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Electrical trees are widely grown in laboratories from metallic needle electrodes to enable the degradation process to proceed rapidly at voltages of ∼10 kV AC. Here it is shown that cone-shaped air gaps may form at a needle tip, even when they are not visible optically. Using X-ray imaging techniques, samples with and without air gaps up to 55 μm long have been characterized. The presence of air gaps led to partial discharge (PD) patterns resembling void-discharges. Prior to tree initiation, discharge characteristics evolved with increased voltage increments and increased phase lag behind the applied voltage. These changes to PD characteristics reflect a change in the physical state of the air gap. Although samples with air gaps generally initiated trees quicker than those with no gap, PD activities did not appear to play a decisive role in tree initiation times. However, initial voids were found to have major impact on the subsequent tree growth. In samples with persistent pre-tree air-gap PDs, trees grew larger branchy structures with higher PD magnitudes. In addition, the PDs became more asymmetric between positive and negative half-cycles than occurred in samples with no air gaps. It is shown that understanding the interface between the needle and polymer at a micron scale is critical to interpretation of laboratory tree growth experiments.

The effect of DC bias on electrical tree growth and partial discharge (PD) characteristics in epoxy resin (Araldite LY ® 5052/Aradur® HY 5052 supplied by Huntsman) is investigated using three waveforms: AC, AC with positive DC bias, and AC with negative DC bias. Needle-plane samples are used. Tree growth is shown to be accelerated by the combined effect of DC and 50 Hz AC, beyond the AC growth rate. Positive and negative DC biased tests result in 62% and 54% reductions in average time to breakdown, respectively. Different tree structures and stages of development are associated with different partial discharge characteristics, with thick dark tree branches associated with high PD magnitudes, whereas fine tree channel growth is linked with PD magnitudes below 1 pC. AC tests showed five distinct stages of tree growth compared to four stages seen in DC biased tests. In particular, trees growing in the ‘reverse direction’ from the planar to the point electrode, which is observed in the latter stages of AC tests, is not seen in the DC biased tests. It is concluded that for composite voltages, the AC component is the essential driver of tree growth but the DC component can accelerate propagation. AC noise may therefore compromise the reliability of insulation in HVDC networks.

Electric tree growth is a key ageing mechanism leading to breakdown of high voltage electrical insulation. Partial discharges (PDs) are invariably associated with electrical tree inception and propagation. In turn, the physical structure of an electrical tree influences the characteristics of partial discharge activity. Interpretation of PD patterns is therefore central to developing an understanding of the tree propagation process, and also to the use of PD patterns as an asset management tool. Our previous research indicates that the phase resolved PD (PRPD) patterns and pulse sequence analysis (PSA) patterns evolve with tree propagation. A method was proposed to estimate the point-on-wave inception and extinction voltages of PDs in tree channels within each power cycle. It was shown that the evolution of PD patterns is a consequence of changes to PD inception and extinction voltages as a tree develops. This paper provides a deterministic model of partial discharge in tree channels. Simulations of PDs in a straight non-conductive tree channel are based on experimental PD inception, extinction and residual voltages. The quantitative simulations reproduce almost all the characteristics of observed PRPD and PSA patterns. It is concluded that PD events are determined by five key parameters: tree structure, applied voltage, PD inception voltage, PD extinction voltage and PD residual voltage. Key parameters estimated by the method, and the models proposed explain PD activity in non-conductive trees. It is suggested that the PSA and PRPD patterns should be discussed together to fully understand the PD events. This model forms a platform for generating robust information for asset managers using PD measurements from high voltage equipment in service.

Electrical tree initiation in LDPE under positive DC stresses with controlled AC ripple is investigated. Needle-plane geometries were used and partial discharge (PD) signals recorded. Tree initiation occurred around +45 kV, and the initial growth within the first second was long compared to the more gradual processes seen with AC excitation. In each case, when the first tree image was captured, they were of the branch type and at least one order of magnitude longer than those seen under pure AC stresses. 5 kV peak AC superimposed onto +45 kV led to rapid sample failure. Unlike in pure AC cases, PD detection did not correlate well with the progress of electrical treeing (initiation, growth or breakdown) under pure DC stresses. Phase resolved PD analysis did however provide useful information in the presence of an AC ripple.

Measurement systems for determining the space charge distribution within the insulation of full-size extruded HVDC cables gained significantly more importance over the last years. This is induced by an increasing number of HVDC cable projects and increasing voltage levels and hence insulation thicknesses of these cables available on the market. Driven by this evolution, this contribution focuses on the development of a space charge measurement system based on the pulsed-electro-acoustic (PEA) method. The system is designed in order to realize space charge measurements on 30 mm thick cable insulations in principle. Besides tackling critical issues of the hardware and software development, deconvolution and acoustic recovery, some example measurements, which were conducted on a 80kV XLPEinsulated DC-cable with 8 mm insulation thickness, are shown to validate the performance.

Electrical trees are artifacts resulting from aging of polymeric insulation in high electrical fields. Whilst there is some debate concerning the mechanism by which they grow, there is no doubt that their growth can lead to the ultimate failure of the host insulation. Studying electrical trees is mainly confined to measurement of associated partial discharges and observing the physical growth of the tree structure optically. This paper reviews developments in observations of the growth of trees in the laboratory. In particular, consideration is given to the benefits of generating three-dimensional replicas of real trees from X-ray computed tomography (XCT) and serial block face scanning electron microscopy (SBFSEM), and how these can facilitate better understanding of tree development mechanisms. It is concluded that both two- and three-dimensional imaging are required, and these need correlating with partial discharge measurements to develop models of tree growth and effective asset management tools.

The impact of air gaps between a metallic needle tip and the polymer on electrical tree growth has been studied in low density polyethylene (LDPE). Gaps of $115\ \boldsymbol{\mu} \mathbf{m},\ 212\ \boldsymbol{\mu} \mathbf{m}$ and $1037 \boldsymbol{\mu} \mathbf{m}$ have been experimentally investigated. Corona discharges of different types were measured within those air gaps. Accelerated treeing to breakdown was found in the sample with a $115\ \boldsymbol{\mu} \mathbf{m}$ air gap. In this case the streamer coronas were slightly higher in magnitude than the maximum partial discharge (PD) during the tree growth in the no-gap sample. In contrast, lower corona discharge magnitudes were associated with larger air gaps, and electrical trees gradually stopped propagating to the counter electrodes and developed into pine-branch tree structures. This work highlights the differences between electrical trees that effectively grew from a metallic electrode in direct contact with the polymer and a point of corona source.

The influence of electrode separation on electrical treeing in a glassy epoxy resin of needle-plane geometry has been studied under a fixed 50 Hz AC stress of 15 kV peak voltage. Growth of electrical trees is compared between samples of 1 mm, 2 mm, 4 mm and 6 mm electrode separation. The tree inception times and their distributions imply a change of mechanism for tree initiation when the electrode separation is above 4 mm. Subsequent tree growth was not affected by electrode separation. The trees were of the filamentary non-conductive type and did not feature a classical runaway to breakdown. Rather, tree propagation along the needle axis has been found to follow a rate independent of electrode separation. This implies that the filamentary tree growth is not dominated by modification of the electrical field in front of the tree tips, although such a conclusion is in conflict with existing treeing models. Breakdown does notoccur rapidly after a filamentary tree fully crosses the dielectric, but is still primarily determined by the average electrical field across the sample.

Two types of electrical trees have been observed to grow in a glassy epoxy resin of needle-plane geometry under 50 Hz AC voltage. They differ in their constituent channel radii and the associated partial discharge activity. The so-called ‘filamentary tree’ has finer channels and much smaller dischargemagnitudes than the ‘PD tree’. This work describes an electrical treeing process containing a clear transition from filamentary tree growth to PD tree growth with associated changes to partial discharge measurements. Apart from emphasizing the importance of recognizing tree channel thickness, it provides experimental evidence for an electro-mechanical treeing process in which filamentary channel propagation is followed by enlargement of the channel radius by partial discharges. Moreover, the PD characteristics during the transient stage areassessed. The magnitude of partial discharges during the transition stage was found to be polarity dependent.

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.