Abstract
In this paper, dynamic analysis of a multi-battery dual mode charger, powered by a single solar array and suitable for lead-acid and lithium-ion cell-based batteries is presented. Each battery is interfaced to the solar array by means of a current-controlled buck power stage, operating either in constant power or constant voltage mode. Operation in former/latter charging mode implies regulating input/output voltage of the converter, which is a non-trivial situation since while feeding different batteries, all the converters share the same input terminals, connected to the solar array. It is revealed that when at least one of the batteries operates in constant power charging mode, open-loop instability occurs whenever converter input voltage is lower than maximum power point voltage of the solar array. Consequently, input voltage regulating controller must be designed to stabilize closed-loop dynamics for the worst case of instability, which is also derived. Moreover, it is shown that the dynamics of the converters operating under output voltage control are perceived as disturbances by input voltage control loop and must be properly rejected. Simple loop shaping design is proposed based on a PI controller, allowing stabilizing the system in case of worst case instability and rejecting output voltage control induced disturbances at the expense of non-constant, operating-point dependent closed-loop damping.
Highlights
The intermittent nature of photovoltaic sources limits their operation to systems with sufficient storage capacity
Current-controlled buck power stages were assumed as power-processing interfaces, powering batteries operating either in constant power or constant voltage mode, implying input or output voltage converter regulation, respectively
While output-voltage control loops analysis was shown to be trivial in case suitable static operating point exists, input-voltage control dynamics turned out to be problematic
Summary
The intermittent nature of photovoltaic sources limits their operation to systems with sufficient storage capacity. Off-grid systems (often referred to as microgrids) fed by photovoltaic sources require careful sizing and energy management aimed to instantaneously match generation, storage and consumption [1,2,3,4,5,6,7,8]. In both the abovementioned cases, the photovoltaic source is usually connected to a voltage-source type storage (battery or voltage-regulated capacitor) via a power processing interface (DC-DC converter).
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