PROTECTIM DEVICE COORDINATION - IDEAL AND PRACTICAL J. C. Das, Senior Member IEEE

Simons-Eastern Consultants, Inc. Atlanta, Georgia

Abstract: The object ives o f protect ive device coordination i n a r a d i a l system o f dis t r ibu t ion is t o achieve s e l e c t i v i t y without sacr i f ic ing s e n s i t i v i t y and f a s t f a u l t c learance times. An ideal time-current coordination is, however, ra re ly achieved. Though the s e t t i n g ranges and characteristics o f the pro tec t ive devices selected may be as f l e x i b l e a s practicable, y e t compromises may be required. In case these compromises a r e not acceptable, addi t ional pro tec t ive devices , changes i n t h e type and characteristics o f the protect ive devices or the spec i f ica t ions o f the equipment being protected may be needed. Coordination o f protec t ive devices is undertaken towards the comnissioning s tage o f an e l e c t r i c a l d i s t r i b u t i o n system, however a proper se lec t ion o f protec t ive devices i n the design s tage itself is important t o ensure a properly coordinated protect ive system. lhe paper examines pro tec t ive device coordination i n an i n d u s t r i a l d i s t r i b u t i o n system, covering both low and medium voltage d is t r ibu t ion wi th in-plant generation and a u t i l i t y tie. The coordination o f protect ive devices for transformers, motors, in-plant generator, and feeder cables is examined. The compromises necessary i n each s i t u a t i o n and the possible improvements t h a t could be made are discussed.

I. INTRODUCTION

The elements i n a pro tec t ive system include relays, direct act ing so l id-s ta te t r i p devices and fuses . Low voltage power c i r c u i t breakers and insu la ted case c i r c u i t breakers are general ly provided with sol id-s ta te t r i p devices. For medium and high voltage systems, re lays a r e exclusively used. A time current coordination o f these devices should ensure s e l e c t i v i t y and back-up protect ion t o u n i t p ro tec t ive devices so that a minimum of unfaulted load is interrupted. A downstream device must operate f a s t e r a s compared t o the next upstream device, though the magnitude o f f a u l t current flowing through these series connected devices may be t h e same. While maximizing protect ion and minimizing the area o f shutdown, it should be ensured t h a t a l l t h e system components l i k e transformers, cables , switching and ro ta t ing equipment a r e protected w i t h respect t o continuous overloads, f a u l t withstand c a p a b i l i t i e s and thermal damage curves according to applicable ASNIIIEEE Standards [l, 2, 3, 4, 51.

Enactment of the Federal Occupational Safety and Health Act o f 1970 (OSHA) has made strict compliance with National Electric Code [l] a l e g a l requirement on a l l new construct ion a f t e r March 1972. Retroact ive requirements were introduced by OSHA for a l l e l e c t r i c a l i n s t a l l a t i o n s and for i n s t a l l a t i o n s after April 16, 1981.

The coordination o f protec t ive devices should meet the specific requirements o f the operating processes. There may be s i t u a t i o n s where protect ion can be sacrificed for cont inui ty o f operations, e.g. a f i r e f igh t ing punp. While nuisance t r ipp ing o f a pro tec t ive device may r e s u l t i n a loss o f production, an equipment damage due to lack o f protect ion can lead t o more ser ious l o s s o f production due t o a prolonged shutdown. Coordination o f protect ive devices i n a given s i t u a t i o n is not a s u b s t i t u t e for proper system planning and adequacy o f the system protect ion t o perform the required functions.

11. DATA FOR COORDINATION STUDY

Depending upon the extent o f coordination study t o be undertaken, the following data may be needed:

A ) A s i n g l e line diagram o f the d is t r ibu t ion system. A s a minimum, t h i s should show a l l protect ive devices which w i l l be coordinated on a time-current basis. It is desirable t o include u n i t protect ion and a l l o ther protect ive device funct ions i n accordance with Reference [SI. The switching condi t ions o f breakers, a l t e r n a t i v e routes o f power flow, and which breakers or switching devices w i l l be opened on occurrence o f a f a u l t condi t ion can also be shown.

B) The equipment current ra t ings and magnitude o f current flow under normal and emergency loading conditions.

C) Current transformers r a t i o s , burdens, secondary res i s tance and relaying accuracies.

D) Time cur ren t c h a r a c t e r i s t i c s o f a l l protect ive devices t o be coordinated. Total c lear ing and minimum melting time-current c h a r a c t e r i s t i c s o f fuses , let-through c h a r a c t e r i s t i c s o f fuses , and current limiters. Time-current characteristics o f sol id-s ta te t r i p devices and relays. Relay burdens, current s e t t i n g ranges for time delay, and instantaneous functions.

E ) Short-circui t cur ren ts a t the point o f appl icat ion o f t h e pro tec t ive devices. Firs t -cycle , in te r rupt ing , and 30 cycles cur ren ts may be required. The ca lcu la ted f a u l t cur ren t decrement curves o f in-plant generators.

F) F u l l load current , locked r o t o r cur ren t , s t a r t i n g time and s a f e locked rotor withstand t ime o f the medium voltage motors. The thermal damage curves o f I the motors.

G ) Power transformers impedances, primary and secondary winding connections and through f a u l t withstand capabi l i ty curves constructed according t o Reference [71.

H) Short-circui t withstand capabi l i ty curves of t h e cables, depending upon t h e i n i t i a l conductor temperature and allowable conductor temperature rise on short-circui t [SI.

111. INITIAL ANALYSIS

I n i t i a l considerat ions t o be appl ied t o a d i s t r i b u t i o n system protect ion ana lys i s before proceeding with a c t u a l coordination o f the protect ive devices are:

A) The coordination for phase f a u l t s is carried out for a three-phase bolted type o f f a u l t . This gives the maximum avai lab le f a u l t current a t a point i n the d i s t r i b u t i o n system. The flow o f this current can widely change when the p lan t is operated a t minimum generation or outage o f a source. In some r a r e cases, the f a u l t cur ren t magnitude may s ink below t h e load cur ren t presenting s p e c i a l re laying and coordination considerations.

Majority o f e l e c t r i c a l c i r c u i t f a u l t s or ig ina te as a phase t o ground f a u l t . The flow o f ground f a u l t cur ren t is dependent upon system grounding and can vary over la rge values.

Future system expansion o f motor loads w i l l contr ibute t o the short-circui t currents . An increase i n the f a u l t a v a i l a b i l i t y can occur from a u t i l i t y t i e source. The short-circui t d u t i e s o f the high voltage switchgear a t primary d i s t r i b u t i o n w i l l

89CH27920/89/~1861$01 .OO 0 1989 IEEE

be selected with a sa fe margin with respect t o calculated short-circui t du t ies . These switchgear ra t ings may be a more acceptable basis o f calculat ion o f f a u l t currents on downstream equipment f o r protect ive device coordination.

B) Recommendations o f Reference [l] should be followed for se lec t ion of protect ive devices and their s e t t i n g s on transformers, feeder cables , and f o r ground f a u l t s . Power transformer's neut ra l grounding and winding connections impact the flow o f primary short-circui t current on a secondary f a u l t 181. The ANSI through f a u l t c h a r a c t e r i s t i c s [7] should be accordingly modified.

C) The tolerances on relay and fuse time-current c h a r a c t e r i s t i c s should be considered. Due t o ambient temperature var ia t ions, pre-loading and manufacturing tolerances, the time-current c h a r a c t e r i s t i c s o f power fuses may s h i f t . One approach employs a 25 percent sa fe ty zone i n time f o r a given value o f current , and the other uses a 10 percent sa fe ty zone i n current f o r a given value o f time. me operating time o f induction pat tern overcurrent re lays within a range o f 1 t o 1.5 o f the pick-up current s e t t i n g is not defined .

D ) The saturat ion of the current transformers under f a u l t condition can a l t e r the relay operating times and much higher primary currents may be required f o r operation than the chosen s e t t i n g s [91. Considerations o f ANSI accuracy c l a s s i f i c a t i o n s [ lo] should be applied. Saturat ion o f current transformers can a f f e c t even the operation o f induction pat tern current re lays 1111.

E) An i n i t i a l analysis o f the protect ive functions should reveal t h a t none o f the system components is exposed t o damaging overloads or shor t -c i rcu i t currents which w i l l go undetected under various conditions o f operations o f the system. A l l switching devices, i.e. c i r c u i t breakers and s t a t i c equipment (such as bus bars and cables ) , a r e applied within t h e i r assigned continuous current ra t ings and shor t -c i rcu i t ra t ings.

F) The s t a b i l i t y l i m i t o f in-plant generators t o feed i n t o an external f a u l t should be considered. I f these external f a u l t s a re not c leared selectively and f a s t enough, the s t a b i l i t y may be l o s t , in te r rupt ing the very e s s e n t i a l loads which should have continued operating.

G) The coordination o f protect ive devices on co-generation and u t i l i t y t i e should receive spec ia l considerations. These have been br ie f ly discussed i n Section I X o f the paper.

I V . COORDINATING TIME INTERVAL (CTI)

The sequent ia l operation o f the series-connected protect ive devices depends upon maintaining a c e r t a i n minimum coordinating time i n t e r v a l throughout the operating range. A graphical representation o f t h e time-current c h a r a c t e r i s t i c s o f the protect ive devices is an accepted method, though it is possible t o determine s e l e c t i v i t y by comparing a t the most th ree c r i t i c a l values o f the f a u l t currents and ascer ta ining the associated relay operating times.

The C T I takes i n t o account the c i r c u i t breaker opening time, relay over-travel, and an a rb i t ra ry safe ty f a c t o r t o take i n t o account the current transformer sa tura t ion and s e t t i n g e r r o r s [SI. Table 1 shows the C T I ' s normally used.

Low voltage c i r c u i t breakers a r e ra ted and applied i n accordance with Reference [12]. These breakers may operate rapidly t o p a r t t h e i r contacts during f i r s t cycle o f short-circui t current . Low voltage power c i r c u i t breakers have a short-time ra t ing o f 30 cycles and s tored energy moulded case breakers have short-time r a t i n g s o f 18 cycles t o 30 cycles. Fuses a r e f a s t act ing devices which operate i n the f i r s t cycle o f f a u l t current . Instantaneous

TABLE 1 Commonly Used C o o r d i n m i m e In te rva ls ( C T I ' s )

Switching Device

Relayed Medium Voltage Breakers

Relayed Weaker and )ownstream

Fuses

.ow Voltage C i r c u i t

Breakers ri th Solid- i tate Trip

Devices

t- uses

Coordinatina Time In te rva l ( c'ir )

Very inverse and extremely inverse electromagnetic re lays:

0.4 sec. 5 cycle breakers 0.45 sec. 8 cycle breakers

(pre 1964 basis) Inverse t ime c h a r a c t e r i s t i c s electromagnetic re lays:

0.43 sec. 5 cycle breakers 0.48 sec. 8 cycle breakers

(The relay over-travel or impulsation time is longer for inverse c h a r a c t e r i s t i c re lays a s compared t o very inverse relays) .

Sol id-s ta te re lays:

(Relay over-travel is eliminated)

Above times can be reduced by 0.5 sec. for properly ca l ibra ted and f i e l d tes ted relays.

0.3 sec. 5 cycle breakers 0.35 sec. 8 cycle breaker

Electromagnetic re lays: 0.20 sec.

Sol id-s ta te re lays:

Relay over-travel and breaker opening time is eliminated. t o coordinate w i t h a s low a s 0.1 sec. C T I f o r opening times below 1 sec.

The s l i g h t time margin provided between operating time bands w i l l provide required coordination. Moulded c i r c u i t breakers which do not have short-time ra t ings should have instantaneous t r i p s .

Coordination between fuses for a time duration l e s s than 0.1 sec. should not be evaluated on a time-current basis. Two s e r i e s connected fuses which see the same maanitude of f a u l t current

0.1 sec.

It is possible

I w i l l coordinate, i f the maximum 1% I let through o f the downstream fuse is I below the minimum 1% l e t through o f I the upstream fuse. I

hstantaneous Se t t ings must recognize p o s s i b i l i t y o f Relays I asymmtricity on f a u l t , a s these relays

(Electro- I operate equally well on ac and dc

Type) magnetic I currents . Coordination without an

I in tervening impedance should not be I attempted.

devices operate i n 2 t o 3 cycles. F i r s t cycle shor t -c i rcu i t cur ren ts should be considered for operation o f these devices, though there may be some decrement due t o ac and dc decay o f shor t -c i rcu i t current components [131.

The time current coordination o f instantaneous devices below 0.1 second when these may see the same magnitude o f asymmetrical current is generally not attempted. o f protect ive devices extended down t o lr4 second. For such a coordination the maximun let-through current excursion o f a downstream device under a l l condi t ions should be below the '*no operate" boundary o f the upstream protect ive device.

Reference [14] shows time-current p l o t s

1862

For appl icat ion o f t ime delay relays beyond s i x cycles, the motor cont r ibu t ion t o f a u l t current can be omitted. The generators are represented by t rans ien t or l a rger impedance re la ted t o the magnitude o f decaying shor t -c i rcu i t current a t the speci f ied ca lcu la t ion time [ 5 1.

V. DISTRIBUTION SYSTEM STUDIED

Figure 1 shows a s ing le l i n e diagram o f an a rb i t ra ry d i s t r i bu t i on with a u t i l i t y t i e and in-plant generation. The ANSI p ro tec t ive device nunbers are shown; however, the coordination discussed i n the paper i s l i m i t e d t o the device nunbers shown i n bold types. A discussion o f the philosophy o f select ion o f p ro tec t ive device

V I . 2000 KVA 480 VOLT UNIT SUBSTATION, LOW VOLTAGE SWITCHGEAR AND LOW VOLTAGE

MOTOR PROTECTION COORDINATION

The pro tec t ive device coordination f o r phase and ground f a u l t s f o r the low voltage system is shown i n Figures 2, 3 and 4. Figure 3 shows the possible improvements i n the pro tec t ion and coordination over tha t o f Figure 2. Referr ing t o Figure 2, which shows coordination fo r phase overcurrent devices , the fol lowing observations can be made:

A) A complete p ro tec t ion with respect t o ANSI through f a u l t withstand charac ter is t i cs o f 2000 kVA transformer phase and ground f a u l t s i s no t provided by the transformer primary fuses. These fuses provide pro tec t ion for three-phase f a u l t s exceeding

Fig. 1. S i n g l e l i n e d i a g r a m o f t h e a r b i t r a r y d i s t r i b u t i o n sys tem s t u d i e d fo r p r o t e c t i v e d e v i c e coordination

funct ions i s excluded. The modifications t o the re lay ing required on coordination attempts are a lso not shown i n Figure 1. These modifications appear on the time-current coordination p l o t s i n Figures 2 through 10.

The system normally operates with bus t i e breaker B between 13.8 kV buses A and B and the reactor t i e breaker C closed. The reactor t i e normally car r ies a load o f 6 MVA; however, i t can suddenly increase t o 25 MVA, which the system can support without excessive voltage drops. The essent ia l service load i s connected t o p lan t generation bus B. The d i s t r i bu t i on connected t o breaker H i s studied for coordination o f p ro tec t ive devices. This load i s an essent ia l service load and should remain i n service when load shedding has i so la ted the u t i l i t y t i e and dropped other loads connected t o bus B. The magnitude o f shor t -c i rcu i t currents are not shown i n Figure 1, but i n the i nd i v idua l t ime-current coordination p l o t s i n Figures 1 through 10.

approximately 45% o f the maximun f a u l t current. Hardly any pro tec t ion i s provided by the primary fuses for secondary ground-faults. This i s an o f ten drawn conclusion tha t the primary fuses cannot provide complete through f a u l t p ro tec t ion o f the transformer.

The pro tec t ion f o r the phase and ground f a u l t s on 480 v o l t switchgear bus i s provided by the sol id-state t r i p devices on 4000 ampere main secondary breaker. The secondary phase and ground f a u l t s tha t may occur ins ide the transformer or interconnecting feeder cables between the transformer and the 480 v o l t switchgear are o f concern. I f these cables are longer than 25 feet, i t i s necessary t o provide overcurrent feeder protect ion as required i n Reference 113. A ground-fault i n t h i s region may be sustained with a probable damage t o the transformer. I f a r a t e o f pressure r i s e relay, device No. 63, i s provided on the transformer, i t may a f fo rd some protection. Ul t imately, the f a u l t may be cleared by opening o f one or two o f the primary fuses.

1863

I U ld 4 5 &

0

a, U ..i 3

5

m e U .d

a,c E 3 a 0 > e o m a C E O H .d

m

D .d E

1864

This may give r i s e t o a ferroresonant condi t ion fu r ther discussed below i n paragraph B.

The possible improvements tha t can be considered are :

i) Add medium inverse time overcurrent re lays connected t o phase current transformers located i n the transformer tank. These provide requ is i t e sho r t - c i r cu i t p ro tec t ion as shown i n Figure 3, and must t r i p the 13.8 kV breaker H. A medium inverse charac ter is t i cs allows primary fuses t o se lec t ive ly c lear higher magnitude o f secondary phase f a u l t currents.

ii) Add a very inverse time charac ter is t i c ground f a u l t relay, device 51G, connected t o a current transformer i n the transformer neutral . Figure 4 shows tha t with the add i t ion o f t h i s relay, the transformer i s protected with respect t o ANSI through-fault charac ter is t i c f o r ground fau l ts .

When devices 63 and 51G are provided, these w i l l be required t o t r i p a remotely located breaker. The voltage drop tha t may occur i n the extended t r i p c i r c u i t due t o inrush currents? as w e l l as i t s p ro tec t ion and f a u l t supervision should be considered. A 63 device i s generally used with a spec ia l aux i l ia ry re lay t o prevent nuisance t r ipp ing . I n addition, a high-speed lockout relay, device No. 86, w i l l normally be used. The extended t r i p c i r c u i t can be separately fused and the fuse f a i l u r e monitored.

iii) It i s unusual t o consider d i f f e r e n t i a l p ro tec t ion f o r a transformer o f the s ize under review; however, a 7500 kVA, 13.8 - 2.4 kV transformer connected t o the same 13.8 kV feeder breaker H i s provided w i th d i f f e r e n t i a l re lay ing (Fig. 1). It i s easy t o extend t h i s d i f f e r e n t i a l re lay ing t o 2500 kVA 13.8 - 0.48 kV transformer by select ing a three-winding type d i f f e r e n t i a l re lay connected t o the secondaries o f both the 7500 kVA, as we l l as 2500 kVA transformers. For a s o l i d l y grounded 0.48 kV system, t h i s relay w i l l be responsive t o ground fau l ts also and the transformer neut ra l connected 51N re lay may no t be required. This a l te rna t ive w i l l require add i t ion o f only three current transformers, preferably on the bus side o f the main 4000 ampere transformer secondary breaker, so tha t the feeder cables, as w e l l as transformer w i l l be covered i n the d i f f e r e n t i a l zone o f protection, as shown i n Figure 5. A disadvantage o f t h i s a l te rna t ive i s t ha t select ive t r i pp ing cannot be obtained.

B) Opening o f one or two o f the primary fuses on a transformer secondary f a u l t resu l t s i n energization o f a transformer phase through the cable capacitance. This c i r c u i t i s o f ferroresonance, as i t involves exc i ta t ion o f one or more saturable reactors (transformer windings) through cable capacitance [151. When ferroresonance occurs, h igh peak voltages, i r regu la r voltage and current waveforms and loud noises i n transformer due t o magneto-striction can be produced. Phase-to-phase and phase-to-ground capacitance o f the l a t e r a l c i r c u i t and transformer i n t e r n a l capacitance are important parameters t o be considered. There are other fac to rs l i k e transformer connections, grounding arrangements and transformer secondary loads which inf luence ferroresonance C163. Reference C173 indicates tha t ferroresonance i s un l i ke ly under the s i t ua t i on being studied as small length o f primary cables are involved and transformer s ize i s f a i r l y large. Thus the r i s k o f a ferroresonant condi t ion due t o operation o f a primary fuse seems t o be a minimum. A negative sequence voltage balance relay, ANSI device number 604, can be used t o detect t h i s condition.

In a s o l i d l y grounded system, the magnitude o f ground f a u l t current can be higher than a three-phase shor t -c i rcu i t current. The fuse l i m i t e r s

IJd KV IUS B I

FIGURE 5

D i f f e r e n t i a l re laying for 13.8 kV feeders and transformers

provided on 480 v o l t feeder breakers and the fuses i n the motor s ta r te r c i r c u i t s a t each o f the 480 v o l t motor con t ro l centers can also create single-phasing. The 480 v o l t power c i r c u i t breakers can open a l l the poles on operation o f a l im i te r . This f a c i l i t y w i l l not be avai lable for the motor c i r c u i t fuses. Three-pole thermal relays required by Reference [11 may a f fo rd only a p a r t i a l p ro tec t ion on s ing le phasing. The motor negative sequence impedance i s much lower as compared t o i t s pos i t i ve sequence impedance; and for a motor drawing 6 times the f u l l load s ta r t i ng current, a mere 5% voltage unbalance can give r i s e t o 3OW: negative sequence current. 118, 191 Recent a v a i l a b i l i t y o f molded case c i r c u i t breakers and motor c i r c u i t protectors without fuses i n in te r rup t i ng ra t ings up t o 100 k A (current l i m i t i n g type) can obviate the necessity o f fuses i n 480 v o l t motor cont ro l centers.

C) Referr ing t o Figure 2, the solid-state p ro tec t ive devices of the 4000 ampere main secondary breaker do not coordinate with transformer primary fuses. A la tolerance on the fuse minimum melt ing time charac ter is t i cs and a fu r the r 16% s h i f t due t o secondary phaseto-phase f a u l t s should be considered.

i) A transformer primary cur ren t - l im i t ing fuse of 200 amperes i s selected i n Figure 3, instead o f 150 amperes shown i n Figure 2. This p u l l s the fuse charac ter is t i cs away from the 4000 amperes c i r c u i t breaker sol id-state t r i p device short-time operating delay band.

ii) A fu r the r advantage o f the divergent fuse charac ter is t i cs i s taken by lowering the short-time delay band o f 4OOO amperes c i r c u i t breaker sol id-state t r i p device. This, however, requires tha t short-time delay band o f 800 ampere feeder breaker t r i p device should also be lowered. This device must be equipped with a 12t func t ion t o c lea r the 200 hp motor s ta r te r fuse.

Figure 3 shows tha t the f a u l t clearance times are reduced and the pro tec t ive device coordinat ion i s improved.

D) Coordination o f p ro tec t ive devices i n s o l i d l y grounded system requires due considerations. The phase f a u l t as w e l l as the ground f a u l t devices w i l l

The improvements are shown i n Figure 3.

1865

-

L L J I I I I I 1 1 1 1 1 1 I I I 1 1 1 1 I I 1 I I 1 1 1 1 1 1 I I 0

9 0 SaN033S - : 0

d . o w & a u c c a O & Uc,

&a: O > u x E O 0

& & 0 0 4J.Q o u E r d u a c C O 0 0 0 m & N O

c,

2 g .4 5 u c a a .4 (0 5 & ha , o x old

W

tn .4 E

0 a, s r l 3 1 L i d x

1866

be operative, a s t he ground-faul t current can approach or even exceed three-phase short-circui t current . Coordination is thus required not only w i t h respect. t .0 ground-fault devices, but a l s o w i t h t h e phase-fault devices. The phase-fault o r ground-fault devices on the main secondary breaker should not operate f a s t e r than s imilar protect ive devices on the feeder breaker t o prevent a shutdown o f t he t o t a l load served from the main 480 vo l t dis t r ibut ion.

The arc f a u l t damage t h a t can occur on a ground f a u l t i n 480 vol t d i s t r ibu t ion has been invest igated and limits o f the acceptable damage t o bus mater ia l on release of a rc f a u l t energy have been establ ished [20, U ] . A s m a r y o f t he recent f indings i s given i n Appendix I, These form a bas i s o f s e t t i n g s adopted on ground - f a u l t protect ive devices 151. Reference [l] gives the maximum limits o f the acceptable ground - f a u l t cu r ren t s e t t i n g s and time delays. Lower s e t t i n g s should always be provided, where possible, t o l i m i t t he a r c f a u l t damage. Figure 4 shows ground - f a u l t coordination. The s e t t i n g s chosen w i l l r e s u l t i n t he following arc f a u l t damages:

i ) 4000 amperes main secondary breaker ground f a u l t pickup s e t t i n g i s 800 ampere, ground f a u l t delay is 0.33 second and clear ing time is 0.05 seconds. The p rac t i ca l damage l i m i t is 1.0 x 1 U 6 (amperes)l. seconds. Thus the maximum to l e rab le arcing current w i l l be 1 9 070 amperes and acceptable damage t o copper 0.72 in3. Figure 4 shows t h a t f a u l t s above 2000 amperes are cleared with a time delay o f 0.38 seconds and thus maximum a r c f a u l t damage t o copper is .0245 in3. Appendix I may be seen f o r fu r the r de t a i l s .

i i ) 800 amperes secondary breaker ground f a u l t pickup s e t t i n g i s 120 ampere, ground-faul t delay 0.18 second and clear ing time is .05 second. The p rac t i ca l damage l i m i t is 0.2 x lo6 (amperes)l.5 seconds. Thus the maximum to l e rab le arcing current w i l l be 9114 amperes and acceptable damage t o copper w i l l be 0.144 in3. Figure 4 shows t h a t f a u l t s above 500 amperes w i l l be c leared i n 0.23 seconds and thus maximum arc f a u l t damage t o copper is 0.0018 in3.

The 480 vol t motor con t ro l center ( K C ) i s not provided wi th ground - f a u l t protection on each motor starter. Qnsider t h a t t he smallest disconnect i n the MCC is rated a t 100 amperes. The p r a c t i c a l damage l i m i t i s .025 x 106 (ampere)ls5 second. Reference [U] indicates t h a t an arcing ground-fault protect ion w i l l not be required t o protect against arcing f a u l t s through a i r , i f t h e avai lable ground f a u l t current a t every point on branch or feeder c i r c u i t is a t least 263% o f instantaneous t r i p se t t i ng . In pract ice , it is d i f f i c u l t t o ca l cu la t e the magnitude of bolted ground-fault current . If i t i s above 3000 amperes, addi t ional arcing f a u l t protect ion is not required for 100 ampere disconnect, a s shown i n Figure 4. Appendix I may be seen fo r fu r the r de t a i l s .

The above analysis shows t h a t the calculated ac tua l damage is much lower than the permissible damage on a r c f a u l t s and the system is adequately protected. The ground- f a u l t t r i p s e t t i n g s on 480 vol t feeder breaker connected t o the motor con t ro l cen te r w i l l operate i n 0.18 seconds for a ground f a u l t current o f approximately 500 amperes. This shows t h a t conductors up t o 10 amperes can be protected without exceeding the permissible damage limits. It is, however, evident from Figure 4 t h a t t he ground-fault s e t t i n g s on feeder breaker w i l l not coordinate with overcurrent devices on the motor starter feeders i n the MCC. If t h i s is unacceptable, an instantaneous ground-fault protect ion, coordinated with the 0.18 second ground-faul t band o f t he 480 v o l t feeder breaker device, should be added t o each MCC feeder. The pick-up s e t t i n g s on the

motor-starter and feeder ground-fault devices should a l so be coordinated.

V I I . 7500 KVA TRANSFORMR, 2.4 KV SWITCHGEAR AND 2500 HP MOTOR PROTECTION

The protect ive device coordination is shown i n Figures 6 and 7, which a re analyzed as follows:

A ) Vacuun contactors have a lesser interrupt ing r a t ing than the a i r break contactors. Though the interrupt ing r a t ing o f 50 MVA fo r t h e 700 ampere vacuum contactor shown i n Figure 6 is higher than the crossover point (where the thermal re lay curve crosses over t he fuse cha rac t e r i s t i c s ) , the fuse clear ing time exceeds the dropout time o f the contactor. Thus a poss ib i l i t y o f a contactor interrupt ing a short-circui t current exceeding i t s interrupt ing r a t ing does e x i s t . Figure 6 shows a dropout time o f 0.02 seconds f o r t h e vacuum contactor [22]. It is, however, real ized t h a t t he contactor dropout time is not very consis tent and is dependent upon the magnetic energy s tored i n t h e contactor magnetic c i r c u i t . In order t o prevent multiple re igni t ions, microprocessor based devices a r e avai lable which w i l l delay the opening o f the contactor and prevent opening near a current zero [22, 231. It is, however, seen from Figure 6 t h a t even with a delay o f 5 cycles, a complete coordination i s not achieved with the contactor interrupt ing rat ing.

If an a i r break contactor o f 75 MVA interrupt ing r a t ing i s used, the s i tua t ion is much improved, a s shown i n Figure 6, though a s l i g h t lack o f coordination s t i l l ex i s t s . A dropout time o f 0.04 seconds is assumed for the a i r break contactor.

The remedial measures can be: ( a ) delayed opening o f the vacuum contactor or latched contactors with undervoltage protection, (b) c i r c u i t breaker con t ro l , ( c ) connecting the 2500 hp motor t o a 4.16 kV system, (d) spec ia l design o f 2500 hp motor t o reduce s t a r t i n g inrush current so t h a t a lower s i z e o f motor fuse could be used.

Considerations should a l s o be applied t o the select ion o f an appropriate fuse s i z e fo r motor protection. The fuse selected should be t h e smallest fu se whose minimum melting time cha rac t e r i s t i c s does not c ros s the motor overload relay curve fo r currents less than the adjusted locked ro to r current withstand time o f t he motor [51. The adjusted locked ro to r current is taken 10% higher than the ac tua l locked r o t o r current t o account for system voltage var ia t ions and manufacturing tolerances a s shown i n Figure 6. For the case under review, a smaller fuse s i z e could not have been used t o provide a b e t t e r coordination with the interrupt ing capab i l i t y o f the vacuum contactor.

B) In order t o coordinate with the 2500 hp motor fuse, t he phase overcurrent r e l ays connected t o 1200 amperes breaker L feeding the line-up o f motor s t a r t e r s have been set a t a pick-up o f 1920 amperes, a s shown i n Figure 6. This can expose the 1200 amperes breaker and its associated current transformer t o 160% o f their rated continuous current ra t ings.

The poss ib i l i t y o f a 160% overload on breaker feeding the line-up o f 2.4 kV motor s t a r t e r s i s examined: The motor protection w i l l not permit such an overload on a sustained basis . It is unlikely that a l l motors w i l l experience a simultaneous overload condition due t o driven load. A poss ib i l i t y o f overload does e x i s t due t o sustained voltage dips, which w i l l r e s u l t i n a proportional increase i n the l i n e currents . Again it is unlikely t h a t plant operations can be sustained a t f u l l load i f t he voltage remains below 5 t o 1R% o f i t s ra t ed value. Thus p rac t i ca l ly a 16cB6 overload on breaker L w i l l not occur due t o operating conditions o f t he connected

1867

- -

I l l I I I I I 1 1 1 1 I 1 I I I I I I I I I I I 1 1 1 1 1 I I I I 1 1 1 1 1 I I 1 I

Id CO3 .d . a m

U U

1868

load. The p o s s i b i l i t y o f a sustained high resistance phase fau l t , when the current w i l l remain l i m i t e d t o 160% o f pick-up se t t i ng o f the relays f o r a considerable period o f time, i s next examined: It i s un l i ke l y tha t f a u l t w i l l remain sustained a t t h i s value. The f a u l t w i l l "burn through" and the overload protect ion should operate. Reference E11 allows long time current se t t i ng o f 6 times the f u l l load current f o r systems above 600 vo l t s f o r shor t -c i rcu i t protect ion o f the feeders. I t i s , however, always desirable t o achieve overload pro tec t ion without exceeding the thermal l i m i t s o f the protected apparatus.

Two phase overcurrent relays set t o operate a t the maximum operating load current o f the 2.4 kV motor cont ro l center can be added and connected f o r alarm only. Undervoltage relays can be used t o p ro tec t against sustained voltage depressions. These should, however, remain inoperat ive f o r motor s ta r t i ng voltage drops and momentary voltage depressions due t o system fau l ts .

A s im i la r s i tua t ion ex is ts f o r coordination o f main 2000 ampere breaker K overcurrent relays with the 1200 ampere feeder breaker relays. Figure 6 shows tha t these relays must be set a t 2800 amperes t o coordinate with the feeder breaker L relays.

The instantaneous elements on feeder and main secondary breaker relays are bypassed as these w i l l not coordinate with the motor fuse (Figures 6 and 7 ) .

C) Figure 7 shows the coordination of phase overcurrent relays o f 13.8 kV breaker H w i th the 2000 ampere, 2.4 kV breaker K overcurrent relays. I n order t o provide required C T I o f 0.4 second a t the maximum avai lable f a u l t current on secondary o f 2.4 kV, 7500 kVA transformer, the 13.8 kV feeder breaker H relay operating time w i l l be approximately 1.0 second. (The decrement in 13.8 kV sho r t - c i r cu i t current due t o presence o f generator i s neglected f o r the present discussion). Though 13.8 kV buses are provided with d i f f e r e n t i a l protect ion, a short c i r c u i t i n the feeder c i r c u i t may cause i n s t a b i l i t y o f in-plant generator, and i t i s desirable t o have faster f a u l t clearance time [241. The fol lowing considerations apply:

i) D i f f e r e n t i a l p ro tec t ion f o r 7500 kVA transformer can be extended t o include 13.8 kV feeder cables from breaker H.

ii) A lack o f coordination can be accepted between 2000 ampere, 2.4 kV secondary breaker. K and 13.8 kV feeder breaker H overcurrent relays. This lack o f coordination w i l l r esu l t i n t r i pp ing o f the 13.8 kV feeder breaker fo r a bus f a u l t on 2.4 kV switchgear. However, the probab i l i t y o f such a f a u l t i n metalclad switchgear i s low. I f t h i s s i t ua t i on i s not acceptable, d i f f e r e n t i a l protect ion can be added t o 2.4 kV switchgear. Al ternat ively, a lack o f coordination between 2.4 kV feeder breaker L and main 2.4 kV 2000 amperes breaker K overcurrent relays may be acceptable. I n each o f these a l te rna t ives the 13.8 kV feeder breaker overcurrent relays (o f very inverse type) need not be se t higher than 0.7 seconds a t the maximm coordinating point . This i s shown i n Figure 7.

0) Figure 7 shows the thermal withstand charac ter is t i cs o f 500 kcmil, 13.8 kV feeder cables. I t i s seen tha t the cables are not f u l l y protected by the feeder overcurrent relays. An advantage of accepting a lack o f coordination between 13.8 kV feeder breaker 4 re lays and 2.4 kV main 2000 ampere breaker K relays i s tha t 13.8 kV feeder relays can be set low and a be t te r p ro tec t ion o f the 13.8 kV feeder cables can be obtained. A n increase i n pick-up se t t i ng o f re lays a t L, K, and H and decrease i n the time d i a l se t t i ng can be t r i ed . This w i l l , however, fu r ther compromise the pos i t i on described above i n paragraphs 0 and C.

This i s shown i n Figure 5.

V I I I . 13.8 KV GENERATOR AND FEEDER OVERCURRENT RELAY COORDINATION

This time-current cooraination i s shown i n Figure 8. The fol lowing observations are o f in te res t :

A) The calculated three-phase f a u l t decrement curve o f 24 MVA in-plant generator i s shown i n Figure 8. This i s calculated based upon the calculat ions described i n Reference E53 f o r a stuck regulator condition, which gives the minimum f a u l t current f o r re lay operation. Appendix 11 may be seen f o r de ta i l s .

The ca lcu la t ion o f the operating .time o f overcurrent relays on delaying currents i s a cu t and t r y process [251. The ana ly t i ca l procedure described i n reference [261 has been used i n Figure 8. I t recommends ca lcu la t ion o f operating time based on ismr, the square roo t o f the mean roo t current. The curve o f ismr i s also p lo t ted i n Figure 8. Appendix 111 may be seen f o r deta i l s . The calculated sett ings of the 13.8 kV feeder breaker H overcurrent relays, as shown i n Figure 7, are rep lo t ted i n Figure 8. It i s seen tha t feeder relays w i l l operate instantaneously f o r f a u l t currents exceeding 11 kA, when 13.8 kV bus t i e breaker B and the reactor t i e breaker C are closed. However, when the d i s t r i bu t i on system i s operating only with in-plant generator, with bus t i e breaker B open, the 13.8 kV feeder H re lay takes 7 seconds t o operate. S t a b i l i t y o f generator should be checked f o r t h i s operating time. The operating times of overcurrent devices i n 2400 v o l t and 480 v o l t system should also be examined when only in-plant generator i s i n operation.

Figures 2, 3 and 4 show tha t on 480 v o l t d is t r ibu t ion , the f a u l t clearance time w i l l not appreciably change; however, from Figures 6 and 7 , on the 2400 v o l t system, a feeder f a u l t w i l l be cleared i n 1.2 seconds, while a bus f a u l t may pe rs i s t f o r approximately 3 seconds. These operating times must be coordinated with respect t o generator s t a b i l i t y l i m i t s . A study may reveal t ha t the generator w i l l be stable f o r f a u l t s on the 2.4 kV and 480 V d i s t r i b u t i o n f o r the durat ion o f f a u l t clearance times involved. However, fas te r f a u l t clearance times on 13.8 kV system are required.

The above discussion shows tha t add i t iona l p ro tec t ion w i l l be required on the 13.8 kV d i s t r i b u t i o n when the generator alone i s supplying the essent ia l loads connected t o i t s bus B.

One obvious so lu t ion w i l l be t o add d i f f e r e n t i a l re lays t o p ro tec t a l l 13.8 kV feeder cables. As shown i n Figure 5, these relays can be used t o include transformers also i n the d i f f e r e n t i a l zone o f protect ion.

The operating time of the pro tec t ive devices on 13.8 kV bus A, w i th bus t i e breaker B open, should a lso be s im i la r l y examined.

I X . UTILITY TIE TRANSFORMER RELAY COOROINATION

Figure 9 shows the coordination o f phase overcurrent relays. The maximum set t ings on feeder relays on buses A and B should be considered for coordination with u t i l i t y t i e transformer.

The overcurrent re lay set t ings on 13.8 kV feeder breaker H were calculated i n Section V I I I . Feeder breaker C on bus A car r ies the maximum load and i t s assumed overcurrent re lay se t t i ng i s shown i n Figure 9. This does not seem t o coordinate with the generator voltage res t ra in t overcurrent device 51V. A study o f Figure 8, however, shows tha t coordination has been achieved f o r the f a u l t s on the load terminals o f feeder breaker C close t o the pick-up sett ing.

The se t t ings on the primary and secondary sides of 25 MVA transformer overcurrent relays coordinate

1869

&

E ' o m W & m a , C Y m m & a ,

m C & .ri a, naY O a J O h u n

n a m C & 3a, o c & a , w m

0 rl

In N

.lJ C &

I870

with the transformer through f a u l t character is t ics and provide se lect ive t r ipp ing.

The set t ings on 67 re lay on breaker A are provided t o coordinate with the u t i l i t y re lays f o r a f a u l t i n the u t i l i t y system. This re lay w i l l see a rap id ly decaying f a u l t current. Assuming t h a t the motor contr ibutions on buses A and B w i l l decay i n 6 cycles, and a 30% decay i n the f a u l t current cont r ibut ion from the reactor t i e , the shor t -c i rcu i t currents seen by 67 re lay w i l l be as shown i n Figure 9. A f a u l t decrement curve can be constructed and response o f the re lay examined by constructing e f f e c t i v e current curves as described i n Appendix 111. The momentary power swings t h a t may occur on sudden loading and synchronizing should receive consideration. There i s a p o s s i b i l i t y o f a sustained low magnitude o f ground-fault current on 115 kV side o f 25 MVA transformer being fed from the in-p lant generator, i n case the transformer 115 kV side breaker i s open. A sens i t ive reverse power relay, device 32, with a time delay can he provided or i n t e r t r i p p i n g channels between primary and secondary breakers can be ins ta l led . Distance relays, device Zl, can also replace 67 re lays f o r coordination with the u t i l i t y re lay ing [41.

Figure 10 shows the coordination o f the ground-fault protect ive devices. The del ta primary windings o f the u t i l i t y t i e transformer and the p lan t u n i t transformers r e s t r i c t the primary ground-faults t o primary re lay ing only. The u t i l i t y transformer and the generator are each grounded through res is to rs l i m i t i n g the ground-fault current t o 400 amperes. The coordination shown i n Figure 10 assumes t h a t the reactor t i e , breaker C , contr ibutes 200 amperes ground-fault current t o the system. The 50N/51N devices on the primary side o f 25 MVA u t i l i t y t i e transformer can have low sett ings, long enough t o r i d e through transformer inrush currents. The instantaneous ground relay, device 50N, i s set above the transformer inrush current.

Though the ground-faults i n the zone between the secondary o f the 25 MVA u t i l i t y t i e transformer and bus A w i l l be se lect ive ly cleared by devices 51N and 51G shown i n Figure 10, yet a d i f f e r e n t i a l ground-fault relay, device 87TG, i s added t o provide more sensi t ive ground-fault protect ion i n t h i s zone. The transformer d i f f e r e n t i a l relay, device 87T, w i l l not be responsive t o low magnitudes o f ground-fault current. Device 87TG i s po lar ized by the current i n transformer neutra l c i r c u i t and has a product range o f 0.25 t o 4 amperes. A s e n s i t i v i t y o f 20 amperes can be obtained, though the re lay burdens and current

. transformer saturat ion may increase the actual pick-up. So l id s ta te type 51N, 51G and 87TG devices can be used t o reduce burden.

I n order t o provide se lect ive ground-fault clearance on 13.8 kV buses A and B devices 67N are added on each side o f the bus breaker B. Device 67N on feeder breaker C i s by-passed when bus t i e breaker i s closed.

The voltage dips tha t may occur on the p lan t load buses on occurrence o f a f a u l t i n the u t i l i t y system requi re care fu l considerations. The vacuum o r a i r break contactors may drop out i n 2 t o 8 cycles on a 20% t o 70% o f the supply voltage. Thus motors contro l led through NEMA E-1 and E-2 s ta r te rs w i l l a l l drop out, though these may be able t o r i d e through the momentary voltage dip. Stabl izat ion o f the contactors, latched contactors, or c i r c u i t breaker c o n t r o l may be required. The SCR loads and dc dr ive loads w i l l be more sensi t ive t o voltage dips.

Figure 1 shows a phase sequence and undervoltage device No. 47 connected t o the 13.8 kV bus A. I t s operation is blocked by device No. 60 f o r a p o t e n t i a l transformer fuse f a i l u r e . The coordination o f undervoltage devices i s not discussed i n the paper.

An autoclosing operation on the u t i l i t y l i n e s t o

c lear t rans ient f a u l t s may subject the motors t o current and torque surges, which may be damaging t o the motors. Such operation, or a f a s t bus transfer, shoulQ be care fu l l y analyzed before implementation. Location o f surge protect ive devices, t h e i r coordination with system grounding and current l i m i t i n g fuses, load shedding on loss o f u t i l i t y or in -p lant generation are some other re la tea concerns not addressed i n the paper.

X. CONCLUSIONS

A re lay coordination study should ensure maximum protect ion o f the system components with minimum load removed from the service. Compromises w i l l be necessary i n t h i s ob ject ive and each compromise should receive c r i t i c a l considerations i n terms o f spec i f i c requirements o f the p lan t operations, a v a i l a b i l i t y o f the equipment f o r r e s t a r t and the possible damages. The paper demonstrates that protect ive device coordination i s no t only a fag-end a c t i v i t y , bu t also a front-end a c t i v i t y too. I t impacts the system design and performance. A coordination engineer can manipulate w i th d i f f e r e n t re lay character is t ics , se t t ing ranges, addit ion/subtraction o f re lay ing and sometimes a l t e r i n g the equipment speci f icat ions t o achieve the desired objectives. Many p o s s i b i l i t i e s ?ay be present, and imp l ica t ion o f each a l te rna t ive i s required t o oe examined. The paper goes through a step-by-step analysis o f coordination o f protect ive devices i n an arb i t ra ry mult i-voltage l e v e l d i s t r i b u t i o n system wi th in-p lant generation and a u t i l i t y t i e . Coordination a t 180 vo l t , 2400 v o l t and 13.8 kV d i s t r i b u t i o n i s examined under f u l l p lan t operations and when the shor t -c i rcu i t l e v e l s on the system are considerably reduced, with only in-p lant generator i n operation.

The paper shows how the i n i t i a l coordination study f o r 480 v o l t phase and ground f a u l t re lay ing could be improved by:

A) Addit ion o f phase overcurrent re lays on transformer secondary feeders. The impact o f the re lay character is t ics and set t ings with respect t o transformer primary fuses f o r coordination has been examined.

B ) Addit ion o f a ground f a u l t re lay on the transformer neut ra l t o provide ground f a u l t protection.

C) Manipulation o f short-time charac ter is t i cs on the main and feeder breakers so l id-s tate t r i p devices and use o f 12t ramp funct ion t o provide fas te r f a u l t clearance times.

D) Philosophy o f ground f a u l t re lay ing i n s o l i d l y grounded system t o l i m i t the f a u l t damage and provide an optimum protection.

E) P o s s i b i l i t y of s ing le phasing tha t can be caused by shor t -c i rcu i t current l im i te rs , motor s t a r t e r and transformer primary fuses and the remedial measures.

For 2400 v o l t d i s t r i b u t i o n system, the

A ) Problems o f coordinating with fuses and i n t e r r u p t i n g rat ings of the motor contactor. Possible solutions, c i r c u i t breaker and latched contactor controls, and d i s t r i b u t i o n system design changes.

B ) Coordination o f feeder protect ive re lays with 2500 hp motor fuses. Problems due t o h igh set t ings tha t have t o be adopted on these relays.

C> E l iminat ing a coordinating time i n t e r v a l f o r 2.4 kV bus f a u l t re lay ing and discussions o f the impact o f t h i s compromise.

coordination of p ro tec t ive devices discusses:

1871

For the 13.8 kV system the coordination problems discussed are:

The generator decrement curve and calculat ions o f operating time o f re lays on decrement. Lack of protect ion on 13.8 kV feeder cables from shor t -c i rcu i t considerations. The generator i n s t a b i l i t y due t o excessive operating relay times f o r a 13.8 kV feeder fau l t . Reduction o f shor t -c i rcu i t currents with only in-plant generation i n service, e f fec t on relaying and the add i t iona i feeder d i f f e r e n t i a l re lays tha t w i l l be required f o r fas te r f a u l t clearance times. Coordination f o r phase and ground f a u l t s on 25 MVA u t i l i t y t i e transformer.

APPENDIX I

Arcing Faul ts on 480 Volt So l id ly Grounded Systems

Single-phase 277 v o l t arc ing f a u l t tes ts using spacings o f one t o four inches from bus bars t o ground a t current leve ls o f 3000 t o 26000 amperes indicates tha t f a u l t damage i s proport ional t o (I)1.5t [U]. The damaged volume o f mater ia l , VD i s given by the expression:

VD = ks ( I ) l S 5 t (in3 /A1s5S) ------ (1) where k, = 0.72 x 1(r6 f o r copper

k5 = 1.52 x 1F6 f o r aluminum k s = 0.66 x 1F6 f o r s tee l

For coordinated sho r t - c i r cu i t and ground-fault protection, an arb i t ra ry p rac t i ca l l i m i t i s assumed i n NEMA PB1.2 [271, so tha t ( I ) 1 * 5 t i s not numerically greater than 250 times the ampere ra t i ng o f the conductor, bus or disconnect t o be protected. This gives:

(2) (1)1.5t = 250 I~ ---_--

VD = 250 k 5 b ------

where IR = current ra t i ng

The acceptable damage then becomes: (3)

The ground-fault se t t ings on the s o l i d s ta te t r i p device o f 4000 ampere breaker shown i n Figure 4 are evaluated i n terms o f equations (11, (2) and ( 3 ) . The ground-fault pick-up i s set a t 800 amperes, and ground-fault currents exceeding 2000 amperes w i l l be cleared i n 0.33 seconds. Assuming a breaker c lear ing time o f .05 second, the maximum tolerable arcing current can be calculated from equation (2). This gives (I)l.5(0.38) = 1.0 x l o 6 and I = 19 070 amperes. Thus the g r o m b f a u l t p ro tec t ive device se t t i ng a t 0.33 sec. should not exceed 20134 amperes t o l i m i t damage t o as acceptable leve l . The acceptable damage i s calculated from equation (l), giv ing V = 0.72 in3 f o r copper. As lower ground f a u l t current set t ings are provided, the ac tua l damage w i l l be .0245 in3, approximately 3.4% o f the permissible arc f a u l t damage.

The avai lable ground-fault current w i l l be given by the fo l lowing expression:

- If = 3E, amperes ( 4 ) 3Rg+Rf+(R1+R2+Ro)+J(X1''+X2+Xo) -----

where E,, = phase-to-neutral po ten t i a l i n vo l t s Rg = resistance o f the ground g r i d Rf = minimun f a u l t resistance

X l " , x2, xo 1 reactances = Sequence resistancesand R1, Rz, Ro

Reference E211 shows tha t 277 vo l t s t o ground arcing fau l ts , having arcing currents less than 36% of the avai lable bol ted f a u l t current, w i l l be self-extinguishing . Reference [ 281 gives arcing f a u l t currents i n percentage o f bol ted L-G f a u l t s f o r various f a u l t c i r c u i t X/R ra t ios . The arc f a u l t current varies from 30-41%, w i th arc durations o f 38-70% of a cycle.

This sel f -ext inguishing property can be very he lp fu l fo r the system design and se t t i ng o f ground f a u l t p ro tec t ive devices. I n case the instantaneous devices are set t o operate, say a t X amperes, a ground f a u l t p ro tec t ion i s not necessary i f the bol ted L-G fau l t current i s a t leas t 2.63X amperes. Figure 4 shows the sel f -ext inguishing zone o f a 100 ampere fuse w i th 3000 ampere L-G f a u l t current.

APPENDIX I1

Calculation o f Generator Fau l t Decrement Curve

The generator par t i cu la rs are: output = 24 MVA, = 26.25%, rated power fac to r = 0.85, X ' k = 17.5%, X k

.012 sec., and TA = 0.19 sec. , a n d T k = 0 . 3 4 sec.

Xx = subtransient reactance, saturated valve Xl, = t ransient reactance, saturated valve X d = synchronous reactance I F g = f i e l d current a t no load, rated vo l t s IF = f i e l d current a t given load condi t ion T% = subtransient shor t -c i rcu i t time constant

T & = t ransient shor t -c i rcu i t time constant

TA = armature shor t -c i rcu i t time constant i n

A sudden shor t -c i rcu i t o f a generator w i l l r esu l t i n a changes i n the f l u x l inkages i n d i rec t and quadrature axes. A change i n the d i rec t ax is tends t o change the f l u x l i n k i n g w i th main f i e l d , which i s res is ted by an induced current i n the ro to r . As the magnetic f lux represents a considerable amount o f stored energy, i t s decay i s dependent upon the time constant associated with e lec t r i c c i r cu i t s . A t instance o f shor t -c i rcu i t subtransient reactances and time constants are considered, a f te r a few cycles when the e f fec ts o f damper windings and eddy currents i n pole faces disappear, the t ransient conditions prevai l . These s e t t l e down t o a steady-state shor t -c i rcu i t current a f t e r a l l damping currents i n the f i e l d windings have decayed.

The decay w i l l also be dependent upon exc i te r c e i l i n g voltage, pre-loading and regulator response. The ins tan t of f a u l t on the voltage wave w i l l determine the presence o f a decaying dc component. Reference [51 gives the fol lowing expression f o r the t o t a l ac component o f armature current:

x d = 13%, fg = 1 Pu, IF = 3 Pu, T"d =

i n sec.

sec.

sec.

. // L d a n d ib- are ?c decaying components o f the c u r r e n t and Ld i s the steady-state component.

I

------ 2 = e f t x k s ~ w e ,, (7) = m a h i n e i n t e r n a l v o l t a g e b e h i n d X d

( 8 ) ------ I

( 9 ) e'= et+ %&Sine ------ I

= m a c h i n e i n t e r n a l v o l t a g e b e h i n d X&

1872

(14) ----- The dc component i s given by

.4 i& = C L & e

e t = machine i n t e r n a l voltage 0 = load power fac to r angle

For the decrement curve shown i n Figure 8, the generator i s assuned unloaded. A stuck regulator condi t ion i s considered as no load f i e l d current resu l ts i n the longest re lay operating times. Thus f o r the 24 MVA generator under consideration 8 = 0; I ~3 = I t = , id = 803 amperes. The generator s h o r t - c i r c u i t decrement curve i s as shown i n Figure 8.

= 6057 amperes, i& = 4038 amperes,

APPENDIX I11

Set t ing o f Generator 51V Device

The set t ings of a voltage r e s t r a i n t overcurrent re lay and other induct ion re lays on decaying shor t -c i rcu i t currents can be calculated by one o f the fo l lowing three methods:

Step-byzstep method: The percentage o f the t o t a l t r a v e l that the re lay d isc w i l l move when subjected t o an a r i t h n e t i c average o f each incremental i n t e r v a l o f the decrement curve i s calculated. The t o t a l distance traveled i n successive increments i s summed up. T r i a l and er ro r method [25]: For a given re lay set t ing, an operating time i s assuned. For t h i s operating time, the average current i s calculated and a new approximation o f the operating time made from the re lay curve. The method i s recommended f o r the saturated por t ion o f the curve.

Mathematical solut ion: Reference [261 re la tes various calculat ions o f e f f e c t i v e current t o the d i f f e r e n t types o f re lay operating characteri- s t i c s and t h e i r slopes. The e f fec t i ve currents considered are:

Decrement From Generator Decrement

Curve

Time t Current i Average Current are i a v

0 0.01 0.04 0.08 a. 12

6057 -- 4831 5444 3751 4291 3362 3557 3076 3219

The resu l ts o f the f ind ings can be summarizea i n

TABLE 2 Ef fec t i ve Current w a t i o n s f o r Various

Table 2.

Relay Types and Sett ings

Relay Type Sett ings I

Voltage r e s t r a i n t 1 ismr (5-20 a t 25% tap set t ing) I i a v (2-5)

I

I Inverse re lays I ismr (above 4) and i a v (2-4)

I

I irms (2-4) Very inverse re lays I ismr (above e), i a v (4-81,

I

Extremely inverse I i a v (above 8), irms (2-8) I irms (2-4) 1

Table 2 shows tha t a s ing le curve o f operation f o r the e f f e c t i v e current w i l l requi re in te rpo la t ion between various e f f e c t i v e current types.

Reference C261 recommends construction o f a "composite curve" f o r generator 51V re lay operation. Between ismr and the generator 3-phase f a u l t decrement curve, i t fo l lows the 51V zero r e s t r a i n t character is t ics . Below steady-state 3-phase f a u l t current, i t breaks from zero r e s t r a i n t curve upwards and crosses each curve f o r higher r e s t r a i n t a t a current given by:

(15) I" = i & X d --v ----- XCL

V = r e s t r a i n t i n p.u. o f generator voltage lv = steady s tate 3-phase current a t which

i d dr X d are as defined i n Appendix I1

Table 3 shows a step-by-step ca lcu la t ion o f ismr f o r 24 MVA generator. The resu l t ing ismr curve i s shown i n Figure 8.

According t o Table 2, i a v should be used f o r the operating range o f 51V device i n Figure 8. However, the ca lcu lat ions do not show much d i f ference between the two character is t ics a t the operating times shown i n Figure 8.

r e s t r a i n t equals V

TABLE 3

Calculations o f ismr

Square Root Average

Current J E T --

73.78 65.50 59.64 56.74

-- -- -- 0.738 0.738 73.80 1.9651 2.703 67.57 2.386 5.089 63.62 2.260 7.454 62.00

1873

ismr = ( & . I F S --

5446 4566 4096 3844

REFERENCES NFPA 70, National E lec t r i c Code. ANSI/IEEE C37.96 - 1976, IEEE Guide f o r AC Motor Protection.

ANSI/IEEE C37.91 - 1985, IEEE Guide f o r Protect ive Relay Applications t o Power Transformers.

ANSUIEEE C37.95 - 1973, IEEE Guide f o r Protect ive Relaying o f Utility-Consumer Interconnections.

ANSI/IEEE Std. 242 - 1986, IEEE Recommended Pract ice fo r Protect ive and Coordination o f I n d u s t r i a l Power Systems.

ANSUIEEE C37.2 - 1979, IEEE Standard E l e c t r i c a l Power System Device Function Numbers.

ANSI/IEEE C57.12.00 - 1980, IEEE Standard General Requirements f o r Liquid-Immersed Dis t r ibut ion, Power and Regulating Transformers.

John C. Dutton, Frank J. McCunn and Robert L. Smith, Jr., "Transformer protect ion then and now," I n d u s t r i a l Power System, March 81.

Applied Protect ive Relaying. Coral Springs, Flor ida, Westinghouse E l e c t r i c Corporation, 1982.

[ l o ] ANSI/IEEE C57.13 - 1978, IEEE Standard

[U] Alex Wu, "The analysis o f current transformer t rans ient response and i t s e f fec t on current re lay performance," Trans. IEEE lAS, Vol. I A - 2 1 , No. 4, pp. 793-862, May/&ne 1985.

[12] ANSUIEEE C37 - 1981, IEEE Standard f o r Low Voltage AC Power C i r c u i t Breakers used i n enclosures.

[131 B. K. Mathur. "A c loser look a t the aml i ca t i ons

Requirements f o r Instrument Transformers.

. . and se t t i ng o f instantaneous devices," i n Conf. Rec. 1979, Seattle, WA, IEEE ICPdrs Technical Conf., pp. 107-118.

[141 R. H. Kaufman, R. 0. Ohlson, J. R. Linders, R. L. Smith, "Coordinating ra t i ng structure": In ter im Report, IEEE Task Force 111.

[ 151 R. H. Hopkinson, "Ferroresonance dur ing single-phase switching o f three-phase d i s t r i bu t i on transformer banks," IEEE Trans.

[16] R. H. bpkinson, "Ferroresonant overvoltage

- PAS, Vol. PAS-86, pp. 1258-1265, Oct. 1967.

con t ro l based on TNA tes ts on three-ohase de l ta wye transformer banks,It IEEE Trans. PAS, Vol. PAS-87, pp. 352-361, Feb. 1968.

[17] ANSI/IEEE C57.105 - 1978, IEEE Standard Guide f o r Application o f Transformer Connections i n Three-phase D is t r i bu t i on Systems.

[18] L. L. Gleason, W. A. Elmore, *IProtection o f three-phase motors against single-phase operation," AIEE Trans. Part 111, Vol. 77, pp. 1112-1119, Dec. 1958.

[19] P. G. Cummings, J. R. Dunki-Jacobs, R. H. Kerr, "Protect ion o f induct ion motors against unbalanced voltage operation," i n Conf. Rec. 1984, Canada, IEEE Fulp & Paper Industry, pp. 56-69.

E201 J. R. Dunki-Jacobs, "The e f fec ts o f arc ing around f a u l t s on low voltaae svstem design." YEEE Trans. E, Vol. IA-8, pp. 223-230, May/June 19/2.

[U] H. J. Stanback, "Predicting damage from 277 v o l t single-phase t o ground arcing- faul ts," IEEE Trans. lAS, Vol. IA-13, No. 4, pp. 3 0 7 - r n &ly/Aug. 1977.

E221 Charles A. L is ter , 'Vacuum, SF6 and a i r break contactors f o r mediun voltaae control lers." IEEE Trans. PAS, Vol. PAS-103, No. 10, pp. 3 0 2 i - 3 m Oct. 1984.

[23] E. A. Long, "An e lect ron ic method o f con t ro l l i ng mu l t i p le r e i g n i t i o n switching transients i n vacuum contactors", i n Conf. Rec. 1988, Chicago, TAPPI Eng. Conf.

r241 David H. Smith. "Problems involv ing i n d u s t r i a l - - p l a n t - u t i l i t y bower system enti