Impulse impedance of grounding systems and its effects on tower crossarm voltage.

摘要

Grounding system is an important part of the power systems that protects power lines from severe ground faults and lightning currents. It is, therefore, necessary to design a grounding system with a low-magnitude ground impedance so that large current can be diverted effectively to the ground without raising insulator-string voltage in power lines. This research addressed the modeling of grounding systems under lightning impulse. Impulse impedance of grounding systems was studied for a single ground rod and counterpoise.; A generalized equation for soil critical electric field was proposed from extensive laboratory tests on various soils considering significant soil parameters, which is desirable for dynamic modeling of grounding systems. Dielectric constant k of the soil was measured by the wave propagation technique using a soil-filled coaxial transmission line, which avoids the problem due to polarization effects.; Chowdhuri's dynamic model of ground rod was checked critically by extensive laboratory and field tests to estimate ground-rod impulse impedance. Instead of assuming one single soil critical electric field value, this model considered the generalized soil critical electric field equation, which is applicable for any soil condition. Estimated impulse impedance by this model was validated with experimental results.; A model with uniformly distributed counterpoise parameters such as inductance L, capacitance C, leakance G, internal impedance Zc and ground-return impedance Zg was proposed by Telegrapher equation in s-domain. Impulse characteristics of an infinitely long counterpoise were studied by this model. Finite Difference Time Domain (FDTD) method was incorporated to compute impulse voltage, impulse current, and impulse impedance of a finite length counterpoise with and without soil ionization effects, considering counterpoise far-end reflection. Field test on counterpoise was performed to validate the proposed model. Furthermore, tower crossarm voltage was analyzed separately for a single ground rod and counterpoise with soil ionization effects under different current magnitudes, waveshapes, and soil parameters. Tower crossarm voltage was also analyzed for various counterpoise lengths to recommend an effective counterpoise length to minimize tower crossarm voltage.