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Photovoltaic systems are composed of several elements as PV source, DC/DC or DC/AC converters, DC or AC machines, accumulators, pumps, fans, and refrigerators. These systems are therefore of hybrid type and their modelling is complex. This is why we used the bond graph methodology. The photovoltaic generator is a special source of finite energy with a non-linear current-voltage characteristic. Thus, performances of linear electrical circuits and systems supplied by conventional sources must be re-evaluated when a PV generator is used as energy source. In the first objective of simulation, we use an averaged bond graph model of a photovoltaic system composed of a PV generator coupled to a DC motor-pump through a buck-boost converter. This converter assures a MPPT (maximum power point tracking) behaviour of the system. In the second objective of stability study of the PV system, a direct linearization of the bond graph model is done. The results show a non minimality of phase of the velocity response. So the PV generator voltage is chosen as variable to be controlled in order to assure an MPPT functioning.