31-03-2012, 03:59 PM
A Review of Active Rectification in aircraft AC systems
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INTRODUCTION
Traditionally the three phase system has
been the most common Power Generation
design. For Aircraft Systems 115/200V 3
Phase with a fixed frequency of 400 Hertz
(Hz) was and still is widely used until the
adoption of Variable Frequency (VF)
AC/DC COMPARISON
In terms of generation this in all likelihood will
remain AC driven from the engine for the
foreseeable future for Aircraft primary power.
There has been some interest in Fuel Cell
technology which would produce a DC output
for ground power where its quiet operation
would compare favorably with the Auxiliary
Power Unit (APU) however this is probably
still some way off for both the military and
Commercial Aircraft market.
DC DISTRIBUTION
High Voltage Direct Current (HVDC) power
distribution from its inception was perceived
to provide a mass and volume advantage
over its AC counterpart due to the reduction
in feeders from three to one. (Traditionally the
return of such a system being by the
airframe)
AC DISTRIBUTION
AC Distribution systems have been the
standard for Aircraft primary power
distribution for many years and 115/200V AC
3 phase 400Hz is available at airports world
wide.
In common with its acceptance across
Airports, all the major Aircraft Avionics
System and Component manufacturers
design a full range of equipment qualified to
meet the standards laid down for 115/200 V
AC and a large range of Commercial Off the
Shelf (COTS) equipment is available for use
with 115/200V AC.
POWER QUALITY
In addressing Aircraft Power Systems we
also need to consider Power Quality and the
effect of harmonics.
Harmonics are caused by the nature of the
loads placed upon a Generation system. If
the load is non-linear in nature it will draw non
sinusoidal currents from its power supply.
Furthermore if a number of such loads are
impressed upon a supply their effects are
additive and the supply current and voltage
waveforms can become complex.
CONCLUSION
The next generation of Aircraft will not be
faced with an entirely clear choice of Power
Distribution Systems to choose from. The
situation is not clear cut and should not be
seen as a stark choice between AC and DC.
As outlined in this paper each has its merits
and disadvantages and perhaps the best
choice overall would be a hybrid system
making optimal use of what is available.