DC Side Impedance Modelling of an Actual Line Commutated Converter High Voltage Direct Current System and DC Resonance Characteristics

Abstract

The stability analysis of Line Commutated Converter High Voltage Direct Current (LCC-HVDC) is of considerable engineering importance. Establishing its analytical mathematical model helps to analyse stability and the factors that affect stability of the system. This paper developed the six-pulse model based on the principles of synchronous firing control of the six-pulse converter and single sideband modulation principle. By precisely solving the firing and commutation end time offsets for each valve, the small-signal model for the six-pulse converter is derived. Subsequently, the harmonic state-space model and its corresponding harmonic transfer function model in the Laplace domain for LCC are constructed, which can be further transformed into the impedance matrix on the DC-side. Based on the impedance analysis method and maximum peak Nyquist criterion, the established DC impedance model is used to study the resonance mechanism. The result shows that the small stability margin of the system at 125 Hz are the reasons for resonance. Further research was conducted on the influence of control system parameters and control delays on system stability. A Butterworth band-stop filter has been added to the constant current control system of Xing-An HVDC project for suppression of harmonic oscillation. This article has certain reference significance for the planning and operation of HVDC transmission systems.