Modular Multilevel Photovoltaic Interfaced Converter with Low Voltage Energy Integration for DC Systems

Abstract

This paper presents a multilevel, cascaded converter topology that allows for integrations of combined photovoltaic and battery storage units. The modular features of the system enable the series connection of PV panels that, in turn, have a parallel connection to a specified voltage level. These voltage levels stem from a series string of power electronic submodules. Likewise, low voltage energy storage units are interfaced to the upper arm of the system via identical converter modules and provide various directions of dc and ac active power flow, depending on the mode of operation. By implementing additional, nestled current loops, guided by passive filters, the system controls the power flow between the photo-voltaic string, batteries and the low voltage dc terminal. The enhanced level of power flow allows the converter to achieve both a controlled energy exchange as well as various auxiliary services in the DC system. This paper demonstrates three distinct types of power flow that are achievable with by combining the multilevel topology along with multifrequency operations. Simulation results from a representative system model are utilised to demonstrate the three possible modes of operation.