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Nonlinear power electronic circuits operating in unbalanced power systems are the cause of many unwanted uncharacteristic harmonics. In case of 12 pulse HVDC converter, fundamental unbalance causes positive sequence third and negative sequence ninth harmonics, among others. This paper investigates the use of a linearised frequency domain technique, modelling an unbalanced system impedance as a small signal deviation from a balanced operating point. The time domain simulation package, PSCAD/EMTDC, is used to verify the results from the linearised solution method. The frequency domain technique is efficient and easily expanded to more complex networks with multiple converters.