Download high voltage cable jointing

HIGH VOLTAGE CABLE JOINTING
1.0
Introduction
Restrictions imposed by manufacturing process, transportation and site conditions necessitate
production of power cables in specified lengths, which after installation, are joined and
terminated at pre-pre-determined locations. Joints and terminations are an integral part of the
cable system, and are required to perform all the functions expected of the cable itself. The
complexities of a jointing system increases with the voltage grade of the cable, the higher the
voltage the more stringent are the requirements to be met.
The following paragraphs make a brief introduction to the concept of H.V. cable jointing, the
requirements on which the design is based, and the techniques employed to achieve the desired
result.
2.0
Fundamental Principles
The principles governing the design of a jointing system are:
3.0
1.
Conductor connections should be suitable for the full rating of the cable.
2.
Insulation of the joint should be as effective as that of the insulated cable cores. For
high voltage applications, special consideration is necessary to ensure that joints are
protected against undue stresses, arising out of transitory changes in the shape of the
conductors to be joined or termination of core screens maintained at earth potential (See
Glossary).
3.
The joint termination should be protected against ingress of moisture, and enclosed to
prevent mechanical damage.
4.
The joint should be able to withstand thermo-mechanical stresses under short-circuit
conditions or thermal effects of normal and permissible fault currents.
Design Concepts Applicable to Joints and Terminations
Let us now see how the above design principles apply to joints and terminations
3.1
Terminations
The basic requirement of a termination must fulfill is that the cable cores should be adequately
separated to permit connection to the end equipment. This is critical for HV and EHV systems,
since the separation should be such that the dielectric medium between phases, which may be
air or an insulating compound, does not break down under normal service a.c., switching surge
or impulse voltages. In addition, there should be no failure, either through the filling medium or
by tracking along the interface between the filling medium and the cable cores. For paper
cables, the filling medium is usually a bituminous compound or rosin oil, but with the
development of polymeric insulation for cables for high voltage applications, synthetic resin
compounds have been introduced which are finding wide acceptance. A more recent concept is
the use of heat shrinkable materials applied on the cable cores, which provide the necessary
insulation level and thereby eliminate the filling medium.
Where screened cables are employed, as is the case for EHV/HV applications, further
consideration is necessary. It is necessary to remove the insulation screen for a sufficient
distance so as to prevent tracking from the conductor along the core surface. The screen being
maintained at earthed potential for confining the electric field within the cables and also for
obtaining radial distribution of the electric stress, any abrupt termination for the purpose of
jointing will naturally affect the stress gradient. A method for overcoming the problem should,
therefore, be included in the design.
Many terminations are encapsulated inside the metallic enclosures, but outdoor terminations are
required to be additionally protected against atmospheric hazards, environmental pollution and
moisture ingress. The practice is to encapsulate the termination inside suitable porcelain
bushings, which apart from affording the desired protection, are capable of withstanding
conductor thrust set up under the action of thermomechanical forces. Insulator bushings made
of polymeric and synthetic materials are also becoming popular for H.V. applications, because
of their lightweight and ease of installation.
H.V. and EHV outdoor terminations should also be designed to overcome what is known as
surface tracking, caused by carbon deposition on the insulated surface under the effect of
atmospheric pollution. The traditional approach is to employ bushings with sufficient number
of sheds which increase the effective creep age distance from the live conductor. With heat
shrinkable terminations, which do not require any additional encapsulation, an adequate
number of skirts is introduced for increasing the creep age distance.
3.2
Joints
The aspects to be considered in joint design are almost similar. In a joint, conductor
connections are to be made in a manner to ensure both electrical and mechanical integrity.
Reinstatement of the insulation is necessary to the extent to guarantee the same design criteria
as in the cable. For screened cables, some form of stress control is called for to take care of the
electrical stresses. In addition, the insulated cores should be bound together, or restricted
otherwise, to prevent buckling and intercore movement under the action of thermomechanical
forces. The joint should also be capable of carrying through fault currents, and therefore some
designs require an additional connection, or cross bonding of sheaths, which is often employed
for single core cables of EHV transmission circuits for minimizing the induced sheath voltages
and thereby increasing the current rating of the metallic envelope. The joint should be protected
against ingress of unwanted substances like moisture and finally sealed and encapsulated in an
enclosure containing a filling medium.
4.0
Stress Control
The most critical aspect of high voltage cable jointing is control of the dielectric stress
originating at the point of screen termination. Without the application of stress control,
discharges would occur, adversely affecting the life of the joint and termination. The following
section discussed the requirements for stress control as also the methods in vogue to provide
relief.
4.1
Why Stress Control
The stress distribution at the conductor joint varies considerably due to changes in the profile
introduced by the use of a ferrule. Sharp edges and protrusions at the joint, if left unrelieved
also result in abrupt change of the stress gradient. It is therefore essential for the conductor to
have a smooth profile so that there is no undue concentration of stresses.
However, the more important aspect of stress control applies to the location where the
insulation screen is terminated. Referring to Fig.1, it can be seen that not only the dielectric
stress increases in termination region, but also a potential gradient is set up along the interface
between the dielectric and the surrounding medium. The stress in the dielectric at the screen
termination will be well above the design stress and may lead to premature failure. In addition
if the surrounding medium is air, or there is a void between the dielectric and the filling
medium, then the stress in the area may cause the air to permit discharge even at the working
voltage. Paper is somewhat resistant to these discharges, but for polymeric insulation, such
discharges will rapidly erode the dielectric and eventually result in failure.
4.2
Methods of Stress Control
The methods employed are described below:
4.2.1
Capacitive Method
The traditional approach to provide stress relief is by means of a stress cone. The stress cone is
a means of controlling the capacitance in the area of screen termination, thereby reducing the
dielectric stress along a gradient to tolerable limits at the point of termination. The stress cone
is extended beyond the screen termination, so that the potential gradient at the dielectric surface
is reduced to a level where discharges will not occur. (See Fig.2).
In joints of high voltage paper cables, the stress cone is usually built to a predetermined contour
by hand application of insulating paper tapes, while in terminations the stress cone is either
hand-applied or performed. With the development of polymeric and elastomeric cables,
premoulded stress cones have also been introduced.
Before the stress cone is applied, it is necessary to reduce the electrical stress at the conductor
joint, arising out of reasons explained earlier.
The concept is to provide a smooth profile so that the stress is evened out. This is obtained by
‘stepping’ of cable papers, which is achieved by removing the paper insulation in a set of steps,
having risers and treads from the inner conductor surface to the outer insulation surface (Fig.3).
With the two cable ends so treated and joined together, hand applied impregnated paper tapes
are applied over the assembly to form the joint dielectric.
The capacitive method of stress control had been an accepted approach to stress relief of high
voltage paper cables having a ‘solid’ dielectric, new concepts were soon introduced.
4.2.2
Resistance Method
An effective method of stress control at the screen termination of both paper and synthetic
cables is by the use of high-resistance tapes or coatings, and materials with non-linear
resistance layers, the material being of constant surface resistivity passes a small current and
thereby sets up a linear voltage gradient along its length. A better stress distribution is achieved
(Fig.4) by using materials of non-linear resistivity, which also allows small current in the layer
increase, the resistance of the material drop, and a smooth linear voltage gradient is achieved
along the applied length.
4.2.3
High Permittivity Materials
Stress control is also achieved by employing materials having relative permittivity significance
higher than the cable dielectric (Fig.5). The method is based on the principle that when
materials of dissimilar permittivities are subject to a potential gradient across their combined
thickness, the highest stress is experienced by the material having the lowest permittivity. It can
be seen from the schematic diagram that the equipotential lines emerge gradually from the
dielectric, thus producing a smooth gradient at the dielectric surface.
5.0
Jointing Systems
Cable jointing is a precise craft is skill-oriented, particularly when working on HV cables.
Although, a number of systems has been developed, there is no ‘universal’ joint or termination,
and the type of product is selected depending on technical, economic and physical constraints
of the particular installation.
The accent today is on simplicity and ease of installation without sacrificing reliability and
efficiency. The conventional techniques for paper cables are easily understood, impregnated
paper tapes being used for building the insulation and providing stress relief. For synthetic
cables having elastomeric or polymeric insulation, self-amalganating tapes made of EPR
provide the joint insulation. Stress control is achieved by employing premoulded stress cones
generally made of EPDM, and in case of joints, by using semi-conducting layers moulded into
a rubber like material. For HV polymeric cable joints where a high degree of finish and
accuracy is involved, systems employing factory moulded components which are fitted ‘in situ’
are also available. XLPE or EPR cable joints operating at transmission voltages sometimes
utilize epoxy premoulded bushings, which have excellent dielectric properties and can
withstand the high thermomechanical forces set up under short circuit or heavy load conditions.
Heat shrinkable joints for paper and polymeric cables of HV grade are also in use. The system
comprises tubings made of cross-linked polymers, which provide insulation, stress control as
well as encapsulation of the joint. These are used in combination with mastic and adhesive
materials to form watertight seals and fill up the voids within the joint.
6.0
Conclusions
Cables are manufactured under carefully controlled conditions. This cannot be said of joints
and terminations, which are sometimes to be made under adverse site conditions. Nevertheless,
when completed, they should be as reliable as the cable to obviate the risk of supply
interruption at a later date. High voltage cables from an integral part of power transmission and
distribution systems, and it is essential to ensure a ‘risk free’ operation, based on sound design
of the cable and its accessories.
Whatever be the jointing system employed, reliability can only be ensured if the fundamental
principles governing the design and construction are given due consideration. For HV cables,
the parameters are sharply defined, and it is as much important to safeguard against inadequacy
in design as against indifferent workmanship or poor quality materials.
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GLOSSARY
Screening (shielding) of power cables
Screening or shielding of h.v. insulated cables consists of conductor screening and insulation
screening.
Conductor Screening
Experimental investigations have revealed that conductor stranding can increase the maximum
electrical stress by about 20 per cent. To alleviate this effect all paper-insulated cables with a ratingof
5kV and above are manufactured in the form of semiconducting carbon, with synthetic or polymeric
insulation of the same rating, conductor screens (shields) are employed to preclude excessive electrical
stress in voids between the conductor and the insulation. To be effective they must adhere to and
remain in intimate contact with the conductor and the insulation under all conditions.
Insulation Screening
The concept of using an insulation screen (shield) for h.v. cables was developed and introduced by
Hochstadter. An insulation screen has a number of functions to perform, each of which is of equal
importance for either paper- or synthetic-insulated cables of multi-core or single-core constructions,
namely:
1.
2.
3.
4.
5.
To confine the electric field within the cable.
To obtain strictly symmetrical radial distribution of electrical stress within the dielectric, thereby
minimizing the possibility of surface discharges by precluding tangential and longitudinal
stresses.
To reduce the hazard of shock, achieved by the screen being well earthed. If it is not, the hazard
of shock is increased.
To protect the cable connected to overhead lines from being subject to induced potential.
To limit radio interference.
Metallic shields, used as insulation screens, vary considerably in their construction but should be made
of non-magnetic metallic layers, such as thin aluminum or copper tapes, or in the form of thin extruded
conducting sheaths. Insulation screens (shields) must be earthed at least at one but preferably at two or
more locations thus being always at earth potential.
Conductor and Insulation Screens For Cables with Synthetic Insulation
For the h.v. range it is most important to use high quality screens, both over the conductor and over the
insulation, in order to keep the discharge level to a minimum, thereby preventing the growth of ‘trees’.
Modern conductor screens are always in the form of extruded semi-conducting layers. The idea
insulation screen is an extruded layer which must be applied and vulcanized in a triple extrusion
process including the conductor screen and the insulation. To avoid discharge the screen should adhere
well to the insulation and the move with it during expansion and contraction due to load cycles or
bending. However, at the same time it must be easily removable during jointing operations. This type
of insulation screen, in the form of an extruded vulcanized layer, has only recently been introduced
after a long period of research. The successful development of this insulating screen represents an
important achievement in the long struggle to improve the x.l.p.e. cable design.
Mechanism of Insulation Breakdown due to Ionisation
To eliminate the weakness of the belted-type cable caused by the weak filler insulation, spaces
between cores and the tangential flux, the screened cable was introduced. The screen round the
individual cores confines the stress to sound core insulation and also ensures that the flux is
substantially radial.
With the introduction of the screened or H type cable, many of the weaknesses of the belted
construction were eliminated and 33 kV solid H type cables have given extremely good service
performance for many years. On increasing the transmission voltage above 33 kV, however, the one
weakness remaining, though not manifestly harmful at 33 kV, had to be eliminated. The final weakness
was a gaseous ionization in voids formed within the insulation by compound migration resulting from
cable loading and steep inclines.
The breakdown of paper insulation due to ionization occurs through the formation of carbonaceous
‘fronds’ on the insulation paper. This is generally known as ‘treeing’. The carbonaceous paths start at
an almost imperceptible carbon core, generally at the conductor surface and gradually spread outwards
through the insulation, increasing in width and complexity as progression takes place.
It will be understood that the above concept of void ionization would equally apply to a paper cable
joint where the insulation is built up with impregnated paper.
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