Analysis of the Impact of AC System Single-Phase Earth Fault on HVDC

The paper analyses the dynamic process of HVDC converter, including voltage characteristic of converter AC bus, phase-locked loop (PLL) and the commutation characteristic of valve, when single-phase earth fault occurs in AC system, which is the most typical fault among all AC system faults. An improved method based on switching function is presented to determine voltage harmonic component in dc side. In the method, voltage in dc side is divided into three groups, corresponding to positive sequence of AC side voltage, negative sequence of AC side voltage and variation of valve conduction time respectively. An equivalent circuit for calculating dc side harmonics is set up, in which the converter at fault side is represented by an equivalent harmonic voltage source while the converter at the other side is represented by average commutation impedance accounting for the commutation courses. A test system is established in PSCAD/EMTDC to verify the analysis. Results show that the analysis of converter's dynamic process is correct and high accuracy is gained in harmonics calculation. The improved switching function method solved the problem that switching function cannot be used to unsymmetrical operation of converter, and thus can be used in harmonic analysis in single-phase earth fault and unsymmetrical operation of converter. The presented method can be applied to the value determination and verification of harmonic protection in HVDC control and protection system.