Grid harmonic impact of multiple electric vehicle fast charging

Abstract

Fast charging is perceived by users as a preferred method for extending the average daily mobility of electric vehicles (EV). The rated power of fast chargers, their expected operation during peak hours, and clustering in designated stations, raise significant concerns. On one hand it raises concerns about standard requirements for power quality, especially harmonic distortion due to the use of power electronics connecting to high loads, typically ranging from 18 to 24kWh. On the other hand, infrastructure dimensioning and design limitations for those investing in such facilities need to be considered. Four sets of measurements were performed during the complete charging cycle of an EV, and individual harmonic's amplitude and phase angles behaviour were analysed. In addition, the voltage and current total harmonic distortion (THD) and Total Demand Distortion (TDD) were calculated and the results compared with the IEEE519, IEC 61000/EN50160 standards. Additionally, two vehicles being fast charged while connected to the same feeder were simulated and an analysis was carried out on how the harmonic phase angles would relate. The study concluded that the use of TDD was a better indicator than THD, since the former uses the maximum current (IL) and the latter uses the fundamental current, sometimes misleading conclusions, hence it is suggested it should be included in IEC/EN standard updates. Voltage THD and TDD for the charger analysed, were within the standard's limits of 1.2% and 12% respectively, however individual harmonics (11th and 13th) failed to comply with the 5.5% limit in IEEE 519 (5% and 3% respectively in IEC61000). Phase angles tended to have preferential range differences from the fundamental wave. It was found that the average difference between the same harmonic order phase angles was lower than 90°, meaning that when more than one vehicle is connected to the same feeder the amplitudes will add. Since the limits are dependable on the upstream short circuit current (ISC), if the number of vehicles increases (i.e. IL), the standard limits will decrease and eventually be exceeded. The harmonic limitation is hence the primary binding condition, certainly before the power limitation. The initial limit to the number of chargers is not the power capacity of the upstream power circuit but the harmonic limits for electricity pollution.