What is a distribution system?

The final stage in the delivery of the electric power is the electric power distribution system. From the transmission system to the individual customers it carries electricity. The distribution system is connected with the transmission system and is used to lower the voltage of the transmission to medium voltage in the range of 2 kV to 35 kV with the help of transformers. This medium voltage to the distribution transformers located near the customer's premises is carried by the primary distribution system lines. These distribution transformers in turn lower the voltage to utilization voltage used in lightings, household appliances, industrial equipments etc. Through secondary distribution lines often several customers are supplied from one transformer. It is through service drops, the commercial and residential customers are connected to the secondary distribution system lines. Customers who demand a larger amount of power can be connected directly to the primary distribution system level or the sub-transmission level.

Functions of Transition

The transition which happens in a power substation from the transmission to distribution system has the following functions :

• The switches and circuit breakers enable the substation to disconnect from the transmission grid or to disconnect the distribution line.
• The transformers help to step down the voltages up to 35 kV or more down to primary voltage. Usually, up to 600-35000 V, they are considered medium-voltage circuits.
• The power from the transformer device goes to the busbar from where the distribution system power gets split or gets segregated off in multiple directions. To the distribution lines, the bus distributes the power which fans out to customers.

In common utility ducts, the urban distribution system is mainly underground. With the utility poles, the rural electric distribution system is always above the ground and the sub-urban distribution system is a mix.

Generation and Transmission

When the potential difference is as high as 33000 volts, the electric power begins at a generating station. The AC or the alternating current is used. To provide DC for the public supply, the users of some large amount of DC power such as some telephone exchanges, railway electrification systems, and industrial processes use rectifiers. To control the quantity of electricity transmitted and for isolating alternating-current systems, the high-voltage DC can be advantageous.
In the case of generating stations, it goes to the switchyard of generating stations where a step-up transformer is present. It increases the voltage to a level suitable to provide for transmission from 44 kV to 765 kV. The electricity from each generating station is combined with electricity produced somewhere else once in the transmission system. As soon as the electricity is produced it gets consumed. Close to the speed of light, it is transmitted at a very high speed.

Primary distribution

The voltages in the primary distribution system range from 4 kV to 35 kV phase-to-phase and 2.4 kV to 20 kV in case of phase-to-neutral. From distribution system voltages only large customers are fed directly and most of the utility content customers are directly connected to the transformers that reduce the voltage of distribution to low-voltage utilization, mains, or supply voltage used by interior wiring systems and lightings.

Network configurations

The electrical distribution systems are divided into two radial categories or networks. A distribution that is arranged like a tree where every customer has one supply source is radial type whereas the distribution that has multiple sources of supply in parallel is network type. For concentrated loads, spot networks are used. The radial system is commonly used in rural or suburban areas. In case of problems such as fault, the radial systems usually consist of emergency connections where the system can be reconfigured. To isolate a certain section from the grid, this can be done by opening or closing switches. The capacitors or voltage regulators are required when long feeders experience voltage drops.

Rural service

Because of the long-distance covered by the distribution lines, the rural electrification systems tend to use higher distribution voltages. The rural service tries to minimize the number of overhead distribution wires and poles. In urban distribution, they use higher voltages which in turn permits the use of galvanized steel wire. The steel wire allows for wide pole spacing, is less expensive and cost-effective. A pole-mounted transformer can serve only one customer in rural areas.

Secondary distribution

Depending on the region the electricity is delivered at a frequency of 50 or 60 Hz. A single-phase electric power is delivered to domestic type customers. A three-phase supply may be made available for larger properties in the case of some countries such as Europe. In terms of power content delivered per cable used, the three-phase electric power is more efficient. For the customer's system as well as for the equipment owned by the utility, a ground connection is normally provided.

Modern distribution system

The distribution system would only operate from the transmission networks, as a result, the electricity would be shared among the customers. Using distributed generation resources such as wind energy or solar energy resources the distribution system in the present time is heavily integrated with renewable energy generations at the level of distribution of the power systems. Day-to-day the distribution networks are becoming more independent from the transmission networks. It is extremely challenging to balance the supply-demand relationship at these modern distribution networks, and also it requires various use of technological and operational means to operate. Such tools include storage power station, battery, optimization tools, data analytics, etc.

Context and Applications

This topic is significant for undergraduate, graduate, and postgraduate courses like:

• Bachelors of Technology in Electrical Engineering
• Masters of Technology in Electrical Engineering

Practice Problems

Q1. What is the final stage of delivery of electric power?

1. Transmission system
2. Generation system
3. Distribution system
4. Both a or b

Answer: Option c

Explanation: The final stage in the delivery of electric power is the electric power distribution system.

Q2. Which of the following disconnects the distribution lines or the transmission grid?

1. Switches
2. Circuit breakers
3. Both a and b
4. None

Answer: Option c

Explanation: The switches and circuit breakers enable the substation order to disconnect from the transmission grid or to disconnect the distribution line.

Q3. What kind of distribution voltages does the rural system use?

1. Low
2. High
3. Very low
4. Vary from low to high

Answer: Option b

Explanation: The distribution lines cover long distances, therefore the rural electrification systems tend to use higher distribution voltages.

Q4. Which transformer increases the voltage in generating stations?

1. Step-up transformer
2. Step-down transformer
3. Autotransformer
4. None

Answer: Option a

Explanation: In the case of generating stations, it goes to the switchyard of generating stations where a step-up transformer is present. It increases the voltage to a level suitable for transmission from 44 kV to 765 kV. 

Q5. From where does the distribution system power gets split?

1. Busbar
2. Overhead lines
3. Conductor
4. None

Answer: Option a

Explanation: The power from the transformer device goes to the busbar from where the distribution system power gets split or gets segregated off in multiple directions. 

Related Concepts

• Backfeeding
• Cost of electricity source
• Electric utility
• Electric generation
• Power distribution unit
• Transmission system operator