PV Module Degradation Due to Frequent and Prolonged Inverter Clipping: A Preliminary Study

Photovoltaic (PV) plant owners usually install PV arrays that are larger than the inverter's rated capacity. With a DC to AC ratio greater than unity, the PV array occasionally operates at a sub-optimal point imposed by the inverter. This is common practice in the USA, and is known as ‘clipping.’ Clipping may be implemented by operating the array at voltages below the maximum power point (MPP), or more commonly, above the MPP. Advantages of clipping operation are the utilization of an inverter's full capacity and improved financial break-even time. Regulatory requirements may also mandate reduced power output from an array in a forced clipping situation known as curtailment. However, the long-term impact on the PV array's life, array degradation, aging, hot spots, and module warranty has not been adequately investigated for prolonged off-MPP operation. This paper presents preliminary studies of the potential long-term impact of clipping on the PV array beyond energy production. For this study, the module and string level data, primarily through I-V curves and IR imaging, is investigated along with analysis of inverter electrical data. This preliminary work shows that the PV array exhibits significantly different temperature signatures if operated below the maximum power point as opposed to above, conditions which could be experienced when clipping. In addition, the initial data revealed unique IR imaging patterns: checkerboard patterns at voltages below the MPP and uniform elevated temperature patterns at voltages above the MPP.