Connections Between Induction Generator Effect and Open-Loop Modal Proximity to Cause Sub-Synchronous Oscillations in Series-Compensated Power System

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.