MATERIALS, DESIGN AND PERFORMANCE REQUIREMENTS FOR HEAT SHRINKABLE POWER CABLE ACCESSORIES

A L I H I R J I, B. E. (ELECTRICAL)FORMER:1) GENERAL MANAGER (EPD), RAYCHEM-RPG LTD, MUMBAI 2) COUNTRY MANAGER (ENERGY DIVN), TYCO ELECTRONICS, INDIA

Introduction: Installation of Cable Accessories – Joints and Terminations - is a very precise craft in which a considerable degree of skill is required, particularly for MV and HV cables. Cable Accessory manufacturers have been constantly endeavouring to develop material technologies and products to make the installation of Cable Accessories less craft sensitive and shorten the installation time. In general the development efforts by various accessory manufacturers has led to the modern cable accessory being based on factory engineered components which are either heat or cold shrunk over the prepared cables or slipped over or pushed onto the prepared cables. Older designs based on field fabrication techniques involving tapes or casting resins are now being phased out.

The purpose of this paper is to introduce to the reader the various functional requirements of cable terminations and how these are achieved with correctly formulated heat shrinkable polymeric components. Please see Figure 1.

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Figure 1 - Heat Shrinkable Cable Termination Systems

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Figure 1 - Heat Shrinkable Cable Termination Systems

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FUNCTIONS OF CABLE TERMINATION:

The various functions of a cable termination used for connecting a cable to an overhead line or to switchgear or a motor or a transformer are:

a) To prevent air entrapment and eliminate voids in critical areas.b) To control electrical stress and prevent discharge activity.c) To provide environmental protection to the cable insulation under

electrical stress.d) To provide effective and reliable earth connection for the cable screen

shield, metallic sheath and armour.e) To prevent ingress of moisture.f) To provide an effective connection to the terminals of the electrical

equipment and to insulate and seal exposed connections to prevent flashovers.

The most common cause of failure of XLPE cable accessories is the cable insulation breakdown through partial discharge brought about by either improper cable end preparation which results in nicks and scratches on the XLPE insulation or poor installation either or both of which lead to void entrapment.

Improper cable preparation is invariably traced to a careless removal of the semi-conducting insulation screen, which is co-extruded with the XLPE insulation during manufacture. The use of sharp knives used for the removal of the semi-conducting screen usually results in the knife cutting through the thickness of the extruded screen and into the insulation, thereby creating a permanent site for partial discharge activity. The partial discharges gradually breakdown the insulation over a period of days or months or years depending on the extent of the depth of the cut. A variety of tools are available which ensure correct removal of the “bonded” semi-conducting screen from the XLPE insulation Figure 2 (a) shows a typical tool.

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Figure 2(a) – SCREEN REMOVAL TOOL

A correctly prepared cable still presents an area of potential problem at the screen end. This area is a site for void entrapment as the stress control system may not be able to follow the profile of the terminated screen at this point. Figure 2 (b).

Figure 2(b)

With heat shrinkable cable termination systems presently two techniques are available which help to address the potential problem of void entrapment at the terminated screen.

i) Use of conductive paint to extend the screen layer on the XLPE insulation. This does not prevent the void formation as a result of the stress control systems inability to follow the contour of the step provided by the terminated screen, but it prevents stress and discharge activity in the void since the surface of the XLPE insulation is at earth potential at this point see Figure 3.

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Figure 3

ii) Use of a void filling, stress-relieving tape which is carefully wrapped around the area of the screen cut to provide a smooth profile see Figure 4.

Figure 4

STRESS CONTROL SYSTEM: At the end of the semi-conducting insulation screen, a capacitive divider effect results in a intense electrical field in the air causing discharge activity Figure 5. Partial discharges on a 11kV cable can be detected at a voltage of about 4KV and for 33kV cable at approx. 8KV using a discharge detector. These discharges will over a period of time erode the insulation leading to a failure.

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Figure 5 Capacitive Divider Effect Causing Intense Electrical Field

To address the occurrence of the high stress and prevent failure, a heat shrinkable tubing with a correctly defined impedance property is shrunk over the screen end with an overlap on the semi-conducting screen (approx. 50mm) and an overlap over the insulation the extent of which depends on the voltage rating of the cable. The stress distribution without the stress control tubing and with the stress control tubing is shown in Figure 6 and Figure 7 respectively.

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Figure 6

Figure 7

Since the stress distribution over the length of the stress control tubing depends on the impedance provided by it, it is very important that the impedance does not change appreciably with respect to the temperature of the cable (within its permissible range) nor with thermal ageing.

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In case of 3-Core cables it is important to prevent discharge activity between cores as a result of inadequate spacing between the cores as a result of “crossing” of the cores in the confined space of a terminal box. This is necessary for all types of 3 core cable terminations, which are installed, mainly indoors. High electrical stresses in the region where two insulated (unscreened) cores come very close to each other results in breakdown of the air between cores producing ozone and with moisture producing acid causing corrosion of the box. This may also lead to total breakdown of the termination outer sleeve and cable insulation. Please see Fig 8.

Figure 8 Maintaining correct spacing Between cores to prevent discharge activity

It is extremely important that for all types of 3 core cable terminations to do the core crossing, when it is necessary, in the area of the cores where the screens are not removed. In 3 – core cable terminations, cable preparation involving maximum screen lengths to enable cores to cross in the screened region should be preferred. Please see Figure 9.

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FIGURE 9 EXTENDED SCREEN LENGTHS

ENVIRONMENTAL PROTECTION TO THE CABLE INSUALTION UNDER ELECTRICAL STRESS:

A need for providing environmental protection under electrical stress for the cable insulation arises as a result of moisture through condensation in an indoor environment or rain and sea spray in an outdoor environment in combination with pollution causing surface leakage currents to flow and consequent heating resulting in the formation of “dry bands” on the surface of the termination. Once a “dry band” is formed, typically 3-4mm in width, the majority of the system voltage is applied across the dry band due to the voltage divider effect. The result is that the breakdown strength of air is exceeded over the ‘dry band’ and arcs or scintillations jump across the dry band. The higher the surface leakage current, the more intense is the arcing or scintillation and the likelihood of the arcing moving around and elongating to caused flash over of the termination – see Figure 10

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Figure 10

Even if a flash over does not occur, the arcing and scintillation over polymeric insulation can initiate surface degradation as result of carbonization (tracking) or erosion of the material. Once this process starts the outer surface will carbonize (track) over a period of time resulting in a permanent failure or erode through the thickness to cause an insulation failure.

While selecting a cable termination due consideration must be given to the non tracking and erosion resistance of the outer insulation of the cable termination used for protecting the XLPE cable insulation.

There are several established test methods to evaluate the tracking and erosion resistance of polymeric materials and these include the ASTM D2303

“Inclined Plan Liquid Contaminant Tracking and Erosion Resistance Test”. Independent testing for this requirement as well as the experience of the manufacturer must be given due consideration.

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In outdoor installations, the material must possess weathering resistance i.e. the ability to resist degradation as a result of solar radiation, temperature changes, atmospheric precipitation and solid, liquid and gaseous pollutants. Due considerations must be given to the materials weathering resistance as evaluated through accelerated laboratory and field studies as well as actual field performance monitored over several years to correlate both accelerated laboratory tests and actual field performance.

SEALING AGAINST INGRESS OF MOISTURE

It is essential that moisture is not allowed to enter a termination and this requires that any interface should be effectively sealed. Seals are usually provided on heat shrinkable terminations by means of hot melt sealants applied to the inside of the component parts. Failure to seal the termination effectively will allow moisture to enter and once it has entered will result in “water treeing” and or “electrical treeing” failure of the cable insulation.

SHIELD / SHEATH / ARMOUR EARTH CONNECTION:

A reliable and effective earth connection system must:a) Not employ materials, which will cause corrosion when in contact

with each other.b) Employs tinned copper mesh wrapped around the strip or wire armour

to provide a good contact with the earth lead.c) Ensures that moisture cannot enter the system and cause corrosion. A

solder block on a copper braid is essential to prevent this.d) Employs supports under the armour to prevent the clamping pressure

of the clamps, damaging the cable insulation.e) Ensures adequate cross section of the copper braid for safely carrying

the expected short circuit current without excessive over heating.f) Uses of nonferrous support rings and clamps for single core cables.g) Be satisfactorily tested and proven.

Typical earthing system for steel wire armoured, XLPE cables is shown in Figures 11 & 12.

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Figure 11

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Figure 12

CONDUCTION CONNECTION:

The importance of connectors (lugs) and the method of connecting them to the conductor ends cannot be over emphasized. It is extremely important that connectors are sourced from established manufacturers who have close control over the composition of the metal used and the annealing and manufacture the lugs is in accordance with the dimensional requirements of Standard Specifications. It is equally important for the lug manufacturer to state the type of tool(s) to be used and the Die nos. for crimping the connectors as well as the sequence of crimping. There are two basic dies used for crimping namely indent and hexagonal. The former places indents along the barrel of the lug while the latter creates hexagonal pressure around the connector. In this case it is likely that sharp edges are formed and these must be filed down to give a smooth profile. In both methods, cold welding takes place between the lug and the cable conductor to give good, electrical contact. Always use an oxide inhibiting compound over the conductor prior to crimping to prevent oxidation / corrosion.

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Earthing Connection for 3 Core, Wire Armoured, XLPE Cables.

Connection to Equipment Bushings

When a cable termination is connected to an electrical equipment, it may be necessary to insulate the connection. Present day equipment, tends to be compact and therefore it is usually necessary to insulate termination connections to ensure that the electrical test requirements are met. There are four basic methods of providing this insulation viz:

a) Heat Shrink Bushing Boots.b) Push fitted Bushing Boots.c) Unscreened Elbows.d) Screened Elbows.

Heat Shrink Boots : These products have been used for some thirty years and are well proven. There are two types depending upon the bushing arrangements and whether it will be at right angles to the termination or in line with it. Figure (13) shows the two types.

Figure 13 Heat Shrinkable Boots

Push Fitted Bushing Boots: These provide the same function with the following advantages:

One size will cover all bushings and cable sizes, although an additional solar may be required to ensure a good seal onto the bushing.

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The same product can be used for both right angle and in line applications. Please see Figure 14.

Figure 14 Push Fitted Cable Boots

Unscreened Elbows : They can be used to connect more than one cable per phase, since adaptors allow other elbows to be added. They are moulded with thick walls and therefore withstand higher voltages. Please see Figure 15.

Figure 15 - Unscreened Elbows

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Screened Elbows: The cost of these elbows is high compared to alternative products discussed above and the equipment bushing must be to a recognized standard. These screened elbows however present a number of advantages of which some are

They eliminate the need for a separate termination. They provide a reliable screen over the insulated connection,

eliminating the need for a cable box. Can be used in indoor and outdoor locations. They can be designed to be load break and fault make and therefore

can be used to isolate circuits. Figure 13 shows a typical screened elbow. Essentially it consists of three parts via.

An inner, conductive for a day cage in the area of the cable lug and connecting pin

An insulating middle section of EPDM or Silicone rubber. The advantage of silicone is that it is more flexible and electrically stronger, although it tends to be more expensive than EPDM.

An outer conductive layer, which is earthed.Please see Figure 16.

Figure 16. Screened Elbow

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Qualification Tests for Cable Terminations

In India customers usually follow the Indian Standards Specifications IS-13573-1992 with latest amendments for evaluating the terminations suitability for use in the distribution system. At the International level two principal bodies for issuing specifications i.e. IEC and CENELEC have brought some harmonization and recently CENELEC published a specification embracing all the tests deemed necessary to evaluate any cable termination, be it heat shrink, cold shrink or push on, including screened elbows. This specification is CENELEC HD 629.ISI.1996 for polymeric insulated cable accessories and CENELEC HD 629.2 for paper cables.

A proposal has been made to BIS to revise the IS 13573:1992 to incorporate the additional tests required in this specification. Please see Figure 17 for the sequence of tests.

Figure 17

CONCLUSION: There has been a considerable change in the cable termination field over the past 30 years and it is likely that more will follow, as customers request for more easily installed and reliable systems. Efforts are on to reduce the components required to install a termination and a significant breakthrough in this regard has been made by the author in developing indigenously, a heat shrinkable termination system for 3 Core cables with greatly reduced components. A patent has been filed for this new design of a 3-core termination.

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Ten Questions on Contents of Paper by Ali Hirji

1. Please explain why does the end of the semi-conducting screen of a XLPE cable present a problem?

2. How is the problem mentioned in 1) resolved in a Heat Shrinkable Cable Termination?

3. Why do surface leakage currents flowing over the moist, contaminated surface of a cable termination present a problem?

4. What is weathering resistance of a polymer?

5. Is it always safe to use a sharp knife to remove the semi-conducting insulation screen of an XLPE cable?

6. While fixing a clamping arrangement to the wire / strip armour of a cable, why is it necessary to use a support ring beneath the armour?

7. How is the size of an earthing braid determined for a cable termination?

8. Why is sealing against moisture ingress important for an XLPE cable termination?

9. What are the insulating arrangements for lug connections to terminals of an electrical equipment?

10. What is the Indian Specification No. for Performance Requirements of Cable Accessories?

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