Research About Grid-Connected Power Generating System With PV Module Integrated Converters

According to the connection structure of photovoltaic(PV) system, solar power generating system can be divided into three categories:centralized mode, string mode and modularized mode. Among them, the modularized mode, which has advantages of high energy conversion rate, strong ability resisting shadow, easy expansion and maintenance, is very suitable for small and medium-sized power occasions. So the modularized mode has drawn the attention of researchers in recent years.Comparing power generating systems composed with PV module integrated converters(MIC) including parallel type, bypass type, serial type, it was concluded that serial type has some merits, such as low voltage level components, high efficiency, etc. Then the paper compared different DC converter topologies and concluded that the same polarity Buck-Boost circuit is better than other circuits in high efficiency, wide regulating range, simple structure, etc. Therefore, this paper chose serial type power generating system with the same polarity Buck-Boost PV MICs as research object.This paper studied Buck-Boost converter’s operating characteristics, modulation method and designing parameters in single PV MIC. According to different states of the PV system, the Buck-Boost converter can switch flexibly between Buck, Boost, Buck-Boost modes through the overlapping carrier modulation.This paper mainly explored the output characteristics of PV system with MICs connected in serial and parallel. And the mathematical expressions of some voltage values had been given. Researches showed that for a single string of PV MICs, if the unbalanced degree of a and DC converter voltage ratio k meet:α>kmin/kmax,there must be a bus voltage value making all PV MICs work in the maximum power point(MPP). For a PV system with MICs connected in serial and parallel, the output characteristics may have multiple peaks.In order to track the maximum power point of the system, a reasonable bus voltage control strategy should be set.The integrated PV system is connected to the grid through an inverter. In order to control the bus voltage to locate at the maximum power point, this paper introduced particle swarm optimization (PSO) algorithm. In the algorithm, particles’ positions are set to be bus voltage references and the objective function is the output power of the PV system. PSO algorithm updates particles’ positions constantly and finds the global maximum power point in the solution space.Simulation module and software and hardware experiment platform with PV MICs were set up. Through simulations and experiments, voltage control of single PV MIC, MPPT control, the output characteristics of PV system with MICs, bus voltage control of the inverter, bus voltage tracking strategies based on PSO algorithm were verified to be correct and practical.