Research On Single-Phase Multilevel PV Grid-Connected Inverter

Multilevel inverter has low output voltage harmonic content, low electromagnetic interference, and small output filter, can help PV grid-connected system to improve efficiency and reduce the volume. This kind of inverter has a wide range of applications in the field of photovoltaic power generation. This dissertation takes the single-phase multilevel PV grid-connected inverter as the research object. Base on the analysis of single-phase control system, this dissertation focus on single-phase three-level PV grid-connected inverter and a kind of multilevel hybrid cascaded PV grid-connected inverter which have ability to restrain common mode current.First, the classification of photovoltaic power generation structures and the type of grid-connected inverter are introduced. For single-phase grid-connected control strategy, this thesis studied various controller and the effect on AC current loop in double closed loop control system. Among them, this thesis focused on the analysis of proportional-resonant control.Then, this dissertation analysed the work mode of symmetrical and asymmetrical single-phase three-level inverter, the three-level bridge arm effect on toss of the inverter and their respective in application fields, and studied the center potential balance method of this two kinds of inverter.Based on traditional transformerless three-level half-bridge PV grid-connected inverter, a kind of multilevel hybrid cascaded PV grid-connected inverter was researched. This structure can reduce input DC voltage level, further reduce output filter volume and switching toss. The work status and common mode model of this kind of inverter were analysed. As the result, the optimum modulation mode and the DC support capacitor voltage control strategy based on that mode are reached. According to dead-time effect of this kind of inverter, a dead-time compensation mode is giveaIn order to solve the problem that multilevel SPWM method take up to much system resources, a single carrier SPWM method can be implemented efficient for the inverter. A vector modulation technique is introduced in this dissertation, which have the same effect and different usefulness with SPWM.To verify the feasibility of the system, a single-phase three-level grid-connected inverter and a multilevel hybrid cascaded inverter are constructed. The analyze result of symmetrical and asymmetrical single-phase three-level inverter output characteristics and hybrid cascaded inverter output characteristics are improved by experiment. The feasibility of DC support capacitor voltage control strategy and single carrier SPWM method are improved.