Abstract
Cascaded H-bridge (CHB) multilevel inverters are widely used in industrial applications, such as medium-voltage conversion and motor drives. However, the DC bus voltage in the electric vehicles is limited and it might not meet the requirements of the inverters for conventional motor drives. This paper presents a solution to drive the conventional motor (3Φ/AC 220 V) with inadequate DC bus voltage (DC 144 V) in an electric vehicle without any extra step-up circuits. The solution consists of a CHB inverter as a motor drive and a high-frequency (HF) transformer to balance the voltage. The multilevel CHB inverter improves the voltage and the current waveforms. High-frequency link (HFL) is used to create several isolated DC sources for the system and it can improve the power density. Besides, it replaces bulky line-transformers in the conventional CHB inverters, and the volume is reduced. Also, the inverter has bidirectional power flow ability, which can improve the efficiency in motor drives. As a result, the reduction of the step-up circuits is achieved and the topology can be used in the electric vehicles that are powered by only one 144 V-battery. The details and the principles of the control algorithm is discussed and an experiment based on a four-level CHB inverter with one DC 30 V power source is carried out to validate the proposed characteristics.
Highlights
In electric vehicles, one of the typical battery rating voltages is 144 V
This paper proposes a solution to eliminate the line-frequency transformers in Cascaded H-bridge (CHB) inverters and to inherit the advantages, such as high modularity and low total harmonic distortion (THD)
The High-frequency link (HFL) is made up of a multi-winding transformer and several H-bridges, where each phase is connected in the dual-active bridge (DAB) configuration
Summary
One of the typical battery rating voltages is 144 V. People can realize high-voltage power conversions with low-voltage power electric devices [2,3], while the inverter in [2] drops to two-level output at high-frequency, and in [3], full-bridge in series with level doubling network is used to achieve multilevel output and several isolated sources are needed. Combination of a double level circuit in [19], the output voltage level is increased nearly twice, while the device count is increased either These solutions focus on the partial drawbacks and they cannot meet the requirements of the motor drives in electric vehicles, in which there is only one DC power source and the voltage is comparatively low to drive conventional motors directly.
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