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UNIVERSITY OF SALENTO – SCHOOL OF INDUSTRIAL ENGINEERINGDEPT. OF ENGINEERING FOR INNOVATION – Lecce-Brindisi (Italy)

MASTER OF SCIENCE IN AEROSPACE ENGINEERINGMASTER OF SCIENCE IN AEROSPACE ENGINEERING

PROPULSION AND COMBUSTIONPROPULSION AND COMBUSTION

Aircraft Systems Mechanical, Electrical and Aircraft Systems Mechanical, Electrical and Avionics.pdfAvionics.pdfChap. 5 - Electrical SystemsChap. 5 - Electrical Systems

LECTURE NOTES AVAILABLE ON LECTURE NOTES AVAILABLE ON http://www.ingegneria.unisalento.it/scheda_personale/-/people/antonio.ficarella/materiale

Prof. Eng. Antonio FicarellaUniversity of Salento - antonio.ficarella@unisalento.it

REVIEW R00

DATE 14/12/2013

FILE propASelectricalR00.odp

RESPONSIBLE Antonio Ficarellaantonio.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 GenerationDC 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

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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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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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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.

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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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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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