Grid-tied PV system modelling: how and why

A critical part of Ascension Technology´s operation and monitoring of numerous grid-tied photovoltaic (PV) power systems for the US Environmental Protection Agency´s PV Demand-Side-Management Demonstration Projects is to insure that these PV systems are operating properly and that problems can be diagnosed by scrutiny of the data being retrieved from these systems. The objective of the work reported here was to develop accurate, flexible PV array and inverter models, and apply them to help understand the performance of the PV systems and address issues which are important to inverter designers/manufacturers, systems designers and system operators. Graphical display of measured PV system performance, along with modelled performance has proven useful for remotely diagnosing a wide range of PV power system phenomena and quantifying their impact: partial array shading, partial array snow cover, PV array max-power voltage lying outside the inverter tracking window (both an array sizing concern, as well as an inverter design issue) and inverter power limiting. Modelling was also key to discovering and understanding an inverter control problem