Power line grounding can be a serious concern for the power supply industry. Only a small amount of ground resistance is acceptable because a good electric contact of power line towers with the earth reduces the possibility of the natural hazard associated with lightning. We investigated the possibility of using helicopter-borne electromagnetic (HEM) apparatus to detect the ground resistances of power line towers. Two induction systems were considered: a transmitter-earth-receiver system and a power line loop-earth-receiver system. The transmitter-earth-receiver system built the background signal in the HEM receiver, whereas the power line loop-earth-receiver system produced the signal used for resolving the power line ground resistances. By ignoring the mutual coupling between the two systems, we calculated the induced electromotive forces in the power line loop above which the HEM bird was flown and its two immediate neighbor loops. We replaced other loops along the power line with the Norton-Thevinin equivalence and solved the current flow in the power line loop by the mesh current method. Using the current in the power line loop as induction source, we calculated the electromagnetic responses in the HEM receiver from the power line loop-earth-receiver system. For identification of the poorly grounded towers, we introduced ratios of in-phase and quadrature to signal amplitude and the HEM phase. We evaluated numerical experiments showing that these parameters can well identify power line towers with high ground resistances, and they were insensitive to the flight altitude and direction. Moreover, we also took the catenaries of the power line into consideration so that the elevation differences between towers resulting from rugged mountains could be easily incorporated into the interpretation. We considered vertical coaxial (VCA) and horizontal coplanar (HCP) coil configurations. It was found that the signal for HCP coil has multiple peaks, whereas the VCA coil signal has only a single peak, so we recommend the VCA coil array for power line ground resistance detection. The inversion of synthetic data showed that the power line ground resistances can be resolved from the HEM survey.

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