Keywords: BEM, numerical methods, grounding analysis, transferred potentials.

Designing safe grounding systems for large electrical installations has been a challenging problem since the early days of the industrial use of electricity. Main objectives of the grounding systems are to carry and dissipate electrical currents produced during fault conditions into the ground, in order to ensure the safety of the persons in the vicinity of a grounded installation and to guarantee the power supply and the integrity of the equipment. To attain these goals, it is required that fault electrical currents dissipate into the soil mainly through the grounded electrode, while potential values between close points on the earth surface must be kept under certain maximum safe limits (step, touch and mesh voltages), established in most of the guides and legal procedures for grounding system design [1,2]. In practice, most of grounding grids of electrical substations consist of a mesh of interconnected cylindrical conductors, horizontally buried, and supplemented by vertically thrusted ground rods in certain places of the substation site [1,2].

Equations governing the electrical current dissipation into the soil through a grounded electrode are well-stated from Maxwell's Electromagnetic Theory. For real cases, no analytical solutions can be obtained due to the complexity of the boundaries shapes. For this reason, several methods for the grounding analysis and design of electrical substations have been proposed since the sixties. These methods (generally based on professional practice, semi-empirical works, experimental data obtained from scale model tests, or intuitive ideas) represented an important improvement in the grounding analysis field. However, some problems (such as large computational requirements, unrealistic results when segmentation of conductors is increased, and uncertainty in the margin of error) have been reported [1].

On the other hand, the numerical application and resolution of the boundary problem in practical cases for grounding grids of large installations presents some troubles. Widespread numerical techniques commonly used in other fields of Engineering (such as the FEM and the FDM) are precluded in grounding analysis. The reason is that the required discretization of the soil would imply an unacceptable computational cost due to the characteristic geometry of these systems: a mesh of interconnected bare conductors with a small ratio diameter/length.

In the last years the authors have developed a numerical formulation based on the Boundary Element Method for the analysis of grounding systems embedded in uniform soils. This approach has been implemented in a CAD system that currently allows to analyze real grounding grids in real-time in personal computers [3]. Recently, this formulation has been extended for the grounding analysis in layered soils [4,5].

In this work we present a CAD system based on this BEM numerical formulation for the analysis of a common problem in electrical engineering practice, that is, the existence of transferred potentials in a grounding installation [1]. The transfer of potentials between the grounding area to outside points by buried conductors, such as communication or signal circuits, neutral wires, pipes, rails, or metallic fences, may produce serious safety problems [3]. Thus, in this paper we summaryze the BE numerical approach and we present a new approach for the transferred potential analysis. Finally, we show some examples by using the geometry of real grounding systems.

1

ANSI/IEEE Std.80. "Guide for Safety in AC Substation Grounding", IEEE, New York, USA, 2000.

2

J.G. Sverak, W.K. Dick, T.H. Dodds and R.H. Heppe. "Safe Substation Grounding", IEEE Transactions on Power Apparatus and Systems, Part I: 100, 4281-4290, (1981); Part II: 101, 4006-4023, 1982.

3

I. Colominas, F. Navarrina and M. Casteleiro. "A Boundary Element Numerical Approach for Grounding Grid Computation", Computer Methods in Applied Mechanics and Engineering, 174, 73-90, 1999. doi:10.1016/S0045-7825(98)00278-3

4

I. Colominas, J. Gómez-Calviño, F. Navarrina and M. Casteleiro. "Computer Analysis of Earthing Systems in Horizontally or Vertically Layered Soils", Electric Power Systems Research, 59, 149-156, 2001. doi:10.1016/S0378-7796(01)00148-1

5

I. Colominas, F. Navarrina and M. Casteleiro. "A Numerical Formulation for Grounding Analysis in Stratified Soils", IEEE Tr. Power Delivery, 17, 587-595, 2002. doi:10.1109/61.997943

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