Study on mismatch losses in large PV plants: data analysis of a case study and modeling approach

The mismatch effect is of fundamental importance in the operation of photovoltaic (PV) power plants because it causes significant losses in energy production. Mismatch originates from several factors such as non-uniform modules aging, shading, dust accumulation, faults in tracker systems and so on. Evaluation of mismatch level is a difficult task because it is necessary to take into account many factors: power configuration, geographical position, weather conditions, quality of measurements provided by dataloggers, etc. Moreover, since different criteria can be applied to calculate the percentage of mismatch, different results might be obtained for the same PV system leading in some cases to wrong conclusions. With the purpose of introducing some approaches for the assessment of mismatch losses in large PV fields, this paper presents a study carried out from data acquired in an existing 300 MW PV plant in Brazil in which an experimental 2.5 MW subfield was realized using 60 kW string inverters whose input stage has more DC/DC input converters. This particular power configuration allows the calculation of mismatch at string level as well as at array level at the DC side input of each string inverter. A multi-criteria approach has been implemented with the purpose to evaluate the coherency of results. A modelling strategy, based on an integrated state-space average model, has been exploited in order to get all the electrical quantities in case of faults, missing or wrong measurements.