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

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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.
Electric grid, Electric vehicle (EV), FCS, Power quality, Harmonics