Accelerated aging of extruded dielectric power cables. II. Lifetesting of 15 kV XLPE-insulated cables

For pt.I see ibid., vol.7, no.2, p.596-602 (1992). Preliminary results are described in which 15 kV XLPE cables were subjected to accelerated aging tests under a variety of controlled voltage stress and thermal load cycle conditions, with loss of life being calculated for each set of conditions in terms of the geometric mean time to failure (GMTF). The relative influence of voltage stress and load cycle temperature are discussed. Accelerated aging results show a reduction in GMTF for 15 kV XLPE-insulated cables as the voltage stress or conductor load cycle temperature is increased in a controlled manner. The relative influence of voltage stress versus load cycle temperature can be compared. The GMTF increases more in going from 90°C to 60°C at constant applied voltage stress than in going from 4X to 2X rated voltage at constant load cycle temperature conditions     

Thermal characteristics of energized coal mine trailing cables

The US Bureau of Mines conducted research to determine the relationship between current load and temperature rises in coal mine trailing cables. Six low-voltage unshielded portable power cables were continuously and intermittently loaded with direct current of various magnitudes. Relationships between average temperature at the conductor/insulation interface and current load were established. The steady-state and intermittent currents that produce a 90°C average conductor/insulation temperature were then determined by mathematical modeling. Comparisons to Insulated Cable Engineers Association steady-state ratings revealed that 10-25% more current is required to reach rated insulation temperature. Examination of the maximum intermittent temperatures attained showed that auto-ignition of coal dust and burns to personnel handling the cable would not be concerns if the temperature at the conductor/insulation interface averaged 90°C     

Effect of testing parameters on the outcome of the acceleratedcable 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°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     

105°C/140°C rated EPR insulated power cables

The excellent long-term operation and field performance of existing EPR cables under the current industry operating temperatures of 90°C normal and 130°C overload is well known. This paper describes a study for the implementation of a higher temperature rated EPR cable system including accessories up to a range of 105°C continuous service and 140°C emergency operation. The investigation has been carried out into the electrical, mechanical, and thermal behavior of the system as a function of temperature. Compatibility has been studied under higher temperature in relation to conductor filling compound and the semi-conducting stress control layer. The results indicate that insulation degradation does not occur under accelerated and elevated temperature testing, concluding that properly formulated and selected materials are both compatible and suitable for the higher temperature ratings     

Long-term reliability testing of 500 kV DC PPLP-insulatedoil-filled cable and accessories

This paper describes the results of a long-term reliability test of a newly developed 500 kV DC oil-filled cable and accessories that transmit up to 2800 MW. This is the largest capacity ever used for submarine cables. The test line was 500 m and consist of submarine cables, land cables and several kinds of accessories. An optical fiber sensor was first installed in the oil duct of the cable to measure the conductor temperature directly under application of DC voltage. From 1994 to 1996, the authors carried out an accelerated electrical and thermal aging test equivalent to 40 years, a load cycle test, a polarity reversal test and a transient overloading current test. Satisfactory breakdown strength was confirmed in the residual test which followed the long-term test. These results have been instrumental to the decision to construct 500 kV DC commercial submarine cable lines     

Effects of DC testing on AC breakdown strength of XLPE insulatedcables subjected to laboratory accelerated aging

The results of a study performed on crosslinked polyethylene (XLPE) insulated cables tested in the laboratory under accelerated conditions are described. URD-type cables rated at 15 kV were aged at 60 Hz, 6 kV/mm, (150 V/mil) and load cycled daily to 90°C conductor, with water inside the pipe and outside of the samples, for periods of up to 3 years. DC testing was performed before and during the aging. An anticipated significant reduction in the AC breakdown strength is observed in control cables (not DC tested), from 44 kV/mm (1100 V/mil) for unaged cables, to 10 kV/mm (250 V/mil), for aged cables. However, there was no further reduction in the AC breakdown strength of cables subjected to DC testing as compared to those that are not tested with DC. It is concluded that AC breakdown strength is not an effective diagnostic tool for determining the effect of DC testing of URD cables aged under the conditions described     

Mechanical performance of thermally aged trailing-cable insulation

As part of an overall effort relating cable life and current load, the US Bureau of Mines conducted research to determine the effects of elevated temperatures on the mechanical performance of coal mine trailing cable insulation. Insulation samples from six low-voltage unshielded portable power cables were thermally aged in air ovens. The specimens were exposed to constant temperatures ranging from 230 to 290°F for periods up to 10 months. At regular intervals over the test terms, samples were extracted and subjected to mechanical tests in accordance with Insulated Cable Engineers Association Standard S-68-516. These tests included tensile strength, elongation at rupture, hot creep elongation, and set. Characteristic variations over time were plotted for each aging temperature. Arrhenius (useful life) graphs showed that 80% retention of tensile strength and 50% retention of elongation were equally critical determinants of thermal degradation. Extrapolation to 90°C yielded useful lives ranging from 4.4 to 7.5 years for the cables evaluated. The findings support the tentative conclusion that, in service underground, cables fail mechanically long before they deteriorate thermally     

Field monitoring of parameters and testing of EP and TR-XLPEdistribution cables

Five commercial ethylene-propylene rubber (EP) and one tree-retardant cross-linked polyethylene (TR-XLPE) 15 kV cables are being aged in the laboratory, and in field service on a utility distribution system. The cables were obtained from six different manufacturers. This study subjects commercially available, widely used EP cables, and one TR-XLPE cable to similar conditions at three different sites. Two test sites are located at Orange and Rockland Utilities' (O&RU) distribution system. The first is in normal 15 kV service; the second is a normal 35 kV service site, with the cable aging under accelerated voltage conditions. Line voltage, voltage surges, load current, earth, duct and cable temperatures on the systems are being field monitored. Cable Technology Laboratories (CTL) has provided an accelerated aging set-up for the third site. This paper describes the methodology, characterization testing and some preliminary results from this project     

Elevated temperature operation of XLPE distribution cable systems

The long-term operation of existing and new cross-linked polyethylene (XLPE) cables at elevated temperatures of 90°C or even higher is of concern to utility operating engineers, particularly where thermoplastic shields are used. By deferring the installation of new cables and operating existing cable systems at higher temperatures near or above the normal maximum, economic advantages can be gained. An investigation was initiated to understand how medium voltage cables perform at elevated temperature and how much damage is done during high temperature operation. Evaluation of specific material properties and the use of bench-top testing and full-size cable load cycling aid this understanding     

Distributed measurement of conductor temperatures in mine trailingcables using fiber-optic technology

Mine trailing power cables operated above safe thermal limits can cause premature insulation failure, increasing electrocution and fire hazards. Previous US Bureau of Mines research showed that, under static test conditions, electrical current levels permitted under present regulations may not limit cable temperatures to less than the 90°C rating of reeled trailing cable. Continuing research under the National Institute for Occupational Safety and Health (NIOSH) addresses thermal characteristics of reeled trailing cable under dynamic test conditions more representative of field conditions, where operators constantly reel in and pay out cable. This research is in support of efforts by industry associations and the Mine Safety and Health Administration to establish safety guidelines for cyclically rated reeled machines. This paper describes a unique approach to measuring temperatures within reeled cable under dynamic test conditions. Fiber-optic sensors embedded within the metallic conductors measure temperatures at 1 m intervals along the entire length of cable. Temperature measurements are reported to be accurate to within ±1°C. The test setup requires access to only one end of the trailing cable, allowing researchers to freely reel in and pay out cable while temperature measurements are made, simulating field conditions. Manufacture of a fiber-optic-embedded trailing cable is described, along with initial test results that indicate the fiber-optic approach is viable     

Accelerated life tests on a new water tree retardant insulation forpower 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 TR XLPE. 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     

Effect of water on aging of XLPE cable insulation

Cables as elements of power distribution system have great influence on its reliable service and overall planning requirements. During last years, crosslinked polyethylene (XLPE) cables have been more and more used in power systems. This paper presents the results of an investigation of changing of (XLPE) cables insulation breakdown stress (AC BDS) due to water absorption. The paper deals with AC BDS of the following kinds of XLPE cable insulations: steam and dry cured with water tree retardant crosslinked polyethylene (TR-XLPE) and non-tree retardant crosslinked polyethylene (XLPE). During tests, the tap water was injected into, (1) conductor with cable ends closed; (2) into cable conductor with ends opened; and (3) into metallic screen with cable ends opened. The presence of water in XLPE cables was subjected to electrical stress and heating. AC BDS tests were performed as a function of aging time and water content in the cable insulation at different aging temperatures. Also, in this investigation, tests with the changing of AC BDS in the radial direction of unaged and aged XLPE cable insulations were carried out.     

Comparative laboratory evaluation of TR-XLPE and XLPE cables with Super-smooth conductor shields

This paper provides data on four commercial tree retardant crosslinked polyethylene (TR-XLPE) and one cross-linked polyethylene (XLPE) insulated 15 kV cables supplied by three manufacturers. The cables have "super-smooth" conductor shields and "extra-clean" insulation and insulation shields. AC and impulse voltage breakdown and selected other characterization data are presented for cables that were aged immersed in room temperature water (15-30/spl deg/C) up to 24 months of a planned 48 months aging program. The five cables have high ac voltage breakdown strength, three of the TR-XLPE cables, actually increased in breakdown strength during aging. The one TR-XLPE cable that had the lowest ac voltage breakdown had vented trees at the insulation shield and high dissipation factor, which the other cables did not have. The impulse voltage breakdown strength of all cables decreased during aging; the cable with the lowest ac voltage breakdown also has the lowest impulse voltage breakdown. The dissimilar performance of the TR-XLPE cables and the excellent performance of the XLPE cable indicates evaluations at longer times are required to differentiate between modern TR-XLPE and XLPE insulated cables.     

Development of 500 kV DC PPLP-insulated oil-filled submarine cable

This paper outlines the development of a 500 kV DC oil-filled submarine cable capable of transmitting 2800 MW with ±500 kV 2800 A bipole system. Polypropylene laminated paper (PPLP) was employed as the insulation material, which is the world's first application to DC cables. The conductor size is 3000 mm², which is the largest size for submarine cables ever put into practical use. Through various fundamental and prototype tests, the cable proved to have excellent electrical characteristics for DC voltage as well as transient overvoltage. The cable and accessories are undergoing a long-term accelerated aging test as the final confirmation of their reliability and stability     

Computerized method for evaluating the suitability of trailingcables

A program to develop models for the enable computer calculation of conductor temperature rise versus time in trailing cables has been initiated. To date, a common model equation has been developed and parameters determined for two round trailing cables typically used on continuous miners, type G-GC, sizes 2/0 AWG and 4/0 AWG, rated 90°C. The resulting cable models have been tentatively confirmed in laboratory trials using repetitive four-level constant-current cycles and triangular currents in which the peak current varied on succeeding cycles. Model refinement and additional laboratory trials are projected using randomly varying current, simulating actual current conditions in mine trailing cables     

Rating power cables in wrapped cable trays

Conductor temperatures for a given ampacity loading is a function of ambient temperature inside the tray. In other words, the ampacity of cables included in these tray systems has to be rated at the ambient temperature inside the tray. Cable overheating and eventual failure can result if cables are overloaded or not derated for operation. IPCEA Pub. No. 54-440/WC lists cable ampacities in air ambient temperature of 40°C. Cables operating at temperatures above this have to be derated accordingly. An algorithm is presented for determining ambient temperatures in the cable tray for conditions of natural air convection with different cable loading. Hence, derated cable ampacities can be derived from those at 40°C. Although at present, there is no industry standard for wrapped cable trays, the method used here can be used to develop such a standard     

Accelerated aging of extruded dielectric power cables. I. Controland monitoring methodology

An improved accelerated cable life test is described. Through the use of programmable logic controllers (PLCs), very precise and consistent control of the accelerated aging process has been achieved. A computer has been utilized to make continuous real-time acquisition and storage of key operating parameters possible. This precise control and monitoring methodology has permitted the study of the synergistic effects of voltage and temperature on the accelerated aging of full-sized cables in the laboratory     

Effect of Thermal Overload on the Voltage Breakdown Strength of Service-Aged URD Cables

Present industry specifications allow thermoset insulated polymeric cables to be subjected to emergency conductor temperatures of up to 130°C. The effect of the high temperatures on cable integrity has been questioned. This study shows that cyclic, long-term or fast-rise application of 130°C to service-aged, water treed underground residential distribution (URD), crosslinked polyethylene (XLPE) insulated cables, result in an increase in dielectric strength. Contrary to what happens in new cables, an increase in temperature from ambient to 130°C also results in an increase in voltage breakdown strength. It appears that at high temperature, moisture and some remnant by-products of the crosslinking reaction such as volatiles, diffuse from the insulation, contributing to the higher levels of dielectric strength. It is shown that thermoplastic insulation shields on XLPE service-aged cables are adversely affected by emergency temperatures.     

硫化工艺对抗水树交联聚乙烯绝缘电力电缆工频击穿性能的影响

为了研究不同硫化工艺对抗水树枝交联聚乙烯绝缘电缆击穿性能的影响,建立了相应的试验手段和评价程序。将使用相同导体屏蔽料、抗水树绝缘料、绝缘屏蔽料,并采用5种不同硫化工艺(A、B、C、D、E)生产的电压等级、型号规格相同且结构相似的电缆作为研究对象,每种电缆取6段作样品,共30段。分别对老化前和老化180d后的5种样品进行工频击穿试验,并观察击穿后样品切片的水树枝、界面微孔、突起和绝缘中的微孔、杂质。试验结果表明:经过180d的加速老化后,5种样品中均无微孔、界面光滑、有少量尺寸较小的杂质,不会导致击穿性能下降;不同硫化工艺生产的电缆工频击穿性能表现出明显差异,其中,A硫化工艺生产的电缆工频击穿强度下降了53.53%,击穿后的样品中观察到了水树枝,B、C硫化工艺生产的电缆工频击穿强度也有不同程度的下降,而D、E硫化工艺生产的电缆的工频击穿强度没有降低,说明硫化工艺对工频击穿强度有直接的影响,并建议实际生产中确定硫化工艺时,各区温度设定应逐渐降低,且初始硫化温度不应过低,生产线速度应适当。     

Long term accelerated aging tests on distribution cables under wetconditions

Two accelerated aging test procedures are proposed for use on solid dielectric extruded distribution cables under wet conditions. One test method is a fixed time duration test in which the degree of cable specimen aging is assessed in terms of breakdown tests, while the other test method is a time to breakdown test in which the cable specimens are voltage stressed until failure ensues