Grid-connected wind farm power control using VRB-based energy storage system

Abstract

To improve the power quality and stability of the grid-connected wind farm, and regulate the grid-connected power effectively, a new type of environmentally-friendly vanadium redox flow battery (VRB) based energy storage system (ESS) with many advantages are added at the exit of wind farm. A dynamic mathematic model of VRB based on the equivalent circuit is built. A bi-directional AC/DC converter is used to achieve the power conversion of VRB-based ESS and its corresponding control strategy is developed. The simulation model of the grid-connected wind farm with VRB-based ESS is established. The given wind speed in this paper is used for an example to validate the scheme, the simulated results show that the output active power of wind farm are effectively smoothed and a certain amount of reactive power support can be provided for the grid and the operating performance of grid-connected wind farm is well improved by the VRB-based ESS. The good charging-discharging performances of the VRB are also verified by simulated results.

Full text

Grid-Connected Wind Farm Power Control using
VRB-based Energy Storage System
Wenliang Wang1, Baoming Ge1, Daqiang Bi2, and Dongsen Sun1,
1 School of Electrical Engineering, Beijing Jiaotong University, Beijing 100044, China
2 State Key Lab of Power Systems, Dept. of Electrical Engineering, Tsinghua University, Beijing 100084, China
E-mail: 08121981@bjtu.edu.cn, gebaoming@tsinghua.org.cn, bidaqiang@mail.tsinghua.edu.cn
Abstract -- To improve the power quality and stability of the
grid-connected wind farm, and regulate the grid-connected
power effectively, a new type of environmentally-friendly
vanadium redox flow battery (VRB) based energy storage
system (ESS) with many advantages are added at the exit of
wind farm. A dynamic mathematic model of VRB based on the
equivalent circuit is built. A bi-directional AC/DC converter is
used to achieve the power conversion of VRB-based ESS and its
corresponding control strategy is developed. The simulation
model of the grid-connected wind farm with VRB-based ESS is
established. The given wind speed in this paper is used for an
example to validate the scheme, the simulated results show that
the output active power of wind farm are effectively smoothed
and a certain amount of reactive power support can be provided
for the grid and the operating performance of grid-connected
wind farm is well improved by the VRB-based ESS. The good
charging-discharging performances of the VRB are also verified
by simulated results.
Index Terms-- Energy storage; power control; smoothing
methods; VRB; wind power generation.
I. INTRODUCTION
In recent years, the wind power generation technology is
developing rapidly and is becoming more mature [1]. The
wind speed presents intermittent and random characteristics,
which leads to relatively large fluctuations of the wind power.
The power fluctuations can result in the deviations of the grid
frequency and voltage [2], and affect the stability and power
quality of the grid operation [3]. If the wind power in the
power system is up to 20% or more, the peaking capacity and
safe operation of the grid will face enormous challenges. In
particular, it needs to construct a number of the fossil fired
power or hydropower stations around the wind farms to
adjust the wind power and improve the stability of the grid
operation. However, it goes against the original intention to
develop the wind power. With a growing number of large-
scale grid-connected wind farms and the continuous
extension of installed capacity, the wind power ratio is
becoming higher. Therefore, the fluctuations of wind power
This work is supported in part by the State Key Lab. of Power System under
grant No.SKLD09KZ10, Tsinghua University, Beijing 100084, China, and
the Power Electronics Science and Education Development Program of Delta
Environmental & Educational Foundation under grant No.DREG2009006.
should be overcome urgently to avoid its negative effects on
the grid.
At present, researchers have proposed several solutions to
smooth the output power fluctuations of the wind farm. In [4],
the wind turbines’ operation state is directly regulated to
smooth output power, but its ability is limited. In [5], an
active power smoothing control strategy is proposed through
the pitch angle control and the variable speed control of the
generator for whole operating regions. This method aims at
smoothing the power fluctuations of a single unit, but it can
not effectively smooth the power fluctuations of whole wind
farm. In [6] and [7], STATCOM is used to adjust reactive
power fluctuations and maintain the grid voltage stability of
wind power access point, but it can not smooth active power
fluctuations [8].
The large-scale energy storage technology provides an
effective approach for the large-scale grid-connected wind
farms and improves the performances of the wind power,
which not only can smooth the active power [9] but also can
regulate the reactive power [10]. To a large extent, the issues
of random fluctuations for the wind power can be resolved
effectively so that the large-scale wind farms can be easily
and reliably connected to the conventional grid. In [8], the
application of battery-based energy storage is researched to
improve power quality and stability of the grid-connected
wind farm, but the specific characteristics are not considered.
In [11], the flywheel based ESS is used to improve power
quality and stability of the wind farms. In [12] and [13], the
superconducting magnetic-based ESS is used to smooth the
wind power. In [14] and [15], the supercapacitors are used to
adjust the wind farm output power. Now, the practical
applications of the flywheel-based ESS, the superconducting
magnetic-based ESS, and the supercapacitors based ESS are
limited due to the high cost or low capacity. The operation
temperature is very high for the sodium sulfur batteries with
explosion dangerous. The security and consistency of large
capacity lithium-ion battery are not guaranteed. At present,
the lead-acid batteries are widely used with mature
technology and low price, but the cycle life is very short.
The vanadium redox flow battery (VRB) is well suited for
the applications of large-scale power energy storage when
compared to other energy storage batteries, because of its
978-1-4244-5287-3/10/$26.00 ©2010 IEEE 3772

large capacity, long life, low materials price, low
maintenance requirements, and fast response to rapid changes,
etc [16]-[18]. The VRB-based ESS has been applied to some
wind power projects in other countries, such as Hokkaido of
Japan, Australian, etc. At present, the VRB has already
started to achieve its commercial operation and is expected to
play an important role in the development of wind power and
other renewable energy sources.
In this paper, the VRB-based ESS is dirctly added at the
exit of the wind farm based on the direct-drive wind turbines
to regulate the wind farm output power. The VRB-based ESS
can be used to absorb the output power fluctuations of wind
farm and provide an amount of reactive power support and
effectively improve power quality and stability of the grid-
connected wind farm.
II. VRB MODEL
A. Operating Principle of VRB
Fig. 1 shows the operating principle of the VRB [17],
[18]. The VRB is an electrochemical cell divided into two
compartments by an ionic membrane where the battery
reaction takes place, the positive and negative vanadium
electrolytes are stored in two tanks. The electrolytes are
pumped from the tanks to the cell for circulating through a
pump in each compartment to improve battery performance
and efficiency.
The total power available is related to the electrode area
in the cell stacks and the total energy stored in the VRB
depends on both the state of charge (SOC) and amount of
active chemical substances. The simplified electrode reaction
processes are as follows:
(1) For the positive electrode, it is
Charge
Discharge
−+
−eV
4+5
V
(2) For the negative electrode, it is
Charge
Discharge
−+
+eV
3+2
V
B. VRB Modeling
The VRB model based on the equivalent circuit [17], [18]
takes into account the physical and mathematical
characteristics, as shown in Fig. 2. The proposed model has
the following characteristics: ① the SOC is modeled as a
dynamically updated variable; ② the stack voltage is
modeled as a controlled voltage source; ③ the variable pump
loss model as a controlled current source is controlled by the
pump loss current Ipump, which is related to the current Istack
following through the battery stack and the SOC. The VRB
power losses include the loss with the internal resistances
Rreaction and Rresistive, the loss with the parasitic resistances Rfixed,
and the pump losses.
Negative
Electrolyte
/
V
Positive
Electrolyte
AC/DC
Generator Load
Charge Discharge
Ionic
membrance
Pump
Electrode
Pump
V
/
VV
5+ 4+ 2+ 3+
Fig. 1. VRB operating principle.
s
s
b
V
pump
I
fixed
R
s
V
reactor
R
resistive
R
electrodes
C
stack
I
Fig. 2. The equivalent circuit model of VRB.
The calculation of VRB equivalent circuit parameters is
based on the losses of 21%, where the loss of 15% is due to
the internal resistance and the parasitic losses is 6 %, for the
worst case operating point around the SOC of 20%. In order
to ensure the VRB providing the rated power PN with 21%
losses [18], the cell stack output power should be:
N
1 21%
P
P=−
stack (1)
A single cell stack voltage Vcell is related to the SOC, as
follows:
cell equilibrium 2lg( )
1
SOC
VV k SOC
=+⋅
− (2)
where k=0.059, the constant that affects the battery operation
and is related to the temperature; Vequilibrium=1.25V, standard
potential difference of each cell.
stack
parasitic fixed pump fixed '( )
I
PPPPk
SOC
=+ =+ (3)
2
b
fixed
fixed
V
RP
= (4)
stack
p
ump b
'( ) /
I
I
kV
SOC
= (5)
where Vb is the output terminal voltage of the VRB; k′ is a
constant related to pump losses.
3773

The internal resistance losses of 15% can be approximate-
ly divided into two parts, i.e., the loss of 9% from Rreactior and
the loss of 6% from Rresistive. Each cell has 6 F capacitance.
The single cell voltage is low, so the VRB is made up of a
number of cells in series generally.
The SOC can be defined as:
Energy in VRB
SOC Total Energy Capacity
= (6)
1tt
SOC SOC SOC
−
=+∆ (7)
stack stack b
NN NN
OC PtIVt
E
SEE PT
⋅∆ ⋅ ⋅∆
∆
∆== = ⋅ (8)
where SOCt and SOCt-1 are the SOC at the instants of t and t-
1, respectively; ∆SOC is the change of the SOC in a time step
∆t.
C. Charge-Discharge Characteristics of VRB
The charge-discharge characteristics of VRB are studied
by simulation based on the equivalent circuit model above.
The parameters are as follows: rated power PN=270 kW,
rated capacity EN=405 kWh, initial voltage value VN=810 V,
the cell number n=648, Rreaction=0.174 Ω, Rresistive=0.116 Ω,
Rfixed =60.5 Ω.
Fig. 3 shows the SOC variation in a charge-discharge
cycle, charging the VRB at a constant current of 320A for 1.5
hours, and discharging the VRB for 1.5 hours later. Fig. 4
shows the curves of open-circuit voltage Vs and the operation
terminal voltage Vb.
It can be seen from Fig. 4 that, in the charging and
discharging process of VRB, Vs is continuously variable with
the SOC; Vb changes with Vs and the voltage drop on the
internal resistance in the VRB.
00.5 1.0 1.5 2.0 2.5 3.0
0
0.2
0.4
0.6
0.8
1.0
SOC
t (h)
Fig. 3. The SOC during a charge-discharge cycle.
00.5 1.0 1.5 2.0 2.5 3.0
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Vb,Vc (kV)
t (h)
Fig. 4. The VRB voltage during a charge-discharge cycle.
There is a voltage difference between Vb and Vs, during a
continuous charging or discharge process. At switching
instant from charging to discharging, Vb is discontinuous due
to the mutation of internal resistance voltage polarity.
Moreover, in the process of charging or discharging, Vb and
Vs will vary greatly during 0-20% SOC and 20%-100% SOC,
with the approximate linear changing during 20% -80% SOC.
Thus, in practical applications, the VRB should work in the
linear region during 20%-80% SOC to avoid some issues
caused by over charge or over discharge.
At present, most of existing VRB-based ESS use two-
stage power converter, namely, AC/DC plus DC/DC, to
charge and discharge, which decreases the overall efficiency
and the reliability though the DC/DC converter can be used
to manage the battery. Therefore, according to the charac-
teristics of the voltage range for the VRB operation, this
paper only uses an AC/DC converter to control the VRB
charging and discharging.
III. WIND FARM WITH VRB-BASED ESS
A. System Structure
The VRB-based ESS is directly added at the exit of the
grid-connected wind farm to regulate intentionally the grid-
injected power from the wind farm, without changing the
existing status of every generation unit including the wind
turbines, generator, converters, and controllers. Since a wind
farm consists of many generation units, there are the random
complementarities occurring for the total power among all
units, which may mitigate the fluctuation of total output
power from wind farm. As a result, the proposed scheme
requires a smaller total VRB capacity when compared to the
distributed installation of VRB-based ESS at the exit or DC
link bus of every wind generation unit. The resultant benefits
include the low maintenance, the improved system reliability,
and low cost, etc.
As shown in Fig. 5, the wind farm includes 10 generator
units, with a total installed capacity of 25 MW. Each unit is a
direct-drive permanent magnet synchronous wind turbine,
with the rated capacity of 2.5 MW. The VRB-based ESS
consists of 30 units of VRB energy storage devices, with a
total rated power of 8.1 MW, and the rated power of 270 kW
per unit. To simplify the system, the paper supposes that
every generator unit is same in the wind farm, and every
VRB-based ESS is same.
Every direct-drive wind turbine mainly includes the wind
turbine, the permanent magnet synchronous generator
(PMSG), dual-PWM converters, and inductors, through the
690V/10kV step-up transformer connected to the grid. This
paper only uses single-stage AC/DC converter as power
converter to control VRB charging and discharging, through
380V/10kV step-up transformer connected to the exit of wind
farm. There are the local loads at the common coupling point.
Vb Vs
3774

w
P
w
Q
Local loads
PCC
1
2
VRB
b
P
b
Q
g
P
g
Q
AC/DC
Wind Farm
690V/10kV
10kV /38 0V
690V/10kV
10kV/110kV
PMSG
PMSG
AC/DC
VRB
30 units
10kV /38 0V
10 units
1
Grid
Note: Pw、Qw — wind farm output active power and reactive power;
P
g、Qg — active power and reactive power flowing into the grid;
Pb、Qb — active power and reactive power absorbed by the VRB.
Fig. 5. Configuration of wind farm with VRB-based ESS.
B. Direct-Drive Wind Turbine Control System
Fig. 6 shows the control strategies of the dual-PWM
converters for the direct-drive wind turbine [19]. The
decoupling control of the torque and the reactive power can
be achieved by controlling the d-axis and q-axis current
components of the generator-side converter, respectively.
The active power P and reactive power Q flowing into the
grid can be controlled by the d-axis and q-axis current
components of the grid-side converter, respectively. It is very
convenient to adjust the power factor and make the system
provide the reactive power support for the grid; also the
maximum power point tracking (MPPT) control will ensure
the utilization of maximum wind energy.
C. VRB Control System
The control principle of AC/DC converter for the VRB-
based ESS is shown in Fig. 7. The active power and the
reactive power of VRB-based ESS are controlled by the bi-
directional AC/DC converter. There are two given references
denoted as P*
ref and Q*
ref, respectively.
Every direct-drive wind turbine could operate at unity
power factor by controlling the grid-side converter during the
normal operation of the grid. If the grid needs the reactive
power support, the VRB-based ESS can provide the required
reactive power to the grid by controlling the AC/DC
converter, and for this case a given reactive power Q*
ref will
equal to the required value. The studies have shown that the
active power components over the frequency of 0.01Hz, in
the output active power of wind farms, have greatly negative
impact on the grid. Therefore, a first-order Butterworth High
Pass Filter (HPF) is used to achieve cut-off frequencies of
0.01Hz in this paper [20], which has a selectable time
constant. The HPF transfer function GW(s) is expressed by:
W
16
() 116
s
Gs
s
=+ (9)
As shown in Fig. 7, the PW represents the output active
power of wind farm, which is filtered by the HPF GW(s), as a
dc
U
SVPWM
αβ
*
d
u
*
q
u
u
α
u
β
dq
dq
abc
dq
abc
PI PI
PI PI
d
iq
i
*
d
i
*
q
i
dc
U
∗
Q
∗
P
Q
−−
−
−
gd
u
gq
u
g
θ
g
θ
gcgbga uuu
d
/
dt
abcdq
SVPWM
PI
PI
dq
PI
-
-
-
-
αβ
s
u
s
θ
*
sd
u
*
sq
u
s
θ
u
α
u
β
*
sd
i
m
ω
*
m
ω
dc
U
d
i
q
i
d
i
q
i
sq
i
sd
i
AC
ACDC
DC
sq
i
sd
i
s
θ
s
detection
θ
*
sq
i
Power
calculation
)(
fsdds
iL+
ψ
ω
sqqs
iL
ω
),(
m
ω
w
Pf
wu
iii
v
ca
iii
b
qgd
Liu
ω
−
dgq
Liu
ω
−
Fig. 6. Control principle of dual-PWM converters.
Wind Farm
w
P
w
Q
PCC
B
P
B
Q
g
P
g
Q
AC/DC
d
i
ref
Q
∗
calculation
PI PI
PI PI
d
i
∗
q
i
∗
q
i
∗
q
u
d
u
∗
VRB
VRB
V
L
V
C
sd
u
sq
u
abc dq
θ
SVPWM
q
u
d
u
w
P
ref
P
VRB
V
ref
P
∗d
Li
ω
sq
u
q
Li
ω
sd
u
Local loads
PQ
P limit
judge
VRB
V
αβ
dq
)(sGw
bauu baii
Fig. 7. Control principle of VRB-based ESS.
result of the given active power reference P*
ref. During
smoothing fluctuation of the active power, the over-charge or
over-discharge of the VRB should be avoided, since it will
affect the VRB performance seriously. Therefore, an energy
management unit is required to ensure the safe operation of
the VRB, which may limit charging and discharging power
within the allowable range. The VRB-based ESS will stop
working if the VRB terminal voltage is greater than the upper
limit or less than the lower limit, that is, VVRB>VVRBmax or
VVRB<VVRBmin, there will be P＊
ref=0 and Q＊
ref=0. The VRB-
based ESS will be limited to the rated power if the reference
power P
*
ref is greater than the rated power P
N. The active
power and the reactive power of the VRB-based ESS can be
controlled by the d-axis and q-axis current components,
respectively.
3775

IV. SIMULATED RESULTS
The simulation model of wind farm with the VRB-based
ESS is established. The parameters of every VRB are listed
in the section II-C, and the parameters of direct-drive wind
turbine are as follows: air density ρ=1.225 kg/m3, wind wheel
radius r=38.5 m, pitch angle β=0°; The rated capacity of
PMSG PSN=2.5 MW, pole pairs np=40, rated frequency
fN=15.885 Hz, stator resistance Ra=0.001 Ω, d-axis and q-axis
inductance Ld=Lq=1.5 mH; The grid-side inductance L=0.5
mH; DC-link capacitor C=12 mF, given voltage Udref=1200 V.
Fig. 8 shows the wind speed variation of wind farm
during 20s-100s. The given reactive power of VRB-based
ESS is 0 M Var during 20s-60s, and 1 M Var during 60s-
100s. The PMSG speed ωm, the output active power P and
the reactive power Q of every wind turbine, the DC–link
capacitor voltage Udc, the grid-injected active power, and the
grid-injected reactive power are shown in Figs. 9-13,
respectively.
It can be seen from Fig. 9 that, even if the wind speed is
changing, the rotor speed of the generator also changes to
capture the maximum wind energy through the MPPT control.
From Fig. 10, we can find that the active power fluctuat-
ions of the wind turbine is significant due to the variation of
wind speed and the use of maximum wind energy, with a
maximum variation of 0.8 MW. The output reactive power of
wind turbine is kept on 0 Var by controlling the grid-side
converter. Fig. 11 shows that the DC-link capacitor voltage
can be well maintained at 1.2 kV through the voltage closed-
loop control of the grid-side converter.
Fig. 12 shows that the instantaneous active power fluct-
uations of wind farm reach 8 MW. When there is the VRB-
based ESS, the power fluctuations of the wind farm can be
rapidly smoothed, and the extra power is absorbed or the
absent power is compensated by the VRB-based ESS, as a
result of a smooth grid-injected power with a maximum
fluctuation of 2 MW. As shown in Fig. 12, when Pb<0, the
VRB-based ESS absorbs the extra active power. On the other
hand, when Pb>0, the VRB-based ESS provides the active
power to the grid for the purpose of smoothing the grid-
injected power.
Fig. 13 shows that the output reactive power of the VRB-
based ESS is 0 Var during 20s-60s, and 1 M Var during 60s-
100s, which is injected into the grid. The reactive power of
wind farm is kept at 0 Var during whole operation. When
Qb<0, the VRB-based ESS provides the reactive power to the
grid. Therefore, the VRB-based ESS would achieve the
purpose to provide the reactive power compensation for the
grid.
V. CONCLUSION
The VRB is well suited for the applications of large-scale
power energy storage with many advantages. When it works
in the linear region during the SOC of 20%-80%, the terminal
20 40 60 80 100
8
10
12
14
Vw (m/s)
t (s)
Fig. 8. Wind speed.
20 40 60 80 100
1.0
1.5
2.0
2.5
3.0
(rad/s)
t (s)
m
ω
Fig. 9. Rotor speed of generator.
20 40 60 80 100
-0.5
0
0.5
1.0
1.5
2.0
2.5
3.0
P (MW),
Q
(MVar)
t (s)
Fig. 10. P and Q of a wind turbine.
20 40 60 80 100
1.0
1.1
1.2
1.3
1.4
Udc (kV)
t (s)
Fig. 11. DC-side voltage of wind turbine.
20 40 60 80 100
-20
-10
0
10
20
30
40
P ( MW)
t (s)
Fig. 12. Smoothed grid-injected active power.
20 40 60 80 100
-4
-2
0
2
4
Q (MVar)
t (s)
Fig. 13. Grid-injected reactive power.
P
w
P
g
Pb
Qw Qg
Qb
P
Q
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voltage changes little. This paper only uses a single-stage
AC/DC converter as power converter of the VRB-based ESS
to control charging and discharging, as a result of the simple
system structure. The simulated results of wind farm with the
VRB-based ESS show that the VRB-based ESS can quickly
absorb the power fluctuations of wind farm and effectively
smooth the grid-injected active power when the wind speed
varies. The VRB-based ESS also provides the reactive power
to the grid with a rapid dynamic response. The operating
performance of the grid-connected wind farm is effectively
improved, due to the VRB-based ESS.
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