Analysis of Harmonic Oscillations Caused by Grid-Connected VSCs

In this paper, a method of harmonic modal analysis to examine the harmonic oscillations caused by grid-connected voltage source converters (VSCs) is proposed. The linearized harmonic model of grid-connected VSCs is derived. Based on the model, analysis was conducted, which indicated that the damping of harmonic oscillation modes is mainly determined by parameter settings of the VSCs' control systems. Interactions between the VSCs' control systems affect the damping and frequency of the harmonic oscillation modes. High-order harmonic interactions are superimposed on the low-frequency variations of variables associated with the VSCs' control systems and consequently pass through the output filters of the VSCs. Subsequently, strong harmonic interactions between the VSCs occur over the AC network. In addition, harmonic resonance occurs when one of the harmonic oscillation modes of one VSC is closed to another harmonic oscillation mode of the other VSCs on the complex plane. When harmonic resonance occurs, damping of one resonant harmonic oscillation mode degrades, and growing harmonic oscillations may occur. Hence, the parameters of the VSCs' control systems must be carefully tuned to avoid harmonic resonance. An example power system with grid-connected VSCs is presented to demonstrate and evaluate the correctness of the harmonic modal analysis.