| In recent years, with the continuous enhancement of the capacity of photovoltaic power generation system, multi-level PV inverting system has been developed rapidly. The multi-level power converter has the advantages of high withstand voltage, low switching stress, and can achieve high-quality waveform output in the condition of lower switching frequency.There are still more problems need to study for multi-level photovoltaic power generation system, such as balance of midpoint potential, leakage current, DC voltage utilization, switching frequency optimization, predigesting of control strategy, three-phase unbalanced anti-issues and comprehensive optimization of the problems above. To solve the problems mentioned above, corresponding research for three-phase three-wire and four-wire hybrid clamped three-level PV inverting system has been done in this subject, and then multi-objective optimization program has been obtained for each of the two different structures.Firstly, corresponding analysis for the development of PV system and background and significance of multilevel PV system has been given in this paper. The researching contents and objectives of the subject have been proposed after the study of problems exiting in traditional multi-level PV system.And then, the research of the capacitors’ charging and discharging loops and control method for a three-phase hybrid clamped three-level three-wire system has been done. Based on traditional SPWM and Triple Harmonics Injection method, two-phase control method which is suitable for this topology has been proposed. By analyzing the charging and discharging loops of clamped capacitors in two-phase control, clamped capacitors can be successfully charged and discharged, which can ensure reliable balance of the midpoint potential. However, the methods are based on traditional phase-shifted PWM method no matter SPWM and two-phase method. By analyzing the vectors and combining the three new discharging conclusions, the traditional phase-shifted PWM method can not fully use of all the discharging loops which will lead to the discharging process of clamped capacitors are much slower. The slower discharging of clamped capacitors can cause the enhancement of THD for dynamic system and increase the possibility of clamped capacitors’voltages getting higher and higher. To solve the problem above, the PWM signals have been improved based on traditional phase-shifted SPWM and two-phase method. The improved PWM signals can make the clamped capacitors discharge faster which not only improves the dynamic output but also make sure the voltage of clamped capacitors not higher than Vdc/2. Moreover, the midpoint potential can keep balance all the time.Two-phase control method can also improve the DC voltage utilization by15%and decrease the switching loss by1/3. Control method of independent and grid-connected operation mode for this topology in the condition of two-phase method has been analyzed. What needs to say is that two-phase method is also suitable for inverting systems with more levels.Furthermore, the research of the capacitors’charging and discharging loops and control method for a three-phase hybrid clamped three-level four-wire system has been done. Based on the study above,3D-SPWM method for this topology has been proposed. This method can not only own simple control, but also guarantee the balance of midpoint potential, reduce leakage current of system, and depress three-phase load imbalance effectively. For the inverting system with this topology,3D-SPWM can meet the control of system without the need for complex3D-SPWM. Control method of independent and grid-connected operation mode for this topology in the condition of3D-SPWM method has been analyzed.Finally, hardware and software parts for a three-phase hybrid clamped three-level PV generation system have been analyzed in detail and an experimental platform based on FPGA has been constructed. Some experimental verification has been obtained in this platform after corresponding simulation. The simulation and experiment results show the effectiveness of two-phase method in hybrid clamped multi-level three-wire system and3D-SPWM method in four-wire system. |
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