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Electricity Distribution

System

Jasjit Singh Jolly

EEE-7thsem

0311324908

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Distribution system layout

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The generation generally takes place at

voltages around 3.3kV to around 22kV which

is medium voltage.

The voltage is then stepped up to a level of

110kV or 220 kV (high voltage) or even 400kV

(extra high voltage) depending on the amount

of power to be transmitted .

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Saket D-block substation

Total no. of transformers: 2

Oil immersed step down delta/star transformers

Capacity : 1250kVA

Rated Primary Voltage : 11KVSecondary Voltage : 416/240V

Input HT lines: 11kV from Malviya Nagar grid

& 11kV from Lado Sarai

Integral part of the ring main distribution system

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RMU: Ring Main Unit

RMU panel generallyforms ring system inwhich distributor issupplied from more than1 feeders as input to RMUso that in case of failurefrom any one feeder,power can be feduninterruptedly fromother feeders at thesame point. Now a days itis of SF6 gas insulatedtype.

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Contd

RMU is used for HT side and it has a

total number of 3 switches

(Circuit Breakers or Isolators),

it is used for two inputsand one outgoing to the load or

one input with two outgoings.

Primarily used for feeding purposes .

Also protects secondary side transformer from the occasionaltransient currents.

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Electricity supply to consumers

The secondary winding of

the transformer is mostly

of wye connection type.

This supply system with a

neutral wire is termed as 4

wire 3 phase supply.

4 wire supplies are normally

used to distribute domestic

supplies since they canprovide an earthed neutral.

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Contd

The three phase wires

together give a 3-phase 3-wire

supply (240x1.73=415Volts)

suitable or heavy machinery

such as 3-phase motors.

Three single phase supplies

are available between each

phase line and the neutral wire.

The three phase single phase 415V supplies are available between

any of the three phases.

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Contd

For distribution to consumers either overhead or

underground lines are used.

The underground system is much expensive as

compared to overhead system and is only used inareas where safety and good appearance are of

prime importance.

For domestic users a phase and a neutral connection

is supplied whereas for those with heavy load

requirements 3 wire connection is supplied.

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Faults in distribution lines

Transient Faults: A transient fault is a fault thatis no longer present if power is disconnectedfor a short time. For eg - momentary tree or

bird contact. Persistent Faults: A persistent fault does not

disappear when power is disconnected. Faultsin underground power cables are oftenpersistent. These occur due to some externaldamage.

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Contd

Faults can further be classified as symmetric

or asymmetric faults.

Symmetric faults: Equal fault currents in line with

120 deg displacement. Rare in nature. Eg - (L-L-L)

or (L-L-L-G) faults.

Asymmetric faults: Unequal fault currents with

unequal phase displacement. Most likely to occur.Eg (L-L) or (L-L-G) or (L-G) faults.

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Fault detection

To detect the exact location of a high

resistance fault in underground transmission

lines Arc Reflection Method (ARM) is used.

LSG 300 surge generator manufactured by

sebaKMT is use to testing purposes.

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Contd

LSG 300 along with a surge

generator and TDR teleflex

helps in detection of thelocation of fault.

In a first step, a TDR Teleflex does a low voltage reflection measurement

of the cable, not seeing the high resistive fault. This picture is stored as

reference. In a second step the energy of a surge generator is released

into the faulty cable, passing through the arc stabilising filter LSG 300.

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Contd

This surge causes a flashover at

the weak spot in the cable.

Simultaneously, the

LSG 300 triggers the TDR Teleflex .

The TDR can now see the lowresistive arc at the weak spot as a

short-circuit. This picture is stored

as fault-picture and compared

with the previously measured

reference picture. The point of

separation of the two traces is the

fault location and the TDR Teleflex

will indicate the distance to the

fault.

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Thank You