Effects of Positive-Negative Sequence Separation Method on the Impedance Characteristics of HVDC System

The voltage source converter based high voltage direct current (VSC-HVDC) technology, which utilizes the modular multilevel converter (MMC), is a crucial solution for facilitating large-scale renewable energy integration. The issue of harmonic oscillations between renewable energy and VSC-HVDC has gained significant attention in recent years, and the impedance-based method being an effective tool for analyzing such problems. The control of MMC involves positive and negative sequence inner loop control, with the positive and negative sequence separation (PNSS) method forming the basis of this control. This study aims to establish a generic impedance model of MMC AC- and DC-side, considering three PNSS methods: time delay-based method, notch filter-based method, and decoupled double synchronous reference frame-phased locked loop (DDSRF-PLL) method. The accuracy of the impedance model is verified through PSCAD/EMTDC simulation frequency scan, and the impact of different PNSS on MMC AC/DC-side impedance is revealed. The finding is significant in analyzing renewable energy DC access systems.