B777 Electrical

Introduction

The electrical system generates and distributes AC and DC power to the aircrafts system and it consists of the main AC power, the backup power, the DC power, standby power and the flight control power. The system operation is automatic and electrical faults are automatically detected and isolated.

AC Electrical System

The AC electrical system is the main source for the aircrafts electrical power. The entire aircrafts AC electrical load can be supplied by any two main AC power sources. The main AC electrical power sources are:-

1. Left and right engine integrated drive generators (IDGs).

2. Auxiliary power unit (APU) generator.

3. Primary external power.

4. Secondary external power.

1. Integrated Drive Generators (IDGs):-

Each engine has an integrated drive generator and automatically controls and protects the system functions. When an engine starts with the GENERATOR CONTROL switch selected ON, the IDG automatically powers the respective main bus and the previous power source is disconnected from that bus. The IDG can be electrically disconnected from the busses by pushing the

GENERATOR CONTROL switches to OFF or by selecting an available external power source before engine shutdown.

The DRIVE light illuminates and the EICAS (Engine Indicating and Crew Alerting System) message ELEC GEN DRIVE L or R displays when low oil pressure is detected in an IDG. The integrated drive generator can be disconnected from the engine by pushing the respective DRIVE DISCONNECT switch which then cannot be reconnected by the flight crew. Even high drive temperature causes the IDG to disconnect automatically.

2. Auxiliary Power Unit (APU) Generator:-

The APU generator is electrically identical to the IDG generators. The APU generator can power either one or both main busses, and may be used in flight as a replacement to an integrated drive generator. In flight when both transfer busses are unpowered, the APU starts automatically regardless of APU selector position and automatically connects to both main AC busses.

If the primary external source is powering both main busses then the APU powers the left main bus while the primary external source continues to power the right main bus. But if the primary and secondary external sources are powering the right and left main buses respectively then the APU powers the left main bus while the primary external source continues to power the right main bus.

If the secondary external source is powering both main busses, the APU then powers both main busses. The APU generator OFF light illuminates when the APU is operating and the APU generator breaker is open because of a fault or the APU GENERATOR switch is selected OFF.

3. Primary External Power:-

The primary external power can power the left and right main busses only when the primary power source voltage and frequency are within limits and the primary external power AVAIL (available) light illuminates. If no AC power is applied either of the power sources will power the aircraft if the BATTERY switch is ON. When primary external power is connected to a main bus, the PRIMARY EXT PWR ON light illuminates and the AVAIL light extinguishes. Switching the primary external power ON has the following effect in different conditions as follows:-

a) If a single integrator drive generator powers both busses, pushing the PRIMARY EXT PWR switch ON connects primary external power to both busses and removes the IDG source.

b) If both IDGs are powering their respective busses while the APU generator is not running and secondary external power is not available, pushing the PRIMARY EXT PWR switch ON connects primary external power to both busses and removes the IDG sources.

c) If both IDGs are powering their respective busses and secondary external power is also available therefore pushing the PRIMARY EXT PWR switch to ON connects the primary external power to the right main bus while leaving the left main bus being powered from the left integrated drive generator.

d) If the APU generator is running, pushing the PRIMARY EXT PWR switch ON connects the primary external power to the right main bus, leaving the previous source connected to the left main bus.

e) If both busses are powered from secondary external power, pushing the PRIMARY EXT PWR switch ON connects the primary external power to the