Synchronous condensers are commonly-used source devices for the purpose of providing voltage support in the renewable energy generation system, using power electronic devices (PEDs) as the interface connected to the grid. It has been pointed out that the grid-connected PEDs may result in small signal instability issues, especially in weak grids. However, how the synchronous condensers influence the small signal stability of the multi-infeed power electronic system (MIPES) has not been theoretically revealed. This paper aims at filling this gap. We demonstrate that the synchronous condensers can be simplified as an equivalent inductance connected to an ideal voltage source in the concerned sub/super synchronous issues caused by PEDs. On this basis, the interconnection of synchronous condensers is equivalent to increase the grid strength and thus improving the system small signal stability, wherein the power grid strength (or small signal stability margin) is our previous work, quantified by the generalized short-circuit ratio (gSCR). The validity of the theoretical analysis proposed in this paper is verified by a two-area four-machine system.