UNIVERSITY OF SALENTO – SCHOOL OF INDUSTRIAL ENGINEERING
DEPT. OF ENGINEERING FOR INNOVATION – Lecce-Brindisi (Italy)
MASTER OF SCIENCE IN AEROSPACE ENGINEERING
PROPULSION AND COMBUSTION
Aircraft Systems Mechanical, Electrical and
Avionics.pdf
Chap. 5 - Electrical Systems
LECTURE NOTES AVAILABLE ON
http://www.ingegneria.unisalento.it/scheda_personale/-/people/antonio.ficarella/materiale
Prof. Eng. Antonio Ficarella
University of Salento - antonio.ficarella@unisalento.it
REVIEW
DATE
FILE
RESPONSIBLE
R00
14/12/2013
propASelectricalR00.odp
Antonio Ficarella
antonio.ficarella@unisalento.it
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INTRODUCTION
aircraft have become more dependent upon electrically powered services
A typical electrical power system of the 1940s and 1950s was the twin 28
VDC system. each engine powered a 28 VDC generator which could
employ load sharing with its contemporary if required. One or two DC
batteries were also fitted and an inverter was provided to supply 115 VAC
to the flight instruments.
much greater power requirements - four 115 VAC generators (60-73 kVA
generators)
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Electrical Power Evolution
introduction of powerful new AC electrical systems
In order to generate constant frequency 115 VAC at 400 Hz a constant
speed drive or CSD is required to negate the aircraft engine speed
variation over approximately 2:1 speed range (full power speed:flight idle
speed). These are complex hydro-mechanical devices which by their very
nature are not highly reliable.
The advances in high power solid state switching technology together with
enhancements in the necessary control electronics have made
variablespeed/constant frequency (VSCF) systems a viable proposition in
the last decade.
‘more-electric aircraft’ concept where it is intended to ascribe more aircraft
power system activities to electrical means rather than use hydraulic or
high pressure bleed air
Schemes for the use of 270 VDC are envisaging power of 250 to 300 kW
and possibly as much as 500 kW per channel
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The sensible limit for DC systems has been found to be around 400 amps
due to the limitations of feeder size and high power protection switchgear.
Therefore for a 28 VDC system delivering 400 amps, the maximum power
the channel may deliver is about 12 kW, well below the requirements of
most aircraft today.
Airbus A380 utilising 150 kVA per channel and the Boeing 787 being even
more electric with 500 kVA per channel
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Aircraft Electrical System
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Power Generation
DC Power Generation
usually the voltage is 28 VDC but there are 270 VDC systems
The natural load characteristic of the generator is for the voltage to ‘droop’
with the increasing load current- For this purpose a voltage regulator is
used to ensure that terminal voltage is maintained while the aircraft
engine speed and generator loads vary
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AC Power Generation
Most AC systems used on aircraft use a three-phase system
the phase voltage of a standard aircraft system is 115 VAC, whereas the
line voltage measured between lines is 200 VAC. The standard for aircraft
frequency controlled systems is 400 cycles/sec or 400 Hz.
higher voltages require better standards of insulation
a higher voltage relates to an equivalent lower current. - The lower the
current the lower are losses
Also as current conductors are generally heavy is can be seen that the
reduction in current also saves weight
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Constant Frequency/IDG Generation
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Variable Frequency Generation
In this technique no attempt is made to nullify the effects of the 2:1 engine
speed ratio and the power output, though regulated to 115 VAC, suffers a
frequency variation typically from 380 to 720 Hz.
This wide band VF power has an effect on frequency sensitive aircraft
loads - In many cases variations in motor/pump performance may be
accommodated but in the worst cases a motor controller may be needed
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VSCF Generation
the variable frequency power produced by the generator is electronically
converted by solid state power switching devices to constant frequency
400 Hz, 115 VAC power
UNIVERSITY OF SALENTO – DEPT. OF ENGINEERING FOR INNOVATION – Lecce-Brindisi (Italy)
Primary Power Distribution
the aircraft may accept power from the following sources
• Main aircraft generator
• Alternate aircraft generator
• APU generator
• Ground power
• Backup converter
• RAT generator
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Generator Control Breaker (GCB) - using Electronic Load Control Units
(ELCUs) or ‘smart contactors’
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UNIVERSITY OF SALENTO – DEPT. OF ENGINEERING FOR INNOVATION – Lecce-Brindisi (Italy)
Power Conversion and Energy Storage
• Conversion from DC to AC power – inverters
• Conversion from 115 VAC to 28 VDC power – Transformer Rectifier
Units (TRUs)
• Conversion from one AC voltage level to another
• Battery charging
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UNIVERSITY OF SALENTO – DEPT. OF ENGINEERING FOR INNOVATION – Lecce-Brindisi (Italy)
Batteries
• To assist in damping transient loads in the DC system
• To provide power in system startup modes
• To provide a short-term high-integrity source during emergency
conditions
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Secondary Power Distribution
Power Switching
Load Protection
Circuit Breakers
Solid State Power Controllers
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Typical Aircraft DC System
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Typical Civil Transport Electrical System
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Electrical Loads
• Motors and actuation
• Lighting services
• Heating services
• Subsystem controllers and avionics systems
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Emergency Power Generation
The aircraft battery offers a short-term power storage capability, typically
up to 30 minutes
Extended Twin OPerationS (ETOPS) flights now means that the aircraft
has to be able to operate on one engine while up to 180 minutes from an
alternative or diversion airfield
• Ram Air Turbine (RAT)
• Backup power converters
• Permanent Magnet Generators (PMGs) - also called Permanent Magnet
Alternators (PMAs)
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Backup generators are driven by the same engine accessory gearbox but
are quite independent of the main IDGs.
UNIVERSITY OF SALENTO – DEPT. OF ENGINEERING FOR INNOVATION – Lecce-Brindisi (Italy)
the backup converter hosts PMGs which may supply several hundred
watts of independent generated power to the flight control DC system
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UNIVERSITY OF SALENTO – DEPT. OF ENGINEERING FOR INNOVATION – Lecce-Brindisi (Italy)
Recent Systems Developments
• Electrical Load Management System (ELMS)
• Variable Speed Constant Frequency (VSCF) – Cycloconverter
• 270 VDC systems
• More-Electric Aircraft (MEA)
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Electrical Load Management System (ELMS)
Load management and utilities systems control is exercised by mean of
Electronic Units (EUs) mounted within the power management panels.
Each of these EUs interfaces with the left and right aircraft systems digital
data buses and contain a dual redundant architecture for reasons of
dispatch availability.
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Approximately 17–19 Electrical Load Control Units (ELCUs) supply and
control loads directly from the aircraft main AC buses.
These loads can be controlled by the intelligence embedded within the
ELMS EUs.
A major advance is the sophisticated load shed/load optimisation function
which closely controls the availability of functions should a major electrical
power source fail or become unavailable. The system is able to
reconfigure the loads to give the optimum distribution of the available
power.
substantial reduction in volume, wiring and connectors, weight, relays and
circuit breakers
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Variable Speed Constant Frequency (VSCF)
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Recent Electrical System Developments
Airbus A380 Electrical System Overview
AC power generation
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DC System
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B787 Electrical Overview
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