basic power distribution system007

Prepared By:

007- Patel Vaishnavee K.

Distribution system:

That part of power system which distributes electric

power for local use is known as distribution system.

In general, the distribution system is the electrical

system between the sub-station fed by the

transmission system and the consumers meters.

It generally consists of feeders, distributors and the

service mains.

Components of distribution system:

1.Feeder:

• A feeder is a conductor which connects the

sub-station (or localised generating station) to

the area where power is to be distributed.

Generally, no tappings are taken from the

feeder so that current in it remains the same

throughout. The main consideration in the

design of a feeder is the current carrying

capacity.

2.Distributor:

A distributor is a conductor from which

tappings are taken for supply to the

consumers. In Fig. , A B, BC, CD and DA are

the distributors. The current through a

distributor is not constant because tappings

are taken at various places along its length.

3.Service mains:

A service mains is generally a small cable

which connects the distributor to the

consumers’ terminals.

Classification of distribution system:

A distribution system may be classified

according to ;

1. Nature of current:

According to nature of current, distribution system

may be classified as,

• (A) d.c. distribution system

• (B) a.c. distribution system.

2. Type of construction:

• According to type of construction,

distribution system may be classified as,

• (a) overhead sys- tem

• (b) underground system

3. Scheme of connection:

• According to scheme of connec- tion, the

distribution system may be classified as,

• (a) radial system

• (b) ring main system

• (c) inter-connected system.

A.C Distribution system:

Now-a-days electrical energy is generated,

transmitted and distributed in the form of

alternating current.

A.C voltage can be step up or step down easily with

the help of transformer.

The a.c. distribution system is classified into

(i) primary distribution system and

(ii) secondary distribution system.

1.Primary distribution system(3 phase 3 wire system):

When the distribution is some what higher

then utilization level then the system is called

primary distribution.

The most commonly used primary

distribution voltages are 11 kV, 6·6 kV and 3·3

kV.

Primary distribution system:

• Due to economic considerations, primary distribution is carried out by 3- phase, 3-wire system.

Electric power from the generating station is

transmitted at high voltage to the substation

located in or near the city.

At this substation, voltage is stepped down to 11

kV with the help of step-down transformer.

Power is supplied to various substations for

distribution or to big consumers at this voltage.

This forms the high voltage distribution or

primary distribution.

2.Secondary distribution system(3 phase 4 wire system):

When the distribution voltage is at utilization

level then it is called secondary distribution.

The secondary distribution employs 400/230

V, 3-phase, 4-wire system.

Secondary distribution system:

The primary distribution circuit delivers power to

various substations, called distribution sub-

stations.

At each distribution substation, the voltage is

stepped down to 400 V and power is delivered by

3-phase,4-wire a.c. system.

The voltage between any two phases is 400 V

and between any phase and neutral is 230V.

D.C Distribution system:

For certain applications, d.c. supply is absolutely

necessary.

For this pur- pose, a.c. power is converted into d.c.

power at the substation by using converting

machinery e.g. rectifiers.

The d.c. supply from the substa- tion may be obtained in

the form of (i) 2-wire or (ii) 3-wire for distribution.

1.2-wire d.c. System:

As the name implies, this system of

distribution consists of two wires.

One is the outgoing or positive wire and the

other is the return or negative wire.

The loads such as lamps, motors etc. are

connected in parallel between the two wires.

2-wire d.c system:

This system is never used for transmission purposes due to low efficiency but may be employed for distribution of d.c. power.

2.3-wire d.c system:

It consists of two outers and a middle or neutral wire

which is earthed at the substation.

The voltage between the outers is twice the voltage

between either outer and neutral wire.

V between any outer and the neutral and 2V

between the outers. Loads requiring high voltage

(e.g., motors) are connected across the outers.

3-wire d.c system:

whereas lamps and heating circuits requiring less voltage are connected between either outer and the neutral.

Connection schemes of distribution system:

According to method of connections

distribution system is classified as :

a) Radial system

b) Ring main system

c) Interconnected system

(a) Radial system:

In this system separate feeders radiates from the

substation to each area and it feeds the

distributor at one end only.

This is the simplest design of distribution system

and has the lowest initial cost.

This system is used where power transferred is

low and the distance between substation and

load is low.

LESS RELIABILITY:

As the consumers are fed by single radial

feeder only , therefore fault on the feeder or

distributor causes complete shut down of the

consumers who are on other side of fault.

Owing to above disadvantage this system is

used only for short distances.

(b) Ring main system:

This consist of interconnection of primary of

different distribution transformers which

form a loop through the area to be supplied.

The feeder is closed on itself.

The feeder SMOQS forms a complete ring.

Ring main system:

The advantages of such arrangement is that:

It offers a greater reliability of supply.

In the event of a fault on any section of the

feeder, say at “F” the supply to all consumers can

continue to be available by isolating the faulty

section between ‘S’ & ‘M’.

There are less voltage fluctuations at consumer’s

terminal.

(c) Interconnected system:

When the feeder ring is energized by two or

more than two substations then it is called as

interconnected system.

Fig. Shows the single line diagram of

interconnected system where the closed feeder

ring DPCOBRAQD is supplied by substation s1 &

s2 at D & C.

Interconnected system:

It has following advantages:

It offers greater reliability of supply.

It reduces reserve power capacity & increases

efficiency.

Requirements of good distribution system:

There are certain requirements of distribution

system to provide the continuous electrical

energy to consumer with minimum

fluctuations.

These requirements are as follows:

1. LOW VOLTAGE FLUCTUATIONS:

• The variations of consumer’s terminal

voltage should be a low as possible.

• This variation should be within +/-6 of the

rated voltage at consumer’s terminal.

• If the voltage variations goes beyond this

limit, certain equipments may damage.

2. AVAILABILITY OF POWER ON DEMAND:

• An electrical energy cannot be stored, therefore

it should be supplied whenever there is required

by the consumer.

• To maintain the continuity and to fulfil the

requirement of electrical power, previous load

curve should be studied by supply authorities

and should predict the future demand.

3. RELIABILITY:

Reliability is very important factor; because

everywhere electrical energy is used such as

industries, commercial complexes; irrigation

purpose, domestic appliances etc.

Therefore supply authority should provide

electrical power with good quality and

continuously.

4. STABILITY:

• Fault on the neighbouring system should not

affect the stability of distribution system.

• Stability and reliability can be improved by

interconnected system, automatic switch

gear system etc.

Design considerations:

Design of feeders and distributors requires

careful consideration to fulfil above

requirements.

Design consideration for feeder:

1. Current carrying capacity:

• While designing the feeder, current carrying

capacity is the most important factor.

The current carrying capacity indicates the

cross sectional area required by the

conductor.

2. Type of conductor:

• The type of material depends on the service

provided.

• Generally in urban & suburban areas, A.C.S.R

conductor is used as a feeder.

3. Distance :

• While designing of feeder distance plays an

important role because it suggests the total

voltage drop in the feeder by which voltage

drop in the feeder can be compensated by

means of voltage regulating equipment at

substation.

4. Cost :

• Cost plays an important role to choose the type

of construction i.e. Overhead or underground.

• Generally in the city areas where safety is

important criteria, underground cables are used

as feeder.

• In rural areas as the cost is main criteria,

Therefore overhead conductor is used as feeder.