Three-level Inverter Discontinuous Hybrid Modulation Strategy And Neutral-Point Voltage Balancing Control

The photovoltaic grid-connected inverter is the key equipment that convert the initial DC power generated by the solar panels into the AC power used directly for the user.The requirements for performance of grid-connected inverter is high.This thesis research NPC-type three-level photovoltaic grid-connected inverter deeply focusing on the two questions that optimal modulation algorithm and neutral-point potential fluctuating?Firstly,based on consulting a number of domestic and foreign literatures,this thesis reviews the status of domestic and foreign development on photovoltaic inverters,and background and significance of photovoltaic power generationdescribes modulation algorithm,meanwhile the research status of modulation strategy and neutral-point potential balancing of NPC-type inverter.Secondly,focuses on the analysis of several important topological structure of inverter,and compares the characteristics of inverter with different structure and summarized the mathematical model of inverter.In order to achieve the purpose of reducing the switching loss while ensuring low harmonic distortion and output voltage quality,analyzing the characteristics and disadvantages of the seven segment modulation and the five segment modulation.The seven segment can ensure the stability of the output voltage,and the five segment can reduce the switching frequency.Combine both advantages,According to the five segment discontinuous modulation strategy,the switching range is 120 degrees,switching times less,each time only one bridge arm switch for action principle,Deduction discontinuous hybrid modulation strategy,in a fundamental period between the adjacent 30 sectors adopted five section and the seven section modulation method,Based on the calculation of the switching time of each phase,two-level algorithm,is corrected to the three-level algorithm,By calculation,and the calculation process is simplified summarized discontinuous hybrid modulation strategy realization methodFinally,contrary to the inherent neutral-point volatility of the three-level diode clamped converter topology,a set of neutral point potential control scheme is proposed based on the discontinuous hybrid modulation strategy,Compare the difference between the two capacitors with the standard values,By judging the difference,adjust the width of the pulse width of thepositive and negative half cycle,change the time of small vector.If the positive half cycle of the clamping part of the extended,the time to maintain the high state of the corresponding extension,therefore,the small vector time increases at high level.At the same time,the negative half cycle of the clamping part is shortened,the low level of small state vector time shortened,ensure a cycle clamping angle is fixed,...