Evaluation of grounding system impedance of a near sea water HV electric power installation

Electric power transmission lines and substations located alongside salt water are frequent cases in the world. In situations where soil resistivity is high, a conductor connecting the grounding system of an electric power line to salt water can reduce substantially the grounding impedance. This deals with an already in service 69 kV transmission line belonging to Hydro-Quebec, built without shield wire neither counterpoise in a high soil resistivity and skirting the Atlantic ocean close to Labrador region in Quebec (Canada). Experimental measurements performed by Hydro-Quebec show that grounding resistance of some towers is high that in case of a short circuit on the line, a voltage rise exceeding the security limits can be developed. A continuous counterpoise connected to salt water can stabilize the potential profile. Method of images is applied to calculate the resistance to earth of different conductor arrangements. Results are validated by experimental measurements.