NATIONAL INSTITUTE OF INDUSTRIAL ENGINEERING, MUMBAI

CABLE CONSTRUCTION & MANUFACTURING PROCESS

UNDER THE GUIDANCE OF

Dr. KVSS NARAYANA RAO

Submitted By

PRASENJIT HOJAI                                       SARVESH HIREMATH

ROLL NO-66                                                 ROLL NO-88

Cable Construction & Manufacturing Process

                                               fig-power cable parts

Power Cable mainly subdivided into parts-

1. Conductor

2. Insulation

3. Metallic Sheath

4. Bedding

5. Armouring

6. Outersheath

Conductor

Copper or Aluminium used for the Conductors obtained in the form of rods. The 8.0 mm Copper or 9.5mm aluminium rods. After testing, rods are drawn into wires of required sizes. These wires are formed into final Conductor in the stranding machines under strict Quality Assurance Program. 

Insulation

Cross linked polyethylene compound or PVC is insulated over Conductor by Extrusion  process.  XLPE insulated cores are cured by steam curing in vulcanizing chamber to provide thorough cross-linking.

The raw materials & thickness of Insulation are maintained under strict Quality Control and conform to B.S. 5467 / IEC 60502 Part-1 or B.S. 6346 / IEC 60502 Part-1 Standards for XLPE & PVC cables respectively. 

Laying Up

The insulated cores are laid up with a right hand, or alternating left & right hand, direction of lay in the sequence of the core numbers or colours. Where ever necessary non-hygroscopic PP / PVC Fillers & binder tape are used to form a compact and reasonably circular cable. 

Bedding

All armoured cables have extruded PVC bedding. The PVC used for bedding is compatible with the temperature of Insulation material. 

Armour

When armouring is required, the armour consists of single layer of Galvanised steel wire. The armour is applied helical, with a left hand direction. We also provide other armours such as steel strip, tape or tinned copper. Single core cables are armoured with Aluminium or copper wires.

OuterSheath

The standard cables are manufactured with Extruded black PVC Type-9 of B.S. 7655 or ST-2 of IEC 60502. Outer sheath is embossed or printed with the information required by the related standards. Special FR, FRLS compounds are used for outer sheathing of cables, to suit customer’s specification requirements.

Cable Manufacture

An extruded cable production line is a highly sophisticated manufacturing process that must be run with great care to assure that the end product will perform reliably in service for many years. It consists of many sub processes that must work in concert with each other. If any part of the line fails to unction properly, it can create problems that will lead to poorly made cable and will potentially generate many metres of scrap cable. The process begins when pellets of insulating and semiconducting compounds are melted within the extruder. The melt is pressurised and this conveys material to the crosshead where the respective cable layers are formed.  Between the end of the screw and the start of the crosshead.it is possible to place meshes or screens, which act as filters. The purpose of these screens was, in the earliest days of cable extrusion, to remove particles, or contaminants that might be present within the melt. While still used today, the clean characteristics of today’s materials minimize the need for this type of filter. In fact, if these screens are too tight, they themselves can generate contaminants in the form of scorch or precross linking.

Nevertheless, appropriately sized (100 to 200-micron hole size) filters are helpful to stabilize the melt and protect the cable from large foreign particles that most often enter from the materials handling system.

The most current technology uses a method called a true triple extrusion process where the conductor shield, insulation and insulation shield are coextruded simultaneously. The cables produced in this way have been shown to have better longevity (Figure 3) [7]. After the structure of the core is formed the cable is crosslinked to impart the high temperature performance. When a CV tube is used fine control of the temperature and residence time (linespeed) is required to ensure that the core is crosslinked to the correct level.

 Jackets

In most MV, HV and EHV cable applications, the metal sheath/neutral is itself protected by a polymeric oversheath or jacket. Due to the critical performance needed from the oversheath, there are a number of properties that are required, such as good abrasion resistance, good processability, reasonable moisture resistance properties, and good stress cracking resistance. Experience has shown that the material with the best composite performance is a PE-based oversheath, though PVC, Chlorosulfonated Polyethylene and Nylon have been used as jacket materials.

Tests on XLPE cables retrieved after 10 years of operation show that the mean breakdown strength falls by almost 50% (from 20 to 11 kV/mm – HDPE & PVC, respectively) when PVC is used as a jacket material. Many utilities now specify robust PE based jackets as a result. The hardness of PE is also an advantage when protection is required from termite damage.

Jackets extend cable life by retarding the ingress of water and soluble ions from the ground, minimizing cable installation damage and mitigating neutral corrosion. Ninety three percent of investor-owned utilities in the USA specify a protective jacket. The semiconductive jacket or oversheath is recommended for high lightning incident areas or joint-use trenches where telecommunications cables co-exist with power cables.

Inspection and Test Plan for Power Cable

The inspection and test plan for power cable article provides you information about power cable test and power cable inspection in manufacturing shop.

• Witnessing voltage and insulation resistance tests or alternative spark tests.

• For 33Kv cable, witness dielectric power factor voltage test.

• Dimensional checking on sample off-cut i.e. construction consistency, insulation thickness, external sheath screen, armours and mans of main components.

• Visual checking in respect of cable formation, core and external sheath colors, marking legibility.

• Testing on material sample i.e. conductor coating, insulation external jacket for elongation, heat strocle blending and characteristics of armour, metal and sheath components including zinc coating.

Third Party Inspection for Power Cable Configuration

Third party inspector checks the power cable configuration in accordance to drawing and datasheets. Following items is taken in account:

• Cable type
• Number of conductors
• Conductor size
• Conductor color coding
• Insulation type and size
• Fillers
• Water stoppers
• Armour
• Shield
• Outer diameter
• Other specified elements/dimensions
• Cable identification

References:-

•    Electrical Power Cable Engineering by  Bruce S. Bernstein and William A. Thu
•    L.V. Power and Control Cables by Oman Cables Industry
•   TR-101670 “Underground Transmission Systems Reference Book: 1992 Edition,” Electric Power Research Institute
•  http://ieeexplore.ieee.org/Xplore/home.jsp