Study and Development of Multilevel Inverter Drive for e-Transportation

Due to increasing demand for high power and efficiency, low operating cost, and most importantly reduced air pollution, mobility is changing drastically, and great attention is focused on the electrification of transportation. Hence, power electronics and electric machines are needed to be much more reliable, efficient, compact, quieter, and have high power density. To accomplish increasing high power demand, the electric vehicles (EV) consisting of an electric powertrain will need very high power and medium voltage drives. It can be realized either by implementing traditional converter topologies with conventional semiconductor technology or by developing new multilevel topologies with lower voltage semiconductor devices. The conventional semiconductor devices have limitations on the rating of the devices when employed in the conventional power converters for high power applications, except when they are operated in the parallel mode to meet the high power requirement, which increases complexity. While new multilevel topologies allow the use of easily available lower voltage semiconductor devices to reach high power demand. Traditionally a 2-level inverter (TLI) is used in medium voltage drives but suffers from many disadvantages such as large dv/dt in the phase voltages and causes electromagnetic interference (EMI) and increased stress on the motor winding insulation, higher blocking voltage across the switches, lower switching frequency needs to be used to reduce the switching losses, poor harmonic performance. Thus, multilevel inverters (MLI) have become preferred alternatives in high and medium power applications since it offers quasi-sinusoidal phase voltage, low total harmonic distortion (THD), reduced torque ripple, reduced dv/dt in the phase voltage, reduced EMI. Among multilevel topologies, Cascaded H-Bridge provides an attractive solution with numerous advantages in terms of high efficiency and modularity. It appears to be the most appropriate for an electric vehicle (EV) application as on-vehicle low voltage battery modules can be arranged in such a way that each H-bridge has its own dc source. In this thesis, a comparative study of electric powertrain between TLI and MLI (different levels like 5, 7, and 9) considering the factors like torque ripple, THD and switching losses is presented. The research study presents a two-fold solution for developing a prototype of an electric powertrain. Firstly, it discusses the trade-offs to decide the best suitable number of level and then validates the same with the simulation and experimental studies by using field-oriented control method with PI controller for the speed control of permanent magnet synchronous motor (PMSM). PMSM is a widely accepted machine due to its cardinal features like high power density, low inertia, high efficiency, less maintenance and reliability. But, it is also vulnerable to variations in the motor parameters and external disturbance. Thus, higher order controller called super twisting sliding mode (STSM) controller design is proposed to track the desired rotor velocity for PMSM fed with the nine-level inverter for critical applications like full electric aircraft. The selected controller maintains the advantages of first-order sliding mode control and removes the chattering phenomenon; MLI further improves the drive performance. Simulation and experimental studies show the efficacy of the proposed PMSM drive. The proposed controller is compared with the conventional PI controller which proves the superiority of the STSM controller. The bidirectional DC-DC converter between the battery module and the inverter is also simulated in order to keep the dc link voltage constant. In the boost mode, the motor is fed by the battery, while in the buck mode, the power flows in reverse and the braking energy is exploited for charging the battery. All the works are experimentally implemented in real-time using DSP TMS320F28379D and FPGA cyclone V controllers.