This paper investigates the dynamic interactions between large-scale PV plants and nearby synchronous generators. To this end, a comprehensive model of a PV plant, including the PV arrays characteristic and their voltage controller, DC/DC converter, DC-link dynamic, DC/AC inverter controllers, and output filter, is obtained. Then, the eigenvalues of the PV plant model are calculated, and it is revealed that the PV plant has a dynamic mode in the sub-synchronous oscillations (SSOs) range. Furthermore, the modal analysis method is employed, and it is shown that the inductance of the DC/DC converter and the DC-link capacitor significantly contribute to the PV sub-synchronous mode. Also, it is shown that this mode oscillations penetrate the grid and increase the risk of sub-synchronous resonance (SSR). Then, the mechanism of dynamic interactions between large-scale PV plants and torsional modes of nearby synchronous generators is revealed. This paper also investigates the required conditions for these interactions and shows that solar irradiation level and the DC-link capacitance are the essential factors that can intensify these interactions.

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