Before discussing the details of an electrical transmission system, we first try to understand why transmission of electricity requires a high-voltage transmission system.
Requirement of Electrical Transmission System
All the generating stations connected to a grid generate electric power at a low voltage level. Because generating electrical power at a low voltage level is economical. Actually, in an alternator, the magnetic system rotates and the stator holds the armature. At high voltage levels, this stator requires thick and complex insulation. Obviously, the thick and complex insulation makes the alternator heavier and larger.
Although at a low voltage level, the current is high for the same quantity of power. Certainly, this causes an increase in the diameter of the armature conductor. Obviously, this also makes the stator bigger. However, insulation is less here due to the low voltage level. This optimizes the overall size of the alternator. This is the main reason why we generate power at a low voltage level.
However, we should not transmit this low-voltage power to the receiving end through a transmission line. We normally transmit this power to the receiving end at a high voltage level with a 3-phase system. This is the most economical approach.
Advantages of High Voltage Transmission
There are many advantages that make high voltage transmission essential. When we need to transmit power over large distances, it is always desirable to transmit it at a very high voltage level in a 3-phase system. 3-phase transmission offers a more economical approach than a single-phase system.
When we transmit a certain amount of power from one point to another point at a high voltage level, the current level of the power reduces. This is the main reason, or you can say this is the only reason, why we decide to transmit power at a high voltage level. Reducing current reduces the I²R loss or copper loss in the conductor. Additionally, the thinner conductors can transmit that low current power. So, we can also cut the transmission cost.
Consumer End Requirements
Normally, at the consumer end, consumers use electric power at a much lower voltage level. Obviously, here, we have to step down this higher voltage level to a lower voltage level for this purpose.
Total Structure of Electrical Transmission
The total structure of electrical transmission is as follows.
We generate power at a low voltage level. Then we step up the low voltage to the high voltage level. Then we transmit that high voltage power to the receiving end. At the receiving end, we again step down the power.
Main Parts of a Transmission System
There are three main parts of a transmission system:
- Voltage Transformation: The arrangement for stepping up and stepping down of power. This means stepping up towards the generating end and stepping down towards the receiving end.
- Transmission line: It consists of both overhead and underground electrical transmission system networks.
- Maintenance & Protection System: There are protection and maintenance systems associated with the system.
Components of Transmission System
One by one, we can describe the main components of a transmission system:
Generating Station
The generating station generates electric power. There are many types of generating stations, for example:
- Thermal power station
- Diesel power station
- Atomic power station
- Hydro power station
- Wind power station
- Solar power (nowadays)
All these generating stations generate power at a low voltage level.
Generating Substation (Step-Up Substation)
The step-up substation consists of step-up transformers along with their maintenance and protection systems. A generating substation is a substation situated just after the generation point. This substation consists mainly of step-up transformers. The step-up transformers step up the low-voltage power to high-voltage power for transmission.
Transmission Line
Either of high voltage, extra high voltage, or ultra high voltage transmission line transmits the power to the receiving end. The transmission line is either an underground or overhead electrical line. At the receiving end substation, step-down transformers lower the voltage level for sub-transmission and distribution purposes.
Step-Down Substation
The step-down substation consists of step-down transformers and associated maintenance and protection systems.
Sub-Transmission System
The sub-transmission system is a kind of transmission system that transmits power from the main receiving end substation to distribution substations. After the main transmission, the step-down transformers feed sub-transmission lines. Then we have primary distribution and secondary distribution with their associated step-down transformers.
Distribution System
From the distribution substation, the power ultimately reaches the consumer end via the distribution network. This network includes intermediate step-down pole-mounted transformers.