Three different aproaches to control a photovoltaic system connected to an unbalanced three-phase power grid

Abstract

This work deals with control for a photovoltaic (PV) system with a three phase inverter connected to an unbalanced three phase electrical grid. Three control strategies are proposed based on the Double Synchronous Referential, which uses two structures capable of independently controlling the positive and negative components of the current injected into the grid by the PV system. Each one of the strategies tries to act on an electrical quantity, improving some characteristics of these. Thus, strategy 1 targets eliminate the negative sequence symmetrical components of the current, and strategies 2 and 3 aim to mitigate the oscillations of active and reactive powers, respectively. The simulation model is a power grid composed of 7 buses, which is executed using Matlab/Simulink® as the simulation software, where the performance of the three strategies is analyzed and evaluated. The grid is unbalanced due to the uneven distribution of loads between its phases, and its primary voltages are determined by voltage sags arising from electrical faults. The results of the simulations are presented, making it possible to observe and compare the performances of the controls in attenuating the current unbalances and oscillations in active and reactive power. Besides, a comparison is made between the strategies using two indices: the unbalance factor of the grid voltages and the electrical line losses. Through the analysis of the results of these indices, it was determined that strategy 2 reduces voltage unbalances in the grid voltages, and losses are lower when strategy 3 is applied to the PV system.