• PDF / 309,015 Bytes
• 5 Pages / 612 x 792 pts (letter) Page_size
• 26 Downloads / 270 Views

DOWNLOAD

REPORT

---

IED PHYSICS

Electric Breakdown during the Pulsed Current Spreading in the Sand L. M. Vasilyak, S. P. Vetchinin, V. A. Panov, V. Ya. Pecherkin, and E. E. Son Joint Institute for High Temperatures, Russian Academy of Sciences, Izhorskaya ul. 13-2, Moscow, 125412 Russia e-mail: [email protected] Received August 14, 2014

Abstract—Processes of spreading of the pulsed current from spherical electrodes and an electric breakdown in the quartz sand are studied experimentally. When the current density on the electrode exceeds the critical value, a nonlinear reduction occurs in the grounding resistance as a result of sparking in the soil. The critical electric field strengths for ionization and breakdown are determined. The ionization-overheating instability is shown to develop on the electrode, which leads to the current contraction and formation of plasma channels. DOI: 10.1134/S1063780X16030077

INTRODUCTION Interest in the spreading of high pulsed currents and the emergence of electric breakdown in the soil is now connected with the necessity to modernize the lightning-protection systems and to prevent the equipment of transmission lines from overvoltage [1], as well as with studies of the earth’s crust structure by using powerful current pulses, e.g., from the explosive magnetic generators [2]. During a lightning stroke, a fast-varying multi-kiloampere current emerges, which affects the soil and causes the sparking, as a result of which both soil properties and grounding characteristics may significantly differ from the values obtained during the passage of a stationary current. Until now there has been no understanding of physics of the breakdown and plasma-channel formation processes during the spreading of high pulsed currents in the soil. While estimating parameters of overvoltage in designing the lightning-protection systems, the calculated and experimental data on grounding resistance, obtained with spreading of weak low-frequency currents, are taken as basic values. Additionally, a model of homogeneous distribution of currents flowing in the earth is used, while the sparking is taken into account using the simplest model of increasing an effective size of the grounding [3, 4]. When plasma channels arise in the soil, a value of the current flowing to the protected elements may exceed multifold the values calculated according to the model of homogeneous spreading of the current. Solving this problem is especially urgent now due to the fact that the information and control systems based on microelectronics, having the increased sensi-

tivity to the arising overvoltage, are widely introduced in power engineering. By using a current that imitates a lightning current in one way or other, the laboratory and field tests of grounding devices have been long carried out in different soils [5–9]. With a current growth, an appreciable reduction is the soil resistivity ρ0 and the pulsed impedance of the grounding Z is observed as compared to its stationary value R0 [3, 4, 10] obtained with a weak direct current. When the am