Cross-linked Polyethylene Insulation Research Articles

Backscattered electron imaging and energy-dispersive X-ray studies of water-treed polymeric insulation

The backscattered-electron imaging mode in a scanning electron microscope has been used to detect and image water trees in electrically stressed cross-linked polyethylene (XLPE) insulation. The location and distribution of inorganic contaminants, such as Cu or Cl, within the water-treed regions of the insulation were revealed using backscattered-electron imaging. Energy-dispersive X-ray (EDX) spectroscopy and mapping were subsequently used to ascertain the chemical nature of the contaminants. Contaminants were only detected by EDX inside the treed areas delineated by the backscattered-electron images. A careful comparison of backscattered-electron and optical images of thin microtomed XLPE samples did not show any contaminant penetration beyond the visible-treed region. Results from a variety of service-aged and laboratory-stressed samples are presented, as well as a preliminary attempt to extend backscattered-electron imaging to optically opaque ethylene-propylene-rubber insulation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effects of voltage impulses on extruded dielectric cable life

Laboratory tests were performed to investigate the effects of lightning and thumping impulses on the performance of various 15 kV cable designs. The cable designs were aged in the laboratory using an accelerated water treeing test. Some samples were subjected to lightning impulses, some were subjected to thumping impulses, and some were aged without impulses applied. The impulse effects were evaluated using time to failure data, AC breakdown data, and impulse breakdown data. The results show that lightning or thumping impulses do not affect cable AC or impulse breakdown strength. There is also no visual difference between wafers taken from impulsed and control samples. However, there is a strong indication that lightning impulses can reduce cable life. Almost all the aging failures occurred while the cables were warm. No failures occurred during the impulse application. It is also shown that increased cable wall thickness extends cable life for the tree-resistant high-molecular-weight insulation material tested, but not for the crosslinked polyethylene insulation material tested.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Evolution of compound pellet inspection [power cables

The history and merits of various techniques developed to achieve the current levels of cleanliness in the cable industry are reviewed, with regard to the fabrication of polyethylene and cross-linked polyethylene insulation and shield materials. The detection methods now used include magnetics, eddy currents, and electrooptics. A description of commercially available equipment and statistically based reasons for a 100% inspection program at the site of cable manufacturing are included.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A new concept for medium-voltage cables: improved voltage life of belt-type cables

Crosslinked polyethylene (XLPE) insulation has in many cases replaced paper insulation in medium-voltage cables. There is one important exception; in 10- or 11-kV networks in Europe an extremely efficient and simple paper-insulated cable is in use: the unscreened three-phase belt-type cable. This type of cable cannot easily be replaced by XLPE cable, because the required semiconducting screens make the XLPE cable expensive and its jointing more complicated. For this reason, an XLPE cable has recently been developed, designed as a three-phase cable, but without semiconducting screens. In the interstices between the cores of this cable, large discharges occur. However, the authors have discovered that these discharges can be decreased considerably by taping a fibrous material over the cores. In this way the life expectancy of this cable is substantially increased. A description is given of this entirely new phenomenon, and an explanation for the unexpected behavior of these interstice discharges is offered. >

Physico-chemical analysis of a failed cable

Studies have been conducted of a section of crosslinked polyethylene insulation from an 80-kV cable that had failed under field aging. The cable had been deliberately treated to excessive conductor temperatures for a long period of time. Both steady and cyclic conditioning had been imposed. The cable sections showed multiple melting peaks in differential scanning calorimetry studies, dependent on location relative to the conductor, which are interpreted in terms of the the known laboratory aging behavior. The section nearest the conductor showed evidence of degradation, increased void content, and the presence of water trees, together with the most complicated melting curves. All the evidence demonstrates that significant chemical and physical degradation occurs within the insulation close to the conductor during combined thermal and electrical aging under moderate conditions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effect of testing parameters on the outcome of the accelerated cable life test

The effects of various testing parameters on the accelerated life test results of 15 kV underground residential distribution cables prepared from thermoplastic and crosslinked polyethylene insulation are considered. Testing parameters included dry versus water in the strand, voltage stresses 4X(34.6 kV) and 3X(26.0 kV), and hot spot temperatures of 90 and 75 degrees C. The effect of shelf aging on the life of crosslinked polyethylene insulation is reported. Microscopic examinations were made for a number of the tested samples, and tree observations are discussed. The data presented provide insight to a number of the factors which reduce the life of polyethylene cable insulation used in the URD applications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The Influence of Cure Method on the Sol Content and Physical Structure of Crosslinked Polyethylene Insulation

A series of transmission cables insulated with crosslinked polyethylene have been studied. The cables were cured using a variety of heat transfer methods, but were all based on the same resin system. Insulation cured with steam or dry gas gave approximately constant sol fractions across the wall thickness. Cables cured using an experimental heat transfer method gave rise to a sol fraction which increased across the wall thickness as the conductor was approached. Correlations exist between sol fraction, swelling ratio, and melting data. It is suggested that a simple melting curve determination can be used to estimate both sol fraction and crosslink density for low density polyethylenes crosslinked using dicumyl peroxide.

An Accelerated Life Test for Evaluating Power Cable Insulation

For many years, the power cable industry has sought an accelerated test that would quantitatively evaluate the life of polyethylene insulation for high voltage cables. This paper describes preliminary results leading to development of such a test. This test employs full size cables with accelerating factors such as temperature, water, electrical stress, frequency and mechanical stress. Times-to-failure for thernnplastic and crosslinked polyethylene insulation materials are canpared at different temperatures and the effect of water in the strand is illustrated. Data are presented showing the performance differences of two peroxide comipounds in crosslinked polyethylene. The influence of a tree-retardant additive to thermo-plastic polyethylene is described. General observations are made of the failure mode and nature of tree formation in test specimens.

Life Expectancy of Crosslinked Polyethylene Insulated Cables Rated 15 to 35 kV

Life Expectancy of Crosslinked Polyethylene Insulated Cables Rated 15 to 35 kV

"Life Expectancy of Crosslinked Polyethylene Insulated Cables Rated 15 to 35 kV"

Ever since it was demonstrated that installed extruded dielectric underground distribution cables lose significant dielectric strength due to formation of electrochemical trees in the insulation users have sought a means to determine the remaining useful service life. Subjecting miniature and full size crosslinked polyethylene (XLPE) insulated cables to accelerated aging tests and comparing the results to aging of I5kV cables removed from service the authors show how the remaining service life of installed cables can be estimated from dissipation factor measurements. This non-destructive test is made utilizing the available system voltage and would require the cable to be disconnected only several hours from service. The method devised to estimate service life is applicable to both polyethylene (PE) and XLPE insulated cables and its accuracy will be increased as more dissipation factor data is taken on cables now in service.

Application of submillimeter wave lasers to high voltage cable inspection

An experimental device designed for real-time, nondestructive inspection of cross-linked polyethylene insulation during cable manufacture has been built, analyzed, and tested under laboratory conditions. The device detects scattered far-infrared (FIR) laser energy at 118.8 μm from imperfections, such as voids and contaminants, known to have a serious deleterious influence on cable in-service lifetime. As presently configured, the device can readily detect voids as small as 100 μm in diameter in a cable 2.8 cm in diameter moving past the sensor at 5 cm/s. Better performance of the liquid helium cooled Ga-doped Ge detectors could lead to a significant improvement in void detection capability.

Radiation processing of polyethylene

This paper covers two areas (a) the use of high energy radiation for the synthesis and improvement of polymer properties and (b) the formulation of radiation curable compounds for automotive/appliance wire applications and high voltage insulation. The first part discusses the use of gamma radiation for the bulk polymerization of ethylene and the properties of the polymer produced. The use of low dose radiation to increase polymer molecular weight and modify polydispersity is also described together with its projected operational cost. An update is provided of the cost savings that can be realized when using radiation crosslinked heavy duty film, which expands its applications, compared with noncrosslinked materials. The second section of the paper considers the advantages and disadvantages of radiation vs. peroxide curing of wire and cable compounds. The formulation of a radiation curable, automotive/appliance wire compound is discussed together with the interactions between the various ingredients; i.e., base resin, antioxidants, flame retardant filler, coupling agents, processing aids and radiation to achieve the desired product. In addition, the general property requirements of a radiation curable polyethylene for high voltage insulation are discussed; these include crosslinking efficiency, thermal stability, wet tree resistance and satisfactory dielectric properties. Preliminary data generated in the development of a 230KV radiation crosslinked polyethylene insulation are included.

Radiation Resistance of Cable Insulating Materials for Nuclear Power Generating Stations

The radiation resistance of sixteen types of representative rubber and plastic materials for cable insulation was studied. The endurance of some crosslinked polymers under the combined environment of thermal aging-radiation-steam (high temperature) was also examined in relation to the chlorine contents of the base polymer and additives (antioxidant, vulcanizers, etc.). Crosslinked polyethylene insulation and polyvinylchloride jacket cables were tested by the IEEE Std. -383 method. The results showed that those cables were not satisfactory under loss of coolant accident (LOCA) simulating conditions. The cables utilizing EP rubber insulation and special chloroprene jacket have been found to pass the IEEE-383 test.

The Morphology of Cross-Linked Polyethylene Insulation

Cross-linked polyethylene, now commonly used for insulation of underground electric distribution cables, is susceptible to electrochemical treeing, in common with other polymeric dielectrics. This treeing phenomenon sometimes originates at holes or cavities present in the cable. A polishing and etching technique has been developed which, when combined with scanning electron microscopy, permits the relation of the cavity structure to the crystalline morphology to be determined. The larger cavities, 5 to 10 μm in size, are to be found at the points of impingement of spherulites. Smaller cavities, revealed by the etching techniques, occuir in the boundaries of spherulites. These cavities are believed to be due to condensation of aromatic by-products of the initiator. A cavity network exists throughout the material giving rise to a weak pathway which is likely to facilitate tree formation.

A Study of Inception and Growth of Water Trees and Electrochemical Trees in Polyethylene and Cross Linked Polyethylene Insulations

This paper presents an experimental investigation into the effect of different factors, such as contaminants, humidity content, temperature, field strength and frequency on water treeing in polyethylene (PE) and cross linked polyethylene (XLPE). For this investigation, model test specimens have been used to supplement cables for testing. Colored trees and their points of initiation have been analyzed for their content of impurity elements and always are found to contain sulfur and some metal. The results show that the relative humidity in the insulation must exceed approximately 70% to produce a significant number of water trees. The average size of the trees shows a very small dependence upon time and temperature and a slight decrease with increasing field strength. An increase in frequency from 50 to 5000 Hz approximately doubles the average size of the trees. The trees appear during the first 300 hours of voltage application, but since no growth rate can be observed at either frequency, it is not justified to establish a frequency acceleration factor in general.

Thermal optical analysis and differential scanning calorimetry of chemically crosslinked polyethylene

Differential scanning calorimetry (DSC) and thermal optical analysis (TOA) were applied as part of a series of characterization methods to solid chemically crosslinked polyethylene insulation. The DSC was used to study the melting and recrystallization. The ΔH of fusion has a lower value than expected. The TOA shows premelting behavior and is more sensitive to crystalline charges, on cooling, than DSC.

Electro-Chemical Treeing in Ethylene-Propylene Rubber Compounds

A series of Ethylene-Propylene Diene (EPDM) compounds, designed to be transparent or translucent in nature, were formulated for the purpose of studying electro-chemical treeing, Systems were tested in sheet form for evaluation of physical properties and crosslinking behavior, and extruded under laboratory conditions over 14 AWG bare copper wire into thin wall mini-cables. Cables were then subjected to the Phelps Dodge Accelerated Treeing Test. All EPDM systems exhibited electro-chemical tree formation to some extent; trees originated at the site of contaminants and at the conductor. Treeing patterns observed are of similar appearance to those formed in Polyethylene and Crosslinked Polyethylene insulation. The nature of the EPDM systems, preparation techniques, physical prop erties and treeing effects are discussed. Results indicated that electro- chemical treeing is a phenomenon characteristic of both rubbery and semi crystalline polymeric materials.

Insulation properties of cross-linked polyethylene cables cured in inert gas

A new method of cross-linked polyethylene cables, vulcanization in inert gas, was researched and developed. In this paper, the principle of the new method of the gas curing process is explained. And then the electrical and physical properties of the cables cured in inert gas are outlined and are compared with those of the cables cured in steam. It is found that the curing method and condition have heavy influence on the micro-structure of the XLPE (cross-linked polyethylene) insulation and electrical strength. In conclusion, we can cure the polyethylene cables in the flow of the high temperature and high pressure inert gas. A.C and impulse breakdown voltage of the cables cured in inert gas (gas-cured cable) were respectively higher about 50∼70% and 20∼ 70% than those of the cables cured in steam (steam-cured cable). By our new method of the gas curing process perhaps insulation thickness will be reduced to 70∼60% of that of the present steam-cured cables. It will be of great advantage to use this method in manufacturing EHV XL-PE insulated power cables.

Development of EHV Cross-Linked Polyethylene Insulated Power Cables

To apply cross-linked polyethylene for EHV cable insulation, several cables are designed, and manufactured and their characteristics are measured. By manufacturing, normal cross-linked polyethylene are used for comparison together with special cross-linked polyethylene insulations which have been newly developed. It is found that non-carbon filled and voltage-stabilized insulations possess excellent properties in corona and treeing deterioration resistance, and electrical breakdown strength which indicate the possibility of application these cables to real service. Problems and aspects of plastic-insulated power cables for higher voltage grades, such as 275kV class, are discussed.

Development and Installation of 138 kV Cable for Tests at EEI Waltz Mill Station: Cable 22

This paper covers General Cable Corporation's research, development and qualification testing of a 138 kV cable having extruded cross-linked polyethylene insulation. Also discussed is the manufacture, factory testing and installation of a length of this cable at the Edison Electric Institute test site at Waltz Mill, Pennsylvania. The accelerated life tests being performed at Waltz Mill will serve to evaluate and establish the performance of 138 kV cross-linked polyethylene insulated power cable designed and manufactured in accord with current technology.