Single line diagram of AC power transmission systemA typical single line diagram that represents the flow of energy in a given power system is shown below:
Electrical power is normally generated at 11kV in a power station. While in some cases, power may be generated at 33 kV. 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 I2R losses in transmitting the power (when voltage is stepped up, the current reduces by a relative amount so that the power remains constant, and hence I2R loss also reduces). This stage is called as primary transmission.
The voltage is the stepped down at a receiving station to 33kV or 66kV. Secondary transmission lines emerge from this receiving station to connect substations located near load centers (cities etc.).
The voltage is stepped down again to 11kV at a substation. Large industrial consumers can be supplied at 11kV directly from these substations. Also, feeders emerge from these substations. This stage is called as primary distribution.
Feeders are either overhead lines or underground cables which carry power close to the load points (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. End consumers are supplied through a service mains line from distributors. The secondary distribution system consists of feeders, distributors and service mains.
Different 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 system
- DC two wires
- DC two wires with midpoint earthed
- DC three wires
Main elements of a transmission lineDue to the economic considerations, three-phase three-wire overhead system is widely used for electric power transmission. Following are the main elements of a typical power system.
- Conductors: three for a single circuit line and six for a double circuit line. Conductors must be of proper size (i.e. cross-sectional area). This depends upon its current capacity. Usually, ACSR (Aluminium-core Steel-reinforced) conductors are used.
- Transformers: Step-up transformers are used for stepping up the voltage level and step-down transformers are used for stepping it down. Transformers permit power to be transmitted at higher efficiency.
- Line insulators: to mechanically support the line conductors while electrically isolating them from the support towers.
- Support towers: to support the line conductors suspending in the air overhead.
- Protective devices: to protect the transmission system and to ensure reliable operation. These include ground wires, lightening arrestors, circuit breakers, relays etc.
- Voltage regulators: to keep the voltage within permissible limits at the receiving end.