HV Cable Termination Method and Precaution. (PART-1)

Introduction:

• Power cable is most important part of power transmission and distribution systems.
• Terminations and joints are the essential part for the power cables. It makes connections between Cable or between Cable and electrical apparatus.
• Cable terminations make physical and electrical connections between the cable and the equipment to flow electricity in the desired manner.
• Cable terminations are weak parts for electrical system and most of the cable faults happen at this section, hence the quality of cable terminations directly affects the safe operation of the cable lines. The mistake will cause troubles, widespread power outages and cause great loss of finance, the people’s life and property.
• There are three types of cable terminations. it can be heat-shrink type, cold-shrink type or pre-moulded push on type cable terminations.

Type of Cable Termination:

• There are various types of High / Medium Voltage cable termination for substation switchgears, transformer, poles, and cable boxes.
• Termination Kit are classified by its application.
• It can be pre-molded push-on, cold shrink or heat shrinkable type.
• Selecting the appropriate termination method is essential for maintaining the mechanical integrity of the cable and it depends on the cable types, operating parameters, voltage applications and site conditions.
• Each technology has specific advantages depending on the needs of the user.

Heat shrinkable terminations:

• It names suggests that it required Heat and have shrinkable type tubing.
• when we applied heat (with an electric or gas heat gun) to shrink tube. It expanded shrinkable tubes to the size of the substrate beneath and enabling quick and easy installation.
• Material:
• Heat shrinkable products are usually made from polyolefin type plastics which have been modified to give additional properties such as improved weathering and enhanced insulation levels.
• Heat Shrink is resistant to most chemicals. It will become rigid once it has been recovered and making it a good option for mechanical protection.
• Application:
• Use on low and medium voltage cables,
• Heat shrink termination kits can be used for XLPE cable in both indoor and outdoor applications, even for extreme hazardous atmospheric conditions.
• The cable terminations provide non-tracking stress control connections for medium to high voltage cables with water, UV, erosion and corrosion resistant performance.
• Advantage:
• When stored correctly, there is unlimited shelf life for the product.
• Drawback:
• the Material rigidity prevents flexing with the cable during normal operation hence an effective environmental seal cannot be maintained without the mastic tapes.

Cold shrink terminations: 

• It names suggests that it does not require heat.
• They can be used for medium-high cable installations which do not require naked flame or heat source to install especially in explosive atmospheres.
• By removing the supporting cord during the installation process causes the tube to shrink so that it fits onto the desired place.
• The cold shrinkable cable terminal offers excellent insulation and high resilience.
• Material:
• Cold Shrinkable products are made from elastomeric materials such as Silicon or EPDM rubbers, which are pre-stretched onto a tubular hold-out made from plastic tape in a tight spiral.
• By unwinding the spiral tube, the material recovers to its original size.
• Application
• The cold shrinkable cable joints are especially suitable for installation in hazardous environment, such as coal mine, oil field etc, where fire is strictly prohibited.
• Advantage:
• The Cold Shrink eliminate any heat source required for installation.
• The rubber material will follow the normal expansion and contraction of cables without need for additional adhesives or mastics.
• Disadvantage:
• Care is needed to store product and there is a finite shelf life.

Push On type termination:

• Similar to Cold Shrink, these are made from elastomeric material and are not expanded before installation.
• The product is applied by sliding onto cable cores with the use of silicone grease as a lubricant.
• It has similar benefits to Cold Shrink, but with restricted application diameter range.

Parts of Termination Kit

(A)  Environment Sealing Shrinkable Tubes:

• Breakout Boot
• Anti-tracking Heat Shrink Sleeve Tubes
• Stress Control Tubes
• Lug Sealing Tubes
• Rain Sheds

(B)  Mastic Taps:

• Stress Control Yellow Mastic
• Red Sealing Mastic
• Black Sealing Mastic
• PVC Insulation Tape

(c)  Earthing:

• Worm Clips:
• Tinned Copper Braid
• Copper Binding Wire / Small Copper Braid

(D) Cleaning Accessories

• Cleaning Solvent
• Aluminum Oxide Tape
• Silicon Grease

(E) Other:

• Lugs
• Nylon Thread

(1) Heat Shrinkable Breakout Boot

• Purpose:

• Breakouts Boot is used to provide insulation and protection against binding force. It also provides sealing over the crutch of three core cables at the branching point.
• They are coated internally with waterproof, non-tracking rubber based red sealant.
• When heat is applied to the heat shrink tubing, the adhesive material inside of the tube is activated and shrink the boot which provide protection to the crutch of three core.

• Installation Location:
• It normally installed on over the crutch of multi-core cables to provide protection to Cable termination earthing assembly.
• Materials
• The base material of the breakouts is thermally stabilized, cross linked polyolefin. Which offer resistance against UV radiation, oxidation, ozone and other environmental effects.

(2) Anti-tracking Heat Shrink Sleeve Tubes

• Purpose:
• Heat shrinkable Sleeve Protects cables from harmful elements such as water, oils, acids, and other environmental hazards.
• When heat is applied to the heat shrink tubing, the adhesive material inside of the tube is activated, which shrink and provide protection to the cable.
• A common gas torch is required to heat and recover the tubes onto the power cables or accessories. It has a high shrink ratio which allows it to be used with a wide range of cable sizes.

• Installation Location:
• It is normally used as an integral part of cable joints for protection & insulation.
• Non tracking tube (red) is used as an external protective covering for the core for both indoor and outdoor applications.
• Minimum length of the non-tracking tube depends on the system voltage and it’s application (Indoor or Outdoor).
• Materials
• The material of the Anti-tracking Sleeve is thermally stabilized, cross linked blended polyolefins which has high insulation, non-tracking, erosion resistant, weather proof and flame-retardant properties.
• It has excellent insulating properties combined with toughness, resistance to impact and abrasion thus providing mechanical protection.
• It offers good protection from environmental effects, ultra violet radiation and also resistant to oil and solvent attacks

(3) Stress Control Tube:

• Purpose:
• Stress control tube (black) is used to control the high electrical stress present at the end of the semi conductive screen of the cable.
• Suitable size of heat shrinkable stress control tubes is to be used to reduce stress at cut back of screen.
• The minimum length of the stress control tube is 130 MM for 12 kV, 190 MM for 24 kV and 260 MM for 36 kV for effective stress control of Cable.

• Installation Location:
• The heat shrinkable semi conductive stress control tubes cover the earth connection point to the conductive screen end. It should not be extended further this point onto the outer cable sheath.
• Ensure the tubes are wrinkle free and have an even wall thickness.
• Material:
• Polyolefin is the most popular material for shrink tubing due to its thermal resistance and has a combination of resistive and capacitive characteristics.

(4) Lug Sealing Tubes

• Purpose:
• To protect the interface between the termination lug and the cable core covered by the non-tracking Lug Sealing tube

• Installation Location:
• At the end of XLPE insulation area near Cable Lugs.
• Material:
• It is made from the same material as the non-tracking tube i,e Polyolefin and coated internally with the red sealant.

(5) Rain Sheds

• Purpose:
• Heat Shrink Single Sheds are used for extending the creepage path for the medium voltage cable termination, thereby saving the length of the cable and reducing the size of the switch gear cabinet required for this purpose.

• Installation Location:
• On anti-tracking Heat Shrinkable Sleeve near Cable Lugs.
• The minimum number of sheds designed to be used in the termination system depends on system voltage and whether the application is indoor or outdoor.
• Typical Applications
• Single sheds are suitable for termination of the complete range of electrical cables of XLPE and PILC types. The rubber based red sealant seals the single shed to the cable insulation.
• Materials
• The base material of the HV sheds is thermally stabilized, cross linked blend of polyolefin and a compatible grade of synthetic rubber.
• The properties of the HV Sheds are electrical insulation, non-tracking, erosion resistant, weather proof and flame retardant.
• HV Sheds are internally coated with waterproof, non-tracking, butyl rubber based red sealant mastic

(6) Stress Control mastic (Yellow Mastic)

• Purpose:
• Stress Control mastic is used in MV terminations for providing stress relief of the electric field.
• It also serves purpose of filling all the gaps and uneven surfaces.
• The mastic is water resistant and has high adhesion property.
• The color of the stress grading mastic is yellow.

• Installation Location:
• Conductive paint / Stress grading mastic is applied around the step formed at the screen-cut point i.e., at end of the semi-con screen and start of the XLPE insulation of the cable.
• Material:

• The base material is thermally stabilized, blended polyolefins.
• The basic resin is mixed with chemical additives offering electrical stress control properties.

(7) Non-Tracking – Water Sealing Mastic (Red Mastic)

• Purpose:
• Anti-tracking / Weather Resistance red mastic sealants Tapes are used in MV terminations on breakout boots, non-tracking tube and on lugs for sealing, insulation, and waterproofing.
• The red sealant also acts as a filler and insulating material to fill in the step formed at the end of conductor insulation.
• It forms a waterproof seal at the end of the termination, which is essential for the long service life of the termination.

• Installation Location:
• Red sealant is applied around the base of the terminal lug where it is crimped to the cable conductor.

• Materials
• The base material is thermally stabilized, blended rubber.
• The base rubber is mixed with chemical additives offering non-tracking and insulating properties.

(8) Insulation-Sealing Mastic (Black Mastic)

• Purpose:
• Black mastic is suitable for LV and MV applications. It is used for sealing and waterproofing of electrical installations such as joints and terminations.
• Insulating mastic is used in joints to protect the heat shrink material by covering the sharp edges of metallic parts and for filling voids and irregular shapes.
• It also acts as an environmental seal to prevent ingress of water and moisture and other environmental contaminants
• Installation Location:
• To protect the cable breakout, PVC tape/black mastic is required to be applied over earthing arrangement (worm drive clip installed over copper braid and armor) for suitable length of the termination part.

• Materials
• base material of the black insulating mastic is thermally stabilized, blended rubber. The base rubber is mixed with chemical additives offering good insulating properties
• The black insulating mastic provides excellent sealing and insulating functions.
• These offer excellent adhesion to rubbers, synthetic cables, metals, and jackets

(9) PVC Insulation Tape:

• Purpose:
• PVC Insulation tape is used for Insulation ,sealing of electrical installations such as joints and terminations.
• PVC Insulation Tap covers the sharp edges of metallic parts and for filling voids and irregular shapes.

• Installation Location:
• To protect the cable breakout, PVC tape is required to be applied over earthing arrangement (worm drive clip installed over copper braid and armor) for suitable length of the termination part.
• Materials
• PVC

(10) Lugs :

Purpose:

• Cable lugs are connectors that used to connect/terminate cables 

Installation Location:

• At the end of Cable Termination.

Material:

• copper or aluminum or their respective alloys

 (11) Cleaning Solvent Pouches:

Purpose:

• To clean the XLPE insulation surface.

 

Installation Location:

• It is applied on XLPE Insulation to Clean XLPE Insulation Surface.
• To clean the XLPE insulation surface.

Material:

• Isopropyl Alcohol solution

(12) Worm Clips:

Purpose:

• Worm drive clip (jubilee/hose clips) for tightly securing the earthing braid.

Installation Location:

• On Copper Wire of Copper Screen and armor at crutch of three core cables at the branching point.

Material:

• The material of support ring to be steel (G.I.) or Aluminium.

(13) Tinned Copper Braid:

Purpose:

• Earth Continuity between armour and earth to be provided by tinned copper braid of adequate cross section.
• This is required for proper earthing of the Cable termination.
• This armour earthing arrangement provides sound electrical contacts at armour bonding points and handling large current and low voltage to avoid excessive temperature rise during earth faults.

Installation Location:

• Earth Continuity between amour and earth to be provided by tinned copper braid of adequate cross section.

Material:

• Copper

(14) Aluminum Oxide Tapes

Purpose:

• For cleaning of cores, removing burrs and roughness of XLPE insulation.

Installation Location:

• It used to smooth surface of XLPE Insulation.

Material:

• Abrasive Material on Textile Backing

(15) Silicone Grease

Purpose:

• The use of insulating silicone grease in MV Cable termination for high-voltage resistance, waterproof and anti-creep performance.
• It is used to smoothen out any uneven insulation surfaces of XLPE Insulation of cables and to lubricate mount moulded parts of cables termination.
• Cuts and scratches on the XLPE insulation may cause electric discharge under the stress control tube. Application of a thin layer of silicone-based discharge suppression compound will make it free of partial discharge.

 

Installation Location:

• It applied on XLPE Insulation and other parts of Cable Termination for insulation, sealing, lubrication purpose.

Material:

• Insulating silicone grease is an insulating paste made from modified silicone oil.

(16) Copper Binding Wire / Cooper Strips:

Purpose:

• It is used to provide earth connectivity between copper screen to armour of cable to earth

Installation Location:

• Between Copper Screen and armor of cable

Material:

• Copper