Electrical Tree in Epoxy Resin under Combined DC-harmonic Voltage

Harmonic voltage can be transferred by the converter transformer on AC and DC sides in high-voltage direct-current (HVDC), which can distort the DC voltage quality. This work investigates the electrical treeing process in molded epoxy resin under combined DC-harmonic voltage. A digital microscope system was employed to observe the electrical treeing process. With the DC voltage ranging from −20 kV to +20 kV, different combined waveforms of DC and harmonic voltage were used as the applied voltage. Width/length ratio was employed to describe the propagation process of electrical tree in two dimensions. Results show that the DC voltage amplitude is a key factor for the electrical treeing process. The distribution of tree structures (branch and bush-branch) depends greatly on the polarity of DC voltage in room temperature. Under the same harmonic voltage, the width/length ratio with negative DC voltage is larger than that with positive DC voltage. Meanwhile, positive DC voltage can lead to the electrical breakdown easier. In addition, ‘reverse tree’ has been found in epoxy resin in pure harmonic voltage, which grows from the plate electrode toward the needle electrode when the electrical tree has bridged the insulation.