Behaviour of Non-Deterministic PWM Methods and Influence of DC Link Fluctuation in Harmonic Spreading Effect of VSI Output Voltage Spectrum

Abstract

The pattern of harmonic spectrum at any output voltage level of a voltage source inverter (VSI) is determined by the characteristics of the pulse width modulation (PWM) adopted. The conventional, deterministic PWM methods like sinusoidal PWM (SPWM) results mechanical vibration, acoustic noise, and limit the application scope when used in drives. This is due to the incapability of deterministic PWM in spreading the harmonic power. The random pulse width modulation (RPWM) approach creates continuous harmonic profile instead of discrete dominant harmonics and hence reduces the acoustic noises. Insufficient filtering of dc link results in the amplitude distortion of the input dc voltage to the VSI and has the most significant impact on the spectral errors (difference between theoretical and practical spectra). It is evidenced from the earlier research that deterministic PWM methods have been modified for VSIs working with fluctuating dc input. The influence of dc link fluctuation on harmonics and their spreading effect in the VSI remains invalidated. Case study is done with four different capacitor values. This paper studies the performance of both conventional SPWM and random carrier PWM (RCPWM) in both VSI working constant dc input and fluctuating dc input. MATLAB based simulation study reveals the fundamental component, total harmonic distortion (THD) and harmonic distortion factor (HSF) for various modulation indices.