Damping Subsynchronous Oscillations using an HVDC-based Single-Mode Damping Controller

The use of series capacitors in transmission lines is seen as a necessary means to improve the power transfer capability. Literature has shown that series capacitors can induce torsional oscillations of frequencies below the power system's rated frequency, which leads to fatigue of the turbogenerator shaft and causes fluctuations in the power transfer. For several decades, the controls of an HVDC line added parallel to a resonant AC line have been seen as an effective way to overcome this problem. This paper suggests an alternative Power System Stabilizer (PSS) for inertial mode monitoring and an HVDC-based Single-Mode Damping Controller (SMDC) to improve the stability of unstable torsional modes. The damping controller design consists of two approaches: the time domain simulation-based test signal method and the phase correction method. These approaches analyze the phase difference between a reference DC current deviation test signal (i.e., injected into the rectifier current control loop) and the electromagnetic torque, thereby designing the corresponding phase compensator. Fast Fourier Transform (FFT) is used to verify the controllers' performances.