Research On The Three-phase Photovoltaic Grid-Connected Current Source PWM Inverters

As the world's energy shortage and environmental pollution is worsening, more attentions are paid to the development of new energy sources all over the world. Solar energy, as a clean and green energy, has broad development prospects in photovoltaic power generation system.Photovoltaic grid-connected power generation is a trend for solar photovoltaic system. In this system, gird-connected inverters are needed. In traditional photovoltaic grid-connected power generation system, voltage source inverter is more used, it can only work in the state when the DC voltage is constant and is higher than the peak voltage of power grid. Two or multi-stage circuit topology is generally used for photovoltaic grid-connected system, not only increases the power dissipation but also makes the circuit structure more complex. However, the input dc voltage of current source PWM inverter is lower than the peak grid voltage and can vary in a wide range, which fit well with the dc voltage of the photovoltaic. And the boost circuit which is needed in the voltage source inverter is not necessary in the current source inverter of the photovoltaic power generation system, which makes the circuit structure more simply and reduces the system cost. This paper studied three-phase photovoltaic grid-connected current source PWM inverter and its control strategies, mainly including following aspects.The working principle of three-phase photovoltaic grid-connected current source PWM inverter is analyzed and its low frequency and high frequency time space mathematical model have been founded in abc stationary coordinate. SPWM modulated signal generation and distribution technology of current source PWM inverter are proposed. Through natural sampling rules bi-logical signal of voltage source inverter can be transformed into tri-logical signal of current source inverter and zero state vectors can be allocated according to the minimum switch rule. A general simulation model of photovoltaic cells has been founded based on the study of the output characteristics of photovoltaic cells and the simulation analysis of the whole photovoltaic grid-connected system has been finished based on this general model. The variable step size incremental conductance method is proposed after comprehensive comparison of different maximum power point tracking algorithm (MPPT). Main circuit parameters of three-phase photovoltaic grid-connected current source PWM inverter and PI parameters of its control system are designed and direct current control strategy which is used in the gird-connected system is described in detail.The entire system's hardware circuit has been designed based on the theoretical analysis and a set of 1kW experimental prototype has been built to verify the above theoretical analysis results. Results of simulation and experiment show that this method could not only make output ac currents sinusoidal and in phase with grid voltages, but also make the input dc voltage of current source PWM inverter vary in a wide range and improve dynamic characteristic of the power system, which fit well with photovoltaic grid-connected power generation.