EPR CABLE TECHNOLOGY CONSORTIUM
Members
The Electrical Insulation Research Center, University of Connecticut has formed an EPR Cable Technology Consortium for the purpose of advancing EPR cable technology to the benefit of Consortium Sponsors and the public through (i) improved understanding of properties of EPR cable dielectric and the resulting cable and (ii) the implications of such properties for user applications. The intention of the Consortium is to advance knowledge and understanding of EPR cable technology, and to this end, all data and knowledge resulting from work sponsored by the Consortium can be made available to the public and published subject to appropriate review by Consortium members to assure that such information is technically accurate.
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].
Downloadable
Technical Literature in PDF Format
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
The 25 year service record
of EPR cables installed at Memphis Light, Gas and Water is described.
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
This paper reports data after 9-14 years
of service aging under normal and accelerated voltage at MLGW. The cables have
now been in service for 21 years, and the results of tests after 21 years of
aging should be available soon. However, a brief summary of experience can be
found in [8].
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.
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 ns risetimes 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.
Last updated:
August 18, 2011. For further information, please get in touch
with Steven Boggs.