Impact of Modulation Schemes on the Power Capability of High-Power Converters with Low Pulse Ratios

The design of modulation schemes at low pulse ratios is a known topic. However, most of previous work has focused on optimized pulse patterns, e.g., selective harmonic elimination. For industrial applications, standard carrier modulators are still highly attractive in many cases. Unfortunately, few papers presented insightful studies on carrier modulators at extremely low pulse ratios. This paper covers this gap. At extremely low pulse ratios, several special phenomena of carrier modulators are introduced for the first time. For example, a significant performance variation effect can be observed when carrier modulators are used. Moreover, the thermal unbalance of the hotspot devices in the converter may lead to significant power capability degradation of the converter unit. In this paper, the mechanisms behind this are explored and explained in detail for the first time. A quantitative comparison between different carrier-based modulation schemes is presented. Based on that, several design instruction for modulators at low pulse ratios are summarized at the end.