Calculation of shielding to earth loop of power cables

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Supervisor:
Dr. Ladányi József
Department of Electric Power Engineering

In the course of planning of substation earthing systems the value of the potential rise of the earthing grid must be determined which is given by the product of the resultant impedance to earth of the earthing system and the current to earth. The current to earth can be determined by calculation with the consideration of mains parameters (transmission line, cable, transformer, resultant impedance to earth of substation). The resultant impedance to earth of a substation is the parallel resultant of the value of the individual resistance to earth of the earthing grid itself and the input impedance to earth of passive conductors (ground wires of overhead lines, cable shields, rails, etc.) connected to the grid. So the knowledge of the resultant input impedance of the shielding to earth loop of power cables is also indispensable from the viewpoint of planning of earthing systems. Because this impedance is connected to the resistance to earth of the earthing grid parallel, it influences the current distribution between the earthing grid and the cable shields in case of an earth fault as well as it determines the potential rise and touch voltages which will be formed in the substation. The allowable value of these voltages and the planning of earthing systems are controlled in detail by the MSZ EN 50522:2011 standard. The shielding to earth loop impedance of armoured cables shows non-linearity in the function of frequency so it is reasonably to use a computer simulation with adoption of iteration to calculate the input impedance. The aim of my thesis is to make a target software which is suitable for this task.

In the first part of my thesis I overview the relevant technical literature, then I present the values of surface impedance of two medium voltage power cables with extruded plastic cover in the function of current and frequency as a result of my laboratory measurements. I utilized the measured direct-current resistance of cable shields as an input data in the calculating software. The further needed measurement results of cables with armour and bitumen cover were available from earlier measurements at the Department. The instruments and devices, which were needed to the measurement, were provided by the Power System and Environment Group of Department of Electric Power Engineering.

In the second part of the thesis I present the target software made in MATLAB environment. The realized program considers the several influencing factors (current to earth, collective co-running of cables, specific conductance to earth, non-linear surface impedance of armour) and gives the current distribution between the cable lines and the potential rise of the substation as a result. The method contains approaches, neglects so I validated the obtained values through comparison with exact calculation results.

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