Investigation on Transient Behavior of Grounding Systems In Multilayer Earth Structure

Abstract

A Grounding system is one of the most important parts of the electrical network. Grounding system is a pivotal one to ensure the risk of life in the situation of grounding faults and to guarantee the safe and reliable operation of the power system. In order to better quantify the behavior of an earth electrode subjected to a lightning current impulse, it is necessary to understand the commonly encountered scenario of ground with various layers resulting from geological stratification. Thus the proposed work introduces a better understanding of the lightning transient behavior of an earth electrode in multilayer soil and develops a simplified approach to quantifying this behavior. Multilayer soil structure studies with grounding rod buried in the earth structure analysis gives close agreement with the measured site data. An optimization methodology is proposed to estimate the parameters of multilayer earth structure by using the hybrid Genetic Algorithm and Particle Swarm Optimization (GA-PSO). The objective function of the optimization problem is obtaining (2N – 1) variables of N layer soil structure. Calculated apparent resistivity has taken as a parameter to compute the theoretical resistivity as well as the parameters of horizontal multi structure earth. It is understood that the thickness of soil’s bottom layer is infinity. Steepest Descent Method (SDM) is also introduced for the estimation of Transient Ground Potential Rise (TGPR) in Substation. The SDM is also known as the gradient descent method. By using four wire Wenner method on the ground is to acquire the experimental apparent resistivity curve. With the measured experimental apparent resistivity, can compute the theoretical apparent resistivity curve and estimate the soil parameters such as a number of layers, thickness of each layer (Nth layer thickness is infinity) and its resistivity. The design of Air Insulated Substation (AIS) grounding systems may become inaccurate if the average value of resistivity measured is taken in the design calculation especially when the variation of resistivity of different probe distances is more than 25%. It is suitable to use more than two soil layers in the AIS grounding system. In the second work, AIS selection of optimal system touch voltage and step voltage depends on the load to be served and the distance between the generation source and the iiload. Soil Resistivity Measurements were carried out at site, by Wenner four point methods in location based on site condition. Observed that, measured soil resistivity readings in a direction were exceeding 30 percent, hence Multi-layer soil modelling chosen as recommended in IEEE 80-2013. In the AIS grounding system, number of layers, resistivity of each layer and thickness are the parameters to be estimated with the measured site data. Identifying specific areas on the grid are unsafe for touch and step voltage in AIS. Current Distribution, Electromagnetic Fields, Grounding and Soil Structure Analysis (CDEGS) software by using RESAP is used for optimizing the parameters of soil structure. Ground Potential Rise (GPR) of the substation is to be computed when the fault current is injected into the grounding grid in power frequency. The results of AIS are evaluated via the voltage levels such as step and touch with respect to earth design. In order to minimize the required substation area and enhance the results of grounding system, Gas-Insulated Substations (GIS) is widely used, mainly in urban cities nowadays. An interpretation of the soil resistivity measurements was carried out and analyzed for GIS by CDEGS. In the event of a short circuit, earth fault current can in the surrounding buried metallic infrastructure where it will be dissipated into the soil. It is therefore legitimate to determine if this will threaten the integrity of adjacent resident facilities and become a concern to public personnel safety. At the beginning of the GIS technology, the grounding design was designed for limiting the power frequency enclosure potentials to safe levels based on the maximum expected fault- current conditions by computing touch voltage levels and step voltage levels. IEEE 80- 2000 is used to design grounding system based GIS for soil models and help of software is taken for two or more than two-layer soil models. The performance of the grounding grid is heavily dependent on the soil structure. The results of GIS are evaluated via the voltage levels such as step and touch with respect to earth design.