Research On Key Technologies Of Three-Level Boost For Photovoltaic Grid-connected Power System

There is a strong trend to use non-isolated photovoltaic grid-connected inverters in order to acquire higher efficiency, smaller size and weight. Meanwhile, with the inverter power rating and voltage level increasing, ordinary non-isolated photovoltaic grid-connected inverters have been difficult to meet people’s needs, while non-isolated three-phase three-level photovoltaic grid-connected inverters have been widely used. This paper focuses on the TL-Boost (Three Level Boost) circuit, and the TL-Boost that is applied to non-isolated three-phase three-level photovoltaic grid-connected inverter in parallel serves as the front-end DC/DC converter, to solve the drawbacks of ordinary Boost in high-voltage power applications. The second stage DC/AC adopts diode-clamped three-phase three-level inverter with the advantages of high efficiency, low harmonic and so on.Common-mode leakage current problem must be solved in non-isolated photovoltaic grid-connected inverters. This thesis analyzes the different modulation strategies of TL-Boost including phase-shifted modulation and synchronizing modulation method, establishes the common-mode equivalent model of system, and deduces the formula of common-mode leakage current. It can be concluded that the synchronizing modulation strategy should be adopted to guarantee the low common-mode leakage current. In the paper soft-start method is also studied to construct dc-link voltage of the TL-Boost circuit, a fuzzy control method is proposed to implement the neutral point potential balance, and the MPPT (Maximum Power Point Tracking) algorithms combining constant voltage method with the variable step perturbation and observation method is employed to achieve fast tracking speed and stable performance.In order to solve the problem of neutral point potential imbalance even when the inverter is still out of normal operation, the proposed fuzzy controller is based on the TL-Boost circuit. The inputs of the fuzzy controller only are from the voltage error of dc-link two capacitors and the corresponding variation rate, and then the fuzzy decision is given in light of control rules in advanced to modify the output duty cycle rate that is from MPPT control algorithm. It is obvious that the implement method can not consume the extra hardware resource and cost is low.Finally, a simulation model is constructed and a17kW experimental prototype developed with floating-point DSP (Digital Signal Processor) TMS320F28335from TI, and the full digital control of TL-Boost circuit is employed. And the simulation and experimental results verify the feasibility of the theoretical analysis and the design.