HVDC converters can generate harmonic voltage, which distorts the DC voltage quality and threatens epoxy resin insulation reliability. In this paper, the electrical tree growth was investigated in epoxy resin under harmonic superimposed DC voltage. The DC voltage ranged from -20 to +20 kV and harmonic frequency ranged from 50 to 450 Hz. This experiment was carried out with needle-plate electrodes system. The effects of DC amplitude and harmonic order were characterized by electrical tree length and accumulated damage. Results show that the electrical tree varies with different combinations of DC amplitudes and harmonic orders. The tree length and accumulated damage experience a non-linear trend with the rise of harmonic order. The low-order harmonics bring more damage to epoxy resin under superimposed voltage. The time to breakdown shows a minimum value at 3rd harmonic superimposed voltage. The DC amplitude has an acceleration on the electrical treeing process. With higher DC amplitude, the electrical tree breakdown happens immediately when the electrical tree reaches the plate electrode. The charge transport process accounts for the tree initiation characteristics under different superimposed DC amplitudes and harmonic orders. Meanwhile, “reverse tree” has been found in epoxy resin sample. The field-driven tree growth model has been employed to interpret reverse tree growth from the perspective of dynamic electric field distribution.
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