Progress Update: Task Group 4
Task Group 4 is focused on testing diodes, which are essential to protecting the module during partially shaded conditions.
There is increasing evidence that shading or other non-uniformities in modules causes localized stress that can lead to overheating and, in some cases, to fires. Not only is this failure a serious safety issue, but there is some evidence that aging modules show increasing non-uniformity, implying that this may turn out to be a significant wear-out mechanism. The details of the stresses that lead to these failures are not well understood but may be related to shading (and reverse bias operation associated with shading or other situations), high temperatures, and lightning. Thermal cycling may also be important because thermal fatigue failures can cause stress on the diodes.
The Task Group has been studying ESD failures in the United States using a human body model; and using a machine model in Japan. A New Work Item Proposal submitted to IEC in June 2013 related to electrostatic discharge (ESD) testing. A technical specification/Guideline IEC 62916 was approved early this year 2014. NWIP draft for "Thermal runaway test for bypass diodes" was submitted by team from Japan to TC82/WG2 on Jan 18th 2014 and is expected to be discussed in the next WG2 meeting in June 2014. Preparation of special measuring equipment for establishing Vf-Tj relation to calculate Tj will be undertaken by the team from Japan. The group in the USA is working on a simpler model for predicting thermal runaway in bypass diodes. Additional tests to validate theoretical models related to critical temperature using different diode manufacturers will be performed. This can potentially result in a NWIP for guidance in diode selection. Further test runs such as extended temperature soak tests will be performed, as performance and reliability is known to degrade with exposure to high temperatures for longer periods. Evidence of diode failures in the field has been observed by several groups and the intent would be to investigate field diode failures to co-relate relevant tests that can generate a NWIP within the next 6 months. Suggestions will be made based on test results to modify qualification test protocols.
Vivek Gade — Jabil, representing the Americas
Paul Robusto — Miasole, representing the Americas
Yasunori Uchida — JET, representing Japan
Volunteers representing other parts of the world are welcomed.