TRANSIENT AND STEADY STATE ANALYSIS OF GROUNDING GRIDS AFFECTING POWER SYSTEM OPERATION

The objective of this thesis is to analyse the transient and steady state behaviour of grounding grids with special reference to current outstanding problems affecting power system operation, viz. a low resistance grounding system design for a complete hydro-electric plant, ground fault current distribution in such a plant in relation to that in substation, towers and ground wire etc. and an analysis for the impulse impedance of grids involved, with their impact on insulator flashover under lightning impulse inputs.;The investigation begins with a review of the general background behind the present analysis. Under steady state analysis, a method to achieve a reasonably low grounding resistance for a complete hydro-electric plant (i.e. generation and transmission complex) situated on extremely resistive rock-bed terrains is presented. Advantage is taken of nearby volumes of water and low resistivity soil deposits to decrease the otherwise high resistance to ground of the plant. A new expression to calculate the ground resistance of a grid embedded in an isolated low resistivity soft medium (e.g. clay or water in a lake) lying on high resistivity hard medium (e.g. rock) is developed, depicting as to how the dimensions of this top superficial medium play an important role in controlling the value of the grid's resistance. This phase of the work also gives a description of the grid being used, together with possible methods of calculation for the various parts of the grounding network, taking into account the existence of multi-layer grounding paths. The methods described are general and can be applied to other types of plants as well with suitable adaptation.;Steady state analysis is also extended to develop a new analytical method to determine the ground fault current distribution in grounding network of the plant forming a parallel path with the substation grounding grid and with the ladder type network constituted by tower ground, overhead ground wire and counterprise of the associated transmission complex, taking into consideration the various system parameters detailed in the first chapter.;Under transient analysis, apart from dealing with basic concepts involved in the study of impulse impedance of grounding grids, their equivalent circuit is formulated and analysed for their response to practical impulse inputs, developing expressions for their impulse impedance so as to gauge not only the initial potential rise of a power plant, substation or tower under ground fault conditions, but also to study their impact in controlling insulator flashover potential. Inductance of grounding grids is shown to be the governing factor contributing towards their impulse impedance, and the necessary formulae to evaluate the inductance of a few typical grids are also developed.