HV Cable Termination Method and Precaution. (PART-2)

Basic Structure of High Voltage Cables

• High-voltage cables generally consist of nine layers.

• The basic structure of high voltage cables is

(1) Outer Sheath:

• It protects cable from the moisture and environment. It also provides protection against mechanical impact on cable.
• Outer sheath material is mostly Polyethylene

(2) Armor Layer.

• Armor provides mechanical Strength to the cable, certain resistance to external force, and prevents animal bite, external mechanical impact on cable.

(3) Inner Sheath:

• Inner protective sheath can keep Cable XLPE Insulation layer away from water, air and other objects, avoiding moisture and mechanical injury on internal insulation layer.
• It protects the cable core.
• Inner Sheath Material is same as outer sheath material, mostly Polyethylene

(4) Package and Filler Layer.

• It helps to organizes several cable cores into circular shape, for the convenience of packaging and cabling.
• It also provides protection to the cable Core.

(5) Copper Shielding screen.

• The main function of copper screen is to equalize the electric field and help improve the electric field distribution.
• The other function is to ground the short circuit current.
• When cable is charged it’s produced strong electrical filed around the core, Copper Shielding equalize this electrical filed and improve uniformity of electrical field distribution surrounding the core which restrict the interference of strong electric field around on core in the cable.
• Hence, if copper shielding layer in cable doesn’t exist, then insulation breakdown between core and core will be damaged.

(6) Semi Conductive Layer (Outer Semi Con Layer).

• There may be small clearance or air gap between in XLPE insulation and copper shielding screen which is one of the main factors causing partial discharge.
• Semi Conductive material have good contact properties hence semiconductive layer is provided between XLPE Insulation and copper screen to avoid the partial discharge between insulation layer and protective layer.

(7) XLPE Insulation Layer.

• The cable insulation provides electrical insulation to the conductor at voltage from the outer screens at ground potential.
• The insulation will be of sufficient thickness to withstand the electric field under the rated and transient operating conditions.
• XLPE (cross-linked polyethylene) is good insulating materials.
• XLPE has high breakdown strength, high insulation resistance, low dielectric loss, excellent tree discharge- resistance performance and long insulation performance period, etc.

(8) Conductor Shielding Layer (Inner Semi Con Layer).

• Conductor shielding layer can improve the electric field distribution.
• This layer reduces the probability of occurrence of partial discharge.
• The cable conductor is made stranding of wires hence it surface is not smooth which creates air gap between insulation layer and conductor. This will cause the concentration of electric field.
• Conductor is covered by Inner Conductor shielding layer of semiconductor materials on the surface of the conductor for good contact with insulation layer.

HV Cable Termination Procedure:

General Instructions

• Use a propane gas torch with a soft yellow flame for shrinking components.
• Avoid a pencil type flame which is caused by unregulated supply
• Keep the flame on the moving direction to ensure even shrinkage of all the materials and also helps to reduce scorching
• Ensure that all components are kept clean and grease free during installation
• Allow to cool before applying any mechanical strain
• Read the instructions carefully before starting.
• Clean and degrease all parts which will be in contact with tapes and adhesives.
• Personnel should be proficient and knowledgeable for preparing and installing medium voltage terminations.

 (1) Remove Outer Cable Insulation Sheath:

• Calculate approximate Terminate length of Cable from following Table.

Voltage	Indoor (L)	Outdoor (L)	x
7.2KV	650mm	700mm	Length 0f Lugs +5mm
12KV	650mm	700mm
17.5KV	650mm	700mm
24KV	700mm	800mm
36KV	800mm	900mm
The “L” dimension should not be longer than the distance between bushing centers and base plate.

• Strip of and removed Outer Sheath of Cable for Length “L”.
• Removed Armored from Length “L”.
• Make smooth edge of Sharp armour.
• Bend / Fold Armour up to 50mm.
• Bind Armour on the outer sheath with use of Copper Wire / Clamp.

(2) Remove Inner Cable Insulation Sheath:

• Removed Inner Cable Sheath 10mm length from Armour with the help of knife.
• Removed extra parts of cable which used to make cable round, i.e., Filler, Binding rope / Strip

(3) Earthing arrangement of Copper Screen.

• Marking with the help of tape from100mm length from inner sheath on Cu Screen.
• Removed extra Copper Screen from this marking tap to cable end.
• Make sure that cu screen is not damage during this process.
• Some Manufacture provides small Copper Braid but if not provided than make it by use of copper wire provided with the kit (Twist together to form a conductor)
• Tight Copper Braid / Copper Wire to Cu screen. Make sure Connection should not be loose.
• Ensure that Free end of copper braid / wire extended up to armour wires or up to tinned copper braid.

(4) Earthing arrangement of Armor.

• Installed Long Tinned copper braid on Armor with used of Warm drive clamp.
• Extend the small copper braid (Strip) / copper wire which are already fixed on copper screen to the Tinned copper braid.
• Make sure this connection should be over lapping to each other.
• Firmly tight these arrangement with the use of Warm drive clip.
• Wrap Black sealing masking Tape around Sharpe edge and over the earthing assembly.
• Further after completion of Cable termination, these braids will either be bonded to the earth gland or other suitable earth point.

(5) Installation of Heat Shrinkable boot on earthing arrangement

• Slide the three-legged adhesive-coated breakout over the cores and pull it as far down into the crutch as possible.
• Apply heat to heat shrinkable Boot by use of gas torch with a soft flame.
• Keep the flame on the move to ensure even shrinkage of all the materials and also helps to reduce scorching.
• Shrink the breakout into place starting at the centre. Work first towards the lower end, then shrink the turrets onto the cores.
• Make sure that there is no any void or air is remaining inside boot during this process and shrinkable boot is firmly tight fixed with the earthing assembly.
• Make sure that Tube is shrink uniformly and free from void and wrinkles.

(6) Removing Semi Conductive Layer.

•  Remove the semi-conductive screen layer using a suitable tool to a point 50 mm above the copper tape screen.
• It is very important that the screen is removed leaving a clean straight cut and that no scoring or damage is done to the XLPE Insulation (Primary insulation).

(7) XLPE Insulation Grinding, Polishing & Cleaning

• The cable insulation surface must be smooth to avoid air gaps where discharges can occur.
• Cable Surface can be smooth by use of grinding (abrasive) paper.
• The Cable grinding / Polishing process must be carried out carefully to avoid damage to the cable insulation screen.
• After grinding / Polishing Cable Insulation must be clean by approved Cable Cleaner solution. Cable Insulation should be clean by using a motion of travel towards the insulation screen.
• Never use the same side of a cleaning tissue twice and the insulation must be clean of conductive particles and dirt.
• After Proper Cleaning, Cable Cleaning Wipes are used to remove all traces of the semi-conducting screen from the cable end towards the cable termination/joint point. The cable XLPE Insulation must be wiped along the available length to ensuring complete removal of contaminants.
• It is recommended that a non-flammable cable cleaning solvent be used. Any solvent that leaves a residue should be avoided. Do not use excessive amounts of solvent as this can saturate the semi-con layer and render it non-conductive.

(8) Apply Stress Control Mastic Tape

• Stretch Stress Control Mastic around the core.
• Start 20mm from Sem conductive Layer to extend onto the XLPE Insulation (Primary insulation) by 10mm.
• Secure copper screen end on semi conductive layer by use of PVC Tape or binding with copper wire.
• Apply thin film of silicon grease on the XLPE Insulation.

(9) Heat Shrinkable Stress Control Tube

• Insert stress control tubes over core and position lower end of stress control tube 10mm from copper screen.
• Start shrinking the tube from the lower end and process upward. Shrink tube one at a time.
• Make sure that Tube is shrink uniformly and free from void and wrinkles.

(10) Installation of Lugs

• Insert Terminal Lug on end of core and clip it properly as per standard practice.
• In case of outdoor termination insert the rain sheds before crimping the lug.
• Remove any burrs or sharp points that may be present during clipping process.

(11) Apply Red Sealant Mastic Tape

• Fill the gap between insulation and terminal lug with the help of red mastic tape.
• Red Sealant Tape over the Lug barrel and extend onto the Insulation by approximate 10mm.

(12) Heat Shrinkable Anti Tracking Tube

• Insert Anti tracking heat shrinkable tube over each core.
• Position the anti-track tubes so that they cover the turrets of the breakout boot and the barrels of the lugs.
• Make sure that tube cover the fingers of breakout by 25mm at least.
• Start shrinking the anti-tracking tube from bottom end of tube and proceed upward to avoid any air void.
• Make sure that Tube is shrink uniformly and free from void and wrinkles.

 (13) Lug Sleeve

• Remove released paper which is inside the lug sleeve.
• Insert Lug sleeve on core. Make the position of lug sleeve so that it covers Lug barrel and core equally.
• Start shrinking sleeve from top and proceed downwards

(14) Rain sheds

• For outdoor application insert Rain shed on each core and keep 150mm distance from cable end and shrink it on the core.
• Make distance of 80mm on next rain sheds.

HT Termination kit