There is a recently installed frequency converter with a long run to the aircraft parking spot. For some aircraft everything just works but other aircraft won’t accept the power or rejects the power, and the diesel unit must be dragged out to provide the power. Everyone is upset about the downtime, lost productivity and increased costs. What is happening?
The Cable Culprits
While there is a chance that there is something wrong with the frequency converter, the more likely culprit is the cable providing the power to the aircraft. On an aircraft the voltage drops in the 400 Hz wiring are well within the allowed tolerance levels due to the short distances and lower currents. The same can not be said for the cable providing power to the entire aircraft. So what is happening at 400 Hz that makes the cable such a critical factor?
There are three basic types of aircraft power cables: three power cables and one neutral banded together every three to four feet; the same number of cables twisted together and enclosed in a scuff-resistant jacket; and six smaller power wires (typically AWG #2 for 90 kVA rated cables) twisted around a neutral wire and enclosed in a scuff-resistant jacket. All of these cables can have from 2 to 18 smaller control wires for the E&F circuit, ON/OFF pushbuttons, UP/DOWN/IN/OUT pushbuttons and for status LEDs.
In all of these cables, the number of wires in the cable, the configuration of the power wires to the neutral and to each other, as well as the size of the wires all affect the voltage presented to the aircraft from the cable plug. Per STANAG 3456, ATA 101 and MIL-STD 704F, the voltage at the external power receptacle of an aircraft must stay within 113 to 118 VAC.
Four wire banded cables are the least expensive cables and are typically used in low-power applications (< 40 kVA) where the voltage drop is lower due to lower currents. This is crucial because the inductance of the cable is different each time the cable is pulled out to an aircraft. Since the cable is only banded every three to four feet, the distance between the phase wires changes, and thus the voltage drop and the phase-to-phase voltage differs with the changing inductance. An additional disadvantage of this cable configuration is that the cables have to be replaced more frequently since they don’t have the extra insulation of a single jacket to provide scuff protection as the cable is dragged across the concrete apron to the aircraft.
Single-jacketed four-wire cables are more expensive than the banded cable, and are typically used in low- power applications up to 60 kVA. These cables have a consistent phase-to-phase voltage (though the voltage drops are different between phases since the core distance is different between phases). If the phase-to-phase voltage is critical in the aircraft being supported, then the frequency converter or voltage-drop compensation equipment must be capable of adjusting each phase voltage independently.
The standard single-jacketed six-around-one cable can support up to 90 kVA loads (this is the maximum rating for a standard aircraft power receptacle). Due to the construction of these cables, they provide the best voltage drop per foot characteristics of any aircraft cable and have the tightest phase-to-phase voltage tolerances due to the configuration of the cable. While these cables cost more to purchase, they also reduce power inconsistencies at higher loads which increases uptime and lowers maintenance costs and headaches.
Voltage Drop Compensation
Some frequency converters come equipped with an automatic voltage- drop compensation circuit. These units require separate voltage sensing wires in the aircraft cables and typically provide 7 percent to 12 percent voltage compensation. Piller frequency converters can use these wires as well, but they also have a separate capability (Intelligent-Boost) which measures the output current and the load power factor and automatically and precisely adjusts the output voltage without requiring the sense wires.
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