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Based on the steady state's parameters of the HVDC system, this paper presents an algorithm to calculate the three-pulse harmonic voltage source in the DC side. In the algorithm, various asymmetries in firing angle, commutation reactance and transformer winding as well as the imbalance of ac voltage are considered. According to the state of valves, the waveform of the three-pulse voltage can be divided into 6 segments in one cycle. In each segment, the voltage of the three-pulse model is calculated by considering the AC fundamental and harmonic voltages respectively. Then, through applying Fourier decomposition to the results of three-pulse voltage and merging the same orders of the harmonics, every harmonic order of the three-pulse voltage can be obtained. The algorithm can treat all the asymmetries effectively and calculate harmonics correctly. The validity of the algorithm is verified through a study case.