A Bi-directional Z-source Inverter for Electric Vehicles

A Bi-directional Z-source Inverter for Electric Vehicles Makoto Yamanaka and Hirotaka Koizumi Tokyo University of Science 1-14-6 Kudankita, Chiyoda-ku Tokyo 102-0073 Japan Email: hosukenigou@ieee.org littlespring@ieee.org Abstract In this paper, a novel power converter for electric vehicles (EVs) is proposed. The proposed converter has a Z- source inverter which drives an ac motor, and performs as a current-fed Z-source dc-dc converter against reverse power flow. Thus, the proposed circuit does not need a bi-directional buck-boost dc-dc converter. The operation of the proposed converter was simulated by MATLAB SIMULINK and tested by circuit experiments for each operation as a Z-source inverter and a current-fed Z-source dc-dc converter. Keywords-Z-source inverter; bi-directional; electric vehicles; S 1 D 1 D 3 D S 6 4 Fig. 1. Z-source inverter. D 6 L A L B L C v A R A C A v B R B C B v C R C C C I. INTRODUCTION EVs, which don t exhaust CO 2, are different from conventional internal combustion engine powered cars. From the view point of the environmental issues, EVs are strongly expected to be put into practical use [1]. For EVs, power converters are needed to drive motors. The power converter is composed of an inverter to make a boosted ac voltage for a motor from a dc voltage of a battery, and a buck dc-dc converter to charge the battery during reverse power flowing [2]. A Z-source inverter, which is shown in Fig. 1, is a kind of inverter invented by F. Z. Peng [3]. By using shoot-through switching, the Z-source inverter can boost the output voltage without a boost converter. However a dc-dc converter is needed to accept a reverse power flow and to reduce the regenerative voltage to a battery voltage. The Z-source inverter includes a Z-network which is an X-shaped combination of two capacitors and two inductors. On the other hand, a currentfed Z-source dc-dc converter, which is shown in Fig. 2, has been also proposed [4]. The current-fed Z-source dc-dc converter performs as a buck converter when the duty ratio is over 50%, and as a polarity reversed buck-boost converter when the duty ratio is under 50%. This paper proposes a novel Z-source inverter for EVs. The proposed circuit has one Z-network which works as a Z-source inverter in the case of driving a 3φ motor and as a current-fed Z-source dc-dc converter in the case of reverse power flow. Therefore the proposed circuit does not need an external bidirectional buck-boost dc-dc converter compared with conventional power converters for EVs. The operational principle of a conventional Z-source inverter, a current-fed Z- source dc-dc converter and the proposed circuit are described in section II. Experimental circuits and results of the proposed circuit in the case of Z-source inverter mode and current-fed Z- source dc-dc converter mode are described in section III. II. L G A BI-DIRECTIONAL Z-SOURCE INVERTER FOR ELECTRIC VEHICLES A. A conventional Z-source inverter A conventional Z-source inverter is shown in Fig. 1 and the switching scheme of the Z-source inverter is shown in Fig. 3. Fig. 3 shows a carrier waveform e s, reference waveforms e A, e B, e C, shoot-through lines e 1, e 2, and driving waveforms S 1 ~S 6. In the case that e s is higher than e 1 and e s is lower than e 2, all driving waveforms S 1 ~S 6 are high. In this time, inverter legs are in short circuited state called shoot-through. The Z-source inverter takes two states which are the shoot-through state and the inverter drive state. The two equivalent circuits of the Z- source inverter are shown in Fig 4. In shoot-through state, double capacitor voltage 2 impresses the circuit. Therefore, the Z-source inverter can boost the output voltage. The voltage gain of the Z-source inverter can be expressed as: C2 R I Fig. 2. Current-fed Z-source dc-dc converter. 574

Fig. 6. A bi-directional Z-source inverter for electric vehicles. :OFF :ON Power flow :OFF Fig. 3. Switching scheme of a Z-source inverter. v O C1 Fig. 4. Equivalent circuits of a Z-source inverter, inverter drive state, shoot-through state. I G R I M VI vˆ x = (1) 2M 1 2 where vˆ x is the output peak phase voltage (x = A, B, or C), M is the modulation index, and is the input voltage [3]. B. A current-fed Z-source dc-dc converter A current-fed Z-source dc-dc converter is shown in Fig. 2. A current-fed Z-source dc-dc converter is also construct of a Z-network. The switch and are driven by pulse width modulation (PWM). The current-fed Z-source dc-dc converter is divided into two states which are :ON, :OFF and :OFF, :ON. The equivalent circuits of the current-fed Z- Fig. 5. Equivalent circuits of a current-fed Z-source dc-dc converter, :ON, :OFF, :OFF, :OFF. I G R I v O :active :OFF Power flow : active source dc-dc converter are shown in Fig. 5. The voltage gain of the current-fed Z-source dc-dc converter can be expressed as: 2D 1 V = D I where is the input voltage, D is the duty ratio of, and is the output voltage. Equation (2) implies that the current-fed Z-source converter performs as a buck converter when the duty ratio is over 0.5, and as a polarity reversed buck-boost converter when the duty ratio is under 0.5. When the duty ratio is 0.5, the output voltage unlimitedly approaches 0 [4]. C. A bi-directional Z-source inverter for electric vehicles Fig. 6 shows a bi-directional Z-source inverter for electric vehicles. Compared to the conventional Z-source inverter shown in Fig. 1, the input diode in Fig. 1 is replaced to a bidirectional switch composed of two IGBTs, and two diodes, in Fig. 6. A smoothing capacitor is set in parallel to the battery. Also a switch with an antiparallel diode is placed between the Z network and the 3φ bridge. Fig. 7 shows an equivalent circuit when the proposed Fig. 7. Equivalent circuits when the proposed converter performs as, Z-source inverter, and current-fed Z-source dc-dc converter. (2) 575

v GSA v dc S 1 S 6 L A L B L C L G v A R A C A v B R B C B v C R C C C III. EXPERIMENTAL RESULTS A proposed circuit was built and tested in the cases of the Z-source inverter mode and the current-fed Z-source dcdc converter mode. The proposed circuit was simulated by MATLAB SIMULINK and tested by an experimental circuit. Experimental circuits are shown in Fig. 8. In both cases, the designed value of the circuit elements, inductors = = 1[mH], capacitors = = 147[μF], and = 100[μF], switching frequency = 30[kHz] were given. The MOSFETs IRF510 were used for the switches in the circuit experiment. The internal diode forward voltage = 2.5[V] of the MOSFET was set in the simulation. The diodes SR340 were used in the circuit experiment. The forward voltage = 0.5[V] of the diode was set in the simulation. In the Z-source inverter mode, a voltage source = 10[V], and the modulation index M = 0.8 were given. Maximum boost control [5] was used. The voltage gain of the maximum boost control can be expressed as: v GSC πm VI vˆ x = 3 3M π 2 (3) Fig. 8. Experimental circuits, Z-source inverter mode, and current-fed Z- source dc-dc converter mode. converter performs as a Z-source inverter loaded with a 3φ motor. Fig. 7 shows an equivalent circuit when the proposed converter performs as a current-fed Z-source dc-dc converter which charges the battery by the reverse current flow from the 3φ motor. In the case of the Z-source inverter mode (Fig. 7), the state of switches is :OFF, :ON, :OFF, and the state of diodes is :active, :OFF :OFF, where the active state of means works as an input diode of Z-source inverter. The 3φ bridge is driven by PWM driving signals including the shoot-through switching states. In the case of the current-fed Z-source dc-dc converter mode (Fig. 7), the state of switches is :active, :OFF, :active, and the state of the diodes is :active :active :active. and are driven by PWM, and the diodes block the current according to the switching states of and. Assuming that the 3φ motor performs as a current source during reverse power flow, the 3φ bridge performs as a 3φ full bridge rectifier composed of the diodes D 1 -D 6. The reverse current is fed to the Z-network. In the case of the current-fed Z-source dc-dc converter mode, when the charge voltage is lower than the battery voltage, the unintended current flows from the battery to the motor. To prevent the unintended current flow, the switch maintains OFF state. Therefore, if the charge voltage is lower than the battery voltage, the unintended current prevented by the diode. where vˆ x is the output peak phase voltage (x = a, b, or c). The inverter was loaded with a 3φ RLC network instead of a 3φ motor, where the resistances R A = R B = R C = 100[Ω], the inductors L A = L B = L C = 1[mH], and the capacitors C A = C B = C C = 0.47[μF]. In the simulation, the solver ode14x was used with the fixed time step of the sampling time 4e-9[s]. In the current-fed Z-source dc-dc converter mode, a dc voltage source = 15[V] with a series inductor L G = 1[mH] was used instead of the motor. An electronic dc load FK- 200L2 at = 10[V] in the circuit experiment. In the simulation a dc voltage source = 10[V] was used with a series resistance 0.7[Ω]. The capacitor was used with a series resistance 0.1[Ω]. The duty ratio D = 0.8 was given for the switch and the duty ratio D = 0.2 was given for the switch. In the simulation experiment, the solver ode14x was used with the fixed time step of the sampling time 4e-9[s]. Fig. 9 shows the driving waveforms of switch S 1 and. The shoot-through state which S 1 and are ON at the same time was found in Fig. 9. Fig. 10 shows the simulated waveforms of the output voltages v A, v B, and v C and the inverter bus voltage v dc in Z-source inverter mode. The output frequency was 1.00[kHz] and the theoretical value of the output voltage was 24.75[V]. From the simulated waveforms, it was confirmed that the proposed circuit operated following the theory. Fig. 11 shows the observed waveforms in the circuit experiment. As shown in Fig. 11, the peak to peak value of the boosted output voltage is 24.0[V]. The difference of the experimental result and the theoretical value is about the same to the forward voltage drop of the diodes. The output frequency is 1.00[kHz]. From these results, the proposed circuit performs as a Z-source inverter in this mode. 576

Fig. 9. Driving waveforms of switch S 1 and, vertical: 10[V/div], horizontal: 10[μs/div] (c) Fig. 12. Simulated waveforms of the driving waveforms v GSA for and v GSC for, and the current in current-fed Z-source dc-dc converter mode, driving waveform of, vertical: 0.5[V/div], driving waveform of, vertical: 0.5[V/div], and (c) charging current waveform, vertical: 0.02[A/div], horizontal: 10[μs/div] Fig. 10. Simulated waveforms of the output voltages v A, v B, and v C and the inverter bus voltage v dc in Z-source inverter mode, vertical: 10[V/div], horizontal: 500[μs]. Fig. 13. Observed the gate source voltage SA, SC and the charging current waveforms in current-fed Z-source dc-dc converter mode, vertical: v GSA 10[V/div], v GS0[V/div], 0.1[A/div], horizontal: 5[μs/div]. Fig. 11. Observed output voltages v A,v B, v C and the inverter bus voltage v dc waveforms in Z-source inverter mode, vertical: v A, v B, v 0[V/div], v dc 50[V/div], horizontal: 500[μs/div]. Fig. 12 shows the simulated waveforms of the driving waveforms v GSA for and v GSC for, and the current in current-fed Z-source dc-dc converter mode. From the simulated waveforms, the input current decreases when is ON and is OFF. On the other hand, the input current increases when is OFF, and is ON. The average input current = -0.84[A] means that the proposed circuit charged the battery in the case of reverse power flow. Fig. 13 577

shows the observed the gate source voltage SA, SC and the charging current waveforms in the current-fed Z-source dcdc converter mode. The average input current = -0.80[A] was measured. The input current value makes a difference by the internal resistance of the battery. PEDS2009 IV. CONCLUSION This paper has presented a bi-directional Z-source inverter for electric vehicles. The proposed inverter has a Z-source inverter which drives an ac motor, and performs as a currentfed Z-source dc-dc converter against reverse power flow. The operation of the proposed converter was confirmed by the simulation and circuit experiments for each operation as a Z- source inverter and a current-fed Z-source dc-dc converter. REFERENCES [1] C. C. Chan, The state of the art of electric, hybrid, and fuel cell vehicles, in Proc. of the IEEE, vol.95, No.4, pp. 704-718, April 2007. [2] Jih-Sheng Lai and Douglas J. Nelson, Energy management power converters in hybrid electric and fuelcell vehicles, in Proc. of the IEEE, vol95, No.4, pp. 766-777, April 2007. [3] F. Z. Peng, Z-source inverter, IEEE Trans. Industry Applications, vol.39, pp. 504-510, Mar/Apr 2003. [4] Xupeng Fang, A novel Z-source DC-DC converter, in Proc. IEEE International conference on Industrial Technology 2008, pp. 1-4, April 2008. [5] F. Z. Peng, Miaosen Shen and Zhaoming Qian, Maximum boost control of the Z-source inverter, in Proc. IEEE Power electronics specialist conference 2004, vol.1, pp. 255-260, June 2004. 578