Design and control optimisation of scalable power electronics converters for DC off-board electric vehicle charging systems

Electric vehicles (EVs) play a major role in mitigating global warming and environmental pollution; they can reduce fossil-fuel emissions, if vehicle-propulsion energy is obtained from sustainable sources or a clean-energy mix. EV charging systems are essential for recharging zero-emission-vehicle batteries. A suitable off-board charging infrastructure is required to efficiently and effectively charge EVs. Off-board charging systems enable the charging of EV batteries directly at high voltage and high power; thus, shorter charging times are required to replenish a battery. State-of-the-art (SotA) chargers still exhibit considerable power losses, which has a negative impact on the charging cost of EVs and the optimal usage of available energy. Wide bandgap (WBG) materials, such as silicon carbide (SiC) semiconductor switches, are a promising technology for the optimised design of power electronic converters (PECs) and are an alternative solution to silicon-based devices. In this PhD study, the use of WBG semiconductor devices, which have superior properties and characteristics (e.g., high-temperature operation, high switching frequency, and more compact and power-dense designs), have been investigated and applied to optimising the design of high-power off-board charging systems. The EV charger is a PEC, which converts three-phase alternating current (AC) grid power into controllable direct current (DC) power for charging. In this PhD thesis, the topologies of PECs are firstly discussed, followed by the design of high-power off-board chargers. Two main charger topologies are considered in this research: one based on the low-frequency (LF) transformer concept and the other on the high-frequency (HF) transformer. Moreover, the concept of a modular converter is introduced to design an ultra-fast high-power charging system for EVs. A new co-design optimisation framework that considers hardware design in combination with the modular converter topology is proposed to design and optimise off-board charging systems in a complete and integrated manner formulated as a multi-objective problem. The co-design optimisation framework adopts a non-dominated sorted genetic algorithm (NSGA-II) to solve the multi-objective optimisation problem. Three objectives are considered for the design optimisation, namely power-density maximization, power-loss minimization, and the enhancement of the lifetime of the converter. The algorithm returns the optimal solutions with information about the design parameters of the charger, switching frequency, and passive filter components. Additionally, the PECs are designed to maintain a sinusoidal grid current and its frequency to ensure the grid-quality requirement. The control-related objective of this PhD study is the designing of a high-power off-board charger with increased efficiency and low harmonic distortion on the grid. Moreover, the control-design optimisation for bidirectional charger modes, namely grid-to-vehicle (G2V) and vehicle-to-grid (V2G) power flow, is investigated and developed. An accurate analytical model of the charger is proposed to precisely design the dual-loop voltage/current control strategy based on charger’s specifications. The controllers are designed to enable the EV chargers to control active/reactive power and offer constant-current (CC) and constant-voltage (CV) charging. A virtual resistance concept is introduced into the bi-directional control algorithm to suppress the inrush currents. In addition, this study presents a grid voltage unbalance and harmonic compensation (VU & HC) control strategy for unbalanced voltages and harmonic compensation. Besides the CC–CV charging control, the VU & HC control strategy effectively ensures better voltage quality under nonlinear peak load conditions. A robust advanced model predictive control (MPC) algorithm is introduced to control the charger’s parameters, aiming to enhance disturbance rejection capability. On the subsystem level, there is a lack of accurate and practical models to deal with high switching frequencies and fast-dynamics devices for the PECs of the off-board charger. In this PhD study, an accurate virtual prototype of high-power chargers is developed for bidirectional AC–DC and isolated DC–DC converters. This simulation makes it possible to investigate voltage-current behaviour, system current ripples, total harmonic distortion (THD) of current, power factor (PF), power loss, temperatures and system efficiency. Finally, a considerable part of this study focuses on implementing real-time (RT) control on a field-programmable gate array (FPGA) using rapid prototyping control platforms to demonstrate and validate the designs with high-power off-board charger prototypes with WBG switches operating at a high switching frequency. The high-frequency discrete FPGA-based controller is developed and implemented using the RT dSpace MicroLabBox™ platform using Xilinx Vivado. This PhD study develops a SiC off-board charger prototype that is based on parallel modular converters with an LF grid transformer. The developed 150 kW charger is experimentally tested at a rated power with an integrated RT control system. Two advanced charger prototypes are developed: a 175 kW charger based on the dual-side cooled (DSC) SiC modules and 150 kW charger based on SiC standard modules. These prototypes are tested and investigated with bidirectional control, and their performances and power densities are compared. Furthermore, multiple tests of battery charging (G2V) and discharging (V2G) are performed to examine the prototype performances.