Single Line Diagram of Power Supply System

The electrical energy is produced at generating stations, and through the transmission network, it is transmitted to the consumers. Between the generating stations and the distribution stations, three different levels of voltage (transmission, sub-transmission and distribution level of voltage) are used.

The high voltage is required for long-distance transmission and, the low voltage is required for utility purposes. The voltage level is going on decreasing from the transmission system to the distribution system. The electrical energy is generated by the three-phase synchronous generator (alternators) as shown in the figure below. The generation voltage is usually 11kV and 33 KV.

This voltage is too low for transmission over a long distances. It is, therefore, stepped up to 132, 220, 400 KV, or more by step-up transformers. At that voltage, the electrical energy is transmitted to the bulk power substation where energy is supplied from several power substations.

The voltage at these substations is stepped down to 66KV and fed to the sub-transmission system for onward transmission to the distribution sub-stations. These substations are located in the region of the load centres.

The voltage is further stepped down to 33KV and 11KV. The large industrial consumers are supplied at the primary distribution level of 33KV while the smaller industrial consumer is supplied at 11KV.

The voltage is stepped down further by a distribution transformer located in the residential and commercial area, where it is supplied to these consumers at the secondary distribution level of 400V three phase and 230V single phase.

Advantage of Interconnection of Generating Stations

The power system consists of two or more generating stations which are connected by tie lines. Interconnection of generating stations has the following important advantages.

1. It enables the mutual transfer of energy from surplus zone to deficit zone economically.
2. Lesser overall installed capacity to meet the peak demand.
3. Lesser standby reserve generating capacity is required.
4. It permits the generation of energy at the most efficient and cheapest station at every time.
5. It reduces the capital cost, operating cost and cost of energy generated.
6. If there is a major breakdown of a generating system unit in an interconnected system, then there is no interruption of the power supply.

The interconnection provides the best use of power resources and greater security of supply. It enables overall economic generation by optimum use of the high capacity economical generating plant. The interconnection between networks is done either by HVAC (high voltage alternating current) links or through HVDC (High Voltage Direct Current) links.