Stability analysis of reduced leakage current modulation techniques for Z-source inverters in transformerless photovoltaic applications

This paper presents three pulse-width modulation techniques capable of reducing leakage currents in transformerless photovoltaic systems based on a modified three-phase Z-source inverter topology. In order to find a direct relation between leakage currents and common-mode voltage in the transformerless photovoltaic system, its equivalent common-mode circuit is determined. Three reduced leakage current pulse-width modulation techniques are described, based on the common-mode voltage for each combination of the Z-source inverter switches. Furthermore, in order to avoid unstable operation of the system, a detailed stability analysis for each modulation technique is carried out. Then, the safe operation ranges for the modulation index, output power factor and inductance of the Z-source impedance are calculated. Simulation and experimental results of transformerless three-phase Z-source inverter connected to a RL load are obtained to prove the effectiveness of the system in reducing the leakage currents without entering in unstable modes.