A Bidirectional Power Converter for Electric Vehicles in V2G Systems Xiangwu Yan, Member, IEEE, Bo Zhang, Xiangning Xiao, Member, IEEE, Huichao Zhao, Liming Yang high power density, high power factor, low harmonic, and electrical characteristics of isolation [5]-[7]. To enable power transfer in both directions between the grid and an electric vehicle, the AC-DC converter in V2G system is needed with bidirectional power flow capability. Generally, the bidirectional power converter in V2G system usually adopts two-stage structure: AC-DC stage and DC-DC stage. The traditional voltage-source PWM converter (VSC) [8], APFC [9], and Interleaved PFC [10] is considered to be a good choice for AC-DC stage converter, and there are some types of bidirectional DC-DC converter (BDC), among which The resonant converter based on soft switch technology obtains a certain development [11]-[13]. Presently, the research for bidirectional converter mainly focuses on the topological structure of the selection, integration and discussing the control strategy. This paper presents a novel power converter topology with two-stage architecture. The voltage source PWM converter is used as the front-stage, and the post-stage is composed of the symmetrical half-bridge LLC resonant converter. The power flow is bidirectional between electric vehicle (EV) and the grid through the power converter proposed in this paper, which also realize the unit power factor (UPF) and low harmonic in grid side. And the post-stage power conversion circuit using symmetrical half-bridge LLC resonant converter improves the conversion efficiency, dynamic performance and power density, reduces size and weight of the converter, gets wide output voltage range. Through the high frequency converter transformer, the electric contact between the power battery and the grid is completely isolated, which improves the system safety and reliability.
Abstract—Focused on the rational utilization of large-scale electric vehicle energy storage medium, the bidirectional and efficient power converter technique is important for the vehicle-to-grid (V2G) system. This paper explores the structure design ways of reducing electrical stress of LLC resonant circuit components, presents a novel power converter topology with two-stage architecture and its control strategy. The voltage source PWM converter (VSC) is used as the front-stage, and the post-stage is composed of the symmetrical half-bridge (SHB) LLC resonant converter. The power flow is bidirectional between electric vehicle (EV) and the grid through the power converter proposed in this paper. It also has the advantages such as isolated, high efficiency, high power density, small size, wide output voltage range, good dynamic performance and low cost. The operational theory and control strategy of the power converter are introduced respectively. The validity of the proposed concept is verified by simulations and experimental results. Index Terms—V2G, bidirectional converter, two-stages, LLC resonant converter, symmetrical.
I. INTRODUCTION
A
Long with the Electric vehicles (EV) quantitie increasing and smart grid level rising substantially, the EV power battery become mobile the storage unit in the smart grid, which transmits energy to the power grid in the peak load time of power grid, and absorb energy for charging in power load time of grid. This kind of bidirectional power flow between power grid and EV called Vehicle-to-Grid (V2G) [1]-[4], which is proposed by Kempton William at the university of Delaware in 1997. Under this concept, Electric vehicles are not only power load, but also power energy storage. It is pointed out that the successful interactive between electric vehicles and grid needs three factors: bidirectional power transformation, communication and control, GPS positioning and intelligent measuring instrument. Power converter technique is one of the key techniques in V2G system and also become a hot issue recently. The V2G system, especially on-board form, should have high efficiency,
II. THE TOPOLOGY AND OPERATING PRINCIPLE Voltage-source PWM converter could achieve sinusoidal current in grid side, operate under the unit power factor, and transform power bilaterally. In the low-to-medium power occasion, a well-known single-phase full-bridge VSC is usually used, as shown in Fig. 1. Asymmetrical half-bridge (AHB) resonant DC/DC converter as shown in Fig. 2 is a basic form of LLC resonant converter [14], which has good characteristics, such as zero-switching (ZVS) for primary-side switches, zero-current switching (ZCS) for secondary-side rectifiers, low switch voltage stress, and small circulating current. Hence, the AHB resonant DC/DC converter [15]-[17]can operate at a high switching frequency with lesser switch loss. The resonant inductor Lr, the
This work was supported in part by the Key Project of the National Research Program of China under Grant 2011BAG02B14, by the National High-tech R&D Program (863 Program) of China under Grant 2011AA11A279, and by the National Natural Science Foundation of China. X. W. Yan, B. Zhang, and X. N. Xiao are with State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources (North China Electric Power University), Changping District, Beijing 102206, China (e-mail:
[email protected];
[email protected];
[email protected]).
978-1-4673-4974-1/13/$31.00 ©2013 IEEE 254
magnetizing inductor Lm of the high-frequency transformer, and the resonant capacitor Cr constitute the resonant tank. Because there are three dynamic components in circuit, the converter can output different dc voltage under different working frequency.
L
V2 VD1
V1
VD 2
C10
Grid
V4 VD 3
V3
with complementary with primary switch pulse (on 50% and off 50%) which is given to the resonant tank as an input. One resonant tank is consisted by the resonant inductor Lr1, the magnetizing inductor Lm of the high-frequency transformer, the resonant capacitor C11 and C12. The other is consisted by the resonant inductor Lr2, the magnetizing inductor L'm of the high-frequency transformer, the resonant capacitor C21 and C22, similarly. The uncontrolled full-bridge rectifiers in the two side are composed of the diodes (VD11, VD12, VD15, VD16) and (VD17, VD18, VD21, VD22). Depending on the direction of power transmission, the proposed converter has two modes: the forward mode and the reverse mode, as shown in Fig. 4.
U DC
VD 4
Fig. 1. Single-phase full-bridge VSC converter
L
VD1
U in V2
Lr Lm
VD 2 Cr
np
* *
V3
C0
L
V11
VD2
V3
V4 VD3
Front-stage
VD4
VD11
C11
V12
VD14 VD12
VD16 C12
C21
T
L
L 1:1
VD 21
VD18 C22
VD22
VD12
C20
1 :1
C12
VD18
EV Battery
VD 22 Resonant Network
Rectifier Network
P
V1
V2 VD1
VD15 VD11
VD 2
V4 VD3
VD 21
V21 C20
' m
L VD16
VD 4
C21
T
1 :1
L2
C22
VD12 Rectifier Network
Resonant Network
VD 22
V22
EV Battery
Switching Network
(b) The reverse mode Fig. 4. Two modes of the SHB LLC resonant converter
The SHB LLC resonant converter could realize ZVS/ZCS operational mode in the whole load range. It reduces the switching loss effectively and slows down the transient over-voltage and over-current of the switches. This topology can solve the problem which is accepted that it is difficult for the lagging arm to achieve soft switching using the traditional ZVS bridge phase-shift PWM converter or the bridge ZVS PWM converter. Compared with the single resonant capacitor topology, using split resonant capacitor, the ripple and root mean square (RMS) of the input current through resonant capacitors are both smaller. The split resonant capacitor receives only half RMS current of the single resonant capacitor and the capacitance of the split resonant capacitor is also only half of the single resonant capacitor. On the resonance side, the clamp diodes circuit can be used as the over-voltage protection of the resonant inductor in the resonant network, while the symmetrical clamp diodes automatically convert to a rectifier arm of the single-phase full-bridge rectifier and separate the unused resonant inductor on the output side from the main circuit. It avoids large internal impedance voltage drop in the output loop. Similarly, the clamp diodes can also be used as over-voltage protection of the resonant capacitors in the resonant network, while the symmetrical clamp diodes on the output side. It can effectively suppress the LC resonance phenomena that may occur in the output-rectifier circuit. Therefore the diode clamp circuit of the resonant capacitors has a complex function of resonant voltage clamping protection and inhibiting resonance in the rectifier circuit.
V21
V22
L1
Switching Network
Grid
C20
VD20
Lm V12
VD 4
VD 21
T
C10
RL
L2
' m
Lm
VD19
VD17
C11
(a) The forward mode
ns2
DC/DC Converter VD15 VD17 L1
C10
Grid
VD13
VD11
P
Fig. 3 shows the proposed topology of bidirectional power converter for V2G system. The topology has two-stage: AC/DC converter (front-stage) and DC/DC Converter (post-stage). The front-stage is a single-phase full-bridge of voltage source PWM converter, which makes the voltage of DC bus steadily at a constant value. The post-stage is a novel half-bridge LLC resonant (HB LLC) converter which is called symmetrical half-bridge LLC (SHB LLC) resonant converter. V2 VD1
V4 VD3 Rectifier
VD R2
AC/DC Converter
VD 2
Inverter
ns1
V11
Grid
VD R1 *
V2 VD1
C10
Fig. 2. Asymmetrical Half-bridge LLC resonant DC/DC converter
V1
V1
V3
V1
P
P
By changing the working frequency of the converter, it can be obtained what output voltage you need within a certain range. It is very suitable for the on-board charger of EVs, whose nominal dc voltages are different. However, for the V2G system, the most basic requirement of the converter is transforming power bilaterally, which is impossible for traditional AHB resonant DC/DC converter because of the diodes in the secondary-side rectifier. So, if one would have a LLC resonant converter used in V2G system, some necessary improvement is must to be done.
EV Battery
Post-stage
Fig. 3. The proposed topology of bidirectional power converter for V2G
The SHB LLC converter means (post-stage) that the circuit topologies on both sides of the high-frequency transformer are exactly the same. Each side of the SHB LLC converters could work as a typical HB LLC resonant converter. When the converter on one side of the high-frequency transformer works in the high-frequency inverter mode, the one on the other side will work in the high-frequency rectifier mode. The SHB LLC resonant converter is composed of a switching network, a resonant network and a rectifier network in series. V11, V12 and V21, V22 constitute the two side switching network, respectively. It is used to get symmetrical square wave signal
255
III. CONTROL THEORY AND DESIGN
1 1 Gdc = ⋅ 2 2 2 ⎛ 1 1 ⎞ ⎛ 1⎞ 2 ⎜1 + − 2 ⎟ + ⎜ f n − ⎟ ⋅ Q k kf f n ⎠ n ⎠ ⎝ ⎝
Fig. 5 shows the control diagram of bidirectional power converter proposed above. The control method of front-stage (VSC converter) adopts double-closed-loop control based on dq transformation with outer voltage loop and inner current loop. It could realize the unit power factor and low harmonic in grid side, whether VSC work in the rectifier mode or inverter mode. The control method of post-stage (SHB LLC resonant converter) has two parts: and reverse control module. The two modules share a VCO (voltage-controlled oscillator) circuit and a driving signal producer circuit. When the bidirectional power converter works in the forward control mode, the switch S connects to the forward control module circuit. Hence the Voltage and current of the EV battery are under control in order to adapt different voltage level and capacity of the EV batteries. And when the bidirectional power converter works in the reverse control mode, the switch S connects to the reverse control module circuit. It can control the current of the DC bus for the VSC at a constant value. Then the VSC inverter the power flow to the grid side. PWM Converter
V1
Grid
V2 VD1
VD 2 C
SHB LLC Converter VD15 VD17
VD13
V11
C11
VD11
10
L1
V3 PLL
3
2
V4 VD3
VD14
V12
VD 4
i =0 id id*
C12
VD12
VD 21
V21
VD18 C22
VD 20 VD 22
where: normalized switch frequency f n =
quality factor Q = π 2
TABLE I THE PARAMETER TABLE OF BIDIRECTIONAL POWER CONVERTER
EV Batter
V22
ADC VCO
S
Forward controller
Symbol
Value
Parameter
Udc Uo Lr Lm Cr fr1 fr2
400V 24V 72μH 216μH 17.5nF 108kHz 51.3kHz
voltage of the DC bus nominal output voltage resonant inductor magnetizing inductor resonant capacitor design value of resonant frequency design value of resonant frequency
Fig. 7-8 show the AC voltage and current waveforms, DC bus voltage waveform of the front-stage converter in the rectifier mode. One can see that the current is in phase with voltage, so the converter operates unit power factor. The DC bus voltage is stable at 400V.
Fig. 5. Control diagram of bidirectional power converter
ia(A)
For the SHB LLC resonant converter, the circuit in the secondary side can be easily reflected to the one in the primary side and the equivalent circuit can be described as shown in Fig. 6. The resonant inductor is Lr (Lr1=Lr2=Lr), the resonant capacitor is Cr (C11=C12=C21=C22=Cr), and the equivalent resistance in the primary side is ZL. One can see that there are three dynamic elements in Fig. 5. The equivalent circuit has two resonant frequencies which are given by (1) and (2) [18]-[20].
200
ua
100
ia
ua(V)
0
Lr
-100 -200 0
uin
Lm
0.2 0.3 t(s) Fig. 7. AC voltage and current waveforms of the VSC (front-stage) in the rectifier mode
ZL
shown in
Fig. 6. Equivalent circuit of the SHB LLC resonant converter
f r2 =
1 2π Lr ⋅ 2Cr 1
2π (Lr +Lm ) ⋅ 2Cr
0.1
Switch gate pulse UG1, resonant current ir and magnetizing current im are Fig. 9. These waveforms are obtained when the SHB LLC resonant converter operates between the two resonant frequencies. The converter operates in CCM (continuous conduction mode). In this mode inductor current lags behind the applied gate pulse which is clear from Fig.9. It illustrates that low switching losses of a switch.
2Cr f r1 =
8 RL
Simulation results prove the effectiveness of the proposed bidirectional power converter for V2G system. A digital simulation was performed with the parameter as shown in Table I.
3
Reverse controller
Lr Cr
Lm Lr
IV. THE SIMULATION AND EXPERIMENTAL RESULTS
Driving signal
2
fs f r1
proportion factor of resonant inductor k =
C20
ADC
Inner current 1
Inner current 2
1:1
VD19
L2
L'm
Lm
VD16
PWM
iq * q
C21
T
(3)
(1) (2)
The DC voltage gain equation of the SHB LLC resonant converter can be obtained in the form 256
load, quarterly load, and low load. The converter operating frequency varies with the load change, distinctly. It can be seen that ZVS is ensured for any load condition. Fig. 11 shows the operation waveform of the SHB LLC resonant converter under different input voltage (250V-420V): (a) middle input-voltage with 24V output, (b) low input-voltage with 24V output, (C) normal input-voltage with 22V output, (d) normal input-voltage with 26V output. In the output-voltage design range, the converter can work normally.
Udc(V)
400 200 0 0.1
0.2 0.3 t(s) Fig. 8. DC voltage waveform of the VSC (front-stage)
30
Voltage(V) Current(A)
0
ir
2
im UG1
1
0
-1
V(HB) I(L11) I(Lm) I(VD21) V(OUT) I(VD22)
25 20 15 10 5 0 -5 22.85
-2 53.59
53.60
53.61
53.62
53.63
53.64
53.65
53.66
Voltage(V) Current(A)
53.58
t(ms)
Fig. 9. Waveforms of the SHB LLC resonant converter V(HB) I(L11) I(Lm) I(VD21) V(OUT) I(VD22)
25 20 15
22.9
22.95
t(us)
30
23 V(HB) I(L11) I(Lm) I(VD21) V(OUT) I(VD22)
25 20 15
10 5 0
22.9
0 -5 22.85
22.9
22.95
t(us)
30
23 V(HB) I(L11) I(Lm) I(VD21) V(OUT) I(VD22)
25 20 15
20 15 10 5 0
22.9
23 V(HB) I(L11) I(Lm) I(VD21) V(OUT) I(VD22)
25 20 15 10 5 0
22.9
22.95
23
t(us)
5
Fig. 11. Waveforms of the SHB LLC resonant converter with different input-voltage and output-voltage
0 -5 22.85
22.95
t(us)
-5 22.85
10
23 V(HB) I(L11) I(Lm) I(VD21) V(OUT) I(VD22)
25
30 5
22.95
t(us)
-5 22.85
10
Voltage(V) Current(A)
Voltage(V) Current(A)
Voltage(V) Current(A)
0 -5 22.85
Voltage(V) Current(A)
15
30 5
23 V(HB) I(L11) I(Lm) I(VD21) V(OUT) I(VD22)
20
-5 22.85
10
22.95
t(us)
25
22.9
22.95
t(us)
30
23
30
V(HB) I(L11) I(Lm) I(VD21) V(OUT) I(VD22)
25 20 15
U0 (V)
Voltage(V) Current(A)
30
Voltage(V) Current(A)
22.9
30
10 5
20 10
0 -5 22.85
22.9
22.95
23
0
t(us)
Fig. 10. Waveforms of the SHB LLC resonant converter with different load (a) full load (b) half load (c) quarterly load (d) low load
0.05
0.1
0.15 t(s)
0.2
0.25
0.3
Fig. 12. Final DC output voltage waveform of the converter
Fig. 10 shows the operation waveform of the SHB LLC resonant converter under different load, such as full load, half 257
Fig. 14-17 show the experimental results of the laboratory prototype (300W, 400V/24V). Fig. 14 shows the switch gate pulse UG and resonant current iLr, when the converter operates with full load and normal input-voltage. The switches of the converter post-stage are all turn on with ZVS. Similarly, Fig. 15 shows the switch gate pulse UG and resonant current iLr, when the converter operates with low load and normal input-voltage. One can see the switches of the converter post-stage are turn on with critical ZVS. Fig. 16 shows the rectifier diode voltage waveform on the output side. And the output voltage is 24V as shown in Fig. 17, which is the same as the simulation results
2.5
I0(A)
2 1.5 1 0.5 0
0.05
0.1
0.15 t(s)
0.2
0.25
0.3
Fig. 13. Current waveform of the EV battery
Fig. 12-13 shows DC output voltage waveform and the current waveform of the EV battery. Both the output voltage and current have low ripples which are important for power flowing between EV battery and grid.
V. CONCLUSION This paper focuse on the rational utilization of large-scale electric vehicle energy storage medium, explores the structure design ways of reducing electrical stress of LLC resonant circuit components, presents a novel bidirectional power converter for electric vehicles in V2G Systems with two-stage architecture and its control strategy. The single-phase voltage source PWM converter (VSC) is used as the front-stage, and the post-stage is composed of the symmetrical half-bridge (SHB) LLC resonant DC/DC converter. The SHB LLC resonant DC/DC converter has a symmetrical structure and split capacity. It has the advantages such as isolated, high efficiency, high power density, small size, wide output voltage range, good dynamic performance and low cost. Simulation results verify the validity of the proposed topology and its control strategy. Finally, a 300W laboratory prototype is implemented and tested.
Fig. 14. Full load under normal input-voltage
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VD 22
[4] [5] [6]
Fig. 16. The rectifier diode voltage waveform on the output side
[7] [8]
[9]
Fig. 17. Output voltage waveform 258
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