다층 대지구조 모델을 위한 변전소 그리드 접지전극 설계

Abstract

‘Electrical Grounding’ originally began as a safety measure used to help prevent people from accidentally coming in contact with electrical hazards. Grounding system refers to metallic wires of different geometrical structures, which are buried in the earth. This metallic wire is used for establishing and maintaining the potential of the earth, or approximately that potential, on the circuit or equipment connected to it. The grounding resistance as well as step and touch voltages determine the performance and quality of grounding grids. The estimation of grounding resistance values and the step and the touch voltages an usually carried out by means of formulas and algorithms that take into account the mutual influence between the grid electrodes. In the past time , grounding system were designed to achieve ground resistance below a specified value. Current practice, however, dictates that such grounding systems are designed to control step, touch and mesh voltages within and around the electrical equipment, and limit both the extent of dangerous zones and the magnitudes of transferred potentials to remote sites. A safe grounding system has the following objectives : 1) Ensure such a degree of human safety that a person working or standing in the vicinity of grounded equipments is not exposed to the danger of a critical electric shock. The touch and step voltages produced in a fault condition have to be at safe values. A safe value is one that will not produce enough current within a body to cause ventricular fibrillation. 2) Provide means to carry and dissipate electric currents into earth under normal and fault conditions without exceeding any operating and equipment limits or adversely affecting continuity of service. 3) Provide grounding for lightning surges and the over-voltages occurring from the switching of substation equipment, which reduces damage to equipment and cable. 4) Provide a low resistance for the protective relays to see and clear ground faults, which improves protective equipment performance, particularly at minimum fault.

This paper describes significance of appropriate soil model to be considered while designing any grounding grid in order to take into account the variation in characteristics of soil. In the past, grounding systems were designed to apply the uniform soil model. But, usually soil models have two or more horizontally and/or vertically stratified layers of different resistivity, former being more common. While the most accurate design and analysis of the grounding system should certainly be based on the actual variations of soil resistivity present at the electrical equipment site, it will rarely be economically justifiable or technically feasible to model all these variations. However, in most case, design and analysis of a ground electrode based on the equivalent two-layered soil model are sufficient for design and analysis of grounding system. This paper will present the analytical methods that used for calculating the grounding resistance and earth surface potential distribution by finite element method(FEM) for grounding grids of electrical substations. In this paper, proposed algorithm was the expressions of the grounding resistance and the potential at any point due to a point source in multi-layered soil models can be obtained by image technique or solution of Laplace’s equations. Determination of potential from such expressions, therefore, forms significant part of computational effort in analysis and design of a grounding system in equivalent two-layer soil model. The validation of methods was explained by comparing their results and the other results that formulated in IEEE guide for safety in AC substation grounding(ANSI/IEEE Std. 80-2000).