Mv Cable Jointing & Termination

PRINCIPLES of M.V. CABLE JOINTING AND TERMINATING

CONSTRUCTION OF XLPE CABLES

Construction of XLPE cables

Conductor Conductor Screen XLPE insulation Insulation Screen Copper Screen Filler Binder tape Lead Sheath Bedding Armour Over sheath

Construction of XLPE cables

Conductor Conductor Screen XLPE insulation Insulation Screen Copper Screen Filler Binder tape Bedding Armour Outer sheath

Construction of XLPE cables

Conductor •Carries the load current.

Conductor screen •Ensures smooth and void free interface with insulation

Insulation •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 screen •Ensures that the electrical field is confined within the polymeric insulation •Strippable type and bonded type.

Copper Screen •Keeps the insulation screen at Earth potential. •Carries the charging current. •Serves as a path for earth fault currents.

Filler •Fills gaps between cores •Rounds up the cable

Construction of XLPE cables

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

Bedding •Serves as a cushion for the armour •Extruded bedding protects ingress of water

Construction of XLPE cables

Armour •Protects against mechanical stresses •Serves as a path for earth fault currents.

Types of armour: •Steel Tape Armour (STA) •Galvanized Steel Wire (GSW) or (SWA) •Galvanized Steel Strip •Aluminum wire armour (single core cables).

Construction of XLPE cables

Oversheath •Protects against external impact or pressure •Protects against corrosion •Protects from water ingress Types of outer sheath •PVC

•PE

•LSF(LSZH)

Conducting Coating on the outer sheath • Enables to check sheath integrity •Graphite coating •Extruded semi-conducting layer (For HV cables)

Construction of XLPE cables

Construction of XLPE cables

Special Constructions

Interstice conductor

Graphite coating

BASIC CONCEPTS

Basic Concepts

What is Electrical Stress?

Basic Concepts

V Max stress 11kV ---- 2.2kV/mm 33 kV---- 3.5kV/mm 132kV--- 8 kV/mm

Uo

Potential Distribution within XLPE insulation

Basic Concepts

Break Down Voltage (BDV) Voltage

Insulation

• Air – 3 kV/mm • PVC – 20-30 kV/mm • XLPE - 40-50 kV/mm

Basic Concepts

Air void in solid insulation Voltage

δt

V

δV

Stress across air void = δV/ δt

t

Basic Concepts

Eliminating 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 insulation

Insulation screen XLPE Insulation

Conductor

Conductor Screen

U0

Basic Concepts

Deep Cut on XLPE insulation

Insulation screen XLPE Insulation

Conductor

Conductor Screen

U0

Basic Concepts

Lifting of semi-conducting screen

Insulation screen XLPE Insulation

Conductor

Conductor Screen

U0

Basic Concepts 95% 75% 55% 35% 15% Insulation screen XLPE Insulation

Conductor Screen

U0

Conductor

Flux lines

Equipotential lines

Stress concentration at the screen termination

Basic Concepts 35%

55% 75%

95% 15%

75%

95%

55% 35% 15% Insulation screen

Stress Control Layer Conductor Screen

U0 Flux lines

Equipotential lines without control Equipotential lines with control

Stress control layer at the screen termination

STRESS GRADING BY GEOMETRICAL PROFILE

STRESS GRADING IN ‘M-SEAL PUSH-ON’

QS-1000 Field Plot

E1 : E2 : E3 : E4 :

1.45 kV / mm 1.66 kV / mm 2.07 kV / mm 2.32 kV / mm

Max. Electrical Stress in Connector Area Max. Electrical Stress in the Insulation Max. Electrical Stress at Semi-con Edge Max. Electrical Stress in the Original Cable

Basic Concepts

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. •While removing strippable type semi-conducting screen, take care not to lift up the edge of the screen termination. •After removing the semi-conducting layer,polish the insulation surface with Al. oxide cloth and cleaned with cleaning fluid.

CONDUCTOR CONNECTION

Conductor connections

Methods of conductor connections: •Soldering •Compression •Mechanical Connector

Conductor Connection Crimping •Correct connectors •Good crimping tool •Correct dies •Required no. of crimps •Always smoothen any sharp edges by filing. •Use corrosion inhibiting paste for Aluminium conductors

Important Connector is the heart of a joint. Wrong selection of connector and/or crimping tool/die can lead to overheating of ferrule and premature failure of joint/termination.

REQUIREMENTS OF JOINTS AND TERMINATIONS

Requirements of Joints & Terminations

Requirements of Joints • • • • •

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

Requirements of Joints & Terminations

Requirements of Terminations • • • • • • •

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

JOINTING & TERMINATION SYSTEMS

Jointing & Termination Systems

• • • • •

Cast Resin system Taped system Heat shrinkable system Premoulded system Cold shrinkable system