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747-400 MAINTENANCE MANUAL
 AUTOMATIC CONTROL, PROTECTION, AND REGULATION - DESCRIPTION AND OPERATION
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General
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 A.  The generator control unit (GCU), auxiliary generator control unit (AGCU), and the bus control unit (BCU) monitor the entire electrical power generating system from inputs provided by the generator control current transformers (GCCTs), differential phase current transformers (DPCTs), and the ground power current transformers (GPCTs). The inputs provided by the CTs enable the GCUs, AGCUs, and BCUs to monitor, regulate, control, and protect the electrical power generating system automatically.
Generator Control Unit (Fig. 1)
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 A.  The GCU provides system protection and control, voltage regulation, and speed control for its associated integrated drive generator (IDG). The GCU monitors and controls its respective IDG and operates in conjunction with the BCUs, GCCTs, and the DPCTs for protection. The GCU also provides excitation to the IDG for voltage regulation, compares IDG frequency to a reference frequency from the BCU for speed control, isolates generator faults, and controls the GCB and BTB to protect the electrical power generating system. The GCUs are identical and are interchangeable.
Auxiliary Generator Control Unit (Fig. 1)
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 A.  The AGCU provides system control, protection, and voltage regulation for its associated APU generator. The AGCU monitors and controls its respective APU generator utilizing POR voltage and the DPCTs. The AGCU controls the APB to apply APU generator power to the syncronizing bus or to remove the power when a fault causes a protective trip. The AGCU also functions as an interface between the BCU and the APU generator. The AGCUs are identical and are interchangeable.
Bus Control Unit (Fig. 1)
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 A.  The BCU monitors the synchronizing bus, utilizing inputs provided by the GCUs, AGCUs, GPCTs and DPCTs to provide control and protection for the electrical power generating system. The BCU also performs the no-break power transfer, load management, and sync bus paralleling. The BCU also contains built-in test equipment (BITE) circuitry used for electrical power generating system fault isolation. The BCUs are identical and are interchangeable.
 B.  The parameters and system status information is transmitted from each BCU on the ARINC 429 bus to the engine indication and crew alert system (EICAS) and the central maintenance computer system (CMCS) by way of the EFIS/EICAS interface unit (EIU).
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747-400 MAINTENANCE MANUAL 
GCU NO. 3 BSEE 
SEE  A  BSEE GCU NO. 4  C BCU NO. 2 SEE  SEE D AGCU NO. 1 
AGCU NO. 2 
SEE  D  BCU NO. 1 CSEE  BSEE GCU NO. 2 
GCU NO. 1 
SEE  B 
GCU (TYPICAL) 

A ELECTRICAL AND ELECTRONICS RACK 
B 
BCU (TYPICAL)  (TYPICAL) AGCU 

C D
Auxiliary Generator Control Unit (AGCU), Generator Control Unit (GCU),
and Bus Control Unit (BCU) Location
Figure 1

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747-400
MAINTENANCE MANUAL

 5.  Current Transformer (Fig. 2)
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 A.  The current tranformers located throughout the electrical power generating system provide inputs to the BCUs, GCUs and the AGCUs for various control functions. The GCCTs work in conjunction with the GCUs to provide:
　
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