FedEx called on Power Quality International, Inc. for assistance. They were experiencing dangerous airframe-to-ground arcing, when supplying their aircraft from their hanger’s 400Hz electrical power system. Inside a hanger is considered a potentially explosive environment. Arcing is an unacceptable hazard.
Arcing results when zero-sequence harmonic currents, flowing through the zero-sequence impedance of the aircraft’s metallic airframe, generate zero-sequence harmonic voltages (Eh = Ih x ZH). Any grounding of the airframe will cause arcing. Arcing occurred when the hangar’s grounded hydraulic lines or static grounding cable come into contact with the aircraft. The zero- sequence current, which flows on the airframe, is generated by:
- Unbalanced loading in the three-phase, fourwire
- Single-phase nonlinear loads, which generate unbalanced positive- and negative- sequence harmonic currents, and
- Single-phase, nonlinear loads, which generate third-order, zero-sequence harmonic
In order to eliminate arcing, zero-sequence currents and arc voltage must be eliminated.
The measured arc voltage was 4.1 volts, sufficient to cause continuous arcing. The predominant arc frequency is 1200Hz, the 400Hz system’s third harmonic.
Solution
PQI was able to eliminate arc voltage with the application of a 400Hz IoFilter™ – Zero-Sequence Harmonic Filter.
The filter also improved the system’s power quality by:
- Reducing peak phase current,
- Reducing average phase current,
- Reducing transformer losses,
- Reducing system losses,
- Reducing total harmonic distortion,
- Improving power factor,
- Improving phase current balance, and
- Improving phase voltage balance.
The zero-sequence current at the aircraft was reduced by 95%. At this level, there is not enough current to sustain an arc should arc voltage increase. In addition, the arc voltage was reduced from 4.1 volts to less than 0.1 volts, which is a reduction of 97.6%. The reduction of zero-sequence current and airframe to ground voltage can be accompanied by applying an ultra low zero-sequence impedance at the load or aircraft end of the three-phase, four-wire radial feeder circuit. The zero sequence filter will shunt all zero-sequence currents, at its point of connection, in proportion to the zero-sequence impedance of the source and the filter.
