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Using classical transmission line steady-state model and based on the three-pulse harmonic voltages of the rectifier side and the inverter side, this paper describes an approach to calculate the DC side harmonic currents in HVDC systems, including the harmonic currents flowing directly in the DC line and the grounding electrode line, as well as the harmonic currents induced in their lightning shield wires. Detailed computing procedures are discussed to calculate the DC harmonic currents. The approach has two steps. In the first step, the DC side system is treated as a general electric network and analyzed by the nodal analysis method. The purpose is to get harmonic voltages on both DC lines' ports. The DC transmission line is treated as a network element, and its coupled phase model is used to form the network nodal admittance matrix. Then Norton equivalent principle is used to convert sources of three-pulse harmonic voltages into injection currents. The harmonic nodal voltages can be calculated by the nodal analysis method. In the second step, harmonic voltages on line ports obtained in the first step are converted into positive and zero sequence. Also the DC lines network are converted into positive and zero sequence according to the operation mode. Then the harmonic currents along the lines are calculated in positive and zero sequence respectively. Then the whole harmonic currents along the DC line are synthesized by the two sequences. In the end of the paper, the calculation of performance index in hvdc system is discussed, namely, equivalent interference current. In the same condition, the results are compared with that of PSCAD/EMTDC simulation, and the validity of the approach is verified through the comparison.