AC & DC distribution system

AC distribution system

In today's world, one important reason for the widespread use of alternating current in preference to direct current is the fact that alternating voltage can be conveniently changed in magnitude by means of a transformer. High transmission and distribution voltages have greatly reduced the current in the conductors and the resulting line losses.

The AC distribution system is classified into:

1. Primary distribution system and

2. Secondary distribution system.

Primary distribution system

This system operates at voltages somewhat higher than general utilization and handles large blocks of electrical energy than the average low-voltage consumer uses. Primary distribution is usually carried out by 3-phase, 3-wire system.

The voltage used for primary distribution depends upon the amount of power to be conveyed and the distance of the substation required to be fed. The most commonly used primary distribution voltages are 11 kV, 66 kV and 33 kV, but this differs from country to country.

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.

Secondary distribution system

This system includes the range of voltages at which the ultimate consumer utilizes the electrical energy delivered to him.The secondary distribution employs 400/230 V, 3-phase, 4-wire system.

The primary distribution circuit delivers power to various substations, called distribution substations.The substations are situated near the consumers’ localities and contain step-down transformers. At each distribution substation, the voltage is stepped down to 400 V and power is delivered by 3-phase, 4-wire system. The single phase domestic loads are connected between any one phase and the neutral, whereas in 3-phase 400 V motor, power transformer loads are connected across 3-phase lines directly.

DC distribution

It is a common knowledge that electric power is almost exclusively generated, transmitted and distributed as AC. However, for certain applications, DC supply is absolutely necessary. For instance, DC supply is required for the operation of variable speed machinery (i.e., DC motors), for electro-chemical work and for congested areas where storage battery reserves are necessary.For this purpose, AC power is converted into DC power at the substation by using converting machinery.

The DC supply may be obtained in the form of:

• 2-wire or

• 3-wire for distribution

2-wire DC system

As the name implies, this system of distribution consists of two wires (+ and -). 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.This system is never used for transmission purposes due to low efficiency but may be employed for distribution of DC power.

3-wire DC system

It consists of two outer and a middle or neutral wire which is earthed at the substation . The voltage between the outer terminals is twice the voltage between either outer and neutral wire.The principal advantage of this system is that it makes available two voltages at the consumer terminals between any outer and the neutral and between the outer terminals.

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