| The purpose of this dissertation is to evaluate different techniques in the field measurement of ground impedances of electric power substations, with particular reference to those cases where this ground impedance magnitude is below 1.0 (OMEGA), and to analyze problems associated with these measurements, primarily the mutual coupling effects between auxiliary current and potential leads.;The investigation begins with a review of the two types of ground testing used: high current injection and low current injection. Following this, extensive experimental data is presented, collected from a series of grounding tests performed at two substations of Hydro-Quebec's power network. In particular, it is concluded that low current injection methods with magnitudes as low as 100 mA, can give results comparable to a more expensive and less safe high current method.;Also, as a result of numerous field measurements with low current methods, the undesirable effect of mutual coupling between auxiliary test leads was experimentally verified and favorably compared to curves determined analytically. It is also shown how accurate results can be obtained, no matter the value of the angle between the auxiliary leads, provided the mutual effects are minimized by the use of proper analytical tools. Subsequently, a technique is presented by which the value of the ground impedance is determined, using interpolating and smoothing techniques applied to a curve of ground impedance magnitudes at different angles between the auxiliary leads.;The final part of this dissertation presents an extension to the work previously done in calculating the mutual impedance of grounded conductors of finite length lying on the surface of the ground. Equations are developed taking into account the fact that one of the conductors may be at any height above the earth's surface. This has particular applications in urban and suburban areas, where it is not always possible to lay cables on the ground. A sensitivity analysis is presented to show the influence of the different parameters involved. |
