Some Thoughts On MV HV Heat Shrink Cable Joints & Cable Terminations

Published 30 May 2018

Heat Shrink Cable Joints & Cable Terminations | Manufacturers & Assemblers


uploaded by Chris Dodds - Thorne & Derrick Sales & Marketing Manager

Although modern cable manufacturing technologies evolve and develop the essential factors to consider when installing heat shrink type joints or terminations on medium/high voltage power cable systems remain essentially unchanged. Materials quality allied with installation by competent trained Jointers is the fundamental basis to ensure reliability, safety and operational service of MV HV cables, joints and terminations.

Here, from the archive we reproduce an important Technical Article by Norman Poulter – Norman was the Managing Director of Shrink Polymer Systems, the UK’s leading specialist manufacturer and assembler of MV HV cable joints and terminations for standard and non-standard cable applications up to 33kV.

Today, SPS’s Managing Director Richard Poulter continues to support the UK and overseas market with the competitive supply of MV HV heat shrink cable accessories for all cable voltages including 11kV/33kV and types such as XLPE, EPR, PILC and Triplex cables.

Medium & High Voltage Cable Accessories

author Norman Poulter

Power cable installation throughout Europe and the UK until the early 1960s exclusively used impregnated paper for the primary insulation of the conductors for both single- and three-core MV-HV cables.

The UK adopted both aluminium and lead-sheathed cables with and without steel wire armouring (SWA); these are still in use today. In the very early days of paper-insulated cable terminations, dry-type systems were employed along with compound-filled end box designs and cast-iron boxes with hot pour bitumen for cable joints.

PILC SWA Paper-insulated Lead Covered & Steel Wire Armoured Cable

Circular stranded copper phase conductors
Conductive paper screen
MIND paper insulation
Metallised paper insulation screen
Circularising fillers
Lead sheath
Bitumenised hessian bedding
Galvanised steel wire armour
Bitumenised hessian serving with whitewash coating

11kV PILC Cable

Cold shrink products such as push-on, pre-stretched tube and grease applied slip-on types also became popular in the 1960s and are still specified by specific electric authorities and utilities for terminating and jointing MV-HV cables.

In the early 1970s polymeric (XLPE) cable types began to emerge in Europe and the UK, mainly on three-core cables, while the USA employed the single-core cable concept at 10kV, 20kV and 35kV medium voltages.

The heat shrink concept began to be employed at this period by the utility companies and has now spread internationally as the preferred method of cable terminating and jointing.

There are many advantages of using heat shrink cable accessories and techniques, such as:

Wide shrink ratio – one joint/termination kit assembly to cover numerous cable ranges
Heat concept dries out moisture from the MV HV cables
Mastic seals are activated by heat so sealants are usually visible at sleeve ends
They are not size sensitive and can be used on sector-shaped conductors

Some Thoughts On MV HV Cable Joints & Cable Terminations 1

Figure 1 – Cable Box Air Insulated Termination

Cable Joint & Termination Kit Instructions

Shrink Polymer Systems also realised the importance of good cable jointing and terminating instructions. By using pictorial drawings with a minimum of text, non-English speaking countries can identify the important highlighted areas in an easily identified format.

Cable Terminations 11kV 33kV – Instruction Excerpt

Failures are nearly always attributed to poor cable preparation by the jointer and failure to observe the correct jointing procedures in the areas where the electric cable stresses are prudent, such as the semicon screen cut back.

Although the USA shares a language with the UK, there are many differences in the selection of words to describe various things. Below is a list of some UK terminology and the common equivalent used in the United States.

Core – Conductor
Screen – Shield
Joints Jointing – Splice Splicing
Cable Jointers – Cable Splicers
Earth Earthing – Ground Grounding
Armour Support – Reinforcing
Metal Sheaths – Armour
Self-Amalgamating – Self-Fusing

Heat Shrink Cable Joints

& Cable Terminations

Terminations

Terminations | Indoor or Outdoor

The major MV HV switchgear and transformer manufacturers have, for many years, designed dry-air filled cable boxes, resulting in a much greater demand for heat shrink-on terminations.

All heat shrink cable terminations and joints have to be tested to various international standards, and while cables prepared by experts in perfect laboratory conditions will undoubtedly meet these requirements under test, field experience shows that failures still occur at working voltage due to a variety of reasons.

Typical List Of Weakness Resulting In Failures

Of Heat Shrink Terminations

Compression lugs, or crimp lugs, fitted to outdoor terminations of the compression tube type with inspection holes allowing moisture to penetrate the conductor cores.
Failure to eliminate air pockets on paper-insulated, lead covered (PILC) three-core “belted” cables in the crotch area.
Core crossing resulting in discharge if cores are too close to each other in an unscreened area at the core cross point. This results in the air “breaking down” electrically at approximately 4kV on an 11kV cable, 6kV on a 24kV cable and 9kV on 36kV cable. The anti-track heat shrink material then begins to erode due to the ionisation of the air, which over time will inevitably cause failure of the cable termination.
Poor cable preparation, in particular on extruded dielectric types where insufficient care is taken on semiconductive screen removal at the crucial area of the screen cut-off. Cable jointers are generally reluctant to purchase engineered screen removal tools and rely on knives, blades and homemade tools for removal. This can result in cutting through the screen and into the primary insulation, leaving voids which result in the discharge phenomena described in the third item above. Even well-prepared screen removal at the cut-off point can result in a possible void, as the stress control tube may not follow the semi-conductive edge profile. (See figure 2.)
Moisture penetration due to poor heat shrink and mastic sealing techniques.
Inadequate phase-to-phase and phase-to-earth clearance.
Tracking.
Poor cable jointing instructions.

Figure 2

There are solutions and remedies to these weaknesses relating to MV HV cable accessories which will be described later in the article.

Joints

Many of the points discussed regarding cable terminations are also relevant to joints.

The object of the cable jointing exercise is to replace all the materials that were removed to joint the conductors and replace them in such a way as to replicate the cable as closely as possible to its original state.

There are many techniques used in conductor jointing from “sweating” the weak-back ferrule (normally associated with the original cast iron paper joints) to compression and the present popular “shear bolt” designs.

Connector jointing is a complex subject due to the variety of designs for copper and aluminium conductors in circular-solid, circular-stranded, and sector-shaped styles.

There is also the consideration of copper being jointed to aluminium, cables of unequal cross sectional area being jointed to themselves, and transition jointing where paper is being jointed to polymeric at medium or high voltages.

Figure 3 An 11kV 3 Core Heat Shrink Cable Joint

Let’s look at some of these designs.

Compression Connections – Crimps

There are a substantial number of manufacturers, such as Cembre, who specify suitability of their electrical connector design for voltages up to 33kV/36kV. Caution is needed if the body of the barrel is not smooth and does not have tapered ends.

Also, responsible MV HV connector manufacturers will be able to manufacture appropriate crimping tooling for their designs or confirm the compatibility of other cable tooling and die combination for their connectors. There are three types of crimping configurations currently in use: hexagonal, oval, and indent.

For instance, the Cembre HT131-C is the industry standard hydraulic crimping tool used by LV HV cable jointers to install copper type lugs and splices onto stranded cable conductors.

Oval and hexagonal crimping can leave sharp “ears” if incompatible tooling is used.

These “ears” must be filed smooth to avoid a highly stressed area which will be subjected to electrical discharge. Indent crimping will leave void holes which must be filled with high permittivity, stress relief tape.

Mechanical Connections – Shearbolts

Mechanical split type connectors are now very popular in the UK, as this design comes in two halves which are easy to apply to three-core cables at 11kV or 33kV where the cable conductors do not have to be bent. The heads shear-off at a given torque; therefore, no compression tooling or die combinations are necessary.

As this connector is “blocked,” it is also suitable for transition cable jointing for paper to plastic to stop the migration of the paper oils. There are several disadvantages of this type of connector, however. The ends of connector are non-tapered, resulting in high “step downs”, and the conductor insertion is non-centralised. (See figure 4.)

Figure 4 Mechanical Shearbolt Connector

These two conditions result in areas of high electrical stresses leading to probable discharge.

Shrink Polymer systems have now standardised on a shearbolt connector design where these critical areas of high stresses are removed by tapering the ends and centralising the conductor. (See figure 5.)

Figure 5 Mechanical Shearbolt Connector (Tapered)

Stress-Relieving Tapes

This type of stress relief tape generally has permittivity values between 7 and 13 (test method IEC 250). (See figure 6.) This does not necessarily mean that a value of 13 will perform better than a value of 7, as void filling characteristics are equally as important.

Figure 6 MV HV Connectors & Stree Relief Tape

Shrink Polymer Systems have a yellow stress control tape, reference number TS 31785Y, which possesses high tack, high stretch, and low viscosity void-filling qualities with a permittivity value of minimum 9. Whichever type of cable connector is selected, stress-relieving tape must be used in conjunction with heat shrinkable installations.

This is applied in a half width overlap with stretch by the cable jointer and must also be applied to any indents left by the tooling.

Gaps between end of primary insulation and connector must also be filled in.

Push-on molded components are also widely used, eliminating the need to fill voids and use stress tapes.

They rely on the Faraday Cage principle, in which conductive rubber-ribbed moldings are in contact with the connector. As the potential difference across the air is very low, discharge should not occur. Push- on molded components, joints and terminations have several disadvantages however.

On three-core cables the molded components prove to be very bulky and have no design features to eliminate moisture penetration unless used in conjunction with large diameter shells and resins. On large aluminium conductor sizes the ferrule could possibly be longer than the moulded component. The positioning over the ferrule is critical.

Stress Control Heat Shrink Tubing

Shrink Polymer Systems employ the heat shrinkable, high permittivity, and low resistivity stress control tubing which is shrunk onto the stress relief tape using heat applied by a jointers gas torch previously applied.

This has the effect of achieving a more uniform distribution of the field lines. This heat shrink tube extends over the ferrule or connector and onto the prepared screen cut-off points of the medium/high voltage power cable (See figure 2.)

Insulation Thickness

When designing cable jointing systems, the thickness of the insulation over the bare conductor (i.e. ferrule) should have a safety factor in excess of 15% of the original cable.

Shrink Polymer Systems employ a one-piece, combined dual wall (insulation / semiconductive) heat shrink tube of appropriate diameter to match this insulation at voltages to 11/12kV.

At 17.5kV, 24kV and 36kV additional heat shrink insulation tubes are added to meet these cable specifications.

The cable jointer must remember to shrink this material all around the heat shrink tube to avoid inconsistent wall thicknesses on full recovery.

Typical List of Weaknesses Resulting in Failures

Incorrect crimping of connector.
Air trapped in connector (if indent crimps not filled).
Air trapped between end of insulation and end of connector.
Discharge at screen ends caused by poor stress taping / cuts to primary insulation.
Moisture ingress entering cable sheathing through poor sealing.
Inadequate insulation levels over connector.
Poor cable termination or cable joint kit instructions.

The solutions and remedies to these weaknesses are described below.

Earthing Cable Terminations

Earthing must be provided to carry any circulating currents to core screens, metal sheaths and armour wires. It must also have the ability to carry fault current. On indoor MV HV cable terminations the use of tin plated copper, solder-blocked braids, metal canisters, armour support, clamps and a complete corrosion protection system should be employed.

Figure 7 Low Voltage Heat Shrink Cable Joint For Multicore Cables

Earthing Cable Joints

Connecting the earthing components across a heat shrink cable joint requires correctly choosing and fitting the components to take care of both circulating currents and the short circuit requirements.

The outer semiconducting layer of the core/connector insulation should be wrapped in a tin copper mesh bandage and connected to the cable earth at each end.

There are a great many variations and earthing complexities and such a wide variety of cable types to consider.

Once a cable type, size, and voltage are specified it should be left to the manufacturer to supply the correct type of earthing system to meet both the national and local standards.

Remedies & Solutions To Overcome Cable Termination Failures

Always use one-piece solid lugs for outdoor termination, not squashed tube type.
Wrap butyl self-amalgamating tape around crotch and under lead cut on three-core belted cables to eliminate air. Check clearance dimensions on three-core cables.
Care must be taken in semicon screen removal not to nick the primary insulation at the screen cut-off point.
Ensure all mastic seals are in place on bushing boots, rain sheds and core tubes.

Remedies & Solutions To Overcome Cable Joint Failures

Ensure cable connector is free from “burrs,” sharp points, not squashed tube type.
Fill in all gaps with stress tapes before applying stress control tubing.
Ensure correct application of stress tape at screen cut-off points. (See figure 2.)
Fit all seals as supplied, in particular at crotch area, under armour beddings and end of connector insulations.

Cable Jointing & Terminating Instructions

It is the responsibility of cable accessory manufacturers to supply easy-to-read, simplified, pictorial jointing instructions and to avoid heavy reading of text manuals.

This point cannot be overstated, as, in the writer’s opinion, far too many jointing instructions are not read or understood, resulting in the installer compromising on the areas of importance previously mentioned. This all too often results in that first failure.

Thorne & Derrick International

How Can We Help?

Since 1985 Thorne & Derrick have been servicing the UK and Export market with Joints, Terminations & Connectors for LV MV HV cables and power systems – call us to discuss your requirements including specialist LV cable joints for high performance applications including hazardous areas, marine and offshore cables, fire resistance cables and low smoke zero halogen cables.