Analysis and Minimization of Output Current Ripple for Discontinuous Pulse-Width Modulation Techniques in Three-Phase Inverters

Abstract

This paper gives the complete analysis of the output current ripple in three-phase voltage source inverters considering the different discontinuous pulse-width modulation (DPWM) strategies. In particular, peak-to-peak current ripple amplitude is analytically evaluated over the fundamental period and compared among the most used DPWMs, including positive and negative clamped (DPWM+ and DPWM´), and the four possible combinations between them, usually named as DPWM0, DPWM1, DPWM2, and DPWM3. The maximum and the average values of peak-to-peak current ripple are estimated, and a simple method to correlate the ripple envelope with the ripple rms is proposed and verified. Furthermore, all the results obtained by DPWMs are compared to the centered pulse-width modulation (CPWM, equivalent to the space vector modulation) to identify the optimal pulse-width modulation (PWM) strategy as a function of the modulation index, taking into account the different average switching frequency. In this way, the PWM technique providing for the minimum output current ripple is identified over the whole modulation range. The analytical developments and the main results are experimentally verified by current ripple measurements with a three-phase PWM inverter prototype supplying an induction motor load.

A Comprehensive AC Current Ripple Analysis and Performance Enhancement via Discontinuous PWM in Three-Phase Four-Leg Grid-Connected Inverters

“…As visible in Figure 25a, reducing switching frequency of the CPWM (taken as a benchmark due to its popularity in industrial applications) by a factor 1/3, DPWM peak-to-peak ripple profile has lower values in comparison with the corresponding CPWM (displayed as 3/2CPWM) for m > 0.21. Similarly, in Figure 25b, the DPWM3 (best among studied DPWM injections in terms of peak-to-peak and rms current ripple performance) rms current ripple profile starts to provide advantages already for m > 0.26 rather than from m = 0.39 as it happens in the three-leg counterpart [26]. The same outcome could have been achieved by increasing the DPWM switching frequency by a factor 50%.…”

“…Equation (26) is the base of the following analysis of current ripple peak-to-peak and rms values, and it is valid regardless of the kind of injection γ(t) has been considered. It must be emphasized here that the total current ripple is composed by primary and secondary peaks, where the latter ones are usually omitted by researchers, despite playing significant role in true rms computation, as it will be shown in Section 4.2.…”

“…By extending Equation (26) to the whole fundamental period [0; 2π], one could find the actual current ripple envelopes that enclose primary and secondary peaks. It can be demonstrated that when u ≤ 0, primary and secondary envelopes can be mathematically described as:…”

Analysis of Equivalent Inductance of Three-Phase Induction Motors in the Switching Frequency Range

“…In Figure 8, the following waveforms are presented: Measured current ripple (orange trace, obtained by downloading the experimental data with a high sample resolution and post-processing in Matlab/Simulink), current ripple envelope analytically calculated by using Leq = 17.36 mH, measured at fsw = 3 kHz (blue trace), and the current ripple envelope, analytically calculated by using Leq = 24.98 mH, measured at 50 Hz (red trace). The procedure of the current ripple envelope calculation is explained in detail in [9,14,15].…”

“…When analysing the output current ripple regarding different PWM techniques and inverter configurations [9,14,15], the switching frequency and the load inductance are directly involved. In case of a squirrel-cage IM supplied by an inverter operating in the typical switching frequency range from 1 kHz to 20 kHz, and considering its equivalent inductance measured at a standstill at 50 Hz, this will result in a wrong current ripple estimation due to the frequency dependency of L eq , which could generally decrease more than 50% ( Figure 5).…”