Page 212 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

VARIABLE-SPEED DRIVES

VARIABLE-SPEED DRIVES 10.11

300 9
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This failure mode is normally caused by incorrect application. The designer of the system
must ensure that the maximum fault current in the bridge cannot exceed these limits. This
failure mode occurs normally during commissioning. However, it can occur at random
intervals during the lifetime of the device.

Device Explosion Rating. This failure mechanism can occur in any of the power devices.
Thyristors can break down in the reverse direction due to a fault. This is usually followed
by a large surge current. The resulting arc at the edge of the device could be strong enough
to blow open the ceramic housing. The explosion rating for a thyristor is normally 50 to 100
percent above the surge current rating.
This type of failure can have serious consequences because conductive plasma is vented
from the failed cell into the bridge, resulting in extensive arcing and destruction through-
out the whole bridge. These failures are normally caused by inadequate fault coordination
(design deficiency). They usually occur during commissioning or at any time during the
lifetime of the device.
Device Application. These failure modes normally occur in the middle region of Fig.
10.12 (slowly increasing failure rate) which extends over several years of operation. The
failure rate depends on these application factors:
● Type of device application
● Voltage applied (as percentage of PIV)
● Junction temperature (at normal running load)

In general, the lifetimes of all semiconductors decrease when the applied voltage or tem-
perature (as a percentage of the rating) increases.

THYRISTOR FAILURES AND TESTING

Recognizing Failed SCR or Diode
It is easy to recognize a failed SCR or diode because it normally (99.9 percent) becomes a
short circuit. Any fuse in the circuit is usually blown due to the overcurrent. Shorted SCRs
or diodes give a resistance around 10 when measured by a volt-ohmmeter. The remain-
ing 0.1 percent, the gate of an SCR, is open. The gate to cathode resistance of a good SCR
is normally 15 to 30 while that of an open-gate SCR is infinity.
Another failure mechanism occurs near the end of the useful life of the device. The leak-
age will increase due to degradation of all junctions. This results in operating problems
such as numerous overcurrents. These symptoms should be taken as an indication that the
devices should be replaced.

Testing of SCRs or Diodes
If the device is not experiencing problems such as intermittent overcurrent trips or blowing
fuses, it should not be tested. The chance of damaging the devices by disconnecting and
testing them is higher than that of finding a suspicious one. In general, if the device is oper-
ating properly, do not test it.

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