A Novel Bidirectional T-type Multilevel Inverter for Electric Vehicle Applications

 Abstract

This paper introduces a new configuration of bi-directional multilevel converter in electric vehicle (EV) applications. It has multilevel DC-DC converter with a direct current (DC) link capacitor voltage balance feature. The multilevel DC-DC converter operates in bi-directional manner, which is a fundamental requirement in EVs. Compared to the conventional configurations, the proposed one only implements two extra power switches and a capacitor to balance the voltage of the T-type MLI capacitor over a complete drive cycle or at fault conditions. Therefore, no extra isolated sensor, control loops and/or special switching pattern are required. Moreover, the proposed configuration due to the high frequency cycle-bycycle voltage balance between 𝑪𝑪𝒏𝒏 and 𝑪𝑪𝒑𝒑 the bulky electrolytic capacitors used in T-type MLI are replaced with longer life more reliable film capacitors. This will result in a size and weight reduction of the converter by 20%. This allows more real estate for the EV battery in the chassis’ space envelope; to increase its capacity. The proposed configuration is tested and validated using a Matlab/Simulink simulation model. A laboratory prototype 1 kW is built to provide the proof of Concept results as well.

Keywords

Multilevel inverter, T-type MLI, Multilevel DC-DC converter, bi-directional converter, Capacitor voltage balance, natural voltage balance;

Proposed Diagram:

 

Fig. 1. The proposed configuration, a multilevel bidirectional DC-DC converter connected to the T-type MLI.

Expected Simulation Results:

Fig. 2. Three phase line to line output voltage.

Fig.3. Low frequency ripple on capacitors voltage Vcn, Vcm and Vcp in corresponding to the three phase output power; three phase output voltage; and three phase output current.

Fig. 4. Capacitor voltage at steady state.

Fig. 5. Duty cycle of the dc-dc boost with the low frequency 180 Hz superimposed ripple; and inductor current at motoring

(a) Step change in the dc link voltage reference from 275V to 375V at t=0.05s; capacitors voltage Vcn, Vcm and Vcp; three phase output voltage; and; three phase output current.

(b) Capacitors voltage; dc-dc duty cycle; and the input current at fault condition by connecting 20 ohm resistor to Cn at t=0.02s.

(c) Voltage of CP and CN at fault condition in conventional configuration.

Fig. 6 The voltage of the converter’s capacitor at step change and fault condition.

Conclusion

This paper presents a new integration of the five levels Ttype multilevel inverter with a  modified bi-directional dc-dc multilevel converter for electric vehicle applications. While the T-type MLI utilize more power switches compared to the conventional voltage source inverter. It generates a higher number of output voltage levels utilizing power switches with half of the peak inverse voltage. However, if such converter is connected to a conventional bi-directional dc-dc converter, the converter power switches have to be designed to withstand the full voltage of the dc bus. Moreover, such conventional configuration needs an addition of voltage balance circuit or special switching pattern with feedback and control loops to insure the voltage balance of the dc capacitors. On the contrary, the proposed configuration takes advantage of the high frequency cycle-by-cycle voltage balance between the dc bus capacitors CN and CP as explained in section II, these capacitors are designed according to the dc-dc input stage high frequency ripple not the line low frequency ripple at 180Hz (triple the rated frequency). Therefore, the required capacitance is reduced from several hundreds uF capacitors to tenth uF capacitors, allowing for replacing electrolytic capacitors with film capacitors. Such advantage of the proposed converter doesn’t interfere with its ability to operate in step-up mode in motoring and in step-down mode in braking the electric motor. Moreover, the peak inverse voltage of all the power switches and the rated voltage of all capacitors is limited to half of the peak ac output voltage, which reduces the voltage stress and allow for implementing higher efficiency power switches in the dc-dc side similar to the ones in the T-type MLI side. The proposed configuration has been tested and validated though simulation model and experimental prototype. The results are compared and discussed, to demonstrate the advantages of the proposed configuration over the conventional one available in the market. In addition; efficiency levels are measured to be around 90%, which is far better than the market range. This work is to be extended by testing the proposed configuration against a complete EV driving cycle at low, medium and high speed to study evaluate the performance of operating EV with a higher rated voltage motors and its impact on the kWh per mileage consumption.

References

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