POWER CABLES
Power cables are basically insulated conductors which carry electricity. Insulation is provided to the conductor for it to not be grounded or electrically shorted with some other conductors.
Table of Contents
Mostly, power cables are classified on the basis of insulation. The insulating material is the single most important thing of consideration in cables.
Common insulating materials include polyvinyl chloride PVC, natural polythene PE, cross-linked polythene XLPE, and ethylene propylene rubber EPR.
POWER CABLE COMPONENTS:
THE CONDUCTOR:
The purpose of the conductor is well known which is to conduct power to serve the load. The choice of metal may be aluminum or copper. The central conductor is basically solid or stranded.
THE CONDUCTOR SHIELD
A semi-conducting shield is placed over the conductor to provide a smooth conducting cylinder around it. This layer is a semi-conducting plastic polymer with carbon filler, extruded directly over the conductor. The layer is generally very smooth and because of direct contact with the conductor, it is elevated to the applied voltage of the conductor.
THE INSULATION:
It is a dielectric material that isolates the conductor of the power cable. The most used types of insulating materials in the industry are cross-linked polyethylene XLPE or ethylene EPR. Both have a maximum operating temperature of 90 degrees Celsius and a short circuit temperature of 250 degrees Celsius. Both XLPE and EPR are thermos-setting materials and do not soften to any degree under their decomposition temperature, hence are not capable of being remolded.
THE INSULATION SHIELD:
It is a semiconducting layer that provides a smooth cylinder around the outside surface of the insulation. A typical shield layer is a compound of polymer with carbon filler that is extruded directly over the insulation. For medium voltage applications, this layer is not bonded to the insulation. However, in high-voltage cables, this layer is bonded to the insulation which requires shaving tools to remove it for cable termination.
The metallic shield wires which is composed of wires, tapes, and corrugated tubes are connected to the ground which keeps the insulation shield at ground potential and provides a return path for fault current. The insulation shield can also serve as a neutral if the cable is sized accordingly but only for medium voltage applications. Typical sizing criteria are:
- It should be equal in capacity to the central conductor for single-phase application.
- One-third capacity of central conductor for three-phase application
- Fault duty type for three-phase feeders and transmission application.
NON-MAGNETIC METALLIC SHIELDING TAPE:
The purpose of the shielding tape is to prevent any distortion to the flowing power from electromagnetic interference from outside sources.
ARMOUR
Armour basically provides the cables with increased mechanical durability and protection against external pressure, impact abrasion, etc. Armour is optional but essential for power cables used in underground transmission and areas where the installed cables can be subjected to various external forces like vehicular traffic etc. Armour also supports the pulling load on cables because of their longitudinal reinforcements, which help in supporting the cable cleats and hangers in the vertical run of the cables.
The most used types of armour are:
Steel wire armouring SWA: This armour consists of galvanized steel wires surrounding the cable. It protects the cable from high mechanical stress. Typically used in low to medium voltage levels.
Steel tape armouring STA: This consists of steel tapes surrounding the cable. It provides greater flexibility to the cable and is typically used for low-voltage applications.
OVERALL JACKET:
This is a plastic layer applied over the metallic shield for physical protection. The polymer layer may be extruded as a loose tube or directly encapsulated over the metallic shield. Although, both provide physical protection, the encapsulated jacket removes the space present in a loose tube design which may allow longitudinal water migration.
The typical compound used for jackets is linear low-density polyethylene LLDPE because of its ruggedness and low water vapor transmission rate.
FAQ’s
What is AC power cable?
AC power cables are used to conduct alternating current from an AC source to load.
Why are power cables used?
Power cables are used in transmission and distribution in urban areas where overhead conductors cannot be used. The power cables because are insulated have little to no electrical clearance issues and hence are best for space optimization, underground application also enhances safety. These find its best application in various industries and domestic usage.
