■■> ^ j^^\ "^m^ .'^x /'} ■"^"•-^ ^^ a"^-' »•? ,0^^ <3. ''^.T*' A * y ^^^ \. *0 o^ 'bV" /\ ^^/ /-^ ^^^ /^. ^;^:^ /^ ^ ' y<-^i-\ .^°*]rB.'> y«-^k-\ ^°.i^^'> ,^*'-"--' > ■^^/^••\/.. %--^-/ *^/^-'\/ %--3^-/ \'-.- , O^ » o „ - , ,/^ o H a ,V ' o_ * 3 VI ^^0^ .-^o*. ^0* ^'^.^ <•*• •<^j. 'o « * * <.G °'.-^v"°o ./\c;^.X " IC 8944 Bureau of Mines Information Circular/1983 Computerized, Remote Monitoring Systems for Underground Coal Mines Faults in Power Systems By Jeffrey H. Welsh UNITED STATES DEPARTMENT OF THE INTERIOR Information Circular 8944 Computerized, Remote Monitoring Systems for Underground Coal Mines Faults in Power Systems By Jeffrey H. Welsh UNITED STATES DEPARTMENT OF THE INTERIOR James G. Watt, Secretary BUREAU OF MINES Robert C. Norton, Director Library of Congress Cataloging in Publication Data: Welsh, Jeffrey H Computerizjed, remote monitorinj; systems for underground coal mines. Faults in power systems. (Information circular / United States Department of the Interior, Bu- reau of Mines ; 8944) Bibliography: p. 8, Supt. of Docs, no.: I 28.27:8944. 1. F.lectricity in mining— Safety measures— Data processing. 2. Coal mines, and mining— Safety measures— Data processing. 3. C-CRMMS (Computer system). 4. Ivlectric fault location— Data processing. I. Ti- tle. II. Series: Information circular (United States. Bureau of Mines) ; 8944. JLN295JJ4 ITN3431 622s [622'. 48'0289] 83-600132 CONTENTS Page Abstract 1 ^ Introduction 1 .^ Need for improvement In power system safety 2 K} Types of power system faults 2 Monitoring power system faults 2 Analysis of regulations 4 Systems currently Installed 6 Production benefits 7 Conclusions 7 References 8 Appendix. 9 ILLUSTRATIONS 1 . Mine monitoring system block diagram. 3 2. Underground power system distribution 4 TABLES 1 . Fatalities in underground coal mines 2 2. Functions of a mine monitoring system 5 3. Causes of downtime 7 A-1 . Current mandatory standards 9 UNIT OF MEASURE ABBEIEVIATIONS USED IN THIS REPORT A ampere min minute ft foot pet percent ml mile V volt COMPUTERIZED, REMOTE MONITORING SYSTEMS FOR UNDERGROUND COAL MINES Faults in Power Systems By Jeffrey H. Welsh ^ ABSTRACT The Bureau of Mines studied the use of computerized, continuous, re- mote monitoring systems for safety from power system faults in under- ground coal mines. In this report the need to improve protection against power system faults is documented, and types of faults are identified. The relationship between mine safety regulations and com- puterized, continuous, remote monitoring is analyzed. INTRODUCTION Electrical power systems are important in underground coal mines. Ventilation fans, pumps, compressors, hoists, and battery chargers use electricity. It is also used by coal mining equipment (continuous miners , roof bolters , shuttle cars , cutting machines , longwalls , and loaders). Electricity runs both the conveyors and railroads used in transporting coal out of the mines. Mine electrical systems vary with the method of mining and the type of haulage. Faults in these power systems not only cause many accidents each year but also effect a de- crease in production. Computerized, continuous, remote mine monitoring systems (CCRMMS) can improve safety in underground coal mines and increase production. Equipment is available commercially to monitor power system faults. This equipment includes (1) sensors that detect impending power system faults and (2) microprocessor-based computers that collect and analyze the data from these sensors. 'Operations research analyst, Pittsburgh Research Center, Bureau of Mines, Pitts- burgh, PA. NEED FOR IMPROVEMENT IN POWER SYSTEM SAFETY Electricity injures twice as many pro- duction workers as maintenance workers. Most (4^)2 electrical accidents are caused by arcs, electrically generated heat, or shock. Table 1 (2^, T) shows that electrical accidents are the fourth lead- ing cause of fatalities in underground mines. In 1981 (2^) there were 246 in- juries caused by electric current that resulted in days away from work. MSHA citations {V) show that Mandatory Safety Standards for electrical equipment have twice as many violations as any other category. Fires can be related to electrical faults. Electrical ignitions started 47 pet {b) of the mine fires reported from 1970 to 1980. Explosions also can be re- lated to electrical faults. Electricity was the source of 21 pet (4^) of methane ignitions investigated through 1972. The CCRMMS can improve safety from power sys- tem faults. TABLE 1. - Fatalities in underground coal mines Causes Fall of roof Powered haulage Machinery Electrical Explosives and breaking agents Ignition or explosion of gas or dust Falling, rolling, or sliding material Hoisting Fall of face, rib, side, or highwall. , . . , Handling material Inundation Hand tools Other Total 1979 62 24 10 8 1 3 5 1 1 115 1980 30 25 16 10 7 5 2 2 1 1 1 100 1981 37 20 6 9 3 36 1 4 1 3 1 121 TYPES OF POWER SYSTEM FAULTS (1) A fault in a mine's power system can cause overloads, short circuits, under- voltages, or grounded conductors. Three types of faults can occur: quality, stress, and wear. Quality faults result from Improper design, bad workmanship, damage dur- ing transportation, and errors in installation. voltage. These faults also result from accidents, for example, when a continuous miner runs over a trailing cable or a roof fall damages electrical equipment. Wear on the power system results from its use and exposure to changes in the environment. Frictional wear, insulation breakdown, and corrosion are examples. Maintenance helps reduce wear. Stress faults result from increased stress levels or combinations of stress such as excessive temperature and Computerized monitoring can both pre- dict and quickly detect power system faults. MONITORING POWER SYSTEM FAULTS A coii^)uterized monitoring system con- sists of an aboveground computer that collects, records, and prints routine and alarm data from sensors in the mine. An alarm indicates that an abnormal ^Underlined nxunbers in parentheses re- fer to items in the list of references preceding the appendix. condition exists in the mine, which needs immediate corrective action, such as an equipment failure or a methane accumu- lation. The computer is continuously manned; therefore, underground miners can be quickly alerted to danger. An unin- terruptible power supply operates the computer during power failures. Intrin- sic safety requirements must be met if sensors are to be placed in return air. Figure 1 shows a block diagram of a CCRMMS. Many miles of cable are needed to carry electricity throughout a mine. Because of the complexity of a power system, de- termining the location of a fault may be difficult. Figure 2 shows how the power system network is distributed. Monitor- ing these power system components can increase safety and production. Safety improves because power system faults or potential faults are rapidly detected and alarmed. Production improves because downtime is reduced when faults can be located quickly. Monitoring enables the status of cir- cuit breakers and sequence of circuit breaker trips to be determined. A short- to-ground in one section can trip other power centers. Knowing the sequence of the circuit breaker trip helps locate the origin of the problem. Another benefit results from monitoring the dc trolley-wire system. Both normal and unwanted load currents flow through the ground. An unwanted load can cause a fire even though it may draw less current than a normal load. A remotely monitored discriminating circuit breaker, which has the ability to detect an unwanted load, warns the mine operator of dangers. Again, safety increases. Once installed, the system can also monitor and control the environment, production, energy use, and maintenance. Control signals sent to equipment under- ground are based on monitored inputs or programmed time intervals. The more parameters the CCRMMS monitors , the greater the return in benefits. Central , manned location Above ground Backup power supply Routine data printer Alarm printer [Communications Micro- processor- based computer CRT Underground Belts Pumps Auxiliary fans Switch- gear Circuit breakers Electric face equipment Power center Fan .• J ■<■. \ Telephonel FIGURE 1. - Mine monitoring system block diagram. From utility company Surface loads Borehole shaft or slope [7j[« **0 (Continuous mining section) <\i "> 'J- Ki I „,\ >o K * p> IT) lO If) It, [£/J lO IT) f) If) (Longwoll mining section) (Conventional mining section) LEGEND / Utility company metering 3Z 2 Main substation 53 3 Surface substation 34 I Disconnect switchhouse, mine 35 5 Portable switchhouse, rectifier 36 6 Power center, miscellaneous loads 37 7 Rectifier, trolley system 38 8 Portable switchhouse, continuous mining 39 9 Distribution transformer, belt drive 40 10 Power center, continuous mining 41 II Portable switchhouse, 4Z distribution transformer 43 12 Distribution transformer, individual loads 44 13 Portable switchhouse, longwoll 45 14 Portable switchhouse, conventional 45 15 Power center, conventional mining ^7 16 Rectifier, shuttle cars 4g 17 Power center, longwoll mining 49 18 Distribution box, longwoll 50 19 Distribution box, longwoll 5/ 20 Conveyor belt starter and drive 5^ 21 Master control, longwoll jj 22 Prep plant."?) s4 ^J Fan / 55 24 Hoists V o . u • /,. 56 25 Shop / 2"^*°" substation W ^^ 26 Office I SB 27 Pumps. . . ._^ s9 28 Shop -1 29 Pumps . . . ./ _ „ n i. , . > Power center (W 30 Belt drive. I 31 Bunker . . ) Trolley system Continuous miner." Shuttle car.. Shuttle car. Bolter , _ Feeder.. ) Power center^; Pump... Charger. Spare , . . Pump... . Conveyor..^ Distribution transformer «?; Spare.. Loader. Cutter . . Bolter V \«i„4», „. / Power center (75/ Water pump. ' Car spotter. Spare . Shuttle can Shuttle car j''«<="««^^''^^ Shearer 71 Stage loader. .1 „ _ > Distribution box 18) Conveyor ( Water pump. .J Conveyor 7) Hydraulic pump. ( _. ,, . UvMronii^ n.,™^ > Distribution \>w.09) Hydraulic pump.i Spore .\ FIGURE 2. = Underground power system distribution. ANALYSIS OF REGULATIONS To determine the role of CCRMMS in pro- viding an increased safety level in mines, and to determine the relationship between mine safety regulations and CCRMMS, an analysis of CFR Title 30, Part 75, Electrical Equipment, subparts F, G, H, I, J, and K, was made. It should be noted that it is assumed by this analysis that the CCRMMS is opera- tional, available, and accurate at all times. Functions a CCRMMS can perform to en- hance mine safety over that provided with CFR Title 30, Part 75 are shown in ta- ble 2. Safety is increased since moni- toring is continuous; faults are de- tected, located, and diagnosed rapidly; and alarms are sent to a central, manned location aboveground. Sensors are commercially available to monitor all parameters listed in the Functions colximn of table 2 except: (a) temperature of a cable over its en- tire length (75.513); (b) dielectric strength of the insulation (75.513); and (c) determining fault location on dc trolley wires (75,1001). In these cases, prototypes are available. No lOlC petitions, in which an operator requests to use CCRMMS in lieu of apply- ing current safety requirements , have been filed with MSHA by mine operators for CFR Title 30, Part 75, subparts F, G, H, I, J, or K. I .tMbu-'.MXjar^ TABLE 2. - Functions of a mine monitoring system CFR Part and title^ Function of mine monitoring system Subpart F, Electrical Equipment — General: 75.513 — Electrical conductor; capacity Monitor temperature, voltage, and current and installation. the cable is carrying. Check dielectric strength of the insulation. 75.518 — Electric equipment and circuits; Monitor status of fuses, circuit breakers, overload and short circuit protection, and relaying devices leading to the cir- cuit interrupts. Monitor voltage, cur- rent, ground continuity, ground wire cur- rent, and phase sequence of the cable. 75.519 — Main power circuits; disconnect- Monitor status of switches and power going ing switches. through them. 75.520 — Electrical equipment; switches... Monitor status of switches. 75.524 — Electric face equipment; electric Monitor if protecting device is in place equipment used in return air outby the and if the device is functional, last open crosscut; maximum level of al- ternating or direct electric current be- tween frames of equipment. Subpart G, Trailing Cables: 75.601 — Short circuit protection of Monitor status of circuit breakers, fuses, trailing cables. protective relaying devices. Monitor maximum levels of current the cable is carrying. Subpart H, Grounding: 75.702 — Protection other than grounding.. Monitor if ground is physically present. 75.705 — Work on high-voltage lines; de- Monitor power, voltage, current pass- energizing and grounding. ing through lines. Monitor status of switches. 75.706 — Deenerglzed underground power Monitor status of circuit breakers, circuits; idle days, idle shifts. Subpart I, Underground High-Voltage Distribution: 75.800 — High-voltage circuits; circuit Monitor status of circuit breakers, breakers . 75.801 — Grounding resistors Monitor continuity of grounding resistor. See footnote at end of table. TABLE 2. - Functions of a mine monitoring system — Continued CFR Part and title^ Function of mine monitoring system 75.803 — Fail-safe ground check circuits Monitor if ground is physically present, on high-voltage resistance grounded systems. 75.808 — Disconnecting devices Monitor status of switches (open or closed) . 75.812 — Movement of high-voltage power Monitor status of circuit breakers, centers and portable transformers; permit. Subpart J, Underground Low- and Medium-Voltage Alternating-Current Circuits: 75.900 — Low- and medium-voltage circuits Monitor status of circuit breakers; moni- serving 3-phase alternating-current tor current and voltage levels, equipment; circuit breakers. 75.902 — Low- and medium-voltage ground Monitor status of grounds, check monitor circuits. 75.903 — ^Disconnecting devices Monitor status of disconnecting devices. 75.906 — Trailing cables for mobile equip- Monitor status of grounds, ment , ground wires , and ground check wires. Subpart K, Trolley Wires and Trolley Feeder Wires: 75. 1000 — Cutout switches Monitor status of switches and relay devices. 75. 1001 — Overcurrent protection Monitor current inline and status of re- ^ lays. Determine fault location. 'U.S. Code of Federal Regulations, Title 30. SYSTEMS CURRENTLY INSTALLED Approximately 30 U.S. coal mines have monitoring systems; most are recent in- stallations. Several of these systems monitor power system components. An example is Company A (8^) , which has fans and circuit breakers spread over the countryside, some 5 mi from the mine por- tal. If a fan circuit breaker trips, it may take considerable time for someone to get to the site. The reduced ventila- tion may cause methane to accumulate. This produces an explosion hazard. Also, miners must be withdrawn from the mine if ventilation cannot be restored within 15 min. This causes a loss in production. Company A, therefore, monitors and re- sets fan circuit breakers remotely. Af- ter an interruption, the company resets the circuit breakers; if there is a sec- ond interruption, someone is sent to the site. Remote resetting of the circuit breakers may permit miners and equipment to keep working. I Company A's system also remotely trips circuit breakers and monitors power con- sumption of the face equipment. The ability to remotely trip circuit breakers provides the capability to rapidly de- energize power in the mine during emer- gencies. By analyzing time histories of face equipment power consumption, inefficiencies such as logistical bottle- necks , equipment problems , or sections that need extra help are identified. As the production and safety benefits of CCRMMS become known to the mining in- dustry, power system monitoring will increase. PRODUCTION BENEFITS Monitoring power systems can decrease downtime and, therefore, increase pro- duction. A decrease in downtime results from being able to rapidly locate and diagnose power system faults. A per- son with the right skills, tools, and repair parts can be sent to the fault location. Table 3 shows causes of downtime and amount of time down for 126 time stud- ies (_5) . Downtime costs (during cut- ting time, per section) used are as follows: Continuous miner - $20/min; Longwall - $100/min. It was also estimated (_5) that comput- erized monitoring and control could decrease downtime by 60 pet. This has a significant impact on production. Cost- benefit analyses (8_) have shown that com- puterized monitoring systems can provide substantial economic benefits to the mine operator. Continuous monitoring of power system components may also predict faults. Problems can be corrected on maintenance shifts before the fault occurs. TABLE 3. - Causes of downtime Aver- Downtime cost age down- per section Cause of downtime Cont. time, miner Longwall man AC power at neck of section 20 $400 $2,000 AC power at OCB submains 24 480 2,400 AC power at outside 48.4 968 4,840 Ground fault 91 1,820 9,100 DC rail haulage..,. 23.4 468 2,340 Belt knocks power.. 20 400 2,000 Fan down 0) C) (2) is down be with- ^Loss of shift. If the fan more than 15 min, miners must drawn from the mine. 2 Varies depending on amount of time left on shift when fan goes down and on face equipment efficiency. CONCLUSIONS The CCRMMS can monitor many power sys- tem parameters. This will improve safety and production. Safety will improve because: (a) Power system status is monitored continuously. (b) The risk of accidents from un- knowns is all but eliminated. Faults in the power system are detected, located. and diagnosed rapidly. Downtime on equipment critical to mine safety is reduced. (c) Alarms are sent to a central sta- tion aboveground. Trained personnel can quickly notify miners of the danger and arrange for immediate correction of the situation, (d) With remote status of circuit breakers , it can be determined from the central station aboveground where power is energized or deenergized in a mine. This is useful on idle days to insure the proper equipment is on or off, (e) Parameters not required to be monitored, according to the CFR, can be monitored to provide increased safety. Production will improve because: (a) Downtime is reduced, (b) Inefficiencies are eliminated. Although it has only been discussed briefly in this report, remote control of power system components, such as circuit breakers , is possible and may be benefi- cial to the mine. The current state-of-the-art in sensor development is such that sensors are com- mercially available or the technology is available to monitor many parameters (ta- ble 2) that can increase mdne safety from power systems faults. Possible limitations of CCEIMMS are — (a) Large mines may require an exten- sive layout of sensors and cable. (b) Monitoring will not replace the need for visual, manual exams of the pow- er system, because a person may be able to detect potential hazards the CCRMMS cannot, such as deteriorating cable insulation. (c) Sensors installed at the face would have to be advanced with mining. Monitoring electric face equipment could be done at the load center, and cabling for the CCRMMS would advance with the load center. (d) Monitoring certain power system parameters may require an innovative en- gineering effort to develop the necessary sensors. REFERENCES 1. Anderson, R. T. Reliability Design Handbook (Rome Air Development Center contract). IIT Res. Inst., Chicago, IL, Catalog RDH-376, March 1976, pp. 3-14, 2. Bureau of National Affairs, Inc. Final Tables of Fatal and Nonfatal Mining Injuries. Mine Safety and Health Report- er; 1980, V. 3, No. 7, Aug. 26, 1981, p. 133; 1981, V, 4, No. 5, July 28, 1982, p. HI. 3. Bureau of National Affairs, Inc. Selected Statistics on MSHA Citations. Orders of Withdrawal From 1979 Annual Report. Mine Safety and Health Reporter, V. 3, No. 9, Sept. 23, 1981, p. 173. 4. Elswick, J. E. , and F. R. Schwam- berger. Electrical Hazards in Mining. Nat. Mine Health & Safety Academy, Beck- ley, WV, Safety Manual 9, undated, p. 3. 5. Lyons, J. C. Underground Super- visory Control, Pres, at Mining Electro-Mechanical Maintenance Assoc. (ME-MMA) Annual Meeting, Wheeling, WV, April 1981; available for consultation at the Pittsburgh Research Center, Bureau of Mines, Pittsburgh, Pa. 6. McDonald, L. B. , and W. H. Pomroy, A Statistical Analysis of Coal Mine Fire Incidents in the United States From 1950 to 1977, BuMines IC 8830, 1980, 42 pp. 7. U.S. Mine Safety and Health Admini- stration (Department of Labor) . Injury Experience in Coal Mining. Selected Sta- tistics, 1979. MSHA IR 1122, 1980, p. 249, 8. Wright, H, A., R. Madden, and M. N. Rubin (Bolt Beranek and Newman, Inc., BuMines contract J0100039) . Guide- lines for Environmental Monitoring in Underground Coal Mines. Phase 1 Report. BuMines OFR 180-82, 1982, 128 pp.; NTIS, PB 83-147777. APPENDIX Table A-1, Current Mandatory Standards, lists a brief description of CFR 30, Part 75 requirements for each regulation discussed previously in the section, Analysis of Regulations. The rationale or hazard to be prevented by each regula- tion is also given. TABLE A-1. — Current mandatory standards CFR part and requirement^ Rationale 75.513 — All electric conductors must be Prevent fires, large enough to carry the necessary load current without creating excessive heat, 75.518 — All electric equipment, circuits, and 3-phase motors must be protected against short circuits and overloads. 75.519 — Disconnecting switches must be installed in all main power circuits within 500 ft of the bottom of shafts, boreholes , and all other places through which main power circuits enter the mine. Prevent fires. Disconnect mine power in an emergency such as a fire or explosion or when a condi- tion exists that could result in a mine fire or explosion. Enable operation without danger of shock, fire, or faulty operation. Prevent methane ignitions and explosions. 75.520 — Switches or controls on all elec- tric equipment must be safely designed, constructed, and installed. 75.524 — The maximum level of ac or dc that exists between the frames of 2 units of electric face equipment that come in contact with each other in work- ing places or in return air outby the last open crosscut must be <1 A. 75.601 — Each conductor of all trailing Prevent fires, cables must be protected against short circuits by a device of adequate curent-interrupting capacity. 75.702 — Methods other than conventional There are other safe methods of grounding grounding to limit the voltage that can such as electronic devices and diode appear between the frame of a machine grounding. Prevent shock, and ground to a safe value may be used. 75.705 — Before maintenance work is per- Prevent shock, formed on underground high-voltage lines, they must be deenergized and grounded. See footnote at end of table. 10 TABLE A-1. — Current mandatory standards — Continued CFR P art and requirement^ Rationale 75.706 — Unused underground power circuits Prevent fires. Rectifiers and transform- must be deenergized on idle days and ers should remain energized to combat shifts. Rectifiers and transformers may moisture, remain energized. 75.800 — High-voltage circuits entering a mine must be protected by suitable cir- cuit breakers of adequate interrupting capacity which are equipped with devices to provide protection against undervolt- age, grounded phase, short circuit, and overcurrent. Prevent shock and fires. 75.801 — The grounding resistor must be of Provide protection against shock and fire the proper ohmic value so that the volt- hazards, age drop in the grounding circuit be- tween the grounded side of the resistor and equipment frames be no more than 100 V. The grounding resistor must be rated for the maximum fault current con- tinuously and insulated from ground for a voltage equal to the phase-to-phase voltage of the system. 75.803 — High-voltage resistance grounded Prevent shock and fires, systems must include a fail-safe ground check monitor. 75.808 — Disconnecting devices with visi- For isolation of faulted circuits and de- ble contacts must be installed at the energizing a circuit when not in use. beginning of all branch lines in high- voltage circuits. 75.812 — Power centers, portable trans- .Prevent shock and fires, formers, and high-voltage cables, con- ducting power to these units, must be deenergized before they are moved from one location to another. 75.900 — Circuit breakers equipped with devices to provide protection against undervoltage, grounded phase, short cir- cuit, and overcurrent must be installed to protect all low- and medium-voltage power circuits serving 3-phase ac equipment. Prevent electrocution and fires. See footnote at end of table. 11 TABLE A-1. — Current mandatory standards — Continued CFR Part and requirement^ Rationale 75.902 — Low- and medium-voltage resist- ance grounded systems must include a fall-safe ground check circuit. Prevent shock, 75.903 — Disconnecting devices, which pro- Prevent shock, vide visual evidence that the power is disconnected, must be installed in con- junction with circuit breakers. 75.906 — Trailing cable for mobile equip- ment must contain at least one ground conductor (cross-sectional area > 1/2 the power conductor) and an insulated conductor for the ground continuity check circuit. Prevent electrocution. 75. 1000 — Cutout switches must be in- Facilitate removal of power during emer- s tailed in trolley wires and trolley gencies , idle periods, and at times when feeder wires at Intervals of 2,000 ft or maintenance work is performed, less and near the the beginning of all branch lines. 75.1001 — Automatic current interrupting Prevent fires, devices must be Installed to protect trolley wires and trolley feeder wires against damage by over current. 'u.S. 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