Техническое чтение для энергетиков. Бухарова Г.П. - 59 стр.

UptoLike

ВУЗ: 

УлГТУ | Ульяновск

Составители: 

Рубрика: 

61
of one phase being broken.
In order to reduce the dynamic load on the tower Masts when a conductor breaks, it
was decided that the crossarm should be designed to fail at 60 per cent of the actual
broken-wire load, that is, a safety factor of one applied to the broken-wire load as
described above. Upon failure the crossarm would swing into the line, thus reducing
both the dynamic and the static load on the tower.
A total of 340 towers were constructed on this line over a period of 8 months.
The great majority of towers were erected by completely assembling the masts on the
ground and then erecting them by means of a mobile crane. A 2-masted tower took
approximately one day to assemble on the ground and 2 hours to erect.
Twenty-three months after commencement the line was completed; it was energized
at 280 kv on November 30, 1952.
TRANSFORMERS
The transformer is a device for changing the electric current from one voltage to
another. As a matter of fact, it is used for increasing or decreasing voltage. A simple
transformer is a kind of induction coil. It is well known that in its usual form it has no
moving parts. On the whole, it requires very little maintenance provided it is not mis-
used and is not damaged by lightning.
We may say that the principal parts of a transformer are two windings, that is
coils, and an iron core. They call the coil which is supplied with current the "primary
winding", or just "primary", for short. The winding from which they take the current
is referred to as the "secondary winding" or "secondary", for short. It is not new to
you that the former is connected to the source of supply, the latter being connected to
the load.
When the number of turns of wire on the secondary is the same as the number
on the primary, the secondary voltage is the same as the primary, and we get what is
called a "one-to-one" transformer. In case, however, the number of turns on the
secondary winding is greater than those on the primary, the output voltage is larger
than the input voltage and the transformer is called a step-up transformer. On the
other hand, the secondary turns being fewer in number than the primary, the
transformer is known as a step-down transformer.
The transformer operates equally well to increase the voltage and to reduce it.
By the way, the above process needs a negligible quantity of power. It is important to
point out that the device under consideration will not work on d.c. but it is rather
often employed in direct-current circuits.
Fig. 8 shows how transformers are used in stepping up the voltages for
distribution or transmission over long distances and then in stepping these voltages
down. In this figure, one may see three large step-up transformers which are used to
increase the potential to 275,000 volts for transmission over long-distance
transmission lines At the consumers end of the line, in some distant locality three
step-down transformers are made use of to reduce hat valued (i.e., 275,000 volts) to
2,300 volts. Local transformers, in their turn, are expected to decrease the 2,300 volts

Страницы