MV Cable Jointing & Termination

PRINCIPLES PRINCIPLES

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M.V. CABLE JOINTING AND TERMINATINGM.V. CABLE JOINTING AND TERMINATING

CONSTRUCTION OF XLPE CONSTRUCTION OF XLPE

CABLESCABLES

Filler

Bedding

Armour

Outer sheath

Binder tape

ConductorConductor ScreenXLPE insulation

Copper ScreenInsulation Screen

Construction of XLPE cables

ConductorConductor•Carries the load current.

Conductor screenConductor screen•Ensures smooth and void free interface with insulation

InsulationInsulation•Prevents the flow of electricity from the energized conductors to the ground or an adjacent conductor

•Withstands the electrical stresses produced by the alternating voltage

•Withstands superimposed transient voltage stress on the conductor

Construction of XLPE cables

Insulation screenInsulation screen•Ensures that the electrical field is confined within the polymeric insulation

Copper ScreenCopper Screen•Keeps the insulation screen at Earth potential.

•Carries the charging current.

•Serves as a path for earth fault currents.

FillerFiller•Fills gaps between cores

•Rounds up the cable

Construction of XLPE cables

•Strippable type and bonded type.

•Binds the cores & filler together and separates the cores from the next layer

Binder TapeBinder Tape

BeddingBedding•Serves as a cushion for the armour

Construction of XLPE cables

•Extruded bedding protects ingress of water

•Protects against mechanical stresses

ArmourArmour

•Serves as a path for earth fault currents.

•Steel Tape Armour (STA)•Galvanized Steel Wire (GSW) or (SWA)

Types of armour:

Construction of XLPE cables

•Galvanized Steel Strip •Aluminum wire armour (single core cables).

OversheathOversheath•Protects against external impact or pressure•Protects against corrosion•Protects from water ingress

Construction of XLPE cables

Types of outer sheathTypes of outer sheath

•PVC •PE •LSF(LSZH)

Conducting Coating on the outer sheathConducting Coating on the outer sheath

•Graphite coating

•Extruded semi-conducting layer (For HV cables)

• Enables to check sheath integrity

Construction of XLPE cables

BASIC CONCEPTSBASIC CONCEPTS

What is Electrical Stress?What is Electrical Stress?

Basic Concepts

Potential Distribution within XLPE insulation

Max stress

11kV ---- 2.2kV/mm

33 kV---- 3.5kV/mm

132kV--- 8 kV/mm

Basic Concepts

V

Uo

Break Down Voltage (BDV)

• Air – 3 kV/mm

• PVC – 20-30 kV/mm

• XLPE - 40-50 kV/mm

Voltage Insulation

Basic Concepts

Air void in solid insulation

Voltage

t

V

t

V

Stress across air void = V/ t

Basic Concepts

Eliminating air voids in cablesEliminating air voids in cables

•Triple extrusion

•Super clean XLPE

•Dust proof extrusion area

•Smooth interface between conductor screen and XLPE insulation

Basic Concepts

Nick on XLPE insulationNick on XLPE insulation

U0Conductor

XLPE Insulation Conductor Screen

Insulation screen

Basic Concepts

Deep Cut on XLPE insulationDeep Cut on XLPE insulation

U0Conductor

XLPE Insulation Conductor Screen

Insulation screen

Basic Concepts

Lifting of semi-conducting Lifting of semi-conducting screenscreen

Basic Concepts

U0Conductor

XLPE Insulation Conductor Screen

Insulation screen

Stress concentration at the screen terminationStress concentration at the screen termination

Equipotential lines

15%

35%

55%

75%95%

U0Conductor

XLPE Insulation Conductor Screen

Insulation screen

Basic Concepts

Flux lines

Stress control layer at the screen terminationStress control layer at the screen termination

Equipotential lines without control

Basic Concepts

Flux lines

95%

75%

55%

15%

35%

Insulation screen

Conductor Screen

U0

15%

35%

55%

75%95%

Equipotential lines with control

Stress Control Layer

STRESS GRADING BY GEOMETRICAL PROFILE

STRESS GRADING IN ‘M-SEAL PUSH-ON’

QS-1000 Field Plot

E1 : 1.45 kV / mm Max. Electrical Stress in Connector AreaE2 : 1.66 kV / mm Max. Electrical Stress in the InsulationE3 : 2.07 kV / mm Max. Electrical Stress at Semi-con EdgeE4 : 2.32 kV / mm Max. Electrical Stress in the Original Cable

•After removing the semi-conducting layer,polish the insulation surface with Al. oxide cloth and cleaned with cleaning fluid.

IMPORTANT IMPORTANT While preparing the cable:

• Take care to avoid any damage on insulation

•The end of semi-conducting screen is the most stressed area of a joint or termination. Take care to avoid any damage or nick on the insulation in this area.

Basic Concepts

•While removing strippable type semi-conducting screen, take care not to lift up the edge of the screen termination.

REQUIREMENTS OF JOINTS AND REQUIREMENTS OF JOINTS AND

TERMINATIONSTERMINATIONS

Requirements of JointsRequirements of Joints

• Conductor connection• Stress grading• Insulation • Screen continuity• Interface pressure(Elasticity)• Earth continuity• Sealing • Mechanical strength

Requirements of Joints & Terminations

Requirements of TerminationsRequirements of Terminations

• Conductor connection• Stress grading• Interface pressure (Elasticity)• Creepage Insulation • Earth continuity• Sealing • Resistance against UV light (for Outdoor)

Requirements of Joints & Terminations