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In this paper, the effects of harmonic superimposed DC voltage on electrical tree growth characteristics in ethylene propylene diene monomer (EPDM) for HVDC cable accessories insulation are studied. Harmonic component with a frequency from 50 to 450 Hz is superimposed on DC voltage to analyze the mechanism of harmonic amplitude, order and voltage polarity on electrical tree growth. The results show that the electrical treeing process is promoted with an increase in harmonic amplitude and order, which brings a longer tree length and more accumulated damage to the insulation. Especially when the amplitude of harmonic component exceeds that of DC voltage, the electrical treeing processes are significantly deteriorated, resulting from the large amount of energy released by the recombination of hetero charges. The growth of electrical tree has an obvious polarity effect which tends to be branch-like tree under positive voltage and bush-like tree under negative voltage. The polarity effect may be caused by different propagation processes of electron avalanche under different voltage polarities. As harmonic order increases, the main channel of positive electrical tree is broadened, while the negative tree becomes denser with a lot of bifurcation. These results indicate that harmonic component accelerates the initiation and propagation of electrical tree by affecting the charge transport behavior under the superimposed electric field. [ABSTRACT FROM AUTHOR]