Background |
Shielded distribution cable is employed over the range from a few thousand volts (kV) to about 69 kV to distribute electric power on a local basis in urban and suburban areas without the use of overhead wires. The key element of such cable is the electrical insulation which supports the very high voltage between the conductor and the grounded shield which assures that the electric field remains within the cable. Two insulation technologies compete in the market; insulation based on mineral filled ethylene propylene rubber (EPR) and insulation based on relatively pure cross linked polyethylene (XLPE or TR-XLPE).
Resistance to Water Treeing |
Cross linked polyethylene cable technology has progressed through three generations of cable, each of which was relatively unreliable as a result of a phenomenon known as “water treeing”, which is described in more detail in PDF files [1 ], [2]. This phenomenon results in the growth of a tree-like pattern of water penetration into the insulation which lowers the electrical strength of the cable and eventually results in premature failure [3], [4], [5], [6], [7]. EPR cable is relatively immune to the water treeing phenomenon [2], and has provided reliable service for over 30 years [8],[9],[10],[11].
Thermal and Chemical Stability |
The base polymer of EPR cable was first developed in the early 1960s [12] can vary in crystallinity, chemistry, etc. [13],[14],[15]. As the base resin is too soft for direct application in cable, a range of fillers, dominated by highly refined clay [16], is employed to achieve the required mechanical and electrical properties. EPR cable compound is highly stable chemically and thermally[17],[13], and EPR-based cables can be rated at temperatures of 140 to 150 oC [18].
Partial Discharge and Corona Resistance |
EPR-based cable insulation has much greater resistance to the degrading effects of corona and partial discharge, which also contributes to the high reliability of EPR cables [19],[8].
Partial Discharge Testing |
Field partial discharge testing of distribution cable has become a “hot” topic as a result of the need to prioritize replacement of the large amounts of HMWPE and XLPE cable which must be replaced over the next few years as a result of water tree induced degradation. The topic of partial discharge testing in the context of distribution cable is discussed in [20]. Field partial discharge testing of EPR cable has not been a priority as a result of its high reliability in the field [8].
Protective Effect of High Frequency Loss |
EPR cable has the advantage of greater high frequency loss which can protect motors and transformers from premature failure caused by fast transients generated by variable speed drives and power system switching devices such as vacuum and SF6 switches and circuit breakers [21],[22],[23],[24],[25].
History |
A Brief History of Rubber Cable and Polymers on which they are Based
A short history of rubber cable, from its earliest use by Morse (telegraph) and Edison (power) to the present, including the evolution of polymers from natural rubber to the ethylene propylene rubber (EPR) of today.
Reliability |
AC and Impulse Performance of Medium Voltage EPR Cables
AC and Impulse Performance of Medium Voltage Ethylene Propylene– Rubber Cables With Over 25 Years of In-Service Aging in a Wet Underground Environment indicates high service reliability.
EPR Insulation Cuts Treeing and Cable Failures
EPR cable properties are compared with those of PILC, XLPE, and butyl rubber.
EPR-Based URD Insulation: “A Question of Confidence”
The properties and reliability of EPR cable insulation is discussed in comparison with XLPE.
Manhole Events are Reduced Greatly by EPR-Insulated Secondary Cable
EPR dielectric has much greater tracking resistance than older insulations such as SBR, which results in very few manhole events being associated with EPR-insulated secondary cable relative to SBR insulated secondary cable.
Water Treeing |
Fundamentals of Water Treeing in Solid Dielectric Cable
This article discusses how the difference in propensity toward water treeing between XLPE and EPR cable insulations can be understood from first principles which involve differences in (i) hydrophobicity of the dielectric and (ii) wet conductivity of the dielectric.
Water Treeing Filled versus Unfilled Cable Insulation
This article discusses the difference in water treeing characteristics of filled insulations, such as EPR, and unfilled insulations, such as XLPE and explains these characteristics in terms of ion content and hydrophobicity.
Mechanism for Impulse Conversion of Water Trees to Electrical Trees in XLPE
A lightning impulse causes substantial capacitive current in a water tree channel which, as a result of its small cross section, has relatively low conductance. Transient, nonlinear finite element computations with coupled thermal and electric fields for the geometry of a 15 kV XLPE dielectric cable indicate that an 80 kV lightning impulse can cause the temperature of water within a water tree channel to rise to the boiling point over a range of four orders of magnitude in water conductivity. The temperature rise reduces substantially the yield stress of the XLPE, raises the pressure within the water tree channel, and is likely to leave a cavity which can support partial discharge resulting in electrical tree initiation.
Mechanisms for Degradation of TR-XLPE Impulse Strength during Service Aging
This paper proposes a physical mechanism for the degradation of the impulse strength of TR-XLPE cable while the AC strength remains relatively stable.
Field Aging and Wet Electrical Aging |
Analysis of AC and impulse breakdown strengths of EPR cables aged on the MLGW system in a wet underground environment under normal and accelerated voltage since 1983 indicates highly reliable operation of these EPR cables over the test period of over 30 years and for the foreseeable future. To date, a systematic failure mechanism has not been identified for any of these EPR insulations subjected to normal and moderately (150% overvoltage) “accelerated” operating voltage stress in a wet underground environment.
Field Monitoring of Parameters and Testing of EP and TR-XLPE Distribution Cables
This paper reported the degradation in the impulse strength of TR-XLPE cable during wet electrical aging in the field, both at normal operating voltage and during accelerated aging, although the AC strength was relatively stable. A mechanism for this degradation is proposed in one of the papers below.
This accelerated aging study on model cables again demonstrates the substantial decrease in the impulse strength of TR-XLPE insulation during wet electrical aging, although the AC strength is very stable. TR-XLPE insulations in this study failed even though the AC strength appeared to remain high. Thus the impulse strength of TR-XLPE cable appears to be a much better indicator of insulation condition than the AC strength, which appears to remain high until shortly before failure.
Service Aged Medium Voltage Cables -A Critical Review of Polyethylene Insulated Cables
This paper discusses available data related to the reduction in dielectric strength of service-aged XLPE and TR-XLPE cable.
Accelerated Life Testing of EPR-Insulated Underground Cable
Summarizes accelerated life tests and testing of EPR, XLPE and TR-XLPE cables, providing details of the test protocol and test results.
EPR Polymer, Clay Dielectric Grade Filler, and Compounding |
The Chemistry of Ethylene Propylene Insulation, Part I
The history, polymeric structure, and properties of EPR are discussed.
The Chemistry of Ethylene Propylene Insulation, Part II
The compounding and mixing of EPR cable insulation is discussed.
From Dust to Quality Dielectric – The Formulation of Dielectric Clays
Describes the process by which high technology clays are produced for dielectric applications.
Compounding of Ethylene-Propylene Polymers for Electrical Applications
The role of the various fillers, processing aids, and compatibilizers in ERP cable compound is discussed.
Compounding of Ethylene-Propylene Polymers for Electrical Applications
Thermal and Mechanical Properties |
Thermal and Mechanical Properties of EPR Cable Compound
the thermal and mechanical properties of four EPR cable dielectrics are described in comparison with XLPE.
105 oC/140 oC Rated EPR Insulated Power Cables
Describes a study for implementation of EPR cable and accessories with a 105 oC continuous rating and 140 oC emergency rating.
Corona and Partial Discharge Resistance |
Evaluation of Discharge Resistance of Solid Dielectric Power Cable Insulations
The discharge resistance of cable insulations varies from the very poor discharge resistance of XLPE to nearly complete discharge resistance of some EPR formulations. Discharge resistance brings increased reliability, as some forms of defects can be tolerated without failure.
Protective Properties of EPR Cable |
Surge Protective Properties of Medium Voltage Underground Cable
Fast transients in power systems can be generated by switching of vacuum and SF6 insulated devices as well as by solid state devices such as those used in variable speed drives. Transients with nsrisetimes can be generated which, in a cable-connected system, propagate down the cable to inductive devices such as motors and transformers. In general, the amplitude of such surges is not out of the ordinary; however, the very short risetime can cause unacceptable voltages across the first turn of an inductive device. A cable with high frequency loss does not generally decrease the amplitude of such surges appreciably but can lengthen the risetime substantially by absorbing high frequency energy from the surge. This reduces the voltage across the first turn of inductive devices and thereby protects them from damage and failure caused by such surges. This contribution presents experimental evidence for such protection through the use of EPR cable to feed distribution transformers and was motivated by circumstantial evidence from the field that the failure rate of such transformers was smaller when they were connected to EPR cable.
Effect of Shielded Distribution Cable on Very Fast Transients
This contribution quantifies the high frequency losses of distribution cable and effect thereof on very fast transients for four types of shielded, 15 kV distribution cable, three made from various EPR compounds and one made from TR-XLPE.
Effect of Shielded Distribution Cables on Lightning-Induced Overvoltages in a Distribution System
The high frequency attenuation of EPR cable can protect underground infrastructure, such as cables, transformers, etc., from the effect of fast transients such as lightning impulses. This paper assesses this issue based on an example from a utility distribution system.
Effect of High Frequency Cable Attenuation on Lighting-Induced Overvoltages at Transformers
This paper is similar to the one above; however, it focuses on transformers and provides a better description of the basic mechanisms by which high frequency cable attenuation protects transformers in an underground distribution system.
Partial Discharge Testing |
Fundamentals of Partial Discharge in the Context of Field Cable Testing
Field partial discharge testing has become a hot topic as a result of the large amounts of XLPE cable which are reaching end of life. Utilities are looking for ways to prioritize replacement of such XLPE cable. No such issue has arisen concerning EPR cable.