Download Mobile Electric-Discharge Blasting Unit for Splitting off and

Mobile Electric-Discharge Blasting Unit for Splitting off and Destruction
of Rocks and Concrete
VOITENKO Nikita1,a, YUDIN Artem1,b
1
National Research Tomsk Polytechnic University, 30 Lenina av., Tomsk, 634050, Russia
a
[email protected], [email protected]
Keywords: high-current pulse generator, electric-discharge blasting, electro-explosive cartridge,
plasma channel.
Abstract. The article describes the pulse power system for electric-discharge blasting of rocks and
concrete, the main equipment of which is a high current pulse generator with operating voltage up
to 15 kV. The test of short-circuit experiment was produced, the main parameters of the discharge
circuit L = 1.01 µH, R = 9 m were calculated. Based on the results of the experiments the
impedance Z = 42.5 m of discharge circuit was defined. The pulse power system with mentioned
parameters has maximum power condition deviation of about 8.6 %.
Introduction
High voltage electric-discharge blasting unit is designed to conduct high voltage and high current
investigations. Electric-discharge blasting unit based on high current pulse generator (HCPG) with
operating voltage up to 15 kV and maximum current 300 kA. Maximum stored energy in the battery
is 63 kJ. The industrial applications of designed pulsed power system include rock fragmentation,
utilization of strong concrete, dismantling of monolithic structures, construction or expansion of
tunnels and so on. The numerous studies have shown conclusive advantages of electric-discharge
technology, such as absence of toxic substances, destructive acoustic and seismic shock waves, an
ability to adjust transferred energy to destroyed object.
The amount of stored energy is one of the main parameters of pulsed power system, which
determines the maximum range of application of electric-discharge equipment. Thus, increasing this
parameter is an important objective. The energy of capacitive storage is proportional to square of
the voltage, so from one side increasing of operating voltage is the easiest way to increase the
volume of stored energy. On the other side, providing reliable insulation of conductor lines is
difficult when operating voltage is high. The development of more effective destruction of solid
non-conductive materials by electric-discharge with a relatively low (5-15 kV) operating voltage is
an actual problem for successfully application of this technology in industry [1-5].
Equipment’s descriptions
Mobile electric-discharge blasting unit includes a power supply, a capacitive energy storage, a
high voltage switch, control panel and measuring equipment. All parts of the equipment were
mounted on rigid metal frame and placed in a container. The internal dimensions of the container
are 275×175×180 cm. An external view of the equipment is presented in Figure 1, technical
characteristics are listed in Table 1.
Table 1 – technical characteristics of electric-discharge blasting unit
Stored energy, kJ
Charge voltage , kV
Supply voltage, V/50 Hz
External dimensions of container , cm
Weight , kg
up to 63
up to 15
380
300x200x270
3000
Fig.1 External view of electric-discharge blasting unit: a) high current pulse generator:
1–capacitor bank, 2–power supply, 3–control panel, 4–high voltage switch, 5–measuring
equipment .b) capacitor bank
Design and fabrication of the equipment
Capacitive energy storage is based on capacitors K75-01-15kV-14 μF designed by “ELCOD
CSC”. At the step of the design the modular arrangement of the capacitors was chosen. Each
module includes 4 capacitors protected by a fuse. The total quantity of modules is 10. The total
bank capacitance is 560 μF. Modular arrangement of the capacitors allows varying the capacitance
by a number of the connected modules. The bus bars were made from copper of 3 mm thickness
and 60 mm width. An assembled capacitor bank is shown in Figure 1b. The power supply was
designed to get output power up to 3kVA. It consists of industrial frequency step-up transformer
with single-stage voltage doubling circuit and protective inductor in output. All elements of the
circuit are arranged in a sealed, oil-filled metal box. Such implementation of the power supply
provides an easy operation and a reliable performance of the equipment. For successful switching of
current pulses when HCPG discharges, high voltage switch should be able to transmit high peak
current with high di/dt. Furthermore, the switch should ensure safe switching control under the
electromagnetic noise. A high-pressure trigatron type switch was designed and fabricated to satisfy
those conditions. The switch has flat graphite electrodes placed in the atmosphere of mixture of
argon with krypton, and it is capable to switch pulses with current amplitude up to 500 kA.
Safe operation of pulsed power system is provided by automatic grounding device with dumping
resistor designed to remove residual voltage after main current commutation or discharge the
capacitor bank in case of emergency. For registration operation modes of HCPG measuring system
is used. A current pulse is provided by Rogowski CWT-1500 coil. The voltage measurements are
provided by compensated resistive voltage divider DNV-25.
The stored energy is transferred to load by coaxial cable RK 50-17-17. To reduce the equivalent
inductance and equivalent resistances of discharge circuit the parallel connection of eight cables
was used. All cables are separated in two bundles with 4 cables in each one. It allows using them
with two separate electrode systems. The length of cables is 12 meters.
The electro-explosive cartridges (EC) were used in experiments of electric-discharge blasting of
rocks and concrete. EC is a coaxial system, in axis of which due to the explosion of thin copper
wire in the moment commutation of capacitor bank an electrical discharge is developed. The length
of the interelectrode spacing and the size of the EC respectively can vary from 50-100 mm. EC was
attached to a special electrode which was fabricated from the piece cable RK50-17-17 (Fig.2a). The
electrode was connected with terminals of cable, such assembly was called electrode system [5]. As
the result of electric-discharge blasting the electrode undergoes high mechanical and thermal
stresses, it leads fracture of electrode and complicates it reusing. Therefore the prototype of metal
electrode with higher life time was designed and fabricated. The role of the ground conductor plays
thick-walled brass tube. To prevent destruction of the central conductor a polycarbonate insulator is
mounted. (Fig. 2 b, c) [7].
a)
b)
c)
Fig.2 Electro-explosive cartridges (EC) and electrode system: a) electrode fabricated from cable
RK50-17-17, b) electrode system in assembly, c) EC and prototype of metal electrode with higher
life time
Determination of discharge circuit parameters
The amplitude of pulse current is the most important parameter of electric-discharge, its value
directly depends on the resistance and inductance of discharge circuit. The parameters of inductance
and resistance were determined from the short-circuit experiment by means of decrement of
damping current curve. The experiment was conducted for three cases, on equivalent schematic
diagram (Fig. 3). The nodes were marked by the numbers, where short-circuiting was realized
successively. The results of the experiments are listed in table 2.
Fig.3Equivalent schematic diagram of the discharge circuit
Table 2 Parameters of the discharge circuit at the short-circuit experiments
Experiment
T, µsec
L, µH
R, m
1
Without coaxial cable
124
0.65
3
2
With connected coaxial cable
150
1.01
9
3
With coaxial cable and electrode system
177
1.349
12.9
The information obtained from short-circuit experiments allowed to analyze the parameters of
the discharge circuit. Calculate inductance and resistance of each elements. Rel.sys. and Lel.sys. of the
electrode system can vary and it depends on the configuration and length of electrode. The
calculations of the parameters were based on data obtained from experiment №2. The average
values of Rsw and Lsw of high voltage switch were specified by the manufacturer. The resistance of
plasma channel during the electric-discharge is nonlinear. An analytical calculation of its value has
a high deviation from the real values, but the average value can be estimated from the results of
experiments. The average value of Rload and Lload was calculated from the experiments of the
electric-discharge concrete blasting. The charging voltage was 12 kV and the capacitance of HCPG
was 560 µF. The initiation of the discharge was performed by explosion of a conductor with 90 mm
length and 0.2 mm diameter [7].
Table 3.Description of circuit parameters
Parameter
Rg
Lg
Rsw.
Lsw.
Rcab.
Lcab.
Rel.sys.
Lel.sys.
Rload
Lload
Description
Equivalent resistance of capacitor bank and conductor lines, m
Equivalent inductance of capacitor bank and conductor lines, µH
Resistance of high voltage switch, m
Inductance of high voltage switch, µH
Resistance of supply cable, m
Inductance of supply cable, µH
Resistance of electrode system, m
Inductance of electrode system, µH
Average Resistance of load, m
Average inductance of load, µH
Value
3
0.65
0.05
0.020
6
0.36
3.9
0.34
29.8
0.037
The distribution of energy in the discharge circuit basing on the experiments was analyzed. In
Fig.4 the chart of energy allocation during discharge of capacitor bank is presented.
Fig. 4- Distribution of energy in the discharge circuit
Results and discussion
Calculation of discharge circuit parameters allows the mode of energy release in the plasma
channel to estimate. The diagram in Fig. 4 shows that, in fact about 65-67% of stored energy
releases in discharge channel, and 33-35% dissipates as resistive losses in capacitor bank, switch,
coaxial cables and bus bars. According to [8], maximum power released in resistance of a circuit
develops at condition Rc = 1,1∙Z, where Rс is resistance of the circuit, calculated from experiment
of electric-discharge blasting of concrete Rc = 42.75 m, Z is impedance of the circuit, calculated
from short-circuit experiment №2 Z  L/C  42.5mΩ . In fabricated pulsed power system ratio is
Rc / Z = 1.006, thus, maximum power condition deviation in discharge circuit is 8.6%.
Conclusion
The mobile electric-discharge blasting unit was designed, fabricated, and successfully tested.
Modular arrangement of the capacitors was applied in the design allowing to vary capacitance by
changing the number of connected modules. Such implementation of the power supply provides an
easy operation and reliable performance of the equipment. The parameters of inductance and
resistance were determined from the short-circuit experiment by means of decrement of damping
current curve. Based on the results of the experiments the distribution of energy was analyzed and
unproductive energy losses were determined. The electric-discharge blasting unit can be applied to
mining and building industries for demolition and splitting off monolith of large natural or artificial
solid blocks, for dismantling and repairing the buildings, construction or expansion of tunnels,
removing the debris and other pulse power applications.
Acknowledgement
This work is supported by Federal target program, project No 14.575.21.0059.
References
1 H. Hamelin, F. Kitzinger et.al. Hard rock fragmentation with pulsed power, Digest of 9th IEEE
International Pulsed Power Conference, Albuquerque, New Mexico, USA,(1993) 11-14.
2 H. Inoue, I. V. Lisitsyn, H. Akiyama, and I. Nishizawa, Drilling of hard rocks by pulsed power,
IEEE Electr. Insul. Mag., 3 (2000) 19-25.
3 Y. SikJin, Y. Kim, J. Kim, Fabrication and testing of a 600-kJ pulsed power system, IEEE
Transactions on plasma science, 10 (2013).
4 C. Silva, A. Stellin, W. Hennies , E. Costa, Electrohydraulic Rock Blasting: An Alternative for
Mining in Urban Areas, International, Journal of Surface Mining, Reclamation and Environment,
(2002) 261-269.
5 H. Bluhm, W. Frey, H. Giese et. al., Application of Pulsed HV Discharges to Material
Fragmentation and Recycling, IEEE Transactions on Dielectric and Electrical Insulation. 5
(2000) 625-636.
6 N. Kuznetsova, V. Lopatin, V. Burkin et.al., Theoretical and experimental investigation of
electro discharge destruction of non-conducting materials, IEEE International Pulsed Power
Conference: Digest of Technical Papers, (2011) 267-271.
7 A. Yudin, N. Kuznetsova, V. Lopatin and N. Voitenko, Multi-borehole electro-blast method for
concrete monolith splitting off, Journal of Physics: Conference Series, 552(2014) 1-4.
8 V.A Avrutsky et al. Testing and electro physical equipment’s. Experimental Techniques –M.:
MEI, 1983.