cables - seminar (15-jul-08)

8/6/2019 Cables - Seminar (15-Jul-08)

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Construction of cable


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1. Core Number of conductors.

2. Insulation Impregnated paper.

3. Metallica sheath To avoid entering of moisture

.

4. Bedding To prevent the sheath from mechanical injury.

5. Armouring To prevent cable from mechanical injury.

6. Serving Layers of fibrous material similar to bedding

but named as serving.


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INSULATINGMATERIALS

Rubber better mechanical props., but easily affected by moisture.

Vulcanized India Rubber good mechanical strength but quickly

reacts with copper.

Impregnated Paper high insulation strength but hygroscopic.

Varnished Cambric called as empire tape and is a hygroscopic.

Polyvinylchloride high insulation resistance but low mechanicalproperties.


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Cross Linked polyethylene (XLPE):

An electrical cable insulation is provided for a submersible pump used in

oil and gas wells which is stable at moderately high temperatures, does

not swell or deteriorate when subjected to well fluids,

which can beobtained by using XLPE as insulating material.

Advantages:

Excellent corrosion resistance.

Reduces the potential of leaks.

Can balance the water pressure. Better flexibility.


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Parameters of cables:a. Physical Parameters.b. Fundamental Electrical parameters.

Physical parameters: O

utside diameter of inner conductor,d.

Inside diameter of shield,D.

Dielectric constant of the insulator, . The dielectric constant is oftenquoted as the relative dielectric constant r referred to the dielectricconstant of free space 0: = r0. When the insulator is a mixture ofdifferent dielectric materials (e.g., polyethylene foam is a mixture ofpolyethylene and air), then the term effective dielectric constant eff is

often used. Permeability of the insulator. Permeability is often quoted as the

relative permeability rreferred to the permeability of free space 0: =r0. The relative permeability of air will almost always be 1.


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Fundamental Electrical parameters:

Shunt Capacitance per unit length, in.

C=2/ln(D/d) in farads per metre.

Series Inductance per unit length,

L=0ln(D/d)/2 in Henrys per metre. Series Resistance per unit length, in ohms per metre. The resistance per

unit length is just the resistance of inner conductor and the shield at lowfrequencies.At higher frequencies, skin effect increases the effectiveresistance by confining the conduction to a thin layer of each conductor.

Shunt Conductance per unit length, in mhos per metre. The shuntconductance is usually very small because insulators with good dielectricproperties are used (a very low loss tangent).At high frequencies, adielectric can have a significant resistive loss.


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Selection of cablesCables are selected based on following parameters

System voltage.

Conditions of installation.

Current rating.

Short circuit level of the system.

Bending radius.

Current rating:

Current ratings are chosen based on normal laying conditions, such as

Maximum operating conductor temperature,Ambient airtemperature, Ground temperature, thermal resistivity of soil anddepth of laying.


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Short circuit Ratings:

Short circuit ratings can be calculated by using the following formula,

Ish = kA/tWhere,Ish = short circuit current in amps.

A = area of the conductor in sq mm.t = duration of short circuit in seconds.K = 75.8 and 109 for aluminum and copper cables, respectively.

Bending radius:

While installing the cables, they should not be bent at radius less

than 12*D, (D overall diameter of the cable).


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Classification:

a. POWER CABLES

Low Tension Cable High Tension Cable

Belted cables

Screened cables Up to 66 KV.

Pressure Cables Beyond 66 KV.


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Low Tension Cable:

up to 1 kv.

one core of tinned stranded copper.

serving layer made of fibrous material.

not armoured, so as to avoid excessive

Advantages:

Simple construction with large copper section.


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High Tension Cable

Cables used for Three phase service are called as high tension cables.

Belted Cables:

Suitable for 11 to 22 KV.

Cores are insulated from each other by impregnated paper and another layer of

impregnated paper tape i.e. paper belt, is wound around the grouped cores.

Gap is filled with fibrous insulating material.

Belt is covered by Lead sheath.

Disadvantages: Low insulation resistance.


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Screened Cables

H-Type cables S.L. Type cables

H-Type cables:Has 3-core, where each core insulated by impregnated paper that is

covered by aluminum foil.

Additional conduction belt is provided over the cores.

Advantages: Eliminates formation of air pockets or voids

Utilization of aluminum foil increase the heat dissipating power of thecable.


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S.L. Type cables: S.L. stands Separate lead sheath type cable.

Each core insulation is screened by separate lead sheath.

No overall lead sheath but only armouring and serving are provided.

Advantages: Separate sheath minimized the core-to-core break down in

effective manner.

Bending of cables becomes easy due to limitation of overallsheath.

Disadvantages: Thickness of these type of cables are very thin, so should be take more care

while manufacturing.


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Pressure CablesCables which are used beyond 66 kv are called as Pressure cables, and these are divided as two

types namely OIL FILLED CABLESand GAS PRESSURE CABELS.

Oil filled cables: Has channels of ducts for oil circulation.

Oil under pressure is kept constantly by means of external reservoirs, where the pressure

suppress the insulation layers and is forced into voids, which is formed between the layers.

So, these cables are suitable for higher voltages in the range between 66 230 kv.

Has three types like single core conductor channel, single core sheath channel and three

core filler space channels.


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Gas pressure cables:

Similar to solid type cables but the lead sheath is designed to have triangular shape

with thickness 75% greater than that of the solid type cables.

Triangular shape reduces the weight, provides low thermal resistance and allows

the sheath to act as pressure membrane.

Sheath is protected by a thin metal tape and laid in a gas tight steel pipe.

The pipe is filled with dry nitrogen gas whose pressure closes the voids formed

between the insulations in effective manner.

Advantages:

Can carry heavy load current and operate at higher voltages .

Low maintenance cost.


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INSTRUMENTATION CABLESCables used for transmitting, receiving and controlling signals are calledas instrumentation cables.

WHAT IS TWISTED PAIR CABLE?Twisted pair cabling is a form of wiring in which two conductors (twohalves of a single circuit) are wound together for the purposes ofcanceling out electromagnetic interference (EMI) from external sources.


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Classification of twisted pair cable

Screened unshielded twisted pair Shielded twisted pair

Unshielded twisted pair cable

shielded twisted pair:

When many cables collect behind equipment or travel together under floor tiles or through

cable trunking, other cables will cause interference and interrupt the data flowing through it.

This type of interference is known as Electromagnetic Interference (EMI).

Shielded Twisted-Pair (STP) cables use a thin layer of foil wrapped between the twisted pairs

and the outer sheath to prevent excessive EMI from interfering with the data.


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Screened unshielded twisted cable

Single foil or braided screen surrounding all four pairs in order to minimize EMI radiation

and susceptibility to outside noise.Screened twisted pair is also called Foil Twisted Pair

(F

TP), or Screened UTP (sUTP).


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Unshielded twisted cable

UTP is the most common form of twisted pair wiring, because it is less

expensive and easier to work with than STP (Shielded Twisted Pair).

UTP is used in Ethernet 10Base-T and 100Base-T networks, as well as in

home and office telephone wiring.

The twist in UTP helps to reduce crosstalk interference between wire

pairs.


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FIRE RESISTANT CABLES:

WHERE WE ARE USING?

WHY WE ARE USING?

WHAT IS THE CONSTRUCTIONOF THIS?


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cables - seminar (15-jul-08)

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