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In youth, there was a touch demand for voltage so tiny power stations were engineered to provide lighting and heating masses. However, the widespread use of voltage by fashionable civilization has necessitated manufacturing bulk voltage economically and with efficiency.

Electric Supply System:

Supply System

An electric offer system consists of 3 principal elements viz., the ability station, the transmission lines, and therefore the distribution system. wattage is created at the ability stations that area unit placed at favorable places, typically quite aloof from the shoppers. it’s then transmitted- Teddy boy over giant distances to load centers with the assistance of conductors referred to as transmission lines. Finally, it’s distributed to an outsized variety of tiny and massive shoppers through a distribution network.

Typical a.c. Power Supply Scheme:

The large network of conductors between the ability station and the shoppers will be generally divided into 2 components viz. transmission system and distribution system. every half will be further sub-divided into 2 primary transmission and secondary transmission and first distribution and secondary distribution.

(i) Generating station :

G.S. represents the generating station wherever electrical power is created by 3-phase alternators operational in parallel. the same old generation voltage is eleven potential units. For economy within the transmission of electrical power, the generation voltage (i.e., eleven potential units) is stepped up to 132 kV at the generating station with the assistance of 3-phase transformers. The transmission of electrical power at high voltages has many benefits as well as the saving of conductor material and high transmission potency.

Supply System

(ii) Primary transmission:

The electric power at 132 potential unit is transmitted by 3-phase, 3-wire overhead system to the outskirts of town. This forms the first transmission.

(iii) Secondary transmission:

The primary line terminates at the receiving station (RS) that sometimes lies at the outskirts of town. At the receiving station, the voltage is reduced to 33kV by reduction transformers. From this station, electrical power is transmitted at 33kV by 3-phase, 3-wire overhead system to numerous sub-stations (SS) settled at the strategic points within the town. This forms the secondary transmission.

(iv) Primary distribution:

The secondary line terminates at the sub-station wherever voltage is reduced from thirty-three potential unit to 11kV, 3-phase,3-wire. The eleven potential unit lines lie the vital roadsides of town. This forms the first distribution. it’s going to be noted that massive customers (having demand over fifty kW) are usually equipped power at eleven potential units for more handling with their own sub-stations.

(v) Secondary distribution:

The electric power from the first distribution line (11 kV) is delivered to distribution sub-stations (DS). These substations are settled close to the consumers’ localities and step down the voltage to four hundred V, 3-phase, 4-wire for secondary distribution. The voltage between any 2 sections is four hundred V and between any phase and neutral is 230 V. The single-phase residential lighting load is connected between anybody section and neutral, whereas 3-phase, four hundred V motor load is connected across 3-phase lines directly.

The electric supply system can be broadly classified into d.c. or a.c. system:

1. D.C. transmission: 

Supply System

For some years past, the transmission of electrical power by d.c. has been receiving the active thought of engineers because of its varied blessings.


The high voltage d.c. the transmission has the subsequent blessings over high voltage a.c. transmission :

  •  It needs solely 2 conductors as compared to 3 for a.c. transmission.
  •  there’s no inductance, capacitance, section displacement, and surge issues in d.c. trans-
  •  because of the absence of inductance, the fall during a d.c. the line is a smaller amount than the a.c. the road for constant load and causation finish voltage. For this reason, a d.c. the line has higher voltage regulation.
  •  there’s no electrical phenomenon during a d.c. system. Therefore, the complete cross-sectional of the road conductor is used.
  • For constant operating voltage, the potential stress on the insulation is a smaller amount just in case of d.c. system than that during a.c. system. Therefore, a d.c. the road needs less insulation.


  • electrical power cannot be generated at high d.c. voltage because of commutation issues.
  • The d.c. voltage cannot be stepped up for transmission of power at high voltages.
  • The d.c. switches gate breakers have their own limitations.

2.A.C. transmission: 

Supply System

transmission. Nowadays, power is sort of completely generated, transmitted and distributed within the kind of a.c.


  • the facility may be generated at high voltages.
  • the upkeep of a.c. sub-stations is straightforward and cheaper.
  • The a.c. voltage may be stepped up or stepped down by transformers with ease and potency. this allows to transmit power at high voltages and distribute it at safe potentials.


  •  An a.c. the road needs a lot of copper than a d.c. line.
  •  the development of a.c. the line is a lot of difficult than a d.c. line.
  • thanks to an electrical phenomenon within the a.c. the system, the effective resistance of the road is accumulated.
  • An a.c. the road has capacitance. Therefore, there’s


From the higher than comparison, it’s clear that prime voltage d.c. transmission is superior to high voltage a.c. transmission. though at this time, the transmission of electrical power is carried by a.c., there’s AN increasing interest in d.c. transmission. The introduction of mercury arc rectifiers and thyratrons have created it potential to convert a.c. into d.c. and vice-versa simply and with efficiency. Such devices will operate up to thirty MW at four hundred kV in single units. The contemporary trend is towards a.c. for generation and distribution and high voltage d.c. for transmission.

Various Systems of Power Transmission:

It has already been got wind that for transmission of electrical power, 3-phase, 3-wire a.c. the system is universally adopted. However, different systems also can be used for transmission underneath special circumstances. the various attainable systems of transmission square measure :

1. D.C. system
(i) D.C. two-wire.
(ii) D.C. two-wire with mid-point earthed.
(iii) D.C. three-wire.

2. Single-phase A.C. system
(i) Single-phase two-wire.
(ii) Single-phase two-wire with mid-point earthed.
(iii) Single-phase three-wire.

3. Two-phase A.C. system
(i) Two-phase four-wire.
(ii) Two-phase 3 wire.

4. Three-phase A.C. system
(i) Three-phase three-wire.
(ii) Three-phase four-wire.

From the on top of attainable systems of power transmission, it’s troublesome to mention that is that the best system unless and till some methodology of comparison is adopted. Now, the price of conductor material is one among one amongst|one in every of the foremost vital changes in a system. Obviously, the simplest system for transmission of power is that the degree of conductor material needed is minimum. Therefore, the degree of conductor material needed forms the idea of a comparison between completely different systems.