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 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 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: ).
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. 1. 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. 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 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. 1 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. 1 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. 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 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. 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 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. 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 threephase line-to-neutral voltages (2400 V) and are 120 out of


NOVAK: ANALYSIS OF VERY-HIGH-RESISTANCE GROUNDING

Fig. 4.

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 4. 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. 5.

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. 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
ms, instead of
, because the
Fig. 5 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. 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
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. 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.


110

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 37, NO. 1, JANUARY/FEBRUARY 2001

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

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.