Performance characteristics of XLPE versus EPR as insulation forhigh voltage cables

The physical and electrical properties of crosslinked polyethylene (XLPE) and ethylene propylene rubber (EPR) are compared in the context of their use in transmission class cables. Results indicate that the 138-kV XLPE cable has AC withstand/breakdown strength at least 25% higher than the 150-kV EPR cable. The XLPE cable exhibits about 70% higher impulse strength than the EPR cable. The loss factor of the XLPE cable is at least 20 times lower than that of EPR cable. Thus with XLPE cables, the yearly energy savings can be on the order of 15 MWh/cct. km for a 69-kV system, 52 MWh/cct. km for a 138-kV system and 127 MHh/cct. km for a 230-kV system     

Performance of reduced wall EPR insulated medium voltage powercables. II. Mechanical characteristics

For the replacement of paper-insulated lead covered power cables (PILC), reduced insulation wall designs have been developed. They provide a reliable lower diameter cable design for installation in existing PILC conduits. This paper presents the results of mechanical tests conducted on both reduced- and full-wall EPR insulated power cables. Both jacketed and nonjacketed cable designs have been subjected to mechanical pulling forces which greatly exceed recommended industry limitations, followed by electrical testing to ascertain cable performance. The results of this study conclude that reduced-wall EPR insulated cables can safely withstand the same pulling forces as recommended for conventional walls and can be designed for installation under the same pulling limitations     

EPR-based URD insulation: a question of confidence

Concern about the possibility of failure of miles of medium voltage (15-35 kV) underground residential distribution (URD) power cable is discussed. This concern centers on the cable specification decisions that were made years ago an the consequences of cable failure and subsequent replacement. The factors that degrade URD are recalled. Choice of insulation material is discussed. The effects of thermal conductivity and thermal expansion are examined. Accelerated cable life test (ACLT) results are considered in the context of the debate over ACLT vs. underground testing. It is concluded that ethylene-propylene rubber (EPR) has performance characteristics that permit cables to be used successfully under conditions of electrical stress, heat, and moisture, and that outstanding performance in ACLTs of cables insulated with EPR-based compounds is obtained     

Development of 500-kV XLPE cables and accessories for long distanceunderground transmission line. III. Electrical properties of 500-kVcables

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     

Development of 500-kV XLPE cables

A 500-kV XLPE insulated cable with an insulation thickness of 27 mm has been developed for long-distance transmission lines. Basic studies on the 500-kV XLPE cable have shown that contaminants in the insulation may determine 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 electrical characteristics of full-size cables. Voltage-withstand and long-term tests have confirmed design values for minimum breakdown stress, ac and impulse voltage, and degradation coefficients. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 118 (1): 28–40, 1997     

Development of prefabricated joint for 275-kV XLPE cable

Application of XLPE cables to extra-high-voltage (EHV) transmission lines is expanding due to their excellent electrical properties and ease of maintenance, and 275-kV long-distance underground transmission lines are reaching the stage of practical utility. Extrusion molded joints (EMJs) are widely used as the XLPE cable joint for EHV transmission lines. However, EMJs have presented problems because of their insulation. They are formed on site through extruding and molding of crosslinkable PE like the cable, and need a long time for installation on site and higher jointing technique. The prefabricated joints (PJs), which are already applied to 154-kV transmission line, are installed on site by using the parts manufactured and tested in the factory, they require a shorter installation environment. This report introduces the results obtained regarding the improvement of the electrical property, workability and the performance of PJs, designed for 275-kV XLPE cable. These joints are planned to be used for 275-kV transmission lines from 1995     

Performance of reduced wall EPR insulated medium voltage powercables. I. Electrical characteristics

Paper-insulated lead covered power cables (PILC) have had a long and successful heritage. After almost 100 years, this design of cable is still in operation and continues to be manufactured. However, electric utilities are now looking for a reliable replacement for PILC cables. This is due to two primary reasons: (1) the difficulty in installing and maintaining this type of cable; and (2) increasing pressure to replace these cables due to environmental concerns. To date, diameter limitations of conventional extruded dielectric cables has impeded their replacement in existing PILC conduits. This paper describes a study for the evaluation for reliably reducing the insulation thickness to achieve a lower diameter cable to effectively replace PILC cable in existing conduits. Part I of the investigation reviews the theory of insulation wall determination and the test program carried out to evaluate electrical performance of reduced wall EPR cables. Additionally, cable design concepts and constructions are discussed     

Development of extruded ethylene propylene rubber insulated superconducting cable

A superconducting power cable is one of the promising ways of underground transmission of huge electric power in the future. The authors have long proposed the idea of the extruded polymer insulation for superconducting cables. The prominent features of the design are to exploit the excellent electrical properties of polymer in the cryogenic temperatures and to separate the helium coolant from the electrical insulation. Although the extruded cross-linked polyethylene cable has proved ability at the liquid nitrogen temperature, the cable insulation cracked due to mechanical stress during cooling to the liquid helium temperature. To overcome this problem, ethylene propylene rubber (EPR) was selected as a new insulating material considering the good results of mechanical and electrical tests of EPR samples at cryogenic temperatures. An extruded EPR insulated superconducting cable 15 m in length was fabricated and a cooling test down to the liquid helium temperature and a voltage test at the liquid helium temperature were carried out with fair success. This is a breakthrough in terms of the electrical insulation design of cryogenic cables.     

Development of 500-kV XLPE cables and accessories for long-distanceunderground transmission lines. V. Long-term performance for 500-kV XLPEcables and joints

500-kV XLPE-insulated cable with an insulation thickness of 27 mm and its joint have been developed for long distance transmission lines. First, in the basic research made from 1989 to 1992, elicited were the performance-determining factors of XLPE cables and joints. Their respective insulation designs were made in terms of these factors. On the basis of these designs, full-sized cables and joints were manufactured in the course of the development research performed from 1992 to 1993. The next step was to evaluate the initial electrical performance to estimate the suitability of our designs. As the final step of our series of researches so far carried out, a long-term loading cycle test was carried out by means of a model line with joints as installed and using the same techniques as employed in the actual commercial lines for full eight months. We could thus verify the long-term reliability for commercial use. The authors discuss EMI, performance dominating effects and quality control, and initial and long term electrical performance testing     

Electrical Power System Design Features of a Modern Large Polyethylene Plant

A modern large polyethylene plant requires a sophisticated electrical power supply to meet the requirements of a continuous process industry. The principal features of such an electrical power system are summarized. The power system was designed to achieve project objectives which stressed safety of operation, ease of maintenance, and high reliability. Some of the modern design features discussed are dual 230-kV power supply lines; low-profile 230-kV bus work; personnel safety grounding of main substation to reduce step and touch potential to safe levels; transfer trip signals to utilize the utility company's switching equipment; extensive protective devices to detect and initiate switching equipment action for power equipment malfunctions or power system faults; vacuum switched under-load tap changers on main power transformers; vacuum breakers for 13.8-kV switching equipment; climate-controlled elevated substation buildings to permit bottom entrance and exit with all power and control cables; nonmetallic sheathed power and control cables in open cable tray; medium resistance ground derived by zig-zag grounding transformers in the 13.8-kV distribution system; high-resistance grounding at the secondary wye connection of all 4.     

Technical trends in medium voltage URD cable specifications

It has been more than thirty years since high molecular weight polyethylene (HMWPE) was first introduced as a commercial insulating material for medium voltage (15-35 kV) underground residential distribution (URD) cables. That began a dramatic expansion of utility electrical service that has led to the installation of several million miles of URD cables by over 3,000 utility companies across America. When early cables began failing prematurely, the industry was challenged to diagnose the causes and find corrective solutions that would give utilities the confidence to continue their commitment to underground cable. The past 25 years have been a story of constant technical improvements, increasingly tighter industry specifications and positive advancements in URD cable life performance. This paper discusses the significant technical changes that have taken place during this period and their impact on cable performance. The results of a 1983-1993 ten-year study of the major technical specification changes of the 25 largest Investor-Owned Electric Utility Companies (IOUs) is also presented. The data illustrate how quickly new cable technology is specified by these leading utilities and provide a clear picture of the key technical trends occurring throughout the utility industry during the 1990s     

Evaluation of service aged 35 kV TR-XLPE URD cables

This paper provides information on the aging of URD power cable insulated with a tree-retardant crosslinked polyethylene (TR-XLPE) compound, installed in a typical utility environment, Numerous evaluations were performed on samples of power 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     

On Single-Phase Earth Faults in a 20-kV Power Network

Single-phase earth faults in per-phase shielded cables of a 20-kV electrical network that is relatively new in Russia are studied experimentally. In contrast to the widely used 6- to 10-kV electrical network with an isolated or compensated neutral, the 20-kV network has low-resistance neutral grounding. Therefore, calculated conditions for substantiation and choice of nominal parameters of per-phase shielded cables for the 6- to 10- and 20-kV networks can significantly differ. It is shown that shielding for 6- to 10-kV cables is chosen for the case of a double-phase to earth fault (two phases connected with the earth), whereas, for 20-kV cables, this is a single-phase earth fault, which is significantly less strict with respect to thermal stability. Double-phase earth faults in 20 kV networks are practically excluded due to significantly lower levels of overloads in the network upon commutations and a comparatively short time (0.25–1.0 s) of faulted-line tripping. It is established experimentally that the single-phase earth fault in 20-kV per-phase shielded cables does not transforms to a multiphase earth fault. It is substantiated that the choice of shields for these cables based on the current of single-phase earth fault eases the requirements to thermal stability of the cables, which is of principal importance for lowering the cable production cost and power and energy losses in electrical networks.     

A Novel Design for Medium Voltage (15-35kV) URD Cable Joints

A new concept in heat-shrinkable technology has been developed into a range of medium voltage URD cable joints. The new thermo-elastomeric tubing has been used in a range of cable joints for concentric and jacketed concentric neutral distribution cables for 15 to 35kV voltage classes. These joints were designed with the aid of a computer modeling and finite element analysis technique which predicted the electrical stress and assisted in design optimization. The joints were tested to IEEE 404-1986 and ANSI C119.1-1974. The typical stress plots of the joints and those data relevent to performance are also reported.     

Condition assessment of 12- and 24-kV XLPE cables installed during the 80s. Results from a joint Norwegian/Swedish research project

This paper reports the results from the condition assessment of 12- and 24-kV cross-linked polyethylene (XPLE) cables using a technique based on dielectric spectroscopy initially developed at KTH in Sweden. The work aims to examine whether the method could detect water tree degradation for the second generation medium voltage (MV) cables with long, but not bridging, water trees. While the overall cable condition was better than expected for second generation XPLE cables, water trees were found in most of the selected cables. The diagnostic method based on the measurement of the dielectric response could only detect water tree degradation in the examined second generation cables when the water trees bridged the insulation wall. Condition assessment above service stress may, in some cases, be required to detect bridging water trees. The results indicate that there is a correlation between the voltage level and the breakdown voltage of the cable. This can be used as a diagnostic criterion for this group of cables.     

Tan δ of ethylene-propylene rubber in cryogenic temperature region–effects of additives and sample thickness

The authors have been developing extruded polymer insulated superconducting power cables. Dielectric loss in electrical insulation cannot be ignored in superconducting cables since conductor loss in the cables is minimal. Studies so far show that ethylene-propylene rubber (EPR) is suitable as an electrical insulating material in the extruded polymer insulated superconducting cable design because it demonstrates excellent mechanical and relatively good electrical qualities at a cryogenic temperature. Widely used EPR includes some kinds of additives; however, their effect on tan δ of EPR at cryogenic temperature remains unknown. The effect of additives such as crosslinking agent and fillers on tan δ of EPR was examined at temperatures of between 4.2 K and 300 K. Thickness dependence of tan δ was also measured using EPR films of different thickness and an extruded EPR insulated cable sample. The results show that additives increase tan δ of relatively thin films of EPR even in the cryogenic temperature region; however, they do not have an intolerable dielectric loss in comparison with conductor loss and heat inflow of the superconducting cable. The remedy to tan δ increase due to the polymer contraction relative to shielding wires has been established.     

Insulation characterization in multistress conditions byaccelerated life tests: An application to XLPE and EPR for high voltagecables

This paper focuses on accelerated testing procedures that can be applied to XLPE and EPR for high voltage cables and that are aimed to achieve the thermo-electrical endurance characterization of insulation. Several methods and techniques for single and combined thermo-electrical stress are illustrated; indices for the evaluation of electrical, thermal, and multi-stress endurance are reported and their derivation is shown. Indications on the feasibility of the design-stress inference from tests on laboratory specimens also are given. Experimental results mainly refer to cable models, but data relevant to full-sized cables, as well as to simple, flat specimens, are also considered     

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     

Advances in Multiconductor Jointing Systems for Medium Voltage Solid Dielectric Cables Using Heat-Shrinkable Components

Heat-shrinkable polymeric materials are now being used to reinsulate joints in single- and three-conductor armored cables, particularly on cables used by industry for 15-kV distribution systems. A theoretical evaluation is given of the electrical stress control in the joint insulation system and the development of a joint for three-conductor cables, which eliminates the need for complex fittings. A test program is reviewed and the results reported.