MATERIAL AND DESIGN CONSIDERATIONS FOR POLYMERIC HIGH

VOLTAGE CABLE ACCESSORIES

(ALI HIRJI, CONSULTANT, RAYCHEM RPG PVT. LTD.

INTRODUCTION : High Voltage Polymeric Insulated Cables were first used in short sections

for undergrounding of Overhead Transmission Lines, wherever necessary. At the ends of the

cable circuit, Terminations were used to make connections to either Transformers, GIS or an

Overhead Line . After Straight Joints were developed and became available, such cables were

used for long distance transmission.

High Voltage Polymeric Cable Terminations and Joints are key elements in High Voltage

Cable Transmission Circuits. They are required to exhibit a performance which is equal to

the cable to achieve the highest system rating at the lowest component and assembly cost.

Thus, they are included in the prequalification and type tests on the cable systems. The

continued development of the Cable Terminations and Joints is the determining factor in

making possible the application of reliable cable systems particularly at the higher

transmission voltages.

Cable Joints and Terminations have to be designed to suit the types of polymeric

transmission cables being used by the Utilities and the Industries and to take into

consideration the various challenges posed by the cable preparation prior to termination or

jointing. The types of cables typically used in India are shown in Figs 1, 2 and 3.

FIG 1: SINGLE CORE, XLPE CABLE WITH COPPER WIRE SCREEN AND POLY-AL SHEATH

FIG 2: SINGLE CORE, XLPE CABLE WITH COPPER WIRE SCREEN AND LEAD SHEATH WITH PE

OUTER SHEATH

FIG 3: SINGLE CORE XLPE CABLE WITH CORRUGATED ALUMINIUM SHEATH AND PE OUTER

SHEATH

HIGH VOLTAGE POLYMERIC CABLE ACCESSORIES

An extremely important design requirement of a Cable Joint or Termination is Electrical

Stress Control ie limiting the high electrical stresses at the screen termination as well as

within the accessory, mainly in the interface between the Cable insulation and the

Accessories Insulation to permissible levels.

ELECTRICAL STRESS CONTROL AT THE SCREEN TERMINATION:

The Electrical Field within a Single Core High Voltage Cable is shown in Fig 4 below

FIG 4

While terminating or jointing a cable, it is necessary to remove the screen to a point at a

certain distance from the exposed conductor to provide adequate length of insulation

interface between the cable insulation and the insulation material applied over it. The

length to which the screen is removed depends upon the voltage rating of the cable and the

type of insulation material used. This removal of a portion of the screen results in a

discontinuity of the axial geometry of the cable with the result that the field is no longer

uniform axially along the cable, but exhibits variation in three dimensions. This is shown in

Fig 5 below

FIG 5

Fig 5 also shows the electric field in the vicinity of the terminated screen. The electric flux

lines which originate along the length of the conductor are seen to converge on the

terminated screen, with the attendant close spacing of the equipotential lines signifying the

presence of high electrical stress. This stress concentration is of much greater magnitude

than that occurring near the conductor in the continuous conductor. Steps must therefore be

taken to reduce this stress without which the stress can exceed the design stress level of the

cable insulation. Without this control, the high stress can also lead to partial discharge in the

cable dielectric, ionization and breakdown of the air at the screen terminus, causing rapid

ageing of the cable insulation, leading to a dielectric puncture and failure. This is illustrated

in the Photograph taken of a cable end having an abrupt termination of the Screen and

energized at normal voltage.

It is extremely important therefore to reduce the stress level at the screen termination to a

low, acceptable value.

For High Voltage Polymeric Cables the most commonly method employed is the geometric

stress control in which the semi conducting screen is extended by means of a conductive

polymer in the profile of a cone and the space between this conical application of the

conductive polymer and the cable insulation is filled with an insulating elastomer. This

combination of the conically shaped conductive polymer and the insulating elastomer is

commonly called the “Stress Cone”. A typical stress cone used for limiting the electrical

stress at the screen termination is shown in Fig 6 below:

FIG 6

The diameter of the elastomeric stress cone has a diameter slightly smaller than the Core

Diameter and is Slipped/Push Fitted onto the Core.

The Stress Cone has an “interference fit” over the Core by virtue of its elasticity. The

elastomeric stress cone relies on the elasticity of the rubber to produce a radial compressive

force onto the cable core and therefore its material properties play a very significant role in

the reliable performance of the Cable Termination or Joint. The interface of the Stress Cone

and the XLPE Cable Insulation is a critical issue since the dielectric strength of this interface

is lower than the strength of the elastomer as well as the XLPE Insulation. The stress cone

must be designed to keep the electric stress within this interface below the critical value

The profile and length of the Stress Cone play a very important role in limiting the strength

of the electrical field at the screen termination.

After a thorough investigation into the properties of the Stress Cone required to support the

thermal contractions and expansions of the XLPE Insulated Core, as well as parameters

which include Mechanical Properties, Electrical Properties, Thermal and Thermo

Mechanical Properties, Physical and Chemical compatibility of the material with XLPE, long

term ageing and tension set properties, a specially formulated silicone elastomer with a

Shore A Hardness of 39-40 was developed.

This choice is also ideal when considerations of the normal screen removal techniques

employed by cable jointers, are taken into account.

Possible problems when the Semi Conducting Screen is removed:

1) The semi conducting insulation screen is usually removed by scraping it off with a piece of

glass .

2) The Semi Conducting Screen can also be removed using a Screen Removal Tool.

Several Tools are offered for removal of bonded semi conducting screens from the XLPE

Insulation and the Figure below shows some of them

The thickness of the terminated Semi Conducting Screen provides a “step” which can result

in voids being entrapped between the edge of the Semi Conductive profile of the Stress Cone

and the terminated screen. To eliminate the possible discharge occurrence in the voids

entrapped by the “step”, the Semi Conducting Screen is normally extended onto the

insulation with the help of a Conductive Paint as shown below in Fig 7

FIG 7

The exposed insulation surface is then smoothed and polished by hand using successive

higher grades of aluminium-oxide abrasive cloth. The surface roughness achieved with

different grades of aluminium-oxide cloths is shown in FIG 8 below:

FIG 8 SHOWING THE SURFACE ROUGHNESS ACHIEVED ON THE SURFACE OF XLPE INSULATED

CORE WITH DIFFERENT GRADES OF ALUNIMIUM OXIDE CLOTH

All traces of indentation and scratches along this interface must be removed since they will

provide for a weak interfacial strength between the XLPE Insulation and the Cable Joint or

Termination component. Gas filled voids will be formed between the prepared cable surface

and the accessory insulations. These gas filled voids will electrically discharge, rapidly

damaging the polymeric insulation and leading to failure of the accessory. The “peaks” and

“valleys” between the rough XLPE Cable surface and the Cable Joint or Termination Surface

is shown below:

The Interface Dielectric Withstand Capability depends upon the surface roughness of the

materials, the pressure exerted by the Elastomeric Component on the XLPE Cable Insulation

at the interface and also the Shore Hardness of the Elastomeric Component. Fig 9 illustrates

the variation of the Dielectric Withstand Capability and the Interface pressure and

Smoothness of the materials.

FIG 9 SHOWING THE RELATION BETWEEN THE SURFACE SMOOTHNESS, INTERFACIAL

PRESSURE AND ALSO THE HARDNESS OF THE MATERIAL WHICH WILL AFFECT THE

ELECTRICAL STRENGTH OF THE INTERFACE

The effect of the screen cut preparation on the possibility of air entrapment in this crucial

region is shown in FIG 10. The importance of the correct hardness of the insulation material

for providing a discharge free performance at this crucial high stress region is explained.

FIG 10 SHOWING THE IMPORTANCE OF THE HARDNESS OF THE INSULATION IN

SIMPLIFYING THE ACHIEVEMENT OF A DISCHARGE FREE SCREEN CUTBACK REGION

For the above reasons, the choice of material for the Stress Cone for the Cable Termination

and the Insulation System for the Joint has been selected to be a Special Silicone Elastomer

with a Shore A Hardness of 30-40. This allows the Stress Cone Region to correctly follow the

smooth profile of the terminated screen and also present a void free interface between

itselfand the XLPE Core.

The interface design considerations resulting in the choice of the Special Silicone Elastomer

being narrowed down to Shore A 30-40 Hardness are given below:

FIG 11 SHOWING THE BENEFITS OF THE SILICONE RUBBER STRESS CONE WITH HARDNESS

OF SHORE A 30-40

FIG 12 EASY HAND INSERTION OF THE SILICONE RUBBER STRESS CONE WITH A HARDNESS

OF SHORE A 30-40

FIG 13

Fig 13 shows the distribution of the equipotential lines over the interface of the conical

portion of the insulating elastomeric stress cone and air. For Cables upto 110 KV, the field

over this insulation/air interface is sufficiently low to prevent any breakdown of air over

the interface, but for HV/EHV Cables the stress is adequate enough to cause discharge

activity and therefore HV/EHV Terminations need to have either an oil/ elastomeric stress

cone insulation interface or a very tight interface between the solid(usually epoxy cast) and

elastomeric stress cone insulation .

Therefore, HV/EHV Terminations can be categorized as Oil Filled Terminations or Dry Type

Terminations depending upon the type of interface the elastomeric stress cone’s outer

surface has with either oil or an epoxy resin casting. Examples of Oil Filled Terminations and

Dry Terminations for both Indoor and Outdoor Applications are given in Figs 14 and 15.

FIG 14

FIG 15

For the Oil Filled Cable Terminations, a housing containing the oil is necessary and the outer

surface of the housing will then interface with air.Therefore, an outdoor application needs a

non tracking, erosion, weather resistant and hydrophobic insulation with the required

external creepage length

This is provided for by means of a Fibre Glass Epoxy Composite Housing having an

externally moulding of a Shedded, High Performance Silicone Rubber. A detailed view of the

construction of the Outdoor Termination is given in Fig 16

Fig 16

The benefits of the Silicone Rubber Shedded Moulding over the Composite Housing are

given below

The oil used as the filling medium is usually silicone oil because of it consistency and ability

to maintain its viscosity over a range of temperatures to which the Cable will be subjected to.

The termination is fully sealed against ingress of moisture using oil resistant mastic with oil

resistant heat shrinkable tubing. The top bolts and connectors are provided with shear- off

head bolts eliminating the need for crimping tools. A simple spanner is adequate enough for

providing a high quality contact. The Cable Sheath and Shield are also connected with Hose

Clips and Roll Springs eliminating the use of heat and possibility of damaging the cable

insulation.

However for Indoor Terminations used for Equipment Connections like Transformers or GIS

Switchgear, a simple epoxy casting is used for containing the oil since the epoxy casting will

only interface either with Transformer Oil (in case of Transformer Terminations) or SF-6

Gas in case of GIS Terminations. A typical Transformer Termination is shown in Fig 18

below. The Corona Shield is only used when the Cable Terminated in Transformer Oil. It is

not necessary for GIS Terminations as shown below in Fig 17

Fig 17

Fig 18

As compared to the Oil Filled Termination the Compact or Dry Type Terminations have the

Elastomeric Stress Cone tightly interfacing directly with the inner surface of the epoxy

housing as shown in Fig 19 below

Fig 19

This type of an Equipment Termination is also called as an Inner Cone Termination as

opposed to the Outer Cone Termination shown below in Fig 20

Fig 20

The differences between the Inner Cone Type Termination and the Outer Cone Termination

is easily seen in the Fig 21 shown below

Fig 21

A detailed view of the Dry or Compact Termination showing the design features is given in

Fig 22

Fig 22

The “Dry Termination” comprises of two main components ie a) The Epoxy Cast Body and

the Contact Socket. A Field Control Device is integrated into the Epoxy Casting in the area of

the Contact Socket.

b) The Elastomeric Stress Cone with the Mechanical Pin with multiple sliding contacts

installed on the conductor at one end and the Spring Loaded Compression Device with Fixing

Rings and the Solderless Shield Connection along with the metallic housing protected with a

heat shrinkable tubing.

The Elastomeric Stress Cone is correctly placed over the the Cable Insulation and then

inserted into an Epoxy Cast Resin Insulator.

The Elastomeric Stress Cone is mechanically pre-loaded by means of a Metal Spring Loaded

Compression Device to ensure a good interfacial contact between the stress cone and both,

the XLPE Cable Insulation and the Epoxy Resin Casting. The Metal Spring Loaded Device

delivers a very homogenous pressure distribution on the Stress Cone at both electrical

interfaces almost independent from the thermal expansion of the cable or stress cone itself .

The current transfer between the cable conductor and the Connector inside the Epoxy Resin

Casting is achieved through a Cable Connector having Shear Off Head Bolts which is easily

and effectively connected to the cable conductor. The current transfer of the Cable

Connector is delivered by means of a spring contact system which provides a multiple ,

sliding, mechanical contact.

The “Dry” or “Compact” Outdoor Termination has a similar construction and offers the

benefits of fluid free installation allowing a greater angle of inclination of the termination

and freedom from worry of oil leakage.

The construction of the outdoor termination is explained in the Fig 23 shown below

Fig 23 showing the Installation of the “Dry” or “Compact” Outdoor Termination

HV JOINTS

STRESS CONTROL IN HV JOINTS: The approach used for the Stress Control in Joints uses a

semi conductive polymer moulded from a low resistance elastomeric compound, commonly

referred to as a Faraday Cage incorporated in a Composite Elastomeric Insulatinng Housing

having an outer conductive layer and also incorporating two Stress Cones for the Semi

Conducting Screen Cutbacks.

The Stress Cones, Insulation and the Outer Conductive Layer could be provided in a Single

Moulding as shown in Fig 24 or through a 3 Piece Design having two stress cone adapters

and the main body incorporating the Faraday Cage as shown in Fig 25

FIG 24 Single Piece Joint Design

Fig 25 showing a 3 Piece Design which has more interfaces but allows a simpler construction

and cable size transitions. This is possible because the outer diameter of the adapters

remains the same irrespective of the size of the cable.

The stress distribution requirement at the screen termination of both cables to be jointed

together remains the same as for terminations and the stress distribution is controlled by

the stress cones. The only difference in case of joints is that all the stresses are then

confined to the insulation section of the joint body, through the presence of outer conductive

layer of the joint body. This is explained in Fig 26 which shows the stress distribution within

the joint. The high electrical stresses around the connector, around the conductor between

the connector and the end of the insulation and over the cut portion of the insulation

adjacent to the bare conductor are controlled by a semi conductive polymeric layer moulded

inside the joint body ie a Farady Cage.

FIG 26

Three additional critical stresses (besides the stress at the screen cut back) are identifiable

in a joint and these include:

a) The maximum radial stress over the Faraday Cage which acts like the connector (ferrule)

shield

b) The maximum stress off the Faraday Cage tip ie at the tip end of the Farady Cage

c) The maximum stress along the interface ie between the cable dielectric and the joint

insulation.

Besides excellent interfacial pressure, a good electrical interfacial strength for a given

Electric Field Distribution, also depends upon the surface smoothness of the cable

insulation, properties of the silicone grease, temperature and temperature variations

(differential thermal expansion/contraction of the Cable Insulation and the

Termination/Joint Body. The silicone grease used for facilitating the pushing into position

of the Joint also serves to improve the interfacial electric strength. However the silicone

grease over a period of time will diffuse into the Cable insulation and the elastomer body

and then the dependence of a good interfacial strength will entirely be on the contact

pressure exerted by the elastomeric body, its hardness and the smoothness of the prepared

XLPE cable core Fig 27

Fig 27

The length of the Faraday Cage is sufficiently long enough to take into account any XLPE

Insulation “Shrink Back” as a result of the strains in the XLPE Insulation during Extrusion

and subsequent Cross Linking.

The Shield Connection either of the straight through type, ground type or shield break

connection are easily achieved as explained in the Figure 28 below

Fig 28 showing various options for Shield Connections

Several options also exist for Mechanical Protection arrangements and these are shown in

Fig 29

CONCLUSION:The correct material properties and design features play an extremely

important role in the correct installation and long term performance of HV/EHV Cable

Accessories. As explained, the tightness of the interfaces between the components and the

cable determine the partial discharge free performance of the Cable Accessories. It is

preferable for the Cable Accessory manufacturer to be able to supply all necessary

components as well as the tools as well as the shield continuity and cross bonding

arrangements and help the user with suggesting the bonding methods to be employed,

supply the Link Boxes, SVLs etc and also train customer jointers. Complete Cable Systems

comprising the Cable and Accessories have to be qualified according to Type Tests in

accordance with IEC 62067 (2001) including Amendment 1 (2006) which includes

Identification and Description of the Test Object, Electrical Type Tests and Non Electrical

Type Tests. The user is assured of the reliable performance of the Accessories which he is

using, through the successful passing of the complete test sequence.