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This study focuses on efforts to characterize aging occurring in 15 kV distribution cables in a dry climate. It has been shown that similar changes can be produced by a suitably planned accelerated thermal aging testing in the laboratory. The Arrhenius equation is used for establishing the accelerated aging test parameters. Different modes of statistical analysis namely analysis of variance (ANOVA), Andersen-Darling test for normality, F-test, t-test are performed to validate results from accelerated aging tests with field aging. Index Terms--Aging, simulation of aging, laboratory aging, Arrhenius equation.

An impulse current of several kiloamperes was injected to the shield of a shielded cable, which was terminated by a varistor. The induced voltage on the inner conductor caused by this impulse current reaches an amplitude in excess of the varistor's threshold level. The clamped voltage across the varistor and the injected current have been studied for different termination conditions of the shielding cable. Furthermore, this paper also presents the use of a lumped circuit to simulate the transfer impedance of an 'electrically short' shielded cable in the time domain. In combination with the varistor circuit model, the shielded cable with the nonlinear load, a varistor, was also simulated straightforwardly in the time domain. A good agreement was found between the measured voltage and current oscillograms and the calculated waveforms. It is thereby demonstrated the validity of the developed lumped circuit model for the transfer impedance of a shielded cable. Index Terms - Cable shielding, impulse testing, nonlinear system, protection, simulation, time-domain analysis, varistors.

Underground power cables in service are inadvertently subjected to impulses generated by lightning and switching surges that are superimposed on the ac voltage at which they operate. HV transients caused by lightning and switching operations not only radiate large electromagnetic fields but also impose additional stresses on the insulation and could initiate deterioration which can continue under normal operating conditions. Electrical breakdown of HV cables is a local phenomenon and electrical aging at local sites in the polymeric insulation occurs by molecular dissociation of the polymer and formation of new chemical bonds. This process usually involves the electronically excited states of the molecules that give rise to radiative phenomena, such as electroluminescence (EL). Analysis of EL can help to clarify the degradation mechanisms that occur at points of electrical stress enhancement and lead to cable breakdown. The EL technique is several orders of magnitude more sensitive than the commonly employed partial discharge detection and can provide a better understanding of the various time dependent mechanisms such as space charge injection, trapping and decay that can lead to insulation failure. The characteristics of EL in crosslinked PE subjected to impulse voltage, are described in this paper. Impulses having the same polarities as the half cycles of the ac voltage on which they are superimposed give rise to the largest number of EL pulses. Also, more EL pulses are emitted when impulses are applied at the peaks of the positive and negative half cycles than at the zero crossing of the ac voltage. This suggests that the amount of charge injected and trapped into the polymer plays a crucial role for EL emission.

The accumulation of space charge in the crosslinked polyethylene (XLPE) insulation of a power cable sample under dc stress was investigated. The sample was held at 82 [degrees] C and atmospheric pressure, and at 82 [degrees] C and 0.1 Pa, for periods up to several days. Such conditioning removed pre-existing space charge. When an external dc field was applied after conditioning at atmospheric pressure and 82 [degrees] C, space charge accumulated at a rate considerably faster than that observed before conditioning. However the rate of accumulation following conditioning at 82 [degrees] C under reduced pressure was much slower than that observed before conditioning. The faster rate could be recovered by reconditioning the sample at atmospheric pressure after conditioning under reduced pressure. Inversion of the equilibrium space charge profile, following reversal of the applied field, was observed. This observation is consistent with a model incorporating a spatially-inhomogeneous polarization in the amorphous volume of the XLPE, and electron injection/extraction at the electrodes. The electron transfer between electrode and XLPE in either direction involves the same narrow window of combined donor and acceptor states in the insulator, centered on the Fermi level. The spatially-inhomogeneous polarization in the XLPE originates in a spatially-inhomogeneous distribution of dipole complexes. It is suggested that the dipoles are formed by a process equivalent to non-permanent oxidation of the XLPE, requiring water as a catalyst.

Four crosslinked polyethylene (XLPE) and ethylene propylene rubber (EPR) insulated cables were subjected to accelerated thermal, radiation, and sequential radiation and thermal exposure to simulate 20, 30, and 40 yr in-service environments in Ontario Hydro Nuclear Plants. Two sets of samples were evaluated. The first set contained 15 cm cable specimens in tubular form with the conductors removed, while the second set was made up of 4.3 m cable samples wound on mandrels. The cable samples were sequentially irradiated and thermally aged and subjected to an additional accident radiation exposure followed by a simulated loss of coolant accident (LOCA)/main steam line break (MSLB) steam test. During the steam test, samples were energized and their electrical characteristics continuously monitored. The material performance of the insulation was assessed using the 15 cm specimens during the various stages of aging exposure by conventional elongation measurement and by the use of micro-specimens with differential scanning calorimeter (DSC), oxidation induction time (OIT) and infrared carbonyl absorption techniques. The results from these samples were compared with the electrical performance of the cable samples wound on mandrels that had been subjected to the LOCA/MSLB steam test. The test results indicate that with the use of micro-specimens meaningful changes were obtained until the elongation values of the samples were reduced to 50% absolute. Electrical test data obtained for XLPE and EPR during the steam tests indicate that 50% absolute elongation values provide sufficient margin to withstand additional accident radiation, and to function electrically during design basis events.

The propagation features of a streamer discharge in water have been investigated. based on the experimental data obtained in the study of water discharges in a non-uniform electric field, the propagation of streamers is explained as the evaporation of water at the tip of the streamer and around it. The energy balance in the process of the streamer propagation is calculated for a sub-microsecond discharge in distilled water It is shown that the energy released in the prebreakdown process is sufficient to evaporate the liquid in the streamer channels. Similar velocity of the streamer propagation in both tap and distilled water substantiates negligible effect of ionic current density onto the streamer propagation process. These estimations based on the experiment have relevance to the discussion on the nature of the dielectric breakdown of water.

Research was conducted to develop the wide band electronics for superconducting quantum interference devices (SQUID). Short pulse currents simulating a signal in a degraded power electric cable were measured using the system. The cancellation of magnetic field from power line was investigated by using two SQUIDs. The magnetic field was reduced by a factor of 100. Results suggest that the SQUID can be applied to the partial discharge measurement of power cables.

In this paper, the general phenomenological life model proposed by the author for insulating materials with both a threshold or non-threshold behavior, is applied to experimental data pertinent to two insulating materials for power cables. The first material, EPR, was found to be a non-threshold material for stresses close to or larger than the service stress, but extrapolation at low temperature seems to show a tendency toward a threshold. Application of the general model does appear very appropriate for this insulation and a good fit to the data has been found. The case of the second material, an old generation of XLPE, is different and the general model does not fit the data. However, an excellent fit is reached by introducing some improvements to the model, where possible and correct. Further complications seem necessary to conform this model to various practical cases, and make its application more difficult. For practical applications, such as selection of materials and/or design of apparatus, the use of properties and indices obtainable by means of long tests, such as those suggested and carried out by the author's research group, is simpler and more rapid. However, the usefulness of the model also consists in studying the various processes occurring during combined-stress aging of materials, all of which the model can take into account by simple mathematical expressions. This constitutes a significant advance in approaching the complex physical mechanisms involved in aging.

Scanning with the micro-PIXE technique was employed to analyze water trees in the XLPE insulation of a field-aged underground HV cable. X-ray spectra of bow tie and vented water trees, the inner and outer semiconductive compounds, and an insulation spot free from any water tree were acquired. Simultaneously, two-dimensional elemental distribution profiles across the water trees were also measured. Various trace element impurities were identified in the analyzed spots and their possible sources are suggested. Differences in elemental distribution profiles in the scanned areas were observed and have been discussed on the basis of the mechanism of incorporation of these elements into the insulation. This study demonstrates the effectiveness of the micro-PIXE facility available in this laboratory in analyzing water trees in underground power cables.

Recently, several diagnostic methods for measurement of permittivity and dielectric loss have been developed for non-destructive testing of water-tree degraded XLPE cables. Critical degradation is found to be associated with a more than proportional increase of the dielectric properties when increasing the applied test voltage above a certain level. In this paper an explanation for this nonlinearity will be presented, using a mechanical approach to the water treeing phenomenon. The water treed insulation is considered to consist of small water filled voids separated by crazing zones. At relatively low water content and low applied test voltage, several of these crazing zones are likely to be closed and insulating. When increasing the test voltage, Maxwell mechanical stresses will cause water to penetrate into the crazing zones, thus making electric contact between the elongated water droplets. Results from finite element method (FEM) calculations of electric fields and losses show that the effect of this will be to enhance the electric field at the tip of these conductive channels and to increase the dielectric losses of the water treed insulation.

A degradation phenomenon of the inner semicon layer of extruded cable insulation occasionally has been observed in different service aged as well as in laboratory aged polymer insulated cables. To investigate on this effect, accelerated aging tests on XLPE-insulated cable cores as well as on a new type of model sample (aluminum wires, semicon and XLPE insulation layer) were performed. It is shown that under the influence of an electrolyte and mechanical strain, channel-like structures develop in the semicon of both test objects, and that vented trees are initiated when these structures reach the interface to the insulation. Thus, this type of degradation also is of particular importance for the long time aging behavior and testing procedures of polymer insulated cables. It is demonstrated that the newly developed model sample test arrangement is a valuable tool for the study of aging of insulation systems.

Observations of the space charge distribution around water trees are reviewed, and the nature of water trees analyzed from those results is discussed. It was concluded that at least two mechanisms are responsible for the formation of space charges. One is interfacial polarization due to the high conductivity of a tree channel, and the other is carrier injection from the tree tip at high electric fields.

This paper examines the importance of fire-resistant electrical circuit feeders in saving lives and property. It discusses sophisticated fire fighting and evacuation systems and the electrical energy requirements to ensure their operation in commercial high-rise, hospital, and other at-risk buildings. The influence of cost, ease of installation, and subsequent inspection of fire.resistant circuits in a consulting engineer's specifications decision is considered. Analysis of the impact of building code modifications that require increased fire protection for feeders to emergency systems is presented. The paper cites examples of devastating fires and resultant code upgrades. Implications of the 1996 National Electrical Code (NEC) sections concerning emergency electrical feeders are reviewed. Specification writers and product installers have primary influence over these systems. Their roles and responsibilities are discussed. The paper also reviews the various methods of providing fire-resistive protection to circuit feeders. Index Terms - Electrical circuit protective systems, emergency systems, fire endurance tests, fire hose stream quench, fire pump supply conductors, fire-resistive wraps, high-rise fire, mineral insulated cable.

HVDC cables start playing a more and more important role in interconnecting national grids. This paper deals with the calculation of electric fields in HVDC cables. The calculation of fields in an HVDC cable is far more complex than the equivalent case in HV ac cables. This is due to the fact that the conductivity of the cable insulation is temperature and field dependent and due to the fact that the electric fields under dc voltage may be time-dependent. The field distribution in an HVDC cable may be of a capacitive, intermediate (and time-dependent) or resistive nature. The kind of field depends on the stage the cable finds itself in: for instance, whether the voltage has just been applied, whether a polarity reversal has occurred or whether the field distribution has become stable. For each stage, the method of calculating, together with the computed results on a real HVDC cable are discussed. Usually, the effect of heating of the insulation by the leakage current may be disregarded. However, in certain cases, i.e. the cable temperature and applied voltage are high enough, the field distribution is influenced by these insulation losses. They even may lead to an instability that causes breakdown of the cable. A cable in service may be subjected to impulses superimposed on the dc voltage. The most severe case is that of an impulse superimposed on a dc voltage of opposite polarity. The calculation of the field distribution in this situation also is carried out.

This paper presents test techniques which enable electrical endurance characterization of HV cable insulating materials in relatively short times. Three crosslinked compounds are considered, i.e., two kinds of homopolymers, slightly differing in morphology, and a copolymer blend based on PE (polyethylene). The technique relies upon electric strength and life tests performed on plaques, subjected to uniform electrical field, and tree-inception tests, realized by needle-plane electrodes. The entire test cycle lasts [approximately]500 h, but it proves to be able to distinguish among material performance not only in short times, but, probably even in the long run. In particular, it is shown that the copolymer blend behaves better, from the point of view of electrical endurance, than the homopolymers. It is thus emphasized that a test procedure as here proposed allows prompt feedback, which is fundamental for the technological development of insulating materials.

This paper describes an on-line PD (partial discharge) detection technique applicable to power cables. The method can be applied to both wire screen and solid shielded cables. In wire screen shielded cables, PD components with frequencies

The residual ac breakdown voltage decreased to about half of the initial breakdown voltage after 3 months of accelerated aging test under immersion. This decrease was found to be caused mainly by a trace amount of moisture existing in the insulation. However, the breakdown voltage did not further decrease with increasing moisture content. In order to clarify the influence of moisture, the state of water existing in the insulation was investigated using FTIR (Fourier transform infrared spectroscopy) and optical microscopy. From these results, it was concluded that the moisture in the amorphous region and/or at the interface next to the crystal region caused the reduction of the ac breakdown voltage.

EL (electroluminescence) in XLPE (crosslinked polyethylene) subjected to impulses of various amplitudes, shapes and repetition rates has been investigated. Defects that could occur in the polymeric insulation of underground HV cables were simulated by embedding semicon protrusions in XLPE to create highly divergent electric fields. Time resolved measurement of the EL pulses emitted during positive, negative or alternating polarity impulses has shown that the EL pulses are grouped into two distinct time periods between which there is no EL activity. The EL emission probability increased with fall time of the impulse voltage but was independent of the impulse rise time. A study of electroluminescence characteristics from unaged and aged XLPE, subjected to a pair of positive and negative impulses, indicated that the charge trapped in the polymeric insulation during aging plays an important role during impulse application and could affect insulation failure.

Films based on a wide range of thermoplastic and XLPE (crosslinked polyethylene) including compounds containing aromatic voltage stabilizers (fluoranthene, naphthalene, pyrene, oterphenyl, vinylnaphthalene and anthracene) were exposed to external PD (partial discharge) in dry air at 298 K. Very low molar mass PE, [Mathematical Expression Omitted] g/mol, exhibited a very low resistance towards PD. It is believed that the low concentration of interlamellar tie chains and the pronounced mechanical brittleness caused early void formation induced by the mechanical action of the plasma. PE with a higher molar mass showed a PD life that was essentially independent of molar mass, degree of crosslinking, crystallinity and crystal thickness. The added voltage stabilizers had no effect on the PD lifetime.

Surface PD (partial discharge) on an insulator from a TJ (triple junction) are reduced by recessing the TJ at the HV electrode. The insulators have an inner electrode connected to the ground potential. PD pulses whose magnitude is > 10 pC are measured. The applied HV (50 Hz) is 1 to 1.5x higher than that at the initiation of the PD. The thickness of the insulator is in a range from 2 to 7.5 mm. For insulator thickness of 7.5 mm, the integrated magnitude and number of the PV pulses from the recessed TJ are reduced to [approximately]20% and 50% of those from TJ without recess, respectively. The reduction in the PD by the recessed TJ is explained by the field conditions on the insulator near the TJ. It is considered that the reduction in the PD by the recessed TJ results mainly from the following three factors: high surface potential ([V.sub.s]) on the insulator, weak parallel component of electric field on the insulator surface, and strong normal component of the field on the surface. Propagation of the PD is restricted by these three factors. We have designed the improved configuration of recessed TJ in order to further emphasize these three factors. It is experimentally confirmed that the improved one reduces surface PD more efficiently than the recessed TJ before the improvement. At 7.5 mm insulator thickness, the integrated magnitude and number of PD pulses are reduced to [approximately]5% and 15% of those for TJ without recess, respectively.

This paper presents the results of a continuing investigation into effect of water on water absorption and density of crosslinked polyethylene (XLPE). The experimental set up was made for the following XLPE cable insulations: steam and dry cured with water tree retardant crosslinked polyethylene (TR-XLPE) and without water tree retardant crosslinked polyethylene (natural XLPE). During tests, the tap water was injected (1) into the cable conductor with cable ends closed, (2) into the cable conductor with cable ends opened, and (3) into the metallic screen with cable ends opened. The XLPE cable insulation together with the water present in the cable was subjected to electrical stress and heating. The results were analyzed theoretically and experimentally. The aim of this paper is to present the results of the influence of the water on water absorption and density of various kinds of XLPE cable insulation in different service conditions.

This paper describes experiments performed on laboratory aged 12kV and service aged 24 kV XLPE cable samples. Results from measurements of residual ac breakdown strength, degree of water treeing and dissipation factors are presented. The dissipation factor at low frequencies was determined by both time and frequency domain measurements. Relation between these methods are discussed, and it is shown that both can be used for assessing the average ageing state of the cable. The results show that water treeing causes reduced residual ac breakdown strength, high and non-linearly increasing low-frequency dielectric loss.

This paper provides information on aging of URD cable insulated with tree-retardant crosslinked polyethylene (TR-XLPE) compound, installed in a typical utility environment. Numerous evaluations were performed on samples of the cables aged up to 7 years in-service. AC and impulse voltage breakdown data are compared with data for similar 35 kV ethylene propylene rubber (EPR) and crosslinked polyethylene (XLPE) insulated cables removed from the same utility system. The data show that, to date, the rate of degradation of TR-XLPE cables is less than that of the EPR and XLPE insulated cables. Extrapolation of the data, assuming the same rate, indicates TR-XLPE cable will have the longest life.

This paper describes new laboratory test methods employed to evaluate the performance of silicone rubber materials under simulated outdoor contamination conditions. Details of a newly designed chamber also are presented here. The chamber utilizes no moving parts and involves no metallic components.

The same basic principles on which the methods of power cable protection against overloads are based, are summed up by different criteria and formulations within ANSI/IEEE and CENELEC/IEC publications. The studies carried out by the technical committees of these organizations have been examined and compared, in order to point out not only their differences, but also their complementary aspects. By combining the two approaches, it has been possible to outline a 'third way' to determine the admissible duration of overload currents and define intermediate criteria for the emergency ratings of cables and for the coordination of protective devices. Index Terms - Emergency ratings of cables, overload protection, power cables.

This paper describes the results of an investigation in which 15 kV rated cables insulated with a new water tree retardant cross-linked polyethylene (TR XLPE) were subjected to accelerated aging tests under a controlled voltage stress and thermal load cycle conditions. Cables insulated with conventional XLPE and a commercially available TR XLPE were used as reference test populations to affirm the test methodology. Under the chosen conditions, cable life of the new TR XLPE as calculated using Weibull and log normal statistical distributions, was more than twice that for the reference TRXLPE. Extensive diagnostic measurements (water content, dissipation factor, water tree analysis) were performed on failed cable samples to bring out the differences between the three insulations. Electron micrographic investigations revealed the size and distribution of micro voids in the new TR XLPE to be smaller supporting its extended life under these tests. The experimental details of the accelerated life tests are also documented in a clear manner facilitating any archival of the data for future analysis and comparison.

Many processes have been considered over the years to explain the origin of breakdown in cable insulation. Such effects as space charge build-up, tree growth, charge injection, etc. have all been discussed. Various techniques are now available to measure, in a non-destructive way, space charge distributions in insulators. These techniques, for instance the pressure wave propagation (PWP) method, can be used under applied electric stress and thus make it possible to follow the development of space charge in selected regions of the insulators. In this paper we present new evidence linking space charge buildup, tree growth and breakdown in XLPE. We have used the PWP method to monitor the charge distribution as a function of time under dc stress in high insulating thickness cable. We show that for certain insulation systems the space charge buildup can increases the local field to a value which is more than 8 x the applied electric field, leading to breakdown. Post-mortem analysis followed by optical microscopy shows the presence of electrical trees, the breakdown channel being centered on one of them. The study of space charge evolution in practical insulations permits an understanding of the role of space charge in dc breakdowns. This understanding enables the development of technologies to suppress this effect and hence realize practical dc XLPE transmission cables.

A 6.6 kV pin-post insulator, incorporating ZnO varistors, has been developed in order to reduce line faults caused by direct lightning strikes, thereby reducing and simplifying maintenance work on distribution systems. The developed insulators are interchangeable with conventional pin insulators and can withstand internal fault currents twice upon their failures. The simulated results regarding the response of the developed insulators to direct lightning strikes on an overhead ground wire agreed well with the experimental results. Simulation studies also revealed that installing the developed insulators on every two poles reduces the line-fault rate of present distribution lines in case of direct lightning strikes by 33%, while installing them on every pole reduces the rate by 97.5% or about 1/40 of the original value. Keywords: lightning protection, direct lightning strike, distribution line, EMTP, surge arrester, insulator

500-kV XLPE-insulated cable with an insulation thickness of 27 mm has been developed for long-distance transmission lines. Basic studies on 500-kV XLPE cable have shown that contaminants in the insulation may be the factor determining electrical performance. This hypothesis is justified by the good correlation obtained between statistical estimations of the size of the largest contaminant in the insulation and the electrical characteristics of full-sized cables. Voltage-withstand and long-term tests have confirmed the design values for minimum breakdown stress, for AC and impulse voltage, and for the degradation coefficients. Keywords: 500 kV, XLPE cable, performance-determining factor, minimum insulation breakdown strength, design stress

This paper addresses the need to increase the section lengths of electrical distribution cables in underground duct by reducing the uncertainty of present analytical predictions for maximum pulling forces exerted on the cable during installation. New analytical models are proposed to take into account several additional effects related mainly to the bending stiffness of the cable. These bending effects, neglected by the classical Rifenburg-Smith model available to date, are in some cases responsible for substantial increases in pulling forces. The new models consider the geometrical and physical nonlinearity of the behavior of a cable stiff in bending and subjected to high tension. However, they are all fairly approximate in nature because they are based on simplifying assumptions aimed at obtaining conservative analytical results for the pulling forces to use in the line design. Based on these bending models, a new computer program, PULLFLEX, has been developed which is able to calculate and check all the mechanical limits of the cable, including pulling tensions and sidewall bearing pressures. The program was validated by comparison with field measurements taken on several underground sections, some built specially for testing and others part of real lines. All validations were done in typical Hydro-Quebec conditions for a 25-kV three-phase cable with bare concentric neutral, installed without lubricant.

In this paper, the relationship between the frequency and time domain windows is discussed. From this study an appropriate time window for transient current measurements may be chosen so that reliable low frequency dielectric data can be obtained. The suitability of applying either Fourier transform or the Hamon approximation for the derivation of low frequency responses is discussed for transient currents with different shapes. With water-treed LDPE samples aged in the laboratory, measurements of transient currents, with subsequent transformation into the frequency domain, show a loss peak at [approximately] [10.sup.-4] to [10.sup.-3] Hz when moisture is retained in the samples. Even with free water removed, a higher dielectric loss is observed in water treed samples compared to virgin samples. The relaxation behavior may serve as an indicator of water treeing in polymeric cable insulation. The results of both transient measurements and TSDC results can be affected significantly by the treatment conditions of the samples. The application of vacuum to a water treed sample can alter the activation energy of the conduction process and the TSDC spectra.

HVDC cables play an important role in a growing number of HVDC links. For almost all of these cables, mass impregnated paper is used as electrical insulation. The electrical conductivity of this insulation is given by a commonly used empirical formula. This formula takes into account the temperature and stress dependency of the conductivity. This paper derives a physics-based equation describing ionic conduction. In good approximation this theoretical formula gives the same results as the empirical one. This makes it most likely that the conduction in paper mass impregnated cables is of the ionic kind. The derived formula uses some physical parameters like the inter-potential well distance, the potential well depth and the carrier concentration. Numerical values of these quantities for mass impregnated paper are given. According to the proposed model, the stress dependency results from the inter-potential well distance only. The temperature dependency results merely from the depth of the potential energy wells. In addition some possible underlying physical processes are described.

Investigations of fault arc propagation on spacecraft cables under specific conditions, e.g. vacuum, normal and oxygen enriched atmospheres have been performed. Typical patterns of fault arc extinction have been identified and their consequences with respect to causes of damages are discussed. The results indicate that for a given test current and test voltage the behavior of the arc and the damages caused in a cable bundle depend strongly on the kind of insulation material, cable size and design, as well as on the environmental conditions. For some cables the arc propagation effects are increased at higher oxygen partial pressure and significantly increased under vacuum. In many cases vacuum has turned out to be an important worst-case parameter. It was found that the amount of damage assessed by post-test measurements increases substantially, if the value of the test current grows. More than 250 tests have been performed for test currents in the range of 6 to 30 A at the test voltage of 125 V dc.

Space charge distribution was measured, applying an electric field of > 1 MV/cm to 3 mm thick XLPE cable insulation. Improving the conventional pulse electro-acoustic method, a new method was developed to measure space charge when applying HV to cables. Under high field, hetero charges were formed soon after voltage application, followed by an injection from the cathode. Then intermittent injections of charge packets from the anode took place. The space charge distribution kept changing without becoming stabilized. A simulation was done assuming hysteresis of the injection characteristics. The observed intermittent injection near the anode was qualitatively reproduced using computer simulation.

The paper gives a survey of the phenomenon 'Arc Tracking of Wires' which has been observed for the first time in wiring systems of aircraft and which recently has occurred also in spacecraft. Aircraft organizations are aware of this phenomenon and have tried to provide solutions to cope with this new wire failure, i.e. modification of cable design and manufacture, avoiding pure polyimide insulation, and development of test methods. As regards space systems, the recognition of this phenomenon with its possible consequences has led to the development of test methods and to the introduction of new wire requirements within the framework of the Columbus Program. The available data do not establish with certainty a correlation between test results obtained for aircraft systems and the behavior expected on spacecraft. The untidy nature of arc tracking has been shown with the different kinds of events reported. In test laboratories, significant variations of results make this failure phenomenon even more difficult to define. From the data summarized in the survey it is apparent that the failure conditions vary with numerous conditions of electrical network, environment, cable design and aging parameters. Whereas the arcing phenomenon is essentially influenced by the environment and network conditions, the susceptibility to tracking is more dependent on the chemical nature of the insulation. Definitions are also presented in the survey to complete the understanding of the phenomenon.

A detailed examination of the aging processes, that may take place in solid-dielectric extruded-type high voltage transmission cables under dry conditions, is presented. Particular emphasis is placed on the aging process as affected by the separate and simultaneous action of four main aging factors, namely the electrical, mechanical and thermal stresses, and the physical and chemical environment. A number of pertinent aging models are considered and their validity and applicability to accelerated aging tests on solid-type transmission cables are discussed.

Investigations were carried out on water tree retardant XLPE (XLVLDPE) cable using very low density polyethylene (VLDPE) which is a very recent new grade of polyethylene. To begin with, when the water tree retardant effects of various types of VLDPE being sold on the market were evaluated by using sheet specimens, extremely good water tree retardant effects were exhibited when the type of VLDPE was selected appropriately. Next, the above selected varieties of VLDPE were tested regarding the extruding into cables. The extrusion of VLDPE was found to be more difficult compared to conventional XLPE, and consequently required the modification of extrusion conditions and a crosslinking agent. The result of the water tree acceleration test of XLVLDPE cables indicated that the generation of bow - tie trees was suppressed to a larger extent than conventional XLPE cables. Although the AC initial breakdown voltage of XLVLDPE cables was lower than that of the conventional XLPE cables, there was almost no decrease in the breakdown voltage due to the water tree test and the breakdown voltage became higher than that of the conventional XLPE cables after the water tree tests. The superior water tree retardation characteristics of XLVLDPE described above was deduced to be due to the combined effect of the low crystallinity of the base polyethylene and the presence of the additive material (neutralizing agent & antioxidant).

Recently in the snowfall area - northern part of Japan, the snow-retardant insulated wire with two fins structure is mainly used for 600 V and 6.6 kV class overhead distribution lines against snow damage. These two fins are placed longitudinally on the insulation at an angle 180 degrees and these height and width are relatively small. These values are 1 mm, 1.5 mm respectively. Although the snow-retardant insulated wire has good performance against snow damage, the violent vibration of the snow-retardant insulated wire was observed several times under a strong wind and the fatigue of conductor occurred resulting from the repeated high stress vibration at the wire supporting part. Therefore some theoretical aerodynamic analyses were conducted by various wind tunnel tests with computerized simulations in order to make the vibration mechanism clear. From those analyses, it became clear that the leeward snow-retardant insulated wire were aerodynamically unstable and the vibration occurred with the influence of the wind induced, on windward snow-retardant insulated wire under certain condition related to some leeward wire fin's angle, against a wind direction and sag difference of both wires. And also the measures against the above mentioned were investigated and the new type low lift snow-retardant insulated wire; with spiral two fins was developed for the overhead distribution lines. In this paper the aerodynamic vibration occurrence mechanism for the snow-retardant insulated wire and the measures for the vibration suppression are mainly discussed.

The insulation recovery time after arc interruption must be known to select the adequate reclosing dead time of rapid auto-reclosing on transmission lines. In order to study insulation recovery characteristics after arc interruption on UHV (1000kV class) transmission lines, many tests were carried out using the high power test facilities of CRIEPI. In this paper we describe the experimental results of these tests of insulation recovery characteristics after arc interruption of UHV transmission lines, and some factors influencing these characteristics.

Deterioration in the insulation performance of extra-high voltage XLPE cables is believed to be attributable to the deterioration caused by partial discharges. In our study, after using an XLPE cable to investigate the behavior of partial discharges under various adverse conditions, we succeeded in developing a highly sensitive new method of measuring partial discharge in XLPE cable lines. Partial discharges in a 275 kV XLPE cable live line have been measured using this newly developed method. As a result, a detection sensitivity of 1 pC has been achieved.

This paper relates to the new polymer insulating material for HVDC cable. The properties of modified HDPE, selected from various polymer materials by evaluations of DC characteristics were examined in both sheet and cable samples. It was found that the modification, which introduced a small amount of polar group into HDPE, considerably enhanced DC breakdown strength to as high as 1.5 - 2.0 times those, of XLPE. Another evaluation was given with TSC method regarding space charge which was considered as a factor contributing to the enhancement of DC characteristics. Under a poling voltage of up to 30 KV/mm, the modified HDPE exhibits particular behaviors, with its space charge decreasing as stress is increasing. In addition, the additives in HDPE have a large influence on space charge characteristics, thereby affecting the DC breakdown strength. The evaluation of cable insulated with optimum modified HDPE was conducted to determine the breakdown strength under various voltage applications. Compared with the XLPE cable, modified HDPE cable exhibited excellent characteristics under all kinds of voltage applications (DC. Imp. polarity reversal, and DC + Imp), particularly DC dielectric breakdown strength which was almost twice that of XLPE. Thus the test results agree with the sheet evaluations. Furthermore, the insulation thickness dependence of DC breakdown strength is considerably small compared with that of XLPE cable, showing that modified HDPE cable provides excellent DC characteristics even if the insulation thickness is increased. For longterm DC characteristics (V - t characteristics: [V.sup.n] * t = const), a value of n = 13 - 14 was obtained. The space charge characteristics in modified HDPE cable were evaluated using the dust figure method as well as a new 'pulsed - electroacoustic method' where both methods led to the recognition of a homo - space charge.

The use of a one-dimensional model for analyzing nonlinear stress in cable terminations is of considerable engineering interest. This is because the approach utilizes two novel techniques for the evaluation of the voltage profile in the cable terminations, the first of which is a characteristic parameter for describing in quantitative mode the ability of the stress grading material to degrade the field strength at the shield cut. The second technique involves the analysis of the general nonlinear resistive-capacitive electric field control problem. However, the model is undermined by the fact that some meaningless approximations have been used in the evaluation of specific cases.

Manufacturers and users have widely recognized water treeing as a long-term failure mechanism of medium and high voltage (15 kV and above) cable. There are many cable constructions and materials which combat treeing. It is important to keep abreast of new constructions and materials. They have different applications, they work differently, some are more available than others, and they are tested differently. The purpose of this paper is to supply the end user with an overview of the most current information on several different constructions and materials that help combat water treeing. Since the mid-1970's manufacturers and users have known dielectric treeing as a failure mechanism. Since then, many people have studied treeing. The result has been a wide range of materials and constructions to combat this problem. Treeing is the breakdown of the insulation. This breakdown resembles a tree or bush, when examined by a microscope. When treeing starts on the semiconducting conductor shield or insulation shield, the 'trunk' of the tree is on the shielded material and the 'branches' fan away. When the treeing occurs within the dielectric, the tree starts at a small point and fan away in each direction, making a 'Bow Tie' tree. Treeing is a long-term failure mechanism for extruded dielectrics. Sometimes it takes decades to fail. In addition, the formation of trees does not mean the dielectric will fail. Often, trees will begin formation, slow down and then stop formation, so the dielectric will remain operative for an indefinite period of time. Voltage stress and exposure to water accelerate the tendency of the dielectric to tree. Trees forming due only to high electrical stress are electrical trees. Trees forming due to exposure to water are electro-chemical or water trees. On exposure, water molecules penetrate the dielectric material. These molecules congregate in amorphus (non-crystalline) areas in the dielectric and eventually cause deterioration of the insulation material. To evaluate the performance of different methods of coping with treeing, one can do an Accelerated Cable Life Test.

For several years it has been apparent that the mechanical characteristics of insulating materials have a pronounced influence on their electrical breakdown by treeing. As a consequence of this discovery, it is possible to look again from a different perspective at the treeing behavior in materials containing fillers and barriers. The effect of temperature on treeing can be interpreted in terms of the effect of temperature on the various mechanical properties of the material, and of any particulate or solid barrier inclusions. The internal mechanical stresses produced in resin samples containing cast-in metallic electrodes can affect treeing behavior both positively and negatively, depending on whether this stress in the material is tensile or compressive. In the light of this experience it has been found possible to enhance the treeing resistance of electrical insulation by using these mechanical effects in the most advantageous way, viz. by mechanically prestressing the material by the inclusion of fibers in tension; a method which is akin to that employed in the prestressing of concrete.