The effect of AC system impedance on the cross-modulation of distortion in HVDC links

This paper investigates the effect of AC system impedance on the returned harmonic and interharmonic distortion around an HVDC link. A simplified modelling approach is formulated providing insight and understanding into the likely AC conditions required for such problems to occur. Using this approach, general 'rules of thumb' are presented for identifying possible worst-case AC system conditions. A case study consisting of a modified CIGRE benchmark back-to-back HVDC link, with unbalance on the rectifier AC terminals is used to predict the effects of AC system impedance variation on the transfer of distortion through and about the link. In this case, the AC system strength and third harmonic impedances are of significant importance. The simplified approach used is validated using multiperturbation analysis on a full direct linearised solution algorithm.