Distribution system

 Introduction, necessity of DS

A distribution substation is located near or inside city/town/village/industrial area. It receives power from a transmission network. The high voltage from the transmission line is then stepped down by a step-down transformer to the primary distribution level voltage. Primary distribution voltage is usually 11 kV, but can range between 2.4 kV to 33 kV depending upon region or consumer.

A typical power distribution system consists of -

1. Distribution substation

2. Feeders

3. Distribution Transformers

4. Distributor conductors

5. Service mains conductors

Along with these, a distribution system also consists of switches, protection equipment, measurement equipment etc.

 

Distribution feeders:

The stepped-down voltage from the substation is carried to distribution transformers via feeder conductors. Generally, no tapping are taken from the feeders so that the current remains same throughout. The main consideration in designing of a feeder conductor is its current carrying capacity.

Distribution transformer:

 A distribution transformer, also called as service transformer, provides final transformation in the electric power distribution system. It is basically a step-down 3-phase transformer. Distribution transformer steps down the voltage to 400Y/230 volts. Here it means, voltage between any one phase and the neutral is 230 volts and phase to phase voltage is 400 volts.

Distributors: 

Output from a distribution transformer is carried by distributor conductor. Tapping are taken from a distributor conductor for power supply to the end consumers. The current through a distributor is not constant as tapping are taken at various places throughout its length. So, voltage drop along the length is the main consideration while designing a distributor conductor.

Service mains: 

It is a small cable which connects the distributor conductor at the nearest pole to the consumer's end.

 

Distribution Layout, voltage level

Primary Distribution

It is that part of an AC distribution system which operates at somewhat higher voltages than general residential consumer utilization. Commonly used primary distribution voltages in most countries are 11 kV, 6.6 kV and 3.3 kV. Primary distribution handles large consumers such as factories and industries. It also feeds small substation from where secondary distribution is carried out. Primary distribution is carried out by 3-phase, 3-wire system.

Secondary Distribution

This part directly supplies to the residential end consumers. Domestic consumers are fed with single phase supply at 230 volts (120 volts in USA and some other countries). Three phase supply may also be provided at 400 volts for big properties, commercial buildings, small factories etc. Secondary transmission in most countries is carried out by 3-phase, 4-wire system.

Topologies or Connection schemes of distribution system

An electric power distribution system can be classified according to its feeder connection schemes or topologies as follows -

1.  Radial distribution system

2.  Parallel feeder distribution

3.  Ring main distribution system

4.  Interconnected distribution

 

Radial Distribution System

This system is used only when substation or generating station is located at the center of the consumers. In this system, different feeders radiate from a substation or a generating station and feed the distributors at one end. Thus, the main characteristic of a radial distribution system is that the power flow is in only one direction. Single line diagram of a typical radial distribution system is as shown in the figure below. It is the simplest system and has the lowest initial cost. 

Although this system is simplest and least expensive, it is not highly reliable. A major drawback of a radial distribution system is, a fault in the feeder will result in supply failure to associated consumers as there won't be any alternative feeder to feed distributors.

Parallel Feeders Distribution System

The above-mentioned disadvantage of a radial system can be minimized by introducing parallel feeders. The initial cost of this system is much more as the number of feeders is doubled. Such system may be used where reliability of the supply is important or for load sharing where the load is higher.

 

Ring Main Distribution System

A similar level of system reliability to that of the parallel feeders can be achieved by using ring distribution system. Here, each distribution transformer is fed with two feeders but in different paths. The feeders in this system form a loop which starts from the substation bus-bars, runs through the load area feeding distribution transformers and returns to the substation bus-bars. The following figure shows a typical single line diagram of a ring main distribution system.

 

Ring main distribution system is the most preferred due to its following advantages.

1. There are fewer voltage fluctuations at consumer's terminal.
2. The system is very reliable as each distribution transformer is fed with two feeders. That means, in the event of a fault in any section of the feeder, the continuity of the supply is ensured from the alternative path.

 

Interconnected Distribution System

When a ring main feeder is energized by two or more substations or generating stations, it is called as an interconnected distribution system. This system ensures reliability in an event of transmission failure. Also, any area fed from one generating stations during peak load hours can be fed from the other generating station or substation for meeting power requirements from increased load.