rotor to realize VSCF operation. When DFIG is sub-synchronous operation, 2f is greater than 0. The excitation converters provide positive sequence excitation and slip power flows to generator. When DFIG is super-synchronous operation, 2f is less than 0. The excitation converters provide negative sequence excitation and slip power flows to grid. The DC excitation is adopted when DFIG is synchronous operation.
Project supported by the Inner Mongolia Natural Science Foundation majorproject (Grant No. 200711020801); Inner Mongolia Natural ScienceFoundation (Grant No. 20080404MS0907).
978-1-4244-4813-5/10/$25.00 ©2010 IEEE
The grid connection condition of DFIG is that the frequency, amplitude and phase angle of stator voltage are the same as those of grid voltage. The rotor speed of generator and grid voltage are adopted for control system [10]. The converters provide exciting current to set up stator flux and voltage to realize flexible grid connection.
III. GRID CONNECTION CONTROL
A. The mathematical model of DFIG in d-q coordinate DFIG is a multivariable, strong coupling, nonlinearity and
high order system in a three phase static coordinate. The stator flux-oriented vector control is used and stator flux is oriented at d-axis as Fig.2 shows [4,5]. The stator side of DFIG uses generator convention and the rotor side of DFIG uses motor convention. The voltage equations, flux and power ones in d-q coordinate are expressed as:
1
1
d s s d s d s qs
qs s qs q s d s
u R i p
u R i p
ψ ω ψψ ω ψ
=− − +⎧⎪⎨ =− − −⎪⎩
(3)
d r r d r d r s qr
qr r qr qr s d r
u R i p
u R i p
ψ ω ψψ ω ψ
= + −⎧⎪⎨ = + +⎪⎩
(4)
( )
( )d s l s d s m d s d r s d s m d r
qs l s q s m q s qr s qs m qr
L i L i i L i L i
L i L i i L i L i
ψψ
= + − = −⎧⎪⎨ = + − = −⎪⎩
(5)
( )
( )d r l r d r m d r d s r d r m d s
qr l r qr m qr qs r qr m qs
L i L i i L i L i
L i L i i L i L i
ψψ
= + − = −⎧⎪⎨ = + − = −⎪⎩
(6)
s ds ds qs qs
s qs ds ds qs
P u i u i
Q u i u i
= +⎧⎪⎨ = −⎪⎩
(7)
1α
1β
2α
2β
d
q
sθ
rθ
1ω
rω
sψ
su
o
Figure 2. Stator flux oriented coordinate
Substituting the flux equations into voltage equations, the voltage-current equations are obtained in d-q coordinate.
1 1
1 1
ds dss s s m m
qs qss s s m m
dr m s m r r s r dr
s m m s r r rqr qr
u iR L p L L p Lu iL R L p L L pu L p L R L p L i
L L p L R L pu i
ω ωω ω
ω ωω ω
⎡ ⎤ ⎡ ⎤− − −⎡ ⎤⎢ ⎥ ⎢ ⎥⎢ ⎥− − −⎢ ⎥ ⎢ ⎥⎢ ⎥=⎢ ⎥ ⎢ ⎥⎢ ⎥− + −⎢ ⎥ ⎢ ⎥⎢ ⎥⎢ ⎥ ⎢ ⎥− − +⎢ ⎥⎣ ⎦⎣ ⎦ ⎣ ⎦
(8)
Where R s and Rr are the winding resistances of stator and rotor; , , , ,s r m ls lrL L L L L are the self-, mutual and leakage
inductance of stator and rotor; , , ,d s qs d r qru u u u are the d- and q-axis components of stator and rotor voltage; , ,d s qsi i
,d r qri i are the d- and q-axis components of stator and rotor
current; , , ,d s qs d r qrψ ψ ψ ψ are the d- and q-axis components of stator and rotor flux; 1 1, ,r s rω ω ω ω ω= − are the synchronous speed, rotor speed and slip.
B. Power decoupled control Stator resistance is far less than winding reactance and can
be ignored on condition that stator frequency of DFIG is the industrial frequency, so 0sR = . The d- and q-axis components of flux are d s sψ ψ= and 0q sψ = .The induced electromotive force of DFIG is approximately equal to stator voltage and induction electromotive force lags sψ by 090 , so 1e locates at the negative direction of q-axis. The equations 0d su = and
qs su u=− are obtained. 1u is grid voltage vector and constant. When stator is connected to the grid, 1 su u= .
The following equations are obtained combined with flux, voltage, power equations and the conditions conducted above.
0
ds m d rd s s d s m d r d s
s
m qrd s s q s m qr qs
s
L iL i L i i
LL i
L i L i iL
ψψ
ψ
−⎧= − ⇒ =⎪
⎪⎨⎪ = = − ⇒ =⎪⎩
(9)
1
0ds
sqs ds
uu uω ψ
=⎧⎪⎨ =− =−⎪⎩
(10)
'
'dr dr dr
qr qr qr
u u u
u u u
⎧ = +Δ⎪⎨
= +Δ⎪⎩ (11)
Where
2'
2'
( )
( )
m drdr r dr r
s
qrmqr r qr r
s
L diu R i LL dt
diLu R i LL dt
⎧= + −⎪
⎪⎨⎪ = + −⎪⎩
2
2
( )
( )
mdr r s qr
s
m m sqr r s dr s
s s
Lu L iL
L Lu L iL L
ω
ψω ω
⎧Δ =− −⎪⎪⎨⎪Δ = − −⎪⎩
Where ' ',ds qsu u are components for the decoupling of the rotor voltage and current, ,dr qru uΔ Δ are compensational components to eliminate the across coupling of the rotor voltage and current. The rotor voltage is decomposed into decoupling and compensational components to simplify control, guarantee precise control and fast dynamic response.
The active and reactive power of stator is simplified as: s s qs
s s ds
P u i
Q u i
=−⎧⎪⎨
=−⎪⎩ (12)
The control diagram is shown in Fig.3, which can be got according to the power decoupling and no-load grid connection mathematical model. The controllers adopt PI regulator. The control strategy is different before and after grid connection. The control of the rotor-side converter adopts voltage outer loop and current inner loop before grid connection, power outer loop and current inner loop after grid connection. The inner
loop is the same and the outer loop should be switched. It should be noticed that the compensational voltage is different before and after the control strategy switching.
Figure 3. Control strategy before and after grid connection
C. No-load grid connection The switch between grid and stator is opened before grid
connection. Under the constraint conditions, equation (13) is obtained.
0ds qsi i= = (13) Considering (13), (9) can be simplified to
0ds m dr
qr
L iiψ =⎧⎪⎨ =⎪⎩
(14)
Substituting (13) and (14) into (6), the following expressions are obtained:
0dr r dr
qr
L iψψ
=⎧⎪⎨ =⎪⎩
(15)
According to (14) and (15), (9) can be simplified to
( )dr r r dr
qr s r dr
u R L p iu w L i
= +⎧⎪⎨ =⎪⎩
(16)
Considering field orientation error in dynamic regulative process, qri may not be 0. The qri is conducted closed-loop regulation. The anti-interference ability is improved and the system response speed is not changed at the same time. Substituting the (13) into last two lines of the (8), the rotor voltage is revised as
( )
( )dr r r dr s r qr
qr r r qr s r dr
u R L p i L i
u R L p i L i
ωω
= + −⎧⎪⎨ = + +⎪⎩
(17)
Where s r qrL iω− , s r drL iω are compensational components to eliminate the coupling of the rotor voltage and current.
IV. WIND TURBINE SYSTEM The available power in the wind is converted by the
blades to mechanical power. According to Betz Theory, the power captured by wind turbine is expressed as [6]:
31( , )
2 pP Av Cρ λ β= (18)
Where P is the capture power by wind turbine; ρ is the air density; A is the swept area; ( , )C p λ β is the power
coefficient of wind turbine; β is the blade pitch angle; λ is the tip-speed ratio; v is the wind speed.
There is an optimum rotor speed for each given wind speed and blade pitch angle, at which the maximal mechanical power can be obtained from the wind. The wind turbine and generator are connected with gearbox. To extract the maximum power from the wind, the rotor speed should vary with wind speed and maintain an optimum tip speed ratio optλ . To avoid using the wind speed measurement, the equation to compute the power can be rewritten as:
4 5max2 3 3
max max 31 12 2 (30 )
pp wt
opt
R CP R C v n
ρπρπ
λ= = ⋅ (19)
Where maxpC is the maximum power coefficient of wind turbine; optλ is the optimum tip speed ratio; wtn is the speed of wind turbine.
The measurement of rotor speed is much easier, more convenient than that of wind speed. The wind turbines can be controlled according to the value which is computed by the equation mentioned above.
V. SIMULATION RESULTS The parameters of DFIG in simulation: rated
power 2gP MW= , V=690V, f=50HZ, p=4, 0.0108sR pu= ，
3.364sL pu= , 0.0121rR pu= , 3.472rL pu= , 3.362mL pu= , J=0.5s.
Wind turbine: air density 31.225 kg mρ = , blade radius R=40m, 2eP MW= , max 0.41pC = , 6.8optλ = , rated wind velocity 12 m/s, gearbox speed ratio N=93.
In order to verify the effectiveness of control strategy, the simulation is conducted. The rotor speed is set to increase from 1275rpm to 1725rpm. The grid connection is implemented at 1.63s. The control strategy is switched after connected to grid successfully. DFIG is switched to torque control at 1.68s, which is input by wind turbine, and the wind speed is 10m/s. The wind speed is changed to 12m/s at 2.8s. The reactive power is changed to 0.5var at 3.8s. The main simulation results are as follows.
As shown in Fig.4, the stator voltage can track grid voltage quickly and the error between stator and grid voltage decreases gradually. The adjustment is not affected by rotor speed, which is implemented via regulating rotor exciting current. DFIG reaches synchronous speed at 0.8s and DC exciting is implemented at the moment. The rotor current becomes phase-sequence reverse when rotor speed exceeds synchronous speed. DFIG realizes variable speed constant frequency operation.
From Fig.5, it can be seen that stator voltage is clamped as grid voltage at the moment of grid connection. The control strategy is switched after connected to grid successfully. There is nearly no impact current and rotor current has smooth transition in the process. The flexible grid connection is realized. The generator accelerates at all times before grid connection, the reason of which is that there is no electromagnetic torque before grid connection.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6-1000
0
1000
(b)
Vol
tage
err
or (V
)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6-1
0
1
(c)
Stat
or c
urre
nt (A
)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
-1000
0
1000
(d)
Roto
r cur
rent
(A)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.61000
1500
2000
(e)time (s)
Roto
r spe
ed (r
/m)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6-1000
0
1000
(a)
Stat
or a
nd g
rid
v
olta
ge (V
)
a b c a bc
Figure 4. Simulation results before grid connection
1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2-1000
0
1000
(a)
Stat
or a
nd g
rid
vol
tage
(V)
1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2-2000
0
2000
(b)
Stat
or c
urre
nt (A
)
1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2-2000
0
2000
(c)
Roto
r cur
rent
(A)
1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.21400
1600
1800
(d)time (s)
Roto
r spe
ed (r
/m)
Figure 5. Simulation results when grid connection
0 1 2 3 4 5
-20
2
(a)Act
ive
pow
er (M
W)
0 1 2 3 4 5-2
0
2
(b)
Reac
tive
pow
er (M
var)
0 1 2 3 4 5
-20000
2000
(c)Sato
r cur
rent
(A)
0 1 2 3 4 51000
1500
2000
(d)time (s)
Roto
r Spe
ed (/
m)
Figure 6. Simulation results after grid connection
The control strategy is switched and power outer loop is input after connected to grid successfully in order to realize MPPT. From Fig.6, it can be seen that rotor speed is various with wind speed. The amplitude of stator current increases correspondingly when the set value of active power is increased, while the reactive power Qs remain unchanged. Rotor speed is regulated to the optimum speed at the same time. Reactive power of stator is changed correspondingly when the set value of reactive power is changed. Active power is not changed nearly. The MPPT and power decoupled control are realized.
VI. CONCLUSION The system simulation model of DFIG wind power
generation system is established on the basis of the mathematical model of DFIG. It can be seen from the simulation results that the system has fast dynamic response and good steady-state performance. The VSCF operation is realized. DFIG can meet grid connection condition quickly with the control strategy and connected to grid with no current shock nearly. The maximum wind energy capture is realized and the output of active and reactive power can be regulated respectively. The work mentioned above is beneficial to our later research on VSCF DFIG wind power generation.
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