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Abstract

Content

Introduction

Fire – one of the most common disasters on our planet. This is uncontrolled burning outside a special hearth, causing material damage. It is characterized by: the formation of open flames and sparks, increased temperature of air, objects, toxic products of combustion and smoke, low oxygen concentration, damage to buildings, structures and installations, the occurrence of explosions. All this relates to dangerous and harmful factors affecting people. [1].

Ensuring fire safety of industrial facilities, including coal mines, in accordance with the legislation of Ukraine on labor protection, is an integral part of state activities. Reduced coal mining does not, according to statistics, reduce the number of exogenous fires. In Ukraine, between 50 and 80 underground fires of exogenous origin occur annually, of which 36.7% of the short-circuit currents in cable networks that cause economic losses to coal enterprises.

In 6 kV networks, the most dangerous from the point of view of fire initiation are arc single-phase short circuits to earth (EPZ) in cables. The fire hazard of cables is characterized by the following properties of their insulation:

  • the ability to ignite and burn when there is a source of heat supplied from the site of damage (combustibility of the cable);
  • the ability to maintain self-combustion after the cessation of the external source of heat (the ability to propagate combustion).

The burning of the outer armor of the cable with arc OZZ are a serious danger from the point of view of ignition of objects located near the cable, under the influence of sparks, which are hot metal particles and burning insulation [2].

Objective

Theoretical synthesis and development of a method for assessing the fire safety of an underground 6 kV power supply system of a coal mine to prevent ignition of cable networks in mines.

Work idea

Imagine the probability of cable ignition from single-phase earth fault currents as a random process of coincidence in space and time of a number of random factors (frequency of occurrence of arc hazardous protective equipment, presence of combustible material in the cable laying area, reliability of protective equipment and timing of their prevention).

Research objectives

  • to carry out a statistical analysis of the time intervals between adjacent fires in the mine cable networks from arc hazardous zone, on the basis of which to substantiate the possibility of applying the theory of Markov processes to assess the causes of cable ignition;
  • to establish the dependence of the current at the fault site (leakage current) on the resistance value at the fault site at different lengths of the power cable for static and dynamic modes;
  • to determine the dependence of the probability of fire on the frequency of the arc OZZ accidentally appearing in a 6 kV cable, the presence of combustible material in the cable laying place (explosive concentration of the metal-arid environment, the presence of combustible materials in the place of occurrence of the OZZZ), the reliability of current leakage protection to the ground and the timing of its prevention.

Object of study – separate power supply system of 6 kV coal mine section.

Subject of research – factors (arc OZZ, the presence of combustible material in the place of the cable, equipment protection, from the escape of current to the ground and the timing of their prevention), which affect the cause of ignition of mine cable networks during their operation.

Research methods. To achieve this goal, we used analytical methods and methods that are based on experimental studies, which are based on the basic concepts of probability theory and reliability.

The study of the dependence of the current at the site of damage on the magnitude of the resistance and different lengths of the power cable

Task setting – by mathematical modeling of a 6 kV network section with an insulated neutral to establish the dependence of the current together the damage at different lengths of the power cable. Research to perform for static and dynamic modes.

The equivalent scheme of the study area is shown in Figure 1 The diagram indicates: R, С – insulation resistance and capacity of the cable in relation to the ground; Iп, Rп – current and resistance in places of damage.

Assumptions:

The parts of the network, separated from the study area by transformers, do not affect the leakage current under study. Resistance at the site of damage is purely active. The capacity of the phases of the network under study is proportional to the length of the cable (the capacity of connections is not taken into account).

Fig 1 – Scheme of the studied network area

Some features of the results:

In steady state, the leakage current is proportional to the length of the cable (or the capacitance of the circuit under study). In the range of resistance values ​​in the place of damage 0 < Rp < 500 Ohm, the leakage current varies slightly (the network with respect to the resistance of the damage behaves as a current source). As the resistance Rp decreases, a sharp increase in the pulse (dynamic) component of the leakage current is observed, due to the transient initial discharge and redistribution of the charge of the cable capacitances. Of interest is the transient at Rp < 100 Ohm. As Rp decreases, the dynamic component of the leakage current can reach tens and hundreds of amperes. However, the duration of this pulse is microseconds. The danger of this component is better to estimate the amount of energy consumed in the resistance Rп. The analysis showed that when Rp varies within: 20 < Rp < 200 Ohm, the time constant of the transition process changes accordingly within: 22.5 < tau < 225 μs. And the pulse energy, respectively: 13.47 > W > 13.47 J. Thus, for the indicated limit of change in Rp, the pulse energy is almost constant [3].

References

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