Study On The Common-mode Voltage Suppressing Strategy Of Two-level Inverter

Two-level inverters are among the most widely used power electronic devices. The common-mode voltage is generated when they are working, causing damage to the sensitive equipment nearby. With the power switching devices getting higher capacity and frequency, the damage is getting worse and worse. Traditional modulation strategies neglect the damage and its suppressing. This paper researched on the common-mode voltage suppressing strategies of two-level inverters to reduce its damage.Analyzing the composition and establishing the mathematical model can help suppress the common-mode voltage effectively. This paper analyzed the damage of the common-mode voltage and gained its expression by concluding. Then, the spectrum of common-mode voltage was analyzed while the two-level inverter was modulated by SPWM. A modeling method of common-mode voltage source was introduced while the Double Fourier Integral method was used to accurately describe the switching process and different modulation strategies were considered. Then taking SVPWM as an example, the effect of modulation index on the common-mode voltage spectrum was analyzed quantitatively.Inverters using traditional Model Predictive Control generate common-mode voltage with high amplitude. After introducing the basic principles of MPC applied in Active Power Filter, this paper analyzed the feasibility of suppressing common-mode voltage using MPC without zero-vectors. To improve the inverter output performance, MPC based on virtual vectors and its detail implementation steps were presented. Considering the dead time, the generating regularity of common-mode peak voltage was analyzed theoretically. The performance of the improved MPCs was proved by the simulations and experiments. The correctness of theoretical analysis and validity of improved MPCs were also verified.To suppress common-mode voltage, Random PWM strategy uses random switching mode to gain continuous and uniform distribution spectrum. Considering the randomness of the spectra, a method based on power spectrum density was introduced from the statistical communication theory to analyze the spectrum dispersing performance. Taking Random Pulse Position PWM strategy as an example, its ability to disperse spectrum and suppress common-mode voltage was proved theoretically. The basic principle of Variable-delay Random PWM strategy was analyzed in detail, a dispersing coefficient was introduced to remove constraints to the inverter output performance and spectrum dispersing ability from the sampling period. The method to calculate the dispersing coefficient was also presented. Then the ability to disperse spectrum and suppress common-mode voltage was proved experimentally by the comparison between SVPWM and Variable-delay Random PWM. The correctness of theoretical analysis was also verified. The improved method to calculate the switching period and dispersing coefficient was proved reasonable by the experimental results of different variation ranges of switching frequency.