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Energy is the basic necessity for the economic development of a country.

The availability of huge amount of energy in modern times has resulted in a shorter working day. higher agricultural and industrial production, a healthier and more balanced diet and better transportation facility.

Energy exists in different forms but electrical energy is the most important form of energy.

The bulk electric power is produced by generating stations or power plants and it employs a prime mover coupled with alternator for production of electric power

The modern society is so much dependent upon the use of electrical energy that it has become a part and parcel of our life.

The greater per capita consumption of energy in a country, the higher is the standard of living of its people.

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1. The function of an electric power system is to connect the power station to the consumer's load by means of interconnected system of transmission and distribution networks.
2. Therefore, an electric power system consists of 3 principle components:

Power Station

Transmission Lines

Distribution System

3. The power station are located at favorable places, generally quite far away from the consumers
4. The generation voltage is not very high; it is usually 11kV to 25kV.
5. The voltage at power station is stepped to high value (say 220kV) by step-up transformer.
6. The electric power is carried by the transmission lines at this high voltage.
7. At the outskirts of the city, this voltage is reduced considerably
(say 11kV) by step-down transformer.
8. The feeders carry the power to distribution sub-station.

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1. That part of power system which distributes electric power for local use is known as distribution system.
2. It generally consists of feeders, distributors and the service mains.
(i) FEEDERS :

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.

(ii) DISTRIBUTORS :

A distributor is a conductor from which tappings are taken for supply to the consumers.

(iii) SERVICE MAINS :

It is generally a small cable which connects the distributor to the consumers’ terminals.

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Classification of Distribution System (part1)

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Classification of Distribution System (part1)

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According to nature of current, distribution system may be classified as :
(a) d.c. distribution system
(b) a.c. distribution system
A.C. Distribution:

Alternating current is preferred to direct current, the fact is that alternating voltage can be conveniently changed by means of a transformer.

The a.c. distribution system is classified into
(i) primary distribution system
(ii) secondary distribution system

(1) PRIMARY DISTRIBUTION :

It is carried out by 3-phase, 3-wire system

The most commonly used primary distribution voltages are 11 kV, 6·6kV and 3·3 kV

Electric power from the generating station is transmitted at high voltage to the substation located in or near the city.

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Classification of Distribution System (part2)

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Classification of Distribution System (part2)

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SECONDARY DISTRIBUTION SYSTEM

It is that part of a.c. distribution system employs 400/230 V, 3-phase, 4-wire system.

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 230 V.

The single phase domestic loads are connected between any one phase and the neutral.

Motor loads are connected across 3-phase lines directly.

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EHV AC stands for Extra High Voltage Alternating Current

Advantages of EHV AC :

Reduction in the current.

Reduction in the losses.

Reduction in volume of conductor material required.

Decrease in voltage drop & improvement of voltage regulation.

Increase in Transmission Efficiency.

Increased power handling capacity.

The no. of circuits & the land requirement reduces as transmission voltage increases.

The total line cost per MW per km decreases considerably with the increase in line voltage.

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HVDC stands for High Voltage Direct Current

Advantages of HVDC :

HVDC systems are economical for long distance bulk transmission by overhead line (reduced tower cost)

There is greater power per conductor and simpler line construction.

Ground return is possible.

There is no charging current and skin effect.

Each conductor can be operated as an independent circuit.

Line losses are smaller

No reactive compensation of DC line is required.

Corona loss, radio interference and audible emissions are less as compared to AC.

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Classification of Distribution System (part 4)

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Classification of Distribution System (part 4)

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3 WIRE DC 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.

The principal advantage of this system is that it makes available two voltages at the consumer terminals.

Loads requiring high voltage (e.g., motors) are connected across the outers, whereas lamps and heating circuits requiring less voltage are connected between either outer and the neutral.

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Corona Discharge (also known as the Corona Effect) is an electrical discharge caused by the ionization of a fluid such as air surrounding a conductor that is electrically charged.

The corona effect will occur in high voltage systems unless sufficient care is taken to limit the strength of the surrounding electric field.

The corona effect can also produce a violet glow, production of ozone gas around the conductor, radio interference, and electrical power loss.

Corona discharge can significantly reduce the efficiency of EHV (Extra High Voltage) lines in power systems.

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Electrical Corona Effect | Causes, Effects & Ways to minimise | TheElectricalGuy

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Classification of Distribution System (part3) 

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Classification of Distribution System (part3) 

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D.C. Distribution

For certain applications, d.c. supply is absolutely necessary. d.c. supply is required for the operation of variable speed machinery (i.e., d.c. motors storage battery.

The d.c. supply obtained in the form of                                             (i) 2-wire distribution                      (ii) 3-wire distribution

2 WIRE DC 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.

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