String level DC performance analysis in Rooftop photovoltaic systems with multi-MPPT inverters

• A new perfomance analysis approach from monitored data for multi-string inverters. • The new methodology focus on the DC side of the multi-string inverter in Rooftop PV. • A disaggregated approach which provides a performance analysis per string and per MPPT. • This approach may help when detecting faults and malfunctions in these systems. Rooftop photovoltaic (PV) systems are a feasible option to address electricity costs in both residential and industrial sectors. It must be highlighted that multi-string inverters boost the penetration of this type of systems. In the literature many studies have been conducted on the performance analysis of Rooftop PV from monitored data. However, they focus on the inverter output: they don’t assess the performance of the different strings and MPPTs. This fact may make it difficult to assess low performance, which may be masked by the global inverter performance. Therefore, the main goal of this paper is to provide a new performance analysis approach which focus on the DC side of the inverter and will be based on the performance analysis per string, per MPPT and at a global level at the input of the inverter. In this sense, weighted calculated parameters should be considered. Monitored data from Rooftops PV in two olive mills will illustrate this new methodology. Moreover, a Rooftop PV with a gable roof is analysed. Results show that this approach may help when identifying strings that provide low performance values where, if the rest of strings and MPPTs provide a proper performance, this malfunctioning may be difficult to detect in a global performance analysis. Therefore, this disaggregated and embedded approach not only allows a better low performance detection but can also play a fundamental role in maintaining the desired performance and meeting the design requirements.