Investigation of Schottky bypass diodes from a faulty PV plant

Bypass diodes in photovoltaic modules are designed to prevent the solar cells and cell strings in the module from overheating if a module is operated incorrectly. Possible consequences of bypass diode failures are for example power loss, glass breakage or hot spot burns in PV modules. To avoid such situations, the key-role-playing bypass diode must be in good condition and the knowledge of the condition status is of high interest. For this reason, 36 Schottky diodes were randomly selected from the junction boxes of taken-back PV modules and examined using a variety of characterisation methods. These included electrical characterisation and imaging techniques. The investigation identified all three possible bypass diode states: (i) bypass diode functional and conducting in one direction and blocking in the other direction, (ii) bypass diode defective and conducting in both directions and (iii) bypass diode defective and no longer conducting in any direction. Defective bypass diodes that changed from failure state (ii) to failure state (iii) were found in different aged conditions. Conversely, only an abrupt transition was found for the state (i) to failure state (ii). The main objective of having a fast and suitable method for characterising Schottky bypass diodes in PV systems in the field, which is also in simple agreement with results from laboratory measurement technology, was found with the electrician’s multimeter.