Distribution systems can be classified according to the scheme of connection.
1. Radial distribution system,
2. Parallel or loop distribution system,
3. Grid or interconnected distribution system
1. Radial Distribution System :
Fig. shows the loop distribution system.
1. Radial distribution system,
2. Parallel or loop distribution system,
3. Grid or interconnected distribution system
1. Radial Distribution System :
Most distribution systems are designed to be radial distribution systems as shown in Fig.
In a radial system, only one path is connected between each consumer and the substation.
The electrical power flows from the substation to the consumer along a single path. This, if interrupted, results in a complete loss of power to the consumer.
In India. 99% of the distribution of power is by radial distribution system only.
Advantages :
1. Initial cost is minimum.
2. Simple in planning, design and operation.
Disadvantages :
1. Reliability is less.
2. Distributor nearer to the feeding end is heavily loaded.
3. The consumers at the far end would be subjected to voltage fluctuations with variations in load.
Parallel or Loop Distribution System :
This system consists of two or more paths between the power sources (substations) and the consumers.
It is selected to carry its normal load plus the load of the other half of the loop also.
Therefore, the size of the feeder conductor in a loop distribution system is the same throughout the loop.
Advantages :
1. Less conductor material is required as each part of the ring carries less current than in the radial system.
2. Less voltage fluctuations.
3. It is more reliable.
Disadvantages :
1. It is difficult to design when compared to the designing of a radial system
Grid or Interconnected Distribution System :
Fig. shows an interconnected feeder which is supplied by a number of feeders.
The radial primary feeders can be tapped off from the interconnecting tie feeder. They can also serve directly from the substations. Each tie-feeder has two associated circuit breakers at each end in order to have less load interruption due to a tie-feeder fault.
The reliability and quality of the service of the interconnected type distribution arrangement are much higher than the radial and loop arrangements.
However, it is more difficult to design and operate than radial or loop type systems.
Advantages :
1. It increases the reliability of supply.
2. Losses are less and efficiency is more.
3. Quality of service is improved.
Disadvantages :
1. Initial cost is more.
2. Difficult in planning, design, and operation.
Design Considerations In Distribution System:
The main elements of a distribution network are the feeders and distributors. To maintain a required voltage at the terminals of a consumer, proper care has to be taken while designing these elements.
1. Feeder design:
While designing a feeder, the main consideration is its current carrying capacity.
The voltage drop consideration is relatively unimportant. The reason is that the voltage drop in a feeder can be compensated by means of voltage regulating equipment placed at the sub-stations.
2. Distributor design:
While designing a distributor, the main consideration is the voltage drop in it. The current carrying capacity consideration is not that much important.
The reason is that the distributor supplies power to the consumers and there is a statutory limit to the voltage fluctuations at the consumer's terminals.
This limit is +-6% of the rated value (as per Indian Electricity Rules). Hence the size and length of the distributor are designed in such a way that the voltage at the consumer's terminals is within the permissible limits.