Cable Construction & Cable Selection- Part:2

 

(5) Insulation Screen:

  • Code: IS:7098/IEC:60502/ BS:6622/BS:7835
  • Material: Extruded thermo set semi-conducting compound, Carbon paper and carbon loaded polymer.
  • Used for : Cable from 6 to 30kV (MV & HV Cables)
  • Purpose:
  • An extruded layer of semi conducting is applied over the insulation layer to insure that the electric stress is homogeneous around the insulated core. The semi conducting layer shall be firmly bonded to the outer layer of the insulation layer.
  • The Purpose of Insulation screen is same as Conductor Screen.
  • The Purpose of Insulation Screen is to reduce voltage stress at the interface between the conducting and insulating component
  • A cylindrical, smooth surface between the insulation and Metallic shield
  • Insulation screen is a layer of black cross linked semi conductive compound of approx 1mm thickness and is either fully bonded to the insulation layer, or can be “cold strippable” by hand.
  • When terminating or jointing the cables, it is necessary to remove a part of the insulation screen.

 (6) Bedding (Inner Sheath):

  •  Code: IS: 7098, 1554 / IEC: 60502 / BS: 6622 / BS: 7835.
  • Material: Thermoplastic material i.e. PVC, Polyethylene, thermosetting (CSP) compound
  • Used for : LV, MV & HV Cables
  • Purpose:
  • It could be also called inner sheath or inner jacket, which serves as a bedding under cable armoring to protect the laid up cores and as a separation sheath.
  • Inner sheath is over laid up of cores.
  • It gives Circular Shape of the cable and it also provides Bedding for the armoring.
  • IS:1554 permits following two methods of applying the Inner Sheath of thermoplastic material i.e. PVC, Polyethylene etc., Which is not harder than insulation.
  • Inner sheath is provided by extrusion of thermoplastic over the laid up of cores
  • Inner sheath is provided by wrapping at thermoplastic tape.
  • All multi-core cables have either extruded PVC inner sheath or thermoplastic wrapped inner sheath, which is compatible to insulation material and removable without any damage to insulation. Single core cables have no inner sheath.

 (7) Water blocking Taps:

  •  Water blocking is used to prevent moisture migration.
  • Water blocking tapes or Swelling powder should be applied between the conductor strands to block the ingress of water inside the cable conductor (if required).
  • Water blocking Methods to be considered are as follows.
  • Powders: Swell able powders are used as longitudinal water blocks in cables to prevent longitudinal water penetration. These powders swell and expand sufficiently upon contact with water to form a gel-like material to block the flow of water.
  • Water-Blocking Tapes: A water-blocking tape is usually a nonwoven synthetic textile tape impregnated with, or otherwise containing, a swell able powder.
  • Sealed Overlap: To ensure a seal of the overlap, hot-melt adhesives can be used. These adhesives can be extruded or pumped into the overlap seam of a longitudinally formed metallic tape before the seam is closed during cable manufacture.

 (8) Metallic Screen:

  •  Code: IS: 7098 /IEC:60502 / BS:6622/ BS:7835.
  • Material: Nonmagnetic metallic materials Copper Wire / Tape or Aluminum Wire / Strip
  • Used for : MV & HV Cables
  • Purpose:
  • Medium Voltage & High-voltage cables have an earthed metallic screen over the insulation of each core.
  • This screen consists one or multi layers of a lapped Conductive copper wires, copper tape or metallic foil, lead, aluminum helically with overlap over insulation screen.
  • The metallic shield needs to be electrically continuous over a cable length to adequately perform its functions of electrostatic protection, electromagnetic protection, and protection from transients, such as lightning and surge or fault currents.
  • (1) Shield Electromagnetic radiation: A metallic sheath is used as a shield to keep electromagnetic radiation in the Cable.
  • The main function of the metallic screen is to nullify the electric field outside of the cable – it acts as a second electrode of the capacitor formed by the cable. The screen needs to connect to earth at least at one point along the route.
  • The capacitive charging current and induced circulating currents which are generated under normal operating conditions will be drained away through the screen.
  • (2) Earth Path: It also provides a path for fault and leakage currents (sheaths are earthed at one cable end).
  • The screen also drains the zero-sequence short circuit currents under fault conditions; this function is used to determine the required size of the metallic screen.
  • Lead sheaths are heavier and potentially more difficult to terminate than copper tape, but generally provide better earth fault capacity.
  • (3) Water Blocking: The other function of Metallic sheaths is to water block and form a radial barrier to prevent humidity from penetrating the cable insulation system.
  • (4) Mechanical Protection: It also provides some degree of mechanical protection to cable.
  • Cable shields are nonmagnetic metallic materials. The two materials typically used for metallic shields are aluminum and copper. Aluminum requires a larger diameter as a wire or a thicker cross section as tape to carry the same current as copper. At equivalent current-carrying capacity, an aluminum shield will be lighter in weight but about 40% larger in dimensions

 Different Types of Metallic Screen:

 (A) Concentric Copper Wire screens /Tapes

  • Advantages:
  • Lightweight and cost effective design.
  • High short-circuit capacity.
  • Easy to terminate.
  • Drawbacks:
  • Low resistance of screen may necessitate need for special screen connections to limit the circulating current losses.
  • Does not form a complete moisture barrier unless water swell able tapes are used under and/or over the copper wires.

(B) Aluminum foil laminate

  • Advantages:
  • Lightweight and cost effective design.
  • Moisture proof radial barrier.
  • Drawbacks:
  • Low short circuit capacity.
  • More difficult to terminate – requires special screen connections.

(C) Extruded lead alloy sheath

  • Advantages:
  • Waterproofing guaranteed by the manufacturing process.
  • Excellent resistance to corrosion and hydrocarbons (suitable for oil and gas plants).
  • Drawbacks:
  • Heavy and expensive.
  • Lead is a toxic metal whose use is being restricted in some countries.
  • Limited capacity for short circuits.