DESIGN & PERFORMANCE REQUIREMENTS FOR TRANSITION JOINTS BETWEEN MV, XLPE AND MIND/MI, PILC CABLES

BY ALI HIRJI,

CONSULTANT,

RAYCHEM RPG LTD.

1.0 SUMMARY While Cable Manufacturers have in the recent past, phased out the manufacture of

MV, MIND/MI PILC Cables in view of the switchover by the Utlities to the use of MV

XLPE Cables, the need for Transition Joints suitable for jointing the already installed

PILC Cables and new XLPE Cables still exists, in view of repairs and extensions of

the existing network.

A MV (Medium Voltage) Transition Joint between XLPE and MIND/MI, PILC Cables

must essentially:

a) Electrically connect the conductors of the two cables while ensuring the creation

of a block within the connector to prevent the paper cable impregnation oil from

flowing through the strands of the PILC Cable Conductors into the strands of the

XLPE Cable Conductors under the forces caused by thermal expansion during

normal, overload and fault current carrying conditions.

b) Completely isolate the XLPE Cable components including the XLPE Insulation

and Semiconducting Screen from the paper cable impregnant. It is important to note

that the Semiconducting Screen of the XLPE Insulation can undergo a dramatic

increase in resistivity when in contact with the PILC Cable Impregnant.

The increased resistance of the Conductor Screen and the Insulation Screen will

impair their Stress Control Functions and this can result in failure of the XLPE Cable.

c) Control Electrical Stresses at 1) In the region of the terminated XLPE Insulation

Screen, which is maintained at ground potential, within the joint 2) Over the

Connector and 3) In the interface between the Cable Insulation and the Rebuilt

Insulation, from the end of the Connector to the end of the terminated XLPE

Insulation Screen.

d) Adequately rebuild the Insulation over the Connector and also screen the rebuilt

insulation without creating an excessive thermal barrier.

e) Provide an adequate and reliable connection between the Lead Sheath and the

Armour of the MI/MIND PILC Cable and the Shields and Armour of the XLPE Cable.

f) Reinstate the mechanical protection of the cables over the joint and also the

sheathing to provide an environmental seal.

Three types of transition joint designs are now in vogue and these can be

categorized as:

a) Tape and Resin Joints in which the control of the electrical stresses, electrical

insulation and screening are done by self amalgamating EPR/Butyl Tapes and

special barrier tapes like silicone rubber tape are used to prevent contact between

the XLPE Cable components and the PILC Cable Impregnating Compound. Resin

filling is done to provide the mechanical protection and the environmental sealing for

the Joint.

b) Heat Shrink Joints in which the compressive forces of the shrinkable oil barrier,

insulation tubings, conductive tubings, stress control tubing, dual walled (insulation

cum conducting) tubings, shrinkable conductive cable breakouts and special oil

resistant mastics provide the oil blocking, stress control, insulation and screening

functions and an armour case in combination with thick walled adhesive coated

shrinkable tubings provide the mechanical protection and environmental sealing.

c) Cold Shrink Joints in which the oil barrier function, stress control, insulation,

screening functions are provided by cold shrink/cold applied, preformed components

and mechanical protection and environmental sealing are provided either by a

combination of cold shrink/cold applied products either in the form of pre engineered

components or resin encapsulation.

This paper covers the design and installation of a recently developed heat shrinkable

transition cable joint and compares this with the other types of joints and also the

Test Performance of this joint.

However before these are discussed, a short review of the components and design

features of typical MV, PILC and XLPE Cables used by the Industry would help to

understand the significance of the materials, components and design features of the

Transition Joints.

2.0 DESIGN AND CONSTRUCTION OF TYPICAL MV, PILC AND MV,

XLPE CABLES USED BY UTILITIES

The construction of typical MIND/MI,” Belted Type” Construction of 11 KV, PILC

Cables and MIND/MI,” Screened Type” Construction of 22/33 KV, PILC Cables used

by Utilities in India are illustrated below:

The stranded conductors are sector shaped and the conductors are “non

compacted”.In the mass-impregnated construction, the pre-spiralled conductor is

passed through a paper-lapping machine where the paper insulation in the form of

tapes is applied layer by layer in continuous helixes to the required thickness, which

in the case of the “belted construction”, is adequate for half the line voltage. Then,

extra insulation is applied as a circumferential belt over the three cores to provide

sufficient insulation to withstand the phase voltage between each conductor and the

sheath. Before the lead sheathing is extruded, the applied paper insulation is

thoroughly dried under vacuum and then impregnated with a suitable oil or

compound to make the cable to be of either the “MI” or the “MIND” Type.

In a 3-Core Belted Cable, the electrical stresses within the paper insulation have a

radial component and a tangential component. Paper insulation is relatively weak

under tangential stress compared to purely radial stress and the interstices between

the sector shaped cores are filled up with relatively electrically weak impregnated jute

fillers, but for voltages upto 11KV, the tangential stresses are low enough to not

cause discharges within the paper insulation or the weak fillers used in the interstices

between the cores to round up the construction of the cable.

The laid up cores are protected with a seamless extrusion of lead sheathing,

hessian/bitumen bedding, steel wire or double steel tape armour and either hessian

or a Polymeric Outer Sheath.

For voltages above 11 KV and upto 33 KV, a conducting screen which is maintained

at ground potential is used around each core which is oval shaped as compared to

the sector shaped employed in construction for cables upto 11 KV and has an

insulation thickness adequate for the phase to earth voltage. This screen is in the

form of a perforated metallized (aluminium) paper and is applied directly over the

core insulation. The three screened cores are laid up with paper/jute fillers in the

usual way and wrapped overall with a copper woven fabric tape to ensure electrical

contact between the metallic screens and the lead sheath of the finished cable. The

provision of the screen makes the electrical stresses essentially radial and the

comparatively weak fillers used in the interstices of the cores are not electrically

stressed.

Single and Three Core, MV, XLPE Cables are shown in the figure above. The

stranded conductors of MV, XLPE Cables, unlike the conductors of PILC Cables, are

round and compacted. A semi conducting compound for the conductor screen with a

high quality, super clean XLPE insulating compound in the required thickness and a

semi conducting compound for the insulation screen are triple extruded over the

stranded conductor and cross linked . A copper tape or copper wire shield with either

a number/colour tape and water blocking tapes (if required) is applied over each

core. An inner sheath of PVC (either taped or extruded) along with polypropylene

fillers for three core cables, galvanized steel wire/strip armour ( aluminium wires in

case of single core cables) for mechanical protection and earth fault current

capability and an extruded PE/PVC Outer Sheath is used for providing environmental

protection.

3.0 DESIGN REQUIREMENTS FOR TRANSITION JOINTS

A Transition Joint must be capable of meeting the performance requirements of both

the PILC and XLPE Cables. In order to do so, it must address many of the design

criteria inherent in straight through joints for either cable construction. Additionally,

the design must ensure that any interaction between the jointing materials and cable

materials are not detrimental to the performance of the joint.

3.1 CONDUCTOR JOINTING

The issues regarding the jointing of the conductors of a PILC Cable and the XLPE

Cable can be listed as under:

a) Usually, transition joints are used to connect different conductor sizes of the XLPE

Cables and the old PILC Cables the connector has to be designed to accommodate

different sizes of the non compacted PILC Cable conductor and the compacted XLPE

Cable conductor.A reliable joint has to be established between two different

conductor sizes.

b) Some Utilities still have Copper Conductored PILC Cables existing in their system

and these have to be jointed to the newer Aluminium Conductored XLPE Cables and

the connectors used in this case must be Bimetallic to be effective for jointing a

Copper Conductor to an Aluminium Conductor.

c) The conductors of the PILC Cables are non compacted and Sector/Oval Shaped

whereas the conductors of XLPE Cables are compacted and round.

d) The loss of some of the impregnating compound from the paper cable and thermal

ageing (embrittlement) of the paper insulation necessitates the previously laid PILC

Cable Cores to be very carefully bent to a radius of bend sufficiently large enough to

prevent the tearing of paper, especially in the crutch of the cable, while separating

the cores for conductor jointing.

e) The connector used for jointing must be solidly blocked to prevent the oil

impregnant from the PILC Conductors being pumped into the XLPE Conductor

during load cycling and particularly during short circuits.

The development of MV Shear Bolt Connectors is a major step towards resolving all

the issues mentioned above. A MV Shear Bolt Connector is a mechanical splice

connector, made of a corrosion resistant, tin plated, high strength and high

conductivity aluminium alloy with bolts having heads that shear off when a proper

tightening torque is applied. The MV Shear Bolt Connector simplifies and quickens

installation making the installation craft insensitive and extremely reliable.Typical

components of a MV Shear Bolt Connector and its installation are shown in the

Pictures below:

Since the Connector body is made from a high strength, high conductivity aluminium

alloy and is tin plated, the connector can be used for jointing either copper or

aluiminium conductored cables as well as jointing a Copper Conductor to an

Aluminium Conductor. Each size of a MV Shear Bolt Connector can accommodate a

very wide range of conductor sizes and can be provided with a built in solid barrier to

prevent compound migration, as shown in the picture below:

To meet with the requirements of minimum bending of the cores of a PILC Cable

during the conductor jointing, a Split and BlockedType Shear Bolt Connector has

been developed which makes it possible to have minimal bending of the cores

during conductor jointing and therefore eliminates stresses on the paper insulated

cores, eliminating the risk of straining and tearing of the Paper Insulation. A Split and

Blocked Mechanical Connector is illustrated in the picture shown below. The two

“halves” of the connector can be easily slipped over the Paper Insulated Conductor

ends and also the XLPE Conductor ends with a minimum separation requirement

and therefore bending requirement of the cores. The old paper insulation is therefore

prevented from being strained to a point where it could crack/break. The two “halves”

are easily, quickly and reliably connected by means of a bolt equipped with a head

capable of shearing off when the required tightening torque is applied

4.0 HEAT SHRINKABLE TRANSITION CABLE JOINT DESIGN

The basic design of a heat Shrinkable Joint design philosophy involves converting

the PILC Cable into an equivalent “Screened Polymeric Cable”. This design

philosophy is applied to the “Belted” PILC Construction and also the “Screened” PILC

Cable Construction.

4.1 TERMINATION OF THE EARTH ENVELOPE

After removal of the lead sheath, the three cores are separated and three numbers

Oil Barrier Heat Shrinkable Insulating Tubings are shrunk over the cores

simultaneously. The covered cores are then individually screened by covering the

shrunk oil barrier tubes with Heat Shrinkable Conductive Tubes. A High Permittivity

Oil Block Mastic is applied in the crutch between the cores and then a Heat

Shrinkable Conductive Cable Breakout is shrunk into place. The Heat Shrinkable

Conductive Cable Breakout seals onto the lead sheath and also onto the Conductive

Tubings applied to the cores, maintaining an electrical connection from the lead

sheath to the Conductive Tubings. This is illustrated in the Pictures given below:

The “Belted Cable” has effectively been converted to a screened construction. This

design provides electrical stress control by a smooth continuation of the earth

envelope as well as by the use of a high permittivity oil blocking mastic.

In the Screened, “H Type” Construction of 22 KV and 33 KV PILC Cable Joints,

where the perforated aluminium foil screen is removed, a combination of the

electrical characteristics of the Yellow Oil Blocking High Permittivity mastic and a

Heat Shrinkable Conductive Tubing (CNTM) shrunk on top of it provides an improved

performance through reducing the longitudinal stress on the paper insulation at the

aluminium foil screen cutback. The displacement of the electrical field starts from the

edge of the conductive tubing and in the solid dielectric polymeric materials which

therefore reduces the stresses on top of the paper layers. This is illustrated in the

picture given below:

4.2 SCREEN CUTBACK STRESS CONTROL

Some stress control must be applied at the termination of the core screen where

high longitudinal stresses normally develop. The old Tape Type and Cold Applied

Transition Joints utilize a conical build-up in the insulation (Stress Cone) with a

ground plane either hand applied over or bonded to the conical build up of insulation

and in contact with the terminated screens of the cable.

The heat shrinkable system utilizes a matched impedance stress control tubing which

is applied from the XLPE Screen Cutback across the joint, overlapping the

conductive tubing on the PILC side. Use is also made of the Yellow High Permittivity

Mastic to provide a void filling function at the screen cutback of the XLPE Cable to

prevent any void formation as the Stress Control Tubing is shrunk over the step.

Without Stress Control, the electrostatic lines of flux in the cable dielectric will attempt

to flow to the nearest earth point, typically the screen termination causing high

electric stress. The electrical properties of the stress control tubing divert the electric

flux lines through capacitive coupling with the conductor through the insulation.

Electrical stresses are thus minimized and controlled.

4.3 STRESS OVER CONDUCTOR CONNECTION

Most Cold Applied Jointing Systems utilize a conductive envelope or Faraday Cage

over the connector area in order to eliminate stress over sharp points on the

connector.

While the system described above reduces the stress within the connector area, the

termination at the conductive envelop itself can lead to additional stress.

In the heat shrinkable system, the process of rebuilding the insulation over the jointed

conductors commences with the application of the yellow high permittivity oil block

mastic to encapsulate the connector area . Electrically this compound, because of its

high permittivity, aids in the control of the electric field in the connector region. It also

seals in any water trapped in the XLPE Cable strands and prevents it from entering

the joint interface that could result in breakdown and failure. Water in the strands of

aluminium conductored XLPE cable has also been identified as one possible failure

mode due to gas formation and pressure generation. The Oil Block Mastic is retained

by the compressive forces of the heat shrinkable stress control tubing and the

Insulating cum screening tubing over it.

4.4 STRESS IN THE INTERFACE BETWEEN THE CABLE

INSULATION AND THE REBUILT INSULATION

The Stress Control Tubing has intrinsic electrical properties to create a capacitively

coupled circuit with both the conductor and insulation shields. This controls the the

electrical field distribution so that the stress is kept below the maximum allowable

longitudinal design level. The stress control tubing also has the advantage over the

stress cone which requires a joint diameter build up. The field distribution in the joint

with the combination of the oil blocking high permittivity mastic and stress control

tubing is shown in the picture given below:

A newly developed triple extruded heat shrinkable tubing which enables a screened

insulation to be provided in just one step results in a slimmer joint profile while

enabling optimum heat transfer from the connector due to reduced thermal capacity.

Another feature of this recently developed tubing is a combined reduction in shrink

time with advanced shrink behavior and profile – following. A high recovery force of

this tubing enables superior electrical interfaces and better sealing properties of the

joint.

This triple layer joint sleeve combines: 1) A heat shrinkable outer conductive layer 2)

A heat shrinkable insulation layer and 3) An Elastomeric Insulation layer as shown in

the picture below:

INSTALLATION OF THE TRIPLE EXTRUDED JOINT SLEEVE

ON THE XLPE CABLE SIDE AS SHOWN ABOVE AND ON THE

PAPER CABLE SIDE AS SHOWN BELOW

5.0 OIL BARRIER TUBING PROOF TESTING

Recognising that the transition joint performance would be only as good as the oil

barrier system, testing of its oil migration resistance was conducted. The major oil

barrier component is the oil barrier tubing. The oil resistance properties of the oil

barrier tubing were compared to those of silicone rubber, which is used as an oil

barrier in the tape joint construction and also in some other cold applied joint

constructions, to determine their relative performances.

As shown the in the figure below, both silicone rubber and oil barrier heat shrinkable

tubings were applied over a perforated mandrel. A conductive polymeric material was

installed over each oil barrier. Cable oil at a temperature of 70 degrees Centigtrade

was circulated through the mandrel.

The Graph on the right hand side shows the change of resistivity of the conductive

material versus time. An increase in resistivity over time indicates a change in the

insulation shield properties due to oil absorption. The Graph confirms the superior oil

blocking properties of the oil barrier tubing as compared to the silicone rubber since

the change in the resistivity of the conductive material is negligible.

Further testing was performed using similar sample construction with the mandrel

capped and filled with oil. Samples were placed in an oven and maintained at 70

degrees Centigrade. In this test, a control sample consisting of a heat-shrinkable

XLPE Tube was utilized to demonstrate the relative performance of the heat

shrinkable oil barrier tubing. During the course of the one year testing, the conductive

material covering the XLPE Control barrier showed an increase of three orders of

magnitude, while the conductive material covering the oil barrier tubing showed a

very small increase in resistance.

Additional accelerated testing was conducted to establish the suitability of the oil

barrier material. Test samples were constructed using a thin film of oil barrier material

bonded to a layer of conductive polymer as shown below.

Samples were placed, conductive side down over a foil plate with a depressed centre

and filled with mineral oil. Samples were placed in an oven and held at 70 degrees

Centigrade for almost one year. Change in resistance of the conductive polymer, was

negligible.

Selection of a material for use as an oil block mastic followed a similar pattern as the

oil barrier tubing. A stress relieving oil block mastic has been successfully used in

Europe for over twenty years on PILC Cable accessories that have a 70 degrees

Centigrade operating temperature. This experience was drawn upon to formulate a

higher temperature oil block stress relieving mastic employing materials known and

used for high temperature and pressure retention.

Due to the complexity of the oil block system, it was determined that only a functional

test would properly show design viability after screening tests were conducted.

Load cycling testing was conducted with internally applied oil pressure to verify the

suitability of the mastic seal in a completed joint. After six months of continuously

applied oil pressure and load cycling, there was no loss of seal at the lead sheath or

over the connector on test samples.

6.0 LEAD SHEATH, ARMOUR CONTINUITY AND OUTER SHEATH

REINSTATEMENT OF THE TRANSITION CABLE JOINT

The heat shrinkable system provides for continuity of the lead sheathing and

armouring system and fault current carrying capability by utilizing a wrap around

galvanized steel shell and tinned copper braids to reinforce the current carrying

capability to meet with the system requirements. The wrap around galvanized steel

shell by itself has been tested upto 16 KA RMS Short Circuit Current for 1 second,

either plumbed or attached with mechanical clamps.

Additional copper braids connected to the Lead Sheath with “Roll Springs” provide

higher fault current carrying capability to meet the system requirements.

A thick walled, adhesive coated, heat shrinkable tubular or reinforced wrap around

sleeve provides excellent protection against moisture ingress.

The pictures shown

illustrate the

methods used for

mechanical

protection and

earth continuity

connections for the

transition joints.

The Roll Spring

used for connecting

the copper braids

to the lead sheath

is shown.

7.0 TAPED TYPE STRAIGHT THROUGH JOINTS USING SILICONE

RUBBER TAPE OVER THE PAPER INSULATION

The construction of a typical Tape Type Transition Joint between MV, PILC Cables

and XLPE Cables is shown below.

The complex, skilled, labour intensive, worksmanship involved in the installation of a

Taped Type Straight Transition Joints is amply illustrated.The large number of types

of tapes used, different dimensional requirements to be met with and the very difficult

installation procedure has resulted in the abandonment of the use of Tape Type

Transition Joint in preference to the Heat Shrink Joint.

8.0 COLD APPLIED JOINT

Several variants in design and installation practices are used.

In one design, elastomeric tubes which have a semi conductive coating applied over

part of the length of the tubing are used to provide the oil sealing and screening

function as shown below. The oil barrier function of the elastomeric layer is not as

good as the oil barrier provided by the oil barrier heat shrinkable tubing.

An electrical connection between the conductive layer and the lead sheath requires

an elastomeric, conductive, cold applied, shrinkable cable break out with an oil

blocking system in the crutch of the cable and is not as effective as that provided by

the combination of the oil blocking mastic and heat shrinkable conductive cable break

out offered by the heat shrinkable system. Also, the oil barrier used at the end of the

connector on the paper cable side as well as the XLPE Cable side is absent. The

Joint is completed by shrinking of a pre expanded Insulating Elastomeric Body with

built in Stress Cones, Faraday Cage and a Conductive Outer Layer.

Mechanical protection and Sealing is provided by means of either Resin or

Elastomeric Tubes, the latter not having the ruggedness or abrasion resistance of the

Heat Shrinkable Tubings with an inner coating of a hot melt adhesive and a wrap

around mechanical protection. It is significant to note that Straight Through Joints on

HV/EHV Cables made with Cold Applied Components are protected with Heat

Shrinkable Outer Tubings.

9.0 TESTING OF HEAT SHRINK TRANSITION JOINT BETWEEN MV,

MIND/MI, PILC CABLES AND XLPE CABLES

A Test Sequence used to evaluate the performance of Heat Shrinkable Transition

Joints is shown in the Table below:

CONCLUSIONS:

While several different types and variants of designs of Transition Joint Designs for

Jointing MV, MI.MIND, PILC and XLPE Cables are available the Heat Shrinkable

Transition Cable Joint Design based on Split, Blocked, Mechanical Connector, Heat

Shrinkable Oil Barrier Tubings, Oil Blocking ,High Permittivity Mastics, Heat

Shrinkable Conductive Tubings and Cable Break Outs, Triple Extruded Heat

Shrinkable Insulating/Conductive Tubings, Wrap Around Mechanical Protection

arrangement and Thick Walled Heat Shrinkable, adhesive coated tubings offers the

highest reliability combined with ease and simplicity of installation.