An improved compensation method for voltage sags and swells of the electric vehicles charging station based on a UPQC-SMES system

Abstract

The immaturity of electric vehicles (EVs) in voltage regulation and the inconsistency of multiple vehicle-to-grid (V2G) functions are the issues that need to be addressed in V2G technology applications. To simultaneously deal with these issues, this paper proposes an electric vehicles charging station (EVCS) based on a unified-power-quality-conditioner-superconducting-magnetic-energy-storage (UPQC-SMES) system. The EVCS based on UPQC-SMES can regulate the regular power disturbances like the load current harmonics and grid voltage distortions. For voltage sags and swells, an improved compensation method based on V2G is proposed to enhance the protection of the faulty grid. It can be classified into 1) Mode 1, to regulate the power profile of the faulty grid with the maximum capability of the EVCS; 2) Mode 2, to minimize the volt-ampere (VA) loading of the UPQC with a specific compensation power and voltage injection angle which are related to the load impedance angle and fault level. A complete power flow analysis of the system is carried out combined with phasor diagrams, especially under the operation of the proposed method. Simulation results show that the modifications of EVs in the V2G mode can effectively mitigate the overcurrent and overvoltage during sags and swells. The feasibility and desirable dynamic characteristics of the proposed method are demonstrated by simulation, and the adopted 1.193-H/0.462-MJ SMES suppresses power fluctuations and stabilizes the DC-side voltage effectively. It ensures stable operation of the UPQC and a superior charging environment for the EVCS.