Page 813 - Industrial Power Engineering and Applications Handbook

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impracticable to keep a close monitoring of such system again. This switching is therefore more stressful for the
variations and alter the capacitive reactance manually to capacitor as well as for the system, due to surge voltages.
maintain the desired p.f. level. During a switch ON therefore some time delay must be
The situation in an HT installation will, however, be introduced into the switching circuit for the capacitor
different. An HT system is mostly a fixed-load one, where charge to decay to a safe level. In this sequence, the
a variation in the load may be only occasional and at capacitors of each step are normally the same. It is a
certain periodic intervals. To provide an automatic primitive and unscientific switching sequence and
switching in such cases will be a costly arrangement due therefore not in much use.
to expensive switchgear equipment. In addition, it will
also cause cumulative switching surges as a result of 23.16.2 True ‘first in first out’ (FIFO)
rapid reclosing and interrupting sequences unless a static
switching control is adopted as discussed in Section 24.10, Capacitors are switched ON in the sequence of 1-2-3
which will further add to the cost. To mitigate such . . . . n but an additional logic is used to switch OFF in
constraints, the following may be considered as the more the same sequence 1-2-3 . . . n, i.e. the oldest OFF capacitor
recommended methods for p.f. control of such installations: is switched ON first and the oldest ON capacitor is
switched OFF first. This apparently is the best switching
1 Identify the likely variations in load and p.f. during a sequence, giving enough time to an OFF capacitor to
24-hour period. Maintain a minimum fixed kVAr discharge before it is switched again. Each stage capacitor
permanently connected in the system, at the distribution rating must be equal to avoid a wide fluctuation in the
point for the likely constant loads. For the rest, the p.f. correction and hence undesirable subsequent
banks may be selected so that only one or two are switchings, which may be necessitated when the capacitors
sufficient to control the whole system for the desired are not of equal ratings.
p.f. level or system regulation. This will also limit
operations of the banks to only three or four a day 23.16.3 Pseudo (false) ‘first in first out’
and less than a thousand during a year, as recommended
in Section 26.1(2). Control may now be carried out: In a way this is more appropriate than the FIFO, for it
Manually, as variations in load would almost be can have unequal stage capacitors. Capacitors, however,
are arranged in ascending order such as 10, 20, 50 . . .
specific and periodic and easy to monitor or
By making it automatic, which would now be kVAr etc., or large or small capacitors. But, as mentioned
economical with one or two variable banks. above, this may not provide accurate correction in the
2 In a secondary transmission or a primary distribution first instance. Now the relay may have to operate on the
network it is common practice to provide the maximum theory of probability and resort to a lot of sequencing to
possible size of fixed capacitor banks at the trans- arrive at the right correction. Such a sequencing may
mission point or on the pole at the distribution point also lead to hunting and cause voltage fluctuations, because
with the feeder transformer. This is done to maintain of varying p.f. s and also cause switching currents and
the p.f. and the regulation of the system up to a certain voltage surges.
minimum level. Capacitors mounted out of sight or
away from easy reach may sometimes be ignored for 23.16.4 1+2+2. . .
maintenance or periodic checks. This must be taken
into account while installing the capacitor units. To The first capacitor is half the rating of the remaining
monitor the health of capacitor units one may install ones, which are all equal. The p.f. correction is delayed,
an ammeter or a kVAr meter in each phase and maintain due to allowing discharge time. The smallest unit is more
a logbook to monitor the health of capacitor units/ stressed.
banks through the unbalance in the three phases of
the capacitor circuit. When warranted, preventive 23.16.5 Direct combinatorial or auctioneering
measures such as cleaning the insulators, tightening switching
the terminals, and replacing or adding of a few capacitor
units to make up for the lost capacitances and to balance Here the relay assesses the kVAr requirement of the system
the system can be undertaken. and switches ON all the required capacitors
simultaneously. Theoretically any size of capacitor units
can be used. But when a capacitor is taken out for
23.1 6 Switching sequences maintenance, this can create confusion in the p.f.
correction. The relay will not be aware of this and act
A switching sequence that can be employed for a particular accordingly. The bank size may also be large and cause
load cycle may be one of the following: switching surges.
23.16.1 First in last out 23.16.6 Special sequencing
This is the simplest type of switching. Capacitors are These are sequencers and can sequence the switching of
switched ON in the sequence of 1-2-3 . . . n and switched capacitors in any fixed pattern. Capacitors can be
OFF in the reverse sequence i.e. n . . . 3-2-1. In this automatically taken out of the circuit and others introduced
switching the last switched capacitor is made to switch in their place by a device known as ‘the load rotator’.