Distribution System

That part of power system which distributes electric power for local use is known as distribution system.

1. Freeders. A feeder is a conductor which connects the substation (or localized generating station) to the area where power is to be distributed. Generally, no tappings are taken from the feeder so that current in it remains the same throughout.
2. Distributors. A distributor is a conductor from which tappings are taken for supply to the consumers. In Fig. AB, BC CD and DA are the distributors. The current through distributor is not constant because tappings are taken out. 
3. Service mains. A service mains is generally a small cable which connects the distributor to the consumers' terminals.

Classification of Distribution Systems

A distribution system may be classified according to:

(i) Nature of current. According to nature of current, distribution systems may be classified as (a) d.c. distribution system (b) a.c. distribution system. Now-a-days, a.c. system is universally adopted for distribution of electric power as it is simpler and more economical than direct current system.

(ii) Type of construction. According to type of construction, distribution systems may be classified as (a) overhead system (b) underground system. The overhead system is generally employed for distribution as it is 5 to 10 times cheaper than the equivalent underground system.

(iii) Scheme of connection. According to scheme of connection, the distribution system may be classified as (a) radial system (b) ring main system (c) inter-connected system.

D.C. Distribution

The dc supply from the substation may be obtained in the form of (i) 2-wire or (ii) 3-wire for distribution.

(i) 2-wire d.c. system. As the name implies, this system of distribution consists of two wires. One is the outgoing or positive wire and the other is the return or negative wire. The loads such as lamps, motors etc. are connected in parallel between the two wires as shown in Fig.

(ii) 3-wire d.c. system. It consists of two outers and a middle or neutral wire which is earthed at the substation. The voltage between the outers is twice the voltage between either outer and neutral wire as shown in Fig. The principal advantage of this system is that it makes available two voltages at the consumer terminals viz. V between any outer and the neutral and 2V between the outers. Loads requiring high voltage (e.g., motors) are connected across the outers, whereas lamps and heating circuits requiring less voltage are connected between either outer and the neutral.

Overhead versus Underground System for Distribution

Overhead lines are generally mounted on wooden, concrete or steel poles which are arranged to carry distribution transformers in addition to the conductors. The underground system uses conduits, cables and manholes under the surface of streets and sidewalks.

(i) Public safety. The underground system is more safe than overhead system because all distribution wiring is placed underground and there are little chances of any hazard.

(ii) Initial cost. The underground system is more expensive.

(iii) Flexibility. The overhead system is much more flexible than the underground system. In the latter case, manholes, duct lines etc., are permanently placed once installed and the load expansion can only be met by laying new lines. However, on an overhead system; poles, wires, transformers etc., can be easily shifted to meet the changes in load conditions.

(iv) Faults. The chances of faults in underground system are very rare.

(v) Appearance. The general appearance of an underground system is better.

(vi) Fault location and repairs. In general, there are little chances of faults in an underground system. However, if a fault does occur, it is difficult to locate and repair on this system. On an overhead system, the conductors are visible and easily accessible so that fault locations and repairs can be easily made.

(vii) Current carrying capacity and voltage drop. An overhead distribution conductor has a considerably higher current carrying capacity than an underground cable conductor of the same material and cross-section. On the other hand, underground cable conductor has much lower inductive reactance than that of an overhead conductor because of closer spacing of conductors.

(viii) Useful life. The useful life of underground system is much longer than that of an overhead system. An overhead system may have a useful life of 25 years, whereas an underground system may have a useful life of more than 50 years.

(ix) Maintenance cost. The maintenance cost of underground system is very low as compared with that of overhead system.

(x) Interference with communication circuits. An overhead system causes electromagnetic interference with the telephone lines. The power line currents are superimposed on speech currents, resulting in the potential of the communication channel being raised to an undesirable level.

 Requirements of a Distribution System

1. Proper voltage. One important requirement of a distribution system is that voltage variations at consumer's terminals should be as low as possible. The changes in voltage are generally caused due to the variation of load on the system. Voltage variations cause loss of revenue, inefficient lighting and possible burning out of motors.
2. Availability of power on demand. Power must be available to the consumers in any amount that they may require from time to time. For example, motors may be started or shut down, lights may be turned on or off, without advance warning to the electric supply company. As electrical energy cannot be stored, therefore, the distribution system must be capable of supplying load demands of the consumers.
3. Reliability. Homes and office buildings are lighted, heated, cooled and ventilated by electric power. This calls for reliable service. Unfortunately, electric power, like everything else that is man-made, can never be absolutely reliable. However, the reliability can be improved to a considerable extent by (a) interconnected system (b) reliable automatic control system (c) providing additional reserve facilities.