Non-linear controller approach for an autonomous battery-assisted photovoltaic system feeding an AC load with a non-linear component

The conventional controller design of the autonomous photovoltaic (PV) energy system feeding a load is usually based on a dual-loop cascade controller structure. The controllers are designed based on the small signal equations of the non-linear system. However, most of the elements comprising the energy system are non-linear in nature such that controllers based on small signal models are not effective in all operability regions. Since the PV-battery-assisted energy system has four inputs and five controlled outputs, it is a non-square mult-input-multi-output system as well. To consider the non-linear behaviour of the system, the concept of feedback linearisation control algorithm is used which provides satisfactory performance, ensures input-output decoupled control on each of the outputs and gives the opportunity to achieve any level of dynamic performance through appropriate locations of the closed-loop eigenvalues and zeros. Different modes of operation of the autonomous, battery-assisted PV system and their controls are investigated and their boundaries of operation are determined. The proposed control algorithm is programmed to operate in all the modes of operation and it is shown that for various load demands, solar irradiation levels and the status of the battery, the control system set forth faithfully follows the reference points. These assertions are validated with computer simulation results.