Page 316 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

From the above comparison, it is clear that high voltage DC transmission is superior to high voltage AC transmission.
At present, transmission of electric power is carried by AC and effort is making towards DC transmission also.
The convertor and inverter have made it possible to convert AC into DC and vice versa easily. Such devices can
operate upto 30MW at 400KV in single units. The present day trend is towards AC for generation and distribution
at high voltage DC for transmission.

The AC power at high voltage is fed to the convertor which convert AC to DC. The transmission of electric power is
carried at high DC voltage. At the receiving emf DC is converted into AC with the help of invertors. The AC supply
is stepped down to low voltage by receiving end transformer (T ) for distribution.
R
Power transmission (Fg 4)
Introduction
Electrical energy is carried by conductors such as overhead transmission line and underground cables. The
properties for several types of transmission lines are high voltage, low voltage, high power, low power, overhead
and underground cables.

In order to provide electrical energy to consumers in usable form, a transmission and distribute on system must
satisfy some basic requirements they are
i provide power at all times.
ii maintain a stable, nominal voltage does not vary by more than ± 10%.

iii maintain a stable frequency that does not vary by more than ± 2Hz
iv supply energy at an acceptable price
v meet standards of safety
vi respect environmental standards

Fig 4

Tranmission substations
To change the line voltage by means of step up and step down transformers and regulate it by means of
synchronous condensers or transformers with variable taps.
Distribution substations
Change the medium voltage to low voltage by means of step down transformers which may have automatic tap
changing capability to regulate the low voltage.
Interconnecting substations
To increase the stability of the overall network it is essential to tie different power systems together to enable
power exchange between them.

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CITS : Power - Electrician & Wireman - Lesson 106-116 CITS : Power - Electrician & Wireman - Lesson 106-116