Minimization of inter-module leakage current in cascaded H-bridge multilevel inverters for grid connected solar PV applications

Abstract

Cascaded H-bridge multilevel inverters (CHB-MLIs) are well accepted for medium and high power grid connected photovoltaic (PV) applications. Independent MPPT and modularity are the advantages of these converters over the diode clamped and capacitor clamped MLIs. However, CHB-MLIs are associated with inter-module leakage current through the parasitic capacitor of PV panels even though an isolation transformer is used on the AC side. This necessitates the use of an additional isolated DC-DC converter in each power cell, which increases the cost and reduces efficiency. In order to address these limitations, a simple passive solution is proposed to minimize the intermodule leakage current. It comprises of several L-C filters placed among the power cells to mitigate the switching frequency components in voltage across the parasitic capacitors. Moreover, the proposed solution can be easily upgraded to higher number of power cells. In this paper, the expressions for inter-module leakage currents are derived for five level CHB-MLI. The required filter values are designed for various pulse width modulation (PWM) techniques and it is found that the modified carried disposition PWM and hybrid PWM techniques are superior in minimizing the leakage currents. Simulation studies validate the claims of proposed solution.