Robust dynamic control strategy for standalone photovoltaic system under varying load and environmental conditions

Abstract

Standalone photovoltaic (PV) systems are widely considered as an alternative source of utility grid due to the notable merits such as inexhaustible solar energy, pollution and noise free power generation, ease of assembly and relatively low costs. However, the major drawbacks of these systems are their environmentally-dependent characteristics and performance degradation due to sudden load variations. In order to address these challenges, two objectives must be met simultaneously for consistent and reliable output of PV system. First, the efficient tracking of maximum power point of the PV array in changing environmental conditions and secondly, the smooth conversion of the direct current (DC) input voltage into the desired level of alternating current (AC) output voltage in the presence of load variations. In this thesis, a standalone PV system with two independent control strategies have been presented. At the first stage, a hybrid non-linear maximum power point (MPPT) technique based on the perturb and observe and integral back-stepping control algorithm is proposed to extract the maximum power from the PV array. The integral action in the MPPT algorithm significantly reduces the oscillations in the PV array output that is fed to the DC-AC inverter at the second stage. Then, at the second stage, a dynamic disturbance rejection strategy based on super twisting sliding mode control (ST-SMC) has been proposed to regulate AC power for a variety of loads at the system output. The PV inverter load parameter disturbances and their effect on the system dynamics are aggregated into a perturbation, which is then estimated online by a newly designed higher-order sliding mode observer. The estimated perturbation is then compensated by the ST-SMC such that a better control performance could be achieved with significant robustness against load disturbances. The proposed control algorithms are evaluated and benchmarked with the existing backstepping controller (BSC) in terms of dynamic response, efficiency, steady-state error and total harmonic distortion (THD) handling capability under varying environmental and load conditions. The designed control strategy reaches the steady-state in 0.005 sec and gives a DC-DC conversion efficiency of 99.85% for the peak solar irradiation level as compared to the 0.008 sec and 99.7% for BSC. The AC-stage steady-state error is minimized to 0.005V compared to 0.51V of BSC whereas, THD is limited to 0.07% and 0.11% for linear and non-linear loads respectively for the proposed algorithm as compared to 0.34% and 2.04% for BSC.