Abstract
Electrical tree growth is a well-documented process leading to failure of high voltage polymeric insulation under AC stresses. However, tree growth in HVDC insulation failure is not well understood. This work considers electrical tree degradation in polymeric insulation subjected to combined AC and DC voltages. Tests in LDPE samples of needle-plane geometry yield three types of electrical trees, which grow depending on the magnitude of the AC components, irrespective of the DC voltages. In tests with an AC component of 10 kV (peak amplitude), 12 kV and 15 kV trees are distinguished by both the conductivity of tree channels and the tree shape, and are referred to as either conducting trees, non-conducting branch trees or non-conducting bush trees respectively. With 10 kV AC, tree initiation was significantly accelerated by superimposing −20 kV DC. With +20 kV DC, the incepted trees had more bifurcations, but there was no major change to tree initiation time. Space charge measurements on thin LDPE films provide a basis for understanding the difference between tree initiation with +DC and −DC voltages. The subsequent propagation of conducting trees were not influenced by biasing the 10 kV AC. With 12 kV AC and 15 kV AC, the growths of non-conducting branch and bush trees were both accelerated by positive biasing. A retardation was observed in bush tree growth with −15 kV DC. The effects of DC voltage polarity on tree morphology are different for branch and bush trees. Different relationships between PD magnitudes and tree growth were also found between branch and bush trees. The tree length determined PD magnitudes in branch trees and the evolution of PD in bush trees suggest that the additional DC stress has no impact to PD magnitudes. Nevertheless, DC bias effects on the symmetry between positive and negative discharges have been evidenced through PD analysis for bush trees. Moreover, the modifications on the shape of bush trees are believed to be associated with the changes in PD asymmetry. This work has illustrated the importance of AC ripples in the failure mechanisms in HVDC insulation.
Highlights
The development of power electronics has enabled growth of DC power transmission systems globally
The authors have previously reported that the voltage for electrical tree growth in an epoxy resin specimen has been measured at 60–70 kV under DC voltages, which compares to ∼15 kV under 50 Hz AC [4]
Tree morphologies and discharge magnitudes, it can be speculated that the electrical tree grown at 10 kV AC was initially non-conducting but developed into a conducting type between 20 and 25 min of testing, while the branch and bush trees grown under 12 and 15 kV were of the non-conducting type
Summary
The development of power electronics has enabled growth of DC power transmission systems globally. As a major degradation process in solid polymeric insulation, electrical tree growth has formed an extensive literature This is mainly in the context of AC voltages; treeing under DC is considered in relatively few publications. Since different tree structures are found to grow under different AC voltage magnitudes [21], this work is designed to provide a comprehensive view on how a DC voltage may influence the AC treeing process. This is a first step to determining the influence of voltage ripples on the reliability of DC polymeric insulation. In addition the influence of DC voltages on PD characteristics in different tree structures is examined
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