S. M. Rowland

Performance of silicone rubber in DC inclined plane tracking tests

The inclined plane tracking and erosion test IEC-60587 is not specified for DC testing. A dc test has been developed from the current ac standard and three formulations of silicone rubber tested. These materials were tested under three voltage levels (2.3, 2.7 and 3.2 kV) for both polarities. Positive dc tests have the highest average and peak leakage current and exhibit a higher degree of surface damage. The observed surface degradation pattern is heavily dependant on polarity. Consistently higher levels of erosion have been observed in the higher voltage positive cases. Erosion of the surface always starts at the bottom electrode, and spreads toward the top electrode. Puncturing of the 6 mm thick samples or deep erosion over more than half the distance between electrodes has only been observed under 2.7 and 3.2 kV positive polarity tests. Results have been analysed using a variety of leakage current analysis techniques going beyond the criteria specified in the original ac standard. The low-frequency behaviour of the leakage current was monitored using a 15 sample per second current recorder. The leakage current magnitude is investigated in a case study and is shown to follow a normal distribution. Reversing the polarity of tests shows the leakage current seen in a test is largely independent of the surface degradation pattern present, but the surface degradation pattern over the first three hours may dictate the morphology of ensuing deep erosion.

A Performance Analysis of Reconductoring an Overhead Line Structure

A holistic computational methodology is employed in this paper to present an analysis of the widely used aluminum alloy conductors (AAAC) performance on a 33-kV wood pole structure. This analysis highlights the basic system properties that influence its mechanical and electrical performance. A comprehensive comparison of the performance of the common AAAC and aluminum conductor steel reinforced (ACSR) conductors erected on the structure is presented, including the study of the increase in operating temperature on the losses, ampacity and sag, in order to identify the most appropriate conductor for the pre-specified structure. Some recently developed high temperature low sag (HTLS) composite conductors are also studied in terms of power transfer uprating on distribution overhead lines. Their performance is examined at normal temperatures instead of the high operating temperatures for which they are specifically designed for, in order to evaluate the benefits they may offer at distribution level voltages.

Use of image analysis in DC inclined plane tracking tests of nano and micro composites

Leakage current magnitude is used as one of the failure criteria in the tracking and erosion test IEC 60587. The 3rd harmonic is also often used to identify periods of arcing activity. This technique is not available for the equivalent dc test. This paper presents the use of a machine vision camera alongside measurements of leakage current to identify discharge activity during dc testing and so enable determination of arc intermittency. This simple image processing technique is shown to add significant value to the existing leakage current analysis techniques and can provide an understanding of the process of degradation and its dependence on voltage polarity. Tests are reported on a commercial silicone rubber compound, a micro-filled epoxy resin, and the same epoxy resin system with nano particle fillers at 5% by weight. It is shown that the technique is applicable to this wide range of materials. It is also shown that changes to the performance of the epoxy compound by the addition of nano filler is due to more than just improving discharge resistance, reported elsewhere. The wettability of the samples is seen to be important, controlling moisture flow during a test. For this reason, in the case of the top electrode being positive, the nano filled material failed the test earlier than the standard material, with reduced erosion but increased leakage current.

Investigating the impact of harmonics on the breakdown of epoxy resin through electrical tree growth

Many studies have investigated the influence of harmonics on insulation life. A limited number of these studies address electrical tree growth characteristics while even fewer employ harmonics maintaining a constant peak voltage. This experimental study sought to quantitatively identify any distinguishing features of electrical tree growth characteristics as a consequence of harmonic content. The total harmonic distortion (THD) and waveshape (Ks) indices were varied to a maximum of 40% and 1.6 respectively. Electrical trees were developed in point-plane geometry using 2 μm tip radius hypodermic needles and a 2 mm gap in epoxy resin (LY/HY 5052) samples. Upon electrical tree initiation, 50 Hz composite waveforms incorporating harmonic content were applied as the excitation waveform at a constant voltage of 14.4 kV peak for 2.5 hours. No change in the electrical tree growth characteristics was identified using the width/length ratio. Electrical tree growth resulting from low power quality reduced operational insulation life and decreased the Weibull shape parameter β with the 7th harmonic displaying particular characteristics worthy of further investigation.

Effect of long-time electrical and thermal stresses upon the endurance capability of cable insulation material

This paper presents the results of endurance tests that have been carried out on cross-linked polyethylene (XLPE) cable peelings. The peelings were taken from cables that were manufactured from a single batch of XLPE and subjected to electrical (up to 28 kV/mm), thermal T = 363 K (90degC) and electro-thermal stressing for at least 5000 hours. The endurance tests of the peelings (thickness 150 mum) were carried out at the same temperature of T = 363 K as the thermally stressed cable, but at two different AC electrical fields of 55 and 70 kV/mm. The resulting life data for the different sample sets are compared to one another and to that of peelings taken from unaged cables. Weibull analysis of the failures shows that only peelings from cables that had experienced a thermal stress component during their time of stressing as a cable, exhibited a statistically significant reduction in endurance capability. Possible reasons for this reduction of life are discussed.

Stability of alternating current discharges between water drops on insulation surfaces

Discharges between water drops are important in the ageing of hydrophobic outdoor insulators. They may also be important in the processes leading up to flashover of these insulators in high pollution conditions. This paper considers discharges between drops when a limited alternating current is available, as experienced by an ageing insulator in service. A phenomenon is identified in which the length of a discharge between two drops is reduced through a particular type of distortion of the drops. This is visually characterized as a liquid protrusion from each of a pair of water drops along the insulator surface. This process is distinct from vibration of the drops, general distortion of their shape and the very fast emission of jet streams seen in very high fields. The process depends upon the discharge current, the resistivity of the moisture and the hydrophobicity of the insulation surface.

A Framework Linking Knowledge of Insulation Aging to Asset Management - [Feature Article]

A multifactor aging framework is described that links the work of those involved in studying aging of dielectrics to the needs of those involved in asset management and network performance.

Effect of electrical and thermal stressing on charge traps in XLPE cable insulation

Insulation peelings were taken from a single 90 kV ac transmission cable whose segments had been stressed for between ~8000 and 10 000 h either at elevated temperature (363 K) or field 19.5 kV/mm (rms) (T= 293 K). Control peelings were taken from a cable segment that had experienced no stressing. The various peelings were subjected to an endurance test at 70 kV/mm (rms) and T = 363 K, which demonstrated that only those peelings that had experienced thermal stressing had a reduced lifetime compared to the set of control peelings. Space charge measurements made prior to the endurance test showed only negative space charge in all types of peelings and an analysis of the decay dynamics in the control set showed that it resided in two trap distributions at 0.8 to 0.9 eV and 1 to ~1.5 eV. Peelings from a second endurance test (55 kV/mm (rms), T = 363 K) were suspended when the test was terminated at 6088 h. Space charge measurements showed that these samples exhibited positive as well as negative space charge with the positive charge being ~60% of the total. The space charge decay dynamics also showed two trap distributions, ~0.65eV to 0.85 eV and 1eV to ~1.3 eV, with both positive and negative charge in each energy range. Detailed differences in the trap energy range and density in both distributions were found between peelings with different cable histories, and those differences that correlate with a reduction of endurance life are identified. A tentative explanation in terms of anti-oxidant consumption and trap generation by chemical reaction is proffered.

The Relationship Between Insulation Ageing and Power Network Performance

A framework has been developed which links the reliability of insulation and plant to changes in their operating environment. These changes include non-power frequency events such as harmonics and the occurrence of impulses. This work is driven by the new demands put on real networks as a result of changing loads and renewable and distributed generation. The framework presented allows a coherent view of the interaction between models of failure mechanisms and network performance. Such a framework allows a clear interface between system analysts and plant engineers. It provides a platform for integrated asset management using existing knowledge of dielectric ageing mechanisms and condition monitoring. It will also provide a framework for future research in the area.

A Holistic Method for Conductor Ampacity and Sag Computation on an OHL Structure

The rating current (ampacity) of a conductor erected on a particular overhead line (OHL) structure installed at a specified location is influenced by the conductor, the OHL structure, as well as weather and operational parameters. Many studies have been carried out regarding calculating an aerial bare conductors ampacity at a steady-state conductor temperature, but without considering the OHL structure as part of the system. In this paper, a holistic methodology for calculating the conductors ampacity and sag at any temperature and power frequency, erected onto a prespecified OHL structure is developed, considering together the mechanical and electrical parameters of the overall system. This methodology incorporates the conductors basic material properties allowing the calculations to be applied to newly developed high-temperature low-sag composite conductors. In this way, it becomes possible to identify, at the system level, the potential benefits that may result from the improved performance of these conductors as well as to indicate new sizes that may better fit a prespecified system, optimizing its performance. The methodology is also validated with a real system application, resulting in correct predictions of the performance of a four-span double-line system.