Active Mitigation of Subsynchronous Interactions Between PWM Voltage-Source Converters and Power Networks

Pulse-width-modulated (PWM) voltage-source converters (VSCs) are gaining widespread acceptance in modern power systems. It has been shown recently that full-scale high-power PWM VSCs can induce negative electrical damping at subsynchronous frequencies. However, active reshaping of the VSC incremental output impedance to minimize the negative impacts of a VSC on subsynchronous damping is not reported. To fill out this gap, this paper presents: 1) an extended analysis of the output impedance of a PWM-based two-level VSC; and 2) more importantly, three simple and robust active reshaping techniques to maximize the positive electrical damping in the subsynchronous frequencies without affecting the converter control performance. The first reshaping technique uses the grid voltage and an active-damping controller to generate active impedance that modifies the VSC impedance in the subsynchronous range. The second reshaping technique uses an internal active damping controller to modify the dynamics of the phase-locked loop, which has significant contribution to the negative impedance of the VSC. The third reshaping technique combines the first and second techniques. The proposed active mitigation methods show excellent performance in reshaping the VSC impedance and inducing positive electrical damping to mitigate possible subsynchronous interactions between the VSC and the power network. Further, the proposed compensators show robust control performance at different output power levels of the VSC without significant impact on the converter control performance. A theoretical analysis and comparative time-domain simulation and experimental results are presented to verify the validity and effectiveness of the proposed techniques.