high resistance grounding tech paper
TRANSCRIPT
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8/16/2019 High Resistance Grounding Tech Paper
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Francis K. Fox, Howard J. Grotts, and Clyde H. Tipton
IEEE Transactions on Industry and General Applications
September/October 1965 E
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8/16/2019 High Resistance Grounding Tech Paper
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Francis K. Fox, Howard J. Grotts, and Clyde H. Tipton
IEEE Transactions on Industry and General Applications
September/October 1965
Approved by the Petroleum Industry Committee for
presentation at the IEEE Petroleum Industry TechnicalConf., Houston, Tex., September 13-15, 1965.
Francis K. Fox is with the Central Electric Company, San
Francisco, Calif.
Howard J. Grotts is with the Tidewater Oil Company,
Avon, Calif.
Clyde H. Tipton is with the Standard Oil Company of
California, Richmond, Calif.
Abstract - A method is described which has been used to
reduce the difficulties encountered on several old
delta-ungrounded systems. Data are included which will help
in applying this to any particular power system. The scheme
gives an alarm whenever a ground fault occurs, permits every-
thing to keep running even more assuredly than with an un-
grounded system, and provides a method for quickly locating
the fault. Circuit diagrams and photographs of actual equip-
ment are included, and operating practice is summarized.
Introduction
Should electric power systems be grounded and if so, what
is the best method to use? Much has been written, and many
heated arguments, not recorded, center about this subject. Be-
fore describing any one particular scheme, the authors will at-
tempt to summarize the entire subject as objectively and as
briefly as possible.
An ungrounded system is one in which no intentional con-
nection is made between any part of the system and ground.
Such a system is nevertheless grounded by the effect of the
distributed resistance and capacitance (mostly capacitance)which exists between all the conductors (cables, motors, trans-
formers, etc.) and ground.
A grounded system is one in which an intentional con-
nection is made between the power system (preferably at the
neutral junction) and ground, either directly or through an im-
pedance.
Present industrial power system practice seems to indi-
cate a greater need for some form of system neutral ground-
ing, as the voltage of the system increases. Many 480-volt sys-
tems have successfully operated ungrounded for years, but ex-
perience with the higher voltages has been such that almost all
12-kV systems are grounded. Between these two voltages area great variety of grounding conditions: Most of the older
2400-volt systems are ungrounded. Many 6900-volt systems
are still ungrounded. The best results in grounding 6900- and
12 000-volt industrial systems have been obtained by ground-
ing the neutral through a resistor to limit ground-fault current
to a desirable value, but retaining enough to produce selective
tripping of breakers. Some 4160-volt systems are solidly neu-
tral grounded although, for industrial service, there is much
preference for resistance grounding at this, as well
as at other voltage levels above 600 volts. Power
company practice is usually to ground the neutral
solidly, if available. Many 480-volt industrial sys-
tems have been solidly grounded. In the last few
years, many 2400- and 4160-volt systems have been
resistance grounded with ground-fault immediate
tripping of breakers or high-voltage motor control-
lers.
What determines the best power system? The
ideal system would be one in which a failure neveroccurred but, even if such a system could be built,
the cost would be prohibitive. So power systems are
designed to produce as little trouble as possible.
The hooker is in defining the word “trouble.”
First, there is the difficulty of obtaining the money
with which to build the power system: no one can
overlook that trouble. Next, recollect all the trouble
you have experienced from power outages or equip-
ment failures and what it cost you. You have read or
heard about other engineers’ troubles; evaluate them
as they might affect you if those same troubles ap-pear in your plant.
The next step is devising a system with features
for protecting against these troubles. However, the
cost of the various protective features must always
be balanced against the cost of the troubles.
Electrical failures occur in many ways, but most
failures originate as ground faults. In this paper, the
authors are confining themselves to a brief summary
of how system neutral grounding may affect the
trouble which such failures produce.
For most industrial power systems 2400 volts
and above, probably the least trouble will be pro-duced if the system neutral is grounded through a
resistor which will limit ground-fault current to a
few hundred amperes, but will retain enough cur-
rent to trip a breaker immediately and remove the
faulty equipment from the system. This method of
grounding has the following advantages:
1) System overvoltages are reduced both in mag-
nitude and duration.
2) Faulty equipment is immediately known.
3) Damage at the point of fault is negligible.
4) Hardly any voltage disturbance is noticed on
the system, and no other loads are affected.
5) Methods for detecting and removing ground
faults in modem switchgear with the use of breakers
(not fuses) and high-voltage motor controllers are
accurate, sensitive, and economical.
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Francis K. Fox, Howard J. Grotts, and Clyde H. Tipton
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However, this method has several disadvantages:
1) No warning precedes the tripping of a breaker or the
opening of a motor contactor.
2) Sudden stoppage of a motor, or of all loads on an entire
circuit, may cause considerable damage to a process plant.
3) On an old system, adding needed neutral grounding
equipment and ground-fault relaying is expensive.
For various reasons, there are many ungrounded 2400- or
4160-volt systems in operation. These systems are all subject
to the following possible troubles:
1) Certain types of ground faults can produce dangeroushigh transient overvoltages throughout the entire system.
2) These overvoltages can a) produce an immediate fail-
ure of some other equipment on another feeder resulting in the
simultaneous tripping of two breakers, or b) weaken other in-
sulation so that the next failure will take place sooner.
3) A ground fault sometimes goes unnoticed for days or
weeks. Even if no dangerous transient overvoltages are pro-
duced, such a fault usually develops into a phase-to-phase fault
with consequent increased damage at the point of fault.
4) It is annoying, time consuming, and sometimes hazard-
ous to locate the ground fault by switching loads on and off, inorder to remove the faulty equipment from the system.
High Resistance Grounding System Utilizing Pulsing
Ground-Fault Detector Apparatus*
This high-resistance grounding scheme was developed to
overcome the troubles which are associated with an ungrounded
system and which have already been described. Briefly, the
scheme is designed to
1) Eliminate the high transient overvoltages which can
appear during arcing ground faults.
2) Give immediate warning when a ground fault occurs.3) Accomplish this with a minimum of system neutral
grounding, so that the current at the ground fault will be only
slightly greater than (but perhaps even less than) the fault cur-
rent would be if the system were left ungrounded. (In the case
of a sputtering fault, the ground-fault current in an ungrounded
system may be increased to several times the bolted-fault value.
High-resistance grounding will hold the current value to sub-
stantially the steady-state bolted-fault value.)
4 Enable the system to continue operation with a single
line-to-ground fault present, in the same manner as an un-
grounded system. (The high-resistance grounding circuit is anexcellent damper of high-frequency transient oscillations so
that, in some cases, the ability to continue operation might be
enhanced.)
5) Provide a means for pulsing the current into the ground
fault, so that it can be traced to the point of fault. This pu
is accomplished without ever removing the high-resist
neutral grounding connection.
6) Provide a means for measuring the system-char
current so that the proper degree of minimum neutral gro
ing by high resistance can be accomplished.
System-charging current is the highly leading power
tor ground-fault current (on an ungrounded system) requ
to charge the capacitance of the other two phases to gro
The component of ground-fault current controlled by
high-resistance neutral ground must be slightly greater the system charging current. The reason for this, as well
complete explanation of the nature and causes of system
ervoltages can be found in the General Electric Indus
Power Systems Data Book [1].
Description of System
Figure 1. illustrates the manner in which groundin
accomplished. The system neutral is derived by three s
(3- to 10-kVA) transformers, connected wye-broken delt
shown. The primary neutral is grounded through a cur
transformer and ammeter, so that ground fault current cameasured. The secondary neutral is connected to a res
with taps, so that the proper resistance can be used to co
the current which will flow into a ground fault. The s
arrows show the path of the ground-fault current. Notice
the ground-fault current is equally divided in the three s
transformers, and that it also circulates through the delta
ply system. The arrows represent currents which are in ph
*U. S. patent pending; applied for by F. K. Fox for the
General Electric Company.
Fig. 1. Diagram of grounding method for system ha
ungrounded delta power supply of 24UO or 4,160 volts:
trol and protective circuits not shown.
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Francis K. Fox, Howard J. Grotts, and Clyde H. Tipton
IEEE Transactions on Industry and General Applications
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Corresponding currents in the secondary neutral circulate in the bro-
ken delta and through the resistor. This method is equivalent to losing
the delta and grounding the primary neutral through a high resistance.
Figure 2 presents the actual voltage conditions, both normally and
with a ground fault on the system. A typical 2400-volt power system
of around 5000 kVA may require 3 amperes, as shown, at the point of
fault. This would produce 1 ampere in each primary and, if the trans-
former ratio is 20 to 1, a current of 20 amperes through the resistor at
208 volts. This allows using a low-voltage resistor and pulsing contactor.
With no ground fault on the system, the voltage at the broken deltais zero. When a ground fault occurs, this voltage increases to a maxi-
mum of 208 volts, so that the voltage relay VR can give the alarm.
For the typical system shown, this is equivalent to grounding the
system neutral through a 460-ohm resistor. The voltage conditions
shown are for a solid ground fault but, even for a high-resistance fault,
enough voltage will appear across the resistor to operate the voltage
relay and sound an alarm. For example, if the entire ground-fault path
introduced 1000 ohms of resistance, the voltage across the resistor
would be around 55 volts. Relay VR can be set to pick up at approxi-
mately 16 volts, which would actually detect and give the alarm, even
if the incipient ground fault had a resistance of approximate 4000 ohms.
Locating the Fault
The operator then initiates a control circuit which causes the puls-
ing contactor to close approximately 40 times per minute to produce
current pulses of about a half-second duration. These pulses can be
traced to the point of fault, with the use of a hook-on ammeter, as
shown.
Several actual ground faults have been locate
this manner. Several ground faults have also been
liberately placed on systems of different types inder to check the ability of the system to follow t
pulses to the point of fault.
On systems involving bare overhead line
poles, tracing the signal is simple because all the
current is forced to return through ground. On
tems involving conduit, tracing the signal is ha
because the fault current tends to return through
conduit of the circuit involved. To the extent that
happens, the return current in the conduit cancel
the fault current flowing out through the conduct
the point of fault. Fortunately, even on all con
systems, this cancelling effect is not 100 percen
the hook-on ammeter is sensitive enough and if
insensitive to other local magnetic effects, the
can be located. The return current divides into str
unpredictable patterns and appears on conduits
metal structures not associated with the faulty cir
Also, these structures very often carry current at
utable to other causes not associated with the fau
any case, the definite rhythmic pulse of
ground-fault current is extremely helpful.
The signal receiver which has been found m
useful in tracing the pulsing ground-fault curre
the hook-on ammeter shown in Figs. 3 and 4.
device has a split core with a window large enoug
encircle a 5-inch conduit. The handle is insulated f
the core so that it can be used safely on power ca
of 2400- or 4160-volt systems which are not in
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Francis K. Fox, Howard J. Grotts, and Clyde H. Tipton
IEEE Transactions on Industry and General Applications
September/October 1965 E
duit. Several degrees of sensitivity are provided. The split core
completely encircles the conduit, or cables in air, thereby ig-
noring the effect of other local magnetic fields to a great ex-
tent.
Physical Equipment
Several types of power systems have been grounded in
the manner described. One of the first installations is shown
in Fig. 5. The power supply consists of a 3750-kVA 3-phase
2400-volt transformer. Various feeders to a large tank-farm
area originate in the switchgear in the background. Distribu-
tion is entirely by bare overhead lines on poles. The groun
ing equipment at this location is in four separate housin
consisting of a fused oil switch, a 3-phase oil-immers
grounding transformer, a relay and control panel, and a s
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Francis K. Fox, Howard J. Grotts, and Clyde H. Tipton
IEEE Transactions on Industry and General Applications
September/October 1965 E
erator was readily available through three single-pole
connects. Therefore, the grounding transformer had on
be a single-phase unit shown at the lower right. Both
transformer and resistor sections of this equipment meat bolted covers. This 2400-volt system was found to have
amperes charging current with a 13 000-kVA load. Th
sistor was set to produce 4.5 amperes into a ground f
with pulsation to 8.0 amperes while tracking the sign
locate the fault.
Operating Experience At Richmond Refinery
A pulsing ground-fault detector was installed on
2400-volt system at the Standard Oil Refinery, Richm
Calif., in December 1963. Its primary function was to the transient overvoltages, during a line-to-ground fau
the system. Its secondary function was to impress a puls
the fault current so that a portable signal detector coul
used to trace the pulse to the grounded conductor.
The 2.4-kV system consists of three turbine-driven
erators rated at 5000-kVA each, connected through reac
ondary resistor.
At another location, the same scheme has been used ex-
cept that a spare breaker in a switchgear line-up was substi-
tuted for the fused oil switch. This is a superior method but,
tracing easier.
A more compact single-enclosure construction is shownin Figs. 6 and 7. This installation is on a 4160-volt system
involving steam turbine-generators having a total capacity of
15 000-kVA and two 10 000-kVA transformer sources. The
fused disconnect switch and three dry-type grounding trans-
formers are mounted in the left-hand high-voltage section
which is padlocked closed. The low-voltage resistor, relays
and controls are all mounted in the right-hand section and are
readily accessible. Louvres are provided to ventilate the re-
sistor, which must dissipate approximately 15kW when a solid
ground fault occurs on the system. Normally, the resistor car-
ries no current. This 4160-volt system was found to have a
charging current of approximately 6.4 amperes with a totalload of 20 000-kVA. The resistor is set on the tap to produce
6.5 amperes into a ground fault. Pulsing to 9.0 amperes is
utilized when hunting for the fault.
Similar single-enclosure equipment is presented in Figs.
8 and 9. This installation also involves 15 000-kVA of steam
turbine-generation but, in this case, the neutral of each gen-
of course, the breaker costs more than the fused switch.
A total of five such installations are set on a resistor tap to
produce approximately 2.7 amperes into a ground fault, with
pulsation to 3.5 amperes while hunting for the fault. The puls-
ing current will probably be increased to make ground-fault
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Francis K. Fox, Howard J. Grotts, and Clyde H. Tipton
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TABLE I
Values of Actual Tests on 2400-Volt Systems
kVA
Total
Trans- (Motors*
Motors formers and Charging Charging
Running Connected Trans- Current Current,
System hp kVA formers) Measured A/MVA
Tidewater sub-
station 1 2700 4142 6842 2.55 0.373Tidewater sub-
station 2 4500 3000 7500 1.98 0.264
Tidewater sub-
station 7A 1900 1500 3400 0.42 0.124
Tidewater sub-
station 7B (1) 1900 1500 3400 0.29 0.085
Tidewater sub-
station 9 1225 1200 2425 0.34 0.140
Richmond
refinery
2400-
volt power
plant 11 525 11 925 23 450 3.6 0.154
*Assuming 1 hp = 1kVA for motors.
(1) This substation feeds no 2300-volt motors smaller than 200
hp, has only 675 feet of RL (5000 volts 3/C) cable total, 2 to 200 hp
and 1 to 1500 hp motors were running, and 2 to 750 kVA (three 2300/
440-volt) transformers were energized.
to three separate 2.4-kV busses. Each bus is conne
through a a reactor to a common synchronizing
From the three busses, thirteen radial feeders dis
ute power to load centers throughout the refinery
10).
Prior to installing the pulsing ground detector
neutrals of the generators were connected throu
resistor to ground. A 10/5-ampere current transfo
on the grounded line was connected to a recor
ammeter and an alarm panel. When the alarm soun
it was the signal for a long tedious search forgrounded conductor. First, each bus section wit
generator had to be separated from the grounding
tem to determine which bus section contained the f
After this had been established, feeders were p
leled and loads switched from one bus to the oth
order to determine which feeder contained the gro
fault. Next, the faulted feeder was paralleled
sectionalized until, finally, after several hour
switching and various manipulations, the locatio
the ground fault was narrowed down to a comp
tively short section of line supplying perhaps fivsix motors. The motors were then shut down, one
time, until the fault cleared The entire operation, f
start to finish, many times required as many as e
hours.
There have been two ground faults on the 2.4
system since the pulsing ground detector was insta
The first one occurred on August 7, 1964 about
P.M. The operating crew had witnessed a demon
tion of the use of the Pulsator but this was the
“real thing.” It required only 28 minutes from the
the alarm sounded to find and shut down the fa
motor. The elapsed time could have been consably less but, in this case, the fault happened to b
the No. 13 feeder, the last feeder to be checked,
was located quite a distance from the power plant
the end of the feeder.
The second ground fault occurred at 6:00 A
January 4, 1965. The operating crew at the power p
had not been on duty when the previous ground
developed. Therefore, this was also a “first” for th
In 18 minutes, the fault was traced to a forced-d
fan motor on the No. 6 station service feeder.
Thus far, the authors’ experience with the pu
ground detector has been extremely satisfactory
two ground faults at the Richmond Standard Oil
finery were located in minutes, rather than hou
was not necessary to shut down any motor-dr
equipment while searching for the fault; and equ
important, the hazard associated with hasty sw
ing was avoided.
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Francis K. Fox, Howard J. Grotts, and Clyde H. Tipton
IEEE Transactions on Industry and General Applications
September/October 1965
Operating Experience at Avon Refinery
The Tidewater Oil Company, Avon California Refinery, has
eight 2400-volt 3-phase delta systems. Each system is inde-
pendent of the others and is fed by its own 2400-volt substa-
tion. Each substation is located at a load center. Many of these
systems are now provided with the high-resistance grounding
scheme, with ground-fault alarms and the pulsing signal to fault,
as described in this paper. A red alarm light is provided above
each substation and an annunciator alarm panel has been in-
stalled in the refinery electric shop. When a ground fault takesplace, both the substation alarm light and the electric-shop an-
nunciator panel will light up. So far, this alarm system has
worked satisfactorily.
Prior to the installation of this high-resistance grounding
scheme, every time a “dreaded” ground fault arose, a great many
hours were required to isolate, locate, and repair the fault. On
a few occasions, the ground faults did not have to be tracked
down because the trouble was actually multiple simultaneous
failures.
The first grounding system was placed in operation on
October 24, 1962. The other grounding systems were installedin 1963. All the grounding systems have been in service for
about 1 1/2 years.
Four ground faults have occurred on systems involving
open-wire lines since these systems have been in operation.
The first one appeared when a potential transformer on the
2300-volt side of a 2300/440-volt 3-phase transformer bank
went to ground. This transformer bank is fed from an open-wire
overhead line by way of a pole riser from the main 2400-volt
substation switchgear through lead-covered cable in conduit
to the overhead line, then from the overhead line by way of
lead-covered cable in conduit to the transformer. The pulsing
current was 3 and 3.8 amperes. The faulty circuit was readilyfound at the substation. It was then a matter of checking for the
pulse current at each pole riser fed from the overhead-line cir-
cuit until the faulty riser and the trouble was found. All in all,
the actual “tracking down” took an hour or two. The second
ground fault occurred when the terminal box for a 2300-volt
800-hp motor filled with rain water. This motor was also fed
from an overhead line in much the same manner as was the
case with the first fault. Trouble 3 took place on the primary
side of a poIe-mounted series lighting transformer. Both faults
2 and 3 were tracked down with little difficulty. Trouble 4, also
found very quickly, was a ground fault in a current transformer
in one of the switch houses.
A fault-tracking experience which proved more difficult
was on an all-conduit lead-jacketed cable system with parallel
feeders serving a crude unit. The pulse intensities used for this
substation are 3 and 4 amperes. The fault was in
high-voltage terminal box of a lighting transformer. With
ground current dividing between the two parallel feeders,
a resulting pulse current in each feeder of 1 1/2 and 2
peres, and with much of the signal cancelled out by re
current on the conduit, it was not possible for company
sonnel to pick up the pulse with the nonhook-on-type si
receiver used. By a method of elimination, the trouble
narrowed down and the pulse was picked up in the con
feeding the faulty transformer.
System-Charging Current Data
Actual charging current seems to vary from one sy
to another. An actual test must be made on each system
that the lowest current tap of the resistor can be used.
test can most conveniently be made after the grounding
tem is installed; therefore, an estimated charging current
be ascertained at the time the equipment is purchased.
estimated charging current should be on the “high side.”
resistor tap must later be selected so that the ground cu
is slightly above the actual measured charging current2400-volt systems having an operating load of 15 000-k
and less, it has not been the authors’ experience to find a ch
ing current over 4 amperes. The one 4-kV system te
showed a charging current of 6.4 amperes with an opera
load of approximately 20 000 kVA.
The following values have been derived from the
presented in the table:
1) Overhead open-wire lines apparently have very
effect on charging current unless they are many thousan
feet in length.
2) VL or shielded cable appears to have the most e
(in the order of 0.4 A/1000 ft for 2400 volts and 0.7 A/10for 4-kV systems).
3) Non-shielded cables in conduit, transformers, and
tors also have some effect, in the order of:
Non-shielded cables .05 for 2400 V in Amps per 100
In conduit .08 for 4160 V in Amps per 100
Transformers 0.3 for 2400 V in Amps per 1000
.05 for 4160 V in Amps per 1000
Motors .05 to .10 for 2400 V Motors in Amps per 1000
.07 to .12 for 4000 V Motors in Amps per 1000
4) Aerial cable approximates the value of cable in
duct.
5) It is very important to include values for surge cap
tors connected to the motor terminals or at switchgear bu
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Francis K. Fox, Howard J. Grotts, and Clyde H. Tipton
IEEE Transactions on Industry and General Applications
September/October 1965
This equipment will add about 0.75 amperes at 2400 volts or
1.3 amperes at 4160 volts, for each set of surge capacitors used.
6) The authors wish to point out that these values were
derived from limited test data and, therefore, are only approxi-
mate. It should also be realized that capacitance values will
vary considerably with the wide ranges of cable, wire, motor,
and transformer sizes installed in the typical refinery.
7) The authors have noted recent published test data [3]
wherein system-charging current of several 2400-volt mining
power systems was found to be in the order of 1.0 A/MVA of
system capacity. This is much greater than the current foundon any systems with which the authors have worked and is
probably caused by the extensive use of lead-covered or
shielded cables used in underground mining operations and
also by the extensive use of surge capacitors at the terminals of
large 2300-volt motors.
References
(1) Industrial Power Systems Data Book. Schenectady, N.
Y.: General Electric Company, Sec 0.220 and App. C.
(2) D. L. Beeman, Power Systems Handbook. New York:
McGraw Hill, 1955.
(3) W. R. Duffy, “How the Anaconda Company protects
its system against ground faults,” Indus. Power Sys., March
1964.
(4) “Grounding of Industrial Power Systems,” AIEE Spe-
cial Publ. 953 (Green Book), October 1956, editorially re-
vised on September, 1960.
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