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1. Distribution system The transmission & distribution system resemble to human’s blood circulating system: Transmission system = arteries Distribution system = capillaries Distribution is that part of Power system which distributes electric power for local use In general distribution system start from the substation fed by the transmission system to the consumers’ meters. 1 Power distribution

2. Feeders, distributor and service mains Feeder is a conductor which connects the substation ( or local Generation ) to the area where power is to be distributed. The feeders have no current tapping, and therefore it is constant through the length of the feeder.  Main consideration is the current carrying capacity. It connects the main distribution to the secondary distribution, and has same current loading throughout its length. Distributor is a conductor where tapings are taken for supplying power to several consumers. Current is therefore not same throughout its length. Main consideration is the voltage drop which should not be more than ±5% of declared voltage at consumer’s terminals. For transmitting the power through feeder and to choose most economical cross section or current density, the annual financial loss is to be adopted. This is known as Kelvin’s Law. Service Mains is generally a small conductor which connects the distributor and consumers terminals. Power distribution 2

3. Classification of distribution system According to the type of current: ac or dc distribution According to construction: overhead or underground. Overhead is preferred as it is 5 to 10 times cheaper than underground According to scheme of connection: radial , ring mains , interconnected Power distribution 3

4. AC distribution system In general, the ac distribution system between the step down substation fed by the transmission system and the consumers’ meters. The system is classified into a) primary distribution and b) secondary distribution Primary distribution system operates at somewhat higher voltage than voltage at utilization level. Generally, 11 kV, 6.6 kV and 3.3 kV levels are used for primary distribution. Choice of Voltage level depends on amount of power to be conveyed and the distance of the substation ( e.g., PMTs) required to be fed. Secondary distribution is that part which includes the range of voltage at which the ultimate consumer utilizes the electric power. The secondary distribution employs 380/220 , 3- phase - 4 wire system 4 Power distribution

5. DC distribution Some applications such as variable speed dc drives in industrial units, electrochemical work, data centres etc require dc power. Hence ac supply is converted into dc through rectifiers or rotary convertors and then distributed through a dc distribution system. DC supply can be obtained from converter substation in the form of  ( i ) 2-wire distribution , consisting of one outgoing ( or Positive) and one return ( or negative) wire. It has low efficiency   (ii) 3- wire distribution , consists of two outer and one middle or neutral wire which is earthed. Load requiring high voltage is connected between two outer whereas low voltage equipment is connected between an outer and middle wire. Power distribution 5

6. Requirement of distribution system Proper voltage : low voltage cause loss of revenue, inefficient lightning, and possible burning out of motors. Good distribution system must ensure voltage variation at consumers’ terminals within permissible limits Availability of power on demand: distribution system must be capable to meet load demand as it can change time to time without advance warning to supply company. Reliability : can be improved by interconnecting systems, reliable automatic control system and providing additional reserve facilities Power distribution 6

7. Overhead and underground distribution- a comparison Public safety Initial cost Flexibility of system Faults Appearance Fault location & repairs Voltage drop Useful life Maintenance cost Interference with communication circuits Annual cost of operation Power distribution 7