Autonomous Seamless Transition Between Grid-Connected and Islanded Operations for DC-Coupled EV Charging Infrastructures

Abstract

This article proposes a novel seamless transition technique between grid-connected and islanded operations for DC-coupled electric vehicle (EV) charging infrastructures. The main objective of the proposed approach is to ensure that the critical grid and EV requirements will be satisfied during the transition, including frequency, rate of change of frequency (RoCoF), current, rate of change of current ($di/dt$), and voltage ride-through requirements. The proposed approach provides three main functionalities, the first one is to ensure the seamless transition between islanded to on-grid operation, the second one is to control the transition process from grid-connected to islanded operation (intentional islanding), while the third one is to perform a fast transition to grid-forming operation when the utility grid is suddenly disconnected (unintentional outage). The algorithm is primarily based on trajectory-based frequency control to synchronize the charging infrastructure with the grid while adhering to RoCoF constraints. Moreover, the reference currents are adaptively generated to ensure a smooth transition, effectively controlling transient currents by minimizing overshoots and limiting $di/dt$. This approach mitigates surge power and prevents false tripping of the protection system while maintaining stable operation during dynamic conditions. The effectiveness of the proposed technique is validated experimentally.