Block diagram of a typical ac electrical power supply system is shown is Fig. 1.3. Electrical power is generated by 3-phase alternators operating in parallel at the generating stations. The generation voltage is usually 11 kV. It may be 3.3, 6.6 or even 33 kV in certain cases. For economy reasons this voltage is stepped up from 11 kV to 220 kV at the generating stations by means of 3-phase transformers.
The primary (or high voltage) transmission lines transmit electrical power from the generating stations to primary transmission substations. At these primary substations the voltage is stepped down to 33 kV (or 66 kV) by means of step-down transformers. From the primary substations electrical power is transmitted through secondary (or sub) transmission lines at 33 kV to various secondary transmission substations.
At the secondary substations the voltage is further stepped down to 11 kV (or 6.6 or 3.3 kV, as per needs) by means of 3-phase transformers and the power is supplied to distribution substations through primary (or high voltage) distribution lines at 11 kV. The distribution substations are located at suitable places in the area in which power is to be supplied. At these substations the voltage is stepped down to 415 V. The secondary (or low voltage) distribution lines, called the distributors, are laid along the roads and service connections to the consumers are tapped off from the distributors.
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Most of the domestic, commercial and small scale industrial consumers are supplied power at low voltage (at 415 volts for 3-phase loads and 240 volts for single phase loads). However, large commercial organisations, public buildings and industries having load in excess of 100 kW prefer to have bulk power supply at 11 kV (or even at 33 kV in case of large industries). This is accomplished by taking 11 kV line directly from secondary transmission substations right up to the premises of such consumers and connecting it to an 11 kV/415 V. step-down transformer.