Mechanism of High-frequency Oscillations Induced by the Measurement Link of Voltage Transformer in an Islanded Converter Station Connected to the VSC-HVDC

Modular Multilevel Converter (MMC) is widely used in islanded networks. The application of high-ratio power electronic equipment makes the islanded network in VSC-HVDC present nonlinearity, which leads to the phenomenon of high-frequency oscillation occurring many times in the VSC-HVDC. In order to study the causes of high-frequency oscillations, this paper proposes the mechanism of high-frequency oscillations in MMC-type islanded networks due to the measurement link of voltage transformers. First, a mathematical model of the islanded network containing the voltage transformer measurement link under light load is established based on the π-equivalent circuit. Second, the stability of the islanded network under light load is theoretically analyzed, and the mechanism of high-frequency oscillation of the islanded network induced by the voltage transformer measurement link under light load is proposed. Finally, the simulation verification in PSCAD/EMTDC shows the accuracy of the mechanism of high-frequency oscillation of islanded network induced by the voltage transformer measurement link proposed in this paper.