Comparative Analysis of String IV Measurement Methods for Fault Detection in Photovoltaic Systems

This work assesses the extent to which three string current-voltage (IV) measurement methods - a portable IV tracer, an inline IV tracer and an inverter - can be utilized to detect faults in photovoltaic (PV) strings in the field. The analyzed faults include potential induced degradation, cell cracks, ribbon damage, short-circuited and open-circuited bypass diodes and shading. We analyse the imprint of fault penetration in the strings with simple diagnostic markers such as the series resistance and shunt resistance, estimated from the slopes of the IV characteristic curve, as well as the IMP -ISC ratio and VMP -VOC ratio, to evaluate their applicability to detect faults in PV string under various conditions outdoors. Results show all three devices measuring very similar IV curves under the described faults and on healthy strings. We demonstrate, that the shunt resistance is suitable to detect early PID at a power loss smaller than 0.5%. Shading proved to be detectable from the series resistance and VMP -VOC ratio from measurements with all three devices.