Cables

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Under Ground Distribution System

Cable Group

Corporate Operation Services

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ROLE OF CABLE GROUP UNDER COS

CABLE FAULT LOCATION

CABLE FAULT RECTIFICATION

IDENTIFICATION OF SICK CABLES

REDUCTION OF CABLE FAULTS TO A BARE MINIMUM.

CABLE FAULT ANALYSIS.

INTRODUCTION OF NEW TECHNOLOGIES

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WHY CABLES :

CONCERN RATING O/H RATING CABLE

VISUAL 2.5 0.2 MAGNETIC FIELD 2.5 1.1 LAND DEPRICIATION 2.2 1.0 ELECTRIC FIELD EFF. 2.1 0.0 GROUND OPERATION 1.7 1.1 SECURIRTY OF SUPP. 1.6 0.4

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CABLE DEFINED

THE NAME CABLE IS GIVEN TO LONG CURRENT CARRYING DEVICES THAT CARRY THEIR OWN INSULATION AND AND PRESENT AN OUTER EARTHED SURFACE.

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EARLY CABLES 1881 STREET PIPES

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edCABLES CLASSIFIED

VOLTAGE LEVEL

LV – HV- EHV INSULATION

PVC -PILC -XLPE CORES

SINGLE CORE -MULTICORE CONSTRUCTION

SOLID- PRESSURE CABLES -ABC SPECIAL FEATURES

TR XLPE

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Grid Stations 55

11kV substations 3500

Distribution Transformer (Including HVDS) 12000

66kV OH line circuit km 170

66kV UG line circuit km 50

33kV OH line circuit km 170

33kV UG line circuit km 160

11kV UG circuit km 1600

Network in NDPL

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Type of Cables

Solid Cables-No control is maintained over internal pressure, which is liable to fall below atmospheric pressure, during normal working condition. e.g. PILC , XLPE,PVC cables

PILC – Paper Insulated Lead Covered

XLPE – Cross Linked Polyethylene

PVC – Polyvinyl chloride

Pressure Cables – Insulation is maintained at a pressure in excess of atmospheric under all service conditions.e.g.oil filled cables, low and high gas pressure cables

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Construction of cables

Paper Insulated Cable Belted – individual

conductors have paper insulation and a belt of paper insulation is wrapped over the cores.

Screened – individual conductors have paper insulation and a metallic screen

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Construction of Belted/PILCA cables

Conductor – Copper or Aluminium, stranded,sector shaped Insulation over core – Impregnated Paper insulation lapped over each

conductor Fillers – placed in between the cores.Made of Jute Insulation over all cores – Belted paper insulation lapped over all the cores Lead sheath – To protect the paper insulation from foreign elements and

mechanical shock Bedding – Protects the lead sheath against corrosion. Consists of bitumen

compound and impregnated cotton tapes. Armour – Provides mechanical strength to the cable. Made of steel tape or

round galvanised wire Serving – protects the armour from corrosion. Made of Jute yarns coated with

bitumen compound

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Conductor Screening

Maximum electrical stress occurs at the surface of the conductors. In case of stranded conductors where the surface of the conductor is not smooth stresses will be very high. To eliminate this, semi conducting layer is applied on the conductor called Conductor screening

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Insulation Screen

Tangential forces sets up stresses along the layers of insulation which weakens them. To offset this, semi conducting material along with metal screen is used over the insulation which converts the tangential stress to radial stress.

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Screening

Tangential stress converted to radial stress

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Screening

PILC Kraft paper as conductor

screen Metallic screen over each

core. Copper woven fabric tape binds all the cores and is in contact with the lead sheath

XLPE Semi conducting layer as

conductor screen Semi conducting layer and

metallic screen

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Heavy due to lead sheathing Less flexibility Poor resistance to vibration Difficulty in sealing and jointing

Disadvantages of PILC

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Construction of XLPE cable

Conductor Semi-conducting Conductor

Screen Insulation Semi-conducting Insulation

Screen Metallic Screen Inner Sheath Armour Outer Sheath

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Relative Electric Properties

Characteristics PVC PILC XLPE

Max continuous conductor temp

60–70°C 65- 80°C 90°C

Max conductor temp on over load

95°C 100°C 130°C

Max conductor temp during short ckt

150°C 200°C 250°C

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Construction of 66 XLPE cable

Conductor Semi-conducting Conductor

Screen Insulation Semi-conducting Insulation

Screen Semi-conducting water blocking

tape Lead sheath Non conducting water blocking

tape Copper wire Outer Sheath

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ABC 11KV

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Minimum permissible Bending Radius

PILC PVC & XLPE

1C 3C 1C 3C

Upto 1.1kv 20D 15D 15D 12D

1.1kv to 11kv 20D 15D 15D 15D

Above 11kv 25D 20D 20D 15D

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edProper bending radius is necessary

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edBending radius is necessary to take care during fault repairs.Max no of faults occurred close to joints

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edBending radius is necessary to take care during fault repairs.

Abnormal Temp. 62.5 Deg C due to sharp bend/ Mech. Stress at joint

35.1 °C

61.5 °C

40

45

50

55

60

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Current Rating for HV and EHV cables

Cable type PILC XLPE

Size of cable Rated Current in Amp

Derated Current in

Amp

Rated Current in Amp

Derated Current in Amp

11kv 3x150 190 143 240 182

3x240 250 188 315 240

3x300 280 211 360 274

11kv 1X1000 585 445 685 550

33kv 3x300 290 215 352 264

3x400 332 249 402 301

66kv 1x630 560 450

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Current Rating for LT XLPE Cables

No. of cores Size Rated Current Derated Current

4 25 95 85

4 50 133 120

4 150 249 185

4 300 365 275

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Factors considered for Derating

Cable Depth Variation in Ground & Ambient Temp Thermal Soil Resistivity Spacing between cables Installation condition

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Temp rise detected at insulation semicon cut back, due to absence of stress pad

Infrared Thermography for Predictive Maintenance

Cables

3.4 °C

60.4 °C

20

40

60

sp1

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edPartial discharge at Screen edge

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Stress Control in Heat Shrink technique

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Stress control in Premoulded Design

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edInfrared Thermography for Predictive Maintenance

Stress at core crossing in 3 core Cable

28.3 °C

47.3 °C

30

32

34

36

38

40

42

44

46

sp1

sp2

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ed33KV O/D XL Cable Termination (WP2- A. Vihar) Date of Scanning: 23rd July 2007Time of Scanning: 12.30HRSAmbient Temperature: 35 CLoad: 220A Max. Temperature captured at R ph cable core 48.1 C

Air jacket formed between insulation sleeve & cable insulation due to improper shrinking of sleeve.

Dust particles deposited on inner side of the sleeve. During Termination installation, dust free zone was not created.

34.1 °C

50.7 °C

35

40

45

50

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Name of Grid: 33KV SGT NagarDate of Scanning: 26th May 2005Time of Scanning: 16.40HRSAmbient Temperature: 40 CDetails of Hot Spots observed during scanningLocation: Tr.2 Sp1:L.V Cable earth connection Max. Temp. 86.2°C Load: 170 Amps.Nature of problem: Hot spot at cable earth connectionInvestigation: Earth braid was bolted at Tr. and touching to the panel at other end.

Degradation of Insulation at the point of hot spot

Infrared Thermography for Predictive Maintenance

Single core cable

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ABC FailureName of Feeder: 11KV ABC L.B College – Satyawati Mkt. (KPM Zone – 509)Ambient Temperature: 31 CSp1: Max. Temp. 70.4°C Load: 90 Amps.Nature of problem: Corona discharge on 11KV ABC CableInvestigation: The metallic screens of ABC cable are not grounded at the end terminations.

-6.3 °C

73.5 °C

0

20

40

60

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Steps of fault location:- Testing:- To identify cable defects & types of faults.

Pre location:- To determine the distance of the fault.

Pinpointing:- To pinpoint the location of the cable fault

Cable tracing:- To trace the route of the cable.

Cable Identification:- To identify the required cable.

Fault Location in Cables

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Methods of Cable Testing:-Methods Instruments

Insulation Resistance Megger

Voltage Testing DC-voltage tester

AC-voltage tester (For test object with small capacitance)


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Insulation Resistance Measurement:-To Test of the ohmic resistance between two different

conductors (i.e. Phase to Earth & Phase to Phase)

For L.T Cables:- Phase to earth & Phase to phase to be tested with 1KV Megger for 1min./ or till reading stabilizes. Minimum IR value should be 50MΩ.

For 11KV Cables:- Phase to earth & Phase to phase to be tested with 2.5/5KV Megger for 1min./ or till reading stabilizes. Minimum IR value should be 50MΩ.


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Insulation Resistance Measurement:-For 33KV Cables:- Phase to earth & Phase to phase to

be tested with 2.5/5KV Megger for 1min./ or till reading stabilizes. Minimum IR value should be 100MΩ.

For 66KV Cables:- Phase to earth & Phase to phase to be tested with 2.5/5KV Megger for 1min./ or till reading stabilizes. Minimum IR value should be 500MΩ.


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Factor affecting Insulation Resistance :-Due to Moisture entering in the cableDue to deposit of dust/carbon particles at cable terminations.

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Voltage Testing:-Testing of the insulation for break-down or high leakage

current.

DC-voltage Tester:- The cable may be HI-POTTED only If IR value is found not satisfactory.

For LT cable:- Not applicable.

For 11KV Cable:- Each phase to be tested at 6.5KV for 5 min. with respect to ground and with other two phases grounded. The cable must withstand the test and leakage current should not exceed 2mA.


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DC-voltage Tester:-For 33KV Cables:- Each phase to be tested at 19.5KV

for 5 min. with respect to ground and with other two phases grounded. The cable must withstand the test and leakage current should not exceed 1mA.

For 66KV Cables:- Each phase to be tested at 38KV for 5 min. with respect to ground and with other two phases grounded. The cable must withstand the test and leakage current should not exceed 1mA.


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Effect of Space Charges During DC Testing in XLPE Insulation :-

XLPE insulation is a polymer, during manufacturing or cable laid with damaged outer sheath, voids occur in the insulation.

The negative test voltage injects electrons into the dielectric & creates space charges around the weak-spot in the cable insulation. This space charge stays in the insulation after the test voltage is shut off.


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Effect of moisture in the cable

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Pre location:-Arc Reflection Method:- A high voltage pulse is applied to draw an arc at the place of

fault, thus making the fault low-resistive for a short time. A test pulse is transmitted to this low-resistance fault and reflected by the fault.

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Pinpointing:-

Acoustic Method:- A pulse capacitor is charged with a high DC voltage and

discharged through a spark gap into the faulty cable.

A disruptive discharge will occur at the place of fault causing a noise which can be picked up by Digiphone. The volume of discharge noise is greatest directly above the cable fault.


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Cables

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