Harmonic emissions assessment for common DC and AC electric vehicle charging station architectures

Abstract

Future transportation needs are going to be met by Electric Vehicles (EVs) because of global pollution by oil-based vehicles and climate change. In order to meet the charging demand and range anxiety of EV users, Fast Charging Stations (FCS) are required. As these FCS are grid-connected, they are going to be a new non-linear load for the host utility, which will impact its Power Quality (PQ). In this thesis, Common DC and AC bus (CDCB and CACB) architectures for grid-connected FCS are examined. For both architectures, two-level Voltage Source Converter (VSC) is used to connect the EV FCS to the grid, and further cascaded DC-DC converters are used for voltage regulation at the charger end. The Unit Template (UTC) and dq-SRF control methods are implemented for switching control of the VSC. The Constant Current-Constant Voltage (CC-CV) method is used for the control of DC-DC converters. The simulations are run in MATLAB/Simulink®. The following studies are carried out: • Comparison of CDCB & CACB architectures by varying load and transformer connections: - results show that CDCB architecture gives better performance in terms of charging and PQ, and the star-delta configuration of Distribution Transformer (DT) connections provides lower harmonics. • Comparison of two control strategies for the VSC using UTC and dq-SRF control strategies: - results show that UTC strategy performs better than dq-SRF method for control and operation of VSC. • Studying the impact of varying X/R ratio and MVASC: - results show that MVASC and X/R ratio has significant impact on a weak-grid operation connected with the FCS. • Studying the system with and without PV-panel: - results show that inclusion of PV-panel increases the reliability and efficiency of the system. There is small increase in THDV and THDI due to inclusion of PV-panel, but is as per the IEEE-519 standards. • Comparison of a Conventional-Capacitor (CC) with Super-Capacitor (SC) for the common DC bus: - results show that SC escalates the charging speed with fewer harmonics. • Comparison of two architectures in Vehicle to Grid (V2G) mode: - results show low harmonic content and better State of Discharge (SoD) with CDCB architecture.