Noun as adjective’ structure

Unit 8

Insulators, Cables and Capacitors

Reading I (detail reading)

Task 1

Read the text and answer the following questions:

1. How is power transmission carried out?

2. What are the reasons for choosing this or that type of cables?

3. Why has the matter of reliability of cables become more important nowadays?

4. What materials are commonly used for making cables?

5. What materials can be used for insulation?

6. What are the reasons for the choice of different insulation materials?

Cables

Cables have been used in transmission and distribution networks since the early days of the electrical power industry. Generally, long distance power transmission is carried out through overhead lines. However, transmission and distribution in densely populated urban areas mostly uses underground cables. Although significantly more expensive than the overhead lines, the cables are preferred in urban areas due to safety, reliability and aesthetical considerations. As a result of developments in insulating materials and manufacturing techniques, high voltage cable technology has improved significantly over the years. With a continuous increase in the overall length of cable networks, questions regarding reliability, failure modes and diagnostics of such cables have assumed greater significance.

CABLE MATERIALS

Conductors

Cables are constructed using a variety of materials for conductors, insulation, screening and armoring. The most common conductor materials are copper or aluminum of high purity (99,5% purity), since the resistivity of such materials significantly increases with impurity content. Sometimes, sodium is also used as the conductor material. In recent years, the possibility of using high temperature superconducting materials for power applications is also being examined. The choice between aluminum and copper is normally based on resistivity (ρ), cost as well as mechanical and manufacturing considerations. Table 9 compares resistivity values for a few materials used in cables. For a given current rating, aluminum requires a larger conductor cross-sectional area than copper. Both solid and stranded conductors are used; the choice depends upon total cross-sectional area, flexibility and manufacturing considerations. For larger cross sections, stranded construction with alternate layers spiraled in opposite directions is generally preferred.

Insulation

The most important insulation material is air. A variety of solid, liquid, and gaseous insulators are also used in electrical apparatus. In smaller transformers, generators, and electric motors, insulation on the wire coils consists of up to four thin layers of polymer varnish film. Film insulated magnet wire permits a manufacturer to obtain the maximum number of turns within the available space. Windings that use thicker conductors are often wrapped with supplemental fiberglass insulating tape. Windings may also be impregnated with insulating varnishes to prevent electrical corona and reduce magnetically induced wire vibration. Large power transformer windings are still mostly insulated with paper, wood, varnish, and mineral oil; although these materials have been used for more than 100 years, they still provide a good balance of economy and adequate performance.

The cable insulation usually consists of impregnated paper, synthetic polymers and compressed gases. Early cables mostly used impregnated paper insulation. Next stage was the introduction of oil-filled paper insulation. At present, polymers are widely used. The polymers most often used are polyvinyle chloride (PVC) for low-voltage cables, polyethylene (PE), cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR) for medium-voltage cables; and XLPE and EPR for high-voltage cables. Flexible insulating materials such as PVC are used to insulate the circuit and prevent human contact with a 'live' wire – one having voltage of 600 volts or less. Alternative materials are likely to become increasingly used due to EU safety and environmental legislation making PVC less economic.

Table 9Resistivity of Some Conductors at 20°C

Task 2Translate the whole text and say what you have learned about cables from it.

Listening

Task 3 1.Margit, an electrical engineer, is describing a high voltage cable. Before you listen, label the cross-section with the parts (a-e).

a) insulation

b) waterproof membrane

c) outer jacket

d) armoured protection

e) conductor

2.Listen to the description and check your answers.

3.Listen again andmatch the parts of the cable (a-e) to the following categories of materials (1-5). You will need to use some parts more than once.

1. non-metallic _____ _____ _____

2. metallic _____ _____

3. ferrous metal _____

4. non-ferrous metal _____

5. polymer-based _____ _____ _____

Language Study

Noun as adjective’ structure

Task 4. Find examples of nounsin the function ofan attribute in Text I. Translate them.

Reading II (for specific information)

Task 5.

Read the text and find a missing word for the following sentences:

1. A capacitor consists of two __________.

2. A standard instrument cable in its basic form is like a __________.

3. Low frequencies are __________ affected by cable capacitance.

4. Cable capacitance __________ high frequencies.

5. Some manufacturers make special cables for a certain __________.

6.__________ capacitance cables allow better performance of the guitar.

Date: 2015-12-18; view: 672