Reliability Evaluation For Low Voltage Direct Current Microgrid And Converter Life Extension Strategy

With the increasing prominent problems of traditional fossil energy depletion,climate warming,etc.,vigorously developing clean and renewable energy and launching a new energy revolution have become the focus of attention of all countries in the world.For distributed energy accesses to the distribution network,compared with traditional AC microgrid,low voltage direct current(LVDC)microgrid has many advantages such as lower line loss,larger power supply radius,higher power quality and so on,and it is getting more and more attentions from scholars both in the domestic and overseas.While the safe and stable operation of low voltage direct current microgrid is closely related to the reliability of power supply to users,around the LVDC distribution system,the corresponding reliability evaluation and discussion are developed from component level,equipment level to system level based on physics-offailure theory.For the AC/DC converter,the influences of different topology structures and modulation strategies on converter reliability are analyzed by comparing the reliability of components and converters with four typical topology structures(two-level converter,flying capacitor converter,neutral point clamped converter and T-type converter)and four representative modulation strategies(sine pulse width modulation(SPWM),third harmonic injection pulse width modulation(THIPWM),space vector pulse width modulation(SVPWM)and 60°discontinuous pulse width modulation(DPWM)).It provides a reference for the selections and designs of the AC/DC converters in LVDC microgrids.The results show that under SPWM,Ttype converter has the highest reliability among the four topologies;considering reliability and power quality,T-type converter performs more better with THIPWM and SVPWM.For the converter’s weak link–DC-link capacitors,two feasible measures are proposed.To high frequency isolation AC/DC converter,the weak links in the converter with and without redundancy are analyzed and the influence of redundancy on converter reliability is discussed.In DC/DC converter reliability evaluation,according to the operation principles of photovoltaic converter,energy storage converter,power adapter and DC charger,and component electrothermal models,the electrothermal stresses of components in each converter are solved analytically.The converter’s reliability and the vulnerable components are analyzed combined with lifetime model,accumulated fatigue damage model and Monte Carlo simulation.Based on equipment level reliability,the reliability indexes of the LVDC distribution system–system average interruption frequency index(SAIFI),system average interruption duration index(SAIDI)and average service availability index(ASAI)are quantitatively analyzed according to accessibility theory and distribution network partitioning principle.And the reliability of AC system and DC system is compared.Finally,for the AC-DC converter which is composed of several parallel modules,considering the differences in electrothermal parameters of different modules,the failure time of each module is not equal.The converter life extension strategy based on dynamic power routing is adopted in this paper to extend the AC/DC converter’s operating lifetime and improve its overall reliability.