Power and LF integrated grounding
The recent growth of cellular phone base stations lead to a quest for elevated locations for the base stations, particularly in rural areas. The figure shows a common solution, a cellular phone (GSM) antenna placed on top of a high-voltage (HV) tower.
Lightning may induce flashover of the HV insulators and a subsequent phase to ground fault. The power frequency current distributes over the lightning protection wires and the soil near the line via the tower grounding electrodes. A fraction of the fault current flows into the low voltage cable towards the distribution transformer feeding the GSM system. At other customers served by this transformer, this current fraction may cause dangerous touch voltages, and overstress the insulation of electric apparatus, certainly when assisted by the lightning proper.
Power faults often affect a larger region than the higher frequency lightning stroke. To study the effects experimentally, a 150 kV system was made available. On a high voltage tower carrying a cellular phone base station, one phase insulator was shorted to the tower. A current of about 200 A was injected into that phase at a large distance. We measured the current distribution and the relevant voltages in the low voltage net. The experimental current data agree with EMTP calculations. Appropriate measures to guarantee safety for persons and electronic equipment were considered. For practical reasons an isolation transformer proved to be the best option.