Rockwell Automation 1557 MEDIUM VOLTAGE AC DRIVE - Manual (Page 51)

DESCRIPTION OF OPERATION 5-33
1557-UM050F-EN-P –June 2013
Thyristor Diagnostics Bulletin 1557 Drive
The Bulletin 1557 ac drive tests for open or shorted thyristors both when it is started and continuously
while running. The method used to detect failed thyristors is different for starting and for running, but the
same hardware is used in both situations. The drive control receives a feedback signal via a fibre optic
cable from each thyristor gate driver that indicates whether the anode-cathode voltage of the thyristor is
high or low. A high voltage across a thyristor indicates that it is almost certainly not conducting. A low
voltage across a thyristor indicates that it may be conducting, but since it is quite normal for the voltage
across a device to be low even when it is not conducting, this is not a reliable indication that the device is
on.
The following description applies to all air cooled and liquid cooled 1557 drives except the small 1557M
drive which does not have any feedback from its gate drivers and uses a completely different method to
detect failed thyristors.
Detection of Failed Thyristors When Starting
Line Converter
When line voltage is applied to the drive but it is not running (i.e. gating is off), the voltage across the line
converter thyristors is high most of the time but low twice per cycle during an interval around the zero
crossings of the line voltage. Shorted thyristors in the line converter can therefore be detected whenever
line voltage is applied even when the gating is off. If the feedback from the gate driver indicates that the
voltage across a device is continuously low for six cycles, then it is assumed to be shorted. The six cycle
interval is required to eliminate spurious faults.
If the feedback from the gate driver indicates that the voltage across a device is continuously high for six
cycles, then an open thyristor fault will be annunciated. Since it not possible to tell if a thyristor is actually
open without gating it, an open fault at this time indicates that the feedback from the gate driver has failed.
When the drive receives a start command, a gating test of the line converter thyristors is performed.
Thyristors are gated one at a time at intervals of one cycle to determine if they conduct. The one cycle
delay between devices is required to allow the snubbers to recharge. A short gating pulse is applied near
the point in the cycle when the voltage across the device is at its positive peak. If the voltage across the
device does not collapse, then it is assumed to be open. This test can also identify errors in the
connection of the fibre optic cables. An open thyristor fault at this time indicates that the thyristor or gate
driver board is defective, or that the fibre optic cable is plugged into the wrong transmitter or receiver on
the Optical Interface board. When two thyristors in the same leg seem to be open, it is usually because
their fibre optic cables have been interchanged.