Equivalent lumped parameter II-network of standard grounding systems under surge conditions

The authors propose and validate a simple lumped parameter model based on a π-type network, aimed at simulating the ground potential rise of standard ground electrode configurations under surge conditions. Lumped parameter values are estimated by means of an optimization procedure based on micro-genetic algorithms (μGAs), which minimizes the standard deviation between target values (obtained using a full non-linear circuit model) and simulation results (yielded by the π-network model). The proposed model has been validated by means of massive simulations under surge conditions, i.e. varying impulsive current wave shapes, grounding system geometrical configurations, soil characteristics and also accounting for non-linear ionization phenomena, showing a very good agreement between target values and model results. In addition, the proposed π-network model drastically reduces the computational resources required for linear and nonlinear transient analyses of the simulated grounding systems.

Introduction

Experimental results and numerical simulations of ground electrodes excited by lightning currents have shown that the transient responses are affected by marked inductive behaviours and non-linear ionization phenomena [1]–[5]. In order to obtain a correct design of electrical systems, with respect to the protection of installations against anomalous events, it is fundamental to predict the impulse characteristics of grounding systems. In addition, the accurate simulation of the nonlinear transient behaviour of these groundings became a fundamental task in parametric studies related to, e.g., backflashover simulation of HV transmission lines or direct lightning of the transition tower of a long mixed overhead-cable EHV lines [6]–[7]. Although several respected models are available in literature, the circuit ones have been those mainly used to simulate complex power systems in which the grounding system is only one of the components, even if they are very computational expensive when non-linear cases are analysed [2]–[5]. In addition, the non-linear circuit model can be easily coupled with power system models developed by means of transient electromagnetic programs such as ATP [8]. These models generally provide accurate results for very complex scenarios [2]–[3], [6]–[7]. However, the main drawback of such models, especially if statistical analyses must be carried out, is the request of large computational resources (i.e., memory occupation and execution time). In order to drastically reduce this drawback, the authors propose a procedure able to represent the input transient impedance of standard grounding systems by an equivalent -network. Recent studies have demonstrated that typical tower groundings may be represented by an equivalent -network for linear and non-linear cases [9]–[10]. In this paper, it will be investigated the possibility to fit the input transient impedances of standard ground electrode configurations [11] (Fig. 1) by equivalent -type networks. Figure 1.

Standard grounding systems simulated