Below is given the table which shows the ratio of conductor-material in any Comparison of Various Systems of Transmission with that in the corresponding 2-wire d.c. system. Cos Φ is the power factor in an a.c. system.
The following points may be noted from the above table :
- There is a great saving in conductor material if d.c. system is adopted for transmission of electric power. However, due to technical difficulties, d.c. system is not used for transmission.
- Considering the a.c. system, the 3-phase a.c. system is most suitable for transmission due to two reasons. Firstly, there is considerable saving in conductor material. Secondly, this system is convenient and efficient.
Elements of a Transmission Line
For reasons associated with economy, transmission of electric power is done at high voltage by 3-phase, 3-wire overhead system. The principal elements of a high-voltage transmission line are
- Conductors, usually three for a single-circuit line and six for a double-circuit line. The usual material is aluminium reinforced with steel.
- Step-up and step-down transformers, at the sending and receiving ends respectively. The use of transformers permits power to be transmitted at high efficiency.
- Line insulators, which mechanically support the line conductors and isolate them electrically from the ground.
- Support, which are generally steel towers and provide support to the
- Protective devices, such as ground wires, lightning arrestors, circuit breakers, relays etc. They ensure the satisfactory service of the transmission line.
- Voltage regulating devices, which maintain the voltage at the receiving end within permissible limits.
Economics of Power Transmission
While designing any scheme of power transmission, the engineer must have before him the commercial aspect of the work entrusted to him. He must design the various parts of transmission scheme in a way that maximum economy is achieved. The economic design and layout of a complete power transmission scheme is outside the scope. However, the following two fundamental economic principles which closely influence the electrical design of a transmission line will be :
- Economic choice of conductor size
- Economic choice of transmission voltage