Research On An LLC Integrated Three-Port Dc-Dc Converter Fora Stand—Alone PV Power System

Renewable energy systems(RESs) have been attracting much research interest worldwide for its potential role in solving the increasingly serious environmental problem and energy crisis. As a typical RES, the stand-alone photovoltaic power system is widely applied to satellites, microgrids, etc.A pulse width modulation(PWM) and pulse frequency modulation(PFM) hybrid modulated three-port converter(TPC) interfacing a PV source, a storage battery, and a load is proposed for a stand-alone PV power system. The TPC is derived by integrating a two-phase interleaved Boost circuit and a full-bridge LLC resonant circuit. Hence, it features a reduced number of active switches, lower cost, and single-stage power conversion between any two of the three ports. With the PWM and PFM hybrid modulation strategy, the voltage gain from the PV to the load is wide, the input current ripple is low, and flexible power management between three ports can be easily achieved. Moreover, all primary-side switches operate with zero-voltage switching(ZVS) while all secondary-side diodes can achieve zero-current switching(ZCS) over wide voltage and power ranges of three ports, which is beneficial for reducing the switching losses, switch voltage stress, and electromagnetic interference.The topology derivation and power transfer analysis are first presented. Depending on the resonant states included, two different operation modes are defined, and the the operation process is explored in detail. Then main characteristics, including the gain, input current ripple, and ZVS range, are analyzed and compared with conventional solutions. A multi-loop control strategy considering the output voltage regulation, PV MPPT control, and battery charging/discharging control, is proposed for the stand-alone PV power system. In addition, guidelines for parameter design and optimization are given as well. Finally, simulation is conducted and an experimental prototype is built and tested to verify the effectiveness and advantages of all proposals.