On outdoor testing procedures of large samples of PV modules

STC power control of PV module supply requires testing large samples of modules with low uncertainty. This paper analyses the feasibility of outdoor measurements with the modules kept at their operating positions. The classical procedure of recording I-V curves and translating them to STC in accordance with IEC-60891:2021 using the cell temperature directly observed at a few points of the rear of the module entails expanded uncertainties larger than 3% (k = 2), which is too much for this procedure being accepted in quality controls with contractual consequences. A convenient procedure for overcoming this barrier consists in comparing the I-V curves of a tested and a reference module of the same type, both working under the same operating conditions. The latter is mostly secured if they are in adjacent positions. However, when the procedure is applied to large samples of PV modules kept in their operating positions, the distance between both modules can reach tens of meters and significant inter-module temperature differences can arise. A method for counterbalancing these differences consists of correcting the measured power values by considering the temperature difference observed at the back-sheet centres of the tested and the reference modules. That also provides clues to estimating the uncertainty of the results. This procedure has been applied in seven testing campaigns, carried out at commercial PV plants. Dedicated instrumentation, based on two radio linked I-V tracers, allowing the simultaneous measurement of the I-V curves and of the temperature at the centres of the reference and the tested modules, has been developed for that. The resulting uncertainties are similar to those corresponding to high-quality solar simulators and low enough for dealing, in practice, with strict quality control requirements.