Chapter 2 - Fault Tracing Thyristors; How To Detect, If A Thyristor Is Faulty; A Fuse Is Blown; Ripple Monitor - ABB DCS 500B Service Manual

How to detect, if a thy-
ristor is faulty

A fuse is blown

Ripple monitor

DCS 500 / DCS 600 Service Manual

Chapter 2 - Fault Tracing Thyristors

Thyristor problems can be noticed by two ways:
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).
Note In case of parallel fuses: If one of the parallel fuses is
blown, all parallel fuses have to be changed. The 'un-
damaged' fuses might be 'half-blown' and will blow
with the next high current.
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...5200 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 ca-
se, asymmetrical phase shift, uneven phase voltage or critical
designed power factor correction equipment or harmonic reducti-
on equippment can be the reason.
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