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

Abstract

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.