Analysis of very high resistance grounding in high voltage longwall power systems IEEE trans

104 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL 37, NO 1, JANUARY/FEBRUARY 2001 Analysis of Very-High-Resistance Grounding in High-Voltage Longwall Power Systems Thomas Novak, Senior Member, IEEE Abstract—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 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 systems (greater than 1000 V), the Mine Safety and Health Administration 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 Index Terms—High-resistance grounding, longwall mining, mine electrical systems I INTRODUCTION T HE power requirements of high-capacity longwall systems have significantly increased in recent years, such that the combined horsepower for the face conveyor, shearer, stage loader, crusher, and hydraulic pumps can exceed 5000 hp The past practice of using 995 V as the utilization voltage is inadequate for these high-capacity applications because of excessive three-phase and line-to-line fault currents, massive cable sizes, reduced motor torque from excessive voltage drop, and difficulty in maintaining the maximum instantaneous trip settings allowed by the Mine Safety and Health Administration (MSHA) [1]–[3] These concerns were minimized, if not eliminated, by using the higher utilization voltages of 2400 V and 4160 V Paragraph 18.47 (d) (3) of Title 30, Code of Federal Regulations, permits alternating-current machines to have nameplate ratings up to 4160 V if all high-voltage switchgear are remotely located and operated by remote control However, the use of high voltage (greater than 1000 V) to power face equipment still requires approval from the MSHA to modify the application of Paragraph 75.1002 of Title 30, Code of Federal Regulations, which states: Trolley wires and trolley feeder wires, high-voltage cables and transformers shall not be located in by the last open crosscut and shall be kept at least 150 ft from pillar workings To obtain approval from the MSHA, the mine operator must formally submit a 101-c Petition for Modification and show that a proposed alternative method will at all times guarantee no less than the same measure of protection afforded by the existing standards To ensure that the high-voltage systems maintain or exceed the same level of safety as medium-voltage systems, the MSHA developed criteria for high-voltage face equipment to supplement existing regulations [4] One MSHA criterion for high-voltage systems deals with maximum ground-fault current The MSHA expressed a concern with limiting the amount of energy dissipated in an explosion-proof enclosure during a ground fault Title 30, Code of Federal Regulations, requires that maximum ground-fault current be limited to 25 A for low- and medium-voltage circuits However, the industry adopted a more conservative 15-A limit As a result, the maximum power that can be dissipated by the neutral grounding resistor, during a ground fault, for a nominal 1040-V system is kW (1) The MSHA then used this 9-kW value for establishing the maximum ground-fault current limits for high-voltage systems as follows: 2400-V system A (2) 4160-V system Paper PID 00–22, presented at the 1998 Industry Applications Society Annual Meeting, St Louis, MO, October 12–16, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Mining Industry Committee of the IEEE Industry Applications Society Manuscript submitted for review October 15, 1998 and released for publication September 23, 2000 The author is with the University of Alabama, Tuscaloosa, AL 35487-0205 USA (e-mail: tnovak@coe.eng.ua.edu) Publisher Item Identifier S 0093-9994(01)00894-5 A (3) As a result, the MSHA initially required a maximum grounding-resistor current limit of 3.75 A for 4160-V systems 0093–9994/01$10.00 © 2001 IEEE NOVAK: ANALYSIS OF VERY-HIGH-RESISTANCE GROUNDING 105 Fig Ungrounded system and equivalent circuit and 6.5 A for 2400-V systems in 101-c Petitions for Modification [4] However, more recent Petitions for Modification have required lowering these values to 1.0 A, or even 0.5 A These lower values have been readily adopted by the mining industry and used with ground-fault relay pickup settings of less than 100 mA The application of very sensitive ground-fault protection in underground coal mines was demonstrated in the early 1980s [5]–[7], but its application was directed toward preventing ventricular fibrillation and was limited to low- and medium-voltage utilization circuits Surprisingly, the author is unaware of any studies that document improved safety with the 1-A and 0.5-A limits with high-voltage utilization circuits The rationale appears to be—the lower the ground-fault current, the better However, a point of diminishing returns occurs, as the fault current is limited In fact, the undesirable characteristics of an ungrounded system surface with very-low ground-fault resistor-current limits Since these concerns have not been discussed in the literature, the intent of this paper is to present an analysis of a typical 4160-V longwall power system that utilizes very-high-resistance grounding (grounding-resistor-current limit of 0.5 A) Computer simulations are used to determine the prudence of using such a low current limit II GROUNDING SYSTEM CHARACTERISTICS The common grounding classifications found in industrial power systems are ungrounded, solidly grounded, and resistance grounded, although variations of these methods also occur [8] Even though this paper deals with high-resistance grounding, the features of all three systems will be briefly described since high-resistance grounding can exhibit some of the characteristics of the other two systems A Ungrounded System With the ungrounded system, there is no intentional connection between any part of the electrical system and ground However, the term ungrounded is somewhat of a misnomer because each line of the system is actually coupled to ground through the inherent per-phase capacitance of the cables, transformer windings, and motor windings Fig is a simplified representation of an ungrounded system, which illustrates the capacitive cou- Fig Resistance grounded system and equivalent circuit pling to ground The cited advantage of this type of system is that the first fault between a line conductor and ground does not cause circuit interruption, thus there is no loss of power that can disrupt continuous type processes However, the capacitive coupling can subject the ungrounded system to dangerous overvoltages from intermittent ground faults and resonant effects due to ground faults through high inductive reactances [8], [9] Thus, ungrounded systems are generally considered to be susceptible to insulation failures The connection of an inductive reactance between line and ground can produce serious overvoltages with respect to ground The degree of overvoltage is dictated by the ratio of the inductive reactance of the fault to the total capacitive reactance of the system It is obvious from Fig that the highest overvoltage will occur at system resonance, where the magnitude of the two reactances are equal At resonance, overvoltages of 20 times normal can be reached Substantial overvoltages can also be developed by intermittent or sputtering ground faults, which are discussed in detail in [9] B Solidly Grounded System The neutral point of a solidly grounded system is connected to ground through no intentional impedance A line-to-ground fault results in a high current, which can easily be detected by protective circuitry and isolated quickly However, since there is no intentional impedance in the neutral connection, a very high ground-fault current, which may be capable of exploding protective enclosures, starting fires, and causing flash hazards, can occur Overvoltage control is a major advantage of this system, because the system neutral is solidly referenced to ground Placing a short circuit around the system capacitance in the equivalent circuit of Fig can represent a simplified solidly grounded equivalent circuit C Resistance-Grounded System The resistance-grounded system can be considered a compromise between the ungrounded and solidly grounded systems Resistance grounding is established by inserting a resistor between the system neutral and ground [10], [11] Thus, the max- 106 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL 37, NO 1, JANUARY/FEBRUARY 2001 Fig General arrangement diagram for a typical 4160-V longwall power system imum ground-fault current is controlled by the ohmic value of the resistor, provided the resistor current is significantly greater than the system capacitive charging current Fig is a simplified representation of a resistance-grounded system The lower fault current requires additional protective relaying, but practically eliminates arcing and flashover dangers, while limiting the amplitude of overvoltages High-resistance grounding can be applied where immediate service interruption on the first ground fault is to be avoided However, this is not an issue in the mining industry because ground-fault protection is required to react instantaneously, or after a short time delay when relay coordination is necessary Instead, high-resistance grounding is required in underground coal mining because it limits the amount of energy dissipated and controls the elevation of frame potentials, during a ground fault Standard practice 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 mentioned above Fig illustrates how a high-resistance-grounded system approaches an ungrounded system as the ohmic value of the grounding resistor increases 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 associated with 4160-V longwalls, the effects of system capacitance become very pronounced III ANALYSIS An analysis was performed on a typical 4160-V longwall power system that utilizes very-high-resistance grounding (grounding-resistor-current limit of 0.5 A), as shown in Fig This diagram shows a 5-MVA power center, which steps down the 13.8-kV distribution voltage to the 4160-V utilization voltage and to the 480-V auxiliary voltage The power center feeds the 4160-V motor-starting unit, which in turn controls the starting and stopping of the longwall face equipment The power ratings of face equipment and the cable lengths and sizes are also shown in Fig A monorail cable handling system supports the cables connecting the motor-starting unit with the face equipment All high-voltage cables are 5-kV SHD type G-GC The master controller is located near the longwall face equipment and controls the motor-starting unit by means of a programmable logic controller (PLC) and data highway cable Zero-sequence ground-fault protection is located in both the motor-starting unit and the power center To provide selective tripping, all outgoing circuits in the motor-starting unit have instantaneous ground-fault protection Ground-fault protection is also provided in the power center and generally has a time delay up to a maximum of 0.25 s to provide coordination with the protection in the motor starting unit A Model The circuit model of Fig was constructed for performing the simulations Some liberties were taken to simplify the model, but sufficient detail exists to determine whether or not potential problems exist The model consists of the longwall equipment motors, power transformer, neutral grounding resistor, and associated cables All circuits are assumed to be energized and operating at rated load The hydraulic-pump motor circuit and the two parallel-connected 250-kcmil feeder cables, between the power center and the motor starting unit, were not included to simplify the model The secondary of the power center transformer is modeled as three voltage sources with series impedances connected in a wye configuration The voltage sources represent the threephase line-to-neutral voltages (2400 V) and are 120 out of NOVAK: ANALYSIS OF VERY-HIGH-RESISTANCE GROUNDING Fig 107 Simplified simulation model for a 4160-V longwall power system phase with each other The series impedances are based upon a 5% transformer impedance with an X/R ratio of The neutral grounding resistor (NGR) is shown connected between the system neutral and ground The equipment cables are represented as lumped impedances connected in a configuration Cable resistances and inductances are based on the cable’s size and length [12] The system capacitance of the model is only due to the cables; capacitance from transformer and motor windings is ignored Although ca- pacitance is distributed along the cable’s entire length, the cable capacitance is lumped and connected from line to ground at the beginning and end of each cable for simplicity Cable capacitance per unit length was obtained from a cable manufacturer and is calculated from the following equation [13]: (4) 108 Fig IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL 37, NO 1, JANUARY/FEBRUARY 2001 Ground-fault resistor and capacitive-charging currents for a grounding resistor current limit of 0.5 A where per-phase capacitance to ground (pF/ft); 4.0 (for EPR); insulation thickness; diameter under insulation Each motor is modeled with three wye-connected impedances These impedances are sized to reflect rated conditions with typical power factors and efficiencies B Simulation Results The circuit in Fig was simulated using OrCad PSpice version The first simulation was performed to determine if the magnitude of the system charging current exceeds the grounding-resistor current under a ground-fault condition Therefore, the value of the neutral grounding resistor was set at 4.8 k to establish a 0.5-A grounding-resistor-current limit A bolted line-to-ground fault was then located on the main bus at the output of the power transformer Fig clearly shows that, for this situation, with all six motors on line during a ground fault, the system charging current significantly exceeds the current in the neutral grounding resistor In fact, the magnitude of the system charging current is over seven times the resistor current (It should be noted that the simulation results in ms, instead of , because the Fig begin at time first-cycle capacitive inrush current dwarfs the steady-state values.) The initial simulation clearly demonstrates that limiting the maximum ground-fault resistor current to 0.5 A violates the definition of a high-resistance-grounded system (It is interesting to note that if the 3.75-A limit was used, the magnitudes of the system charging current and the resistor current would be approximately equal, and the definition of high-resistance grounding would not be violated) The next step was to determine whether the system begins to exhibit the overvoltage problems of an ungrounded system Therefore, the bolted line-to-ground fault was replaced with an inductive reactance to provide a resonant effect with the system capacitance under ground-fault conditions The worst case scenario occurs when the magnitude of the inductive reactance of the fault equals the magnitude of the capacitive reactance of the system To determine the appropriate value of fault inductance, the system capacitance can be approximated by adding the individual per-phase lumped capacitances; thus, the per-phase system capacitance is approximately 1.38 F, which results in of 1.92 k The required a per-phase system reactance fault inductance to produce the maximum resonant effects is given by H (5) A fault inductance of 1.7 H was then inserted in the model The results of the simulation are presented in Fig Fig shows the three line-to-ground voltages of the system plotted on the same scale The fault is introduced at time ms The resonant effects of the circuit are apparent as the line-to-ground voltages escalate to approximately 27 kV or within a few cycles, which is approximately eight 19 kV times the rated voltage To demonstrate the effect that fault inductance has on the magnitude of overvoltage, simulations were run with fault inductances above and below the resonant value at 2.5 and 1.0 H, respectively The results of these simulations are shown in NOVAK: ANALYSIS OF VERY-HIGH-RESISTANCE GROUNDING 109 Fig Line-to-ground voltages for a grounding resistor current limit of 0.5 A and a 1.7-H fault inductance Fig Line-to-ground voltages for a grounding-resistor current limit of 0.5 A and a 2.5-H fault inductance Figs and Fig shows the line-to-ground voltages with a fault inductance of 2.5 H As expected, the overvoltages are reduced significantly when the fault inductance deviates from its resonant value After a few cycles, the overvoltages reach a steady-state value of 10 kV or 7.07 V , which is approxi- mately three times rated voltage A similar situation occurs with a fault inductance of 1.0 H, as shown in Fig A simulation was then performed with the maximum ground-fault resistor current increased to 3.75 A, which requires a 640- value for the neutral grounding resistor 110 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL 37, NO 1, JANUARY/FEBRUARY 2001 Fig Line-to-ground voltages for a grounding-resistor current limit of 0.5 A and a 1.0-H fault inductance Fig Line-to-ground voltages for a grounding-resistor current limit of 3.75 A and a 1.7-H fault inductance The simulation was performed for the worst case, with the fault inductance set at 1.7 H The results in Fig clearly show that the worst case overvoltage is effectively controlled because the magnitudes of the system charging current and the resistor current are nearly equal For this case, the maximum line-to-ground overvoltage reaches 6.5 kV or 4.60 kV , which is, as expected, approximately equal to the line-to-line voltage of the system NOVAK: ANALYSIS OF VERY-HIGH-RESISTANCE GROUNDING IV CONCLUSIONS With high-voltage (greater than 1000 V) longwall mining systems, the MSHA initially required a maximum ground-fault resistor-current limit of 3.75 A for 4160-V systems in 101-c Petitions for Modification However, more recent Petitions have been required to limit maximum resistor current to 1.0 A, or even 0.5 A Standard practice in other industries 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 unshielded cables, are required for high-voltage applications in the mining industry and compound the grounding problem As a result, an analysis of a typical 4160-V longwall power system was performed to determine whether or not potential problems exist with a grounding-resistor-current limit of 0.5 A The analysis showed that, with all motor circuits energized, which is a common occurrence, the magnitude of the system charging current significantly exceeds the magnitude of the grounding-resistor current under a ground-fault condition (by an approximate factor of seven) Thus, the definition of high-resistance grounding is violated Furthermore, simulations reveal that, at such a low value of grounding-resistor current (0.5 A), the system begins to adopt the characteristics of an ungrounded system The simulations show overvoltages of approximately eight times normal voltage for a worst case resonant ground-fault condition One may question whether these potential overvoltages are significant given that fact that instantaneous ground-fault protection is required However, even with instantaneous protection, a vacuum breaker has a typical clearing time of three cycles Furthermore, the ground-fault protection at the power center may have a time delay up to a maximum of 0.25 s Therefore, if a ground fault occurs on the line side of the motor-starting unit, an overvoltage may exist for the extended period resulting from the time delay One may also argue that the lower ground-resistor current limit of 0.5 A reduces frame potentials during a ground fault This may be true, but a close inspection reveals the reduction is on the order of a couple of volts, which is insignificant In conclusion, the analysis shows that there is no advantage in reducing the ground-resistor-current limit from 3.75 to 0.5 A In fact, this practice may have detrimental effects since the system begins to acquire the undesirable characteristics of an ungrounded system, such as overvoltage problems 111 REFERENCES [1] T Novak and J L Kohler, “Technological innovations in deep coal mine power systems,” IEEE Trans Ind Applicat., vol 34, pp 196–203, Jan./Feb 1998 [2] T Novak and J K Martin, “The application of 4160-V to longwall face equipment,” IEEE Trans Ind Applicat., vol 32, pp 471–479, Mar./Apr 1996 [3] L A Morley, T Novak, and I Davidson, “The application of 2400-V to longwall face equipment,” IEEE Trans Ind Applicat., vol 26, pp 886–892, Sept./Oct 1990 [4] C M Boring and K J Porter, “Criteria for approval of mining equipment incorporating on-board switching of high-voltage circuits,” in Proc 9th WVU Int Mining Electrotechnology Conf., July 1988, pp 267–274 [5] T Novak, L A Morley, and F C Trutt, “Sensitive ground-fault relaying,” IEEE Trans Ind Applicat., vol 24, pp 853–861, Sept./Oct 1988 [6] L A Morley, F C Trutt, and T Novak, “Sensitive ground-fault protection for mines,” U.S Bureau of Mines, Washington, DC, Final Rep for U.S Bureau of Mines Contract JO134025, 1984 [7] T Novak, L A Morley, and F C Trutt, “Analysis of ac mine power systems for the application of sensitive ground-fault protection,” Mineral Resources Eng., vol 1, no 1, pp 51–66 [8] IEEE Recommended Practice for Electric Power Distribution for Industrial Plants, IEEE Std 141-1993 [9] C H Titus, “Evaluation and feasibility study of isolated electrical distribution systems in underground coal mines,” U.S Bureau of Mines, Washington, DC, Final Rep for U.S Bureau of Mines Contract HO111465, 1972 [10] B Bridger Jr., “High-resistance grounding,” IEEE Trans Ind Applicat., vol 19, pp 15–21, Jan./Feb 1983 [11] J R Dunki-Jacobs Jr., “The reality of high-resistance grounding,” IEEE Trans Ind Applicat., vol 13, pp 469–475, Sept./Oct 1977 [12] Mining Cable Engineering Handbook, Anaconda Wire and Cable Company, 1977, p 69 [13] M Fuller, private communication, May 1998 Thomas Novak (M’83–SM’93) received the B.S degree in electrical engineering from The Pennsylvania State University, University Park, the M.S degree in mining engineering from the University of Pittsburgh, Pittsburgh, PA, and the Ph.D degree in mining engineering from The Pennsylvania State University in 1975, 1978, and 1984, respectively He has been an Instructor of Mining Engineering at The Pennsylvania State University, an Electrical Engineer for the U.S Bureau of Mines, Pittsburgh Research Center, and Assistant Division Maintenance Engineer for Republic Steel Corporation, Northern Coal Mines Division He is presently Department Head and holder of the Drummond Endowed Chair of Civil Engineering at the University of Alabama, Tuscaloosa, where he has also held the positions of Interim Department Head of Aerospace Engineering and Mechanics, Professor of Electrical Engineering, and Associate Professor of Mineral Engineering Dr Novak is a member of the Executive Board of the IEEE Industry Applications Society (IAS) and is the current Chairman of the IAS Meetings Department He has also served as Chairman of the IAS Process Industries Department (1994–1998), Chairman (1992–1994) and Vice-Chairman (1990–1992) of the IAS Mining Industry Committee, and Co-chairman of the IAS Mining Industry Technical Conference (1987) He is a member of the Society of Mining, Metallurgy, and Exploration, Inc and the American Society of Civil Engineers He is a Licensed Professional Engineer in the States of Alabama and Pennsylvania  ... results of these simulations are shown in NOVAK: ANALYSIS OF VERY -HIGH- RESISTANCE GROUNDING 109 Fig Line-to-ground voltages for a grounding resistor current limit of 0.5 A and a 1.7-H fault inductance... maximum line-to-ground overvoltage reaches 6.5 kV or 4.60 kV , which is, as expected, approximately equal to the line-to-line voltage of the system NOVAK: ANALYSIS OF VERY -HIGH- RESISTANCE GROUNDING. .. on a typical 4160-V longwall power system that utilizes very -high- resistance grounding (grounding- resistor-current limit of 0.5 A), as shown in Fig This diagram shows a 5-MVA power center, which