Analysis of PEV charging impact on distribution transformer aging for charging station transformer design

As plug-in electric vehicles (PEVs) are expanding, there is a growing need for widely distributed publicly accessible charging stations that supply electric energy for the recharging of the PEVs. In the charging station design, the proper distribution transformers that delivery reliable power from utility to the station are essential and important. To ensure economic and reliable operation of the charging station, the paper proposes a methodology for the PEVs charging station transformer design. A set of dynamic thermal energy models that describe the relationship between the transformer winding temperature and charging station load profile is derived according to ANSI/IEEE standards. The base load, PEVs demand, power quality, ambient temperature, transformer rating and connection all are taken into account. The models are then used to analyze PEVs charging impact on transformer loss-of-life and determine the proper transformer capacity and connection. The proposed method provides a better knowledge of the transformer effects and can be used for evaluating the level of PEVs that might be accepted on a distribution transformer. The models developed can also be used for determining proper transformers in the design of charging station distribution systems. The field testing results of a practical charging station for PEVs pilot projects in Taiwan confirm the validity of the proposed method.