Electrical Power Distribution System | Radial and Ring Main

Electrical Power Distribution System

The main function of an electrical power distribution system is to provide power to individual consumer premises. The distribution of electric power to different consumers is done with a much lower voltage level compared to the transmission of power over long distances (i.e. over long transmission lines).

Distribution of electric power is done through distribution networks. Distribution networks consist of the following parts:

1. Distribution substation
2. Primary distribution feeder
3. Distribution Transformer
4. Distributors
5. Service mains

The transmitted electric power is stepped down in substations, for primary distribution purpose. Now, these stepped down electric power is fed to the distribution transformer through primary distribution feeders. Overhead primary distribution feeders are supported by mainly supporting iron pole (preferably rail pole).

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 tipping point 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 the 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 transformer failure also, 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 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.

In this way, supply to the consumers connected to the healthy zone of the ring can easily be maintained even when one section of the ring is under the shutdown. The number of feeders connected to the ring's main electrical power distribution system depends upon the following factors.

1. Maximum Demand of the System: If it is more, then more numbers of feeders feed the ring.
2. Total Length of the Ring Main Distributors: Its length is more, to compensate for the voltage drop in the line, more feeders to be connected to the ring system.
3. Required Voltage Regulation: The number of feeders connected to the ring also depends upon the permissible allowable, voltage drop of the line.

The sub-distributors and service mains are taken off maybe via a distribution transformer at different suitable points on the ring depending upon the location of the consumers. Sometimes, instead of connecting the service main directly to the ring, sub-distributors are also used to feed a group of service mains where direct access to a ring distributor is not possible.