Small-Signal Model of the NPC + GCC Multilevel Transformerless Inverter in Single-Phase Photovoltaic Power Systems

Abstract

Photovoltaic transformerless inverters are very efficient and economical options for solar-power generation. The absence of the isolation transformer improves the converters’ efficiency, but high-frequency voltage to ground can appear in the photovoltaic string poles. The high capacitance to ground of the photovoltaic generator leads to undesirable high-leakage currents. Using half-bridge topologies dramatically reduces the leakage to ground, and using a multilevel half-bridge inverters improves the output quality compared with classical inverters. The neutral point clamped + generation control circuit (NPC + GCC) topology is a multilevel single-phase transformerless inverter capable of tracking the maximum power point of two photovoltaic sources at the same time. This paper presents the control structure and the dynamic modeling of the NPC + GCC inverter. The pulse-width modulated (PWM) switch model in continuous conduction mode (CCM) was used to obtain the small-signal model of the two switching converters that make up the inverter. The resulting dynamic model was used to quantify the stability margins of both converters’ current and voltage loops.