Cable

Definition

*        In simple term,  electrical cable can be considered as just a conductor with insulation, shield and jacket. Cables are  used to transmit power. 

               

*        Cables are used as an alternate to overhead conductors where it is not possible / viable to use overhead conductor - e.g due to space constrain, right of way , dense population etc.

Power cable 

–        Sizing of cross section and number of run of cable based on

•          Load current

•          Short circuit current

•          Fault clearance time

•          Voltage drop in cable

•          De-rating factors

–        Selection of insulation (especially for HT cable)

•          HT cable insulation based on the type of system i.e. Unearthed or effectively Earthed system

•          Specific care to be taken where impedance earthing is done but system behavior is near to Unearthed system

Cable routing, numbering, grouping and type of laying

Control cable 

–        Sizing of cross section and number of cores

•          Type of signals control cable to carry

•          Voltage (PT, CVT)

•          Current (CT)

•          DC Circuit (Closing, Tripping, Indication)

•          AC Circuit (DC supervision, operating mechanism ckt.)

•          Potential free contact circuit (Indication, Annunciation, logic ckt.)

•          Voltage drop due to longer cable route length

–        Grouping of signals for deciding the number of cores

–        Cable routing and marking and crossing

–        Cable and core numbering

Advantages of Power cables

*        Improved Reliability of power supply

*        Safer operation

*        Less maintenance cost - cable is almost maintenance free

*        No risk of faults due to wind, cyclone, birds & falling trees

*        No problem of right of way

*        No pilferage

*        Environment friendly

*        Lifetime cost of cable is comparable with overhead lines

The main  components of EHV XLPE Cables  are :

*        Conductor                           : Aluminium  / Copper

*        Conductor screen            : Extruded semiconducting XLPE

*        Insulation                            : Extruded insulating XLPE

*        Insulation screen             : Extruded semiconducting XLPE

*        Metallic shield                   : Extruded lead alloy / copper wires

*        Outer sheath                     : Extruded PVC / PE

*        Additional features         : Longitudinal water sealing by water blocking tapes

                                                                : Radial water sealing by lead sheath / Al-laminate sheath

XLPE Cables have following advantages over conventional paper insulated/ Oil filled cables

í          Lower dielectric losses

í          Lower weight

í          Higher permissible continuous and short circuit current withstand capacity

í          Environment friendly - no risk of oil leakage in to environment

í          Easy installation of accessories

í          Maintenance free

í          No problem with vertical  installation

Improved flame retardant  property with PVC sheath, due to absence of any oil etc

EHV XLPE Cables are tested for following tests in accordance with IEC 60 840

Routine Tests

î          Partial Discharge Test

î          High Voltage Test

î          Voltage test on outer sheath

Sample Tests

î          Conductor examination

î          Conductor resistance test

î          Measurement of thickness of insulation and non-metallic sheath

î          Measurement of thickness of metallic sheath

î          Measurement of diameters

î          Hot set test on XLPE insulation

î          Measurement of capacitance

Type Tests-Electrical

í          Bending test followed by partial discharge test

í          Tan d measurement

í          Heating cycle  voltage test followed by power frequency voltage test

í          Impulse voltage test followed by power frequency a.c. voltage test

í          Resistivity of semi conducting screens

Type Tests - Non Electrical

î          Conductor examination

î          Conductor resistance test

î          Measurement of thickness of insulation and non-metallic sheath

î          Measurement of thickness of metallic sheath

î          Measurement of diameters

î          Test for mechanical properties of insulation before and after ageing

î          Test for mechanical properties of non metallic sheath before and after ageing

î          Ageing test on pieces of completed cable to check compatibility of materials

î          Loss of mass test on PVC type ST2 sheath

î          Pressure test at high temperature on sheaths

î          Test on PVC sheath at low temperature

î          Heath shock test on PVC sheath

î          Hot set test on XLPE insulation

î          Measurement of carbon black content on black PE sheaths

î          Shrinkage test on XLPE insulation

î          Test under fire condition

î          Water penetration test