Effect of shading on the switching of bypass diodes in PV modules

Abstract

Bypass diodes are installed in Photovoltaic (PV) modules in order to prevent the application of high reverse voltage across the shaded cells in the event of partial shading of the module. Crystalline silicon (c-Si) modules have one bypass diode per 18-20 cells while thin film modules have at most one bypass diode per module. Ideally, bypass diodes are expected to turn on as soon as a current mismatch is detected between various strings of cells inside the module, which typically occurs in the event of partial shading. However, limited information is available on the actual switching characteristics of bypass diodes in field. In this paper, effect of incremental shading of various cells on the flasher I-V curve of a commercial 60-cell c-Si module was studied. Cell combinations in various strings were shaded with operational bypass diodes in the module and effect on module performance parameters such as Voc, Isc, Pmax and Fill Factor was discussed. Consequently one bypass diode in the module was short circuited and open circuited respectively and again the effect of shading on the I-V curve was investigated. Techniques for identifying short circuited and open circuited bypass diodes from I-V curve are presented.