Electrical power systems Flashcards
Design considerations
- power requirements and type
- mass and space
- distribution
- safety requirements
- redundancy
- dispatch
- maintainability
- cost
Safety and redundancy design considerations
- flight control system
- autoland
- ETOPS
- under regulations, at one engine inoperative, large twin-engined aircraft are limited to less than 60 minutes from a suitable diversion airfield
- ETOPS approval is required to extend the diversion time especially over oceanic or sparse terrain routes
Requirements for ETOPS approval
ETOPS type approval
the aircraft must have sufficient electrical backup power in the design
ETOPS operational certification
adequate procedures and processes in place by the operator including a track record
B777 programme design requirements
- self-sufficient on the ground
- aircraft dispatch with one engine
- aircraft dispatch for ETOPS with APU inoperative
- provide uninterruptible power to the FBW system
- ability to support CAT IIIB autoland in single-engine configuration with 2 separate supplies for autoland channels
- ability to share ground power between APU and external power to provide full ground operations and galley facilities
- A629 based electrical load management system to reduce crew workload
Types of electrical generators
IDG
hydromechanical constant speed drive included with the generator in a single package
caters for 2:1 ratio in engine speed between max power and ground idle
CSD acts as an automatic gearbox maintaining the shaft at a constant speed resulting in a constant frequency output
Drawbacks of CSD
- needs to be correctly maintained
- oil charge level and cleanliness
- frequent overhauls required to maintain high reliability
- expensive to purchase and maintain due to the complexity of CSD
VSCF
frequency wild AC is rectified to DC then reformed into 400Hz AC in the electrical equipment bay
not robust or reliable
PMG
crude but reliable and simple
used as a primary source of power for critical flight control systems(FADEC)
excitation source for primary engine generators when bringing them online
VF
- 115VaC is generated at a variable frequency but may need to be controlled at the load
- the simplest form of generating power
- cheapest and most reliable
- solid-state electronics used
- motor controllers needed for certain aircraft loads
- VF impacts other aircraft subsystems
B777 electrical power generation
- 3 IDGs
- PMGs
- main power system for critical consumers
- Emergency/backup
- RAT
- ram air turbines
- Batteries
- start APU
B777 ELMS
- automatic control over the supply of electrical loads
- main function-
- load shedding-
- optimum distribution of available power during a generator failure
- load shedding-
- control and integration based on ARINC 629 data bus
B777/787 system differences and benefits to the B787
B787 electrical power generation
- 6 VF
- PMG
- Emergency/backup
- RAT
- Batteries
