The transmitted electric power is stepped down in substations, for primary for distribution purposes.
This stepped down electric power is fed to the distribution transformer through primary distribution feeders. Overhead primary distribution feeders are supported by mainly supporting iron poles (preferably rail poles). The conductors are strand aluminum conductors and they are mounted on the arms of the pole by means of pin insulators. Sometimes in congested places, underground cables may also be used for primary distribution purposes.
Distribution transformers are mainly 3 phase pole mounted type. The secondary of the transformer is connected to distributors. Different consumers are fed electric power by means of the service mains.
These service mains are tapped from different points of distributors. The distributors can also be re-categorized by distributors and sub-distributors. Distributors are directly connected to the secondary distribution transformers whereas sub-distributors are tapped from distributors.
Service mains of the consumers may be either connected to the distributors or sub-distributors depending upon the position and agreement of consumers.
In this discussion of the electrical power distribution system, we have already mentioned both feeders and distributors. Both feeder and distributor carry the electrical load, but they have one basic difference.
The feeder feeds power from one point to another without being tapped from any intermediate point. As there is no tapping point (i.e. a point where the voltage and current can be stepped down or stepped up) in between, the current at the sending end is equal to that of the receiving-end of the conductor.
The distributors are tapped at different points for feeding different consumers, and hence the current varies along their entire length.
Radial Electrical Power Distribution System
In the early days of the electrical power distribution system, different feeders radially came out from the substation and connected to the primary distribution transformer.
But radial electrical power distribution system has one major drawback that in case of any feeder failure, the associated consumers would not get any power as there was no alternative path to feed the transformer.
In the case of a transformer failure, the power supply is interrupted. In other words, the consumer in the radial electrical distribution system would be in darkness until the feeder or transformer was rectified.
Ring Main Electrical Power Distribution System
The drawback of a radial electrical power distribution system can be overcome by introducing a ring main electrical power distribution system.
In this network topology, one ring network of distributors is fed by more than one feeder. In this case, if one feeder is under fault or maintenance, the ring distributor is still energized by other feeders connected to it. In this way, the supply to the consumers is not affected even when any feeder becomes out of service.
In addition to that, the ring main system is also provided with different section isolates at different suitable points. If any fault occurs on any section, of the ring, this section can easily be isolated by opening the associated section isolators on both sides of the faulty zone transformer directly.
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