Third Harmonic Current Phase Angle Behaviour During Branch Touching Wire Earth Faults

Abstract

This research scrutinizes credibility of the third harmonic (I_3h) current phasor relationship with respect to the fundamental voltage (V_50-Hz) as a potential signature of Vegetation High Impedance Faults (VeHIFs) using data from continuous-contact ‘branch touching wire’ earth faults. Prior work argued on a distinct phasor relationship between I_3h and V_50-Hz. Common HIF models simulate this distinct phase relationship as ∼ 180° with near perfect stability after only few cycles. This work uses a dataset of 132 phase-to-earth (ph-to-e) VeHIF test recordings to analyse temporal variations in the I_3h phase shift. The over-fault mean was ∼170° with a volatility (standard deviation (STD)) of 7°. Ninety-five percent of tests stabilised to these values within 1.25-s of fault inception. Outlier cases with long phase-shift stabilisation periods may pose a risk in the timely detection of earth faults. However, such cases were usually associated with low initial fault currents (I_f) for extended periods and were unlikely to cause ignition. Fault currents above 33mA are shown to have stable phase-shifts. Finally, a regressive statistical model is presented for modelling time sequences of I_3h phase-shift with respect to V_50-Hz.