Introduction to Distribution Systems

Introduction to Distribution Systems

It is well known that the electrical power is now a days generated in a.c. form. It is transmitted and distributed with the help of transformers, transmission lines, feeders, distributors and service mains. The transformers are used to step up and step down the voltage levels as per the requirement at various service stages. The lines which transmit an mains electrical power from the generating station to the different substations are feeders. There are no tappings on feeders. The electrical power is distributed from the substations with the help of distributors. The distributors can have number of tappings. The small cables which are used to supply consumer from the distributor are called service mains. The Fig. 7.4.1 shows a general distribution scheme showing feeders, distributors and service mains.

The feeders are lines of large current carrying capacity. These are feeding to the substation. The size of the feeder is determined by the required current carrying capacity of the feeder. The voltage drop along the feeder is compensated by compounding the generators.

The distributors allow the tappings. The Fig. 7.4.1 shows the distributors AB, BC, DC, DE and EA. The voltage drop along the distributors is the main criterion to design the distributors.

1. General Aspects of a Good Distribution System

The necessary requirements of a good distribution system are,

1. The continuity in the power supply must be ensured. Thus system should be reliable.

2. The specified consumer voltage must not vary more than the prescribed limits. As per Indian Electricity Rules, the variation must not be beyond ± 5 % of the specified voltage.

3. The efficiency of the lines must be as high as possible.

4. The system should be safe from consumer point of view. There should not be leakage.

5. The lines should not be overloaded.

6. The layout should not affect the appearance of the site or locality.

7. The system should be economical.

Though the a.c. transmission and distribution is used, still for certain applications such as d.c. motors, electro-chemical work, batteries, electric traction etc. the d.c. supply is must. Hence along with a.c., d.c. distribution is also equally important. In a d.c. distribution, d.c. generators are used in the generating stations or a.c. is converted to d.c. using the converters like mercury arc rectifiers, rotary converters etc. at the substations. Then the d.c. supply is distributed to the consumers as per the requirement.

In this chapter both d.c. as well as a.c. distribution systems are discussed in detail.

Review Question

1. What are the requirements of a good distributed system ?