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.