Electrical Power Transmission System
Single line diagram of AC power transmission system:
A typical single line diagram of the flow of energy in a given power system is as follows:
Usually, electric power is generated at 11 kV
in generating stations in India and Europe. While in some cases, generation
voltage might be higher or lower. Basically, generating machines of power
stations are available between 6 kV to 25 kV from some big manufacturers. This
generating voltage is then stepped up to 132kV, 220kV, 400kV or 765kV etc.
Stepping up the voltage level depends upon the distance at which power is to be
transmitted. Longer the distance, higher will be the voltage level. Stepping up
of voltage is to reduce the I2 R losses in transmitting the power. I2 R loss is the copper loss which indicates that the energy lost each second, or power, increases as the square of the current through the windings and in proportion to the electrical resistance of the conductors. When voltage
is stepped up, the current reduces by a relative amount so that the power
remains constant. As a result, I2 R loss also reduces. This step is known as
The voltage is stepped down at a receiving station to 33kV or 66kV. Secondary transmission lines appear from this receiving station to connect substations located near load centers such as cities and villages. Again, the voltage is stepped down to 11kV at a substation. Electricity can be supplied to large industrial consumers at 11kV directly from these substations. Feeders also appear from these substations. This step is known as primary distribution.
Feeders can be either overhead lines or underground cables which carry power near to the load points or end consumers up to a couple of kilometers. Finally, the voltage is stepped down to 415 volts by a pole-mounted distribution transformer and delivered to the distributors. Electricity is supplied to end consumers through a service mains line from distributors. Feeders, distributors and service mains are included in the secondary distribution system.
Types of transmission systems
Single phase AC system
- single phase, two wires
- single phase, two wires with midpoint earthed
- single phase, three wires
Two phase AC system
- two-phase, three wires
- two-phase, four wires
Three phase AC system
- three-phase, three wires
- three-phase, four wires
- DC two wires
- DC two wires with midpoint earthed
- DC three wires
Underground cables can also be used to carry electric power for the transmission. The construction of an underground transmission line generally costs 4 to 10 times than an equivalent distance overhead line. Therefore, the cost of constructing underground transmission lines highly depends upon the local environment. Apart from this, the cost of conductor material required is one of the most substantial charges in a transmission system. As the conductor cost is a major part of the total cost, it has to be taken into consideration while designing. Reliability, efficiency and economy are some important factors for choosing the transmission system. Considering all aspects, overhead transmission system is generally used.
Core elements of a transmission line
overhead system is widely used for electric power transmission considering economic factor. The core elements of a typical power system are as follows:
- Conductors: three for a single circuit line and six for a double circuit line. The cross sectional area of conductors must be sized appropriately which basically depends on the capacity of its current capacity. ACSR (Aluminium-core Steel-reinforced) conductors are basically used.
- Transformers: For stepping up the voltage level, step-up transformers are used. On the other hand, for stepping down the voltage level, step-down transformers are used. Transformers allow power to be transmitted at higher efficiency.
- Line insulators: Line insulators are used to support the line conductors mechanically during electrically isolating them from the support towers.
- Support towers: Support towers are used to support the line conductors suspending in the air overhead.
devices: In order to protect the transmission system and ensure reliable operation, protective devices are used. Ground wires, lightening arrestors, circuit breakers, relays etc. are used as protective devices.
regulators: In order to keep the voltage within permissible ranges at the receiving end, voltage regulators are used.