An Electrical Power Distribution System is a network designed to deliver electricity from the transmission system to individual consumers, such as homes, businesses, and industries. It involves a series of components and processes that ensure an efficient and reliable electrical power supply at the appropriate voltage levels.
Electricity is the lifeline of any economy in the world, and the demand for electrical power is rising exponentially. Power generation companies do their best to meet these demands efficiently and without disruption. Electrical power is generated at war footing using every possible resource – from fossil fuels to solar, wind, and nuclear.
However, generating huge amount of power and meeting the end-user demand are two different things. This is because merely having an efficient power generation mechanism in place is not enough but it should be backed by an efficient transmission as well as distribution system.
The electric power distribution diagram is shown below.
Power plants are located in remote areas from where it has to be transmitted to a distribution station in the city or village. This distribution station then dispatches the power to the consumers. Hence, without an efficient system of transmission and distribution, a huge proportion of the generated power will be lost chiefly as heat.
Moreover, this power is transmitted and distributed through open areas or public spaces. Hence, an efficient and safe system is important. Let’s understand how the generated power is distributed efficiently and reliably among consumers.
Key Components of Electrical Power Distribution System
Power is generated and transmitted at a higher voltage using a power transformer. This is done to reduce the reliance on higher currents for the transmission of this huge amount of power and to improve the power factor. However, at the distribution end, this voltage is again reduced before being distributed among the consumers. There are various parts of a power distribution system.
- Distribution Substation
- Distribution Feeders
- Distribution Transformers
- Distribution Conductors
- Service Mains Conductors
1. Distribution Substation
A distribution substation is the junction which receives the transmitted power. It is located at the outskirts of a city or a village or an industrial area. The transmitted power is at a very high voltage (like 400kV, 800kV etc. depending on the distance) which is stepped down at a distribution substation. The stepping down of the voltage is done at the required levels for Primary distribution which is followed by secondary distribution system.
Primary distribution system
The Primary distribution system caters to the power demand of big consumers, such as factories and industries. It operates at a higher voltage than that required for ordinary or residential consumers. The transmitted power is stepped down to the primary distribution voltage, which is usually 11kV but can be between 2.4kV and 33kV, depending on the demand type.
Commonly used primary distribution voltages in most countries are 11 kV, 6.6 kV and 3.3 kV. Primary distributor also feeds a small substation from where the secondary distribution is carried out. Primary distribution is carried out by three phase three wire system.
Secondary Distribution system
The Secondary distribution system caters to the power demands of residential consumers, small factories, shops, and other commercial setups. Domestic or residential consumers are supplied with single phase power supply at 220~240 Volts or 120 Volts (in the US and some other countries). Three-phase supply is also provided at 415 Volts to commercial buildings, malls, big properties, small factories, etc. Secondary distribution is carried out by a three-phase, four-wire system.
2. Distribution Feeders
The power lines that transfer the power from substation to the distribution transformers are known as distribution feeders. Feeders have no tapping to keep the current constant. The main consideration in the design of a distribution feeder is its current carrying capacity.
3. Distribution Transformers
A distribution transformer is a three-phase transformer that steps down the voltage from the distribution levels to the one suitable for the primary and secondary consumers. It is also known as a service transformer. The distribution transformer steps down the voltage to a level where the voltage between a phase and the neutral is 220V and between phase to phase is 415V. However, in the US and some other countries, the phase-to-phase voltage is 240V, while the single-phase voltage is 120V.
Distribution transformers normally have ratings of less than 200kVA. These transformers must work 24 hours to maintain the power supply; hence, they are designed to have lower iron losses. Usually, they don’t operate at full load; hence, they are designed to have greater efficiency even at smaller loads.
4. Distribution conductors
After the voltage is stepped down by a distribution transformer, the output is taken out by the distribution conductors. These conductors have tappings through which the power is distributed from the distribution end to the consumers. As there are multiple tappings, the current through the distribution conductors is not the same. The voltage drop, however, in the distribution conductors must be as low as possible.
5. Service Mains Conductors
Service mains conductors are the conductors that connect the distribution conductor at the nearest pole to the consumer’s facility.
The following picture shows a simple radial distribution system.
Classification of Electrical Power Distribution System
The electric power distribution system can be classified under various heads. The following are the types of distribution systems.
1. According to the nature of current
DC distribution system: The transmitted AC power can be rectified at the distribution substation using power electronic converters and can be distributed as per the consumer’s requirement. DC distribution is of two types
- Unipolar DC Distribution system (2-wire DC distribution)
- Bipolar DC Distribution system (3-wire DC distribution)
AC distribution system: It is the most widely used system of power distribution as almost all the loads, whether commercial or residential, run on AC power. Hence, the transmitted power at high voltage is stepped down to a suitable voltage for the end users at the distribution substation and then dispatched. There are various ways of AC power distribution
- Single-phase, 2-wire system
- Single-phase, 3-wire system
- Two-phase, 3-wire system
- Two-phase, 4-wire system
- Three-phase, 3-wire system
- Three-phase, 4-wire system
2. According to position
Overhead distribution system: This is the traditional and most common method of power distribution. In this method, the power is distributed through overhead wires attached to roadside poles. Transformers and other important equipment are mounted on these poles or similar supporting structures. This system is cost-effective and easy to maintain. However, being exposed to the outer atmosphere, it is subject to various weather phenomena.
Underground distribution system: Modern advancements in electrical power distribution has given the popularity to the underground distribution system. This system doesn’t need poles and supporting structures and has no overhead hanging wires. However it is costlier than its overhead counterpart and tough to maintain.
3. According to connection types
Radial distribution: This electric power distribution system is used where the distribution substation is located at the center of the consumers from where the feeders radiate and distribute the power in all directions. The flow of power in radial distribution is in one direction.
Radial distribution is cost-efficient, but a major drawback is that a fault in a feeder can result in a power outage for all the consumers connected to it.
Parallel feeder distribution system: The parallel feeder system was introduced to tackle the disadvantage of the radial distribution system. In this system, instead of a single feeder, parallel feeders are used for power supply. The cost of this system is higher, but it is more reliable than its radial system counterpart.
Ring Main Distribution System: A ring main distribution system comprises feeders connected in a closed loop or ring configuration, with distributors tapped from different points along the ring. This system forms a continuous loop network without any terminating end in the feeders.
The advantage of this system is its reliability, as each distribution transformer is fed by two feeders. This ensures the continuity of the power through an alternative path in the event of a fault in any section of the feeder.
