Audioscript

Section 1

Moving on now to the transmission of electric power. Put simply, this means the system for sending electricity efficiently and in large amounts from the generation stations to the consumers. However, transmission is also used to interconnect adjacent power systems, so that one can supply another in case of emergency, or to co-operate in power sharing in order to reduce costs. We all take electric power supply for granted until there is a power failure. But I wonder if you know when power transmission began in the United States and how it has increased? Let's have a look. In front of you there is a graph. The horizontal line, or axis as it is called, shows time divided into ten-year periods. You will see that it begins with 1890, at the base of the vertical line and goes up in ten-year stages to the year two thousand.

The vertical axis shows line-to-line voltage measured in kilovolts. That begins with one hundred, above the year 1890, and goes up to one thousand. You can see it is divided into units of one hundred. These numbers help us to see how transmission voltage has increased during the period 1890 to 1980.

Section 2

If we study the graph, we can see that in 1890 only a maximum of 10,000 volts could be transmitted. That increased to 50,000 in 1900 and in 1910 the voltage transmitted was 100,000 or 100 kilovolts. Between 1910 and 1925, the amount transmitted increased steadily to 220,000 volts. This remained the same for the next ten years until 1935. In that year, the amount increased to 300,000 volts. 300,000 volts was the maximum transmission voltage until 1950.

Between 1950 and 1965 there was a small increase in transmission voltage. In 1965, the maximum that could be transmitted stood at 350 kilovolts. Around this time, transmission systems and techniques developed and, in 1970, 500 kilovolts was the maximum that could be transmitted. This large increase in transmission capability continued and in 1980 line-to-line voltage had reached 765 kilovolts.

Section 3

One final word. The figures on the graph refer, as you can see, to alternating current. Direct current is transmitted for some applications and it has advantages. It is, however, an expensive transmission method at the moment.

Well, I shall finish at that point. My next lecture will be on Thursday.

Дата добавления: 2015-08-03