Research On Parallel Photovoltaic Power Optimizer Based On DC Bus

With the intensification of the energy crisis and the deterioration of the natural environment,the large-scale development and utilization of renewable energy has become the consensus of human society.Photovoltaic power generation is one of the most promising and representative renewable energy projects.In the photovoltaic power generation system,the maximum power point tracking(MPPT)function is indispensable,which can ensure that the photovoltaic array outputs the maximum power as much as possible,thereby improving the utilization of solar energy.According to different applications,photovoltaic MPPT can be divided into photovoltaic array MPPT,photovoltaic string MPPT and photovoltaic module MPPT.The parallel power optimizer(PPO)is an effective method to implement component-level MPPT,and the parallel power optimizer is very suitable for distributed generation systems.This dissertation focuses on the key issues such as the circuit topology and control strategy of the parallel power optimizer.Firstly,the concept of the parallel power optimizer is described,and the main problems studied by the shunt power optimizer and the key technologies involved in the parallel power optimizer are further analyzed.Because the main function of the parallel power optimizer is to realize the MPPT control of photovoltaic modules,the MPPT algorithm is carefully studied in this paper.The photovoltaic cell model is introduced in detail.Based on this,the control principle of MPPT is explained and some classic MPPT algorithms are analyzed.After that,the improved MPPT algorithm based on the combination of constant voltage method and variable-step perturbation observation method is introduced.This algorithm can meet the demand of the parallel power optimizer studied.Secondly,the topology of the shunt power optimizer is carefully studied in this paper.The topology of the parallel power optimizer should be an isolated high step-up conversion topology.In this paper,several typical topologies of isolated high gain are introduced.Then a Forward-Flyback converter based on a coupled inductor voltage doubler unit is proposed.Forward and Flyback converter units are connected in parallel to output in series to realize high gain conversion.The primary side of the transformer transfers energy to the secondary during turn-on and turn-off of the switch,improving the utilization of the transformer.The converter topology is simple and the cost is low.Finally,the article gives a control strategy of a parallel power optimizer.In order to improve the conversion efficiency of the circuit,a soft-switching control method for a Forward-Flyback converter is proposed to achieve zero current switch(ZCS)of the diode and zero voltage switch(ZVS)of the MOSFET.Based on the soft-switching control and the MPPT algorithm,a complete control strategy is given.The hardware design scheme of the parallel-type power optimizer is also given in this paper.A prototype is designed and the correctness of analysis is verified by experiments.