Antonios Tzimas

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.

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 effect of dc poling duration on space charge relaxation in virgin XLPE cable peelings

The effect of dc poling time upon the time-dependent decay of space charge in insulation peelings of cross-linked polyethylene (XLPE) cable that had not previously experienced either electrical or thermal stressing is investigated. Two dc poling durations were used, 2 h and 26 h at an electric field of 50 kV mm−1 and at ambient temperature. Space charge was measured in the two samples investigated both during space charge accumulation and throughout its subsequent decay. The results show that the length of dc poling plays an important role in the subsequent decay. Despite the fact that both samples have had the same amount of space charge by the end of both short and long poling durations the time dependence of the space charge decay is different. Most of the charge stored in the sample that had experienced the short time poling decays rapidly after voltage removal. On the other hand, the charge that is stored in the sample with the long dc poling duration decays slowly and its decay occurs in two stages. The data, which are analysed by means of the de-trapping theory of space charge decay, imply that the charge stored in the material has occupied energy states with different trap depth ranges. The two poling durations lead to different relative amounts of charge in each of the two trap depth ranges. Possible reasons for this are discussed.

DC utilization of existing LVAC distribution cables

Low Voltage (LV) AC cables are predominant in distribution networks across the world. The possibility of transforming such networks to DC may hold significant advantages and it is widely being considered. In particular it may allow a greater flow of electrical energy within urban areas, allowing adoption of electrical vehicles and Smart Grid applications. Distribution networks in UK use well established 4-core Paper Insulated Lead Covered (PILC) belted cables as well as newer cross linked polyethylene (XLPE) screened cables. Optimal utilization of existing conductors in the cables is of paramount importance in order to fully take advantage of the infrastructure. DC can be applied using a unipolar or a bipolar configuration. Both configurations possess advantages in different situations therefore both are considered in this study. This paper investigates different scenarios and proposes the optimal configurations for an LVDC distribution network. The implications for optimizing power flow are considered.

Asset management frameworks for outdoor composite insulators

Power supply utilities are continuously working to maintain reliable and efficient electrical networks that meet the growing demand for electricity. This is a complex task in which appropriate maintenance, refurbishment and replacement policies for all the assets are critical. Optimising business processes through these constitutes a key challenge of balancing service quality and stakeholder value. Here we present two frameworks that can be used to effectively condition monitor both ethylene propylene diene monomer (EPDM) and silicone rubber (SiR) composite insulators during their lifetime in service. The frameworks are tools to assist asset management decision making. The first framework is derived from a generalized dielectric ageing framework and a more specific one on composite insulators that points out the elements that govern composite insulator materials¿ ageing on power transmission and distribution lines. The second framework defines four aged states in relation to the risk to failure that a composite insulator has in service before its replacement. Properties of materials that can be measured in order to identify ageing are reviewed. The techniques available as engineering tools for measuring these properties are introduced. These are distinguished as techniques that can be carried out on-line and off-line, and as destructive and non-destructive tests. These techniques are then reviewed in the context of monitoring and maintaining reliable and efficient operation of power networks.

Low voltage DC cable insulation challenges and opportunities

This paper addresses the challenges arising when considering the conversion of an existing low voltage cable infrastructure in distribution networks from AC to DC. It will explore the ageing mechanisms of low AC voltage cable insulation and how could they differ when operated at DC voltages. In a DC environment, the cables may experience lower voltages but higher currents; hence the thermal ageing could be the dominant stress component that may determine the lifetime of DC cables. A model based on a campus distribution network is used to identify possible constraints. Existing AC cables and their characteristics (based on a 6.6 kV distribution network), such as ampacity, temperature-current characteristics, power requirements of the system and simulation results for cables at different loads are investigated. Properties affected by ageing under AC conditions for the existing cables are considered along with constraint-points of the network.

Electric fields in LVDC cables

The operation of legacy LVAC distribution cables under DC is considered in this work. The electric field distribution in cable insulation under DC voltage is governed by the electrical conductivity of the material unlike the AC case where it is dependent on the permittivity of the materials. Temperature, water ingress and chemical ageing can increase the conductivity of the insulation. A good insulator should exhibit the least conductivity possible in order to minimise the current flowing through the material. Variations in insulation properties leading to reduced uniformity could cause local elevated stresses in the insulating material which could lead to cable failure. This paper shows how the electric field distribution in typical LVAC cables changes as the conductivity of the insulation is altered due to changes in temperature and electric field when operated under DC. A 4-core Paper Insulated Lead Covered (PILC) belted cable is simulated in COMSOL Multiphysics both under AC and DC conditions and the results are compared.

Electric field analysis of 132kv EPDM insulator and correlation with ageing features

A COMSOL multiphysics electric field analysis of an ethylene-propylene rubber (EPDM) insulator is presented. The 132kV insulator is an inverted post insulator that is taken from a jumper loop. It has been in service in a severely polluted environment for eight years. The insulators surfaces are asymmetrically aged with organic dark deposits covering the insulators surface, and with additional surface markings. The insulator has a manufacturing artifact, known as the mold line, which forms an elongated protrusion along the length of the insulator. This artifact is introduced into the COMSOL model to investigate its effect on the local electrical field. The reason for this study is that the area surrounding the mold line is lighter in color, with markings that suggest discharge activity. The regions of the localized field enhancements are identified and correlated with the ageing features, such as hydrophobicity loss, surface roughness and discoloration observed after eight years in service. A discussion whether the environment or the electric field are dominating the ageing process of the insulator is included.

The effect of material interfaces on electrical tree growth and breakdown time of epoxy resin

This study investigates the effect of barriers and interfaces on the lifetime of epoxy resin samples, as well as the growth characteristics of electrical trees. Four sample types are presented; all having been prepared in the point-plane configuration using a hypodermic needle as the HV electrode. Tests were carried out at 13 kV rms and sample images were taken at fixed one minute intervals during the test period. Results show that the incorporation of a barrier improves the time to breakdown of the epoxy resin tested. The inclusion of a major void defect did not accelerate failure of a sample which included an effective barrier.

Implementation of statistical principles to reflect the long term performance of composite insulators

There are numerous advantages of polymeric composite insulators over the traditional ceramic ones. However composite insulators, being comprised of organic elements, age more rapidly over time. Environmental conditions control both the manner in which composite insulators age, and the effective stresses that insulator strings are exposed to in service. The current work presents a statistical methodology to reflect the reliability of a network consisting of composite insulators. This accounts for the working environment and its impact on both the ageing of the insulator and risk of flashover. This is achieved by combining previously established statistical dimensioning principles according to the sites pollution severity and a four-state Markov ageing model. The risk of flashover is estimated according to the sites pollution severity using statistical principles. Then by choosing appropriate transition probabilities between the states of Markovs ageing process the population of insulators at risk of flashover because of ageing is estimated. The combination of the risk of flashover due to the pollution severity and due to ageing of the insulators results in an estimation of risk of flashover for a given number of insulators in a given point in time. The resulting forecast of flashover performance could be used to assist asset management decisions as well as optimising condition monitoring of composite insulators.