Reliability analysis of PV generating systems in the islanded DC microgrid under dynamic and transient operation

Abstract

Dynamic fluctuations in the PV-source power and different DC-fault scenarios in the off-grid PV-battery-based DC microgrid could lead to a rapid decrease in the reliability of the PV-generating system. To make this viewpoint clearer, this paper proposes a novel reliability-evaluating methodology for PV-generating systems in the islanded DC microgrid under dynamic and transient operation conditions. Firstly, the dynamic-voltage-varying failure rate (DVVFR) and the fault-current-varying failure rate (FCVFR) of a PV-generating system in the off-grid DC microgrid are formulated. The DVVFR mainly depends on dynamic fluctuations in the PV-source power and the load power, whereas the FCVFR mostly represents the failure probability due to pole-to-pole and pole-to-ground faults in the DC microgrid. Then, a possible combination of the used-time-varying failure rate (TVFR), the power-loss and temperature-dependent failure rate, the DVVFR, and the FCVFR is proposed for evaluating the system-level and component-level reliability of PV-generating sources. Finally, Markov-state transition diagrams and Chapman–Kolmogorov equations are derived and applied for the PV-system reliability assessment in the off-grid PV-battery-based DC microgrid. Experimental results are analysed to reveal that the operation of DC-DC power converters dominates the PV-system reliability under the dynamic and transient operation. The DVVFR of the PV system is mostly smaller than its FCVFR, however, the system-level reliability of the PV-generating source is significantly reduced by dynamic-voltage-fluctuating cases due to the more frequent repetition of these dynamic cases. Moreover, the MTTF and MTBF of the PV-generating system could also be dramatically decreased by the dynamic and transient operation of the DC microgrid.