Introdution to Distribution System, Lecture notes of Electrical Engineering

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Distribution System

Distribution system is that part of power system which distributes electric power to the consumers for utilization

Distribution System

“Electrical system between the sub-station fed by the transmission system and the consumers meters”.

Consists of

• Feeders
• Distributors
• S ervice mains

Classification of Distribution Systems

• Nature of current

DC distribution system or AC distribution system.

• Type of construction.

Overhead system or Underground system

• Scheme of connection

(a)Radial system ( b ) Ring main system ( c ) Inter- connected system.

A.C. Distribution

Primary distribution system.

• Part which operates at voltages somewhat higher than general utilization and handles large blocks of electrical energy
• Primary distribution voltages are 11 kV, 6·6 kV and 3·3 kV. Due to economic considerations, primary distribution is carried out by 3-phase, 3-wire system.

Secondary distribution system

• Part of AC distribution system which includes the range of voltages at which the ultimate consumer utilizes the electrical energy
• Employs 400/230 V, 3-phase, 4-wire system

Distribution substations

• The substations are situated near the consumers’ localities and contain step down transformers (receives power from primary distribution)

Secondary distribution system

Choice between overhead and

underground system

• Public safety
• Initial cost
• Flexibility
• Faults
• Appearance
  • Fault location and repairs
  • Useful life
  • Maintenance cost
  • Interference with

communication circuits

Depends upon a number of factors

Connection Schemes of Distribution

System

1. Radial System

• Separate feeders radiate from a single substation and feed the distributors at one end only
• A feeder OC supplies a distributor AB at point A.

2. Ring main system

Primaries of distribution transformers form a loop. The loop circuit starts from the substation bus-bars, makes a loop through the area to be served, and returns to the substation

Ring main system advantages

• There are less voltage fluctuations at consumer’s terminals.
• The system is very reliable as each distributor is fed via two feeders. In the event of fault on any section of the feeder, the continuity of supply is maintained. For example, suppose that fault occurs at any point F of section SLM of the feeder. Then section SLM of the feeder can be isolated for repairs and at the same time continuity of supply is maintained to all the consumers via the feeder SRQPONM.

Interconnected system advantages

a) It increases the service reliability.

b) Any area fed from one generating station during peak load hours can be fed from the other generating station. This reduces reserve power capacity and increases efficiency of the system.

Requirements of a Distribution

System

Proper voltage

• Changes in voltage are generally caused due to the variation of load on the system
• Low voltage causes loss of revenue, inefficient lighting and possible burning out of motors.
• High voltage causes lamps to burn out permanently and may cause failure of other appliances
• The statutory limit of voltage variations is ± 6% of the rated value at the consumer’s terminal

Requirements of a Distribution

System

Reliability

• Almost all homes/offices/industries are dependent on electric power for their operation
• Reliability can be improved to a considerable extent by ( a ) interconnected system ( b ) reliable automatic control system ( c ) providing additional reserve facilities.

• Example