A hybrid cascaded multilevel inverter using variable DC-link voltage technique for battery electric vehicles

Abstract

The trend of electrification has been well recognized by the automotive industry in recent years. Today, there are an increasing number of hybrid and electric varieties in the automobile market, pushing towards more efficient and environmentally friendly transportation. Cost, durability and range are the primary concerns for an electric vehicle. The three crucial elements - battery, traction inverter and electric motor - must be given special attention when developing the traction system. This dissertation proposes a hybrid cascaded multilevel inverter (HCMI) to improve the overall efficiency of the traction system. Thus, the longevity of batteries and the range of vehicles can be extended.

By using the proposed HCMI, the dc-link voltage of the traction inverter can be controlled according to various driving conditions. Compared to the conventional topology, which has a constant dc voltage, the switching losses and voltage harmonics are significantly reduced. Moreover, since each battery cell in the HCMI is switchable, charge equalization between the cells can be easily realized by arranging their respective charge/discharge duration. The objective of this dissertation is to develop suitable control schemes for the HCMI, and to investigate how the variable dc-link voltage affects the efficiency of the traction system.

The system efficiency will be evaluated by analyzing the power loss model of the electric motor, traction inverter and battery, respectively. The individual loss model is dependent on the dc-link voltage, which varies with the rotational speed and torque of electric motors. Two optimal modulation schemes are developed to regulate the dc-link voltage of the HCMI. Their respective system losses will be evaluated for a set of operating points. Depending on the employed modulation scheme and driving conditions, the optimal dc voltage, leading to the minimum overall system losses, might be different. As a comparison, the conventional topology using a constant dc voltage is also investigated. Generally speaking, a low dc voltage is more favorable for low-speed operation.

The developed control strategy will be verified on a prototype. The HCMI, consisting of 100 cascaded units, is implemented in the experimental environment. Power loss measurements are carried out on a 5kW synchronous machine. Experiment results illustrate that the efficiency of the traction system is improved by at least 2%. Besides, other advancing features of the HCMI, such as torque ripple reduction and charge equalization, are presented as well.