Environmental Engineering Reference
In-Depth Information
Voltages need to be kept within, typically, 6 per cent of their nominal values.
Therefore, the transmitted power is determined mainly by the power angle d -hence
its name.
Sending-end voltage does not need to be greater than receiving-end voltage for
power to flow to the receiving-end. This can be seen from Figure 2.26 for a load
with leading power factor. Maximum transmissible power
Note that the power transmitted for a loss-less line is a maximum when the power
angle is 90 . The maximum transmissible power will then be, from (2.23),
The sending- and receiving-end voltages will be close to the rated values. Thus
the key factor limiting transmissible power, apart from the conductor current rating,
is reactance. This is determined by line length ( X ¼w l length).
P RM ¼
2.5.4 Voltage regulation
Consider the case of a generator supplying a large system through an overhead line,
as shown in Figure 2.27.
It is assumed that the system is large, and that its voltage is fixed. We wish to
study the variation of the generator's voltage with generated active and reactive
power. The generator voltage is given by
V g ¼ V þð
j X
We will examine the effect of active and reactive power separately. Con-
sidering active power first, the current I will be in phase with the generator voltage.
The relationship between generator voltage and system voltage may be seen from
the phasor diagram shown in Figure 2.28.
It may be seen from Figure 2.28 that the voltage rise due to active power
generation is given approximately by
RV g I
V g ¼
V g
D V a RI ¼
R + j X
V g
V System
Figure 2.27
Generator feeding large system
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