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* Generator drive disconnect (underfrequency protection).
The GCB opens when the TRIP GCR closes.
The TRIP GCR does not close when you shut down the engine
with the IDG on line. The GCU sees an underspeed condition
before the under frequency condition. The GCB opens with an
underspeed condition.
AC GENERATION - GCU - PROTECTION AND EXCITATION
EFFECTIVITY
GUN ALL 24-20-00
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BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
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GCU Protection
The GCU monitors voltage, current, and frequency. The GCU
opens the GCB and de-excites the generator to protect the
electrical channel. The GCU has these protective functions:
* Overvoltage
* Undervoltage
* Overfrequency
* Underfrequency
* Unbalanced phase current
* Generator diode failure
* Phase sequence
* Overcurrent
* Differential fault
* Underspeed protection
* GCU fail-safe.
Overvoltage and Undervoltage Protection
The GCU monitors each of the phase voltages at the point of
regulation (POR). A voltage fault commands the generator
control relay (GCR) and generator control breaker (GCB) to
open.
An overvoltage protection trip happens when a phase voltage
goes more than 130 volts.
An undervoltage trip happens when the lowest phase voltage
goes below 101 volts for more than 7 seconds. The GCU also
sees an open phase (feeder lead not connected) or a short in
the IDG exciter circuit as an undervoltage.
Frequency Protection
The GCU monitors frequency at the PMG. Proper generator
frequency is 400 Hz. The GCU opens the GCR and GCB if the
frequency goes above these values:
* 425 Hz for more than 1.5 seconds
* 435 Hz for 35 milliseconds.
The GCU opens the GCR and GCB if the frequency goes below
these values:
* 375 Hz for 1.5 seconds
* 355 Hz for 150 milliseconds.
The GCU sees an underfrequency condition if a manual
disconnect or thermal disconnect happens while the IDG is on
line. The GCU uses the RTL signal from the DEU to find if the
engine is underspeed or the generator is underfrequency.
Unbalanced Phase Protection
The GCU monitors the three phases of the IDG current
transformer. Unbalance phase protection occurs if the current
unbalance between any two phases is more than 140 amps for
6.3 seconds. The GCU opens the BTB and if the unbalance
continues, the GCR and GCB open.
AC GENERATION - GCU - PROTECTION AND EXCITATION
EFFECTIVITY
GUN ALL 24-20-00
Page 32
D633A101-GUN Oct 10/2005
BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
24-20-00-103
Generator Diode Failure Protection
The GCU monitors current ripple in the excitation return to find
a shorted rotating diode in the IDG. If the current is more than 5
amps, the GCU opens the GCR and GCB.
Phase Sequence Protection
The GCU monitors the phase sequence at the POR. The GCU
does not let the GCB close if a phase sequence problem occurs.
Overcurrent Protection
If the current is more than 274 amps for 300 seconds, the GCU
commands the BPCU to remove secondary electrical loads
(loadshed). The BPCU first removes galley loads and if the
overcurrent condition continues, the main buses are shed. If an
overcurrent condition still occurs, the GCU opens the GCR and
GCB.
If the current is more than 340 amps for five seconds, the GCU
commands the BPCU to start loadshed. If the overcurrent
continues for another 0.1 seconds, the GCU opens the GCR and
GCB.
Differential Fault Protection
The GCU monitors the three phases of the neutral current
transformer (NCT) and the differential protection current
transformer (DPCT). The GCU compares current flow at the
IDG (through NCT) and at the GCB (through DPCT). If the
differential current is more than 20 amps for 70 milliseconds,
the GCU opens the GCR and GCB.
Underspeed Protection
The GCU monitors the ready-to-load (RTL) signal from the
display electronics unit (DEU). The DEU sends the RTL signal
when the engine is at or above idle speed and the engine start
lever is in the idle position. If the RTL signal is present, an
underfrequency or an undervoltage fault will cause the GCB
and the GCR to open. The GCB will open if the underfrequency
condition is due to an underspeed. The GCR stays closed if the
RTL signal is removed.
A normal engine shutdown with the IDG on line is an example of
an underspeed condition.
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