GPU Operation: Cable System Considerations

When it is determined that there is a voltage-drop problem, there are several options to try to solve it.


If the frequency converter in use doesn’t have the ability to automatically adjust the output voltage or it doesn’t have enough range, then voltage-drop compensation equipment should be considered. These units are used to increase the length of an aircraft cable before the voltage drop reduces the voltage to the aircraft below the allowed tolerances (typically 112 – 113 VAC).

The basic line drop compensator (LDC) consists of one or two capacitors and a transformer per phase which are used to add a fixed amount of capacitance to counteract the cable inductance at a specific load, cable type and length. The transformer usually has between seven and 10 taps that are used to adjust the percent of capacitance that is added to the line. An LDC is setup using a resistive and reactive load bank that are typically set to the maximum load that is expected to be supported with the aircraft cable. The major problem for this type of device is that the capacitance is fixed for a specific inductance and power factor. If either of these changes significantly, then the applied capacitance could result in an out of tolerance voltage condition at the aircraft receptacle.

The line voltage regulator (LVR) was developed by Piller to provide varying amounts of capacitance based on the actual current and voltage being supplied to the aircraft. This device can maintain the aircraft receptacle phase-to-phase and phase-to-neutral voltages to within +/- 1.5 volts at any load. Thus, a boarding gate or maintenance area that is equipped with an LVR can handle any aircraft or load without requiring any tuning after the initial setup.

Conclusion

When it is determined that there is a voltage-drop problem, there are several options to try in order to solve the problem. Shortening the cable will lower the voltage drop, especially for higher current loads. If using banded cable, try replacing it with a seven-conductor single-jacketed cable to reduce voltage variations and voltage drops for a given length. If the shortest, single-jacketed cable that is adequate for the aircraft spot is already being used, then consider adding an LDC or LVR to the circuit.

For more information on this subject, download the “Cable Design for 400 Hz networks” at www.piller.com.

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