Research On Neutral- Point Balancing For T-type Three-level Inverter

Three-level inverter has advantages of high capacity, high pressure levels, low harmonic content of the output current and low switching loss. It is one of the ideal choices for high voltage and high power applications. However, the three-level inverter also has a defect that can not be avoided —— the neutral-point potential balance issues.Unbalanced three-level inverter neutral point voltage will cause lower order harmonics in the output voltage, give rise to voltage distortion, severely reducing power quality. At the same time, the imbalance will also increase voltage stress on the power switch and the DC bus side. Therefore, the neutral-point balancing issue of three-level inverter has become a hot research of worldwide scholars. The research for this thesis is to research the neutral-point balancing issue of three-level inverter.This thesis first introduces the topology structure and operating principle of T-type three-level inverter. The thesis analyzes the commutation process and the correspondence between the output voltage and switching state, and builds a mathematical model of voltage space vector of T-type three-level inverter to describe the distribution status of space vector. Research the modulation principle and implementation of space vector pulse width modulation(SVPWM) which has been used widely, and make simulation experiment for SVPWM modulation algorithm. According to the action principle of SVPWM algorithm, the thesis analyzes the influence of midpoint potential imbalance to the space vector role and the output waveform.Then, the thesis analyzes the causes of the midpoint potential imbalances from two aspects of modulation algorithm and active current or reactive current. Based on the different impacts of different vectors from SVPWM modulation algorithm, the thesis analyzes action principle of distribution factor method to control the neutral-point balancing. In order to balancing the neutral-point we use the hysteresis control, PI control, charge conservation control and neutral-point average current zero control four different algorithms to calculate the distribution time factor, by changing the redundant small vectors duration of action to achieve the neutral-point balancing purposes. The four algorithms are simulated and analyzed the pros and cons under different operating conditions.Aim at the problem that when the traditional SVPWM modulation algorithm workat the situation in high modulation and bad power factor can not control the neutral-point balancing well, we use another modulation algorithm——virtual space vector pulse width modulation(VSVPWM) which can make the neutral-point average current in a single modulation cycle always zero. This algorithm make the three-level vector control algorithm in action time vector time set aside part of the time as the role of small vectors, resulting in a control period increased by small vector action time,more conducive to balanced midpoint potential.Finally, to solve the problem that VSVPWM modulation algorithm is unable to correct the bias of neutral-point potential, the thesis gives method to optimize VSVPWM modulation algorithm. The method corrects the bias of neutral-point by selecting the suitable small vectors and changing their action time. What’s more, the thesis compare the results of the simulation that before and after optimization of VSVPWM modulation algorithm. And the simulation results show that, in a variety of work conditions, the optimized modulation algorithm can effectively suppress the fluctuation neutral-point potential, and accomplish the purposes of neutral-point balancing control.