Investigation and analysis of decentralised multilevel modular integrated converters in small scale grid-tied PV systems

Abstract

This research focuses on the analysis of multilevel voltage interleaving for decentralised cascaded micro inverters in small scale photovoltaic (PV) grid tied applications. These decentralised cascaded micro inverters, otherwise known and modular integrated converters (MICs), have previously been implemented both with multilevel voltage interleaving (requires fast and reliable communications for PV power tracking) and without (requires no communications). The approach proposed by this research utilises a hybrid of both multilevel and non-multilevel switching, which reduces the communications requirement down to less than one system-wide update per second (whilst still allowing for a reduced filter size and lower switching frequency). In addition to the benefits of multilevel switching, the cascaded topology does not require a high gain DC-DC boost stage and maintains the ability to track the power of each PV panel independently. It was found that the optimal number of MICs for a cascaded system should be between 4 and 8 and that such a system should utilise a 1st order inductive filter. Prototype MICs were developed and a comparison was made between a parallel and 2-MIC cascaded system that found an increase in both the efficiency (94.8% to 95.9%) and the total harmonic distortion (THD) (4.8% to 5.2%) for the cascaded system. Additionally, a grid zero-crossing detection error of just 4° in the cascaded system generated enough harmonics to exceed allowable THD limits. The implemented 4-MIC decentralised cascaded system utilised a round robin greedy sorting algorithm to sort power blocks for PV multilevel power tracking with an allocation error generally below 2%. Accounting for typical solar irradiance transient conditions and harmonic standards, it was found that a communications update rate of 0.7Hz is required. Additionally, it was found that grid-tied cascaded MICs have fundamental power sharing ratio limitations that restrict the maximum shading of one MIC to 74% in the 4-MIC system.