Underground Cables Presented By:

Sanjeet Kumar Registration No.-1308143

Branch:-Electrical & Electronics Engineering

Introduction• The basic requirements of any underground cable to survive the

expected life period and needs. Different types of cables in used in telecommunication networks and the parameters offered for various utilities are discussed below.

 • An underground cable essentially consists of one or more

conductors covered with suitable insulation and surrounded by a protecting cover. The interference from external disturbances like storms, lightening, ice, trees etc. should be reduced to achieve trouble free service. The cables may be buried directly in the ground, or may be installed in ducts buried in the ground.

CABLE STRUCTURE

Advantages & DisadvantagesAdvantages

Better general appearance

No corona discharge, RI and TVI No bush fire problems Minimal lightning problems

Less chances of faults

Small voltage drops

Disadvantages

Maintenance cost is high Fault location instantaneous, can have longer repair time

Construction of Cables• Core or Conductor

A cable may have one or more than one core depending upon the type of service for which it is intended. The conductor could be of aluminum or copper and is stranded in order to provide flexibility to the cable.

• InsulationThe core is provided with suitable thickness of insulation, depending upon the voltage to be withstood by the cable.The commonly used material for insulation are impregnated paper, varnished cambric or rubber mineral compound.

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

• BeddingBedding is provided to protect the metallic sheath from corrosion and from mechanical damage due to armoring. It is a fibrous material like jute or hessian tape.

• 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 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.

Insulating Materials for Cables

• RubberIt can be obtained from milky sap of tropical trees or from oil

products.It has the dielectric strength of 30 KV/mm.Insulation resistivity of 10 exp 17 ohm.cmRelative permittivity varying between 2 and 3.They readily absorbs moisture, soft and liable to damage due to

rough handling and ages when exposed to light.Maximum safe temperature is very low about 38 C

• Vulcanized India RubberIt can be obtained from mixing pure rubber with mineral compounds i-e

zinc oxide, red lead and sulphur and heated upto 150 C.

It has greater mechanical strength, durability and wear resistant property.

The sulphur reacts quickly with copper so tinned copper conductors are

used.

It is suitable for low and moderate voltage cables.

• Impregnated Paper This material has superseded the rubber, consists of chemically pulped

paper impregnated with napthenic and paraffinic materials. It has low cost, low capacitance, high dielectric strength and high

insulation resistance. The only disadvantage is the paper is hygroscopic, for this reason paper

insulation is always provided protective covering. • Varnished Cambric

This is simply the cotton cloth impregnated and coated with varnish. As the varnish cambric is also hygroscopic so need some protection. Its dielectric strength is about 4KV / mm and permittivity is 2.5 to 3.8.

• Polyvinyl chloride (PVC) This material has good dielectric strength, high insulation resistance and high melting

temperatures. These have not so good mechanical properties as those of rubber. It is inert to oxygen and almost inert to many alkalis and acids.

• XLPE Cables (Cross Linked Poly-ethene) This material has temperature range beyond 250 – 300 C

This material gives good insulating properties

It is light in weight, small overall dimensions, low dielectric constant and high

mechanical strength, low water absorption.

These cables permit conductor temperature of 90 C and 250 C under normal and short

circuit conditions.

These cables are suitable up to voltages of 33 KV.

XLPE cable

CLSSIFICATION OF CABLES

• Low tension (L.T) ----- up to 1000V

• High tension (H.T) ----- up to 11, 000V

• Super tension (S.T) ---- from 22KV to 33KV

• Extra high tension (E.H.T) cables --- from 33KV to

66KV

• Extra super voltage cables ------beyond 132KV

Extra High Tension Cable

Low Tension Cable

3- Core CablesBelted Cables

In these cables the conductors are wrapped with oil impregnated paper, and then cores are assembled with filler material. The assembly is enclosed by paper insulating belt.

These can be used for voltages up to 11KV or in some cases can be used up to 22KV.

High voltages beyond 22KV, the tangential stresses becomes an important consideration.

As the insulation resistance of paper is quite small along the layer, therefore tangential stress set up, hence, leakage current along the layer of the paper insulation.

This leakage current causes local heating, resulting breaking of insulation at any moment

3-core belted Cable

• Screened Cables• These can be used up to 33kv but in certain

cases can be extended up to 66kv.• These are mainly of two typesH-type and S.L type cables

3- Core Cables (H-Type)

• Designed by H. Hochstadter. • Each core is insulated by layer of impregnated paper. • The insulation on each core is covered with a metallic screen which

is usually of perforated aluminum foil.• The cores are laid in such a way that metallic screen make contact

with one another.• Basic advantage of H-TYPE is that the perforation in the metallic

screen assists in the complete impregnation of the cable with the compound and thus the possibility of air pockets or voids in the dielectric is eliminated.

• The metallic screen increase the heat dissipation power of the cable.

S.L - Type: (Separate Lead)

• Each core insulation is covered by its own lead sheath.

• It has two main advantages, firstly the separate sheath minimize the possibility of core-to-core breakdown. Secondly the, bending of cables become easy due to the elimination of over all sheath.

• The disadvantage is that the lead sheaths of S.L is much thinner as compared to H-Type cables, therefore for greater care is required in manufacturing.

• Pressurized Type Cables• In these cables, pressure is maintained above

atmosphere either by oil or by gas.• Gas pressure cables are used up to 275KV.• Oil filled cables are used up to 500KV.

3- Core Cables (Gas pressure)

Oil Filled Cables

• Low viscosity oil is kept under pressure and fills the voids in oil impregnated paper under all conditions of varying load.

• There are three main types of oil filled cablesa. Self-contained circular typeb. Self-contained flat typec. Pipe Type cables

3- Core Cables (Oil filled)

Advantages of Oil Filled Cables

• Greater operating dielectric stresses• Greater working temperature and current carrying

capacity• Better impregnation• Impregnation is possible after sheath• No void formation• Smaller size of cable due to reduced dielectric

thickness• Defect can easily be detected by oil leakage

Gas Pressure CablesIn these cables an inert gas like nitrogen is used to exert pressure on paper dielectric to prevent void formation.These are also termed as Compression cablesThey insulated cores similar to solid typeThe cable is inserted in a pressure vessel which may be a

rigid steel pipe, commonly known as pipe line compression cable.

The nitrogen gas is filled in vessel at nominal pressure of 1.38 * 10 exp 6 N/ square meter with a maximum pressure of 1.725 * 10 exp 6 N/ square meter.

Gas Insulated Cables (GIC)• In GIC cables high pressure sulphur hexaflouride

(SF6), fills the small spaces in oil impregnated paper insulation and suppresses the ionization.

• Most EHV and UHV lines insulated with sulphur hexaflouride (SF6) gas are being used extensively for voltages above 132 KV up to 1200 KV.

• These cables are very popular for short lengths, river crossings and high way crossings.

Advantages of GIC

Gas Insulated Cables have several advantages over oil filled cables,

• Efficient heat transfer hence can carry more current.

• Low dielectric loss and low capacitance• SF6 gas is non-toxic, chemically stable and non-

inflamable.• Terminations of GIC cables are simpler and

cheaper.

Laying of Underground Cables

• The reliability of underground cable network depends to a considerable extent upon proper laying.

• There are three main methods of Laying underground cables

a. Direct Layingb. Draw in systemc. Solid system

Direct Laying• This method is cheap and simple and is most likely to

be used in practice.• A trench of about 1.5 meters deep and 45 cm wide is

dug.• A cable is been laid inside the trench and is covered

with concrete material or bricks in order to protect it from mechanical injury.

• This gives the best heat dissipating conditions beneath the earth.

• It is clean and safe method

Direct Laying

Disadvantages of Direct Laying

• Localization of fault is difficult• It can be costlier in congested areas where

excavation is expensive and inconvenient.• The maintenance cost is high

Grading of Cables

• Since the stresses are maximum at surface of the conductor or inner most part of the dielectric.

• The stress goes on decreasing as outer most layer is reached.

• Since the process of achieving the uniform electrostatic stresses on the dielectric of cables is known as Grading of cables

• The unequal distribution of stresses is undesirable because,

• if dielectric is chosen according to maximum stress the thickness of cable increases or either this may lead to breakdown of insulation.

• The following are the two main methods of gradingCapacitance grading Inter sheath grading

TYPES OF CABLE FAULTS

Cables are generally laid in the ground or in ducts in the underground distribution system. For this reason, there are little chances of faults in underground cables. However, if a fault does occur it is difficult to locate and repair the fault because conductors are not visible. Nevertheless, the following are the faults most likely to occur in underground cables

1) open circuit fault2) short circuit fault3)earth fault

OPEN CIRCUIT FAULTS

• When there is a break in the conductor of a cable, it is called open circuit fault.

• The open circuit fault can be checked by megger. For this purpose, the three conductors of the 3-core cable at the far end are shorted and earthed.

• The resistance between each conductor and earth is measured by a megger and it will indicate zero resistance in the circuit of the conductor that is not broken.

• However, if the conductor is broken, the megger will indicate infinite resistance in its circuit

SHORT CIRCUIT FAULTS

• When two conductors of a multi-core cable come in electrical contact with each other due to insulation failure, it is called a short circuit fault.

• Again, we can seek the help of a megger to check this fault.• For this purpose the two terminals of the megger are

connected to any two conductors. • If the megger gives zero reading, it indicates short circuit

fault between these conductors.• The same steps is repeated for other conductors taking two

a time.

EARTH FAULTS• When the conductor of a cable comes in contact

with earth, it is called earth fault or ground fault.• To identify this fault, one terminal of the megger

is connected to the conductor and the other terminal connected to earth.

• If the megger indicates zero reading, it means the conductor is earthed. The same procedure is repeated for other conductors of the cable.

Physical Limitations of Underground Lines

The main argument against constructing underground systems is usually financial. But costs are not the only limitation.The laws of physics limit how physically long a power line can be.These limits are relatively unimportant on overhead lines but will severely limit high voltage underground cable systems The higher the voltage the shorter the line length must be. The limiting effects become very important at transmission

voltages, especially 100,000 Volts and above. Limiting effects may also be important for subtransmission

voltages, 69,000 Volts and 35,000 Volts.

Physical Limitations: The Effect of Capacitance

o Capacitance causes current to flow even when no load is connected to the cable. This is called “line charging current”.

o Underground line capacitance for power cables is far higher than overhead line capacitance.o Wires are closer to each othero Wires are closer to the earth (within a few inches).

o Underground lines have 20-75 times the line charging current that an overhead line has (depending on line voltage).

o If a line is long enough the charging current could be equal to the total amount of current the line can carry. This will severely limit its ability to deliver power.

Summary of Costs: Overhead vs. Underground

• Transmission: Underground may be 4-20 times Overhead.

• Sub transmission: Underground may be 4-20 times Overhead

• Distribution: Underground may be 2-10 times Overhead

• New underground may be cheaper than overhead in special conditions and costs vary greatly from utility to utility and place to place.

Presented By:Sanjeet Kumarsanjeetkumar2101@gmail.com +91-8968143473