A P-Q capability chart approach to characterize grid connected PV-units

Photovoltaic (PV) generation units are more and more used in order to have a local production cutting down the use of conventional fuels and providing reactive support to the supply. Such gridconnected systems fix mandatory constraints to power electronic converters to meet power quality specifications and draw the maximum power conversion from renewable sources. This paper deals with the possibility to employ a grid-connected PV production unit not only as an active power supply but also as an ancillary service provider. In a recent paper, an advanced control scheme has been proposed in order to control P and Q power flows injected by the PV unit, which can therefore support reactive power compensation. Here, the aim is to develop a modelling approach which allows us to define a sort of capability curve for the PV unit, i.e. the set of points in the P-Q plane which, at steady state, can be reached by properly operating the control system and without exceeding the physical limits of all the involved devices. Basic algorithms are developed in the MATLAB environment and rely on a simplified description of the system, which neglects the harmonics injected by the VSC inverter. Starting form real data of PV units, analytical expressions have been derived for their characteristic in the plane I-V, best fitting the experimental points. Afterwards, the developed model is used to evaluate all the possible working points in the P-Q chart; Finally, simulations with the electromagnetic code PSCAD-EMTDC, which allows representing with high detail all the components, are performed in order to assess the validity of the approximate results. The advantage of the approximate model is that with very low computational cost (less than 1 minute) it is possible to completely characterise a PV unit in terms of possible P and Q injections, being confident that such working points maintain stability properties during dynamic performances.