INVESTIGATION OF THE BREAKDOWN OF MOISTURIZED ROCKS UNDER PULSED VOLTAGE EXPOSURE

V. S. Malakhov, I. I. Kalyatskii, V. V. Krivko

Strengthening the country’s economic base requires the introduction, in all branches of the national economy, of new and more progressive technological processes, an increase in labor productivity, and a reduction in production costs. Under present-day conditions, with the large volume of mining and geological work, alongside the improvement of existing methods for the destruction of rocks, new and more effective methods are being developed and introduced that use new forms of energy—electrical, thermal, electromagnetic, and others [1, 2]. The use of electrical energy without intermediate conversion into the mechanical energy of destruction is the most promising. The creation of fundamentally new installations for the destruction of rocks and the development of technological regimes require extensive knowledge of the electrophysical characteristics of rocks (specific resistivity, dielectric permittivity, electric strength, etc.). Along with a fairly large amount of data on the specific electrical resistivity and dielectric permittivity of rocks [3], information on pulsed electric strength is very limited [4], and data on the influence of various factors on the pulsed electric strength of rocks (moisture content, porosity, mineralogical composition, structure, etc.) are entirely absent.

In the present work, the results are presented of experimental investigations of the pulsed electrical breakdown of several types of dry and moistened rocks over a wide range of voltage-exposure times in a sharply nonuniform field. Data are given on the investigation of the dependence of the breakdown voltage of rocks on their impregnation with an insulating liquid. The data on the volt-second characteristics of pulsed breakdown of rocks, in addition to their use in designing installations for rock destruction, may be of interest for the calculation and design of grounding electrodes in rocky soils [5].

Experimental Method

In the investigations, the source of single high-voltage pulses was a multistage impulse-voltage generator (GIN) rated at 480 kV, with a discharge capacitance of 6250 pF.

For recording pulsed voltages, an OK-19M electronic oscilloscope with a capacitive voltage divider was used. Joint calibration of the oscilloscope and divider with respect to voltage amplitude was carried out using sphere gaps on a standard 1.5/40 μsec wave for pulses of positive polarity. In the investigations, specimens of medium-grained sandstone, plagiogranite, aleuropelite, fine-grained marble, and coarse-grained marble were used. Plane-parallel rock specimens with dimensions \(50 \times 50 \times 10\) mm were cut on a SASP-1 semiautomatic stone-cutting machine. The moisture content in the rock specimens was estimated from the difference in the weights of specimens dried in a thermostat at \(t = 105^\circ\mathrm{C}\) and moistened specimens (the specimens were kept in ordinary technical water with a resistivity \(\rho = 0.15 \cdot 10^{-3}\ \Omega\cdot\mathrm{cm}\) for 10 days).

To eliminate the edge effect [6] during pulsed breakdown, a conical recess with a cone angle of \(45^\circ\) was drilled on one of the surfaces of the rock specimen; the other surface of the specimen was ground.

In order to preclude surface flashover of specimens of the selected dimensions, the experiment was carried out in transformer oil with a strength of 35 kV in a standard breakdown vessel. The thickness of the specimens at the breakdown site was taken to be 10 mm and was measured with an IZV1 instrument to an accuracy of 0.01 mm.

Investigations of the influence of impregnating rocks with transformer oil on their breakdown voltage were carried out on specimens of sandstone and coarse-grained marble. The volt-second characteristics of the above-mentioned rocks were taken on specimens coated with a film of paraffin. This prevented penetration of transformer oil into the pores of the rock and did not affect the breakdown voltage of the rocks. Specimens without paraffin coating were kept in transformer oil for 24 hours before breakdown.

The electrodes were brass plates 120 mm in diameter and a rod 8 mm in diameter with a molybdenum tip pressed onto its end, with an apex angle of \(45^\circ\). Single pulses of positive polarity were applied to the rod; the plane was grounded.

Breakdown was carried out on the front of a single pulse. The breakdown probability was taken as \(\Psi = 90\%\). Each point of the volt-second characteristics was obtained on the basis of processing 20 or more breakdown oscillograms. The maximum deviation of the breakdown voltages from the mean value did not exceed 20%.

Results of the Experiments and Their Discussion

Figure 1 presents the volt-second characteristics of wet and dry marble, wet and dry plagiogranite, wet aleuropelite, and wet sandstone during their breakdown in a sharply nonuniform field with an electrode spacing of 10 mm. The data of Fig. 1 show that, as the duration of voltage action is reduced from \(2 \cdot 10^{-6}\) to \(2 \cdot 10^{-7}\) sec, the breakdown voltage of all the rocks studied (regardless of moisture content) increases on average by 60%. For example, for plagiogranite at \(t_p = 0.2\ \mu\mathrm{sec}\), \(U_{\mathrm{br}} = 217\ \mathrm{kV}\), whereas at \(t_p = 2\ \mu\mathrm{sec}\), \(U_{\mathrm{br}} = 134\ \mathrm{kV}\). The increase in breakdown voltage with decreasing duration of the applied voltage is associated with the phenomenon of discharge delay, by analogy with pulsed breakdown of liquid and solid dielectrics [7].

Fig. 1. Volt-second breakdown characteristics of rocks in a nonuniform field with a positive point. 1—marble, moisture content 0.8%; 2—aleurolite, moisture content 0.5%; 3—plagiogranite, moisture content 0.5%; 4—dry marble; 5—dry plagiogranite; 6—sandstone, moisture content 1.5%.

Fig. 2. Volt-second breakdown characteristics of rocks in a nonuniform field with a positive point. 1—marble and sandstone impregnated with transformer oil; 2—sandstone coated with paraffin; 3—marble coated with paraffin.

The breakdown voltage of moistened rocks over the entire investigated time interval of the applied voltage exceeds the breakdown voltage of dry rocks by about 25%.

It is known [8] that the impulse breakdown voltage of technical-grade water in a nonuniform field, with distances between electrodes of 10 mm and voltage application times from \(3 \cdot 10^{-7}\) to \(1 \cdot 10^{-5}\) s, is 20–70 kV, while the impulse strength of air under the same conditions is 25–30 kV. Filling the air pores of specimens with water therefore leads to an increase in the impulse breakdown voltage of moistened rocks. It should be noted that for rocks containing readily soluble salts, moistening may lead to a reduction in their breakdown voltage, since the electric strength of salt, acid, and alkali solutions is considerably lower than that of technical-grade water [9].

Figure 2 gives the volt-second characteristics of rocks, dry and impregnated with transformer oil. It follows from Fig. 2 that the difference in breakdown voltages between dry rocks and rocks impregnated with transformer oil, over the entire investigated time range from \(1.2 \cdot 10^{-7}\) to \(2 \cdot 10^{-6}\) s, reaches about 25–35%. For example, for dry marble at \(t_p = 0.3\ \mu\text{s}\), \(U_{\text{br.}} = 68\ \text{kV}\), whereas for marble impregnated with transformer oil at \(t_p = 0.3\ \mu\text{s}\), \(U_{\text{br.}} = 132\ \text{kV}\). The increase in breakdown voltage of impregnated marble and sandstone specimens is due to the fact that transformer oil has a higher breakdown voltage under analogous experimental conditions [8]. In these experiments (Fig. 2), the difference in breakdown voltages of impregnated and non-impregnated specimens for sandstone is somewhat smaller than for marble. This is associated with the filling of the open pores of a certain surface layer of the sandstone specimens with paraffin (owing to its greater porosity) during specimen preparation.

Conclusions

The results obtained from experimental investigations of the breakdown of dry rocks, moistened rocks, and rocks impregnated with transformer oil, under positive-polarity impulses, make it possible to draw the following conclusions:

1. The excess of the breakdown voltages of moistened rocks over the breakdown voltages of dry rocks in the voltage application time interval \(2 \cdot 10^{-7}\)—\(2 \cdot 10^{-6}\) s is no more than 25%.

2. Impregnation of porous rocks with a liquid of high electric strength (transformer oil) increases their breakdown voltage by 25–35%.

3. In technological schemes for the destruction of rocks using the energy of electrical discharges, for the destruction of moistened rocks or rocks impregnated with a liquid of high electric strength, it is necessary to increase the voltage amplitude by an average of 25%.

Literature

1. E. F. Epshtein, E. I. Arsh, G. K. Vitort. New Methods for the Destruction of Rocks. Gostoptekhizdat, 1960.

2. S. S. Sulakshin. Fundamentals of the Theory of Rock Destruction and Removal of Destruction Products from Boreholes. Tomsk University Press, 1964.

1. V. N. Kobranova. Physical Properties of Rocks. Gostoptekhizdat, 1962.

2. A. I. Limasov, A. T. Chepikov. Breakdown of dielectrics and semiconductors (collection of reports), Energiya Publishing House, 1964.

3. V. V. Krivko, A. I. Limasov, A. T. Chepikov. Investigation of the pulsed electrical strength of rocky soils. (Proceedings of the All-Union Conference on Grounding Devices), Kharkov, 1966.

4. A. A. Vorob’ev, G. A. Vorob’ev, V. D. Kuchin. On a method for determining the breakdown voltage of solid dielectrics in a nonuniform field. Izv. TPI, vol. 91, 1956.

5. G. I. Skanavi. Physics of Dielectrics. Gostekhizdat, 1949.

6. I. I. Kalyatskii, V. V. Krivko. Breakdown of dielectrics and semiconductors (collection of reports), Energiya Publishing House, 1964.

7. B. V. Semkin, A. T. Chepikov, A. M. Mironov. Pulsed breakdown of electrolytes at short voltage exposure times. Izv. TPI, vol. 149, 1966.