Effect of AC system impedance on the regulation and, hence, the stability margin of HVDC schemes

In the paper, a mathematical analysis of how the AC system network parameters, including the harmonic filter impedances, affect the regulation of an HVDC scheme, is given. The regulation and, in particular, the sign of the regulation will be one of the parameters which will affect the stability of the overall scheme. The paper shows how increasing the impedance at both the rectifier and inverter busbars will either improve or degrade the overall system stability margin, the rectifier impedance improving and the inverter degrading. The importance of a correct assessment of the AC system damping angles is emphasised, by showing the effect on the stability margin and on the correct rating of reactive-compensation equipment. The paper aims to demonstrate the effect of the impedances by carrying out a parametric analysis, which will be helpful to system design engineers in setting up the system which they must model to determine overall stability.