Rockwell Automation 1557 MEDIUM VOLTAGE AC DRIVE - Manual (Page 52)

5-34 DESCRIPTION OF OPERATION
1557-UM050F-EN-P – June 2013
Machine Converter
If no open or shorted thyristors are detected, the line converter is gated at a firing angle of 90 degrees
with the machine converter gating off in order to apply a voltage to the dc link. Although a firing angle of
90 degrees corresponds to an average dc voltage of zero when the dc current is continuous, with the
machine converter gating off the snubber capacitors will charge up to the peak of the voltage waveform,
resulting in a dc link voltage which is close to rated voltage. After a delay to allow the dc voltage to build
up to its maximum value, the feedback from the GTO gate drivers is examined. If the gate driver
feedback indicates that the voltage across a GTO is still low, then that device is assumed to be shorted.
If all devices in one leg of the bridge are shorted, all devices in the other two legs on the same side
(positive or negative) of the bridge may incorrectly appear to be shorted because they are connected in
parallel with the shorted leg through the low impedance of the motor. For this reason, if the drive is
indicating that a number of machine converter thyristors are shorted, all devices should be checked with
an ohmmeter to confirm that they actually are shorted before replacing them.
If no shorted devices are detected, a gating test of the machine converter thyristors is performed.
Thyristors are gated one at a time at intervals of 20 ms to determine if they conduct. The 20 ms delay
between devices is required to allow the snubbers to recharge. Each device is gated for approximately
100 microseconds while the gate driver feedback is monitored. If the voltage across the device does not
go low, then it is assumed to be open. 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 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. The machine converter gating test is not included in firmware
revisions earlier than 4.20.
Detection of Failed Thyristors When Running
Detection of thyristor faults is much simpler when the drive is running than when it is started because no
special gating is required. When the gating is enabled for both converters, the feedback from the gate
drivers is constantly switching on and off, usually several times per cycle. The gate driver feedback
signals from each device are sampled a number of times during each cycle. If any of the conduction
feedback signals indicates that the voltage across a thyristor is continuously high or low for six cycles
then the corresponding device is assumed to be open or shorted respectively. The detection interval of
six cycles, which is required to eliminate spurious faults, makes the fault detection too slow to protect
thyristors from failing, but it does shut the drive off after a device has failed in order to prevent additional
failures from occurring.
Drives that have an extra redundant thyristor in each leg of the bridge will issue a warning and continue
running if a single device in a leg fails short, but will trip if a second device fails in the same leg.
When the drive is running, an open thyristor is treated as a warning, not a fault, because it is not possible
for a device to fail open when high voltage is applied and gating is on. An open thyristor warning while
running usually means that the gate driver feedback has failed.
For the line converter, the detection of open thryistors is turned off when the dc link current is
discontinuous because the voltage across the device does not provide a reliable indication of its
conduction state when there is no current flowing. For the machine converter, all thyristor diagnostics are
turned off when the motor voltage is less than 0.25 pu because the feedback from the gate drivers is not
valid at low voltage.