Previously there was very little demand for electrical energy. A single small electrical generating unit could meet the localized demand. Nowadays the demand for electrical energy is tremendously increasing along with the modernization of human lifestyles. To meet this increasing electrical load demand, we have to establish quite a large number of big power plants. But from the economic point of view, this is not always possible to build a power plant nearer to the load centers. We define load centers as the places where the density of consumers or connected loads is quite high compared to other parts of the country. It is economical to establish a power plant near the natural source of energy like coal, gases, and water etc. Because of that and for many other factors, we have to construct an electrical generating station often far away from load centers.
Thus we have to establish electrical network systems to bring the generated electrical energy from power generating station to the consumer ends. Electricity generated in the generating station riches to the consumers through the systems which we can divide into two main parts referred as transmission and distribution. We call the network through which the consumers get electricity from the source as electrical supply system. An electrical supply system has three main components, the generating stations, the transmission lines and distribution systems. Power generating stations produce electricity at a comparatively lower voltage level. Producing electricity at lower voltage level is economical in many aspects.
The step-up transformers connected at the beginning of the transmission lines, increase the voltage level of the power. Electrical transmission systems then transmit this higher voltage electrical power to the possible nearest zone of load centres. Transmitting electrical power at higher voltage levels is advantageous in many aspects. High voltage transmission lines consist of overhead or/and underground electrical conductors. The step-down transformers connected at the end of the transmission lines decrease the voltage of electricity to the desired low values for distribution purposes. The distribution systems then distribute the electricity to various consumers according to their required voltage levels.
We usually adopt AC system for generation, transmission and distribution purposes. For Ultra High Voltage transmission we often use DC transmission system. The transmission and distribution both networks can be either overhead or underground. As the underground system is much more expensive than an overhead system, the latter is preferable wherever possible from the economic point of view. We use three phase 3 wire system for AC transmission and three phase 4 wire system for AC distribution.
We can divide both transmission and distribution systems into two parts, primary transmission and secondary transmission, primary distribution and secondary distribution. It is a generalized view of an electrical network. We should note that all the transmission distribution systems may not have these four stages of the electrical supply system. As per requirement of the system, there may be many networks which may not have a secondary transmission or secondary distribution even in many cases of localized electrical supply system the entire transmission system can be absent. In those localized electrical supply system generators directly distribute the power to different consumption points.
Let us discuss a practical example of the electrical supply system. Here generating station produces three-phase power at 11KV. Then one 11/132 KV step-up transformers associated with the generating station steps up this power to 132KV level. The transmission line transmits this 132KV power to 132/33 KV step down substation consisting of the 132/33KV step-down transformers, situated at outskirts of town. We will call that portion of the electrical supply system that is from 11/132 KV step-up transformer to 132/33 KV step down transformer as primary transmission. The primary transmission is 3 phase 3 wire system that means there are three conductors for three phases in each line circuit.
After that point in the supply system, the secondary power of 132/33 KV transformer gets transmitted by 3 phase 3 wire transmission system to different 33/11KV downstream substations situated at different strategic locations of the town. We refer this portion of the network as secondary transmission.
The 11KV 3 phase 3 wire feeders passing through roadsides of the town carry the secondary power of 33/11KV transformers of secondary transmission substation. These 11KV feeders comprise the primary distribution of the electrical supply system.
The 11/0.4 KV transformers in consumer localities step down the primary distribution power to 0.4 KV or 400 V. These transformers are called distribution transformer, and these are pole mounted transformer. From distribution transformers, the power goes to consumer ends by 3 phase 4 wire system. In 3 phase 4 wire system, 3 conductors are used for 3 phases, and the 4th conductor is used as the neutral wire for neutral connections. A consumer can take the supply either in three phase or single phase depending on his requirement. In case of three phase supply the consumer gets 400 V phase to phase (line voltage) voltage, and for single phase supply, the consumer gets 400 / root 3 or 231 V phase to neutral voltage at his supply mains. Supply main is the end point of an electrical supply system. We refer this portion of the system that is from secondary of the distribution transformer to supply main as secondary distribution. Supply mains are the terminals installed at consumer premises from which the consumer takes connection for his uses.