Grid-Connected Wind Farm Power Control using VRB-based Energy Storage System Wenliang Wang1, Baoming Ge1, Daqiang Bi2, and Dongsen Sun1, 1
2
School of Electrical Engineering, Beijing Jiaotong University, Beijing 100044, China State Key Lab of Power Systems, Dept. of Electrical Engineering, Tsinghua University, Beijing 100084, China E-mail:
[email protected],
[email protected],
[email protected]
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 largescale 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.
978-1-4244-5287-3/10/$26.00 ©2010 IEEE
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
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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 gridconnected wind farm.
Ionic membrance
Positive Electrolyte
Negative Electrolyte
V 5+/ V 4+
V 2+/ V 3+
Electrode
Pump
Pump
AC/DC Generator
Charge
Discharge
Load
Fig. 1. VRB operating principle.
II.
VRB MODEL
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 V 4+ − e −
Charge Discharge
V 5+
(2) For the negative electrode, it is V 3+ + e −
B.
Charge Discharge
V 2+
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.
I stack
I pump
A.
Vb
Rfixed
s
Celectrodes
s
Rreactor Rresistive
Vs
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: PN Pstack = (1) 1 − 21% A single cell stack voltage Vcell is related to the SOC, as follows: SOC Vcell = Vequilibrium + 2k ⋅ lg( ) (2) 1 − SOC 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. I Pparasitic = Pfixed + Ppump = Pfixed + k '( stack ) (3) SOC Rfixed =
Vb2 Pfixed
(4)
I stack (5) ) / Vb SOC where Vb is the output terminal voltage of the VRB; k′ is a constant related to pump losses.
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I pump = k '(
The internal resistance losses of 15% can be approximately 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 = (6) Total Energy Capacity SOCt = SOCt −1 + ∆SOC
(7)
∆E Pstack ⋅ ∆t I stack ⋅ Vb ⋅ ∆t = = (8) EN EN PN ⋅ TN where SOCt and SOCt-1 are the SOC at the instants of t and t1, respectively; ∆SOC is the change of the SOC in a time step ∆t. ∆SOC =
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. 1.0
SOC
0.8 0.6 0.4 0.2 0
0
0.5
1.0
1.5
2.0
2.5
3.0
t (h)
Fig. 3. The SOC during a charge-discharge cycle. 1.1
Vb
Vb,Vc (kV)
1.0
Vs
0.9 0.8 0.7 0.6 0.5
0
0.5
1.0
1.5
2.0
2.5
t (h)
Fig. 4. The VRB voltage during a charge-discharge cycle.
3.0
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 twostage 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 characteristics 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 System Structure The VRB-based ESS is directly added at the exit of the grid-connected wind farm to regulate intentionally the gridinjected 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.
A.
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PCC Pg Qg Local loads Grid
2
Pw Qw
1
Pb Qb
PMSG
θs
AC/DC
Wind Farm
AC/DC
PMSG
10kV/380V
10kV/380V
1
30 units
DC DC
abc dq
uβ
θs
Note: Pw、Qw — wind farm output active power and reactive power; Pg、Qg — active power and reactive power flowing into the grid; Pb、Qb — active power and reactive power absorbed by the VRB.
dq
isd isq
ωs (ψ f + Ld isd ) ω s Lq isq
VRB Control System The control principle of AC/DC converter for the VRBbased ESS is shown in Fig. 7. The active power and the reactive power of VRB-based ESS are controlled by the bidirectional 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:
-i isd*
16 s (9) 1 + 16 s 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
Power calculation
iq
− iq
iq*
PI
sd
id
PI
id*
ωm*
Q
PI
−
−
U dc∗
Q∗
P
f ( Pw ,ω m ) Fig. 6. Control principle of dual-PWM converters.
Pw Qw
Pg Q g
PCC Local loads
L Pw
Wind Farm
P limit
PB
VVRB
AC/DC CV
iaib
VRB
Q
PI ∗ Pref
SVPWM
QB u sd usq iq id αβ ∗ ref
VVRB
judge
uaub
calculation PQ abc dq
Gw (s)
Pref
C.
GW ( s ) =
ugq
u gd − ωLiq
− id
U dc
-
θg
dq
u gq − ωLi d
PI
PI
abc
ugd
uq*
PI
isq*
B.
αβ
ud*
u gau gbu gc
θg
id iq
-
ωm
abc dq
uβ
dq * sd
PI
isq
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.
uα
u
ia i b ic
SVPWM
αβ
* sq
d /dt
Fig. 5. Configuration of wind farm with VRB-based ESS.
Udc
uα
u
VRB
AC
SVPWM
θs
θ s detection
690V/10kV VRB
AC
iu i v iw
10kV/110kV
690V/10kV 10 units
us
PI
i∗q i ∗d
VVRB θ
dq
u ∗q u q ud usd ω Liq
PI
PI
ud∗
usq ω Lid
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
