lt distribution in cesc ltd

7/25/2019 LT Distribution in CESC LTD.

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CABLES

Cables from the artery system for the transmission and distribution of Electrical energy. The

residential and industrial loads today have a trend towards their growing density. This

requires rugged construction, greater service reliability, increase safety and better

appearance. This interferences from external disturbances like storms, lighting, ice, trees

etc. should be reduced to minimum. These difficulties are easily overcome by the use of

underground cables and a trouble free service is achieved under a variety of environmental

condition.

Earlier underground cables wire mainly mainly used in or near densely polluted areas and

were operated al low or medium voltages only, but the present day requirements seek to

use them even at extra high voltages for longer distances. The underground system

although more costly as compared with the overhead system for the same voltage, is more

acceptable to public from the point of view of its merits mentioned above.

POWER CABLES

Increased working voltages of the overhead lines require the cables to be insulated for such

voltages in order to meet the requirements of the overhead line.

The possibility of supply interruption due to lighting or other external influences is lesser

with underground cables, but if a fault occurs due to any reason it not easily locate. For

long distance transmission, cables can not be used due to their large charging currents.

Components and Construction of Power Cables

Many types of modern electrical cables have been in use for underground distribution and

transmission of power. They vary greatly in their designed constructional techniques.

However, all power cables consists of three essential components.


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CABLE CROSS SECTIONS

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1. The metallic conductor which provides and electrical conducting path.

2.

The insulation of a cable which prevents direct contact and dangerous proximitybetween energized conductor and other objects.

3. the external protection preventing the ingress of moisture, mechanical damage,

chemical or electrochemical attack, fire or any other harmful influences which are

detrimental to the cable itself.

Synthetic Dielectrics

The introduction of synthetic dielectrics instead of impregnated paper gained popularity as :

1.

The elimination of expansive and rather heavy metallic sheaths due to the non-hygroscopic nature of the synthetic insulants.

2. The simplification of cable jointing and termination as well as repair work coupled

with a saving in the cost of labour.

3. Complete freedom from compound drainage troubles, especially in cable

terminations. In the recent years, may synthetic or plastic insulants were employed

in power cable manufacture, of which the following four dielectrics are most

common:a)

Polyvinyl Chloride (PVC)

b) Polyethylene (PE) or Polythene

c)

Crossed linked Polyethylene (XLPE)

d)

Ethylene Propylene Rubber (EPR)

LT CABLE JOINTING

There are three types of cable jointing done.

A). Straight Through Jointing

Here crimping is done when jointing Aluminium conductors MAP OF THE AREA

However, solder basting is also done when Cu & Al conductors are joined

Crimping of Conductors:-

Selection of the Correct Cable Size for Service

The size of cable is selected on basis of load requisitioned by consumer.

The selection of the Cutout to be installed is also dependent on cable size and load.


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Straight Jointing Cable

Tee Joint Cable


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NEW SERVICE INSTALLATION

The procedure of service installation can be divided in to five subgroups namely:

A) Inspection for service;

B)

Laying of service cable;

C)

Fixing of meter board;

D) Termination of cable at meter board; and

E) Tee jointing of cable.

A)Inspection for Service

In Areas Served By Overhead Network:

Selection between Overhead or Underground mode of supply is done weighing both

economic and technical parameters.

In O.H. area if consumer insists on U.G. supply then whole cable is chargable.

If width of road is less than 3.0 meters, then U.G. service is given.

If load requisition is more than 15 kW, then U.G. service is provided.

General Requirement:

If consumers load requirement is more than 40 kW, then dedicated service through

CT metering is done.

A clear space 80 cm wide should be available in front for installation up to 4 way

meter board and 1.2 meters for multiway meter boards.

Consumers Main Switch must be within 3 feet from our meter board and always in

the same room. Consumer has to be informed to make meter board wall 10 thick, if not done.

Length of cable required in MP, PCP, and PP chould be clearly defined.

Cable route is kept as short as possible and free from obstructions.


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B) Laying of Service Cable

Applicants details from paid bill and job slip must be verified.

he meter board position and wall for fixing board must be verified.

Depth of Laying cable is 75 cm. Cable should be laid inside pipes whenever crossing any installation of other agencies and to

be adequately protected.

Precut length of cable is laid in trench prepared and within pipes laid for this purpose.

The service cable should contain a loop in ground below meter board.

This cable must enter meter board through a 75 mm dia PVC pipe 1.0 meter long placed

vertically below board.

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C) Meter Board Installation

The Main Switch of Switches must be identified.

The consumers installation/wiring must be verified for completion.

The Main Switch must be in Off position.

A 2 way/4 way Meter board is installed on wall by 4 Nos.4xNo.14 Screws.

The board must be at least 1.0 meter vertically above floor level.

Earthing Brackets must be provided to all meter boards.

In 2 way Meter Boards the Earth Terminal is prefixed.

A connection from our earth is provided to meter board Earth Bracket for consumers use.

If requirement is more than a 4 way board size, same is fabricated at site by fitting extra 2/4

way board as necessary.

Asbestos millboards are fixed over the rear wall for fire protection.

These boards must have doors and locking arrangements.

Steel meter board is installed with CT Metering arrangement for load above 40 kW.

D) Termination of Cable at 2 way Meter Board with 25mm2 cable:


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MAIN JOBS OF LT DEPARTMENT

o Providing new UG (AC) services to LT consumers

o Shifting and strengthening of existing connection

o Fixing and providing energy meters

o Exchanging with energy meters of better specification

o with increase in load.

o Conversion of overhead service to underground service.

o Detection of underground fault

o Quick restoration of faults

o Installation of new pillar boxes.

o

Installation, replacement and repair of distribution

o Transformer and

o Servicing and maintenance

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REQUIRED EQUIPMENT FOR DISTRIBUTION

STATION

TRANSFORMER

For transmitting a bulk power over a distance, it is desirable to have high voltage

in order to decrease IR loses in the conductor. On the other had a


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Type of insulationliquid immersed or dry type

Number of Phases - single phase or three phase

Voltage class

Basic impulse insulation level (BIL)

Parts of Distribution Transformer

Nameplateof transformer. It has details like year of manufacture, kava rating (e.g. 400) HV

and LV volts at no load, current rating for HV and LV sides, phases, frequency, max

temperature rise in oil, winding material (copper), amount of oil, total weight, impedence on

6/11 kV, details of external CT as well as the phasor diagram for better understanding. Also

informs us whether it is dual ratio, tap changing transfer.

LT box (420V): The phase sequence is N,R,B, Y from the right. The parts are bush bearing at

the top followed by the top flexible (made of copper), connected to the currenttransformers in each phase except neutral. There are fuses in each phase after that,

followed by two cables from each phase including neutral, which terminate at the bottom of

the LT box and finally join the feeder cables in the pillar box.

HT box (6000/11000 V): Has the phase sequence RYB from the right .

Pillar box: The pillar box has a name, and consists of six unit with bus bars horizontal to

these units. There are also two types of earthing done, namely neutral earth and body earth.

Each unit of the pillar box is connected to an ACPB box or else merely linked without being

energized for future emergencies.

There are different types of pillar boxes:

FEB (Feeder pillar box)

ACPB (AC pillar box)

LPB (Link Pillar Box)

MPB (Meter Pillar Box)

Type of Box: There are two typeof box used in our system

a). 6 Way

This is generally installed as Feeder Pillar in conjunction with distribution transformer.

D/S Transformer


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A/C PILLER BOX

Here six nos. Of cables may be terminated.

B). 4 Way:

This is generally installed for terminating distributor cables cables and also for achieving network

flexibility.

Here four nos. of cables may be terminated.

Every Pillar Box has two openings;

A)

The cable side where the cables are terminated

This is usually towards the road and preferably not less than 300 mm from kerb.

B) The fuse side where the fuses are inserted.

This is usually towards the building.

Feeder cables: These feed power from the LT Box to the pillar box and have 400 mm2 cross

sectional area, made of XLPE (crossed linked poly ethylene).

Ring Main Unit (RMU): this unit consists of three legs usually, and serves the pusposes of

connection, supply and protection of one or more transformers on an open ring network ( A

ring network is a network topology in which each node connects to exactly two other nodes,

forming a single continuous path way for signals through each node a ring). RMU has in

built fault indicators ammeters, and gas pressure inductors. RMU cab be used to energy or

earth any particular line.


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lt distribution in cesc ltd

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