In this paper an analysis of the line interactive device UPQC (Unified Power Quality Conditioner) from a reliability point of view is carried out. A brief description of its topology points out the components constituting it, such as the static transfer switch, the converters, the energy storage unit and the input static switch. The device normal and fault conditions are studied in order to define the load voltage magnitude starting from the operational states of the components: we can see that this relationship depends significantly on the compensator topology and the protection system. The series unit protection system is defined and verified by means of numerical simulation. The stochastic process describing UPQC behaviour is studied, in the hypothesis that state durations, namely life and repair times, are exponentially distributed. Assuming stochastic independence for all the components, the whole system follows a continuous time Markov process with a finite state space. System analysis is then performed in stationary conditions, making it possible to estimate the MTBF (Mean Time Between Failures) and the MTTR (Mean Time To Restoration) of the output compensator voltage. Finally load voltage MTBF is computed taking also a mechanical bypass switch into account.
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