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Effects of modulation techniques on leakage ground currents in a grid-tied transformerless HB-NPC inverter

Effects of modulation techniques on leakage ground currents in a grid-tied transformerless HB-NPC inverter

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In this paper, a comparative study of modulation techniques used in a transformerless H-bridge neutral point clamped (HB-NPC) multilevel inverters is presented. Transformerless inverters, usually found in photovoltaic (PV) applications, are more efficient and economically affordable than isolated systems. Nevertheless, they may cause leakage ground currents. The latter are produced by the stray capacitances formed between the PV panel frame and the ground path. There are different ways to overcome the leakage ground current issue in transformerless topologies, for instance using topologies that generate almost zero leakage currents, by using adequate modulation techniques, by inserting additional filters, and by using adequate control schemes, among others. The comparative study, considers five modulation strategies, namely, level-shifted pulse-width modulation (PWM), phase-shifted PWM, three-level PWM, two sectors hybrid PWM and six sectors hybrid PWM. As it will be demonstrated through the common-mode model analysis, the three last modulations strategies are aimed at alleviating the leakage ground current issue. In this study, the two sectors hybrid PWM and the six sectors hybrid PWM are proposed. These modulation techniques lead to obtaining five output voltage levels keeping the RMS value of the leakage currents bounded for the transformerless HB-NPC inverter.

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