Ñîntent
Introduction
Application of electrical power in the mines is exposed to a range of hazards(electrical shock to personnel, fires, methane explosions caused by open arcs, sparks, and fault currents); thus, development of different measures and safety provision tools is of crucial importance.
Development of coal industry promotes increase of power availability for mining equipment and length and branching of electrical grids. The complication of mining power supply systems and electrical facilities requires clarification and improvement of scientific basics and design of new protective switchoff appliances.
An effective instrument for safety provision by operation of mining power supply facilities is application of fault current protection that performs protective switching off of the power supply at emergency events connected with appearance of ground leakage.
In view of these facts, this work describes known fault current protection apparatus, influence of fault current to human's body, and results of studies in subject available at the time of completion of work over the abstract.
1. Purpose and problems of studies
The purpose of this thesis is improvement of electrical safety of mine process area equipment operation based on the substantiation of the parameters of the automatic device for suppression of reverse electromotive forces of consumer electrical motors after protective switching off the grid.
Issues of the thesis studies include:
1. Performance of analysis of influence of reverse power flows on the level of electrical safety when contact of the human body to cable live part;
2. Proof of mathematical model of electrical complex of the mine under consideration of influence of reverse power flows;
3. Development of functional and circuit diagram of motor emf suppression automation device.
2. Relevance of subject
Conditions of mine department electrical power grid carry increased exposure to electrical shock to personnel; in this. Their application is compulsory for mine section power supply circuits.
However, the application practice shows that after protective switching off, personnel is not fully provided with protection because induction motor reverse power flow exposure is still available. Therefore, study of such processes and development of appropriate protective devises are of relevance.
3. Planned practical results
Development of induction motor reverse emf automatic suppression device, after protective switching off of electrical circuit area.
During material preparation, results of studies performed on the base of modeling of mine grid area and processes taking place therein in emergency cases, will be used.
4.Critical Overview of Known Technical Solutions Covering Subject of the Prototype and Equivalents of the Device under Development
At present, low voltage distribution circuits of mine location transformer substations use various kinds of ground wire monitors: AZPB, AZUR, RU-1140, etc. Besides this, to protect from electrical shock in the networks with 127V voltage, simplified circuit apparatus- UAKI-127, are used. The principle of operation of the latter is based on the circuit of direct operative current Iop in insulation resistance circuit and ground leakage, and generation of command of grid protective switch off if value of this operative currency exceeds the set allowed value (25mA). Both measuring and executive device of UAKI-12 is electro-magnet relay which is connected to operative current series circuit. This prevents the self-control of apparatus circuit operative condition. Compensation of leakage current capacitive components is performed for incomplete range because static compensator with fixed inductivity value is used in the circuit of UAKI-127.
Circuits of AZPB and AZUR devices are free of these shortcomings, which's principle of operation is based on comparison of operative current Iop with standard current Ist (which circulates in the internal circuit of device). If Iop > Ist, command to switch off the executive relay (ER) of protection apparatus is generated, which, in its turn, switches of the RPNN automatic switcher of local transformer substation. Before ER switching off, its switching off might occur, or violation of protection apparatus circuit inconsistency as well. This meet requirement of self-control of operability of ground leakage circuit protection circle.
In the operative currency circuit Iop, ground leakage capacitive current automatic compensator regulator is connected in series. The circuit of protective apparatus constantly measures capacity of circuit insulation and adjusts the inductivity of the latter according to circuit insulation capacity by change of the current in automatic compensator regulator in field winding.
Besides this, an effective solution for the account of increase of electrical power safety is short-circuiting to the ground of circuit phase where dangerous ground leakage occurred. This principle is applied in the circuit of apparatus RU-1140.
Before start of each shift, periodic check of functionality of the protective apparatus shall be performed, by way of circuit phase short-circuiting against additional grounding electrode with check pushbutton, through test resistor.
An additional grounding electrode shall be placed not less than 5 m away from the local one, and shall be connected to protective apparatus circuit through the clip insulated from the body. Such circuitry also allows control of grounding electrode operability.
Apparatus of AZPB and AZUR series can be used independently. This kind of use is provided with mining version of the said apparatus in explosion-proof housing. In this case, the protective apparatus shall effect onto AV group switch that is connected with electrical supply power transformed, with the cable not exceeding 10 m.
5.Short Statement of Own Results Available by Completion of the Work over Abstract
For today, principles of process build-up in mine local electrical network after protective switching off are investigated, and so are peculiarities of influence of reverse power flows on emergency processes in electrical and technical complex package of the mine.
Use of the circuit presented at fig.1 is reasonable for research of the process in mine local electrical network after its protective switching off. This circuit provides registration of parameters of induction motors (M1 – Mn) of such consumers as active and inductive stator impedances (Rñ;Xñ); reduced active and inductive rotor impedances (R’r; X’r ); excitation circuit inductive impedances (Xî); and rotation EMF (åâð), induced at stator of each inductive motor by the rotor circuit rotating field.
Figure 1 –Diagram of substitution of electrical and technical section of the mine for research of ground leakage processes and short circuit after protective switching off
For ground leakage occurrence, leakage impedance switch (Róò =1êÎì) is provided with Ê2 contact and registration of active and capacitive impedances of cable insulations (Rèç;Xèç) as well.
Therefore, contactor in switched on state allows influence on point of emergency from the side of series of consumers' inductive motors.
Special features of application of actuators is that circuits thereof, upon receipt of supply from inductive motor rotation EMF (åâð), hold ÊÌ contactors switched on for certain time after circuitry supply voltage switch off. Let assume that switch off voltage of each contactor is within 0,4-0,6 of rated voltage of the circuitry, and switch off of contactors is performed stochastically.
where ð – number of inductive motor magnet pole pars; is and ir – stator and rotor circuits; - inductivity of main magnet field against inductive motor phase, and À – «À» phase index.
If leakage occurs, EMF reduces exponentially with constant Tp of rotor current attenuation:
where Lm – inductive motor main flow inductivity; Lp, w0 and s – full inductivity, synchronous frequency of rotor revolution and slip; Io and rp – idle running current and inductive motor rotor active impedance; Uô – stator phase voltage at idle running, and Ên = 1 - 1,1 – inductive motor saturation ratio.
For non-allowable condition of electrical and technical complex package of mine section caused by presence of ground leakage low impedance, the circuitry (Fig.1) shall be filled with components that imitate active and inductive impedances of cable operative wires (Rk;Xk), àctive impedances and capacities of cable insulations against ground (Rèç;Ñèç), and active impedances of ground leakages (Róò) at joint action between contactors, for example ÊÌn , and respective motor Ìn (Fig.2).
Figure 2 – Circuit of substitution of the section of i-cable by ground leakage through human body: à – total; á – rated
(Ànivation: volume - 52.43 KB; size -976x463; number of shots - 17; delay between shots - 5000ms; delay between the last and first shots - 5000ms; namber of repetition cycles - infinity)
In view of the fact that free motor rundown operating at leakage impedance (Róò) will be more lasting as by the operation at point ê.ç., intervals of contactor switch on states will also be increased. At that, assumptions about stochastic character of switching off of ÊÌ1 – ÊÌn contactors at reduction of back inductive motor EMF (M1 – Mn) up to the level 0,35 – 0,4 îf nominal circuitry voltage.
Demonstrated structures of circuitries of substitutions and assumptions may be used by development of computer model of electrical and technical complex package of mine section after protective switching off of power supply caused by ground leakage in cable connection of i- consumer.
In article 1 from reference list is shown development of the present theory related to research of application of pre- discharged capacity as a mean of inductive motor reverse power flow suppression.
Conclusion
At present, development of coal mining industry led to application of higher voltages in power grids. More complex packages of electrical equipment emerge that complicates and extends tasks of service personnel. In specifically heavy conditions of mining, electrical shock of the people is possible. In paper, ways of contact of the human with live parts are shown, as well as influence of ground leakage to human body. Overview of the most widely used ground leakage protective apparatus is also presented.
Research of ground leakage in electrical and technical complex package of mine section are performed, revolution EMF character is studied, induced at stator of each inductive motor with rotor circuit rotating field. This resulted in mathematical model of ground leakage process in electrical and technical complex package of mine section.
The master degree paper is incomplete at the time of this abstract issue. Date of paper final completion: December 1st , 2009. The full text of paper and materials on the subject can be obtained from author or his scientific adviser after specified date.
Reference List
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