Page 865 - Industrial Power Engineering and Applications Handbook

Making capacitor units and ratings of switching devices 251819
interrupter and the bus system must be suitable to transformer to the other is a case of rapid reclosing. This
withstand such stresses. may result in an overvoltage across the motor windings
Switching surges The switching device must be due to two residual voltages, one of the motor's own
capable of making and breaking the circuit successfully induced e.m.f. at the reduced voltage of y or of tapping
and generate the least possible voltage surges. All of the A/T and the other of the capacitor charge. if the
other equipment and interconnecting cables used in capacitor is connected across the motor windings
that circuit must be suitable to withstand the severities (terminals ai, bl, ci), irrespective of the time of discharge.
surges thus generated. In such cases, the capacitor should be open-circuited
Voltage harmonics When used in a transformer while the motor is in star or auto-transformer position
circuit capacitors may generate harmonics. At light through a special starter. A typical power and control
loads, the capacitors must be switched off or their scheme is illustrated in Figure 25.7.
value reduced to avoid oversaturation and consequent
damage to the transformer. Otherwise they may cause Hoist and crane applications
resonance, between their capacitances and the induc-
tance of the transformer, giving rise to overvoltages, Descending loads may overspeed the motor and overexcite
on the one hand and voltage harmonics, on the other the capacitor when connected across the motor due 10
(Section 20.2.1(2)). In rectifier and arc furnace applica- motor generator action above the synchronous speed
tions, however, inductive reactances of such equipment (Section 6.21). Such a situation may damage the motor
will rise due to high harmonic frequencies and offset as well as the capacitor and must be avoided.
to some extent the effect of the higher voltages
produced by the third harmonic. Electrodynamic braking
When a motor is statically controlled and employs dynamic
Residual voltages (for all ratings) Having energized braking (Figure 6.32) and has a capacitor also connected
once, the capacitor takes time to discharge. There is in the circuit it is possible that on an instant braking the
thus a residual voltage across the capacitor terminals, capacitor may discharge itself into the static circuit and
even after the power supply has been switched off. damage the electronic components used in the rectifier
This residual voltage is detrimental to the connected or the inverter units. This situation must also be avoided
equipment as well as to the operator. It is therefore by suitably designing the capacitor switching circuit.
mandatory that this residual voltage is dampened from
its peak value at fi V, to a maximum of 50 V within Stability of a power network
1 minute in LT and 10 minutes in HT shunt capacitors
and 5 minutes in HT series capacitors after a switch- In this case the next switching may almost be instant to
oft. (See also the footnote in Section 26.3.1.) retain the stability of the system and save it from an
When a capacitor switch is reclosed after an interruption outage. A static switching (Section 24. IO) alone may be
on a capacitor which is not fully discharged the trapped the correct solution for such applications.
charge of the capacitor will be superimposed on the applied
voltage and lead to overvoltages. Suitable resistance units
are fitted across the capacitor terminals as standard features
by the capacitor manufacturers to dampen the residual 25.7 Fast discharge devices
voltage from its crest value to the desired level or less, To accelerate discharge, one of the following method\
within the stipulated time, as noted above. When faster may be adopted. This is a special requirement and the
reclosing becomes essential, as in an automatic p.f. manufacturer must be informed.
correction scheme or for such load applications that change
quickly with little discharge time between two successive Use of extra resistance across the capacitor terminals
reclosings as noted above then either the discharge device, (Figure 25.8).
as discussed in Section 25.7 or the switching scheme Use of high series reactor, with or without resistance
should he designed $0 that the residual voltage across across the capacitor terminals. Open-delta VT (Section
the capacitor terminals decays to at least 10% of the 15.4.3) is one such device, which is a combination of
system voltage by the next reclosing. an inductance and a resistance and is to discharge an
The following are examples where the capacitors should HT capacitor unit quickly.
either he isolated from the circuit with the switching Use of a switch which, in the open position, can chort-
device so that its induced e.m.f. does not impinge on the circuit the capacitor with small resistance.
connected equipment or it is reduced to a safe level by
the next switching.
Notes
I Capacitor5 conncckd directly (without a switching device)
Protection from excitation voltages across a motor or a transformer are automatically discharged
through their windings, being inductive, and need no additional
resistance or inductance for a faster discharpe.
Star/delta or auto-transformer srvitching of (I motor 2 A capacitor being switched through a Y/r> \witchins of a
For motors compensated individually and having a star/ motor, howevci-, poses a different kind of a problem a5 discussed
delta or auto-transformer switching, the changeover from above and requires a different type of sicitching rchrme, as
star to delta windings or from one step of the auto- illustrated in Figure 2.5.7. This is because a charged capacitor