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Compressed-gas-insulated Transmission

Transmission lines in which compressed gas is used as insulator have a number of advantages. The main advantages are simplicity of I construction and low cost.

What is the construction of compressed-gas-insulated transmission 1 lines? It is rather simple. They comprise a number of phase conductors; each phase conductor is placed inside a tube and centered by means of cir­cular spacers. The tube space is filled with compressed gas - usually sul­phur hexofluoride. Each tube in a 345-kV line has a diameter about 50 cm.

The system including compressed-gas-insulated transmission has the following advantages: its losses are rather low, they are considerably lower than those of cable transmission. Unlike cables, compressed-gas-insulated transmission system can be designed for ultra high frequencies. No external electric field appears in the system. The shunt capacitance is considerably less for a gas-insulated line than for a cable. A gas-insulated line can thus transmit power over larger distances than cable lines.

The system should be protected against metallic particles. In case metallic particles get into the system, they cause a fault - a dielectric breakdown.

1. Does the system described have any advantages? What are they?

2. What gas is the space filled with?

3. Why should the system be protected?

Unit VI

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Thermal Steam-turbine Power Plants

Large steam-turbine plants have two forms: condensing plants or electric power plants.

He great masses of hot steam, having accomplished the mechanical work in the turbines of condensing steam-turbine plants, are condensed, i.e. are cooled down and turned back into distilled water, and returned to the boiler for production of steam to activate the turbine.

Condensation of steam takes place in condensers where the hot steam is cooled when it comes in contact with tubes through which cold water, supplied from a water reservoir (river or lake), is circulated. This cooling water, after it takes the heat from the spent steam, is returned to the water source carrying alond with it the unutilized heat energy. This water is called the circulating water. The importance of the distilled water for feeding steam boilers is extremely great since chemically clean water decreases the formation of scale in the boiler tubes, and, thus, makes their service life longer.

Condensing plants of large generating capacity are built close to sources of fuel, in order not to transport large quantities of fuel over considerable distances.

The electric rower generated in such plants is transmitted over long distances for the supply of large industrial regions. So these plants are called regional thermal power plants.

Heat and electric power plants, in addition to electric power generation, also supply heat to closely located consumers (within a radius of 50 km), i.e. serve as district hest plants. To such heat consumers belong all kinds of industrial enterprises that require heat for production purposes, and also municipal consumers such as baths, laundries and the heating systems of dwelling houses and other buildings.

The electric power developed by the generators is fed to the switch-board of the plant, whence it is delivered by overhead transmission and cable lines to the consumers.

For several reasons, the relative importance of the various types of power plants has been shifting. Good sites for new hydroelectric plants have become scarce in many countries. Distribution networks have been extended so that less expensive power from large steam-electric stations has been replacing power from smaller diesel-generator units. Nuclear-electric power plants have been built instead of fossil-fuel steam-electric plants because the cost of coal and oil has been increasing.

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