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

the Mine Safety and Health Administration initially required amaximum 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

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

Mod-ification 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

sig-nificantly 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

THE power requirements of high-capacity longwall

sys-tems 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

Paper PID 00–22, presented at the 1998 Industry Applications Society

An-nual Meeting, St Louis, MO, October 12–16, and approved for publication in

the IEEE T RANSACTIONS ON I NDUSTRY A PPLICATIONS 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.

proval 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 ca-bles 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

The MSHA then used this 9-kW value for establishing the max-imum ground-fault current limits for high-voltage systems as follows:

2400-V system

4160-V system

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 1 Ungrounded system and equivalent circuit.

and 6.5 A for 2400-V systems in 101-c Petitions for

Modifica-tion [4] However, more recent PetiModifica-tions for ModificaModifica-tion 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

pre-venting 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

dis-cussed 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 GROUNDINGSYSTEMCHARACTERISTICS

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

characteris-tics of the other two systems

A Ungrounded System

With the ungrounded system, there is no intentional

connec-tion between any part of the electrical system and ground

How-ever, 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

wind-ings, and motor windings Fig 1 is a simplified representation

of an ungrounded system, which illustrates the capacitive

cou-Fig 2 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 cou-pling can subject the ungrounded system to dangerous overvolt-ages 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 induc-tive reactance of the fault to the total capaciinduc-tive reactance of the system It is obvious from Fig 1 that the highest overvoltage will occur at system resonance, where the magnitude of the two reac-tances 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 1 can represent a simplified solidly grounded equivalent circuit

C Resistance-Grounded System

The resistance-grounded system can be considered a compro-mise between the ungrounded and solidly grounded systems Resistance grounding is established by inserting a resistor be-tween the system neutral and ground [10], [11] Thus, the

max-Fig 3 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 2 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

am-plitude of overvoltages High-resistance grounding can be

ap-plied 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

in-stantaneously, or after a short time delay when relay

coordina-tion is necessary Instead, high-resistance grounding is required

in underground coal mining because it limits the amount of

en-ergy 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 2 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 3

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 3 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 in-stantaneous 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 pro-tection in the motor starting unit

A Model

The circuit model of Fig 4 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 three-phase line-to-neutral voltages (2400 V) and are 120 out of

NOVAK: ANALYSIS OF VERY-HIGH-RESISTANCE GROUNDING 107

Fig 4 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 4 The

neu-tral 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

induc-tances 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 capaci-tance per unit length was obtained from a cable manufacturer and is calculated from the following equation [13]:

(4)

Fig 5 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

imped-ances These impedances are sized to reflect rated conditions

with typical power factors and efficiencies

B Simulation Results

The circuit in Fig 4 was simulated using OrCad PSpice

version 8 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 5 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

Fig 5 begin at time ms, instead of , because the

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 capaci-tance 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

a per-phase system reactance of 1.92 k The required fault inductance to produce the maximum resonant effects is given by

A fault inductance of 1.7 H was then inserted in the model The results of the simulation are presented in Fig 6 Fig 6 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

19 kV within a few cycles, which is approximately eight times the rated voltage

To demonstrate the effect that fault inductance has on the magnitude of overvoltage, simulations were run with fault in-ductances 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 6 Line-to-ground voltages for a grounding resistor current limit of 0.5 A and a 1.7-H fault inductance.

Fig 7 Line-to-ground voltages for a grounding-resistor current limit of 0.5 A and a 2.5-H fault inductance.

Figs 7 and 8 Fig 7 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 8

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

Fig 8 Line-to-ground voltages for a grounding-resistor current limit of 0.5 A and a 1.0-H fault inductance.

Fig 9 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 9 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 111

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

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

re-laying,” 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 protec-tion 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,”

Min-eral Resources Eng., vol 1, no 1, pp 51–66.

[8] IEEE Recommended Practice for Electric Power Distribution for

Indus-trial 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

Com-pany, 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 Pennsyl-vania 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 En-gineer for the U.S Bureau of Mines, Pittsburgh Re-search 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 Appli-cations Society (IAS) and is the current Chairman of the IAS Meetings Depart-ment 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, Metal-lurgy, and Exploration, Inc and the American Society of Civil Engineers He is

a Licensed Professional Engineer in the States of Alabama and Pennsylvania.