Page 132 - Power Electronics Handbook

Overvoltage protection 125

arrangement in which R-C suppressors are connected across the
three-phase supply lines, as well as across the d.c. load. The transient
voltage is partly dissipated in the line impedances and series resistor, and
the rest appears as an increase in capacitor voltage. The larger the value of
capacitance, the lower this voltage rise. The resistors in series with the
suppression capacitors limit the charging current and also damp down the
oscillations, resulting from resonance between these capacitors and
inductances in the lines, which can lead to overvoltages.
The capacitors shown in Figure 5.2(a) carry power frequency
components of current and their presence can also affect the commutation
behaviour when changing from one supply phase to the next. Figure 5.2(b)
shows an alternative arrangement which uses an auxiliary low-power
bridge, but which requires only one suppression capacitor and this can be
electrolytic, since it only cames d.c., enabling it to be physically small. If
the inductance of the capacitance is high it will not be able to suppress
fast-rising transients, and in these cases it is usual to shunt the electrolytic
capacitor with a much lower-valued low-inductance capacitor, such as
ceramic. Resistor R2 limits the charging current of the capacitor and
resistor R1 discharges the capacitor, for safety reasons, after the rectifier is
de-energised.
Suppression components, which convert the voltage transient energy
directly into heat, consist of devices with characteristics such as those
shown in Figure 5.3(a), for unidirectional operation, and Figure 5.3(b) for
bi-directional operation. Very little current flows through the device until
the breakover voltage VB is reached, after which time the current rises
rapidly and the voltage is held substantially constant. These devices are
available in a range of voltage ratings, which are fixed and cannot be
adapted to changes in circuit operating voltage, as is possible with an R-C
suppressor.
The voltage clamping device can be connected across the power supply
lines, or across the components being protected, as in Figure 5.3. The
device should have a flat voltage-current characteristic once breakover has
occurred, but it should also be able to absorb high energies for short
durations. The zener diode exhibits the characteristic shown in Figure
5.3(a) and it can be used as a suppressor. It has a flat characteristic, i.e. low
slope resistance, but it is only able to absorb small energy levels, so it is
used in low-power applications. Transient overvoltages usually have a high
voltage value but then only last for a short time, or they have a relatively
low voltage but last for longer. Therefore the energy requirements are
fairly well defined, and suppression devices are usually specified in terms
of watt-seconds of energy dissipated.
Many different forms of surge suppressors are available commercially,
such as varistors and gas filled tubes. These are all designed to handle high
energy levels and have a low inductance, so that they can suppress steep-
rising waveforms. Their slope resistance is often higher than that of a zener
diode so the voltage will rise with current. Their operating temperature range
is also relatively low. Suppressors are based on diodes operated in the reverse
direction, having a characteristic similar to that of Figure 5.3(a). Two devices
can be connected back to back for bi-directional operation, as in Figure
5.3(b), and several cells can be connected for higher-voltage operation.