Underground Cable Technology Polymeric Insulation Medium Voltage 7.2 kV up to 42 kV

Cable 1 Core and 3 Core

Construction Polymeric Cable 1 Core

Carry Current
Aluminum, Copper & etc (Super conductor)
Solid conductor
- For big size conductor difficult for bending
- High skin effect for small size
Strand Compact
- Less diameter to reduce insulation material
Compact Segmental (Milliken)
- Lower AC resistance for 800 mm2 & above
- Each segment is insulated to reduce skin effect
Hollow core
- For Oil-Filled cable

Type of Conductors

Typical Cross Section Conductor AWG & mm2

Conductor Screen (1)

Uniforms the electrical field around of conductor without high electrical stress (Corona Discharge)

Applied over the conductor to smooth irregular profiles (e.g. over stranded conductor) and to provide an intimate between high voltage and dielectric (prevent voids between dielectric and conductor)

Early conductor screens for polymeric insulated cables used tapes, applied over the conductor (also prevented insulation material penetrating stranded conductor during the extrusion process.

Today, almost 100% of conductor screens for XLPE cables are extruded simultaneously with the dielectric and consist of conducting (carbon loaded) polymer

Some EPR insulated cables have conductor screens that employ fabric tape materials

Critical to successful performance of the cable that there should be no voids between conductor screen and insulation

Insulation

Gas: Gas Pressured SF6
Liquid with Solid insulation: Oil filled and mass impregnated paper (MI & MIND)

Solid insulation (Polymer)

Natural: Elastomer (Large reversible elongation)
Synthetic Elastome
- Thermoplastic (can be melted when reheated)
- Thermosetting (can be not melted) Cross-Linked

Thermoplastic PE, EVA, ETFE, PVC
Cross-Linked Thermoplastic XLPE, XL EVA
Elastomer Natural rubber, EPR, EPDM, Silicone rubber

Insulation Screen (Semi-Conductive Layer)

Potential grading and limiting of the electrical field.
Conduction of charge and discharge currents.
Touch protection (Shielding)

Electrical Field of Unscreen Cable (LV)

Electrical Field of Screen Cable (MV & HV)

Unscreen with Screen Cable

Typical Insulation Screen

A – extruded “easy-strip” screen

B – screen of graphite and conductive paper tape

C – bonded extruded screen

Removing Insulation Screen

A – Extruded Easy Strip

C – Bonded Extruded Screen

Metal Screen (Metal Shield)

The semi-conducting polymer insulation screens can carry very little fault current, so it is necessary to provide an additional current path.

This is accomplished with Copper (Cu) Wire or Tape, Aluminum (Al) Tape, Lead Sheath screens applied over the individual cores
An alternative is to use Copper Wires over the 1 cores or over the laid-up cores of a 3 core cable. When the network design need for high short circuit current (kA)
To prevent moisture ingress & mechanical impact environment are Lead Sheath or Corrugate Sheath.

Corrugated Aluminium Sheath (CAS)
Lead Sheath (LS)
Copper Wire with Aluminium laminate (CW)
Copper Wires (CW)
Copper Tape (CT)

Armour

Armour provides mechanical protection against soil movement, during installation and in case of third party damage.
It may provide earth fault current path.
Different types of armour provide different levels of protection
Selection of armour type is important, to suit installation conditions and level of protection

Important to remember short circuit currents and corrosion protection

Typical Armour

Typical Function

Metallic armour is usually galvanized steel, Aluminum, Lead and Copper braid.

SWA: Steel wire armour for high mechanical impact and short circuit current

STA: Steel tape armour is added for mechanical impact resistance lower SWA for short circuit current. DSTA (Double Steel Tape Armour)

AWA: Aluminium wire armour for single core cables is added for increased tensile rating. for single core cables only because non magnetic requirements.

HDPE: High density polyethylene if not required for fault current

Submarine cable: use two steel wire armour layers can be used to minimise the twisting effect of one layer and this is called torque balancing.

Mine or Marine Cables (Shipyard): use Copper Braid for flexible to prevent vibration effect and torque balancing on the shipyard.

Interlocked, corrugated flexible: aluminum or steel (Std US).

Extruded smooth or Corrugated: aluminum sheath (HV).

Oversheath (Outer Jacket)

Last layer of cable for Environment, Mechanical Impact and Water protection

PVC Polyvinyl Chloride for 3 core with armour or 1 core use Cu tape shield. Flexible, Chemical and flame retardant
PE Polyethylene for high mechanical impact. Use for 1 core wire shield for duct bank application
HDPE High Density Polyethylene for more mechanical impact PE use for HV transmission line cable