Analysis of Sub-synchronous Oscillations in a Sending-End Power System With Series-Compensated Transmission Line

This chapter introduces the analysis of the SSOs in a sending-end power system with the series-compensated transmission line. Firstly, the concept and theory of open-loop modal resonance are introduced to examine the SSR. For the introduction and examination, a single-input single-output (SISO) closed-loop interconnected model of the power system is used. The connection of the open-loop modal resonance with the conventional method to examine the SSR, i.e., the complex torque coefficient analysis (CTA), is presented. In addition, the connection of open-loop modal resonance with the torque interaction (TI) and the induction generator effect (IGE), which explain why the SSR happens, is studied. Secondly, the SSCI in a power system, where a sending-end DFIG is connected to the main grid via a series-compensated transmission line, is analyzed. The analysis is conducted according to the principle of conventional induction generator effect (IGE) and the open-loop modal resonance. Thirdly, the SSCI is examined by applying the open-loop modal analysis. For the examination, dynamics of the phase-locked loop (PLL) are ignored. Subsequently, the closed-loop interconnected model of the grid-connected DFIG system, established in the d-q coordinated of the DFIG, is simplified to have two inputs and one output. The simplification is in order to introduce the theory and principle of the open-loop modal resonance gradually from the simple case of SISO closed-loop interconnected model to the general case of multi-input and multi-output (MIMO) closed-loop interconnected model. Finally, the SSO in a power system with a sending-end PMSG connected to the main AC grid via a series transmission line is studied by using the theory of open-loop modal resonance.