MODELLING OF A GROUNDING GRID USING AN ELECTROLYTIC TANK Ioannis F. Gonos Frangiskos V. Topalis Ioannis A. Stathopulos* National Technical University of Athens, Department of Electrical and Computer Engineering Electric Power Division, High Voltage Laboratory Abstract: The work presented in this paper refers to the problem of transient analysis of grounding grids under impulse lightning currents using scale models (an electrolytic tank). Impulse current tests were performed on several types of grids. The injected current in the grounding system and the developed potential were recorded, resulting in the determination of the variation of the transient impedance upon the time. Key words: Grounding system, electrolytic tank, transient impedance, scale model. 1. Introduction Grounding systems constitute one of the most important parts of building constructions. The grounding systems resistance has an essential influence on the protection of the grounded system. As it is stated in the ANSI/IEEE [1] a safe grounding design has two objectives: • To provide means to carry electric currents into the earth under normal and fault conditions without exceeding any operating and equipment limits or adversely affecting continuity of service. • To assure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electric shock. Grounding systems can consist of a) one or more verticals [2, 3] or horizontal ground rods [4, 5], b) three or more vertical ground rods connected to each other [2, 3] and to all equipment frames, neutrals and structures that are to be grounded. Such a system that combines a horizontal grid and a number of vertical ground rods penetrating lower soil layers has several advantages in comparison to a grid alone. Sufficiently long ground rods stabilize the performance of such a combined system making it less dependent on seasonal and weather variations of soil resistivity. Rods are more efficient in dissipating fault currents because the upper soil layer usually has a higher resistivity than the lower layers. The current in the ground rods is discharged mainly in the lower portion of the rods. Therefore, the touch and step voltage is reduced significantly compared to that of the grid alone. The existing studies are mainly concentrating to the behavior of grounding systems in the steady state operation condition. In this particular condition the value of the grounding system resistance appears to be much lower than the value of the resistance in the transient condition. It must be paid special attention to the above event in order to avoid possible failure or damage of the protected installation due to a high value of the grounding resistance, during a transient effect (e.g. lightning discharges). For this reason an effort has been made to simulate in the high voltage laboratory the operation of a grounding system during a transient phase and to study its behaviour. 2. Fundamentals A typical substation-grounding grid consists of a number of horizontal wires and vertical rods connected together and buried in the earth [6, 7]. Many questions may be raised concerning the best configuration for the mentioned grid. Experiments to answer such questions are both costly and difficult to perform. An alternative to full scale measurements is a numerical computation. Thus while numerical computation is a valuable tool it is not the only method and does need from time to time verification. On the contrary scale models offer a practical and inexpensive alternative solution [8]. The work presented in this paper refers to the problem of transient analysis of grounding grids under impulse lightning currents using scale models (an electrolytic tank). Experiments on full size grid are both costly and difficult to perform. In real grounding systems it is impossible to make experiments for the improvement of the grounding grid characteristics due to its large dimensions. The atmospheric conditions change the behaviour of the grounding impedance as they change the value of soil resistivity [8]. Modelling of ground grid using an electrolytic tank is a known technique over years. The electrolytic