Dynamics of an Integrated PV/Battery System for EV Charging in a Microgrid

Electric Vehicles (EVs) are emerging to play a key role in mitigating fossil fuel reliance and the environmental impacts brought on by the transportation sector. In parallel, it is essential that this growing electricity demand be met mostly with renewable sources. This study, as part of the Irvine Smart Grid Demonstration (ISGD) project, analyzes the effects of a renewable solar photovoltaic (PV) nanogrid — a small-scale, autonomous power system, that ultimately ties into the central electrical distribution grid — for EV charging associated with a controllable battery energy storage system, deployed in a primary circuit on the UCI (University of California, Irvine) Microgrid. The system’s dynamic behavior is characterized for different control algorithms that govern battery dispatch, creating four different energy management strategies.Power quality aspects of EV charging are also investigated, with a focus on harmonic distortion. A robust system design using delta-wye grounded transformers was shown to prevent any excessive harmonic currents from flowing upstream into the microgrid. Additionally, a diversity of EV charging loads promotes favorable harmonic cancellation. A power flow model of the nanogrid was developed in a commercial power flow software (ETAP) and verified against smart meter data, which were used as inputs for the ETAP Real-Time module.Lastly, a linear program was developed for optimally sizing nanogrid assets in the context of minimizing operation costs for the microgrid operator. The combination of an 80 kW PV array and a 95 kWh/45 kW battery energy storage system was shown to be the best option to power 20 Level-2 (7.2 kVA) EV chargers deployed at one of the university campus parking structures