Page 130 - Power Electronics Handbook

Causes of failure in power circuits 123

current when the thyristors are off. If this diode were to fail to a short
circuit it would cause the two thyristors to carry large amounts of current
when they turn on, and if it failed to an open circuit the decay of load
current would cause a high-voltage spike to be generated, destroying the
semiconductors. Failure of thyristor "€32 would result in THI being
permanently on, which could cause it to overheat.
In the bridge circuit, shown in Figure 5.l(b), the devices are controlled
in pairs, TH1 and TH, together and TH2 and TH3 together. The current
and voltage waveforms are as shown in Figure 5.l(c), where each device
conducts for half a cycle. If any component, say TI&, now fails to an open
circuit, then the other thyristors, in this case THI and TI&, will carry the
full-load current continuously, causing them to overheat and perhaps be
destroyed. However, if thyristor TH2 was fired after TH3 had failed, then
the inductive load current would free-wheel through thyristors TH2 and
TI€+, causing overheating. If any of the thyristors fail to a short circuit then
this short circuit is applied across the lines, resulting in a failure of the
other devices. Failure of the a.c. supply line would cause the load current
to free-wheel through the two arms of the bridge, assuming that the
thyristors have been fired.
Circuit-related failure mechanisms can be due to a variety of reasons.
Fault current, as a result of short circuits, can cause heating and loss of
control of the power semiconductor switch, the current building up over
several a.c. cycles to reach a steady state value determined by the circuit
voltage and impedance. The high current can also result from discharge of
circuit capacitances, such as snubbers used across the lines or capacitors
used in power supplies, and is one of the main causes of dildt failure.
Lightning strikes can cause a steep-fronted, line-to-ground, surge of
voltage which destroys the power semiconductors, although the surge may
be attenuated to some extent by output transformers. Lightning arrestors
are usually placed across lines to guard against this. Transformer switching
is another source of high-voltage transients. On switch-on the, inrush
current can result in oscillations within the resonant secondary winding,
due to transformer leakage inductance and the distributed parasitic
capacitance of the secondary winding. On switch-off the magnetising
current is interrupted and this results in a collapse of the core flux,
generating voltage transients on the secondary. This effect is greatest on
light loads, when the primary current is passing through zero. Energising a
stepdown autotransformer results in the interwinding capacitance causing
the primary voltage to be momentarily coupled through to the secondary,
giving an overvoltage.
Overvoltages are also caused by discontinuous current operation in
inductive circuits, the energy stored in the inductance causing a
high-voltage spike. Current interruption can occur due to several causes,
such as power semiconductors turning off too fast, or due to operation of
protection devices such as circuit breakers and fuses. Voltage transients
can be avoided by several techniques, for example:
(i) Switching the secondary of transformers, so as to avoid the inrush
current which occurs when the primary is switched;
(ii) Ensuring that switching devices do not operate too quickly, or have