Abstract

Urgency of the problem: At exploitation of local mine net by strain of 1140 Volt with an isolated neutral, the special danger represents double short circuits of two various phases on ground contour at its various points. Thus the electric equipment of the site which takes place between two points of short circuit on ground contour appears under dangerous potential. The casual touch of the person to the case of an electric equipment results by his defeat by electric current without dependence from a condition local grounding conductor. Besides at presence of the weakened contact in ground net at it, at course of a current, it is possible arcing or arc-wise, dangerous concerning explosion and a fire. Are known cases when at various sites of mine there was a short circuit of a phase on ground net, and the weakened contact was at deadlock development with dangerous concentration methane-air mixture (the system of gas protection did not work). Concurrence in space and time of such events led to explosion at deadlock outturn and to destruction of people. At one of mines of Donbass at double short circuit in various points and phases on ground contour the current of the last one became the reason of a fire in outturn owing to warming of a coal dust to temperature of ignition. The cases described above are possible, if independently from each other there are short circuits on ground contour in various points and phases of net, and protective switching-off is in the given up condition (set point is coarsened, protection is switched - off, refusal of a drive of system of switching-off). Thus maximal current protection can appear not sensitive to the emergency currents proceeding between two points of short circuit on ground contour. It is known, that for such double short circuit on ground contour in various points and phases of a net precedes occurrence of single-phase outflow on the ground at the one of phases. It is determined, that exactly during existence of not disconnected single-phase short circuit, as a rule, and occurs breakdown in a place of the weakened isolation of one of the phases which are taking place under the raised strain.

Proceeding from above-stated, it is necessary to note, that the similar emergency is inadmissible, and, for its prevention it is necessary to exclude preconditions of its occurrence, it means to provide reliable switching-off of single-phase short circuits by system of protective switching-off (the device of protective switching-off (DSPO), the automatic switch).

For this purpose it is necessary to develop and prove the corresponding technical decision.

The purpose of research:
      а) To estimate values of emergency currents of a net and groundcontour at conditions when points of breakdown are carried territorially;
       b) To estimate potentials of the mechanisms which have appeared energized at double short circuits on the ground at carried points of a net;
      c) A substantiation of technical decisions on increase of reliability of system of the protective switching-off, used in local mine nets;
      d) To prove technical recommendations on increase of reliability of system of switching-off of the automatic switch of substation at inadmissible warming windings of the power transformer.

For achievement of lay down objective it is necessary to solve the following problems:
       - to develop mathematical model of mine local net with account ground contour on the basis of a method of central potentials;
       - to prove technical decisions on increase of reliability of system of the protective switching-off, used in local mine nets.

One of the widespread methods of research of complicated linear circuits is the method of central potentials (MCP) which is easily formalized and is convenient for modelling on the COMPUTER. However, methods does not take into account reaction of rotating machines to components of currents of return sequence and reaction of cables to currents of zero sequence that can cause doubt in reliability of the received results.

The method of symmetric components (MSC) is deprived of this lack, though it is labour-consuming, bulky and difficultly formalizable because of necessity of a lot of the "manual" transformations, dependent on from configuration both the basic circuit, and a net of grounding, that raises probability of mistakes of modelling. For carrying out the research, is used the method of symmetric components (MSC). It consists of decomposition of entrance asymmetrical system by pressure for three symmetric systems: direct, return and zero sequences.

Masters
DonNTU