Hualong Zheng

Space charge monitoring in cables at low DC electrical field

Space charge measurements based on the Pulsed Electro-Acoustic (PEA) method were performed on a XLPE based mini-cable at low applied DC voltages and under isothermal temperature (60°C). This work was to study the transient of the space charge accumulation due to the field dependent conductivity of the cable insulation. Considering the small charge quantity of charge accumulated and the resulting difficulties in accurate data acquisition, a new method is proposed based on using the peak area ratio of the electrode peaks to characterize the transient behavior of charge accumulation. The experimental results show that the peak area ratio decays exponentially to an equilibrium value determined by the cable cylindrical geometry and therefore a time constant for space charge accumulation can be estimated. Moreover, the measured peak area ratios, which reflect the ratios of electrical fields at two interfaces, from different initial conditions (charge-free, charged steady-state and following polarity reversal) coincide with that calculated from theory published in the literature.

DC electrical tree growth in epoxy resin and the influence of the size of inceptive AC trees

Electrical tree propagation is a precursor to dielectric failure in high voltage polymeric insulation. Tree growth has been widely studied under AC conditions, but its behavior under DC is not well understood. The aim of this work was to examine the importance of polarity and initiating defect size on DC tree propagation. Electrical tree propagation in epoxy resin under constant DC voltages of +60 kV and −60 kV was measured in samples with classical needle-plane electrodes, but with small initial trees (< 100 μm) incepted under lower AC voltages before the DC tests. Experimental results showed strong polarity dependence. In either polarity, the length of the initial AC tree had a major influence on the inception of the subsequent DC tree. The effect was attributed to the defect being influential if it is larger than the space charge injection region. As a consequence, there is a critical defect size that will accelerate failure of DC insulation dependent on space charge injection behavior of the polymer/electrode system in question — a critical finding for high voltage asset management.

Evolution of partial discharges during early tree propagation in epoxy resin

Electrical tree growth is a precursor to dielectric breakdown in high voltage polymeric insulation. Partial discharge (PD) has a close relationship with electrical tree propagation and can be both used to understand the aging process, and as a diagnostic tool for asset management. In this paper it is shown that PD patterns change through the early stages of tree growth, and consideration of these changes gives insight into the processes of tree growth. Here, trees have been grown in epoxy resin in needle-plane geometries with 2 mm gaps, at 15 kV peak AC. The PD phase-resolved pattern can be regarded as a combination of the well-known turtle-like and wing-like PD patterns. As a tree extends its length, a wing-like pattern is seen and the maximum discharge magnitude has an almost linear relationship with the maximum length of the growing branch. Comparison of the energy released by discharges and the vaporization energy needed for tree growth supports the proposal that the wing-like pattern corresponds to PDs responsible for growth in length of the trees. Implications for mechanisms for tree growth are considered. Results suggest that asset managers may be able to use partial discharge analysis to distinguish different stages of tree growth, providing a valuable prognostic tool for optimising high voltage plant management and replacement.

Electrical treeing and reverse tree growth in an epoxy resin

An electrical treeing process is presented which consists of two types of electrical tree structures that coexist in the same epoxy resin: these have been termed as ‘filamentary trees’ and ‘reverse trees’. In samples with needle-plane electrode geometries, a filamentary tree that consists of fine tree channels propagates from the needle electrode to the plane ground electrode under an applied AC voltage. It is observed that once the filamentary tree has crossed the insulation, trees then grow from the planar electrode towards the needle electrode as reverse trees, eventually leading to dielectric breakdown. The characteristics of the treeing processes have been obtained for a range of samples through optical observations. In addition, partial discharge (PD) activity associated with the growth of a reverse tree is thoroughly characterized. The prior existence of a filamentary tree is a prerequisite for the development of a reverse tree. PD does not appear to be a driving force for the growth of the filamentary trees, whereas high levels of PD are associated with the growth of a reverse tree. This distinction shows that aging can occur undetected by PD, but asset management of the more aggressive treeing stages can use PD analytics.