Study On The SVPWM Control Strategy And The Neutral-point Voltage Balance Of The Three-level NPC Inverter

With the rapid development of the industrial system, the low voltage and lowpower equipment can’t meet the needs of social production，which is becominggradually high voltage and high power. The research of high voltage and high-powermulti-level inverter is becoming gradually the focus in the modern power electronictechnology. Multi-level inverters are used in Frequency Adjustable Speed System, CityElectric Locomotive, Active Filter of Power System, Reactive Power Compensationand Flexible Transmission and Distribution Systems because of the advantages of highdielectric strength, high equivalent switch frequency, small common mode voltage,good output waveform and so on.The Neutral-Point-Clamped (NPC) Three-level inverter is selected as the researchobject. The SVPWM modulation algorithm has been one of the most popular researchpoints because of the main advantages of the strategy are the following： higher DCvoltage utilization radio，sinusoidal output voltage， well digital control；Besides, theneutral-point voltage imbalance is relatively serious problem of the NPC Three-levelinverter. Based on the analysis of the cause of the neutral-point voltage unbalancefactor, the potential impact on the various vector to the Neutral-point voltage isdifferent that is as the research emphasis, a thorough study on that in this paper.According to this topic research goal, the main content of this topic is completedas follows:Firstly, the principle of working and the traditional SVPWM modulationalgorithm for NPC three-level inverters are analyzed detailedly. For a large number oftrigonometric function calculation is involved in the traditional SVPWM algorithmwhich is not conducive to real-time control. A SVPWM algorithm based on60。coordinate system is proposed, simplify the modulation algorithm; meanwhile, theswitch state equation which can realize effective control for neutral-point voltagebalance in a certain range is introduced. Secondly, for the neutral-point potential of NPC three-level inverter, the harm andthe reasons of neutral-point potential offset, and the influence of each vector onneutral-point potential offset is analyzed. The existing hardware and software controlmodes are introduce briefly. The virtual space vector PWM is adopted, which will beoptimized, then applied to the closed-loop control system of the neutral-point potentialfor the NPC three-level inverter.Thirdly, based on the above theoretical research, the SVPWM algorithm based on60。coordinate system and the closed-loop control scheme of the neutral-point potentialoffset are simulinked by the MATLAB/Simulink, the simulation results confirm thecorrectness and effectiveness of them.Lastly, the experimental platform for the NPC three-level inverter has been set up,and the DSP of TMS320F2812is used to create PWM drive signals. The wholehardware system contains main circuit of the NPC three-level inverter, isolating anddriving circuit, detecting circuit and snubber circuit；Software part involves SVPWMmodulation algorithm which is achieve by DSP programming. Through analyzing thewaveforms and data of experiments, conclusions of the strategy of SVPWM is correctand adopted strategy of neutral point potential balancing is effective.