ĐIều khiển thích nghi tuabin gió nguồn kép nối lưới điện truyền tải. A novel control strategy is presented for the backtoback PWM converters of the gridconnected DFIG wind power system to enhance the transient performance and reliability of the overall system during physical parameter uncertainty and certain grid disturbance. The system description is modeled by using the fieldoriented vector of the stator and voltageoriented vector of grid control. The rotorside and gridside converter controllers are designed in integration by utilizing nonlinear adaptive control technology. The theoretical analysis shows that the proposed controller can guarantee the system to achieve the maximal absorption of wind power, constant dcbus voltage, and constant voltage constant frequency output with respect to variable windspeed, parameter uncertainties and disturbance. The effectiveness of the proposed strategy is validated by the simulation comparison with the conventional PID controller.
Trang 1Adaptive Control for Grid-Connected DFIG Wind
Power Generation System
Zhiguo Gao
Institute of Electrical Engineering
Yanshan University
Qinhuangdao 066004, China
Email: gaozhiguo1100@126.com
Xiaohong Jiao
Institute of Electrical Engineering Yanshan University Qinhuangdao 066004, China Email: jiaoxh@ysu.edu.cn
Chaobo Ge
Institute of Electrical Engineering Yanshan University Qinhuangdao 066004, China Email: 568276120@qq.com
Abstract—A novel control strategy is presented for the
back-to-back PWM converters of the grid-connected DFIG wind power
system to enhance the transient performance and reliability of
the overall system during physical parameter uncertainty and
certain grid disturbance The system description is modeled by
using the field-oriented vector of the stator and voltage-oriented
vector of grid control The rotor-side and grid-side converter
controllers are designed in integration by utilizing n onlinear
adaptive control technology The theoretical analysis shows that
the proposed controller can guarantee the system to achieve the
maximal absorption of wind power, constant dc-bus voltage, and
constant voltage constant frequency output with respect to
variable wind-speed, parameter uncertainties and disturbance
The effectiveness of the proposed strategy is validated by the
simulation comparison with the conventional PID controller
Keywords-doubly fed induction generator; back-to-back PWM;
grid connection; wind power generation; adaptive control;
disturbance attenuation
As well known, DFIG is mainly used in variable speed
wind power systems due to its many advantages such as the
improved power quality, high-energy efficiency and reduced
power converter rating, etc Consequently, in the decade, the
research of control problem for the grid-connected DFIG
through back-to-back PWM has received much attention as
one of preferred technology for wind power generation (see
[1-8] and the references therein)
Early research results mainly concentrated on the control
strategy for the rotor-connected converter of DFIG, which
applies the stator-flux-oriented vector technique to describe
model of DFIG, and then design PI controller[1,3] or robust
controller[2] to guarantee the wind power system to achieve the
maximal absorption of wind power and the decoupling control
for the active and reactive power of the generator Before
connecting the stator of DFIG to the grid terminals, the stator
voltage has to be adjusted to be synchronized with the line
voltage Thus, some references handle DFIG control for the
synchronization process, for example, [4] describes a smooth
and fast synchronization scheme of DFIG to the grid as well as
decoupling control of generator active and reactive power by
using the stator flux-oriented control at normal operation
With increased penetration of wind power into electrical grids,
the probability of requirements is that wind turbines should remain connected and actively support to the grid during disturbances Accordingly, there has recently been a growing interest in the context of the grid connected wind turbine with DFIG, such as the dynamic responses[5], maximum power control strategy[6], performance evaluation and control scheme [7,8]
for the operation during abnormal conditions
Motivated by the reason above, this paper provides a control scheme for the grid connected wind turbine with DFIG through back-to-back PWM First, the overall model of a wind power system is described, including the DFIG and a vector-controlled converter connected between the rotor and the grid Adaptive voltage controllers of the rotor-side and grid-side converters are coordinately designed by utilizing nonlinear adaptive control technology under consideration of the system parameter uncertainty and grid disturbance, with the aim to control the generation of wind power in order to maximize the generated power with the lowest possible impact in the grid voltage and frequency during normal operation and under the occurrence of faults Meanwhile, the comparative simulation are presented between the proposed adaptive coordinated controllers and PID controllers, showing that better dynamic characteristics can be obtained using coordinated controllers
II SYSTEM DESCRIPTION AND CONTROL PROBLEM
The basic configuration of a grid-connected DFIM wind power system is sketched in Fig 1
g R g R
g R
g
L
g
L
g
L
1a
u
1b
u
1c
u u dc
2a
u 2b u
2c
u 1 L
1
L
1
1
R
1
R
Figure 1 Diagram of grid-connected DFIG wind power system
A Overall model of the controlled system
For an induction generator, the stator field orientation
control is based on the stator d-q model, where the reference
frame rotates synchronously with respect to the stator flux,
with the d-axis of the reference frame instantaneously overlaps
the axis of the stator winding flux The stator flux linkage keeps constant when the system is in the steady-state operation
Project Supported by Natural Science Foundation of Hebei Province
(F2010001322)
_
978-1-4244-9690-7/11/$26.00 ©2011 IEEE
Trang 2
and the stator resistance is ignored For such a reference frame
selection, the rotor voltage equations can be written as
rd
s
di
dt L
ψ
°
®
°
¯
(1)
d-q axis reference frame, respectively is rotor resistance
is leakage factor, , , represent rotor, stator
inductance and mutual inductance, respectively
is slip frequency, , represent synchronous angular speed
and rotor speed, respectively is the number of pole pairs
The stator active and reactive power can be described as
m s s
sd sd sq sq rq
s
m rd
sq sd sd sq s s
s
L
L
L i
L
ψ ω ψ
ω ψ
°°
°
°¯
(2)
The active and reactive power can be respectively controlled
by controlling the q and d-axis rotor current In addition, in the
case of ignoring the copper and iron loss of the stator, the
power relations for DFIG can be expressed as
' '
1
m m
s
−
− (3)
whereP , e P and m '
m
P represents electromagnetic power, input
mechanical power and the mechanical losses of the generator,
respectively P and 1 '
1
P denote the rotor power and rotor losses,
respectively Furthermore, the power transfer relationship of
wind power generation systems is governed by
' '
p r
P
n
ω ω
− −
= (4)
where PM; P0
M are the mechanical power captured by wind
turbine and mechanical wear of wind turbine, respectively
Under the d-q reference frame, the equivalent circuit equations
of rotor side converter can be described as
rd
rq
di
dt
dt
ω ω
°
®
°
¯
(5)
where denote equivalent resistance and inductance,
are the output voltage of machine side converter in
the d-q axes By combining (1) and (5), the integration model
of generator and rotor side converter can be obtained as
rd
rd s rq d rq
di
dt
dt
ω
°
®
°
¯
(6)
where c L1= mψs/ ,L R s 2= +R r R L g, 2= + σ L g
The motion equation of wind turbine with DFIG is described as
M
g e
P d
dt
ω + = − (7)
moment of inertia, viscous friction coefficient of wind turbine
and generator, respectively Tedenotes electromagnetic torque
generated, which can be calculated by
The power P produced by the wind is given by M
0.5 ( , ) ( )
P = ρπR C λ β v =kω λ ω (9)
where v is wind speed, ρ is air density, λ ω= R v/ is tip-speed ratio, Ris the radius of wind turbine, denotes power coefficient of wind turbine, is pitch angle
In d-q coordinate, the circuit equation of grid side converter
can be described as
d
q
di
dt
dt
ω ω
°
®
¯
(10)
where are the grid EMF components in d,q-axis, respectively are AC voltage and current in d- and q-axis components of the grid converter, respectively are the equivalent resistance and induction
Ignoring the line loss and switching device switch loss, according to energy conservation, grid side input power equals
to stored power of the DC side capacitance and excitation power of the rotor side, therefore:
3 2
dc
du
Conservation of energy for converter side:
2
95 100
dc
du
dt
= + (12) Considering external disturbance and (6), (7), (10) and (11),
we can get the overall model of DFIG wind power system:
2
1
4
1
3 2
m s s rq s
dc
nL
u
ω
ω ω
°
°
®
°
¯
where n=n n p g, w i i ( = "1, , 4) denote external disturbances
B Control problem formulation
Generally, for the wind power system (13), the main goals
of the control strategies are:
(1) Maximize the produced energy in the assurance of a secure functioning of the turbine;
(2) Control the active power supplied by the turbine in order to optimize the operating point and limit the active power in case of high wind speed;
(3) Control the reactive power flow between the generator and the grid, especially in the case of weak grids, where voltage fluctuations can occur, to guarantee the quality of the grid voltage
Moreover, it should be noted that during system operation there exist uncertainties, including parameters uncertainty and
external disturbance, such as, physical variables B and are
be unknown parameters Therefore, the task in this paper is to
Trang 3
design the global controllers for the rotor-side and grid-side
converters to ensure the wind power system (13) to achieve
the above control objective regardless of uncertain parameters
and external disturbance
during normal operation and under the occurrence of faults
To this end, define:
1 , 2 rd rd, 3 rq rq, 4 dc dc, 5 d d, 6 q q
x = −ω ω∗ x = −i i∗ x = −i i∗ x =u −u∗ x = −i i x∗ = −i i∗
then (16) can be rewritten as
2
2
5
1
1
, , , 1
, , ,
rq
rd
J
L
L
x
ωω θ θ ω
∗
∗ ∗
∗ ∗
ª
¬
º
¼
=
,
°
°
°
°
°
°°
®
°
°
°
°
°
°
(14)
with d1=3/(2 )C , 2
f =k xω + kωω∗x ,
f = −R x±Lωx BL n x+ ω ∗ x BL ni x∗ i j= i≠j
2
2
2
2
2
f x x i R x i x x i v x u
R σ x i x x i v x u
ω
1
1
1
1
u u L R i L i u u u L R i L i
L
u u L R i L i u u u L R i L i
L
where ω∗,i rd∗ ,i rq∗,u dc∗, ,i i d∗ q∗
are the reference values of the wind
turbine speed, current d-q components of generator rotor, DC
voltage and current d-q components of grid-side, respectively,
which can be obtained by the following relationship
maintaining safe operation, the wind turbine should be driven
according to the three fundamental modes associated with
wind speed, maximum allowable rotor speed and rated
the requirement for reactive power, the expected reactive
and can be obtained To achieve the control of unity power
factor of converter, the value of is 0
Therefore, the control problem of this paper is to design an
where, is an estimate for the unknown parameter , which
makes the resulting closed-loop system operate safely and
stably and achieve the control goal in the presence of the
parameter perturbation and external disturbance, i.e the speed
of wind turbine achieves asymptotically tracking the desired
speed trajectory based on the maximum capture of wind
and the grid-side converter exports electrical energy of
constant voltage and constant frequency in the required power
factor The diagram of integrated control system of wind
power generation system is shown in Fig 2
dc
u
rd
i
r q
i
d
i
q
i
ˆ θ
ω
1d
ˆ
θ
Figure 2 Block diagram of the grid connected wind power control system
In this section, an adaptive controller based on coordination will be designed for system (14) by utilizing the nonlinear recursive technique First, to design controller, the following coordinate transformation is utilized for the system
4 x4 u dc d x1 2 i rd d x1 3 i rq d u1 dc d i1rd d i1rq
4 d Ex5 f4 I x w1 2 2 c2 3 I x w1 3 3 w4
Thus, we get the following conclusion
Proposition1: For system (14), if a coordinated controller for the rotor-side converter and grid-side converter is designed as:
1 2
2
1 3 2 1 1
2
,
ˆ
( )
ˆ
s
s
L L
J x R
ω γ
α θ θ αω ατ
ω
γ
∗
∗
¬
© ¹
2 4
2 3
ˆ
1 1
rq rq
rq rq
z
ξ
ξ
γ
¸
∂
3 4 5 4 4
ξ
°
°
°
°
°
°
°
°
®
°
°
°
°
°
¯
(16)
1
1
c
ξ
−
°
¯
1( )x1 f x1 1 x12x1
ψ =ª¬ − γº¼,k k k2, ,3 6>0are tuning parametersˈ
1, 2
1 diag r r r{ , , }4 5 6
Γ = ˈr i>0, (i= "1, 6)ˈThen the resulting closed-loop system has the following operation performance:
equilibrium and states x can converge to the origin, namely,
ω→ω∗,i rd→i rd∗ ,i rq→i rq∗ ,u dc→u dc∗ , i d→i d∗, i q→i q∗ ast→ +∞
Outline of Proof: The controller (16) and the adaptive update law (17) are derived by nonlinear adaptive backstepping design technique, where Lyapunov function of the closed-loop
Trang 4
system is recursively constructed The proposition is obtained
according to Lyapunov stability theorem and LaSalle invariant
principle as well as L2-gain disturbance attenuation technique
The effectiveness of the designed controller is validated by
simulation in MATLAB/SIMULINK and the comparison with
that of PID controller is given
A simulated wind speed and the corresponding desired
speed of wind turbine are shown in Fig 3
12
14
16
18
t/s
v
1.92 1.94 1.96 1.98 2
t/s
* (m
w*
Figure 3 Wind speed and the desired speed ω ∗ of the wind turbine
The following two operation cases are discussed The fault
considered in simulations is a symmetrical three phase short
circuit fault which occurs on one of the transmission lines
(1) Uncertain parameters and external disturbance: In the
(2) The occurrence of faults: The system is in a pre-fault
operation state, a symmetrical three phase fault of grid voltage
occurs at t=80s
The simulation results in case1 are shown in Fig.4 and Fig.5
To compare the control performance, the curve is also given
for the system under the action of PID controller The
simulation results in case2 are shown in Fig 6
1.85
1.9
1.95
2
2.05
2.1
t/s
w *
w PID
349.4 349.6 349.8 350 350.2 350.4
t/s
udc*
u
dc
PID
0
5
10
15
20
t/s
i rd
i
rd
*
i
rd
PID
0.05 0.1 0.15 0.2
t/s
i rq
irq*
i
rq
PID
-4
-3
-2
-1
0
t/s
i
d
*
id PID
-5 0 5 10
t/s
iq*
iq PID
Figure 4 The response curves of the closed-loop system in case 1
-2000
-1000
0
1000
2000
t/s
v1
-8000 20 40 60 80 100 120 -600
-400 -200 0
t/s
v3
00 20 40 60 80 100 120
10
30
50
70
t/s
1 3
-1500 20 40 60 80 100 120 -100
-50 0 50
t/s
,2 ,3
1 3
Figure 5 Control inputs and estimates of adaptive parameters
From the simulation results, it can be concluded that:
The nonlinear adaptive coordinated controller
proposed in this paper can effectively improve transient
stability of the system and achieve the control aim
maximizing the generated power with the lowest possible
impact in the grid voltage and frequency during normal operation and under the occurrence of faults with parameter
case 2
In this paper, the adaptive coordinated control of rotor-side converter and grid-side converter is investigated via the vector control strategy The theoretical analysis shows the designed
dynamic performance irrespective of uncertain parameters and external disturbance The simulation results also illustrate that
absorption of the wind power according to three wind turbine
turbine to optimize the operating point, control the reactive power flow between the generator and the grid to guarantee the quality of the grid voltage
1.85 1.9 1.95 2 2.05 2.1
t/s
w *
w PID
3470 50 100 348
349 350 351
t/s
u d
u
dc
*
u dc
PID
0 5 10 15 20
t/s
i rd
* i rd PID
0 0 20 40 60 80 100 120 0.02
0.06 0.1 0.12
t/s
i rq
irq
* rq PID
-40 50 100 -3
-2 -1 0
t/s
i d
id*
id PID -50 50 100 0
5 10
t/s
i q
iq*
iq PID
Figure 6 The response curves of the closed-loop system in case 2
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