Reliability evaluation of an aggregate power conversion unit in the off-grid PV-battery-based DC microgrid from local energy communities under dynamic and transient operation

Abstract

In a rural/local energy community (LEC), a PV-battery-based DC microgrid (MG) normally operates at the off-grid/islanded mode. At this off-grid operation mode, the reliability of power converters in both the PV system and the battery energy storage system (BESS) could be reduced by dynamic operation scenarios of the DC microgrid which are frequently repeated, such as the intermittency of PV-system output power, the random fluctuation of load power. Indeed, the dynamic operation of the PV-generating system and the load system in the off-grid DC microgrid could lead to a certain decrease in the reliability of DC–DC bidirectional converters of the BESS, boost converters of the PV system, and buck converters for low-voltage DC loads because of withstanding different power flows to provide a source-load power balance in the DC microgrid. Moreover, transient operation scenarios of the off-grid DC microgrid can significantly impact the reliability of the mentioned power converters. To make the above assumptions more clear, reliability analysis of an aggregate power conversion unit (APCU) in the off-grid PV-battery-based DC microgrid under dynamic and transient operation in the local energy community is presented in this paper. The APCU contains the boost converter of the PV system, the bidirectional converter of the BESS, and the buck converter of the low-voltage DC load system. The main objective of the paper is to shed light on calculations of the dynamic-voltage-dependent failure rate (DVDFR) and fault-current-dependent failure rate (FCDFR) of the APCU from dynamic and transient operation conditions respectively in the off-grid LVDC microgrid. Then, a novel combination of the useful-time-dependent failure rate (UTDFR), the DVDFR, and the FCDFR is proposed to evaluate the system-level and component-level reliability of the APCU in the off-grid DC microgrid. Markov-state transition diagrams are applied for the APCU’s reliability assessment. Experimental results show that the reliability of the bidirectional power converter is more affected by the dynamic and transient operation than that of the boost or buck converters in the APCU. In addition, the DVDFR of the APCU is almost smaller than its FCDFR, but the system-level reliability of the APCU could be significantly reduced by dynamic cases in the islanded DC microgrid. Last but not least, the MTTF and MTBF of the APCU could be dramatically decreased by the dynamic and transient operation of the DC microgrid.

Keywords

• Bidirectional converter
• Boost converter
• Buck converter
• DC microgrid
• Dynamic operation
• DC fault