Exercise 1. Answer the questions

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Electrical energy from power plants is transferred to substations lo-

cated near cities. Transmission lines are high-voltage transmission net- works. They are usually called “power grids” or simply “grids”. Transmis- sion lines use three-phase alternating current (AC). Railway electrification systems sometimes use single phase AC. It is more economical to transmit electricity at high voltages (110 kV or above) to reduce the energy loss in long distance transmission. Electric power is usually transmitted through overhead power lines. Underground power transmission lines through ca- bles are more expensive but are often used in cities.

Electrical energy cannot be stored and therefore must be generated when it is needed. Electrical energy supply must be equal to the demand. If supply and demand are not equal, generation plants and transmission equipment can shut down. That is why electric transmission networks are interconnected into regional, national or continental wide networks. This interconnected power system provides alternate routes for electric power to another part of the network.

Subtransmission lines carry voltages reduced from the major trans- mission line system (typically,

34.5 kV to 69 kV). This power is sent to regional distribution sub- stations.

Overhead AC transmission lines usually carry three-phase current. The voltages can be dif- ferent according to the particular

grid system they belong to. Transmission voltages vary from 69 kV up to 765 kV.

The DC voltage transmission tower has lines in pairs rather than in threes (for three-phase current) as in AC voltage lines. One line is the posi- tive current line and the other is the negative current line.

High-voltage overhead lines are made of aluminum wires not cov- ered by insulation. Wires have several strands and are often reinforced with steel strands. Copper was sometimes used before for overhead transmission

but aluminum weighs less and is cheaper. Overhead conductors can have cross sectional area from 12 mm² to 750 mm². Transmission voltages are usually 110 kV and above. Voltages 66 kV and 33 kV are sometimes used on long lines with small loads. Voltages less than 33 kV are usually used for distribution.

Insulators are usually made of porcelain or glass and can be stacked in a suspension in- sulator string. They hang from a cross arm on a tower or pole and support the line conductor.

Suspension insulators are used for very high voltage systems.

A pothead is a type of insulator with a bell- or pot-like shape used to connect underground electrical cables to overhead lines. It separates the conductors from one another in the cable. It also seals the cable end from the weather. Potheads are mounted on a distribution pole.

Air circuit breakers are used to break circuits while current flows through them. Compressed air is used to quench the arc when the connec- tion is broken.

Bus support insulators are porcelain or fibreglass insulators that serve to isolate the bus bar switches and other support structures and to prevent leakage current from flowing through the structure or to ground. These insu- lators are similar in function to other insulators used in substations and transmission poles and towers.

Capacitors are used to control the level of the voltage supplied. Oil circuit breakers are used to switch circuits and equipment in and out of a system in a substation. They are oil filled to provide cooling and to prevent arching when the switch is activated.

Circuit switches provide equipment protection for transformers, lines, cables and capacitor banks. They are also used to energize and reen- ergize capacitor banks and other circuits.

Exercise 1. Answer the questions

1. What is the usual way of transmitting electric power?

2. Can electrical energy be stored?

3. What types of transmission lines are there?

4. Why is electricity transmitted at high voltages?

5. Why are electric transmission networks interconnected into wider networks?

6. Which type of insulator is used to connect underground cables to overhead lines?

7. What is used to control the voltage level supplied?