Multi-Source Inverter for Electrified Vehicles: Concept, Analytical Design, Efficiency Analysis, an Implementation

Abstract

This thesis focuses on the concept, analytical design, efficiency analysis, and implementation of the multi-source inverter (MSI) for electrified vehicles (EVs).

The fundamentals of power electronics in EVs, including the main powertain architectures and the characteristics of power electronic converters, are discussed. Vehicle-level modeling methods and simulation tools for power electronics are also analyzed.

Located at the heart of the propulsion system, traction inverters play a major role in the performance and competitiveness of a vehicle. Standard industry solutions are reviewed, and a comparison of alternative candidates is presented. The main modulation strategies are also reviewed. For any topology to be seriously considered by the automotive industry, it must be able to compete on cost, reliability, efficiency, and power density. These driving factors are discussed, and the design trade-offs are explained.

In view of the continuous power increase in modern EVs, current industry configurations show some limitations. As an alternative solution, the MSI is suggested to be used as a traction inverter. The fundamental purpose of this power converter is to connect two independent DC sources to the same AC output using a single conversion stage. The concept of the MSI is presented and two circuits are considered. The operating modes during both DC/AC and AC/DC conversions are detailed and new control strategies are proposed. Closed-loop control simulations are performed to verify the MSI operation in each operating mode. A scale-down prototype is experimentally tested with an electric machine and a load to validate the effectiveness of the proposed topology and concept.

A comprehensive analytical design analysis of the switch configuration is presented, and analytical calculations of the capacitor requirements are suggested to select the proper capacitor banks for the MSI. An efficiency model based on the average and RMS currents of the switches is also proposed to evaluate the performance of the inverter. Experiments with the prototypes of both MSI circuits and an R-L load are carried out to validate the theoretical efficiency analysis. Design and efficiency comparisons of the MSI with the voltage source inverter are conducted as well.

The control and simulation of a new power-split powertrain with the MSI is discussed. As one of the most popular EVs on a sales-weighted basis, the Toyota Prius is analyzed as a case study. The suggested powertrain with the MSI aims to reduce the use of the DC/DC converter by offering an additional commutation path between the battery and the MSI. Another advantage of the MSI is the possibility to extend the battery charging opportunities. A vehicle-level simulation model of the traction drive system with the MSI is developed to estimate the potential benefits of the proposed powertrain. Simulations of the conventional and suggested powertrains are performed by applying the same voltage command of the DC/DC converter and power distribution between the battery and the electric machines.

Besides the hybrid power-split powertrains, the MSI can also be integrated into an active hybrid energy storage system. This new configuration aims to interconnect a battery and an ultracapacitor, without the use of any additional power electronic converters. A new control strategy is developed to manage the current distribution between the two sources and aims to take advantage of the high energy density of the batteries and the large specific power of the ultracapacitors, while minimizing battery degradation. Closed-loop control simulations are carried out to verify the operating principle of this novel configuration. The influence of the additional control parameters on the source currents is further investigated through simulations and validated with experiments on an R-L load.