Page 771 - Industrial Power Engineering and Applications Handbook

23/728 Industrial Power Engineering and Applications Handbook
as discussed in Section 7.22 the number of deratings 0.71, 0.91‘
for all unfavourable operating conditions are assumed
to be occurring at the same time. This results in the
selection of an oversized machine as all the unfavourable
conditions may not be occurring simultaneously.
Wide voltage fluctuations may be prevalent in a rural
distribution system, particularly in developing countries.
In such cases it is common practice for users to select
an oversized machine for their needs. Accordingly,
the motors employed for loads such as pumps, thrashers
and winnowers are normally over-rated and under-
utilized. Also the same motor may have to cater for 0.21,
different types of loads, at different times, and these by0.21, i.e. m o r
f
.
loads may be much less than the motor rating. 28.6% of the existing
system for the same
I‘R tosses
The power factor of a motor decreases sharply at loads I,
lower than rated as discussed in Section 1.8. All the
above factors, contribute to reducing the overall system Figure 23.2 Better utilization of power with improved power
power factor, which is sometimes seen to reach a low of factor
0.6 or even less on an LT distribution network.
Higher line losses result in higher voltage fluctuations
due to the greater line drop (IZ). In an HT system, the As a result of higher voltage drops, the receiving-end
voltage at the receiving end is not as much affected as in voltage may sometimes fall below the required limit which
an LT system due to the lower voltage drop on an HT in, electric motors, is -6% (Section 1.6.2). Motors for
system as a percentage of the system voltage. An LT such locations may be required to be suitable for voltages
system experiences a much higher fluctuation, as a lower than the standard system voltages such as 400 V,
percentage of the supply voltage, due to excessive voltage 380 V or even less, as against the standard 415 V. A
drops. A higher current for a smaller load, due to a low motor designed especially for a lower voltage than standard
power factor, reduces the capacity of the distribution may sometimes require a larger frame size at a higher
network. In other words, it decreases the generating capacity cost.
of the power plant, feeding such a system. For instance,
for the same power generated or distributed ‘P’
23.4 Other benefits of an improved
P= &.V.I-cos$
power factor
Therefore, for a given load current I,
P = cos $ Reduced kVA demand will result in lower tariffs since
the electricity companies usually charge users on the
Thus, at 0.70 p.f., the generating or the distributing capacity basis of their maximum kVA demand.
of the system, compared to a system having a p.f. of In certain countries the consumer may even be entitled
0.90, will reduce to approximately 77.8%, (0.7/0.9 x to a rebate for maintaining a high power factor, instead
100%). In other words, if we consider a distribution p.f. of paying a higher tariff.
0.70 to be improved to 0.9, then the useful power can be Lower kVA demand will reduce the load current (due
enhanced by to reduced Z2R losses) and result in an economical
selection of switchgear components and cables.
0.90 - 0.70 100% Lower voltage drops in the lines and cables and thus
0.70 lesser voltage fluctuations.
i.e. 28.6% of the existing system (see Figure 23.2). The electricity companies would also benefit due to
For the same generation or distribution current I, at a p.f. better utilization of their distribution system and make
cos $,, utilization of the system will be more power available to consumers.
PI = I x cos @I
whereas at an improved p.f. cos 4, it will be 23.5 Behaviour of a power capacitor
P, = I x cos & in operation
Thus while the Z2R loss will remain the same in both
cases, at a lower power factor the utilization capacity of Before we discuss the application of this device, it is
the system will reduce in the same proportion as the important that we study its behaviour during operation.
A capacitor unit behaves like a short-circuit on being
power factors, i.e.
energized and retains its charge for a brief period even
cos @I when the source of supply is removed. This behaviour
cos $2 gives rise to the following: