Controlled Charging of Electric Vehicles to Reduce the Aging of Distribution Transformers

Electric vehicles (EVs) are expected to dominate the road-transport sector in the future. A successful transition to EVs is contingent on the capability of electric power systems to deliver the energy required to recharge the vehicle batteries. Distribution transformers have been identified as the weakest link in the grid infrastructure that could be severely overloaded by widespread EV integration. Transformer overloads raise the operating temperature and lead to faster insulation degradation. The increased likelihood of insulation failure and the need for premature replacement of transformers, which are one of the costliest distribution equipment, represents a highly unfavorable scenario for the system operator. Thus, there is a need for controlled charging schemes in which the power output of individual EV chargers can be optimized to regulate the system demand and prevent transformer overloads. This paper formulates a charging strategy that seeks to minimize the peak load on a transformer and thereby extend its service life. The impact of controlled EV charging on the internal temperature and the aging rate of a distribution transformer is analyzed as per the guidelines given by the IEEE C57.91 standard in this study. Simulation results indicate that the proposed strategy is able to reduce peak load by 39% and extend transformer life by nearly fifteen times.