Precise performance diagnosis of photovoltaic string by operation voltage and current: Experimental verification

Improvement of the quantitative performance characterization of photovoltaic (PV) strings was investigated, based on their monitoring data during maximum power point tracking (MPPT) operation. The maximum power voltage V-mp and current I-mp of the PV strings under MPPT were continuously monitored, and corrected to standard temperature of 25 degrees C by using the temperature correction formulas, which were recently developed for PV modules. The irradiance G was measured by using a PV module irradiance sensor. It was verified that the formulas are applicable to PV strings, enabling reproducible temperature correction of V-mp and I-mp with relative standard deviation (sigma) of about 0.4-0.9% under various weather conditions. Comparison of the outdoor results with indoor current-voltage curve measurements of the constituent modules in the strings showed that the maximum output power P-max under the standard test conditions STC, i.e. 25 degrees C and 1 kWm(-2), estimated from the temperature-corrected V-mp and I-mp, agreed with the indoor result typically within +/- 1.5%. The experimental results also showed that other information of the string such as partial shading and activation of bypass diodes can be also sensitively detected by analyzing the temperature-corrected V-mp and I-mp. The outdoor measurements and data analysis of this study can be carried out without stopping the MPPT operation of the string. The analysis is performed by using the real-time data. The present method is expected to be applicable to most kinds of crystalline silicon PV strings without modification.