Thomas Novak

Technological innovations in deep coal mine power systems

Various innovations and changes have occurred to underground coal mine power systems during the past ten years. Some of these are the use of higher utilization and distribution voltages and associated switchgear, the use of programmable logic controllers for control, monitoring and diagnostic applications, improved protective relaying with built-in test circuitry, power-factor correction near loads for improved voltage regulation and modifications to power system component arrangements. Many of the innovations were necessitated by significant increases in power requirements of face equipment, particularly longwall equipment, while others are the result of improved technology. In either case, improved power system operating characteristics, as well as enhanced levels of safety, have resulted. This paper describes many of the major innovations that have evolved during recent years.

The application of 2400 V to longwall face equipment

Operational and safety aspects of 2400 V longwall systems, as compared with 995 V systems, are presented. A 900 ft face width was chosen for the comparison, and typical longwall equipment, power requirements, and cable lengths were selected. The kVA rating of the power-center transformer was specified as 2500 kVA, with a 0.05-pu impedance and a resistance-to-reactance ratio of 4.0. Maximum fault currents were calculated for each system voltage, assuming a bolted three-phase fault and an infinite bus at the primary of the power-center transformer. Maximum fault currents and interrupting capabilities of circuit breakers at both utilization voltages were then compared. Typical cable sizes were specified for each system voltage, and their respective voltage regulations were calculated. Voltage drops and motor-torque reductions were also calculated and compared for conveyer-motor starting conditions. Better voltage regulation, improved motor torque, and decreased cable sizes are achieved with the higher voltage. Enhanced safety features collectively provide a safer electrical installation at 2400 V than presently required for 995 V systems.<<ETX>>

The effects of very-high resistance grounding on the selectivity of ground-fault relaying in high-voltage longwall power systems

With the advent of high-voltage (greater than 1 kV) utilization circuits on longwall mining equipment in the late 1980s, the Mine Safety and Health Administration (MSHA) initially required maximum ground-fault current limits of 3.75 A for 4160-V systems and 6.5 A for 2400-V systems. Ground-fault relay pickup settings were not permitted to exceed 40% of the maximum ground-fault current. Shortly thereafter MSHA began, and presently continues, requiring a much lower maximum ground-fault current limit of 1.0 A, or even 0.5 A, with ground-trip settings of 100 mA. Shielded cables, which have significantly more capacitance than their unshielded counterparts, are required for high-voltage applications in the mining industry. In an earlier paper, the author showed that with the long cable runs of a high-voltage longwall system, capacitive charging currents could easily exceed grounding-resistor currents under ground-fault conditions. As a result, overvoltages from inductive-capacitive resonance effects can occur. Because of the large system capacitance and low ground-trip setting, the relay selectivity of the ground-fault protection system may also be compromised. Therefore, an analysis of a typical 4160-V longwall power system that utilizes very-high resistance grounding (ground-resistor-current limit of 0.5 A) is performed to determine if potential problems exist with the selectivity of ground-fault relaying.

Sensitive ground-fault relaying

Sensitive ground-fault protection refers to the concept of detecting low levels of ground-fault current that might cause electrocution of any human that becomes part of the ground-current path and providing warnings. The concepts behind sensitive ground-fault relaying for use on AC and DC low-voltage utilization systems are covered. The background for these relaying types is presented, and it is shown that the critical component in sensitive ground-fault relay design is the current sensor. Zero-sequence devices for three-phase industrial utilization systems and a saturable-transformer device for DC utilization are discussed. Relaying schemes for both AC and DC systems are presented. This study is aimed at mine power systems but could be applicable to any portable low-voltage portion of industrial power systems that involve handling by personnel. >

Analysis of very-high resistance grounding in high-voltage longwall power systems

The application of very sensitive ground-fault protection in underground coal mines was demonstrated in the early 1980s for low- and medium-voltage utilization circuits (less than 1 kV), but its commercial application did not occur until the advent of high-voltage utilization circuits on longwalls in the late 1980s. With these high-voltage (greater than 1 kV) systems, MSHA initially required a maximum ground-fault resistor current limit of 3.75 A for 4160-V systems and 6.5 A for 2400-V systems in 101-c Petitions for Modification. However, more recent Petitions for Modification have been required to limit maximum ground-fault resistor currents to 1.0 A, or even 0.5 A. Standard practice in other industries generally requires high-resistance grounding to be designed so that the capacitive charging current of the system is less than or equal to the resistor current under a ground-fault condition. The intent of this practice is to prevent the system from developing some of the undesirable characteristics of an ungrounded system, such as overvoltages from inductive-capacitive resonance effects and intermittent ground-faults. Shielded cables, which have significantly more capacitance than their unshielded counterparts, are required for high-voltage applications in the mining industry, Thus, with the long cable runs of a high-voltage longwall system, capacitive charging currents may exceed grounding-resistor currents under ground-fault conditions. An analysis of a typical 4160-V longwall power system that utilizes very-high resistance grounding (grounding-resistor-current limit of 0.5 A) is performed to determine whether or not potential problems exist.

Detrimental Effects of Capacitance on High-Resistance-Grounded Mine Distribution Systems

Modern underground coal mines can be very large, having a total connected load in excess of 15 000 hp. These mines generally have many miles of high-power conveyor belts and 15 or more miles of high-voltage power cables at distribution voltages of 12.47, 13.2, 13.8, or 14.4 kV. The shielded cables used in mine power distribution systems have a significant level of capacitance, on the order of 110 pF/ft. This level of capacitance, in an extensive power distribution system at todays voltage levels, can cause significant charging currents during a ground fault. This paper addresses the potential detrimental effects of capacitance charging currents during line-to-ground faults in mine power distribution systems. A representative mine power system is modeled, and simulations with faults at various locations are conducted to evaluate the effects of this capacitance on the level of fault current and relay selectivity. This paper also includes results of capacitance measurements made on mine power feeder cables used to validate the simulation model

Safety issues and the use of software-controlled equipment in the mining industry

Equipment control functions that were once hardwired are being implemented with software and very large scale integrated (VLSI) devices. Often this transition has resulted in increased flexibility, improved quality and decreased costs. At the same time, it has created new concerns and challenges concerning worker safety. The visible and well-defined ladder diagram for relay-logic has been replaced by programs in which the exact outcome for varied inputs can be more obscure. In the coal mining industry, efforts to automate longwall mining systems have resulted in semiautonomous machines operating within the same space as workers. This paper describes an effort initiated by the National Institute for Occupational Safety and Health (NIOSH) to identify the safety issues related to the use of processor-controlled equipment in mining. Specific findings in the areas of human factors, hardware and software safety are presented in this paper, and a brief description of a plan to address identified weaknesses is given.

Development of an expert system for diagnosing component-level failures in a shuttle car

The development of a rule-based expert system, called SCAR for assisting electricians in diagnosing electrical faults in a shuttle car is discussed. Insight 2+, a microcomputer-based expert-system development tool, was used in creating the system. The program requires the user to specify the initial symptoms of the failed machine, and the most probable cause of failure is traced through the knowledge base, with the software requesting additional information such as voltage or resistance measurements as needed. A causal-reasoning approach, which was used to develop the production rules, is described. The organization, implementation, and limitations of the knowledge base are also presented, along with a brief discussion of the ongoing development of a new system SCAR.2.<<ETX>>

Use of mine ventilation exhaust as combustion air in gas-fired turbo-electric generators

Methane liberated in coal mines is a potential safety hazard because it is explosive at relatively low concentrations (5-15%) in air. To manage methane, underground mines are ventilated with large quantities of air, and in some cases the gas is also drained with gob wells, and predrained with vertical and horizontal wells. The ventilation air is used to dilute methane emissions to levels well below the explosive limit, and the diluted stream is discharged to the atmosphere. Unfortunately, this waste stream may contain as much as 60% of the total gas energy that was originally in the coal. Also, methane is considered by some to be 24.5 times more detrimental than CO/sub 2/ in contributing to the greenhouse effect. The volume of the waste stream, the high electric power demands of a mine, and the greenhouse effect of methane provide a strong incentive for converting the waste-methane chemical energy to the electrical or mechanical equivalent. A preliminary economic assessment of a proposed test-turbine installation at the Jim Walter Resources No. 5 Mine, shows that such a project makes good sense economically, even without considering the emission-reduction benefits. This unit could produce enough power to drive a ventilation fan, provide a profitable rate of return, and produce a 2% reduction in emissions. A market study indicates that there is the potential to generate 706 to 816 MW of power from mine ventilation gas in the United States.

Sensitive DC Ground Fault Relays for Coal Mines

Recent US Bureau of Mines sponsored research aimed at virtually eliminating electrocutions on dc utilization circuits has resulted in an effective dc circuit protection device, called a sensitive ground fault relay (GFR). This relay can detect and act to interrupt small deadly ground currents that can trigger ventricular fibrillation in humans while ignoring spurious power system signals. Initially relay performance criteria were established to ensure reliability for mine duty as well as effectiveness for shock prevention. Mineworthy prototypes, consisting of a saturable-transformer current sensor and an electronic relay, were constructed. They complied with the performance standards in laboratory tests.