Generator Control Unit (GCU) Protective Functions
Figure 4 (Sheet 1)
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747-400
MAINTENANCE MANUAL
FUNCTION SENSING THRESHOLD DELAY TIME TRIPS REMARKS
GENERATOR ACTUAL MEASURED ACTUAL VOLTAGE > 6.25 GCR, GCB
DIODE FAULT CALCULATED GENERATOR VOLTAGE EXCITER FIELD VOLTAGE VS. CALCULATED GENERATOR VOLTAGE |0.75 SEC
(OE) OVEREXCITATION OE/UE CT 3-PHASE VOLTAGE AT POR AND 130 |3 VOLTS AC INVERSE BTB, GCR
UNDEREXCITA-TION (UE) OE/UE CT 3-PHASE VOLTAGE AT POR AND AC 104.5 |1.5 VOLTS 3.00 |0.25 SEC BTB, GCR
SHORTED PMG (SPMG) PMG RIPPLE VOLTAGE PMG RIPPLE DETECTOR SEC 0.7 |0.5 GCR, GCB
OPEN PHASE LINE DPCT LOWEST PHASE, < 375 |40 BTB
(OP) CURRENT 55 |5 AMPS 2 PHASES, í 6 |5 AMPS, OTHER (CONCUR-RENT) (OP2) (CONCUR-RENT) 9 |1 SEC (OP1) MSEC GCR, GCB
REVERSE POWER CTA OUTPUT 8 |1 kW INVERSE BTB CONDITION ONLY OPERATES IN UNDERSPEED
CPU FAULT (FAILSAFE) WATCHDOG IMMEDIATE GCR, GCB FAULT BTB TRIPS FOR A MAIN BUS
POWER READY NONE OV, UV, OF1, UF1, US 150 |50 MSEC GCB WILL CLOSE IF NO TRIPS ARE PRESENT
Generator Control Unit (GCU) Protective Functions
Figure 4 (Sheet 2)
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400 MAINTENANCE MANUAL
3) Overfrequency 4) Underfrequency 5) Underspeed 6) Difference current 7) Differential current 8) Open phase 9) Generator diode fault 10) Overexcitation 11) Underexcitation 12) Reverse power 13) Central processing unit fault 14) Shorted permanent magnet generator 15) Pole slip 16) Disconnect and fire switch shutdownThe GCU provides the following control functions: 1) Generator field control and indication 2) Generator circuit breaker control and indication 3) Bus tie breaker control and indication 4) Automatic bus isolation control (autoland) 5) DC isolation relay control 6) IDG air/oil cooling valve control 7) Utility load management control functions (sent via ARINC
429 to BCU for control) 8) Bus off indication (dead bus through ARINC 429 data link)During fault conditions, the excitation level to the generator is automatically controlled to limit current and/or power output of the associated generator to within safe levels.The GCU also controls various breakers and relays; they are closed when no fault conditions exist and they can be opened automatically when required. The GCU controls the GCR (a relay in the GCU which opens and closes the generator field circuit), the GCB (a breaker which connects the generator to its associated load bus), and the BTB (a latching-type relay which connects the main generator buses to the sync bus).The voltage regulation portion of the GCU maintains the generator voltage at the point of regulation. The regulator senses the voltage at the point of regulation and compares this against a reference voltage. If a difference exists, it will adjust the generator exciter field current to maintain a constant voltage at the point of regulation.
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