Diodes, Shading, and Reverse Bias: Task Group 4
Task Group 4 is focused on testing bypass diodes, which are essential to protecting the module during partially shaded conditions.
When the PV modules are subjected to partial shading, the shaded cells are forced to operate under reverse bias, which leads to operation at high temperature. In case of bypass diodes failed in open circuit, partial shading can cause severe hotspots and fire hazards. For commercial 60/72 cell mc-Si modules, a bypass diode failed in short circuit results in approximately one-third reduction of module power, and hence PV module failure. Even with working bypass diodes, continuous operation of cells at reverse bias, high temperatures due to regular partial shading can cause permanent degradation in cells.
The details of the stresses that lead to these failures are not very well understood. The group is working towards developing enhanced understanding of the stressors and field failure mechanisms in commonly used silicon based Schottky bypass diodes. Knowledge of shading induced long-term degradation mechanisms in cells and lifetime of bypass diodes in field is essential for estimating service life of PV modules.
The Task Group has been studying ESD failures in the United States using a human body model; and using a machine model in Japan. IEC TS 62916 is in the final stages of review by the IEC Technical Committee 82 National Committees before scheduled publication later in 2016. IEC 62979 "Photovoltaic module bypass diode thermal runaway test" is being reviewed by IEC Technical Committee 82. The group in Europe has identified a number of existing standards that may be easily adopted. The group in the USA led by Jabil Solar and Environmental test Center (JSEC) has developed a model for quantitative risk assessment of bypass diodes in junction boxes for failure by thermal runaway. They have tested junction boxes from several manufacturers and have identified the design changes that would be necessary to pass the thermal runaway test for modules with short circuit currents ranging from 8 A to 10.5 A. In addition, they are working towards recommending revisions to the existing bypass diode thermal test in IEC 61215 for modules deployed in extremely hot climates / roof mount configuration. The US group is also working towards climate-specific High Temperature Forward Bias (HTFB) and Thermal Cycling tests for bypass diodes. The group in Japan reported on their studies and, based on their recommendation, the Tlead method (MQT18.1) has been deleted from the Tj measurement in bypass diode thermal test (MQT8). The Vf-Tjmethod (MQT18.2) will be used for the Tj measurement in order to reduce the measurement error. 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 is to investigate field diode failures to co-relate relevant tests that can generate a new proposal. Suggestions will be made based on test results to modify qualification test protocols.
The Chinese regional Task Group 4 was started in July 2014, with 22 participant organizations, including diode manufacturers, junction-box manufacturers, module manufacturers, system installers, universities, and testing and auditing organizations.
The Chinese Group has initiated efforts to:
- Define a diode test to be used to confirm that a diode is good, since sometimes the diodes increase in resistance without failing completely in the open or shorted configuration.
- Define a high temperature durability test for diodes.
- Support international efforts already underway on ESD and thermal runaway associated with the transition between the forward and reverse bias conditions.
Separately, a draft standard for diode testing was initiated in Chinese. This is being translated into English.
IEC 62979 E.1.0 Photovoltaic module bypass diode thermal runaway test is now at CDV stage. The voting results will be discussed at TC82 WG2 Taipei meeting mid May 2016.
Vivek Gade — Jabil, representing the Americas
Narendra Shiradkar – Jabil, representing the Americas
Paul Robusto — Miasole, representing the Americas
Yasunori Uchida — JET, representing Japan
Xian Dong — Zhongshan University, representing China
Chandler Zhang — Hohai University, representing China
Other leaders are welcome.
- N.S Shiradkar and V. Gade, "Climate and Mounting Specific Testing of Bypass Diodes / Junction Boxes for Improving Long Term Reliability and Cost Reduction", Oral Presentation, 43rd IEEE Photovoltaic Specialists Conference, Portland, 2016.
- N.S. Shiradkar, V.S. Gade and K. Sundaram, "Predicting service life of bypass diodes in PV modules", Oral Presentation, 42nd IEEE Photovoltaic Specialists Conference, New Orleans, 2015
- N.S. Shiradkar, V. Gade and K. Sundaram, " Thermal resistance measurement of bypass diode / junction boxes" PV Module Reliability Workshop, Denver, 2015