Chapter 2 - Fault Tracing Thyristors
DCS 500 / DCS 600 Service Manual
VI A
2 - 1
How to detect, if a thy-
ristor is faulty
Thyristor problems can be noticed by two ways:
A fuse is blown
This is an indication, that a strong overcurrent has happened due to
one of the following reasons:
-
An internal short circuit between the phases (line side) because
of a defect thyristor (short circuit inside a thyristor from anode to
cathode).
-
An internal short circuit between the phases (line side) because
of circulating current in a 4-Q converter (malfunction of the con-
trol electronics, no thyristor defect).
-
An external short circuit at the DC terminals of the converter
without sufficient impedance.
-
A commutation fault during inverting mode operation (active
braking with high current, high EMF and with low AC voltage).
Ripple monitor
The ripple monitor indicates, that the ripple of the DC current is
much higher than normal. In such a case, most often one thyristor
does not work. It´s missing current contribution causes a deep dip
in the direct current.
The structure of the current loop, the current controller will force the
other thyristors to compensate the dip by a certain overcurrent in
order to keep the average current constant. Such a compensation
results in a ripple monitoring fault during motoring mode operation
with
α
≤
90°.
During braking or inverting mode operation most often fuses are
blown.
The reason for a currentless thyristor may be:
-
A fuse has disconnected one of the six thyristors. This is possi-
ble only for converters with 900...5150 A (six internal branch
fuses). A converter with three external fuses stops working com-
pletely at once when one of the three AC input fuses interrupts a
phase input of the converter.
-
A thyristor does not get firing pulses.
-
The current controller may be totally mismatched to the DC load.
-
The AC mains network is causing that fault message. In this
case, asymmetrical phase shift, uneven phase voltage or critical
designed power factor correction equipment or harmonic reduc-
tion equippment can be the reason.
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