50Hz (or 60Hz in America) power when they are parked at an airport apron stand. The aircraft switches its on board aircraft service loads and navigation avionics to 400Hz ground power service (GSE) provided by the airport.
The airport 400Hz “ground power” service is used Instead of the aircraft having to continue running the engines and the on board turbo generator auxiliary power unit (APU).
This 400Hz “ground power” service supply eliminates the noise and air pollution, unnecessary fuel consumption and non essential operating hours of the APU and contributes to the reduction of maintenance on the aircraft APU, generators and batteries.
Centralised 400Hz systems
In the earlier years the older airports provided the 400Hz ground power from multiple parallel connected large, heavy, noisy and inefficient rotary converters which had to be located in a purpose built substation along with special 400Hz distribution boards.
400Hz SD Series frequency converter aircraft ground power systems technical data sheet 30032011bpwR70mdoc
These centralised 400Hz system delivered power at high voltage up to 975V in an
underground ring main to a number of 90kVA service pits embedded in the concrete
apron near to the parking position of the aircraft.
Each of the pits would be fitted with a step down transformers and line drop compensation
equipment and a length of flexible cable with aircraft power connector.
Where the airports embarked on an expansion of a terminal or to construct a passenger
departure pier, (now being demanded by the growing numbers of Budget Airlines) this would
require an extension to a centralised 400Hz system. It was often found impractical to introduce
other loads onto the existing 400Hz ring main because it would have affected the voltage at the
point of use when incoming aircraft were to be connected.
The disruption to apron use and service, the out of action time and the cost to excavate the
concrete apron to run underground cables to extend a centralised system would be very
expensive, hence may users deployed the diesel engine driven 400Hz ground power units
(GPU’s) often as a “temporary measure but in reality became a long term lease revenue cost.
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