Robust Control of Grid-Connected Photovoltaic Systems Under Unbalanced Faults Without PLL

This paper presents the design of a robust control scheme for grid-connected photovoltaic systems subjected to severe operating conditions such as grid faults, abrupt set-point changes, parametric uncertainties, and unknown disturbances. During unbalanced faults, the scheme is able to deliver either constant real and reactive powers, or constant real power with sinusoidal currents to the grid, without requiring a phase-locked loop or symmetrical component decomposition. Moreover, the same controllers are used under normal operation and grid faults. These feats result in a control system having lesser computational requirements and complexity, and a lower number of design steps in comparison to some existing schemes. The dc-link voltage controller is based on active disturbance rejection control, and phase currents are controlled using repetitive control based on the internal model principle. The controllers are designed using linear matrix inequality constraints that can be solved by readily available tools. A number of simulation test cases are presented using the SimPowerSystems toolbox of the MATLAB/Simulink computing environment to demonstrate the performance of the control scheme under various types of grid faults, parametric uncertainties, and abrupt changes under operating conditions. Controller performance is also validated through digital implementation on a low-cost microcontroller.