Research On Control Strategy Of Cascaded Two-stage Photovoltaic Grid-connected Inverter

Due to the intermittent and volatility of photovoltaic power generation,large-scale access to the power grid brings many hidden dangers to the safe,stable,and reliable operation of the power grid.At the same time,high power inverter device is expensive in the microgrid,which makes the economic benefits of PV generation system is low.For the above problems,this article proposes the MPPT control strategy of the pre-stage circuit and the voltage balance control strategy of the rear-stage circuit of the cascaded two-stage grid-connected inverter,which aims to improve the system's working efficiency,reduce the cost of power generation,and realize the safe operation of the grid-connected system.Firstly,the topology classification and the working principle of single inverter and cascade inverter is described in detail,and the mathematical model of cascaded two-stage grid-connected inverter is analyzed emphatically,which has laid a theoretical foundation for later research.Secondly,the output characteristics of photovoltaic modules are analyzed in detail,and the traditional MPPT control strategy of cascaded two-stage inverters is studied in depth.Due to the traditional control method cannot accurately search for the maximum power point when the light intensity or temperature abruptly changes,a MPPT of photovoltaic system by variable acceleration disturbance method based on GA is proposed that optimizes the search step size and introduces GA as an auxiliary search algorithm to reduce the system search time.In order to verify the correctness of the proposed method,a simulation model has built on the relevant simulation platform to verify the simulation.The simulation results showes that the system's dynamic response speed,stability and search accuracy has improved.Finally,the modulation technology of cascaded inverter is analyzed in detail,and the control principle of traditional double closed loop method of cascaded two-stage inverter is deeply studied.In view of the fact that the traditional control method can not solve the problem of DC bus voltage drift between power units caused by uneven illumination,a new control method based on error scaling is proposed.After the module error voltage is standardized and adjusted by a series of PI regulators,the duty cycle is adjusted to balance the unit voltage.The correctness of the proposed method is verified by simulation in the relevant simulation platform.The results show that the system can achieve stable output under uneven illumination,reduce voltage harmonics and restrain bus voltage drift.