1-20 Functional Description
7000-TD002A-EN-P – September 2007
Off-line Detection of Failed SCRs/SGCTs
Line Converter – 6P-SCR, 18P-SCR and PWM
The rectifier diagnostics are performed when medium voltage is
first applied by closing the input contactor and when the drive
receives a start command. The drive also performs off line
diagnostics when a drive reset command is issued. These
diagnostics are capable of detecting a bad device, loss of
feedback fiber optic and loss of gating fiber optic. The
diagnostics consist of two stages. A passive diagnostic test
followed by an active diagnostic test. In the passive diagnostics
test no devices are gated.
SCR Rectifier Passive Off-Line Diagnostics
For SCR rectifiers, when the line voltage is applied to the drive
but the drive is not running, the voltage across the line converter
thyristors is high and positive for half cycle except during
intervals around the zero of the line voltage. The gate driver
transmits light whenever the device is forward biased with a
large enough voltage as shown in Figure 1.9. On the OIBB
diagnostic test point this translates into a feedback signal at 0V
level. Since the drive is not gating (no light, 5V signal on the
OIBB gating test point) the feedback normally toggles state
every cycle of the utility voltage. However the feedback will not
toggle state if the device is shorted, or if the feedback fiber-optic
path is incomplete. This is shown in Figure 1.9. If this occurs,
the drive faults and issues an OfflineShrt fault for the device.
PWM Rectifier Passive Off-Line Diagnostics
For PWM rectifiers the transmitter on the device should send a
light back when the device is healthy (0V on the diagnostic test
point). However the light signal will not be received if a device is
shorted or if the feedback fiber-optic path is incomplete. The
drive presumes the device has failed and determines which
devices would be safe to gate for the more detailed active offline
diagnostic test.
Power Semiconductor
Diagnostics (cont.)
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