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AGCU opens the APB and the VR enable signal. The BPCU
prevents the re-closure of BTB 1 and BTB 2.
AC GENERATION - AGCU - PROTECTION AND EXCITATION
EFFECTIVITY
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Generator Diode Failure Protection
The SCU monitors the current ripple return in the excitation
circuit to detect an APU starter-generator shorted rotating
diode. The SCU sends a built-in test equipment (BITE) message
to the APU electronic control unit (ECU). The ECU gives a
maintenance light on P5. The generator operates with a shorted
diode. The AGCU may see a low voltage condition if more
diodes fail. If this happens, the AGCU removes the VR enable
signal and opens the APB.
Phase Sequence Protection
The AGCU monitors phase sequence. The APB cannot close
when the AGCU finds a phase sequence problem.
Overcurrent Protection
If current is more than 274 amps for 300 seconds, the AGCU
commands the BPCU to shed secondary electrical loads
(loadshed). The BPCU first removes galley loads and if the
overcurrent condition continues, the main buses are shed. If the
overcurrent remains, the AGCU removes the VR enable signal
and opens the APB.
If current is more than 340 amps for 5 seconds, the AGCU
commands the BPCU to start loadshed. If the overcurrent
continues for another.1 seconds, the AGCU removes the VR
enable signal and opens the APB.
Differential Fault Protection
The AGCU monitors the three phases of the starter-generator
current transformer and the feeder current transformers with
the differential protection current transformer (DCPT) in the
power distribution panel. The AGCU compares current flow at
the generator and at the APB. If differential current is more than
20 amps for 70 milliseconds, the AGCU removes the VR enable
signal and opens the APB.
Underspeed Protection
The AGCU monitors the ready-to-load (RTL) signal from the
ECU. With no RTL signal, the AGCU removes the VR enable
signal which opens the SCU GCR. This removes the excitation
from the starter-generator. The AGCU also opens the APB. The
sequence above happens when you put the APU switch to the
OFF position and the APU is supplying electrical power. The
IDG GCU does not open its GCR during engine shutdown. For
the APU, the excitation circuit must be open (no VR enable
signal) to protect the SCU voltage regulator during an APU start.
AGCU Failsafe
The AGCU contains internal protection against its own failure. A
fail-safe condition causes these:
* AGCU removes the VR enable signal which opens the SCU
GCR
* AGCU opens the APB
* AGCU tells the BPCU to close the BTB(s), if open
AC GENERATION - AGCU - PROTECTION AND EXCITATION
EFFECTIVITY
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BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
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24-20-00-108
* GCU fault indication light on AGCU goes on and fault
indication shows on the P5.
VR Enable
The starter-generator receives excitation (generator mode)
when there is a VR enable signal. The AGCU supplies the VR
enable signal when these conditions are true:
* APU RTL signal is available (APU more than 95% rpm and
not in cool-down cycle or at start of protective shutdown)
* Starter-generator frequency is in limits
* GCR (in the AGCU) is closed.
Another GCR in the SCU closes when there is a VR enable
signal. The voltage regulator in the SCU then controls startergenerator
output. There is no VR enable signal during an APU
start. During an APU start, the generator excitation circuit is
open. This protects the voltage regulator in the SCU from the
starter power. If the GCR in the AGCU is closed, the AGCU
supplies the VR enable signal when the APU gets to 95% rpm.
The RTL signal goes away during an APU shutdown. This
makes the VR enable signal go away and the starter-generator
excitation goes away.
AC GENERATION - AGCU - PROTECTION AND EXCITATION
EFFECTIVITY
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BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
24-20-00-108
AC GENERATION - AGCU - PROTECTION AND EXCITATION
AC GENERATION - AGCU - PROTECTION AND EXCITATION
EFFECTIVITY
GUN ALL 24-20-00
Page 51
D633A101-GUN Oct 10/2005
BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
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