Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2000-12-26
2003-09-16
Le, N. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S539000, C174S019000
Reexamination Certificate
active
06621276
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a termination assembly for a power cable. More particularly, the present invention relates to a power cable termination assembly used for power cable testing after manufacture.
Quality testing is performed on a length of finished power cable at the end of the manufacturing process. In the art, one such quality test is a high potential test where the finished cable is subjected to an alternating current voltage of approximately three to four times the tested cable's rated operating voltage. With this voltage applied, partial discharge, also referred to as corona, from the cable is measured to determine the insulation quality. If partial discharge greater than a specified magnitude is present under such high potential testing, the tested power cable is defective and probably contains voids or contaminants within the cable insulation. Basically defined, partial discharge is the phenomenon whereby air ionizes and begins to conduct electricity under high voltage conditions. In some cases, partial discharge can produce light, noise, and even ozone.
During the testing process, however, if the cable end is not properly terminated, partial discharge will occur at the cable termination even when the insulation contains no defects. The existence of partial discharge at the termination can hinder the detection of flaws in the cable insulation because when partial discharge occurs at the cable termination, its origin may be difficult to determine. In this case, testing techniques cannot distinguish the source, which may either be a defect in the cable insulation or ionized air at the cable termination. When a material with a high ionization voltage surrounds a cable termination, however, the ionization at that termination is greatly reduced or nearly eliminated.
FIG. 1
shows an exemplary conventional cable
100
containing a jacket
110
, a primary shield
120
, an insulation shield
130
, insulation
140
, conductor shield
150
, stranded conductor
160
, and strand seal
170
. Those skilled in the art will appreciate that other cable constructions, including various voltage ratings, may be used in conjunction with the present invention.
Jacket
110
provides thermal, mechanical, and environmental protection of the layers underneath it. Jacket
110
is optional and may be constructed of polyethylene, PVC, or nylon.
Next to the jacket, primary shield
120
may be made of a circumferentially corrugated metal tape, drain wires or a concentric neutral. A concentric neutral
120
′, shown in
FIG. 2
, comprises a plurality of electrically conductive strands placed concentrically around insulation shield
130
. The concentric neutral
120
′ serves as a neutral return current path and must be sized accordingly. The insulation shield
130
is usually made of an extruded semiconducting layer that is partially bonded to the insulation
140
. Primary shield
120
, insulation shield
130
, and conductor shield
150
are used for electrical stress control providing for more symmetry of the dielectric fields within cable
100
.
The insulation
140
, contained beneath insulation shield
130
, is an extruded layer which provides electrical insulation between conductor
160
and the closest electrical ground, thus preventing an electrical fault. Generally, insulation
140
is made of polyethylene, crosslinked polyethylene, or ethylene-propylene rubber. Polyethylene is susceptible to degradation due to partial discharge which may in turn lead to “water treeing”. Water treeing is the phenomenon whereby small tree-like voids form and grow in the insulation
140
and fill with water that may have ingressed through the conductor strands. If a tree grows large enough in the insulation
140
, electrical breakdown, and thus cable failure, will occur between the conductor
160
and an electrical ground. Crosslinked polyethylene is a significant improvement to polyethylene and, like ethylene-propylene rubber, is less susceptible to electrical breakdown due to water treeing. Also, ethylene-propylene rubber is more flexible than polyethylene or crosslinked polyethylene.
Conductor shield
150
is generally made of a semiconducting material and surrounds conductor
160
. As stated previously, conductor shield
150
is used for electrical stress control, providing for more symmetry of the dielectric fields within the cable
100
. Conductors are normally either solid or stranded, and are made of copper, aluminum or aluminum alloy. The purpose of stranding the conductor is to add flexibility to the cable construction. The small spaces between the strands of a stranded conductor, however, provide a path for water to ingress the cable
100
. As stated previously, water can aggravate the treeing problem within insulation
140
, accelerating cable failure. In an attempt to alleviate this problem, strand seal
170
is added into the small spaces between the strands. While the strand seal tends to limit the water ingress, it does, however, add to the stiffness of the cable
100
. A solid conductor
160
′ construction is shown with respect to
FIG. 2
, as an example of a construction not requiring strand seal.
FIG. 3
illustrates an exemplary conventional cable prepared for a termination assembly. To prepare the cable for termination, jacket
110
(not shown) and primary shield
120
(not shown) are first removed for a predetermined distance, exposing insulation shield
130
. Second, insulation shield
130
is cut circumferentially at a predetermined distance from the end of the cable and that portion of insulation shield
130
is removed from the insulation
140
. At last, a circumferential cut close to the end of the cable is made through insulation
140
and conductor shield
150
. After this cut is made, a portion of the insulation
140
and conductor shield
150
at the end of the cable are removed, exposing conductor
160
at the cable end. When testing the quality of the insulation, cable manufacturers currently employ several techniques to limit partial discharge not caused by defects in the insulation.
For example, most cable manufacturers currently use resistive paint as a mechanism for limiting partial discharge at a cable termination. Using this method, the insulation shield is cut circumferentially at a certain distance from the cable end. Care must be taken to cut completely through the insulation shield without scoring the insulation. The insulation shield at the cable end is then removed, exposing the insulation surface. A thin layer of resistive paint is applied over the exposed insulation surface around the cable circumference overlapping the insulation shield by about one inch and extending to near the end of the cable. The layer of resistive paint provides a resistive current path, which results in a linear distribution of the electrical stress along the cable end, thus limiting partial discharge.
The effectiveness of this method, however, depends heavily on the cable construction, characteristics of the resistive paint, and the length of the insulation shield. Different cable insulating materials or different cable sizes, therefore, require resistive paint of different electrical characteristics or different insulation-shield-strip-back distances. This requires the technician performing the test to have a variety of different resistive paints available and to apply a different strip-back distance for the variety of different cables tested, which adds to the complexity of the testing process.
Another major problem associated with the resistive paint method is that it requires special handling because the paint dries quickly and chips very easily. Once the paint chips, the chips can act as sharp electrodes and can cause significant partial discharge in the area, which aggravates the very problem the resistive paint was intended to overcome. In addition, the resistive paint is not completely compatible with ethylene-propylene rubber, a commonly used cable insulation. Due to the chemical characteristics of ethylene-pro
Hebert Claude
Kelly Nathan J.
Pion André
Wen Yingli
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Lair Donald M
Le N.
Pirelli Cavi e Sistemi S.p.A.
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