FAULTS IN A OIL IMPREGNATED

PAPER CABLES AND EXTRUDED

CABLES

Introduction

General Constructions of Cable.

Classification of Cables

Properties of Insulating Material

Types of materials used in insulation

Types of cable faults

Oil impregnated paper insulated cables

Extruded (XLPE)cables

Faults in OIPC and XLPE

Conclusion

References

A power cable is an assembly of two or

more electrical conductors, usually held together with

an overall sheath. The assembly is used for

transmission of electrical power. Power cables may be

installed as permanent wiring within buildings, buried

in the ground, run overhead, or exposed.

Flexible power cables are used for portable devices,

mobile tools and machinery.

Core or ConductorA cable may have one or more than one core depending upon the type of service for which it is intended.

InsulationThe core is provided with suitable thickness of insulation, depending upon the voltage to be withstood by the cable.

Metallic SheathA metallic sheath of lead or aluminium is provided over the insulation to protect the cable from moisture, gases or othes damaging liquids

Core

Belted paper

Lead sheath

Bedding

Single wire

armoring

Overall Serving

BeddingBedding is provided to protect the metallic sheath from corrosion and from mechanical damage due to armoring

ArmouringIts purpose is to protect the cable from mechanical injury while laying it or during the course of handling. It consists of one or two layers of galvanized steel wire or steel tape.

ServingTo protect armouring from atmospheric conditions, a layer of fibrous material is provided.

Properties of Insulating Material

The insulating materials used in cables should have the

following properties

High resistivity.

High dielectric strength.

Low thermal co-efficient.

Low water absorption.

Low permittivity.

Non – inflammable.

Chemical stability.

High mechanical strength.

High viscosity at impregnation temperature.

Capability to with stand high rupturing voltage.

High tensile strength and plasticity.

TYPES OF MATERIALS USED IN INSULATION

Vulcanized India rubber

Impregnated paper

Silk and cotton

rubber

Enamel insulation

Varnished cambric

Polyvinyl chloride

1. Low tension cable

2. Belted cable

3. Screened or H type cable

4. SL type cable (separate lead sheath cable)

5. HSL type cable (H+SL)

6.Super tension cable

(a) Oil field cable

(b) Gas pressure cable

1. Paper insulated Cable (PIC)

2. PVC Cable (Polyvinyl Chloride)

3. Oil filled paper insulated Cable

4. High Pressure oil filled Cable

5. Compressed gas insulated cable

6. Vulcanized rubber Cable

7. XLPE insulated Cable (Cross linked polyethylene

insulated)

The following are the faults most likely to occur in Power cables:

A.Fault Conductor - conductor (parallel Fault)

• Connection between two or more

conductors.

• When two conductors of a multi-core cable

come in electrical contact with each other

due to insulation failure

TYPES OF CABLE FAULTS

B. Flashing fault (parallel Fault)

• Very high resistance fault.

• The flashover happens typically at some kV

and is very often located in Joints.

• The insulation resistance of this fault is

typically infinite up to the breakdown voltage.

C. Fault Conductor - shield (parallel Fault)

• Connection between Conductor and shield or

Conductor/Conductor and shield.

• Experience has shown, that most faults are in

this category

D. Serial fault (Open, Interrupt)

• Faults of this type can be very high resistive

up to infinite (complete cut).

• The reason for this being a complete cut of the

cable, or it is pulled out of the joint , which

interrupts everything, but also permits

flashovers in all possible variations.

E. Earth faults, sheath faults

• Faults between the metallic shield and

surrounding soil in case of plastic insulated

cables.

• Especially for these type of faults the highest

precaution must be taken when using high

voltage, this is of utmost importance, since the

voltage discharges directly to earth.

F. Humid / wet faults

• Humidity faults are the most difficult faults to

locate.

• Depending on the cable construction (e.g.

longitudinal water sealing) these faults can be

punctual or widespread throughout the cable.

• Other forms of humidity faults are underwater

faults. Here the water pressure prevents an

effective ignition of the fault during the HV

application.

Paper or paper propylene

laminated (PPL) insulated with

individual metal sheaths and

impregnated with low pressure oil

It is estimated that about 83% of HV

cables in existing systems are paper

insulated [1].

At higher voltages they present

operational problems due to high

dielectric losses and presence of

voids.

• withstand an electric stress about 5 to 10 times higher than dry paper insulation.

• Greater working temperature • no voids present as the oil is present between the layers of the

paper which forms the insulation.• Smaller size of cables due to reduced dielectric thickness

• At higher voltages they present operational problems due to high

dielectric losses

• Risk of oil leakage or fire

• Presence of void

Disadvantages:

Paper insulation is still common, especially for land UG cables.

However, there is an increasing application of polymer cables

(XLPE),especially for in DC and submarine installations.

In polymer insulation the insulating XLPE compounds must act as

thermoplastic materials, be immune to thermal degradation and

must not be prone to defects such as voids ,contaminants [1].

XLPE cables constitute about 2 - 5% of installed HV cable

capacity in the 115-161 kV range, and make up about 50 -

70% of new cable installations in the HV range(220KV and

more) [1].

• No risk of oil release

• Excellent reliability performance

• Reduced operating costs

• lower capacitance of the XLPE cables offers numerous technical

and economical benefits

• insulation failures are much more difficult to monitor and detect.

• The diameter of the XLPE cables increase with voltage

Disadvantages:

Partial discharges (PDs) Partial discharges occur in voids or at protrusions that

occur in the insulation due to manufacturing systems flaws

and contamination.

Electrical trees Protrusions and contaminants can

also cause localized high electric

stresses which lead to discharges

Insulation failure

Influence of moisture Increased dielectric losses, increased conductivity and

lowered breakdown strength are instant consequences of

moisture in the cable.

moisture causes faster decomposition of cellulose .

Influence of temperatureRise in temperature also increases the dielectric loses

and deformation of the cable itself

Increased temperature also has negative impacts as

the lead or aluminum sheath can be stretched

permanently by the pressure of the expanding oil

Thermoelectric ageing

Environmental considerations are forcing more high power cable transmissions.

both high operating temperatures and electric stresses have an impact on the life time of the cable

Design considerations

The factors(e.g. operating temp., insulation parameter, length of operation) need to be taken into account at the design stage.

Test data show that electric stress has a big influence on the ageing rate

As per the requirement, the cables need to be chosen

for suitable application and optimum parameters are

kept for its type and size.

Modern underground power cables are sophisticated

assemblies of insulators, conductors and protective

materials. Within these components are sensors, which

enable cable operators to monitor conditions along

the cable in real time.

The condition of the cable insulation is usually

monitored through the following two main methods:

a) Loss tangent measurements

b) Partial discharge (PD) measurements

1. A Haddad and D Warne (Editors), Advances in High Voltage Engineering, IEE Power Energy Series 40, IEE Publishers, London ISBN 0852961588, Chapter 10, 2000.

2. High voltage cable insulation systems by Prof. Nelson Ljumba, University of KZN

3. Ch-5, pg 134 “High Voltage Engineering” - J R Lucas, 2001

4. Online Incipient Detection and Localization for Low Voltage Faults in Oil Impregnated Paper Cables door: N .G . van Luijk EPS .oZ.A.i65 July 2002

5. Ch-7, Pg 123 “Electrical power system”- AshfaqHusain , 5th Edition, 2010