Asymmetrical Multilevel Inverter for Electric Vehicles Application with Chopper Control

Abstract

This paper deals with cascade multilevel inverters. It focuses on asymmetrical topologies where the cell input voltages are of different values. These hybrid topologies might be used in several applications. The main advantage of asymmetrical multilevel inverters is the optimization of levels with a minimum number of power supplies. However, this optimized multilevel system still needs a large number of isolated and floating dc supplies, which makes these converters complicated to implement in Electric Vehicles (EVs), because the system will require many independent battery packs. In this work, a very simple scheme, based on a small and cheap High Frequency Link (HFL), allows the utilization of only one power supply for the complete multilevel inverter drive, with an inherent regulation of the voltages supplied among the H-bridges. It also allows voltage control with the full number of levels if the dc source is of a variable voltage characteristic. This is focused on a 27-level asymmetric inverter, but the strategy, using only one power supply, which can be applied to converters with any number of levels. In particular, an asymmetrical 27-level converter needs nine isolated power supplies, and the system reduces these nine sources to only one (the battery car). A new scheme of Multi Carrier Pulse Width Modulation (MCPWM) method for the control of a three-level inverter is proposed. Multi Carrier Pulse Width Modulation (MCPWM) works with a constant carrier frequency not synchronized with fundamental stator frequency. MCPWM gives an optimal utilization of switching frequency permitted, therefore PWM carrier frequency may be chosen to a value of two times the permitted switching frequency. The topology also permits full regenerative braking working as a three-level converter. The system is intended for application in EVs from power ratings up to 150 kW.