The electrical power sources are:
- Four Variable Frequency engine Starter/Generators (VFSGs)
- Two variable frequency APU starter/generators
- Three external ac power receptacles
- One Ram Air Turbine (RAT)
- One main battery
- One APU battery
- Three flight control Permanent Magnet Generators (PMGs)
- Two EEC Permanent Magnet Alternators (PMAs)
The power sources remain isolated from each other throughout the generat
ENGINE STARTER/GENERATORS
- The engine starter/generators function as electrically powered starter motors for engine start and as engine-driven generators once the engines are running.
- Each engine has two 235 Vac VFSGs. L1 and L2 are mounted on the left engine. R1 and R2 are mounted on the right engine. They are directly connected to the engine gearboxes, producing variable frequency power proportional to engine rotor speed.
- Electrical power for engine start can be provided by external power, APU generated power, or the opposite engine’s generated power. The electrical system automatically selects both starter/generators to engage in order to provide optimal torque for engine start.
- After start, the starter/generators function as generators to energize their respective main AC buses, also designated L1, L2, R1, and R2. Generator status is shown by generator switch indications, MFD electrical synoptic, and the EICAS message system.
- Each generator has a drive disconnect mechanism that allows the generator to be mechanically disengaged from the engine. Depending on the fault condition, the generator drive is either automatically disconnected or requires pilot action to be disconnected.
- The generator DRIVE light, generator DRIVE synoptic
indication, and EICAS advisory message ELEC GEN DRIVE shows when a condition is detected that requires the pilot to disconnect the generator drive. - The generator can then be disconnected from the engine by pushing the respective generator drive disconnect (DRIVE DISC) switch. The drive cannot be reconnected by the pilot.
APU STARTER/GENERATORS
- The APU has two 235 VAC variable frequency starter/generators, designated
left (L) and right (R). The generators are directly connected to the APU gearbox,
producing variable frequency power proportional to APU speed - Electrical power for APU start can be provided by forward external power, APU battery, or engine-generated power.
- The electrical system automatically selects single starter/generator to provide sufficient torque for APU start. For APU starts with the APU battery as the power source (no engine running and forward external power not ON), the right starter/generator is always used. Either the left or right starter/generator is used for APU start if an engine is running or forward external power is selected ON.
On the ground, both APU generators are available to energize:
The airplane’s 235 VAC electrical buses; L1, L2, R1, and R2.
There are three common motor start controllers (CMSCs) available to drive the two APU starter generators.
Two CMSCs give priority to the main engine start
and the third CMSC gives priority to the hydraulic electric motor pump over the APU start.
If demand pumps are turned on during APU start, the start may be aborted due to the CMSC being prioritized away from the APU start. Similarly, if an engine is shut down during an APU start, the start may be aborted due to apower interrupt to the CMSC performing the start.
EXTERNAL POWER
- The 115 VAC external power system consists of both a forward and aft system, with their corresponding receptacles, controls, and indications.
- The forward system provides the power for general ground operations. It has two (L, R) power receptacles located on the left forward fuselage of the airplane. Connecting one or both power sources to the receptacles illuminates the AVAIL light(s) in the power switch(es) and energizes Ground Handling mode. Only one connection is required to power this mode. If two sources are connected, the first source connected is the one that is used.
- If the APU is not available, engine start may be accomplished using external power.
- At a minimum, two 90 kVA-rated forward external power sources are required
- In this configuration, either engine may be started first
- Two VFSGs are used during start, resulting in longer engine start times compared to an APU start.
- Load shedding occurs, especially during engine start. Connecting the two forward and one aft receptacles only reduces load shedding and does not decrease engine start time.
- The aft external power system can assist the forward system when starting the right engine first. The aft receptacle is located on the airplane fuselage aft of the left wing. To receive the benefit of the aft power source, both forward power sources must also be connected and the right engine must be started first.
RAM AIR TURBINE (RAT) GENERATOR
A RAT serves as an emergency source of electrical and hydraulic power. It has no operating time, airspeed, or altitude limits.
Inflight, the RAT deploys automatically if any of the following occur:
- Both engines have failed.
- All three hydraulic system pressures are low.
- Loss of all electrical power to Captain’s and First Officer’s flight instruments.
- Loss of all four EMPs and faults in the flight control system occur on approach.
- Loss of all four EMPs and an engine fails on takeoff or landing.
The RAT can be deployed manually, if necessary, by pushing the RAM AIR TURBINE switch on the hydraulic panel.
The main battery provides power for:
- Airplane power-up
- APU start (assists APU battery)
- Refueling operations
- Towing operations
- Electric braking (as backup power source)
- Captain’s flight instruments (energizes essential instruments until RAT deployment)
The APU battery provides power on the ground for:
- APU start
- Navigation lights (during battery-only towing operations)
PERMANENT MAGNET GENERATORS (PMGS)
- Three engine-driven PMGs are the primary source of power for the flight control electronics. These power sources are independent from the main airplane electrical system and are also independent from each other.
- A secondary source for flight control power is provided by the airplane’s 28 VDC bus distribution system and the main battery. In addition, a backup system is provided by dedicated batteries to assure positive flight control operation during temporary power interruptions
PERMANENT MAGNET ALTERNATORS (PMAS)
- Two engine-driven PMAs (one per engine) are the primary source of power for the EEC. PMAs are independent from the main airplane electrical system and are also independent from each other.
- A secondary source for EEC power is provided by the airplane’s 115 Vac bus distribution system. During engine start, initial EEC power is provided by the
airplane until the PMA is able to provide power. The airplane also serves as a backup EEC power source if a PMA is inoperative.
The electrical system uses two distinct power distribution methods:
- Primary and remote
- The primary power distribution method energizes higher amperage loads directly from bus power panels located in the forward and aft E/E bays
- The remote distribution method energizes lower amperage loads through a decentralized network of 17 remote power distribution units (RPDUs) located throughout the airplane.
- A limited number of high voltage 235 Vac electrical loads are energized from the aft E/E bay. The majority of airplane electrical loads are 115 Vac and 28 Vdc, energized by the forward E/E bay power panels and RPDUs.
Main 235 VAC Bus Distribution System
- There are four main 235 VAC buses, designated L1, L2, R1, and R2.
- On the ground, with only forward external power selected ON (no engine or APU generator power), the 235 VAC buses are energized by the 115 VAC distribution system through power conversion devices. If the APU or an engine(s) is subsequently started, it energizes the four main 235 VAC buses and external power transitions from ON to AVAIL.
- On the ground, with only the APU power available (no external power or engine generator power), each APU generator energizes two of the four main 235 VAC
buses. - With one engine running (no APU or external power) , that engine’s two related 235 VAC buses are energized directly. The other two 235 VAC buses are energized by the two powered buses.
- With one engine and the APU running, that engine’s two related 235 VAC buses are energized directly. The other two 235 VAC buses are energized by the APU
generators. - With both engines running, each main 235 VAC bus is energized directly from its related engine generator. If any engine generator is not available, its related main bus is energized by another main 235 VAC bus.
The EICAS caution message ELEC AC BUS (L1, L2, R1, R2) shows when:
A main 235 VAC bus is inoperative
115 VAC Bus Distribution System
There are multiple 115 VAC buses.
* To distribute power, the 115 VAC system uses both primary power system buses located in the forward E/E bay and RPDUs located throughout the airplane.
* The higher amperage (greater than 10 amps) loads are distributed directly by the bus system.
* The lower amperage (10 amps or less) loads are distributed by the RPDUs.
* On the ground, with only forward external power selected ON (no engine or APU generator power), the 115 VAC system is energized directly. It then energizes the 235 VAC bus system through power conversion devices.
* With an engine(s) and/or APU running (external power OFF), the main 235 VAC buses energize the 115 VAC system through power conversion devices. If forward external power is then selected ON, it energizes the 115 Vac system directly.
28 VDC Bus Distribution System
- There are multiple 28 VDC buses.
- To distribute power, the 28 VDC system uses
both primary power system buses located in the forward E/E bay, and RPDUs located throughout the airplane. - The higher amperage (greater than 10 amps)
loads are distributed directly by the bus system. - The lower amperage (10 amps or less) loads are distributed by the RPDUs.
The 235 VAC system provides power to the 28 VDC system through power conversion devices.
Four electrical brake power supply units regulate power for the exclusive use of the airplane electric brake system.
* Each of the four power supplies receives two separate 28 VDC inputs: one from the 28 VDC bus system and one from the main battery.
* The 28 VDC bus system is the primary power source and the main battery provides the backup power source.
Large Motor Power Distribution System
- The airplane’s use of variable frequency power requires large motor loads to be energized by motor controllers
- The main 235 VAC distribution system energizes the large motor power system through power conversion devices.
- The Power Electronics Cooling System (PECS) dissipates heat created by these high voltage motor controllers. PECS is energized if one or both engines
or the APU is running, or if forward external power is connected and selected ON.
ELECTRICAL LOAD MANAGEMENT
- The 787 electrical system dynamically manages electrical loads. Using electronic controllers, the system can automatically cycle on or off loads for
efficient use of available airplane power - The Bus Power Control Unit (BPCU) provides flight deck control/indication, power transfer functions, and overall load management and protection. In the event of a BPCU failure, a second BPCU assumes all functions.
Towing Power Mode
- Towing Power mode is active when the TOWING POWER switch is selected to ON (with the main BATTERY switch OFF).
Significant loadsenergized by this mode include:
* Captain’s ACP
* Aisle stand flood lights
* Captain’s flight interphone
* Navigation lights
* Flight deck dome lights
* Brakes
- Note: Radio communication systems are not energized in the towing power mode.
On-Ground Battery Only Mode
On-Ground Battery Only mode is active when the BATTERY switch is selected to ON while the airplane is on the ground and there are no other power sources
connected. This mode is to support the airplane power-up sequence.
Significant loads energized by this mode include:
Captain’s inboard DU
* MFK L
* Lower DU
* CCD L
* MCP
* AHRU L, R
* APU start capability
* FMC
* Wing fueling panel
* CCR L
* Captain’s/First Officer’s ACP
* Ground crew call horn
* Captain’s/First Officer’s flight interphone
* Brakes
* VHF L
* Engine/APU fire detection
* TCP L
* Miscellaneous lighting
* DSP L
- Note: Some overhead panel indicator lights are inoperative in On-Ground Battery Only mode.
Ground Handling Mode
Ground Handling mode is active when the first forward external power receptacle is connected (not selected ON) and there are no other power sources.
Significant loads energized by this mode include:
- Lower DU
- Cargo compartment lighting
- APU start capability
- Cargo door actuation systems
- MFK L
- Cargo handling systems
- CCD L
- Cargo loading lights
- CCR L, R
- Service compartment lights
- Equipment cooling fans
- Wing fueling panel
- Engine/APU fire detection
- Battery chargers
- Note: Ground Handling mode has priority over On-Ground Battery Only mode and initializes regardless of BATTERY switch position.
Ground Service Mode
Ground Service mode is active when the GROUND SERVICE switch on the master attendant switch panel is selected ON while in Ground Handling mode.
In addition to loads energized by Ground Handling mode, other significant loads energized by this mode include:
- Cabin systems (limited operation)
- Cabin lighting (limited operation)
- Flight deck dome lighting
External Power On Mode
External Power On mode is active when at least one forward external power receptacle is connected and selected ON with the APU and engine generators off.
Significant loads energized by this mode with only one forward external power source connected and selected ON include:
- Flight deck equipment, controls, and indications
- Cabin lighting (limited operation)
- Cabin systems (limited operation)
- PECS (single loop only)
Additional loads energized by this mode with two forward external power sources connected and selected ON include:
- NGS (limited operation)
- Cabin systems (basic operation)
- Hydraulics (limited operation)
- Cabin lighting (basic operation)
- IFE (limited operation)
- PECS (both loops)
Note: Packs and cargo heat are inoperative in External Power On mode.
In-Air RAT Only Mode (Standby Power)
In-Air RAT Only mode is active if loss of all electrical power to Captain’s and First Officer’s flight instruments occurs inflight.
The RAT energizes the Captain’s flight instruments and other essential equipment for flight controls, navigation, and communication. The main battery provides standby power until RAT deployment.
After operation in In-Air RAT Only mode, when normal electrical power is restored, some critical systems may not restore. Air conditioning system CACs may not restore. If none of the CACs restore, inflow for cabin pressurization is lost for the rest of the flight. Descent to a lower altitude may be needed.
After operation in In-Air RAT Only mode, the following EICAS messages and flight deck effects can occur:
- FUEL PUMP CENTER R
- AUTOTHROTTLE DISC
- AUTOPILOT
- PACK L+R or PACK L or PACK R
Significant loads energized by this mode include:
- Captain’s inboard DU
- Captain’s outboard DU
- Lower DU
- Autoflight system (limited operation)
- MCP
- Autopilot (limited operation)
- Captain’s/First Officer’s ACP
- Captain’s/First Officer’s flight
- interphone
- VHF L
- TCP L
- DSP L
- ECL
- MFK L
- CCD L
- IRU L, R
- AHRU L, R
- INR L, R
- ISFD
- FMC (limited operations)
- PFC
- CCR L
- Center pitot heat
- Engine/APU fire detection
- Miscellaneous lighting
Note: Inoperative items for In-Air RAT Only mode include: Autothrottle, LNAV/VNAV, FMC predictions, FMC thrust limits, TAP, flaps, slats, stabilizer trim, thrust reversers, auto speedbrakes, packs, head-up displays, HF radios, SATCOM system, TCAS, GPWS, transponder,
weather radar, external lighting, WIPS, and window heat.
