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Understanding the role of power quality in aging of HVDC cable systems is critical to the reliable connection of offshore renewable energy sources, and hence global ambitions to reduce carbon emissions. This work investigates electrical treeing with a needle-plane geometry in low density polyethylene under a high negative DC voltage superimposed with AC ripples (-60 kV DC ± 7 kV AC). Tree initiation showed behavior similar to that widely reported under pure power frequencies. Subsequent tree growth, however, was observed to be confined in a smaller area with limited length and width than seen under pure AC voltages, even after long periods of voltage application. Traditional 2D imaging showed overlapped tree channels which developed to cover the whole area within the tree outline. A distinguishing tree shape resulted, which we have named a ‘slim bouquet’ shape. The volume rendering from XCT showed the structure had a 3D fractal dimension greater than 2, considerably larger than its 2D representation. PD signals during the growth had wing-like PRPD clusters and signal phase concentrations between 10-45° appeared after hours of growth. There was comb-like appearance in maximum PD magnitude variation, which is typical in pure high negative DC fields. Consideration is given to both space charge distribution controlled by high DC fields and continuous degradation by AC fields to explain the slim bouquet tree formation.

This article identifies the threat to high-voltage direct current (HVDC) polyethylene insulation posed by the growth of fine electrical trees on the removal of high DC voltages. Such fine trees have been identified previously growing under AC voltages, especially in glassy epoxy resins. However, fine tree growth is distinguished as not being associated with measurable partial discharges (PDs) and so presents a particular threat to high-voltage (HV) system integrity. In the case reported, fine trees are seen to grow in low-density polyethylene (LDPE) from smaller traditional dark trees, on the removal of a negative DC voltage. The growth is seen even in the case of a relatively slow ramp down over many seconds. The growth of a fine tree leaves the system potentially vulnerable to rapid ageing when re-energizing. For cable systems, the threat is that such a tree will grow and is essentially invisible to asset managers using the traditional method of PD measurement.

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.

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.

Partial discharge (PD) characteristics are inherently linked to associated electrical tree growth characteristics. This paper employs both phase-resolved patterns and pulse-sequence analysis (PSA) to study PDs in a short branch-tree grown from a metallic needle tip. It is found that the shape of PSA dV-dV patterns vary with applied voltage; in particular the values of characteristic voltage difference increase with applied voltage. At 10 kV the value of characteristic voltage difference decreases with tree growth, the average number of PDs per cycle increases, the PD magnitudes decrease, and the characteristic wing-like phase-resolved pattern evolves into a turtle-like pattern. This paper proposes a physical model relating PD activity to tree development, considering space charge accumulation inside the tree channel. All key features of experimental results are explained by the model. It is shown that the characteristic voltage differences are determined by the local PD inception and extinction electric fields. These key parameters determining the PD events can be calculated giving a clear physical interpretation of PD measurements, which is invaluable to those using PD as a condition monitoring or asset management tool.

Electrical tree growth through polymeric insulation is a key degradation process in HV cables. Partial discharges (PDs) are considered to be the energy conversion path for the tree growth. Several methods have been employed to study PDs in electrical tree channels, including the phase resolved PD (PRPD) pattern and pulse sequence analysis (PSA). Our recent work proposed a new method of estimating the PD inception and extinction voltages in non-conductive tree channel based on the PD signals. Here a typical non-conductive tree in polyethylene is considered. Both PRPD and PSA have been employed. It is found that after the PD inception, the PD inception and extinction voltages decrease with the tree growth, and become stable before breakdown. It is also found that the decrease of PD inception and extinction voltages causes an increase in the average number of PDs per cycle, and the shrinking of the dV-dV PSA plot. In turn, the average number of PDs per cycle, and the dV-dV plot can be used to monitor the change of PD inception and extinction voltage during the tree growth.

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.

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.

This work investigates electrical tree propagation in a glassy epoxy resin under constant DC voltages of +60 kV and −60 kV, using samples with classical needle-plane electrodes but having small AC trees (henceforth called ‘initial trees’) incepted prior to the DC tests. The characteristics of DC trees propagating from an initial tree is analysed via a sequence of 2-D projections of trees captured using a CCD camera during the test. The experimental results suggest that DC tree growth rate is polarity dependent even when initiated by the presence of an initial AC tree. The shape of the initial AC tree does not have a profound influence on the shape of the DC tree. The length of the initial tree, however, may have a determinant influence on the growth of DC tree. DC treeing exhibits a runway propagation and statistical analysis suggests that tree growth starts to accelerate after reaching a certain distance across the insulation.

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.