Methods of AC distribution system based on connection scheme

Explain methods of AC distribution system based on connection scheme.

The following connection schemes of distribution system are generally employed:

Radial System:

In a radial system, separate feeders radiate from a single sub-station and feed the distributors at one end only. A single line diagram of a radial system for AC distribution is shown in the figure.

Here substation supplies power to a distributor AB at end A through feeder OC. This system is only employed when power is generated at low voltage and the substation is located at the center of the load.

Drawbacks of Radial system:

• The end of the distributor nearest to the feeding point will be heavily loaded.
• The consumers are dependent on a single feeder and single distributor, therefore any fault on the feeder or distributor cuts off supply to the consumers who are on the side of the fault away from the substation.

Ring Main System:

In this system, primaries of distribution transformers form a loop. it forms a complete ring and hence the name ring main system. Fig. shows AC ring main system, where BCDAB is the closed feeder supplied by the sub-station S at point B. The various distributors are connected to points A, C and D of the feeder.

Advantages of Ring main system:

• Each distributor is supplied via two feeders, for example, the distributor connected at point C is fed via feeder BC and BADC. This reduces the voltage fluctuations at the consumer’s terminals to some extent.
• The system is more reliable since each distributor is fed via two feeders. In the event of a fault on any section of the feeder, the continuity of supply can be maintained. For an instant, suppose fault occurs at any point F of section CD of the feeder, then section CD of the feeder can be isolated for repairs and at the same time continuity of supply is maintained to all the consumers.

Inter Connected Sysem:

In this system, the feeder ring is energized by two or more than two generating stations or substations. The figure shows the single line diagram of AC interconnected system, where the closed feeder ring ABCA is supplied by two generating or substations S₁, and S₂, at point D and C respectively. Distributors are connected to the feeder at point R, Q, P and O.

Advantages of Interconnected Distribution System:

• It increases the reliability of supply.
• During overload hours, the area fed from one generating station can be fed from other generating station. Thus it reduces the reserved plant capacity, improves the service reliability and increases the load factor and efficiency of the system.