Abstract
In this paper, a novel adaptive control method is presented, aimed at robustifying the terminal voltage of the photovoltaic generator, interfaced by the current-mode-controlled buck DC-DC converter load, and based on conductance estimation. The photovoltaic generator, which is integrated into the buck converter and a battery storage unit, is continuously affected by the operating point of the system and environmental variables, thus presenting a nonlinear behavior. Furthermore, the development of small-signal equations reveals a potentially unstable condition when the system is used as a micro-grid with a battery storage unit. This study shows that when the nonlinear behavior of the photovoltaic generator is combined with a typical nominal controller, designed for a single nominal operating point and due to the possibility of an unstable condition, it forces the controller to operate mostly outside the nominal operating point. These conditions result in significantly varied closed-loop performance. In contrast, an almost perfect loop gain performance can be achieved when implementing an adaptive controller based on an online conductance estimation method. Applying estimator results and injecting its value in real-time into the inverse-based plant controller results in an adaptive controller. Therefore, the closed-loop performance of the system integrated with an adaptive controller achieves an almost nominal response throughout the operating range.
Highlights
Renewable energy-based systems, such as photovoltaic generators (PVG) [1], are based on the extraction of the maximum possible energy [2] from a solar source [3] by exploiting a wide range of maximum power point tracking (MPPT) algorithms [4]
To solve this problem at any operating point, a small constant value will be added to the system that depends on the characteristics of the PVG to ensure stability throughout the operating range without a significant influence on the response of the system
The overall base system combined with an estimator is illustrated in Fig. 9, where the measured PVG voltage and inductor current are transferred to the estimator
Summary
Renewable energy-based systems, such as photovoltaic generators (PVG) [1], are based on the extraction of the maximum possible energy [2] from a solar source [3] by exploiting a wide range of maximum power point tracking (MPPT) algorithms [4]. To track the changes in real-time of the dependency of the variable PVG-converter-load on the operating point, an implementation of an adaptive control method is required to achieve consistent and nominal closed-loop performance [25] throughout the whole operation range [26]. A fast and accurate estimation of the main component, internal resistance, and modification of the controller value in real-time creates an inverse nominal plant almost identical to the actual inverse plant, it depends on the operating point and environmental conditions. It maintains an almost constant closed-loop performance throughout the operating range.
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