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A Ground Fault Protection Methodfor Ungrounded Systems

Louis V. Dusang, Jr.

Abstract This paper presents a novel approach tosimultaneous ground fault isolation for ungrounded powersystems. The concept capitalizes on current differential anddirectional overcurrent designs by considering the second groundfault on the system to prevent a phase-to-phase-to-ground fault.Supplying uninterrupted power to consumers is important.Ungrounded power systems have an advantage of ride-throughcapability during single phase-to-ground faults. It is desirableand important, to trip only the appropriate breakers duringfaults. While an ungrounded power system can remainoperational with a single phase-to-ground fault there arecircumstances when a major portion of the distribution systemshuts down upon a second ground fault on another phase

resulting in a phase-to-phase-to-ground fault. A patent pendingconcept exploits prior art designs universally regardless of thevarious relay manufacturers implementation methods.Combining prior art differential protection and ground faultdetection the invention minimizes breaker tripping by addressingmultiple ground faults.

Index Terms Differential protection, ground fault protection,ungrounded power system

I. I NTRODUCTION

For many commercial, industrial, and even residentialenvironments, power system reliability is of utmostimportance. In some manufacturing or textile environments, a

power outage can result in the loss of product in the production process when outage occurred. Further, poweroutages can result in down time for a facility, not only duringthe outage, but also due to production restarting undertakensubsequent to an outage. Losses of product and down timemay also lead to substantial monetary losses for a facility as aresult of the power outage. As such, facilities often takemeasures to improve or maximize power system reliability toavoid such losses.

Utilization of an ungrounded power system is a means ofimproving power system reliability; hence, some textile andindustrial facilities, as well as US Navy ships operate on an

ungrounded distribution system. This paper presents atechnique of determining two unique single phase-to-groundfaults on different line section phases associated withungrounded power systems, causing a double line-to-groundfault, and isolating one of the faults.

_____________________________

Louis Dusang is with Northrop Grumman Shipbuilding, Pascagoula, MS39567 USA (phone: 228-935-2451; email: louis.dusang@ngc.com)

There are two types of distribution systems, grounded orungrounded. These systems are typically derived from a wye-connection or delta-connection. A wye-connected may ormay not utilize the neutral for grounding. A delta-connectedhas no neutral. Since the delta-connected system does nothave a neutral it is an ungrounded system. To ground anungrounded system one generally employs a resistance toground apparatus.

High resistance to ground systems like ungrounded systems permit continued power system operation under single-line-to-ground (SLG) fault conditions. It requires another SLG fault

on a phase other than the one faulted to trip the circuit protecting device. This provides an opportunity to clear theSLG fault without shutting down the system such that the enduser may never no there was a problem The purpose of thehigh impedance grounding system is to lower the fault currentand limit overvoltage transients.

There are no intentional ground paths for ungroundedsystems. Ungrounded systems do have a stray capacitance toground path for current to flow. The impedance for such asystem should be equal to or slightly less than capacitivereactance to ground. The reason for this is to increase theamperage slightly to a value that can be read.

II. U NGROUNDED OR ISOLATED NEUTRAL POWER SYSTEMBACKGROUND

System grounding minimizes voltage and thermal stresses, provides personnel safety, and assists in rapid detection andremoval of ground faults [1]. Operating a power systemungrounded limits ground fault current, but does not minimizevoltage stress. Additionally, locating ground faults on anungrounded power system is difficult.

The advantage of a solidly or low grounded systems, likemost in the U.S., over ungrounded power systems is that theyreduce overvoltage, but not to the extent of permitting un-interrupted service. Phase-to-ground faults on solidly or low

impedance grounded systems must be cleared immediately toavoid thermal stress and human safety hazards.Ungrounded systems (see Fig. 1) have no intentional

ground connections. The system is connected to groundthrough parasitic capacitance, the line-to-ground capacitance(CAG , CBG , and C CG). Additionally, there is distributedcapacitance to ground for the transformers and feederconductors, and phase-to-phase capacitances which are notrepresented. In both delta- and wye-configurations, loads areconnected ungrounded phase-to-phase; therefore, thedistributed capacitance to ground forms the unintentional

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ground. The advantage of an ungrounded power system isthat for a single-phase-to-ground (closing Switch S of Fig. 1)fault, the voltage triangle (see Fig 2) remains intact andtherefore loads can remain in service. When a SLG faultoccurs, the faulted phase potential decreases to near zero andthe healthy phases increase by a factor of 1.73. At the sametime, the zero-sequence voltage increases to three times thenormal phase-to-ground voltage. Fig 2 demonstrates these

two conditions. Fig 2a shows an unfaulted, ungroundedsystem. Fig 2b shows how the voltage triangle shifts relativeto ground for an A-phase-to-ground fault.

A

B

C

G

CB

C CC A

R

R

R

S

a) Delta-configuration

A

B

C

G

CB

C CC A

R

R

R

N

S

b) Wye-configuration

Fig. 1. Three-Phase Ungrounded Systems

Fig 2. Voltage Triangle (a) Unfaulted System.(b) Faulted System (Solid A-Phase Fault, RF = 0)

The major factors in determining the magnitude of groundfault current in ungrounded power systems are the groundreturn impedance (zero-sequence line-to-ground impedance)and fault resistance [2], [3]. Since loads are connected phase-to-phase and there is no return to ground they do not generateany zero-sequence current.

Ground faults in ungrounded systems utilize zero-sequencecurrent or three-phase voltage measurements [4]. While this

method, detects a fault it does not locate the fault [5], [6]. Thetraditional method for locating single-phase-to-ground faultswas to disconnect a bus-tie feeder and determine if the zero-sequence voltage decreased to its prefault value. Since faultcurrent can flow in either direction, forward or reverse,modern relays incorporate a directional element to isolate thedetected ground fault.

III. TWO -TERMINAL U NIT PROTECTION OF U NGROUNDEDPOWER LINES

Pilot channel relaying for two-terminal lines requires arelay at each line end and a communication circuit connected

between the relays (see Fig. 3) [1], [7]. The five commontypes of communication channels for pilot relaying are audiotones over leased circuits, microwave or power-line carrier(SSB), power-line carrier by itself, metallic wire pairs, RS-232or n x 64 kb digital and fiber optics. Securely transferring atrip, block or trip permission signal to the opposite end of the

protected line is the premise of pilot channel relaying [8].Pilot wire usage provides high-speed differential anddirectional signal capabilities.

Fig. 3. Schematic Diagram of Modern Differential and DirectionalComparison System

Differential relays operate on a current summing principlethat is the current flowing into a protected circuit zone equalsthe current flowing out yielding no differential current on a

per-phase basis. When a fault occurs within the protectedcircuit zone, part of the current flows into the fault such thatthe current flowing in no longer equals the current flowing outthe circuit zone. While a differential current flows in therelay, it does not assert unless the current is above a presetvalue.

Directional overcurrent relay consist of a non-directionalovercurrent element in conjunction with a directional function.Directional overcurrent relays provide sensitive tripping forfault currents in forward direction, but not in the reversedirection. Directional elements compare the current flow atthe terminals. Current flows into the line at the terminals forinternal faults in which the relay sends a trip signal to thecircuit breaker. Current flows outward at the terminals for an

external fault and utilizing a blocking signal inhibits thesending of an assert signal to the breakers.A relay may contain backup fault detection to the

differential fault detection. If the differential element or fiberoptic is damaged, the associated relay will not receive remotecurrent information; therefore, a blocking scheme,incorporated into each relay takes appropriate action. Whiledifferential relay action is without intentional delay, the

blocking scheme does include a short coordination time delay.The backup fault detection is typically based on the phase

directional elements (67). The 67P only detects multi-phase

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faults while the 67N element detects phase-ground faults.Both elements are necessary to detect all fault types becauseof the difference in pickup and sensitivity levels. The 67Pelements operate from phase currents and the 67N elementsoperate from the current delivered by the core-flux summing

current transformers. A core flux summing transformer orzero sequence CT encircles all phase conductors and senses

phase current imbalances. Core flux summing transformersconsist of a secondary winding isolated from the core withouta primary winding.

5 4

6

3 1

APS1

2

APS2DG1

DG3DG5

DG4 DG2

DG6

7HB 5HB

5SG

4SG

2SG

3SG

4HB 1HB

1SG6SG

6HA 5HA 4HA 2HA

SHORE

POWER

SHORE

POWER

F1

GROUNDING BANK 4.16kV/460 BANKDIESEL GENERATOR ON-LINE CIRCUIT BREAKER (CLOSED)

CIRCUIT BREAKER (OPEN)DIESEL GENERATOR OFF-LINE

Fig. 4. Example Power System Single-Line Diagram

For the 3-phase fault shown in Fig. 4 (F1 in the Figure), thedesired result is that only Breakers 1 and 2 open. The arrowsshown near Breakers 1, 3 6 indicate the direction of faultcurrent flow. If we were to consider time-overcurrent

protection, we would need to review instantaneous (ANSI 50)

and inverse-time elements (ANSI 51).The arrows in Fig. 4 for Breakers 1, 3 6 indicate the

direction of fault current flow for Fault F1. For those breakerswhere the arrow direct is away from the bus, a phasedirectional element would declare a forward direction fault.Conversely, if the arrow direction were into the local bus, the

phase directional element would declare a reverse directionfault. What would happen if we required forward directiondeclaration by a directional element before allowing the 51element at that same breaker to begin operating? Breakers 1 6 would then include directional protection, and, onlyBreakers 1, 2, 4 and 6 would declare a forward direction fault.(Again note that Breakers 1 and 2 are protecting the faultedline and both declare a forward fault. For all other linesections, only one line terminal directional element declaresthe fault direction as forward.) To achieve selectivity betweenthe protective relays at Breakers 1, 4 and 6, these relays mustsense different magnitudes of fault current.

The system configuration shown in Fig. 4 is such thatBreakers 1, 3, 4, 5 and 6 all sense the same magnitude of faultcurrent. The phase current waveforms flow through Breakers1, 3 6 for Fault F1. It should be noted that the load and faultcurrent waveforms are 180 out-of-phase for Breakers 3 and 4,and Breakers 5 and 6. This is a strong indicator that a line

current differential element scheme applied to these lineswould properly restrain.

IV. SIMULTANEOUS GROUND FAULT LOGIC DESCRIPTION

The zero-sequence component is the primary means todetect and clear phase-to-ground faults. Ungrounded systems

produce very little phase-to-ground fault current.Continued service is possible in ungrounded power systems

under SLG fault conditions. An A-phase-to ground faultalone on 1HA-2HA line as seen in Fig. 5a and Fig. 5b will notresult in tripping of circuit breakers for the ungrounded powersystem. The same is true of a single B-phase-to ground fault.However, if the B-phase-to-ground occurs before clearing theA-phase ground fault the zero-sequence component cannotdetect the resulting phase-to-phase fault. For the radialconfiguration shown in Fig. 5a, the relays associated with

busses 1HA-2HA, 2HA-3HA, and 3HA-4HA will send an

assert signal to their respective breakers resulting indifferential element trip for the phase-to-phase fault. In thiscase the power transformers 2HA and 3HA will have no

power source. Fig. 5b is similar, but a relay racingcondition exists as to which breakers will trip since connectedin a ring bus configuration. In the ring bus configuration case,it is possible that more of the system is shutdown.

Protection against second ground faults is possible whenutilizing a zero-sequence current sensor at both cable ends ofeach bus-tie. In general, time delays are inefficient in that itdelays the differential current trip thereby defeating the high-

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speed pilot function and may cause excessive damage during phase-to-phase fault and short circuit conditions. The proposed concept essentially blocks the differential elementsfrom asserting under ground fault conditions. Utilizing aSupervisory Control And Data Acquisition (SCADA) systemone bus-tie can be isolated in either case minimizingtransformer loss. In the case of Fig. 5, bus-tie 2HA-3HA canopen without removing a fault while maintaining power to all

power transformers.

a) Radial System

X

A-Phase Fault

X

B-Phase Fault

1G

1HA 2HA 3HA 4HA

2G

1HB 2HB 3HB 4HB

b) Ring Bus System

Fig. 5. Power Distribution System

Relay manufacturers have various ways of implementingdifferential and directional protection. Although the methodsdiffer between relay manufacturers the principles are similar.Fig. 6a depicts an oversimplified rendition of present daydifferential protection (analyzing each phase for a differencein current via OR logic) and Fig. 6b depicts an oversimplifiedversion of directional overcurrent protection. For currentdifferential protection if B-phase and C-phase have no

differential current flow, but current flows through thedifferential relay in A-phase the differential relay will assert ifthe value exceeds the pickup setting. Operation of the groundrelay is based on detection of a ground fault without isolatingthe fault condition.

a) Differential Scheme

b) Ground Directional Overcurrent Scheme

Fig. 6. Simplified Fault Protection Logic

V. SIMULTANEOUS GROUND FAULT PROTECTION

Present day differential protection typically operates on a per phase basis. This design is suitable for both grounded andungrounded power systems. However, for ungroundedsystems a single phase fault will not result in a trip unless therelay setting is set such that it results in a signal to trip the

breaker. Employing a phase-to-phase design (see Fig. 7) suchas in the patent pending approach provides for single phase

fault isolation.

Fig. 7. Simplified Patent Pending Protection

The patent does not modify the primary differential,directional or ground fault detection schemes developed by thevarious manufacturers in which CT saturation and otherfactors are considered by the relay manufacturers design.The patent enhances each of these schemes by includingadditional logic to address the second ground fault. Utilizingexisting ground fault detection technology we block thedifferential elements when a ground fault is present on the

power system to prevent tripping when the second groundfault occurs.

To examine the relationship of zero-sequence voltage andcurrent and differential protection the system was modeled asa simple two line ungrounded system connected to two buseswith a source connected to each bus as shown in Fig. 8.

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Fig. 8. Power System Configuration

As stated earlier, each manufacturers relay can apply the patent pending simultaneous ground fault protection.Evaluating the new concept consisted of running scenarios on

power system configuration of Fig. 8 and comparing theresults with present day differential and ground fault

protection with simultaneous ground fault protection, seeTable I.

TABLE IFAULT PROTECTION TEST CASES

TestCase

Scenario Non-Simultaneous GroundFault Protection

Simultaneous GroundFault Protection

1 SLG fault on Line 1 Relay 1 and Relay 2 faultdetection via zero-sequence

Same

2 Phase-to-phase-to-ground fault,Line 1

Relay 1 and Relay 2 isolate faultvia differential protection

Same

3 Phase-to-phase fault on Line 1 Relay 1 and Relay 2 isolate faultvia differential protection

Same

4 Three-phase-to-ground fault onLine 1

Relay 1 and Relay 2 isolate faultvia differential protection

Same

5 SLG fault (A ), Line 1SLG fault (C ) w/ delay, Line 1

Relay 1 and Relay 2 isolate faultvia differential protection

Relay 1 and Relay 2isolate fault via newconcept

6 SLG fault (A ), Line 1SLG fault (C ) w/ delay, Line 2

All relays assert isolating bothfaults via differential protection

Selectively isolate eitherLine 1 or Line 2

Cases 1-5 deal with one line in which fault isolation issimilar regardless of protection scheme applied. Thedifference lies in Case 5. If the SLG fault occurs on a bus andevolves into a double line-to-ground fault, the same relaysassert similar to a differential trip except with a delay. Thistime delay acts as a differential element block while

permitting fault isolation via SCADA system corresponding tothe simultaneous ground fault protection scheme.

Case 6 affects both lines. Here is where the real benefit ofthe concept becomes apparent. The per phase differential

protection approach results in each relay sending a tr ip signalto open all breakers. However, when implementing a two-

phase scheme, blocking differential elements provides time toisolate one of the SLG faults. In other words, with thedifferential phase components blocked, the second groundfault utilizing a SCADA signal isolates one SLG fault.Obviously, only two breakers trip versus four. In the event noSLG fault is cleared a timer will time out tripping theapplicable busses, which operates as a backup.

The advantage in both Case 5 and Case 6 is it providestime to address the ground fault with continued differential protection.

For larger power systems as in Figure 4 when the secondground fault occurs on a different bus (Case 6), the differentialcurrent while remaining internal between the two outermostrelays covers more lines. In Figure 4a, we have three, twoterminal, busses: 1HA-2HA, 2HA-3HA and 3HA-4HA.Implementing a timer in the scheme addresses multiple bussituations.

Personnel may be addressing the first ground faultoccurrence when the second ground fault occurs. Under theseconditions it may be unsafe to trip the breakers. Dependingon whether the power system configuration is ring bus orradial depends on how the two faults will be isolated. Ineither case the option may be to isolate the second faultoccurrence. This option can be safe for personnel andmaximize power system reliability.

VI. CONCLUSIONS

Power system reliability is important for many commercial,industrial, and even residential environments. As such, this

paper introduces a technique of determining two uniquesingle-phase-to-ground faults, creating a double-line-to-ground fault, and isolating one of the faults. The concept isapplicable to ungrounded and high impedance groundedsystems. Additionally, the new scheme capitalizes on currentdifferential, directional overcurrent and ground fault

protection.

High-speed fault clearing is maintained for phase-to-phase, phase-to-phase-to-ground and three-phase faults. Although adelay is added when a second SLG fault occurs on the system,it is insignificant in that it is similar to protection withoutcommunications. Additionally, it enhances power system

performance and reliability.

VII. ACKNOWLEDGEMENT

The author acknowledges the co-inventor, Jeff Roberts, forhis contributions.

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VIII. R EFERENCES [1] IEEE Recommended Practice for Protection and Coordination of

Industrial and Commercial Power Systems, IEEE Std 242-2001[2] Cooper Bussmann Inc, Overcurrent protection and the 2002 National

Electrical Code questions & answers to help you comply, On-lineTraining, March 2002, [Online]. Available:http://www.bussmann.com/library/docs/NE02.pdf

[3] J. Roberts, H.J. Altuve and D. Hou, "Review of ground fault protectionmethods for grounded, ungrounded and compensated distribution

systems," presented at the 28th Annual Western Protective Relay Conf.,Spokane, Washington, October 23-25, 2001. [Online]. Available:http://www.selinc.com/techpprs/6123.pdf

[4] A. A. Regotti and H. W. Wargo, Ground-fault protection and detectionfor industrial and commercial distribution systems, WestinghouseEngineer, pp. 80-83, July 1974.

[5] D. J. Love and N. Hashemi, Considerations for ground fault protectionin medium voltage industrial and cogeneration systems, IEEE Trans.Ind. Applications., Vol. 24, pp 548-553, July/Aug. 1988.

[6] T. Baldwin, F. Renovich, and L. F. Saunders Directional ground-faultindicator for high-resistance grounded systems, IEEE Trans. Ind.Application, Vol. 39, No. 2, pp. 325-332, March/April 2003

[7] J. Roberts, D. Tziouvaras, G. Benmouyal, and H. J. Altuve, The Effectof multiprinciple line protection on dependability and security,

presented at the 28th Annual Western Protective Relay Conf., Spokane,

Washington, October 23-25, 2001. [Online]. Available:http://www.selinc.com/techpprs/6109-Paper-WPRC.pdf [8] J. Benckenstein, System reliability improvements through fiber optic

systems, Pulsar Technical Publication FD45VER01, March 2001,[Online]. Available: http://www.pulsartech.com/pulsartech/docs/FD45-VER01.pdf

IX. BIBLIOGRAPHY Louis Dusang received a Bachelor of Science in Electrical Engineering

degree from Mississippi State University in 1988 and is pursuing his MSEE atthe University of Idaho. He is a Registered Engineer in South Carolina. Hehas been an electrical engineer with Northrop Grumman Shipbuilding since

November 2001. He is the lead project engineer for LDA Power Systems.Prior to joining NGSB, Mr. Dusang worked as both an electrical engineer andcontrols engineer for Jacobs.