3.2. Systems of Transmission
For transmission of electrical power three-phase circuits are generally used because of economical reasons. Transmission lines may be classified as follows:
- Single line
- Parallel lines
- Radial lines
- Ring system
1. Single line:
- The simplest form is the single line, such as obtained from a power plant supplying its entire output to one load centre over a single-circuit line.
- Such a system has the disadvantage that in case of damage to the line the service is interrupted.
- Its use is more or less confined to small power systems and is therefore becoming more and more uncommon.
2. Parallel lines:
- Where continuity of service is necessary, it is best to use at least two circuits in parallel, placed either on the same supports or on separate supports.
- Separate supports afford greater safety against both lines being damaged at the same time, but the cost is much higher than when two circuits are placed on one support.
- In some cases, where very large quantities of power must be handled, more than two circuits may be run in parallel.
3. Radial lines :
- Invariably a power plant or substation supplies power to the neighbouring territory by means of radial lines.
- These radial lines may be either single circuit for the less important loads or double circuit for the more important loads.
4. Ring system:
- For systems covering a large territory the ring system of transmission is very important.
- With this system the main high-voltage power line makes a closed ring, taps being taken off at any advantageous point of the ring, thus supplying a large territory.
- In case of damage to any section of the ring, that section, may be disconnected for repairs, and power will be supplied from both ends of the rings, thereby maintaining continuity of service.
- A network often constitutes several ring systems with sections of single, parallel, or radial lines.