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This study addresses the issue of the control of both active (P) and reactive (Q) power flows injected by grid connected photovoltaic (PV) units participating in the provision of the reactive power compensation ancillary service. A model is derived allowing to define a sort of `capability chart` for the PV unit, that is, 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. To this aim, a basic algorithm is developed in the MATLAB environment relying on a simplified description of the system, which neglects the harmonics injected by the voltage source converter inverter used to connect the PV unit with a medium voltage radial distribution grid. The proposed model is used to evaluate all the possible working points in the P-Q chart; a proof of the stability of those points, using the well-known Lyapunov methods, is also annexed. Finally, simulations with the electromagnetic code PSCAD-EMTDC are performed in order to assess the validity of the approximate results and to improve the developed basic algorithm.