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This paper presents a set of equations for line commutated HVdc systems that can be used to calculate the main voltages and currents of positive or negative sequence on the ac side of the rectifier or inverter and to calculate the main voltages and currents on the dc side that are related to one ac system background voltage harmonic of positive or negative sequence. Furthermore, the frequency-dependent impedances of a line commutated HVdc system on the ac or dc side can be derived. It will illustrate how the converter impedances on both sides are related via an impedance transformation function, which is the product of the transfer functions for currents and voltages. The equations for the impedances differ from those given in a previous IEEE paper, especially with respect to the representation of the impedance for the converter transformers. It did, however, lead to similar results for the CIGRE benchmark model. In order to distinguish between a monopolar and bipolar scheme, the equations take the numbers of poles into account. Furthermore, six- or 12-pulse arrangements are considered.