Abstract
Renewable-energy-based generating systems like photovoltaic (PV) systems, and energy storage devices like batteries are mostly considered as two types of main generation sources for the islanded operation mode of a low-voltage DC microgrid (LV-DC MG). Dynamic characteristics of the off-grid LV-DC MG, for instance, fluctuation in load power or frequent changes in the PV-system output power; as well as transient characteristics such as DC faults, and switching actions of power devices may reduce the converters’ reliability. To explicate the viewpoints mentioned, this paper will evaluate the reliability of DC-DC step-up converters used for renewable energy-based generation systems, typically for the PV systems of the LV-DC MG from different dynamic and fault conditions. To perform the reliability analysis, the dynamic-voltage-dependent (DVD) failure rate (FR) and fault-current-dependent (FCD) failure rate of the PV-system step-up converter will be formularized. More clearly, the assessment of step-up converter reliability and its discrete components is based on a new combination of the used-time-dependent (UTD) failure rate, FCD-FR, and DVD-FR and the use of a Markov-state transition diagram. Experimental results illustrate that the output capacitor of the boost converter is greatly influenced by the transient and dynamic characteristics of the PV-generating system. The DVD-FR of the boost converter is quite small when compared to its FCD-FR. Moreover, the PV-system boost converter’s reliability is most rapidly decreased by multiple repeated dynamic cases when the LV-DC MG is in the islanded mode. In addition, when considering both dynamic and transient characteristics of the LV-DC MG, two indices, namely, mean-time-to-failure (MTTF), and mean-time-between-failures (MTBF) of the PV-system boost converter could also be quickly reduced.
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