High Resistance Grounding Tech Paper

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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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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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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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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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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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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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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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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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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Documents

High Resistance Grounding Tech Paper