B.  The DPCTs in the system in conjunction with the GCUs, AGCUs, and similar control transformers in all the generators provides differential current and open phase protection. The GPCTs work in conjunction with the BCUs to provide load management. The sync bus DPCTs provide input to the BCUs for sync bus differential fault protection.
 6.  Operation
_________
 A.  Functional Description (Fig. 3)

GENERATOR CONTROL
GROUND POWER CURRENT
CURRENT TRANSFORMER
TRANSFORMER
Electrical Power Generating System (EPGS) Current Transformers
Figure 2

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EXTERNALPOWER

EXTERNALPOWER
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MAINTENANCE MANUAL

 (1)  
The electrical power generating power system (EPGS) provides power to four main load buses via a synchronizing (sync) bus. The load buses may be supplied by any of eight power sources: IDG channels 1 thru 4, auxiliary power unit (APU) generator channels 1 and 2, and external power sources 1 and 2.

 (2)  
The IDG channels may operate separately to supply their load buses through their repective generator control breakers (GCB), or the channels may operate in parallel through bus tie breakers (BTB) and the split system breaker (SSB) to provide equal load sharing. The IDG channels may operate as two parallel systems with the GCBs and BTBs closed and the SSB open. Normally, the four IDG channels operate in parallel.

 (3)  
The APU generator channels may be used to supply the load buses through the auxiliary power breaker (APB) while the airplane is on the ground. The APU generator channels cannot be paralleled through the SSB.

 (4)  
The external power sources may be used to supply the load buses through the external power contactor (XPC) while the airplane is on the ground. External power sources cannot be paralleled through the SSB.

 (5)  
The GCUs will isolate generator faults and open the appropriate GCBs to protect the EPGS. The GCUs send control and protective trip data along with the status of the GCBs, BTBs, APBs, XPCs, and SSB through ARINC 429 data buses to the BCUs. The BCUs analyze the incoming data and send commands to the GCUs that operate bus tie breakers (BTBs) for load transfer. The BCU also controls the SSB for sync bus parallel configurations.

 (6)  
On the ground, external power can supply any or all of the load buses. The ground service buses may be supplied by the APU generating channels, external power channels, or by the IDG generating channels. If ground power is not available on the left sync bus, the ground service bus is then powed by ac bus 1.

 (7)  
Generator Control Unit (Fig. 4)

 (a)  
Each of the four GCUs are identical and can be used on any main (IDG) channel. A GCU provides system protection, control, voltage regulation, and speed control for its associated generator.

 (b)  
The GCU provides the following protective functions:
 1) Overvoltage
 2) Undervoltage

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MAINTENANCE MANUAL

FUNCTION  SENSING  THRESHOLD  TIME DELAY  TRIPS  REMARKS 
OVERVOLTAGE (OV)  HIGHEST PHASE POR VOLTAGE  >131 |2 VOLTS  INVERSE  GCR, GCB 
　
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