Reliability Assessment of Bypass Diodes in Photovoltaic Systems Based on Accumulated Damage Calculation Models

During the operation time, the practical lifetime of bypass diodes (BD) in photovoltaic (PV) systems is mostly less than the expected lifetime given by manufacturers. A rapid reduction in the diode lifetime can be because of electrostatic discharge, reverse bias thermal runaway, forward bias overheating, and transition operation from the forward-biased to reverse-biased states of the diode. Indeed, the aforementioned reasons for the failure of BDs are normally caused by dynamic operations, such as partial shading, fluctuation in solar irradiance and temperature, and short-circuit conditions in the PV system. Specifically, dynamic and short-circuit operations of the PV system can cause a sudden increase in junction temperature, the forward current, and reverse voltage which can influence the degradation of bypass diodes. Bypass diodes can undergo thermal runaway and consequently, these bypass diodes of the PV system can be damaged after a certain amount of the used time. To practically evaluate the reliability of bypass diodes in PV systems, this paper proposes to use accumulated damage (AD) calculation models based on the junction temperature, forward current, and reverse voltage. A grid-connected 8kW PV-system testbed is used for collecting dynamic-and-short-circuit experiment data to serve for the reliability analysis of the bypass diodes. As a result, based on the accumulated damage calculation models, the failure rate (FR) and reliability index (RI) of bypass diodes in the PV-generating system can be properly determined, hence, the periods of maintenance, repairment, or replacement of the diodes in the system can be given more practically and logically.