Influence of Sampling Method and Frequency Ratio on PWM Voltage Harmonics in Two-Level Voltage Source Inverters

Analytical evaluation of harmonic spectra of pulse-width-modulated voltages in power electronic converters is a useful tool for practicing engineers. An analytical method which helps in understanding the underlying implications of sampling technique and frequency ratio on harmonic spectrum of PWM voltage is presented. It leads to the derivation of simple and accurate closed-form expressions for Fourier coefficients of output voltage in single-phase and three-phase two-level voltage source inverters (VSI) operated with regularly-sampled sine-triangle pulse width modulation (STPWM). The expressions are derived for both symmetric and asymmetric regular sampling techniques of STPWM. The proposed analytical expressions facilitate the estimation of voltage harmonic spectrum for given dc bus voltage, modulation index and carrier-to-fundamental frequency ratio. This article also presents the effect of frequency ratio on sampled modulating signal and proposes a method to estimate the voltage harmonic spectrum in the case of frequency ratio being a noninteger, which increases its practical utility. Simulation and experimental results under a wide range of operating conditions are presented to validate the analytically evaluated voltage harmonic spectra.