Modular Power-Balanced Resonant Converters for MVdc Distributed PV Systems

Research into the use of medium-voltage dc (MVdc) grids is on the rise due to advantages including lower maintenance costs, lower loss, and simpler controllability compared to their ac counterpart. The operating voltage of a typical PV array system is quite low compared to the MVdc level which has led to advances in module-based input independent output series (IIOS) configurations to improve the output voltage level while providing individual maximum power point tracking (MPPT) for each module. Due to varying solar irradiation, atmospheric conditions, and PV manufacturing differences, converter modules will operate at varying power levels leading to modular power mismatch which can result in overvoltage scenarios, damaged components, and ultimately the failure of the system. In this article, a new interconnecting modular full-bridge step-up LLC resonant converter is proposed for IIOS based MVdc PV energy applications. The proposed system’s converter module is capable of regulating power flow between mismatched modules through the use of pulse-width modulation (PWM) control on a new active voltage doubler based voltage balancer (VD). To achieve individual maximum power point tracking (MPPT) in each module, phase-shift control is used on the input full bridge switches of each module. The use of the LLC resonant converter allows for zero-voltage switching operation on all module switches to enhance the system efficiency while stepping up the PV voltage. These configurations allow the modular PV system to achieve simultaneous MPPT, soft-switching operation, and balanced output voltage operation over a wide operating range of PV irradiation levels. The steady-state and dynamic performance of the designed system is validated on a five module 6kW, 12kV-output simulation and a scaled-down 1.6kW, 1.6kV-output two module laboratory prototype.