Studies On Control Strategy Of Multi-level Grid-connected PV Inverters

With the development of the world economy and human society, the massive consumption of traditional fossil energy makes human face great challenges. As a pollution-free, renewable and widespread energy, the new energy has received more and more attention, particularly in solar energy. The photovoltaic (PV) grid-connected system is the main utilization of the solar energy.Recently, the PV grid-connected system has developed rapidly and the PV grid-connected inverter, as the core part, has always been a domestic and foreign research hot spot. Research on PV grid-connected inverter is developing towards high-efficiency and grid-friendly. In contrast with the traditional two-level inverter, the multi-level inverter has the advantages of lower harmonics, higher waveform quality, lower influence on the grid, as well as lower voltage level and longer service life of the switching devices. As a result, it is widely used in high voltage and high power fields. With the progressive development and advance of power electronic devices and digital control technology, the multi-level PV grid-connected system will get fast development.Firstly, several schematic topologies and control strategies are introduced in this dissertation. Taking the typical NPC three-level inverter as an example, this dissertation discusses the working principle of this topology and the space voltage vector PWM (SVPWM) control strategy in detail. With detailed analysis and computation on the method of selecting big and small sectors, computation of the functioning time of the space vectors and the selecting of working orders, a popular and practical SVPWM control strategy is obtained.Secondly, three control methods of balancing the neutral point (NP) voltage are come up with on the basis of analyzing the scheme of the vibration of the NP voltage. Among the three control methods, hysteresis NP control method and accurate NP control method are effective to balance the NP voltage through regulating the functioning time of the little vectors, and the virtual SVPWM (VSVPWM) NP control method gets best control effect through selecting new and proper space vectors. Simulation results demonstrate the effectiveness of the three control methods.In order to get the control scheme of the current flow with the grid, the dynamic mathematical model of the NPC three-level inverter is analyzed firstly, and the commonly-used direct current control method of the PV grid-connected inverter is finally obtained through the method of coordinate transformation, decoupling control and design of two PI regulators for closed loops. Based on the theoretical analysis, a 10kW three-phase three-level grid-connected PV inverter prototype is designed and the hardware and software parts are discussed in detail. Moreover, the simulation results performed on the Matlab /Simulink platform and the experiment results tested on the inverter prototype are present, which have validated the correctness of the grid-connected theory discussed in this dissertation.In the end, the research of the whole study is summarized and the future work is also planned.