Effect of the Positioning of Single and Double Bypass Diodes in a PV String on its Reliability

One of the more common operational problems in a solar PV system string is the open circuiting of one or more diodes. Open circuit conditions are often created in solar PV strings by mechanical failure of circuit continuity over a period of time or else due to transient conditions like shadow effects which affect one or more cells in the string This affects the reliability of the system. The reliability of a solar photovoltaic (SPV) string can be modified by the use of bypass diodes across affected cells suffering an open circuit. Cells in the string that are enclosed between the ends of the bypass diode, if affected, will be bypassed and the chances of power continuity will be improved. In this paper, we attempt reliability evaluation of a single string of 6 solar PV cells in series using a single bypass diode that can enclose a variable number of cells of the string between its ends. We assume that only one cell is open circuited in the string. Monte Carlo simulations are used to evaluate the LOLP of the string with different bypass diode configurations. Next, we introduce a second bypass diode across the remaining cells not under the influence of the first bypass diode. The object is to examine if any improvements in the system reliability result. The results of the study appear to indicate that both types of bypass diode configurations result in almost identical reliability estimates although some configurations may prove to be better in terms of the LOLP estimates. The evaluation is done for three levels of load i.e. 90%, 80% and 70% of the rated power delivery of the string.