Long-term degradation of polymer insulation systems is a major reliability issue for the electrical generation and supply industries. In the laboratory, unrepresentative needle geometries are ubiquitously used to generate and study electrical tracks and trees. This article describes how tree-like structures and tracking can be generated, without the use of needles, from an engineered cavity at the interface between two polymers. Under these conditions, trees can be grown at relatively low average applied fields of 2 kV/mm. Characteristic partial discharge (PD) activity and light emission are associated with the generation of tree channels. A model describing the growth of such defects is presented containing five stages. This article provides insight into how interfacial stress can lead to failures in high-voltage systems. X-ray computed tomography (XCT) was used to characterize the channel structures and establish a correlation between PD and the shape and type of channel. The use of such voids may present an alternative and improved platform, compared to traditional needle-plane geometries, for investigation of insulation failure.

This study investigates the prediction of audible noise (AN) from transmission lines under complex weather conditions using machine learning (ML) methods based on corona cage experimental data. The original dataset was acquired through experiments conducted in a semi-anechoic chamber, systematically measuring the AN of three types of single conductors under simulated rainfall conditions. Training results demonstrate that the CatBoost prediction model exhibits exceptionally high accuracy, proving that machine learning algorithms can precisely capture the complex non-linear mapping relationships between various input features and AN. To overcome the limitations of a single experimental setup, this study further integrated historical corona cage experimental results to construct an expanded dataset for baseline model optimization. Leveraging this expanded model, the importance weights of various physical factors influencing AN were comprehensively analysed, and the generalization capability of the model was significantly enhanced. Nevertheless, to achieve precise prediction for actual operational transmission lines, the current data-driven model still requires continuous calibration and optimization using in-situ outdoor measurement data in future work.

Electrical tree growth is a key degradation process in the failure of high-voltage cables, but the subtleties behind established methods of reproducing the processes in the laboratory remain unclear. In this work, electrical treeing tests are performed in needle-plane, low-density polyethylene (LDPE) samples with and without initial conical (i.e., needle-shaped) voids around the needle tip. Different growth characteristics are seen among trees originating from needle tips with voids of various lengths (100-250 mu m). Without initial voids, a branch tree develops consisting of stem channels and leaf-type channels, and partial discharge (PD) magnitudes increase almost linearly with tree length. Taking the void-free sample as a benchmark, trees form from voids either grow faster with PD events of higher magnitudes but without leaf-like channels or develop into a distinct dense branch tree consisting of filamentary channel clusters accompanied by smaller PDs. Phase-resolved PD (PRPD) plots reveal that while the accelerated tree growth is dominated by high-magnitude PD clusters, the dense branch structure developed from voids is attributed to low-magnitude PD clusters which are associated with discharges within voids. The influence of voids on electrical treeing highlights degradation threats posed by small voids. The reported PD characteristics may also provide guidance for interpreting measurements of discharges during treeing degradation in insulation materials.

Dielectric interfaces are critical in determining the reliability of high voltage insulation systems. By considering new experimental results and data from electrical treeing in the literature it is shown that not only is the nature of the interface being studied important, but so is the electrode configuration. In particular, voids around the electrodes are critical to inception and development of a track. Unlike in electrical treeing tests where homogeneity is often sought in a laboratory sample, it is concluded that understanding the nature of the interface being modelled is essential and that experimentation must replicate service materials and systems. Comparison of silicone rubber interfaces formed purely by pressure and those which are bonded must be differentiated by whether the source of partial discharges (here a needle tip) is vented, and whether a void is present at the point of discharge initiation.

It has been shown that noise emissions from HV overhead line conductors can be reduced under rain conditions by making their surfaces superhydrophobic. The working environment makes ensuring the longevity of any treatment a major challenge. The degradation of various superhydrophobic surfaces generated by applying a superhydrophobic coating and patterning microscale channels is assessed under an AC electric field (18 kV/cm) with continuous water spray. By examining the droplet distribution on the surfaces during the degradation and the surface roughness before and after degradation, the authors demonstrate that no water droplets were found on the microscale patterned surface, but droplets were formed on the coated surface after degradation. The surface roughness reduction of the coated surface and microscale patterned surface was 29.8% and 11.3%, respectively, indicating that the microscale patterned surface has better durability than the superhydrophobic coating under the AC electric stress.

Power cables connecting floating offshore wind farms will see mechanical stresses not experienced in land-based networks. Cross-linked polyethylene (XLPE) samples from power cable were subjected to compressive and tensile strains of up to 5% to examine how electrical tree growth is affected. A total of 35 samples were tested using needle-to-plane geometry and 50 Hz AC voltage at 7.5 kVrms. Compressive strain widened electrical trees along the direction strain was applied, and tensile strain led to narrower trees, distorting their rotational symmetry, and confirming a mechanical influence on growth. The distortion was linear with strain magnitude. The likelihood of increased tree branch bending or bifurcation angles linearly increased with compressive strain and reduced linearly with tensile strain. The fractal dimension of the trees was unaffected. Strain did not significantly influence the growth rate of trees until a value of 5% was reached. At this value, compressive strain reduced growth rate by a factor of approximately 2, whereas tensile strain showed no influence. Increasing compressive strain was also shown to linearly delay tree initiation. The influence of tensile strain was not quantifiable as trees initiated readily. Evolution of phase-resolved partial discharge patterns (PRPD) was unaffected by the strain applied. Time-to-failure was decreased by a factor of 1.8 at 5.7% tensile strain which could have significance to in-service usage of high voltage cables under dynamic forces found in floating applications.

Audible noise from transmission lines is mainly generated by corona discharge from defects and water droplets on the conductors’ surface. Applying a superhydrophobic coating to the conductors can reduce corona discharge, thereby reducing the level of audible noise produced. In this article, corona discharge and noise measurements from an untreated conductor and a conductor coated with superhydrophobic coatings were carried out in AC electric fields and continuous water spray. The coated conductors show higher partial discharge inception voltages (PDIV) than uncoated ones. PD magnitude and PD per cycle of the coated conductors are lower compared with uncoated conductors, especially at electric fields above 14 kV/cm. A linear correlation between overall sound pressure and total PD magnitude is established. The behavior of corona discharges from single droplets with various sizes and positions and associated droplet vibration were investigated and the local electric field enhancement was analyzed by finite element analysis to examine the influence of droplet size and position on electric field distribution. Superhydrophobic coatings result in reduced PD magnitude and number and therefore reduce audible noise generated. This is attributed to small sessile droplets forming rather than large pendant droplets.

Dynamic cables used for connection of offshore floating renewable energy sources are subjected to strain from tidal motions and there is little knowledge on the effects of strain on electrical tree growth. Laboratory electrical trees are often grown in small-scale opaque samples for visual imaging of degradation. Here, combined electromechanical test techniques have been developed to examine electrical tree characteristics in full-scale cable core experiments to examine the scalability of laboratory work. Testing methodology has been developed for testing on 66 kV power cable samples energized to grow electrical trees from a needle electrode. Partial discharge (PD) data was recorded, and optical microscopy used for imaging of tree growth post-testing. PD behavior was consistent with studies conducted in laboratory scale needle-plane tests, but the final tree geometries were shorter in length and width than expected in many cases. Followup investigations to review the electrode geometry are presented. It has been shown that only by careful control of conditions can samples be fabricated which will allow combined electrical and mechanical testing in future.

This paper explores in detail tree structures grown in negative DC fields superimposed with AC ripples (previously named ‘slim bouquet’ structures). Fine channels which grow during negative DC ramp down are included in the study, and trees grown in purely 50 Hz AC fields are considered for comparison. A state-of-the-art 4K optical microscope was employed to observe tree structures with both transmitted and reflected light. With reflected lighting, dark stem channels and light yellow quasi-2D leaf-type structures were identified in AC trees. Raman spectroscopy suggested the presence of carbon decomposition products within dark stem channels. Bouquet structures were found to have similarities to leaf-type structures as both appeared as light yellow pixels under reflected lighting. Pictures comprising images taken at different focal planes are shown to be powerful tools in understanding tree morphology.

Understanding the role of power quality in the 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 (LDPE) 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 2-D 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 X-ray computed tomography (XCT) showed the structure had a 3-D fractal dimension greater than 2, considerably larger than its 2-D representation. PD signals during the growth had wing-like phase-resolved partial discharge (PRPD) clusters and signal phase concentrations between 10°–45° appeared after hours of growth. There was a comb-like appearance in maximum partial discharge (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.

A comparison is made of electrical tree initiation and growth in LDPE under pure positive and negative DC stresses. Branch trees formed from needles in DC fields, and they comprised channels of ~2 $\mu$m diameter. Positive DC trees exhibited long initial lengths followed by little growth, while negative DC trees had shorter initial lengths with further tree growth. At −45kV, tree initiation length and the length at which tree growth ceases yield an inverse linear fit. PD events were measured before tree initiation in negative DC fields in some cases, and a linear relation was found between initial tree length and the logarithm of the maximum PD magnitude prior to initiation. An electric field threshold for tree growth is proposed. Based on this and an understanding of space charge injection and accumulation, the difference in tree initiation and growth with polarity is explained.

The drive to reduce carbon emissions and meet climate change targets set by governments worldwide has led to the growth of renewable energy sources and electric vehicles in recent years. Resulting intermittent generation and centralised demand require increased transmission capacities when operating associated power systems Existing transmission lines are not capable of meeting planned needs. In many countries, obtaining a new right of way to build an overhead line (OHL) is increasingly difficult. There is therefore an urgent need to study techniques for transmission line uprating. Simply increasing voltages is not feasible because elevated surface potential gradient at conductor surfaces increases audible noise (AN) levels unacceptably. The feasibility of high phase order, especially six-phase, transmission techniques has been previously demonstrated, and it has been proved to be an effective way to uprate existing three-phase double circuit OHLs with increased voltages. This study analyses two typical overhead line structures, whose conductor surface potential gradient and AN level are calculated. The average and maximum surface potential gradient of the six-phase OHL are 18.5% and 18.6% lower than that of the three-phase ones. The AN of the six-phase single circuit OHL is predicted to be 6.33 and 6.45 dBA lower than that of the three-phase double circuit OHL after the voltage is increased by 45% and 50% for the lines considered. It is shown that six-phase OHLs can increase capacity by allowing elevating phase voltages without increasing acoustic noise levels.

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 aging 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.

The acoustic impact of corona discharge from high voltage (HV) conductors is a cause of increasing environmental concern. Excessive corona discharge is initiated under wet weather when a large amount of droplet protrusions is formed on the surface of an HV conductor. The surface condition of the conductor therefore significantly affects the inception, development and intensification of corona discharges through electrohydrodynamic processes. Existing studies on the dynamic characteristics of rain droplets assumes a constant contact angle, which does not conform with the actual profile observed in experiments. This article investigates the dynamic contact angles of charged droplets through a simplified sphere-plane HV experiment. Pendent droplets with a range of sizes have been studied when subjected to AC electric fields. It is found that the variation of dynamic contact angle is closely related to the volume of the water droplet. Clear sinusoidal variation of contact angle with time is seen throughout the whole power cycle for larger droplets $(\ge 6 \mu \text{L})$, while variation is smaller and more difficult to categorize on the smaller droplets $(\lt 6 \mu \text{L})$. The characteristics of dynamic contact angle obtained here can be used for further analysis within the finite element method to investigate its electrohydrodynamic properties.

The influence of DC bias on electrical tree growth under 13 kV square-wave voltages at 50 Hz in epoxy resin is investigated. Tree growth is captured in a needle-plane configuration using a CCD camera and digital microscope. Tree growth is accelerated under positive unipolar square waves in comparison with a bipolar square waves and negative square waves. Tree growth under positive and negative unipolar square wave voltages can be described as a three-stage process, while under bipolar square wave voltages tree growth follows a fivestage process. Reverse tree growth is observed before breakdown only under bipolar square waves. No reverse tree growth was seen under unipolar square waves of either polarity. Damage was observed at the bottom of the plane surface of epoxy where the reverse tree channels initiated.

A void geometry has been developed to generate partial discharge activity and initiate electrical tracking degradation in laboratory insulation systems, providing an alternative to the needle-plane configuration. Reproducible voids were successfully manufactured in epoxy resin at the surface of a rod electrode, creating an enhanced electric field within the voids. Partial discharge measurements have been used to understand the role of discharges in the progression of degradation in the void sample. Fractures formed due to mechanical stresses are observed and investigated by venting the void. Finite element analysis is used to explain damage patterns seen in the epoxy resin samples including a broad region of tracking formation at an associated interface.

The addition of nano-fillers has been widely proposed as a method to enhance the dielectric properties of high voltage polymeric insulation, though there are mixed reports in the literature. Here the potential of silica nanoparticles to extend the time to failure specifically through resistance to electrical tree growth in epoxy resin is determined. The benefit of silane treating the nano-particles before compounding is clearly established with regard to slowing tree growth and subsequent time to failure. The growth of trees in needle-plane samples is measured in the laboratory with loadings of 1, 3 and 5 wt% nano-filler. In all cases the average times to failure are extended, but silane treatment of the nano-particles prior to compounding yields much superior results. The emergence of a pronounced inception time before tree growth is also noted for the higher-filled, silane-treated cases. The average time to failure of silane-treated 5 wt% filled material was 28 times that of the unfilled resin. The improvement in performance between the nanocomposites with untreated and treated fillers is attributed to fewer agglomerations and improved dispersion of the filler in the treated cases. Measurements of Partial Discharge (PD) indicated significant differences in PD patterns during the growth of trees in the treated and untreated cases. This distinction may provide a quality control method for monitoring materials. In particular, long periods in which PDs were not measured were observed in the silane-treated cases. Visual imaging of tree growth in the unfilled material allowed the changing nature of the tree from fine to tree to dark tree to be observed as it grew. Corresponding PD measurements suggest the dark tree is gradually becoming conductive, and that growth of maximum PD measured is dependent on the relative rates of the growth of the tree and its carbonization. X-ray computer tomography identified significant differences in average tree channel diameters (a reduction from 2.8 ?m to 2.0 ?m for 1 wt% and 3 wt% cases). This implies that in addition to tree length changes, evaporated tree volumes also change and may explain the change in partial discharge characteristics observed.

Electrical tree initiation and growth in low-density polyethylene (LDPE) with a needle-plane geometry under negative DC fields was investigated in this work. Many differences have been found between the cases of the needle being energized positive (positive DC fields) and those in which the needle is negative (negative DC fields). Tree initiation in the latter could occur under a wider range of voltage magnitudes and had a shorter initiation length. Negative trees also tended to initiate more quickly than positive trees, and considerable length propagation was observed to immediately follow the initiation. Furthermore, there were differences in partial discharge signals between the two polarities. These distinctions are thought to be caused by the differences in nature between hole injection and electron injection and the subsequent space charge accumulation in the polymer around the needle tip.

Epoxy resin is widely deployed as a high voltage electrical insulation material when compounded with inorganic fillers. However, in the laboratory, the filler prevents visual observation of the long-term degradation known as electrical treeing. To date therefore, much laboratory testing has been conducted on unrepresentative unfilled materials. Here, the impact of micro-sized fillers on the treeing phenomenon in an epoxy system has been explored. Sub-micrometre resolution 3D reconstructions of electrical trees are reported from X-ray computed tomography (XCT) using an advanced ‘pink beam’ synchrotron light source imaging system. The role of filler particles between 1 and 10 μm in size on tree channel propagation is reported. In highly filled materials (30% by weight) a radical change in tree growth behavior is seen, leading to bush tree rather than branch tree growth. The dielectric breakdown time at constant stress was also found to increase as the square root of the filler level. The change in geometry of tree growth may explain the extended life of filled materials in high voltage applications.