In this paper, the input signals and the control model structure of the HVDC link POD control are studied. First, a benchmark is proposed to investigate the input signal selection of the POD controller. Next, by considering different situations, it is shown, based on the residues criteria, that the choice of the input signal depends on the parameters of the system. More precisely, the numerical results show that, the power level of the system and the impedances of its transmission lines play an important role. The proposed benchmark is also used to analyse and to control an inter-area mode. Its validation is done based on a realistic power system of 23 generators. In practice, all these results can be exploited to improve the performances analysis and the control design of modern power systems.