Impact of aggregated electric vehicle home charging in the sub-transmission grid

Abstract

The increasing global concern for greenhouse gas emissions has led to a fast pace in the transition away from fossil fuels in the transport sector. The share of electric passenger cars is increasing quickly, introducing new loads on the power grid. These loads primarily appear at the distribution grid level, where most of the charging occurs in the form of overnight charging at home. Although the impact on the distribution grid capacity from electric car charging is well researched, little effort has been put into understanding the aggregated impact at higher voltage levels. This study develops a probabilistic methodology for estimating the aggregated home charging demand of a 100 % electrified car fleet at the sub-transmission grid level. The resulting risk of overload in the primary substation transformers is estimated through a probabilistic load flow simulation. The full methodology is applied on a case study in southern Sweden using the best available vehicle and transport data, and a real grid model of the area. The results of the study show how aggregated home charging at low voltage levels may increase the average substation transformer loading by up to 72 %. This indicates that the aggregated home charging loads cannot be ignored at the sub-transmission level. More specifically, 11 out of 86 studied substations do not have enough capacity to feed the simulated home charging without transformer overloads.