Common Mode Current Suppression And Neutral Point Potential Balance Of Three-level T-type Inverter

Since 1954,the first pratical photovoltaic cells available,china also began to develop solar cells in 1958.In recent years,the national policy and financial support has also accelerated the industrialization and large-scale development of photovoltaic industry.But the large investment and high cost are the key factors to hinder the development of PV system,thus to explore the low cost,high efficiency,high reliability of PV grid system is particularly crucial.The grid connected photovoltaic inverter is an important part in grid connected photovoltaic system,the common-mode current caused by the resonant circuit composed of ground parasitic capacitance and grid impedance can not be neglected,it is also an vital aspect to be ameliorated.In this paper,through the analysis of the structure and mathematical model of the three level T-type grid connected inverter,according to the common mode voltage expression and the difference of the common mode voltage amplitude of the voltage space vectors,based on the traditional common-mode voltage suppression PWM principle,an improved common-mode voltage reduction PWM principle and an improved model prediction algorithm are proposed,the effectiveness of the proposed algorithm is verified by theoretical analysis and simulation.The main work and achievements of this paper are as follows:By analyzing the principle of the common mode voltage eliminate PWM and the common mode voltage reduce PWM,analysis the fluctuation of neutral point potential,an improved common mode voltage reduction PWM principle is proposed for the existing problems,through using mathematic programming for duration of the three-phase switching states,it is capable to reduce the common-mode voltage,by ensuring that the average value of any c arrier cycle neutral point current to zero can achieve the purpose of reducing the amplitude of the neutral point potential.Presented an improved model predictive control algorithm,given the control feasible domain query table of the improved model prediction for the three level T-type grid connected inverter,the idea of hierarchical optimization is used to reduce the computational complexity and the difficulty of weighting factor design,and to improve the reliability of the algorithm.The simulation results show the advantages of the improved common mode voltage reduction principle in the high power,high power density PV grid connection,and also provides a new idea for the grid-connected control strategy of other types of inverters.