Mitigating Torsional Interactions with MMC Connected to Turbine-Generator Using a Novel Subsynchronous Damping Controller

Modular multilevel converter (MMC) is a new potential power converter architecture for applications that require high voltage or high power.In investigating the subsynchronous resonance (SSR) characteristics of MMC, detailed small-signal modeling and control design is essential.This paper presents the detailed modeling of MMC in D-Q reference frame and analysis of SSR with MMC connected to a turbine-generator supplying a series compensated long AC transmission line.This paper proposes a novel subsynchronous damping controller (SSDC) that uses a band pass filter (BPF) to extract the subsynchronous frequency components of line current.These subsynchronous frequency components are then injected into the transmission line by an MMC controller, which suppresses them through the generator.Using D-Q model of the study system with a rating of 892.4 MVA at 60 Hz, eigenvalue analysis and transient simulation are used to perform SSR analysis for various series compensation levels from 0.20 p.u to 0.60 p.u in MATLAB-Simulink.The results of eigenvalue analysis are validated with transient simulation for a step disruption of 10% decrement in mechanical input of turbine-generator, which is employed at 0.50 sec and restored at 1.0 sec.Simulation results with MMC show that damping of torsional modes (TMs) improves in high-frequency range of subsynchronous network mode (NMsub), on the other hand, the damping of low-frequency TMs reduces.Therefore, it is clear that the damping of torsional interactions increases with MMC.Finally, SSR characteristics are investigated with a novel SSDC connected to MMC that is activated at 1.50 sec.The results clearly demonstrate the improvement of critical TMs in the system.Thereby reducing the potential risk of SSR at all operating conditions.