The electrical energy is generated at the various power (or generating) stations viz. hydro-electric power plant, steam power plant, and whichever which are located at the desirable places where water energy or fuel (coal, etc.) is available in abundance. This generated power at the power stations has to be supplied to various consumers scattered at far off places.
There is a large network of conductors between the power plants and consumers for this purpose. This network can be broadly divided into two parts, namely the transmission and the distribution system.
The system by which bulk power (or energy) is delivered from the power station to load centers (big sub-stations) is called a transmission system.
Layout of Transmission System
The generation, transmission, and distribution of electric power are called power system. The layout of transmission system in India is shown in the figure below.
Generally, a power system consists of the following stages:
- Power Station.
- Primary Transmission.
- Secondary Transmission.
- Primary Distribution.
- Secondary Distribution.
The electric power is generated at the power station by 3-phase, 3-wire system employing the number of alternators in parallel. The generation voltages are 3.3, 11, or 33 kV; however, the most common value adopted in practice is 11 kV. At the power station, the generation voltage (11 kV) is stepped up to 132, 220, or 400 kV (i.e., whichever is economical) depending upon the distance; the amount of power to be transmitted and the system stability.
Primary Transmission System
The bulk of electrical power is transmitted from the generating station to the load centers (i.e., main sub-stations) by overhead lines at 132, 220 or 400 kV. This forms the primary transmission system. For primary transmission 3-phase, 3-wire system is always employed, and the aluminum conductors with steel reinforcement (ACSR) are run over the steel towers.
Secondary Transmission System
On the outskirt of cities, there are substations where power is received at 132, 220, or 400kV and is stepped down to 66 or 33kV depending upon the amount of power to be fed to a particular area. Then the power is transmitted at 66 or 33kV by overhead lines. This constitutes the secondary transmission system. Three-phase, three-wire system is always employed.
At suitable places near the city or load centers, there are substations where secondary transmission voltage is stepped down to 11 kV, and the power is delivered at this voltage by overhead lines. This forms the primary distribution system.
These 11kV lines are run along all the important road-sides of the city, and power is supplied to the bulk consumers (big industries) at this voltage. The bulk consumers install their own sub-station to step down the voltage to 400 V for the utilization of electric power. Three-phase, three-wire system is invariably used.
The electric power from primary distribution lines is delivered to various substations called distribution sub-stations. These sub-stations are located near the consumer’s localities. At these substations, voltage is stepped down to 400 V, and the power is distributed by 3-phase, 4-wire system. This forms a secondary distribution system.
The voltage between any two phases (i.e., line voltage) is 400 V, and the voltage between any one phase and neutral, i.e., phase voltage) is 230 V.
The single-phase domestic loads (e.g., lamps, fans, refrigerators, heaters, etc.) are connected between any one phase and neutral whereas industrial loads (e.g., 3-phase motors, furnaces, etc.) are connected across 3-phase, 400 V lines.
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