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Fault Tracing Thyristors
Converters size D5, D6, and D7 (900...5200 A)
These converters are equipped with fuses in the branches of the
power part.
The converter must be disconnected from the mains.
Blown fuses
In case of a blown fuse, the faulty thyristor or the faulty pair of
thyristors are already isolated at one side from the others and
therefore the faulty branch is known (see picture Anti-parallel
B6-bridges with branching fuses on page 45).
The OHM test should be performed, when the thyristor is still
clamped. Outside the converter a special thyristor clamping
device is needed.
For 4-Q converters with anti-parallel thyristors or BCT’s:
The selection of a forward or reverse thyristor or BCT
(Bidirectional-Controlled-Thyristor) is done during the
disassembly. Continue with related part Exchange of Thyristors
for Size D5
, D6 or D7 section Find faulty thyristor.
After a thyristor was replaced, the OHM test should be done
another time to make sure that all faulty thyristors have been
detected! If the motor is still connected to the converter the
result of the measurement may be wrong.
Ripple monitor
If the ripple monitor fault occurred, a fault tracing as described
above must be carried out:
Check the fuses and the thyristors, according to the statements
before.
If the power section seems to be ok, but still one or more
thyristors don’t take current, something went wrong in between
the firing pulse generation and the thyristor’s gate; in this case
check:
Is a firing pulse present on the primary side of the firing
pulse transformer?
Is a firing pulse present on the secondary side of the
firing pulse transformer?
Is the firing pulse transferred to the gate of the thyristor?
Are there all electrical connections still healthy?
Can the thyristor be fired with the applied firing pulse? Is
the pulse form of the firing pulse identical at all
measuring positions?
Check the settings of the current controller.
Check the AC mains network by taking recordings of the line
voltage and current at all 3 phases at the same time.
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