Comprehensive Spectral Analysis of PWM Waveforms With Compensated DC-Link Oscillations

In the available literature, including reference books on the topic, the spectral analysis of voltage waveforms obtained through the use of pulse width modulation (PWM) relies on the assumption of an ideally flat voltage across the dc-link of a considered switching structure. Nevertheless, certain converter families (e.g., modular multilevel converter, characterized by the presence of distributed and floating dc-links), feature low-frequency oscillations of dc voltage within their switching stages. On these terms, the flat dc-link assumption cannot be considered valid from both theoretical and practical viewpoints. In case carrier-based modulation is employed, unless properly compensated through the modulation signal adjustments, the above mentioned voltage oscillations cause undesirable distortion of the converter currents. However, the very same adjustments of modulation signals introduce additional challenges in the spectral analysis of the voltage across the ac terminals of an observed switching stage. This article provides the ac terminals voltage analysis of a single half-bridge experiencing dc-link oscillations, which are compensated through the modulation signal generation process. PWM with the conventional triangular carrier is considered, while all the results are presented in a generalized form, making them easily extendable to cascaded structures. All of the presented results were experimentally validated.