162941

SELECTION OF SYSTEM NEUTRAL GROUNDING RESISTOR AND GROUND FAULT PROTECTION FOR INDUSTRIAL POWER SYSTEMS

Copyright Material IEEE Paper No. PCIC-91-51

DR. LUKE YU. P.E. AND ROLF L. " R I K S , P.E. The Ralph M. Parsons Company

100 West Walnut S t r e e t Pasadena, CA 91124, U.S.A.

(818) 440-2000 Abs t r ac t - This paper p e r t a i n s p r i m a r i l y t o low- v o l t a g e (LV) and medium-voltage (MV) i n d u s t r i a l power d i s t r i b u t i o n systems, below 1 5 kV. The method chosen f o r system n e u t r a l grounding is s i g n i f i c a n t t o system performance , o p e r a t i o n , maintenance, s a f e t y , e t c . The purpose o f grounding i s t o c r e a t e a f i rm p o t e n t i a l r e f e r e n c e p o i n t and t o minimize t h e hazard t o pe r sonne l and damage t o t h e system when a l i n e - t o - ground f a u l t occurs . S o l i d l y grounded ( S G ) , low- r e s i s t a n c e grounded (LRG) , and h igh - re s i s t ance grounded (HFG) systems are d i scussed and compared. S e l e c t i o n o f n e u t r a l grounding r e s i s t o r v a l u e s and ground f a u l t p r o t e c t i o n d e v i c e s are d i scussed . F i n a l l y , t h e paper proposes t h a t a modif ied high- r e s i s t a n c e grounding (MHRG) w i t h ground fault t r i p p i n g may be t h e b e t t e r p r a c t i c e i n some i n d u s t r i a l a p p l i c a t i o n s .

I. INTRODUCTION

System grounding and ground f a u l t p r o t e c t i o n have been s u b j e c t s o f cont inuous i n t e r e s t f o r y e a r s and have been covered i n many IEEE s t a n d a r d s , t h e Na t iona l Electrical Code (NEC) , and many t e c h n i c a l p u b l i c a t i o n s 11-10 1 .

System grounding is t h e p r a c t i c e o f p h y s i c a l l y connect ing a s p e c i f i c p o i n t o f t h e e l e c t r i c a l system (normally t h e n e u t r a l o f a t r ans fo rmer o r a g e n e r a t o r ) t o t h e e a r t h . The i n t e n t i s t o create a p o t e n t i a l r e f e r e n c e p o i n t f o r t h e e l e c t r i c a l system. Some advantages o f system grounding are :

1) Reduce l i f e hazard by minimizing p o t e n t i a l t o e a r t h

2 ) Reduce t r a n s i e n t ove rvo l t ages

3 ) Reduce p o t e n t i a l stress on c a b l e i n s u l a t i o n

4) Provide adequate system ground f a u l t p r o t e c t i o n

Recent p r a c t i c e has been t o conve r t many ungrounded systems ( such as del ta-connected systems) t o grounded systems f o r t h e s e advantages. In p r a c t i c e , a zigzag-type o r a d i s t r i b u t i o n - t y p e t r ans fo rmer may be used t o create a grounded n e u t r a l system [ 6 ] .

In t h i s pape r , o n l y t h o s e systems wi th a v a i l a b l e system n e u t r a l s are d i scussed . The c u r r e n t t y p i c a l p r a c t i c e is shown i n Table I.

Obviously, from an o p e r a t i o n a l v i ewpo in t , t h e r e is a cho ice o f on ly two responses when a ground f a u l t i s d e t e c t e d : (1) t o t r i p and clear t h e f a u l t , o r ( 2 ) no t t o t r i p and ma in ta in s e r v i c e c o n t i n u i t y .

For c o n t i n u i t y o f s e r v i c e ( i .e. , no t r i p ) , system grounding i s o f t h e HFX type . Otherwise, when t h e f a u l t i s t o be c l e a r e d qu ick ly , e i t h e r SG o r LRG is used.

Consul tant Temple C i t y , CA, U.S.A.

(818 1 286-4913

TABLE I TYPICAL PRACTICES

~~

LV MV

2.4 and System Type 5600 V 4.16 kV 1 5 kV

S G T r i p Tripa Tripa

LR G Not used T r i p T r i p

HRG Alarm A l a r m Not usedb

i s not u s u a l l y app l i ed i n i n d u s t r i a l a p p l i c a t i o n s .

bHRG i s not a p p l i c a b l e t o t h e 15-kV system t o ach ieve s e r v i c e c o n t i n u i t y because expe r i ence does not show a s u c c e s s f u l a p p l i c a t i o n f o r 13.8-kV systems [lo].

Ground f a u l t p r o t e c t i o n i s governed by t h e system grounding method used because t h e l ine- to-ground f a u l t c u r r e n t determines t h e s e n s i t i v i t y and , t h e r e f o r e , t h e r equ i r ed r e l a y type .

11. COMPARISON OF SYSTEM GROUNDING METHODS

SG systems have t h e c h a r a c t e r i s t i c s o f t h e lowest c o s t , h i g h e s t a v a i l a b l e c u r r e n t , h i g h e s t p ropens i ty t o pe r sonne l hazard and a r c i n g damage, and lowes t s u s c e p t i b i l i t y t o t r a n s i e n t ove rvo l t ages . The SG system has been adopted f o r high-vol tage ( H V ) u t i l i t y t r ansmiss ion and d i s t r i b u t i o n systems and i s o f t e n used i n LV systems where it i s t h e only grounding mode when l i n e - t o - n e u t r a l l oad ing i s planned. It can accomoda te t h e most l e v e l s o f ground f a u l t r e l a y i n g c o o r d i n a t i o n , and ample f a u l t c u r r e n t i s a v a i l a b l e t o a c t u a t e ove rcu r ren t r e l a y s 12-4 1 .

Arcing f a u l t s are t h e most common type o f ground f a u l t s i n s o l i d l y grounded LV systems. Because o f low c u r r e n t magnitude ( t y p i c a l l y , about 38% t h a t o f a b o l t e d f a u l t ) , t h e phase r e l a y s may not o p e r a t e . However, a r c i n g f a u l t s d e l i v e r a tremendous amount o f energy i n t h e form o f hea t and p r e s s u r e , s u f f i c i e n t t o m e l t bus and enc losu re materials and produce l a r g e amounts o f carbonized smoke that may damage o t h e r components i n t h e v i c i n i t y . The amount o f damage is d i r e c t l y p r o p o r t i o n a l t o t h e magnitude and d u r a t i o n o f t h e f a u l t . S tud ie s show t h a t t h e r e s t r i k e v o l t a g e o f an arc i s about 375 V ( p e a k ) . For a 277/480-V system, a line-to-ground v o l t a g e o f 277 V w i t h a 392-V peak causes r e p e t i t i o u s r e s t r i k i n g or a r c i n g u n t i l t h e f a u l t i s c l e a r e d [ 11-14].

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LRG h a s t h e characterist ics o f h ighe r c o s t , moderate ground f a u l t c u r r e n t , l i t t l e o r no p ropens i ty f o r a r c i n g damage, less f a u l t damage than SG systems, and l i m i t e d l e v e l s of t r a n s i e n t ove rvo l t ages . In LRG systems, t h e r e s i s t o r i s t y p i c a l l y r a t e d t o a l l o w 50 t o 800 amps o f ground f a u l t c u r r e n t . Th i s method is o f t e n used i n MV i n d u s t r i a l systems. Of pr imary concern when app ly ing t h i s method i s t h e need o f s u f f i c i e n t ground f a u l t c u r r e n t f o r o p e r a t i n g ground f a u l t r e l a y s i2-41.

For t h e s e same r e a s o n s , LRG should a l s o be cons ide red f o r 34.5- and 69-kv systems when t h e s e systems are brought i n t o i n d u s t r i a l p l a n t s r a t h e r t han simply adop t ing t h e u t i l i t y p r a c t i c e . For t h i s arrangement , t h e r e s i s t o r would be connected through a. s ingle-phase t r ans fo rmer . Both t h e r e s i s t o r and t h e t r ans fo rmer can b e r a t e d f o r a s h o r t t i m e .

HRG is used i n LV and MV systems (up t o 4.16 kV) where t h e r e i s a d e s i r e not t o t r i p f o r a ground f a u l t t o avoid an unscheduled outage. HRG systems t y p i c a l l y a l l o w less than 7 amps o f ground f a u l t c u r r e n t t o pas s . A t p r e s e n t , it is t y p i c a l p r a c t i c e not t o t r i p when app ly ing HRG. This method assumes t h a t t h e f a c i l i t y has competent maintenance pe r sonne l who can qu ick ly determine t h e l o c a t i o n o f t h e f a u l t ar,d then schedule an ou tage f o r r e p a i r . I n r e a l i t y , t h i s system i s no th ing more than + means o f d e t e c t i n g a ground f a u l t i n an o the rwise ungrounded system. Because o f t h e s m a l l amount o f ground c u r r e n t t h a t i s al lowed t o f low ( s l i g h t l y more than t h e sys t em ' s c a p a c i t i v e cha rg ing c u r r e n t ) , ground f a u l t damage with such a system i s obviously minimal. I f one phase is grounded, t h e l ine- to-ground v o l t a g e o f t h e o t h e r two phases w i l l be inc reased and t h e p o s s i b i l i t y of a second f a u l t , c o n s t i t u t i n g a phase-to-phase f a u l t and i n c u r r i n g major damage, i s heightened 12,141. The need f o r a qu ick r e p a i r i s u r g e n t .

111. SELECTION OF SYSTEM GROUNDING RESISTANCE

A lower ground r e s i s t o r v a l u e impl i e s h ighe r damage; conve r se ly , a h ighe r r e s i s t a n c e impl i e s less damage. A grounding r e s i s t o r should be s e l e c t e d t o minimize f a u l t damage and y e t keep ove rvo l t ages t o a t o l e r a b l e l e v e l , a s w e l l a s p rov id ing s u f f i c i e n t ground f a u l t c u r r e n t f o r r e l a y o p e r a t i o n .

When s e l e c t i n g t h e r e s i s t o r va lue :

Comply w i t h c r i t e r i a set f o r t h i n Appendix A t o l i m i t t r a n s i e n t o v e r v o l t a g e s t o less t h a n 2.5 p e r u n i t ( p u ) l ine- to-ground crest vo l t age .

Allow enough c u r r e n t t o f low so t h a t t h e ground f a u l t r e l a y s w i l l o p e r a t e r e l i a b l y ; however, h i g h e r r e s i s t a n c e ( i .e. , lower f a u l t c u r r e n t ) is d e s i r a b l e .

Achieve 90% winding p r o t e c t i o n of t h e p r o t e c t e d equipment.

S e l e c t e i t h e r a cont inuous duty-rated r e s i s t o r f o r t h e n o n t r i p scheme o r a short- t ime r a t e d one f o r a t r i p p i n g scheme such as LRG.

I n MV i n d u s t r i a l systems, t h e cho ice o f a 400-amp r e s i s t o r has become common ( s t a n d a r d p r a c t i c e by some). A c t u a l l y , t h e c u r r e n t v a l u e o f t h i s r e s i s t o r should be determined by t h e need f o r s u f f i c i e n t ground f a u l t c u r r e n t t o ach ieve s a t i s f a c t o r y r e l a y coordina- t i o n and p r o t e c t i o n , bu t t h i s v a l u e should be low t o minimize f a u l t damage. Fig. 1 shows two schemes w i t h

d i f f e r e n t r e s i s t o r r a t i n g s . It i l l u s t r a t e s t h a t h ighe r c u r r e n t r a t i n g s a r e e s s e n t i a l when more s t a g e s o f c o o r d i n a t i o n are requ i r ed . Fig. l ( b ) has t h e advantage o f less f a u l t damage.

( a ) 400-amp R e s i s t o r

T

1 ( b ) 200-amp R e s i s t o r

F ig . 1. Typical Ground F a u l t P r o t e c t i o n Scheme

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I V . PROTECTION AND COORDINATION

A. Coordinat ion

F ig . 2 shows several t ime-current cu rves d e p i c t i n g a c c e p t a b l e ground f a u l t c o o r d i n a t i o n o f t h e scheme shown. Fig. 2 ( b ) i s t h e t r a d i t i o n a l time-and-current d i f f e r e n t i a t e d coord ina t ion scheme. Fig. 2 ( c ) shows coordinated t i m e d i f f e r e n t i a t i o n , w i t h a l l r e l a y s having t h e same c u r r e n t pickup s e t t i n g . A t f i r s t g l a n c e , F i g , 2 ( d ) appea r s t o b e miscoordinated i n c u r r e n t b u t coord ina ted i n time d i f f e r e n t i a t i o n . Because o f t h e magnitude o f t h e f a u l t c u r r e n t being determined by t h e c u r r e n t v a l u e o f t h e grounding r e s i s t o r ( I ) , a l l t h r e e schemes a r e accep tab le t o se rve t h e pcrpose.

IQ

B. Relays and S e t t i n g s

Two t y p e s o f r e l a y s are used i n ground f a u l t a p p l i c a t i o n : (1) t h e power system o r u t i l i t y t y p e such as t h e Westinghouse CO o r t h e General E l e c t r i c IFC, normally used i n a r e s i d u a l connect ion scheme, and ( 2 ) t h e ground f a u l t s enso rxGFS) t y p e , which is matched w i t h a window o r c o r e ba l ance t y p e o f c u r r e n t t r ans fo rmer t h a t e n c l o s e s a l l conductors o f t h e c i r c u i t . The c h a r a c t e r i s t i c s o f t h e s e two t y p e s o f r e l a y s a r e shown i n Fig. 3.

The power system type has a s a t i s f a c t o r y o p e r a t i n g r e l i a b i l i t y above 1.5 times i t s t a p s e t t i n g , as shown i n F ig . 3 ( a ) . It i s good p r a c t i c e t o ensure t h a t t h e ground f a u l t c u r r e n t allowed by t h e grounding r e s i s t o r w i l l be a t least one and a half o r twice t h e r e l a y t a p s e t t i n g .

I

Current - ( c ) Coordinat ion wi th Same Pickups

( a ) Scheme

Current - ( b ) Proper Coordinat ion

Current - ( d ) Acceptable Coordinat ion

Fig. 2. Acceptable Ground F a u l t P r o t e c t i o n Coordinat ions

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Fig. 3. Comparison of Time-Current Curves

The GFS t y p e o p e r a t e s r e l i a b l y a t s l i g h t l y h ighe r t h a n i t s pickup s e t t i n g . Fig. 3 ( b ) shows t h e r e l a y t ime-current c h a r a c t e r i s t i c s o f t h e GFS. The GFS type can a l s o b e used i n a zone s e l e c t i v e scheme t h a t p rov ides b e t t e r p r o t e c t i o n by blocking upstream r e l a y o p e r a t i o n b u t r e q u i r e s w i r ing between t h e r e l a y s a t t h e v a r i o u s l e v e l s , which is not always f e a s i b l e f o r i n d u s t r i a l a p p l i c a t i o n s because o f t h e d i s t a n c e s involved.

App l i ca t ion o f t h e power system ground f a u l t r e l a y s i n t h e r e s i d u a l connect ion p r e s e n t s s e n s i t i v i t y concerns. This connect ion i s s u s c e p t i b l e t o misope ra t ion from phase unbalance such a s i s experienced du r ing t r ans fo rmer in rush o r motor starting. To avo id misope ra t ion , t h e common p r a c t i c e i s t o d i s a b l e t h e 50N ( i f so equipped) and set pickup c u r r e n t o f t h e 51N to a t least 10% of t h e c u r r e n t t r ans fo rmer (CT) r a t i n g [4,61. Unfor tuna te ly , t h i s connect ion cannot be used or p rope r ly coord ina ted with r e s i s t a n c e grounding u n l e s s t h e CT r a t i o s are low enough t o ensure s u f f i c i e n t c u r r e n t f o r o p e r a t i o n . This a s p e c t must be considered when s e l e c t i n g a grounding r e s i s t o r , because it is p o s s i b l e t h a t t h e r e s i d u a l connect ion may no t be a v i a b l e method o f ground f a u l t d e t e c t i o n i n some resis tance-grounded a p p l i c a t i o n s . F ig . 4 shows a t y p i c a l example o f coord ina t ion .

50015A

( a ) Scheme

20A 5 O A

2 0 0 A

516

51 N

Current - ( b ) Coordinat ion

Fig. 4. Coordinat ion o f Res idua l ly Connected Relay (51N)

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C. EQuipment Winding P r o t e c t i o n

Normally, a 90% winding p r o t e c t i o n f o r t r a n s - formers or g e n e r a t o r s should be achieved. To accomplish t h i s , common p r a c t i c e is t o select a CT w i t h a pr imary c u r r e n t r a t i n g o f h a l f t h e n e u t r a l r e s i s t o r c u r r e n t r a t i n g and t hen t o s e t t h e r e l a y pickup v a l u e a t 10% o f t h e r e s i s t o r r a t i n g . This i s shown i n F ig . 1.

D. Relay s e n s i t i v i t y

Power system r e l a y s ( f o r use i n t h e r e s i d u a l connec t ion ) are a v a i l a b l e w i t h minimum pickup v a l u e s as low as 0.1 amp. However, t h i s low pickup va lue cannot a lways be used because o f t h e p o s s i b i l i t y o f misope ra t ion ( p r e v i o u s l y d i s c u s s e d ) . Thus, a pickup v a l u e o f 0.5 amp f o r 5 1 N r e l a y s i s assumed i n t h e fol lowing c a l c u l a t i o n . To ensure s a t i s f a c t o r y o p e r a t i o n , it w i l l r e q u i r e an o p e r a t i n g c u r r e n t o f a t least 1.5 t imes t h e minimum pickup v a l u e , i . e . , 0.75 amp, a t t h e secondary o f t h e CT. Normally, t h e CT r a t i o is chosen t o b e 1.35 t i m e s t h e maximum r a t e d load c u r r e n t . The re fo re , t h e minimum primary c u r r e n t f o r o p e r a t i o n is approximately 1.35 x 0.75/5 = 0.2 pu o f r a t e d load c u r r e n t . Th i s means t h a t i f a 5 l N i s a c c e p t a b l e , i t s s e n s i t i v i t y t o ground f a u l t s l i e s a t 9 minimum o f 20% o f t h e r a t e d load c u r r e n t t o t r i p r e l i a b l y . S i m i l a r l y , most LV power c i r c u i t b r e a k e r s a r e equipped w i t h an i n t e r n a l ground f a u l t s enso r normally set a t 0.2 pu o f t h e phase senso r r a t i n g ( t h e minimum s e t t i n g ) . These may o r may not coord ina te wi th o t h e r d e v i c e s depending on t h e senso r magnitude, similar t o t h e r e s i d u a l connect ion. The re fo re , a GFS r e l a y i s p r e f e r r e d when p o s s i b l e : it can be set t o a low pickup v a l u e r e g a r d l e s s o f t h e r a t e d load c u r r e n t . By comparison, t h e GFS can clear t h e f a u l t much more qu ick ly , t h e r e b y minimizing f a u l t damage.

Sometimes, ground f a u l t d i f f e r e n t i a l p r o t e c t i o n may be used f o r maximum s e n s i t i v i t y . The c o s t i s high and should be j u s t i f i e d . However, i n multigrounded systems , such p r o t e c t i o n must be used.

E. Summary

The fo l lowing l i s t summarizes t h e p r i n c i p l e s used i n t h e s e l e c t i o n o f t h e n e u t r a l grounding r e s i s t o r v a l u e and ground f a u l t p r o t e c t i o n dev ices :

1) The n e u t r a l grounding r e s i s t o r r a t i n g and ground f a u l t r e l a y s e t t i n g s are i n t e r r e l a t e d with t h e a p p l i c a t i o n g u i d e l i n e s s e t f o r t h above.

Concepts o f a c c e p t a b l e c o o r d i n a t i o n a r e shown i n Fig. 2.

2 )

3) The 51N d e v i c e ( r e s i d u a l l y connected) i s no t g e n e r a l l y a p p l i c a b l e t o LRG systems, except i n a f e w cases where t h e f eede r c u r r e n t and t h e CT r a t i o are low. The re fo re , t h e 5lGS should be used f o r res is tance-grounded systems t o t h e e x t e n t p o s s i b l e .

4 ) Lower r e s i s t o r c u r r e n t r a t i n g s w i l l minimize f a u l t damage.

V. PROPOSED MHRG SYSTEM

To ach ieve lower ground f a u l t c u r r e n t and r equ i r ed ground f a u l t p r o t e c t i o n , t h e a p p l i c a t i o n o f a MHRG system is proposed f o r LV and MV systems where equipment p r o t e c t i o n has a h ighe r p r i o r i t y than s e r v i c e c o n t i n u i t y .

In t h i s proposed scheme, t h e n e u t r a l grounding r e s i s t o r can be s i z e d i n a manner similar t o t h a t o f t h e HRG system p rev ious ly d i scussed . The GFS-type r e l a y i s recommended. To c o o r d i n a t e , a l l GFS r e l a y s can be set a t t h e same pickup s e t t i n g (2-amp primary c u r r e n t pickup i s used i n t h i s d i s c u s s i o n ; even lower s e n s i t i v i t i e s a r e a v a i l a b l e ) , and each r e l a y can be s e t t o a s e q u e n t i a l time delay. This scheme i s i l l u s t r a t e d i n F ig . 5.

( a ) Scheme

E i=

I

Current

( b ) Coordinat ion

Fig. 5 - MHRG Scheme and Coordinat ion

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The fo l lowing l ist summarizes t h e s e l e c t i o n o f t h e n e u t r a l r e s i s t o r and a s s o c i a t e d CT and r e l a y f o r t h e proposed MHRG system:

1) The r e s i s t o r r a t i n g must no t b e less t h a n t h e system cha rg ing c u r r e n t ; 20 amps w i l l be adequate i n almost a l l LV and MV systems [ l s ] . The CT primary c u r r e n t r a t i n g should be h a l f t h e r e s i s t o r c u r r e n t r a t i n g ( i.e., l 0 / 5 amps) . The r e l a y i s set a t 1 amp ( i .e . , 2 amps a t t h e CT primary s i d e ) .

As shown i n Fig. 5 ( b ) , a l l r e l a y s a r e set a t t h e same c u r r e n t pickup wi th t h e p rope r t i m e d i f f e r e n t i - a t i o n between c o o r d i n a t i o n l e v e l s t o ach ieve good coord ina t ion .

2 )

3 )

The proposed scheme seems t o have no de t r imen ta l e f f e c t and compares f avorab ly wi th t h e o t h e r system grounding methods as shown i n Table I1 1121:

TABLE I1 GROUNDING SYSTEM COMPARISONS

No T r i p T r i p Charac t e r i s t i c UG HRG MHRG LRG SG

Trans i en t ove rvo l t age - F F F F

S e r v i c e c o n t i n u i t y F F - - - a f t e r ground f a u l t

Avoidance o f a r c i n g F F F F -

Damage F F- F+ - Fa - F Serving s ingle-phase - -

l oads

F = f avorab le F+ = more f avorab le F- = less f avorab le UG = ungrounded - - - not a p p l i c a b l e

aThe NEC and Occupat ional S a f e t y and Heal th Admin i s t r a t ion (OSHA) do not a l low single-phase load ing o f HRG systems (NEC 250-5 ex. 5d) o r t h e u t i l i t y may r e q u i r e s o l i d grounding o f 277/480-V s e r v i c e . The r e g u l a t i o n s must be reviewed be fo re implementing MHRG where t h e r e i s , t o be s ingle-phase l o a d s . Single-phase load ing is t e c h n i c a l l y f e a s i b l e because t h e f a u l t w i l l be c l e a r e d qu ick ly .

V I . CONCLUSION

A ground f a u l t can be damaging, and a t t e n t i o n should be g iven t o minimizing i t s e f f e c t . With t h e r u l e s s e t f o r t h i n t h i s paper , a sound p r o t e c t i o n p r a c t i c e can b e achieved by p rope r s i z i n g o f t h e grounding r e s i s t o r and s e l e c t i o n o f t h e a p p r o p r i a t e r e l a y s . The proposed MHRG scheme, w i t h i t s low c u r r e n t and s e l e c t i v e t r i p p i n g , has t h e advantage o f reduced damage when compared t o t h e LRG scheme under ground f a u l t cond i t ions . This scheme i s a p p l i c a b l e f o r LV and

MV systems where s e r v i c e c o n t i n u i t y du r ing a ground f a u l t i s not e s s e n t i a l . Where c o n t i n u i t y i s e s s e n t i a l , t h e HRG system should be used.

V I I . REFERENCES

111 Nat iona l E l e c t r i c a l Code - Nat iona l F i r e P r o t e c t i o n Assoc ia t ion , P u b l i c a t i o n 70.

121 E l e c t r i c a l Transmission and D i s t r i b u t i o n Reference Book, Westinghouse E l e c t r i c Corp., East P i t t s b u r g h , PA, 1964.

131 L.W. Manning, " I n d u s t r i a l Power Systems Grounding P r a c t i c e s ," presen ted a t t h e I n d u s t r i a l and Commercial Power Systems Technical Conference, P h i l a d e l p h i a , PA, October 1964.

14 1 Donald Beeman, I n d u s t r i a l Power Systems Handbook, McGraw-Hill Book Co., Inc ., N e w York, 1955.

15 1 I E E E Standard 241-1983, I E E E Recommended P r a c t i c e f o r E l e c t r i c Power Systems i n Commercial Bu i ld ings , The I n s t i t u t e o f E l e c t r i c a l and E l e c t r o n i c s Engineers , Inc .

16 1 I E E E Standard 142-1982, IEEE Recommended P r a c t i c e f o r Grounding of I n d u s t r i a l and Commercial Power Systems, The I n s t i t u t e o f E l e c t r i c a l and E l e c t r o n i c s Engineers , Inc.

17 I I E E E Standard 141-1986, I E E E Recommended P r a c t i c e f o r E l e c t r i c Power D i s t r i b u t i o n f o r I n d u s t r i a l P l a n t s , The I n s t i t u t e o f E l e c t r i c a l and E l e c t r o n i c s Engineers , Inc.

181 I E E E Standard 242-1986, IEEE Recommended P r a c t i c e f o r P ro tec t ion and Coordinat ion of I n d u s t r i a l and Commercial Power Systems, The I n s t i t u t e o f E l e c t r i c a l and E l e c t r o n i c s Engineers , Inc .

[91 ANSI c62.92-1987, I E E E Guide f o r t he App l i ca t ion o f Neutral Grounding i n E l e c t r i c a l U t i l i t y Systems, P a r t I - I n t r o d u c t i o n , American Nat ional S t anda rds I n s t i t u t e .

[lo] J.R. Dunki-Jacobs, " S t a t e o f t h e A r t o f Grounding and Ground F a u l t P ro tec t ion" , p re sen ted a t t h e I E E E 24th Annual Petroleum and Chemical I n d u s t r y Conference, Da l l a s , TX, September 12-14, 1977.

Ill] J . R . Dunki-Jacobs, "The E f f e c t s o f Arcing Ground F a u l t s on Low Voltage System Design ," IEEE Transac t ions on Indus t ry and General A p p l i c a t i o n s , Vol. I A - 8 , No. 3, May/June 1972.

1121 GET-6098, The Impact o f Arcing Ground F a u l t s on Low Voltage System Design, General E l e c t r i c Co.

1131 Harris I. Stanback, Jr., "P red ic t ing Damage from 2778 S i n g l e Phase t o Ground Arcing Fau l t s , " I E E E Transac t ions on I n d u s t r i a l App l i ca t ions , Vol. 1~-18, No. 13, July/August 1977.

[ 141 J .R. Dunki-Jacobs, "The R e a l i t y of High Res i s t ance Grounding ," presen ted a t t h e IEEE 23rd Annual Petroleum and Chemical Indus t ry Conference, P h i l a d e l p h i a , PA, August 30-September 1, 1976.

115 1 PRSC-4E, System Neutral Grounding and Ground F a u l t P r o t e c t i o n Guide, Westinghouse I n d u s t r i a l a n d Commercial Power Systems App l i ca t ions S e r i e s , February 1986.

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APPENDIX A DETERMINATION OF TRANSIENT OVERVOLTAGE

To l i m i t t r a n s i e n t o v e r v o l t a g e s t o 250% o f t h e normal l i n e - t o - n e u t r a l crest v o l t a g e , t h e fol lowing cr i ter ia should be observed 12,3 ,9 ] :

Grounding System Parameter R e s t r a i n t

LRG R o / X o 2.0 and Xo/X, 5 20.0

HRG Rg I xc

Where Ro = ze ro sequence r e s i s t a n c e Xo = ze ro sequence r eac t ance X1 = p o s i t i v e sequence r e a c t a n c e R = system n e u t r a l r e s i s t a n c e X: = system c a p a c i t i v e r e a c t a n c e

( a l l i n t h e same u n i t s )

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