Analysis of SSR with three-level twelve-pulse VSC-based interline power-flow controller

The interline power-flow controller (IPFC) is a voltage-source-converter (VSC)-based flexible ac transmission system (FACTS) controller for series compensation with the unique capability of power-flow management among the multiline transmission systems of a substation. The reactive voltage injected by individual VSCs can be maintained constant or controlled to regulate active power flow in the respective line. While one VSC regulates the dc voltage, the others control the reactive power flows in the lines by injecting series active voltage. This paper presents the modelling of IPFC with 12-pulse, three-level converters and investigates the subsynchronous-resonance (SSR) characteristics of IPFC for different operating modes. The analysis of SSR is carried out based on eigenvalue analysis and transient simulation of the detailed system. It is illustrated with the help of a case study on a system adapted from the IEEE, Second Benchmark Model. The analysis uses both D-Q model (neglecting harmonics in the output voltages of VSCs) and the three-phase model of VSCs using switching functions. While the eigenvalue analysis and controller design is based on the D-Q model, the transient simulation considers both models.