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MAINTENANCE MANUAL

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747-400 MAINTENANCE MANUAL
IDG power is routed through the generator control current transformer before reaching the GCB. It then passes through a generator Differential Protection Current Transformer (DPCT) before being sent to the load bus. On the way to the sync bus, power passes through a sync bus DPCT, the BTB, and then through a generator DPCT before reaching the sync bus. The GCU provides GCB control and system protection. The GCB trip relay is operated by the GCR, generator diode failure protection, and underspeed protection. The BTB is operated by the BTB close relay, which is activated by signals from the BUS TIE switch on the flight deck control panel, and inputs from auto-parallel circuitry, GCU protection circuitry, and auto-bus isolation circuitry.The auto-paralleling network of the GCU senses C phase voltage of a generator and C phase voltage of the associated sync bus. The network can withstand a difference between the two voltages of 280 volts root mean square (rms). When actuated, the network energizes the GCB and BTB close relay, if the generator field control relay is closed, the underspeed relay is de-energized, and the associated GEN CONT and BUS TIE switches are in the latched position. The auto-parallel network senses frequency, phase angle, and voltage differences between the source voltages.The auto-parallel network operates as follows if the average of the three phase voltages is above 100 |2 volts:
 (a)  The network does not operate if the frequency difference
 exceeds 6 Hz or the phase angle difference exceeds 90 degrees.
 (b)  The network operates if the frequency difference is between 0
 and 5 Hz.  The network may operate if the frequency difference
 is between 5 and 6 Hz and the phase angle difference is 90
 degrees or less.  No slip frequency is required for
 paralleling.
 (c)  If the frequency and phase angle requirements are satisfied,
 the network operates if there is a voltage difference of up to
 10 volts line-to-neutral between the two voltages.
 NOTE:____ Operation of the auto-parallel network is corrected to
 allow for the operating time of the GCB close relay and
 the GCB, and the BTB close relay and the BTB.

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The auto-parallel bypass (dead bus) relay is also part of the auto-parallel network. This relay senses the associated generator Point of Regulation (POR) voltage and sync bus voltage. When this relay is de-energized, it bypasses the auto-parallel network to permit the GCB close relay and BTB close relay to energize in 0.05 second maximum. The bypass relay is in operation when sync bus voltage is between 5 volts L-N and 25 volts L-N. This relay allows an unpowered (dead) bus to be paralleled with another source without meeting frequency and phase angle requirements for paralleling.

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 Power generation responsibilities for one or both halves of the EPGS may be transferred from one source to another as described below. Unless otherwise specified, all generator field control switches, bus tie switches, GCRs and BTBs are assumed to be closed, and the SSB does not change from its initial state as defined in each of the following transfers. For system configurations in which the SSB is open and one half of the sync bus is de-energized or becomes de-energized for any reason, the SSB closes.

 (a)  
External Power Operation to IDG Power Operation - One External Source Being Used (SSB Open) 1) Generator Control Switches ON and Engines Not in Operation

 a)  When an adjacent engine is started and up to normal speed with the associated GEN CONT switch initially in the ON position, the IDG automatically synchronizes with the external power source. When the frequency and phase angle are within acceptable limits, the GCB automatically closes, momentarily paralleling the IDG with external power until the external power contactor automatically trips.
　
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