Constant Frequency Operated Autonomous AC Microgrid with Different MPPT Methods Applied to PV Panels and with Converting Circuit for Powering Unbalanced Loads

Abstract

In this paper, an autonomous microgrid is proposed to supply power to both balanced and un-balanced loads through step-up and step-down transformers and three-phase transmission lines. The microgrid is operated at constant frequency which is maintained by the grid-forming generator, accompanied by a reactive power compensator. The grid-feeding generator outputs maximum power from solar panel to the microgrid while the grid-supporting generator together with the grid-forming generator outputs power to keep voltage variation within limit. Different MPPT methods have been adopted to extract maximum power from PV panels and their effectiveness has been compared. Three-phase transformers in Δ-Y connection with Y’s neutral connected to its local grounding system are adopted to mainly supply power to balanced three-phase loads and also a small percent of unbalanced loads as at the primary Δ-side only positive- and negative-sequence currents can flow and zero-sequence current is effectively blocked. AC-DC-DC-AC converter with medium-frequency transformer isolation and with four-wires at the output is also adopted to mainly supply power to single-phase loads. Fixed-time-step real time modeling in Matlab/Simulink has been conducted to prove the effectiveness of the system. From the simulation results, it is found that such a system can operate with high power quality: frequency being fixed at 50Hz and voltage deviation from rated one being kept well within ±5%.