Power Distribution

Distribution System: - The system by which distributes electric power for local use is known as distribution system. The step down in voltage in voltage is at the distribution substation, where the voltage on lines leaving the substation ranges from 3 to 3.4 KV and is commonly between 11 and 15 KV. 

2.6 Classification of Distribution System:

 

2.6.1 A distribution system may be classified according to:s

(1) Nature of current

     (a) a.c distribution system.

     (b) d.c distribution system.

(2) Types of construction.

      (a)  Over head system.

      (b)   Underground system.

(3) Scheme of connection.

      (a) Radial system.

      (b) Ring main system.

      (c) Inter connection system.

2.6.2 A .C Distribution:

            

     Now a day electrical energy is generated, transmitted and distributed in the form of alternating current. One important reason for the widespread use of alternating current in preference to direct current is the fact that alternating voltage can be conveniently changed in magnitude by mean transformer. Transformer has made it possible to transmit a.c power at high voltage and utilize it at a safe potential. High voltage transmission and distribution voltages have greatly reduced the current in the conductors and the resulting line losses.

There is no definite line between transmission and distribution according to voltage or bulk capacity. However, in general ac distribution system in the electrical system between the down substation by the transmission system and the consumer’s meter.

                                         

        The a.c distribution system is classified  into two types:

(1)   primary distribution  system.

(2)   secondary distribution  system.

(1)   primary distribution system:

It is that of the a.c distribution system which operates at voltages somewhat higher than general utilization and handles large blocks of electrical energy than the average low.

(1)   Secondary distribution system:

The secondary distribution system control of 3­- phase  4­­­- wire 400v.

Distribution lay along roadsides. The service connections to the consumers are tapped of the distribution at convenient points. The service connection may be single phase two wire circuits or three-phase four wire circuit. An attempt is made to divide the single phase load equally (as far possible) on the three-phase. The secondary distribution systems generally used are: radial, open loop and network distributions.

2.6.3 Radial Distribution System:

A radial distribution system takes off from the distribution transformer and runs through the area to be served by it. This is the simplest and the least expensive system but has the poorest reliability.

2.6.4 Open loop distribution system:

An open loop system consists of two distributors taking off from the same distribution transformer and running in different directions and supplying different area. A normally open switching devices ties the far ends of the distribution together. In the even of a fault on one of the distributor .Thus an open loop system provides better continuity of service, and its cost is marginally higher then that of radial system.

2.6.5 Network Distribution System:

A network or grid system of distribution consists of a number interconnected the distributors. Two or more distributor transformers feed the network and operate continuously in parallel. This is suitable for serving high load density metropolitans areas.

2.6.6D.C Distribution:

It is a common knowledge that electric power is almost exclusively generated, transmitted and distribution as a.c. However, for certain applications d.c supply is absolutely necessary. For instance, d.c supply is required for the operation of variable speed machinery foe electrochemical work and for congested areas where storage battery reserves are necessary. For this purpose a.c power is converted in to d.c power at the sub-station by using converting machinery e.g mercury arc rectifier, rotary converters and motor generators sets. The d.c supply from the substation may be obtained in the from of         

(a)   2-wire or

(b)  3-wire distribution. 

2.7 Connection Schemes of Distribution System:

   All distribution of electrical energy is done by constant voltage system. In practice, the following distribution circuits are generally used in GPBS.

(i)                Radial system:

  In this system, separate feeders radiate from a single sub-station and feed the distribution at one end only. Fig. 3.1 (i) shows single line diagram of a radial system for d.c distribution where a feeder oc supplies a distributor. 

AB at point A. Obviously, the distributor is feed at one end only i.e. point in this case .Fig(ii) shows a single line diagram of radial system for a.c distribution. The radial system is employed only when power is generated at low voltage and the sub-station is located at the center of the load.