considerations for generator ground-fault protection in midsize cogeneration plants

628 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 33, NO. 3, MAY/JUNE 1997

Considerations for Generator Ground-FaultProtection in Midsize Cogeneration Plants

Rasheek M. Rifaat,Senior Member, IEEE

Abstract—Generator ground-fault protection aspects are ana-lyzed for midsize cogeneration plants. In these plants, the gen-erators may be connected to a common generator bus. Theoverall bus-connected system should be considered when apply-ing generator high-resistance grounding schemes. Calculationsare reviewed for sizing generator neutral-ground resistance andevaluating third harmonic originated in the scheme. The relevantapplications of third harmonics in applying 100% stator ground-fault protection schemes are examined. Practical considerationsare presented for establishing selectivity of ground-fault protec-tion schemes in the case of common generator bus.

Index Terms—High-resistance grounding, stator protection,third harmonic.

I. INTRODUCTION

RECENT advances in overall generator protection systemshave been noted [4], [15], especially in the areas of

developing new protection schemes and the application ofdigital processors to relay hardware. Adaptation of true numer-ical relays is leading to improvement in the performance andeconomics of relay applications. As a result, comprehensivegenerator protection schemes are finding applications in co-generation plants. The development and maturity of the stator100% ground-fault protection scheme is a relevant example.The correct applications of such comprehensive schemes tocertain cogenerating plant configurations represent notableengineering challenges.

In power systems, the chances of line-to-ground-fault oc-currences are higher than two-phase and three-phase faults.Accordingly, generator ground-fault protection schemes aregiven special attention in cogeneration plants. This paper dis-cusses critical considerations when applying generator ground-fault protection schemes in midsize cogeneration plants. Theeffects of plant electrical system configuration and the overallplant operating duties on selecting and setting ground-faultprotection systems are also discussed. The cases discussed arebased on typical engineering design experience for 100-MWcogeneration plants.

Paper ICPSD 95–23, approved by the Power Systems Protection Committeeof the IEEE Industry Applications Society for presentation at the 1995IEEE/IAS Industrial and Commercial Power Systems Technical Conference,San Antonio, TX, May 7–11. Manuscript released for publication November20, 1996.

The author is with Delta Hudson Engineering Ltd., Calgary, Alta., T2H2N7, Canada.

Publisher Item Identifier S 0093-9994(97)03621-9.

II. COGENERATION PLANT CONFIGURATIONS

Capacities in the range of 25–100 MW are typical formidsize cogeneration plants. Many of these plants are multiple-generator facilities. An individual generator output will be inthe range of 15–60 MVA. Generators are driven by primemovers, such as gas or steam turbines, in simple or combinedcycle arrangements. Economic justifications for most of theseplants are based on continuous duty operation (base load) atnominal outputs.

Configurations of main output systems for cogenerationplants can be different from those of traditional thermalpower plants. Large generators in thermal power plants aretypically arranged in a dedicated generator–transformer ar-rangement [Fig. 1(a)]. In midsize cogeneration plants, a com-mon generator-bus/common-transformer configuration may beused [Fig. 1(b)]. Connecting more than one generator and anumber of loads to a common bus introduces additional factorswhich should be considered when devising the ground-faultprotection system.

III. H IGH-RESISTANCE NEUTRAL GROUNDING SCHEMES

High-resistance grounding of the generator neutral is ex-panding to be a preferred scheme for midsize generators [7],[16]. The scheme confines phase-to-ground-fault currents toa maximum value in the range of 3–25 A, limiting associ-ated damages. High-resistance grounding can be achieved byinsertion of a resistance between the generator neutral andground. Alternatively, an endorsed scheme is composed of adistribution transformer and loading resistance. The primaryside of the distribution transformer is connected betweenthe generator neutral and ground. The loading resistance isconnected to the secondary side of the distribution transformer.The equivalent resistance reflected at the generator neutralequals the value of loading resistance multiplied by the squareof the distribution transformer turns ratio. (Fig. 2). The latterscheme allows using a resistance at lower voltage, reducingits insulation requirements.

IV. NEUTRAL RESISTANCE CALCULATION

Ideally, the neutral resistance shall be as large as possible, tominimize the line-to-ground-fault current. However, maximumresistance size is dictated by the capacitive charging currentsof the electrical system connected to the generator. To avoidthe buildup of destructive overvoltage caused by spitting orrestriking phenomena during a single phase-to-ground fault,

0093–9994/97$10.00 1997 IEEE

RIFAAT: CONSIDERATIONS FOR GENERATOR GROUND-FAULT PROTECTION 629

(a)

(b)

Fig. 1. Typical plant bus configuration. (a) Dedicated bus/transformer usedin large thermal generating plant. (b) Common bus/transformer used incogeneration plants.

the resistive fault current shall be larger than the capaci-tive current [6], [7], [9]. Accordingly, the equivalent neutralgrounding resistance shall be less than the system relevantcapacitive reactance. In a three-phase system , and

(a)

(b)

Fig. 2. High-resistance grounding schemes. (a) Using resistors. (b) Usingdistribution transformer and loading resistor.

Fig. 3. Fault current distribution.

phases) with phase grounded, the total charging currentwill be (Fig. 3)

Charging Current

(1)

where

-phase capacitive current;-phase capacitive current;

line-to-line voltage;line-to-ground voltage;line-to-ground charging capacitance;line-to-ground charging current;

(2)

since

(3)


Fig. 4. An example of system charging-current capacitance in a common bus configuration (single-line diagram).

and

(4)

Therefore,

(5)

To calculate the charging capacitances of systemcomponents should be identified. Information on the gener-ator stator capacitances and surge-protection capacitor can beacquired from the generator supplier. At the early design stage,the charging current capacitances for other system componentscan be estimated from available references. A practical methodto evaluate capacitances of typical system components isdepicted in [7]. Fig. 4 represents typical system charging-current capacitances. An example of numerical values of thesecapacitances may be as follows:

generator 1 F

generator 2 F

surge capacitor 1 F

surge capacitor 2 F

transformer 1 F

transformer 2 F

transformer 3 F

switchgear F

gen. 1 cables F

gen. 2 cables F

trans. 1 cables F

trans. 2 cables F

trans. 3 cables F

Component phase-to-ground capacitances can then belumped by direct addition (parallel capacitances), withresultant capacitance of 1.2536F. The equivalent and

can be calculated as

(6)

(7)

The minimum value of ground-fault current to be used canbe calculated by dividing the line-to-neutral voltage (in thisexample, 8000 V/ A). It may be observed thatconnecting more components to the generator common buswill result in decreasing generator neutral resistance valueand increasing allowable resistive fault current. In the aboveexample, considering the resistive current contributions fromonly one generator is a conservative assumption. The actual re-sistive fault current will be contributed from the two connectedgenerators, as they are both equipped with neutral resistanceschemes. In asimplified calculation, the two generator neutral


(a)

(b)

Fig. 5. Third-harmonic current distribution. (a) Common neutral resistance.(b) Separate neutral resistance.

resistances can be assumed to be in parallel. The equivalenttotal resistance is represented as

(8)

If the two generator neutral resistances are equal, the totalresistive current will be doubled. A substantial increase in thetotal fault current increases fault effect (damage) and defeatsthe original purpose of high-resistance grounding.

V. THIRD-HARMONIC CIRCULATION

Modern, high-efficiency generators are fabricated with anarmature winding pitch factor close to 5/6. These generatorsconstitute third-harmonic voltage sources stronger than thoseconstituted by generators with an armature winding pitchfactor near 2/3. Precautions should be taken to avoid highthird-harmonic circulation between two generators connectedto a common generator bus. Fig. 5(a) shows that the use of

Fig. 6. Generator ground-fault protection.

one common neutral ground for the two generators will allowhigh third-harmonic current circulation. In contrast, having aseparate high neutral ground resistance for each generator addshigh resistance in the pass of the third-harmonic current[Fig. 5(b)], reducing such currents and their heating effects.

VI. GENERATOR GROUND-FAULT PROTECTION

Fig. 6 depicts a single phase-to-ground fault occurring ina generator winding. Fault-current circulation in the neutralresistance will introduce corresponding voltage on the neutralresistance. The fault current decreases as the fault movestoward the neutral end of the generator, vanishing when afault occurs at the neutral bus. An overvoltage relay acrossthe neutral resistance will detect a line-to-ground fault in themachine, as long as that voltage exceeds the relay threshold.For practical considerations, the relay setting will allow pro-tection coverage of up to 90%–95% of the winding, leaving5%–10% of the winding on the neutral side uncovered.

A near neutral fault current is relatively small. Its damagingeffects are also small with respect to high current faults.However, identification of the fault will prevent more seriousdamage, as in the case when a second fault develops in thesystem. Different schemes have been developed to cover theremaining 10% of the winding for a single phase-to-groundfault and also to detect open circuit in neutral groundingconnections. Two of these schemes are well known: the sub-harmonic injection scheme and the third-harmonic detectionscheme. The third-harmonic scheme is more economical and,hence, may be justifiable in some cogeneration cases. Thedebate continues about the type and application of third-harmonic schemes in cogeneration plants. This paper is notintended to referee different relays and types. Instead, itpresents some considerations that should be addressed whenapplying the third-harmonic scheme to common generator-busconfigurations.

The theory of relay operation is based on detecting third-harmonics voltages originating from the generator. The systemshown in Fig. 4 will be considered, with only generator#1 operating (generator #2 breaker open). Fig. 7 depictsthe reduction of generator and system capacitance and neu-tral resistance into two impedance components, one on thegenerator-terminal side and the second at the neutralside The capacitance at the generator terminal side willequal the total of those of surge capacitor cables


Fig. 7. Generator third-harmonic voltage distribution.

switchgear transformers and 1/2generator #1 capacitance (0.5 for a total of 0.5486

F for this example. Assuming a selected neutral resistanceof a value of 740 referred to the primary side of the

neutral-distribution transformer, the corresponding calculatedimpedances are as follows:

(9)

(10)

angle (11)

In this example, generator third-harmonic voltage willbe assumed to be 160 V (given by generator manufacturerupon request). Such a voltage shall be divided between theimpedances on the line and neutral sides of the generator. Thethird-harmonic voltage at the neutral side will then be

V (12)

The neutral third-harmonic voltage is then reflected to thesecondary side of the neutral distribution transformer. Assum-ing a transformer ratio of 8400–240 V, will be reflectedat the secondary side of the distribution transformer as 1.86V. The undervoltage relay is normally set at less than thecalculated value (typically, about 50% of calculated value, i.e.,in this example, it may set at 0.93 V). The relay manufacturerwill normally provide some recommendations in this regard.The relay is generally equipped with necessary filters, to filterout fundamental and harmonics other than third harmonic.Considering the filter attenuation for the fundamental, theoverall scheme performance can be evaluated.

The connection of two generators to a common bus involvesmore elaborate calculations. Such calculations are the subjectof future work. However, to illustrate the concerns that shall beaddressed, Fig. 8 depicts a simplified impedance circuit where

the two generators are included. The following concerns needto be addressed.

1) The capacitance and neutral ground-resistance values ofeach generator and their effects on the third-harmonicvoltage distributions in and must be con-sidered.

2) Does the protection cover all possible operating cases?These cases include the states when two generatorsconnected to the bus are running, or only one generatoris running and the second is down. (Overall systemground-fault protection shall also address the case whenno generator is connected to the bus at the plant start-up)

3) Third-harmonic voltages vary with changes in eachgenerator real and reactive power output conditions [5],[12], [13]. Maximum third harmonic could be typicallyin the order of 2%–9% of the fundamental phase-to-ground voltage. On the other hand, minimum third-harmonic voltage could be about 50% of the maximum,with some cases where the minimum is a mere fractionof a percentage of the fundamental. In most cases whenthe ratio between the minimum and the maximum isin the order of 50%, the relay can be set to cover thebalance of the stator winding under extreme values,without false tripping.

4) Some schemes [5] claim to overcome the above problemin 3) by measurement of the third-harmonic voltages atboth sides of the generator, which could prove to bemore important in the case of two generators connectedto the same bus.

VII. SELECTIVITY OF GROUND-FAULT PROTECTION

FOR COMMON GENERATOR-BUS CONFIGURATION

Typical protection zones are shown in Fig. 9. Ideally, a faultwithin a generator zone will result in tripping of the relevantgenerator or zone breaker, leaving the balance of the systemuninterrupted.


Fig. 8. Two generators third-harmonic distribution.

Fig. 9. Common bus/transformer used in cogeneration plants.

In a high-resistance grounding scheme, locating a phase-to-ground fault may be a challenging task. A simple over-voltage ground-fault relay (59G) mounted on the generatorneutral loading resistance, will be sensitive to faults fromapproximately 10% of the respected machine neutral, past thegenerator terminal, through the bus, up to all connected trans-formers, and into the feeders of other generators connected tothe same bus. Fault discrimination by current values cannot beapplied in this case. Also, discrimination by time intervals isnot applicable in its mere radial sense, as the feeders are notin radial arrangements. Using restricted ground-fault protectionor directional ground-fault protection may provide a solution.However, certain verifications are required.

When applying a restricted ground-fault protection, asshown in Fig. 10, practical limitations emerge. The line-sidecurrent transformers should be designed to carry generatorfull-load current. For example, a generator of40-MVA outputat 13.8 kV will probably have line-current transformers witha ratio of 2000-5 A or 400:1. With a phase-to-ground faultcurrent in the order of15 A, the corresponding CT secondaryground fault will be37.5 mA, or 0.75% of the full range ofthe line-side current transformers circuit. Although such a lowcurrent may satisfy some modern relay operating ranges, the

Fig. 10. Restricted ground-fault protection.

sensitivity of current transformers and their circuits should bechecked. Also, using zero-sequence current sensors may notbe practical in certain bus arrangements. However, if such ascheme can be implemented on one generator, time steppingcan coordinate the rest of the system components. Issues of asimilar nature may be encountered when applying directionalground-fault relay schemes.

VIII. C ONCLUSIONS

Elaborate considerations may be required when devisingand setting ground-fault protection schemes in midsize co-generation plants. Design calculations for such schemes aredependent on the plant main output configurations and theirelectrical parameters. When applying schemes such as third-harmonic generator-stator 100% ground-fault protection, con-sideration should be given to both machine and system extremeconditions. Selectivity may be established by schemes suchas restricted ground fault or directional ground overcurrent.However, prudent examinations should be applied to sizingthe current transformers serving such schemes.


Future work is anticipated to model in detail third-harmoniccircuits for multiple generators connected to the same bus.The objective of such a work will be to establish a calculationprocedure for setting third-harmonic relays and checking theirperformance.

ACKNOWLEDGMENT

The author wishes to thank A. Childs of Delta Catalytic forhis valuable editorial assistance and critique.

REFERENCES

[1] Protective Relaying for Power System, S. H. Horowitz, Ed. New York:Wiley, 1980.

[2] F. A. Woodbury, “Grounding considerations in cogeneration,”IEEETrans. Ind. Applicat.,vol. IA-21, pp. 1523–1532, Nov./Dec. 1985,

[3] D. J. Love and N. Hashemi, “Considerations for Ground Fault Protectionin Medium-Voltage Industrial and Cogeneration Systems,”IEEE Trans.Ind. Applicat.,vol. IA-24, pp. 548–553, July/Aug. 1988.

[4] R. M. Rifaat, “Protection scheme considerations for common generatorbus configurations used in cogeneration plants,” presented at the IEEEWESCANEX Conf., Saskatoon, Sask., Canada, 1993.

[5] R. G. Marttila, “A new generator stator ground relay for 100% cov-erage of the stator winding,” presented at the Pennsylvania ElectricAssociation Spring Meeting, Johnstown, PA, May 21–22, 1987.

[6] J. R. Dunki-Jacobs, “The reality of high resistance grounding,”IEEETrans. Ind. Applicat.,vol. IA-13, pp. 469–475, Sept./Oct. 1977.

[7] D. S. Baker, “Charging current data for guesswork-free design of highresistance grounded system,”IEEE Trans. Ind. Applicat.,vol. IA-15,pp. 136–140, Mar./Apr. 1977.

[8] M. J. Rook, L. E. Goff, G. J. Potochney, and L. J. Powell, “Applicationsof protective relays on large industrial-utility tie with industrial cogen-eration,” IEEE Trans. Power App. Syst.,vol. PAS-100, pp. 2804–2812,June 1981.

[9] J. Bermann, A. Kripsky, and M. Skalka, “Protection of large alternatorsconnected to step-up transformers against the consequences of earthfaults in the stator winding,” presented at the Int. Conf. Large HighTension Electric Systems (CIGRE), Paris, France, Aug. 28–Sept. 6,1972.

[10] L. Pazmandi, “Stator earth-leakage protection for large generators,” pre-sented at the Int. Conf. Large High Tension Electric Systems (CIGRE),Paris, France, Aug. 28–Sept. 6, 1972.

[11] R. L. Schlake, G. W. Buckly, G. McPherson, “Performance of third har-monic ground fault protection schemes for generator stator windings,”IEEE Trans. Power App. Syst.,vol. PAS-100, pp. 3195–3202, July 1981.

[12] J. W. Pope, “A comparison of 100% stator ground fault protectionschemes for generator stator windings,”IEEE Trans. Power App. Syst.,vol. PAS-103, pp. 832–840, Apr. 1984.

[13] C. H. Griffin and J. W. Pope “Generator ground fault protection usingover-current, overvoltage, and under voltage relays,”IEEE Trans. PowerApp. Syst.,vol. PAS-101, pp. 4490–4501, Dec. 1982.

[14] IEEE Guide for Generator Ground Protection, ANSI/IEEE StandardC37.101-1985.

[15] IEEE–PES Relaying Committee, Working Group on Generator Pro-tection with Digital Computers, “Survey of experience with generatorprotection and prospects for improvements using digital computers,”IEEE Trans. Power Delivery,vol. PWRD-3, pp. 1511–1522, Oct. 1988.

[16] M. Stien and J. R. Linders “Ground Fault protection of the completegenerator winding,” presented at the 4th Annu. Western Protective RelayConf., Spokane, WA, Oct. 18–20, 1977.

[17] N. Nichols, “Electrical considerations in cogeneration,”IEEE Trans. Ind.Applicat., vol. IA-21, pp. 754–761, May/June 1985.

[18] R. J. Marttila, “Design principals of a new generator stator ground relayfor 100% coverage of the stator winding,”IEEE Trans. Power Delivery,vol. PWRD-1, pp. 41–51, Oct. 1986.

Rasheek M. Rifaat (M’76–SM’92) received theB.Sc. degree from Cairo University, Cairo, Egypt,in 1972 and the M.Eng. degree from McGill Uni-versity, Montreal, P.Q., Canada, in 1979, both inelectrical engineering.

He was originally with Egyptian Iron and SteelCo. and ELMACO Transformer and Switchgear Co.,both in Cairo, Egypt, and then with Union CarbideCanada Ltd., Beauharnois, P.Q., Canada. In 1981,he joined Monenco Consultants Ltd., Calgary, Alta.,Canada, and Saskmont Engineering Ltd., Regina,

Sask., Canada, where he was involved in thermal-power generating plantprojects, with a special interest in generator protection systems and powerplant systems. He is currently with Delta Hudson Engineering Ltd., Calgary,Alta., Canada, where his work involves large power cogeneration projectsand industrial power systems.

Mr. Rifaat is a Registered Professional Engineer in the Provinces of Alberta,Saskatchewan, Ontario, and Quebec, Canada.

considerations for generator ground-fault protection in midsize cogeneration plants

Documents