The induction generator effect (IGE) and the open-loop modal proximity (OLMP) are two different reasons why sub-synchronous oscillations (SSOs) in a series-compensated power system (SCPS) may occur. The IGE attributes the growing SSOs to negative resistance

while the OLMP explains the SSO mechanism from the standpoint of modal conditions. In this paper

we investigate the connections between the IGE and the OLMP through equivalent RLC circuit and open-loop modal analysis. Our investigation is conducted for two types of series-compensated power systems where either a synchronous generator or a DFIG is connected at the sending end. The investigation reveals the conditions

in which the IGE and the OLMP may jointly cause the growing SSOs

i.e.

both the IGE and the OLMP can explain why the growing SSOs occur. Furthermore

the investigation indicates that the IGE and the OLMP may be totally irrelevant and lead to growing SSOs separately. This implies that it is possible that in a SCPS

the growing SSOs are only due to the IGE

and the OLMP is non-existent

and vice versa. Hence

when the growing SSOs occurs in a SCPS

examination based on both the IGE and the OLMP should be carefully conducted in order to find if the oscillatory instability is due to the IGE

or the OLMP

or both of them.