Study On Controlling Technology Of The Midpoint Potential For Three-level Inverter Based-carrier

Compared with conventional two-level inverter, three-level inverter hasmany advantages, for example, less harmonic content, the lower voltage stresson the power switching devices, as well as high efficiency.It has been used insome of the high-voltage power places, such as reactive power compensation,marine drives, rolling mills, and other variable-speed drive. In some speed drivesystems and other applications, some manufacturers have begun a three-leveldiode clamped inverter commercialization, but it has an inherent problem that isthe voltage sizes of two dividing capacitors in series on the DC side, this iscaused by a variety of reasons. In some of the above applications, when thevoltages on the two dividing capacitors are not equal, the output voltagewaveform quality of the inverter may be decreased, or even distorted, so theDC-sided capacitors and switching devices may be damaged.Therefore, thisissue affects the reliability of the inverter operation, but also indirectly affect thespeed of the motor and so on.Viewing the above issue, the main contents are as follows:1) In theory, the background and application of multi-level inverter; theworking principle of three-level diode-clamped inverter is summarized; severalcarrier modulated methods are described briefly, and triangular carrier layeredwith phase PWM modulation method is analyzed.2) Against the main circuit structure of three-level diode-clamped inverter,the causes and impacts of midpoint potential fluctuation on the DC-side areanalyzed, the midpoint potential regulator is proposed, and this design issimulated and experimented, simulation and experimental results verify themidpoint potential fluctuations to some extent, has been effectively suppressed.3) Based on TMS320F2812controller chip, system hardware circuits aredesigned, including a main circuit of three-level diode clamped inverter, adriver circuit of the12Road MOS tube, protection circuit, sampling circuit, and current polarity detection circuit, lay the foundation for follow-up software andhardware debugging.4) Asymmetry regular sampling algorithm of generating the trigger PWMwave to control power switching devices is introduced. And related programsare written, including the main program and the interrupt subroutine.Three-levelSPWM wave generation subroutine and the midpoint potential controlsubroutine have been conducted a performance evaluation of codes.5) Build an experimental platform in the lab, and debug the program, at thesame time, phase voltage, line voltage PWM pulse experimental waveform andPWM control pulse experimental waveform of switching devices have beenobtained under the phase-laminated triangular carrier PWM modulation, and theDC-side midpoint potential experimental waveform has been obtained underclosed-loop control, these experimental waveforms show that the algorithm togenerate SPWM wave is correct, also show that the midpoint potential regulatorproposed to some extent inhibited the midpoint potential fluctuations in the DCside, experimental results are consistent with the theory.