Page 849 - Industrial Power Engineering and Applications Handbook

System voltage regulation 24/803
network is being fed by more than one generator. When by switching in or switching out a few units. In manual
any transient fault occurs, the current flow through the switching it will be possible only in steps. and may
fault will be shared more by the machine nearest to the not provide a smooth compensation and may also
fault and less by the one installed a little away. This will cause switching transients (Section 23.5. I). Moreover.
upset the earlier tandem operation of the machines and conventional switching methods (mechanical switching
they will become unequally loaded and may fall out of through contactors and breakers), are sluggish due to
step. The one near the fault will slow down more than the time of closing and interruption, which may be ;is
the other. The machine that shares the smaller amount of much as three or four cycles, depending upon the
the load will slow down less and feed more, becoming type of interrupter (Section 19.5 and Table 19. I) as
overmessed. Now it will slow down and the other will well as the minimum time required for the discharge
pick up. The situation will reverse thus and so the situation of the capacitors. Human sluggishness may also
will continue creating a hunting effect. The following introduce some delay. They are therefore ill-suited to
may result depending upon the transient stability limit meet the system’s rapidly fluctuating needs.
of the system. However, power systems that cater to almost fixed
loads at a time and whose variations occur only at
The fault is cleared promptly and the normal condition specific times of the day may not require ii fast
is restored. The setting and the speed of the protective response. In such cases. it is possible to provide manual
relays should be commensurate with such a situation switching methods which will give enough time
and isolate the faulty circuit as quickly as possible. between two switchings. Manual switching, however,
If not, the machine being loaded most may fall out, has certain shortcomings, due to the human factor
which may not necessarily be the one nearer the fault such as its accuracy and diligence, as noted above.
or The recommended practice is therefore to select fast
The situation may have a cascading effect until all the reactive controls as noted below.
machines fall out, resulting in a total blackout. 2 Static VAr compensators (SVCs) Whenever a large
reactive control is required, the SVC is always a
To achieve a better level of stability it is desirable that preferred method. The static VAr controllers are more
the line be loaded a liltle less than the optimum power it expensive, but respond very quickly. They cause no
is capable of transmitting to sustain the system distur- switching transients and limit the magnitude of a
disturbance. through extremely fast controls. They
bances such as load tluctuations, faults and switching
of large machines without an outage. The load curves can handle large currents and peak inverse voltages,
(Figure 24.20) provide a guide to determine the level at except voltage transients, such as switching surges or
lightning strikes, which may have a front time as low
which the line should be operated and from this can be as 1-2 ps only (Section 17.3.3) while the switching
assessed the magnitude of di$turbances that the line can time of a static device (a thyristor) may be as much as
safely sustain and recover promptly without an outage. one cycle, as discussed later. But surges can be taken
Series reactive support will become essential. whenever
the line loading is expected to be more than the STL, P,, care of by a surge arrester. The use of an SVC or a
(generally on 132 and 220 kV networks). But series manual switching will largely depend upon the
reactive support has been found extremely useful on characteristics of the line. the type of load it is feeding
existing lines even up to I I kV, which are required to and its importance. For a system having almost the
cater for higher power demands than were originally same type of load demand during the day. inanual
en\,i\aged. switching may serve the purpose. But for a system
with wide fluctuations, an SVC alone will be suitable.
The decision will vary from one system to another
24.10 Switching of large reactive and the system engineer can make a better choice.
banks /vote
If auto-control is selected through p.f. or voltage control. cate
The \cries capacitors are connected in series with the must be taken against frequent switchings of the capaciroi-s
power lines to provide reactive control to an individual when the load is of a varying nature which may c;iuse the
load or to a power distribution or transmission system. capacitors also to switch frequently. Fast switching?, can be
They are therefore switched with the power lines and are made possible by providing special di\charyc devices. and by
controlling the number of switchings to within permisyihle limit\
thus permanently connected devices. (Section 26.1.1(2) by carefully arranging the unity 21.; diicus\ed
But the shunt capacitors can provide reactive control in Section 23. IS. 1
through unswitched, i.e. permanently connected, banks
(fixed VAr) or through switched banks (variable VAr). In SVCs the number of switchings is of no relevance,
The unswitched VAr may be used to aid stability against as they are free from inrush currents. Switching is
possible overvoltages of the network, during a load performed at the instant when the current wave is
rejection or an open circuit while the switched VAr is passing through its natural zero. Static devices in
used to maintain the level of p.f. during load variations. various combinations and feedback control systems,
VAr switching can be done in two ways. which may be computer-aided, can almost
instantaneously (5 1 cycle) generate or absorb reactive
I Manual control This is through switching devices power, as may be demanded by the system. Correction