electric power supply for a large chemical plant

Electric Power Supply for a Larse

Chemical Plant

A. C. FRIEL MEMBER AIEE

TH E a-c e l e c t r i c p o w e r g e n e r a t i o n a n d d i s t r i b u t i o n s y s t e m of t h e D o w C h e m ­

ical C o m p a n y a t M i d l a n d , M i c h . , m a y be of i n t e r e s t t o p o w e r e n g i n e e r s for t w o r e a s o n s . F i r s t , i t r e p r e s e n t s t h e e x ­pe r i ence of a p l a n t w i d e a -c s y s t e m t h a t has g r o w n f r o m s m a l l b e g i n n i n g s in a pe r iod of 2 3 y e a r s a n d h a s e n c o u n t e r e d a n d s o l v e d t h e p r o b l e m s t h a t g o w i t h r ap id g r o w t h . S e c o n d , b e i n g a c h e m i ­cal p l a n t , m a n y s p e c i a l p r o b l e m s p e c u l i a r to t h e c h e m i c a l i n d u s t r y w e r e s o l v e d in t he p r o c e s s , y i e l d i n g t e c h n i q u e s t h a t m a y b e of i n t e r e s t t o o t h e r s in t h e chemica l a n d p e t r o l e u m i n d u s t r y .

Generation System

G e n e r a t i o n a t t h e M i d l a n d p l a n t t o d a y is p r o v i d e d b y s e v e r a l g e n e r a t i n g u n i t s . T h e s e a r e s u p p l e m e n t e d b y a c o n n e c t i o n wi th C o n s u m e r s P o w e r C o m p a n y , t h e whole s c h e m e b e i n g a m i n i a t u r e of a pubHc u t i H t y s y s t e m . G r o w t h of t h i s s y s t e m is s h o w n b y Figin-e 1.

I n 1927, t h e f i rs t m a j o r a - c u n i t s , a p a i r of 6 ,250 k v a m a c h i n e s , s e r v e d a 1 2 5 - a c r e p l a n t a t 2 ,400 v o l t s . T h e l o n g e s t f e e d e r was a b o u t 3 ,600 f ee t .

I n 1930, t h e n e x t u n i t w e n t i n t o o p e r a ­t ion w i t h a 1 0 , 0 0 0 - k v a r a t i n g . T h i s n e w u n i t w a s fo r 13 ,800 v o l t s . A t t h i s t i m e the e l e c t r i c s y s t e m s e r v e d a p l a n t a r e a of a b o u t 2 3 0 a c r e s . T h e l o n g e s t 2 , 4 0 0 - v o l t feeder w a s n o w 6 ,200 fee t .

A d d i t i o n a l u n i t s w e r e a d d e d a s p l a n t r e q u i r e m e n t s for s t e a m a n d e l e c t r i c e n e r g y grew u n t i l i n 1948 t h e l o a d w a s o v e r s ix t i m e s w h a t i t w a s i n 1927 .

T h e S o u t h P o w e r H o u s e w e n t i n t o o p e r a t i o n i n 1950 w i t h a 3 7 , 5 0 0 - k v a u n i t . A d u p l i c a t e u n i t i s n o w o n o r d e r . T h e p r e s e n t p l a n t c o v e r s a p p r o x i m a t e l y 6 3 0

J. P. SMITH MEMBER AIEE

a c r e s a n d t h r o u g h t h e u s e of a s y s t e m of s u b s t a t i o n d i s t r i b u t i o n , t h e l e n g t h of t h e l o n g e s t 2 , 4 0 0 - v o l t f e e d e r w a s r e d u c e d t o 3 ,000 fee t . N o t e t h a t i n 2 3 y e a r s , p l a n t a r e a i n c r e a s e d five t i m e s , b u t t h e g e n e r a ­t i o n k i l o v o l t - a m p e r e s i n c r e a s e d o v e r e i g h t t i m e s .

T w o o u t l y i n g 5 , 0 0 0 - k v a u n i t s c o m p l e t e t h e p r e s e n t p i c t u r e of t h e D o w g e n e r a t i n g s y s t e m , t h e s e t w o b e i n g l o c a t e d b y r e a s o n of s t e a m r e q u i r e m e n t s .

T h e C o n s u m e r s P o w e r C o m p a n y s u p ­p l i e s p o w e r r e q u i r e d a b o v e o u r g e n e r a t i n g c a p a c i t y w h i c h a t p r e s e n t i s a r e l a t i v e l y s m a l l p o r t i o n of t h e t o t a l .

T h e s e p o w e r s o u r c e s a d d u p t o a t o t a l i n s t a l l e d c a p a c i t y for t h e s y s t e m of o v e r 150 ,000 k v a . W i t h t h e n e x t u n i t a t S o u t h P o w e r H o u s e t h i s wi l l g r o w t o a l ­m o s t 2 0 0 , 0 0 0 k v a .

M a c h i n e s N u m b e r 6 a n d 14 a r e e a c h g r o u n d e d t h r o u g h a 5V3-ohm r e s i s t o r a n d m a c h i n e N u m b e r 15 t h r o u g h a 4 - o h m r e ­s i s t o r . T h e s e r e s i s t o r s will k e e p g r o u n d f a u l t c u r r e n t d o w n t o 5 ,000 a m p e r e s , suff ic ient for r a p i d r e l a y t r i p p i n g b u t i n ­suff ic ient t o c a u s e e x c e s s i v e b u r n i n g of m a c h i n e l a m i n a t i o n s .

Steam-Electric Balance

W i t h D o w , a s p r e s u m a b l y i t is w i t h m o s t i n d u s t r i a l s w h o g e n e r a t e e l e c t r i c p o w e r , t h e p r o c e s s s t e a m d e m a n d g o v e r n s t h e a m o u n t of e l e c t r i c p o w e r w h i c h i s g e n e r a t e d . T h e s t e a m is g e n e r a t e d a t a h i g h e r press iu-e a n d t e m p e r a t u r e t h a n t h e p r o c e s s r e q u i r e s a n d is t h e n r u n t h r o u g h t u r b i n e s o n i t s w a y t o t h e p r o c e s s s t e a m l o a d s . E l e c t r i c p o w e r is t h e r e b y s t r i p p e d o u t of t h i s s t e a m b y t h e t u r b o g e n e r a t o r s . I t h a s b e e n f o u n d t h a t w h e r e a d e m a n d for s u b s t a n t i a l q u a n t i t i e s of p r o c e s s s t e a m a n d e l e c t r i c p o w e r e x i s t w i t h i n t h e s a m e

p l a n t o r w i t h i n r e l a t i v e l y c lose p r o x i m i t y of e a c h o t h e r , t h i s m e t h o d of o p e r a t i o n r e ­s u l t s in r e d u c e d t o t a l c o s t of s t e a m p l u s e l e c t r i c p o w e r .

E l e c t r i c p o w e r g e n e r a t e d in a n i n d u s ­t r i a l p l a n t in t h i s m a n n e r is a b y - p r o d u c t of p r o c e s s s t e a m u s e a n d a c c o r d i n g l y t h e q u a n t i t y of p o w e r g e n e r a t e d is g o v e r n e d b y t h e a m o u n t of p r o c e s s s t e a m u s e d .

T h i s m e t h o d of e l e c t r i c p o w e r g e n e r a ­t i o n c o n t r a s t s w i t h t h e u s u a l p u b h c u t i l i t y p o w e r g e n e r a t i o n w h e r e i n t h e e lec­t r i c p o w e r is t h e e n d p r o d u c t a n d t h e s t e a m , a f t e r h a v i n g g o n e t h r o u g h t h e t u r ­b i n e s , t h e n goes t o c o n d e n s e r s w h e r e 5 5 t o 6 0 p e r c e n t of t h e e n e r g y s u p p l i e d b y t h e fuel i s l o s t .

C o n s i d e r a t i o n of t h e s e f a c t s m a k e s i t a p p a r e n t t h a t i t is d e s i r a b l e u n d e r t h e c o n d i t i o n s o u t l i n e d t o m a i n t a i n a s c lose a b a l a n c e a s p o s s i b l e b e t w e e n s t e a m d e ­m a n d s a n d e l e c t r i c p o w e r d e m a n d s in o r d e r t o k e e p r e q u i r e m e n t s for t h e m o r e e x p e n s i v e p u r c h a s e d p o w e r a s l o w a s p o s ­s ib le .

F i g u r e 1 a l s o s h o w s t h e g r o w t h of e lec­t r i c a l l o a d a g a i n s t s t e a m l o a d . T h e e lec­t r i c a l l o a d h a s d e v e l o p e d f a s t e r t h a n t h e s t e a m l o a d a n d a s a r e s u l t t h e s t e a m g e n ­e r a t i o n t e m p e r a t u r e s a n d p r e s s u r e s h a v e b e e n i n c r e a s e d a n d t h e d i s t r i b u t i o n p r e s -stu-es t o p r o c e s s h a v e b e e n r e d u c e d t o t h e l o w e s t p o s s i b l e in o r d e r t o g e t m o r e e lec­t r i c p o w e r o u t of t h e s t e a m r e q u i r e d b y o u r p r o c e s s e s . I n s p i t e of c o n s i d e r a b l e effor t t o i n c r e a s e p r o c e s s s t e a m d e m a n d v e r s u s p o w e r u s e , p u r c h a s e d p o w e r is s t i l l r e q u i r e d , w h i c h is o f t e n r e f e r r e d t o a s c o n ­d e n s i n g p o w e r a t D o w . R e c e n t s t u d i e s i n d i c a t e t h a t D o w c o u l d p r o b a b l y m a k e i t s o w n c o n d e n s i n g p o w e r for n e a r l y t h e s a m e c o s t a s p u r c h a s e d p o w e r , b u t a c h e m i c a l m a n u f a c t u r e r finds i t m o r e p r o f i t a b l e t o i n v e s t c a p i t a l i n n e w p r o c e s s e q u i p m e n t r a t h e r t h a n in p o w e r - g e n e r a t ­i n g e q u i p m e n t o n t h a t b a s i s .

Paper 5 2 - 8 0 , recommended b y the A I E E Industrial Power. Sys tems Committee and approved by the A I E E Technical Program Committee for presenta­t ion at the A I E E Winter General Meet ing, N e w York, N . Y., January 2 1 - 2 5 , 1952. Manuscript submit ted October 22, 1951; made available for printing December 18, 1951.

A. C. F R I E L is with the D o w Chemical Company , Midland, Mich. , and J. P. S M I T H is with the General Electric Company , Detroit , Mich.

JANUARY 1 9 5 2 Friel, Smith—Electric Power Supply for a Large Chemical Plant

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Figure 1 . Rate of system load srowth

' 4 8 ' 5 0 Figure 3 . South Power H o u s e circuit breaker

assembly

W i t h g e n e r a t i o n of t h e e n t i r e p o w e r r e ­q u i r e m e n t , D o w w o u l d i n c u r t h e e x p e n s e a n d o p e r a t i o n of f r e q u e n c y r e g u l a t i n g e q u i p m e n t , w h i c h i s n o w le f t t o t h e p u b l i c u t i l i t y . F u r t h e r , i t m i g h t b e c o n s i d e r e d t h a t p u r c h a s e d p o w e r i s m o r e firm t h a n t h e M i d l a n d p l a n t g e n e r a t e d p o w e r s i n c e i t i s b a c k e d u p b y a l a r g e r s y s t e m a n d t h e r e f o r e , p r e s u m a b l y a m o r e s t a b l e s o u r c e . H o w e v e r , t h e r e l i a b i l i t y a n d e c o n o m y of p u r c h a s e d v e r s u s h o m e - g e n ­e r a t e d c o n d e n s i n g p o w e r i s a l a r g e s u b j e c t a n d o n e w h i c h wil l n o t b e s e t t l e d h e r e .

Getting Power to the Load

VOLTAGE SELECTION

T h e p o w e r s y s t e m a t D o w h a s g o n e t h r o u g h s e v e r a l s t a g e s i n i t s e v o l u t i o n . O r i g i n a l l y , 2 , 400 v o l t s w a s s e l e c t e d for g e n e r a t i o n a n d d i s t r i b u t i o n . G e n e r a t o r s w e r e s m a l l i n s ize , l o a d s w e r e n e a r b y , t h e p o w e r c o m p a n y w a s a b l e t o f u r n i s h 2 , 4 0 0 - v o l t p o w e r , a n d t h i s v o l t a g e w a s we l l s u i t e d t o m o t o r - g e n e r a t o r s e t s a n d r o t a r y c o n v e r t e r l o a d s .

L a t e r , o t h e r u n i t s w e r e l o c a t e d f a r t h e r

GENERATOR Νδ |5 3 7 , 5 0 0 kvo 1 3 , 8 0 0 V

f r o m t h e l o a d c e n t e r . T h e t u r b i n e s i n ­s t a l l e d w e r e 9 ,375 k v a , q u i t e l a r g e fo r 2 ,400 v o l t s . I t w a s a t t h i s t i m e t h a t 13 ,800 v o l t s w a s s e l e c t e d for a l l f u t u r e ex ­p a n s i o n . T h i s v o l t a g e h a s p r o v e n s a t i s ­f a c t o r y w i t h c o n t i n u e d p l a n t g r o w t h . I t t o d a y p e r m i t s a d i s t r i b u t i o n p a t t e r n of 7 ,000- t o 1 0 , 0 0 0 - k v a l o a d s s e r v e d b y s i n g l e 3 - c o n d u c t o r 5 0 0 , 0 0 0 c i r c u l a r m i l c a b l e s .

SYNCHRONIZING BUS SYSTEM

T h e r e is a n i m p o r t a n t f a c t o r w h i c h is well k n o w n b u t o f t e n n o t c o n s i d e r e d ; s h o r t c i r c u i t c a p a c i t y . A t o n e l o c a t i o n t h e r e a r e foiu* t u r b o g e n e r a t o r s , b u t o n l y o n e m a c h i n e is c o n n e c t e d t o a b u s s e c t i o n . T h r e e of t h e s e g e n e r a t o r s a r e 6 , 2 5 0 - k v a u n i t s . W i t h o n e m a c h i n e o n a b u s s e c ­t i on , a n d a s s u m i n g a 10 p e r c e n t r e a c t a n c e g e n e r a t o r , t h e m a c h i n e w o u l d s u p p l y 6 2 , 5 0 0 k v a t o a b u s o r f e e d e r f a u l t . A d d ­i n g t h e c o n t r i b u t i o n f r o m c o n n e c t e d l o a d s a n d t h e c o n t r i b u t i o n f r o m t i e s w i t h c u r r e n t - l i m i t i n g r e a c t o r s , 1 0 0 , 0 0 0 - k v a s h p r t c i r c u i t c a p a c i t y i s v e r y q u i c k l y r e a c h e d .

GENERATOR NQ I6 37,500kvo 13 ,800V

S Y N C H R O N I Z I N G BUS

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FDR GND, SW

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Figure 2 . South Power H o u s e 1- l ine diagram

POTHEAOS 5 0 0 0 kvo EACH

A\'hen i t i s r e m e m b e r e d t h a t t h e largest s t a n d a r d c i r c u i t b r e a k e r s h a v e a n inter­r u p t i n g c a p a c i t y of o n l y 150 ,000 k v a at 2 ,400 v o l t s , i t is o b v i o u s t h a t t w o gener­a t o r s of t h i s s ize c a n n o t b e c o n n e c t e d to a c o m m o n b u s , s i n c e t h e s h o r t circuit c a p a c i t y w o u l d r e a c h n e a r l y 200,000 k v a .

A s y n c h r o n i z i n g b u s s y s t e m h a s been a d o p t e d . O n e g e n e r a t o r s e r v e s e a c h of t h e f o u r b u s s e c t i o n s . A r e a c t o r con­n e c t s e a c h b u s s e c t i o n t o a m a i n syn­c h r o n i z i n g b u s , w h i c h af fords m e a n s for p o w e r t r a n s f e r b e t w e e n b u s s e c t i o n s in the p o w e r p l a n t .

E a c h b u s s e c t i o n s e r v e s f eede r s whose n o r m a l l o a d wi l l t o t a l a p p r o x i m a t e l y the n o r m a l g e n e r a t e d k i lo v o l t - a m p e r e s of that b u s s e c t i o n . I n t h i s w a y , p o w e r transfer t h r o u g h r e a c t o r s a n d t h e synchroniz ing b u s wi l l b e k e p t t o a m i n i m u m , s o that r e a c t o r losses a r e s m a l l , a n d t h e r e is very l i t t l e v o l t a g e d i f fe rence b e t w e e n t h e vari­o u s b u s s e s .

T h e s y n c h r o n i z i n g b u s t o g e t h e r with c h a n g e s t o b e m a d e s o o n wil l m a k e i t pos­s ib le t o s h u t d o w n a n y m a c h i n e , which m a y b e n e c e s s a r y d u e t o v a r i a t i o n s in s t e a m d e m a n d w i t h o u t i n t e r r u p t i n g serv­ice t o a n y l o a d . F o r e x a m p l e , if one m a c h i n e w e r e s h u t d o w n , p o w e r s u p p l y to t h e l o a d o n t h a t b u s s e c t i o n w o u l d n o t be i n t e r r u p t e d , s i n c e t h e t h r e e o t h e r ma­c h i n e s w o u l d i m m e d i a t e l y s u p p l y power t h r o u g h t h e i r r e s p e c t i v e b u s section r e a c t o r s t o t h e s y n c h r o n i z i n g b u s , and t h e n t h r o u g h a r e a c t o r t o t h e b u s section in t r o u b l e , o r p o w e r c o u l d b e imported f r o m o t h e r s o u r c e s b y t i e l ines . Power n e v e r f lows t h r o u g h m o r e t h a n t w o leac-t o r s b e t w e e n a n y g e n e r a t o r a n d a n y load w i t h t h i s s y s t e m .

D o w ' s o t h e r p o w e r h o u s e s a l so use s y n c h r o n i z i n g b u s s y s t e m s . I n o n e case i t h a s b e e n p o s s i b l e t o c o n n e c t 30 ,000 kva of g e n e r a t i o n t o o n e s e c t i o n , a n d 35,000

84 Friel, Smith—Electric Power Supply for a Large Chemical Plant JANUARY 1952

Fisuie 4 (left). South Power House transfer

bus

kva of utility power to another section since the 13.8-kv circuit breakers have an interrupting capacity of GOO megavolt-amperes. (Circuit breakers on those sec­tions have recently been rebuilt to pro­vide 960-megavolt-ampere capacity, since new tie lines have raised the short circuit contribution from the synchronizing bus.)

vSouth Power House, with 37,500-kva generators at 13.8 kv, has been provided with 1,500-megavolt-ampere switchgear which will be discussed later.

Experience gained from short circuit studies of this system leads to the con­clusion that with a synchronizing bus system, the short circuit capacity of a generator switchgear bus section should be at least 16 times the total name plate kilovolt-amperes of generation connected to that bus (assuming a generator sub-transient reactance of approximately 10 per cent).

The synchronizing bus system at the powerhouses also helps to solve a tie-line problem which has been trcublesome.

Tie Lines Between Power Stations

Between a pair of Dow powerhouses which shall be called X and Y, a 4,687-kva

Fisure 6 (risht). East Side sub­station number 3

line ties bus section Number 10 of power­house X to section Number 1 of power­house F. A second tie of 8,000-kva capacity connects powerhouse X syn­chronizing bus with powerhouse Y section Number 1.

It has never been possible to operate these tie lines at rated capacity. At a time when power must be transferred from X to F, there will be flow through Num­ber 10 section reactor at X. This causes a voltage difi'erence between section Number 10 and the synchronizing bus, the synchronizing bus being at the lower voltage. Thus despite the difi'erence in tie line ratings, they actually carry about the same power, so that actual tie capacity is only 74 per cent of the rated capacity.

The system is being arranged so that both tie lines will terminate at the syn­chronizing bus at both powerhouses.

There is another pair of ties each of 16,000-kva capacity, terminating at sec­tion busses. With present reactances and under certain conditions 70 per cent of the kilovolt-amperes transferred over these ties is carried by line A ; 30 per cent by fine B. Thus instead of 32,000-kva tie

capacity, we have only an effective 23,000 kva. Here also, the system is being ar­ranged with tie lines terminating at sta­tion synchronizing busses, which will make the entire 32,000-k\^a capacity available.

With the system rearranged in this manner, the synchronizing busses and the tie lines will afford a solid backbone for the system. Each section bus will form a practically self-sufficient system with generation equal to load. Upon loss oF generation, or under excess load condi­tions, the power deficiency of any section bus will be made up by power flowing from the synchronizing bus through the section reactor. With this system, there will be only two reactors between any generator and any load in the entire Dow system.

Power House Distribution Busses

At each of the powerhouses, radial feeders carry power directly from the generator bus to the load area. As far as possible, these feeders are so arranged that loads nearest a given powerhouse are

Fisure 5. Reactors and bus duct Figure 7. N T switch house

85 JANUARY 1952 Friel, Smith—Electric Power Supply for a Large Chemical Plant

Figure 8 . Typica l transformer bank

s e r v e d f r o m t h a t s t a t i o n , a n d t h e f e e d e r k i l o v o l t - a m p e r e s o n a g i v e n b u s a r e k e p t t o t h e n o r m a l g e n e r a t e d k i l o v o l t - a m p e r e s c o n n e c t e d t o t h a t b u s .

I n o n e i n s t a n c e , t h e s e t w o r e q u i r e ­m e n t s w e r e in conf l ic t . A 5 , 0 0 0 - k v a s u b ­s t a t i o n l o c a t e d n e a r p o w e r h o u s e A w o u l d l og i ca l l y b e s e r v e d f r o m t h a t p o w e r h o u s e , b u t g e n e r a t i o n fo r t h i s l o a d w a s l o c a t e d a t p o w e r h o u s e B. i t p r o v e d b e t t e r e c o n o m ­ics t o i n s t a l l a r a d i a l f e e d e r t h e l o n g e r d i s t a n c e f r o m p o w e r h o u s e Β t o t h i s s u b ­s t a t i o n , s i n c e c o n n e c t i n g t h i s l o a d t o p o w e r h o u s e A w o u l d i m p o s e a n o r m a l 5 , 0 0 0 - k v a l o a d o n t h e 3 2 , 0 0 0 - k v a t i e l i ne s , t h u s r e d u c i n g t h e i r a b i l i t y t o t r a n s ­fer p o w e r b e t w e e n s t a t i o n s d u r i n g e m e r ­g e n c i e s o r m a c h i n e o u t a g e s . T h e s w i t c h ­i n g e q u i p m e n t a t t h e o l d e r p o w e r h o u s e s is of o p e n t y p e c o n s t r u c t i o n . I n o r d e r t o g a i n t h e a d v a n t a g e s of i n d o o r p r o t e c t e d c o n s t r u c t i o n , m e t a l - c l a d s w i t c h g e a r of 1 , 5 0 0 - m e g a v o l t - a m p e r e s h o r t c i r c u i t c a ­p a c i t y i s u s e d a t S o u t h P o w e r H o u s e , w h i c h is t h e n e w e s t o n e .

F i g u r e 2 s h o w s t h e m a n n e r i n w h i c h t h e m a c h i n e s wi l l b e c o n n e c t e d t o b u s s e c t i o n s a n d a l s o h o w t h e b u s s e c t i o n s a r e c o n ­n e c t e d t h r o u g h r e a c t o r s t o t h e s y n c h r o n i z ­i n g b u s a t S o u t h P o w e r H o u s e . I t wi l l b e n o t e d t h a t t h e r e i s a t r a n s f e r b u s w h i c h , b y p r o p e r m a n i p u l a t i o n of d i s c o n n e c t s w i t c h e s a n d c i r c u i t b r e a k e r s , c a n b e u s e d

t o t r a n s f e r a n y f e e d e r f r o m i t s b u s s e c t i o n t o t h e t r a n s f e r b u s w i t h o u t d r o p p i n g l o a d o n t h e f eede r . I n t h i s w a y , a f e e d e r c i r ­c u i t b r e a k e r c a n b e r e m o v e d f r o m s e r v i c e fo r m a i n t e n a n c e w h e n r e q u i r e d . T h e t r a n s f e r b u s c a n a l s o b e u s e d t o r e d u c e l o a d o n a b u s s e c t i o n w h e n a b n o r m a l c o n ­d i t i o n s r e q u i r e . T h i s a r r a n g e m e n t is s i m i l a r t o t h a t u s e d a t a n y of t h e D o w p o w e r h o u s e s .

F i g u r e 3 s h o w s t h e i n s t a l l a t i o n of 1,500-m e g a v o l t - a m p e r e c i r c u i t b r e a k e r c u b i c l e s a t S o u t h P o w e r H o u s e . T h e a d d i t i o n of n e w f e e d e r s i s b e i n g a c c o m p l i s h e d b y e x ­t e n d i n g t h i s a s s e m b l y o n t h e f a r e n d . T h e s e a r e s t a t i o n t y p e s t a t i o n a r y a i r b l a s t c i r c u i t b r e a k e r s of 1 ,500 -megavo l t -a m p e r e c a p a c i t y i n s e g r e g a t e d p h a s e m e t a l - c l a d c o n s t r u c t i o n .

F i g u r e 4 s h o w s t h e r e a r of t h e c i r c u i t b r e a k e r c u b i c l e s , w i t h o p e r a t i n g m e c h a ­n i s m for c i r c u i t b r e a k e r d i s c o n n e c t s , a n d o n t h e o p p o s i t e s i d e of t h e a i s l e t h e t r a n s ­fe r b u s , w i t h i t s d i s c o n n e c t i n g s w i t c h m e c h a n i s m s .

F i g u r e 5 s h o w s t h e floor b e l o w t h e c i r ­c u i t b r e a k e r . T h e b u s d u c t c o n n e c t i n g t h e c i r c u i t b r e a k e r s w i t h t h e t r a n s f e r b u s a r e o n t h e ce i l ing of t h i s r o o m . T h e s y n ­c h r o n i z i n g b u s r e a c t o r s a n d t h e s y n ­c h r o n i z i n g b u s a r e a l s o o n t h i s floor. O u t ­g o i n g c a b l e s e n t e r p o t h e a d s i n t h e c u b i c l e s a t t h e f a r s i d e of t h e r o o m .

I t i s i n t e r e s t i n g t o n o t e t h a t e a c h f e e d e r c i r c u i t b r e a k e r wi l l s u p p l y t w o c i r -

Figure 9 . Three-phase transformer

c u i t s e x t e n d i n g t o d i f fe ren t p l a n t areas. T h e n o r m a l l o a d o n e a c h f eede r cab l e is 5 ,000 k v a , s o t h i s wil l p e r m i t e a c h circuit b r e a k e r t o s e r v e a l o a d of a p p r o x i m a t e l y 10 ,000 k v a . W i t h a t o t a l g e n e r a t i o n of 3 7 , 5 0 0 k v a p e r b u s s e c t i o n , fotu* feeder c i r c u i t b r e a k e r s a r e n e c e s s a r y for each b u s s e c t i o n . T h e f eede r s a r e so a r ranged t h a t u p o n o p e n i n g of a n y feede r circuit b r e a k e r , t h e e n t i r e 1 0 , 0 0 0 - k v a l o a d c a n be s e r v e d f r o m o t h e r s o u r c e s a s d e s c r i b e d in t h e n e x t d i s c u s s i o n .

Power Distribution System

O u r p l a n t d i s t r i b u t i o n s y s t e m a s it s t a n d s t o d a y a n d a s i t is e x p e c t e d t o be for y e a r s t o c o m e is r a d i a l . T h e p lan t e l e c t r i c a l l o a d is of t w o t y p e s : p o w e r for e l e c t r o l y t i c s e r v i c e , ( l a r g e l y c h l o r i n e pro­d u c t i o n ) a n d p o w e r for p l a n t process d r i v e s , l i g h t i n g , a n d o t h e r p u r p o s e s . The t w o t y p e s of l o a d a r e a p p r o x i m a t e l y e q u a l i n k i l o v o l t - a m p e r e d e m a n d .

T h e e l e c t r o l y t i c l o a d s a r e s e r v e d at 13 ,800 v o l t s b y r a d i a l f e ede r s directly, f r o m p o w e r h o u s e b u s s e c t i o n s . T h i s in­c l u d e s rec t i f i e r s , m o t o r - g e n e r a t o r s e t sub­s t a t i o n s , a n d r o t a r y c o n v e r t e r subs ta ­t i o n s .

P l a n t a n d p r o c e s s l o a d w a s originally fed b y 2 , 4 0 0 - v o l t c i r c u i t s f r o m o n e power­h o u s e b u t a s l o a d s b e c a m e h e a v i e r and

8 6 Friel, Smith—Electric Power Supply for a Large Chemical Plant JANUARY 1952

feeders b e c o m e l o n g e r , a s y s t e m of s u b ­s t a t i o n d i s t r i b u t i o n w a s a d o p t e d .

A 5 0 0 , 0 0 0 c i r c u l a r mi l l a e r i a l c a b l e a t 13,800 v o l t s c a r r i e s p o w e r f r o m t h e p o w e r ­h o u s e t o t h e c e n t e r of a p a r t i c u l a r l o a d a r e a . A 2 , 4 0 0 - v o l t l o a d c e n t e r is i n ­s t a l l ed a t t h i s p o i n t , c o n s i s t i n g of a 6 , 0 0 0 / -7 ,500-kva 3 - p h a s e oil filled o u t d o o r t r a n s ­fer b a n k c o n n e c t e d t o a n i n d o o r 2 , 4 0 0 -vo l t s w i t c h g e a r a s s e m b l y . F i g u r e 6 shows s u b s t a t i o n N u m b e r 3 .

A c o m p a r i s o n of c o s t i n 1950 b e t w e e n an i n d o o r a n d o u t d o o r s u b s t a t i o n s h o w e d t h a t c o s t s r a n $ 1 5 , 0 0 0 p e r c i r c u i t p o s i t i o n for t h e i n d o o r s u b s t a t i o n a n d $ 1 4 , 5 0 0 p e r c i rcu i t p o s i t i o n for t h e o u t d o o r s u b s t a ­t ion . I t w a s d e c i d e d t h a t i n v i e w of t h e r e l a t i v e l y s e v e r e w i n t e r c l i m a t e , a n d t h e chemica l s i n t h e a t m o s p h e r e t h e i n d o o r s u b s t a t i o n w i t h i t s i n c r e a s e d o p e r a t i n g c o n v e n i e n c e a n d r e d u c e d m a i n t e n a n c e r e ­q u i r e m e n t s w o u l d d e f i n i t e l y j u s t i f y t h e inc rea sed first c o s t .

S e v e r a l of t h e s e s u b s t a t i o n s a r e n o w i n service , t h e first h a v i n g b e e n i n s t a l l e d in 1942, a n d s e v e r a l m o r e a r e n o w in t h e process of b e i n g i n s t a l l e d . W o r k i n g l o a d per s u b s t a t i o n wi l l b e k e p t d o w n t o 5 ,000 kva s o t h e r e wi l l b e m a r g i n for l o a d g r o w t h t h a t m i g h t o c c u r f a s t e r t h a n w e a n t i c i p a t e , a n d a l s o t o m e e t e m e r g e n c y c o n d i t i o n s w h i c h m a y a r i s e .

LOW-VOLTAGE TIES

T h e s y s t e m a s e x p l a i n e d s o f a r is a pure ly r a d i a l s y s t e m . H o w e v e r , s i n c e m a n y of t h e p r o c e s s e s s e r v e d f r o m t h e 2,400-vol t s u b s t a t i o n s a r e c r i t i c a l a n d c a n n o t b e i n t e r r u p t e d , a s e c o n d a r y s e l e c ­t ive f e a t u r e is p r o v i d e d b y n o r m a l l y o p e n 2,400-vol t t i e s b e t w e e n p a i r s of s u b s t a ­t ions . B y s i z ing t h e 1 3 . 8 - k v c a b l e for 10,000 k v a , a n d b y u s i n g t r a n s f o r m e r s r a t e d 7 ,500 k v a c o n t i n u o u s w i t h f a n coo l ­ing p r o v i s i o n h a s b e e n m a d e for e m e r ­gency o p e r a t i o n of t w o s u b s t a t i o n s f r o m a single 1 3 . 8 - k v f e e d e r a n d t r a n s f o r m e r . T h u s , in c a s e o n e s u b s t a t i o n is l o s t d u e t o

a p r i m a r y f a i l t u e , i t c a n b e s e r v e d for a s h o r t t i m e a t fu l l c a p a c i t y , o r fo r a n e x ­t e n d e d p e r i o d a t r e d u c e d c a p a c i t y , b y c l o s i n g o u t t h e s e c o n d a r y t i e .

I n s e t t i n g u p t h e s e s e c o n d a r y t i e s o n e of t h e l i m i t a t i o n s of 2 , 4 0 0 - v o l t d i s t r i b u ­t i o n m u s t b e f a c e d . T h e t r a n s f o r m e r c i r ­c u i t b r e a k e r s a r e r a t e d 2 ,000 a m p e r e s , t h e l a r g e s t d r a w - o u t t y p e 2 , 4 0 0 - v o l t c i r c u i t b r e a k e r a v a i l a b l e . T h i s p l a c e s a n a m e p l a t e ce i l i ng of 8 ,000 k v a o n t h e i n p u t t o a s u b s t a t i o n w h e r e a s a l l o t h e r e q u i p m e n t a n d c a b l e i s s i z e d for 10 ,000 k v a u n d e r e m e r g e n c y c o n d i t i o n s . W e r e 4 , 1 6 0 v o l t s b e i n g u s e d , t h i s l i m i t a t i o n w o u l d n o t ex ­i s t .

U s e of 4 , 1 6 0 - v o l t d i s t r i b u t i o n i n c e r t a i n a r e a s h a s b e e n c o n s i d e r e d , b u t a c h a n g e a t t h i s t i m e w o u l d l e a d t o m u c h c o n f u s i o n , w o u l d r e q u i r e r e a r r a n g e m e n t of c i r c u i t s , a n d m i g h t r e q u i r e r e w i n d i n g of s o m e 2 , 3 0 0 - v o l t m a c h i n e s .

I t i s i n t e r e s t i n g t o c o m p a r e t h e o l d w i t h t h e n e w . F i g u r e 7 s h o w s t h e t y p e of c o n ­s t r u c t i o n u s e d a t a n o l d 2 , 4 0 0 - v o l t s w i t c h h o u s e , w h i c h h a s m a i n b u s s e c t i o n s , s y n ­c h r o n i z i n g b u s , t r a n s f e r b u s , a n d r e a c t o r s . A t o n e t i m e a l l 2 , 4 0 0 - v o l t f e e d e r s o r ig i ­n a t e d i n t h i s s w i t c h h o u s e . O n e f e e d e r w a s 4 , 8 0 0 f ee t l o n g a n d a n o t h e r w a s 6 ,100 f ee t l o n g . A s m a y b e s u s p e c t e d , v o l t a g e p r o b l e m s a r o s e w h i c h l e d t o u s e of s t e p -t y p e v o l t a g e r e g u l a t o r s .

T o d a y , w i t h s u b s t a t i o n s s e r v i n g t h e s e c i r c u i t s , t h e a v e r a g e 2 , 4 0 0 - v o l t f e e d e r i s 9 5 0 fee t l o n g . C a r r y i n g p o w e r t o t h e l o a d a r e a a t 13 .8 k v i n o v e r s i z e d c a b l e , k e e p i n g l o a d w e l l w i t h i n t h e r a t i n g of t h e 6 ,000 -k v a s u b s t a t i o n t r a n s f o r m e r , a n d k e e p i n g 2 , 4 0 0 - v o l t f e e d e r s s h o r t a l l c o n t r i b u t e t o h o l d i n g e x c e l l e n t v o l t a g e a t t h e l o a d w i t h ­o u t t h e u s e of r e g u l a t o r s .

LOW-VOLTAGE DISTRIBUTION

L o w - v o l t a g e l o a d is a t 4 8 0 v o l t s , 3 -p h a s e a n d 1 2 0 / 2 4 0 v o l t s , s i n g l e p h a s e . A t p r e s e n t m o s t of t h i s l o a d is s e r v e d b y o p e n o u t d o o r t r a n s f o r m e r b a n k s a s s h o w n

b y F i g u r e 8, c o n s i s t i n g of 3 2 0 0 - k v a t r a n s f o r m e r s i n c l o s e d d e l t a p l u s a f o u r t h t r a n s f o r m e r for s i n g l e - p h a s e l o a d , s u c h a s l i g h t i n g . S i n g l e - p h a s e t r a n s f o r m e r s for­m e r l y w e r e i n s t a l l e d for t h e c lass ica l r e a s o n ; w h e n a f a i l u r e o c c u r r e d o n o n e of t h e s i n g l e - p h a s e t r a n s f o r m e r s , i t c o u l d b e c u t o u t of s e r v i c e a n d 5 8 p e r c e n t of t h e l o a d c o u l d b e c a r r i e d b y a n o p e n d e l t a b a n k u n t i l t h e f a u l t e d t r a n s f o r m e r w a s r e m o v e d a n d r e p l a c e d b y a s p a r e .

T h e M i d l a n d p l a n t h a s r e c e n t l y g o n e t o t h e u s e of 3 - p h a s e t r a n s f o r m e r s , a s s h o w n b y F i g u r e 9 , a n d is i n s t a l l i n g 500- , 750- , a n d 1 ,000-kva u n i t s for o u r n e w e s t b a n k s . I t wi l l b e n o t e d t h a t t h e s i ng l e -p h a s e t r a n s f o r m e r s a r e r e t a i n e d for l i g h t ­i n g . A s c a n b e r e a d i l y s e e n , t h i s m a k e s a m u c h c l e a n e r l o o k i n g a n d s i m p l e r i n ­s t a l l a t i o n . A n d w h e r e i n s t a l l a t i o n is s i m p l e r , i t i s p r e t t y s a f e t o s a y t h a t c o s t s wi l l b e l o w e r . I t i s r e c o g n i z e d t h a t i n c a s e of f a i l u r e of a 3 - p h a s e t r a n s f o r m e r , t h e w h o l e l o a d is d o w n w h i l e t h e t r a n s f o r m e r is r e m o v e d a n d r e p l a c e d b y a s p a r e . H o w ­e v e r , t h e r e l i a b i l i t y of a m o d e r n t r a n s ­f o r m e r is s u c h t h a t t h e n u m b e r of failiu-es d o e s n o t w a r r a n t t h e h i g h e r c o s t of t h e s i n g l e - p h a s e t r a n s f o r m e r c o n s t r u c t i o n .

Open Circuit Insulation

T h e s u c c e s s of o u r 1 3 , 8 0 0 - v o l t d i s t r i b u ­t i o n h a s b e e n p a r t i a l l y d u e t o c a r e i n o v e r -i n s u l a t i o n of o p e n c i r c u i t s . T h e fly a s h a n d c h e m i c a l s i n t h e a t m o s p h e r e h a v e l e d t o u s e of 3 4 . 5 - k v c i r c u i t b r e a k e r b u s h i n g s a n d b u s i n s u l a t o r s a t t h e o u t d o o r s u b s t a ­t i o n , a n d for a n y o t h e r o p e n 1 3 , 8 0 0 - v o l t c o n s t r u c t i o n . E v e n w i t h t h i s i n s u l a t i o n , i t i s n e c e s s a r y t o w a s h i n s u l a t o r s a n d b u s h i n g s p e r i o d i c a l l y .

T h e 2 , 4 0 0 - v o l t l i n e s a r e a l m o s t a l l of o p e n , o v e r h e a d c o n s t r u c t i o n , as s h o w n b y F i g u r e 10 . T h e s e l i k e w i s e a r e o v e r i n -s u l a t e d . W h e r e t h e a t m o s p h e r e is r e l a ­t i v e l y f r e e f r o m c o n t a m i n a t i o n , 1 5 - k v in­s u l a t o r s a r e u s e d ; i n c o n t a m i n a t e d a t -

Table I . Cost Comparison ior Three Types of a 1 3 , 8 0 0 - V o l t Plant Power Transmission,

Based on 5 0 0 , 0 0 0 - C i r c u l a r - M i l Cab le

Max. Kva Instal led for Cost Cost

3 / C per per 500 F o o t / F o o t /

M e m Circuit Mva

Figure 1 0 . Typica l 2 , 4 0 0 - v o l t l ine

One Circuit Overhead Open 1 6 , 8 0 0 . . $ 7 . 0 0 . . $ 0 . 4 2 Overhead Aerial Cable . . 1 0 , 0 0 0 . . 1 2 . 5 0 . . 1 .25 Underground Cable in

D u c t 9 . 5 0 0 . . 1 5 . 0 0 . . 1 .58 Four Circuits

Overhead Open 1 6 , 8 0 0 . . 3 . 5 0 . . .21 Overhead Aerial Cable . . 1 0 , 0 0 0 . . 1 0 . 0 0 . . 1 .00 Underground Cable in

Duct 7 , 4 0 0 . . 8 . 5 0 . . 1 .15

JANUARY 1 9 5 2 FrieU Smith—Electric Power Supply for a Large Chemical Plant 87

Figure 11 ( le f t ) . Tower J showing o p e n construc­

t ion

Figure 1 3 (right). Cable risers at

South Power H o u s e

m o s p h e r e s , 2 2 - k v i n s u l a t o r s a r e u s e d . F i g u r e 11 s h o w s t h e t e r m i n a t i o n of a

g r o u p of o v e r h e a d l i n e s l e a v i n g NT s w i t c h -h o u s e . I t m a y b e n o t e d t h a t t h e l i n e s in t o w e r J a r e c o n s i d e r a b l y o v e r i n s u l a t e d . I n t h e w i n t e r of 1 9 4 2 - 4 3 t h e r e w a s a d r i z ­z l i ng r a i n f o l l o w e d b y s l e e t o n e n i g h t , w h i c h a l o n g w i t h c h e m i c a l c o n t a m i n a t i o n of t h e a t m o s p h e r e in t h i s a r e a c a u s e d s e v e r e flashovers of t h e i n s u l a t o r s o n e v e r y c i r c u i t e x c e p t o n e d u r i n g t h a t n i g h t . T h e s e l i ne s h a d a l w a y s b e e n o v e r i n s u ­l a t e d a s h a v e a l l o t h e r o p e n l i n e s in t h e p l a n t . H o w e v e r , s i n c e t h a t t i m e t h e s e p a r t i c u l a r l i n e s h a v e b e e n d o u b l y o v e r i n ­s u l a t e d a n d in a d d i t i o n t h e y a r e p e r i o d i ­c a l l y w a s h e d h o t b y m e a n s of a fire h o s e

w i t h p r o p e r p r e c a u t i o n s t a k e n a g a i n s t h a z a r d t o p e r s o n n e l .

F a i l u r e s of o p e n 1 3 . 8 - k v l i ne s h a s b e e n r e d u c e d in r e c e n t y e a r s b y a t r i c k . I n t h e p a s t t h e r e w e r e q u i t e a few c r o s s a r m a n d p o l e fires d u e t o c u r r e n t l e a k a g e o v e r t h e i n s u l a t o r s , d o w n t h e p i n s t o t h e a r m s , a n d a c r o s s t h e a r m s t o t h e p o l e s , t h e n c e , p r e ­s u m a b l y t o g r o u n d . T h i s c u r r e n t w o u l d c a u s e h e a t i n g of t h e w o o d a n d e v e n t u a l l y a fire. B o n d i n g t o g e t h e r t h e p i n s a n d o t h e r h a r d w a r e o n t h e p o l e a n d c o n n e c t ­i n g t h e m t o a l a g s c r e w d r i v e n i n t o t h e h e a r t of t h e p o l e h a s a l m o s t e n t i r e l y e l i m i n a t e d t h i s p r o b l e m . T h e c u r r e n t p r e s u m a b l y t r a v e l s f r o m t h e b o n d i n g c i r ­c u i t d o w n t h e h e a r t of t h e p o l e t o g r o u n d .

Aerial Cable Versus Open Wire Versus underground Ducts

T h e s e l e c t i o n of t h e b e s t l ine con­s t r u c t i o n for a c h e m i c a l p l a n t h a s rece ived m u c h s t u d y , a n d a b r ie f r e v i e w of the D o w findings m a y b e of i n t e r e s t . In o r d e r of e x p e n s e , o p e n c o n s t r u c t i o n is r a t e d l o w e s t i n c o s t , f o l l o w e d b y aerial c a b l e , a n d t h e n b y u n d e r g r o u n d duct w h i c h is m o s t e x p e n s i v e .

AERIAL INTERLOCKED ARMOR CABLE

T h e M i d l a n d p l a n t o r i g i n a l l y u s e d only o p e n o v e r h e a d c o n s t r u c t i o n b u t later t u r n e d t o ae r i a l c a b l e . T h e first aerial c a b l e i n s t a l l a t i o n w a s a 1 5 - k v va rn i shed c a m b r i c r u b b e r h o s e j a c k e t e d 500,000 c i r c u l a r m i l 3 - c o n d u c t o r c a b l e w i t h an

Figure 1 2 ( lef t ) . A e r i a l interlocked armor cable

Figure 1 4 ( b e l o w ) . A e r i a l cable installation

8 8 FrieU Smith—Electric Power Supply for a Large Chemical Plant JANUARY 1952

mierlocked bronze armor overall. This cable is a generator lead, connecting a 13.8-kv machine which is physically located at one powerhouse to a bus sec­tion of another powerhouse, a distance of 2,500 feet. It was installed in 1941, and to date there have been no failures.

It is interesting to note that a lead covered 600-volt multiconductor control cable with performance grade insulation which was installed at the same time as the generator cable and followed the same route failed last spring at several places where cable rings supported it from the messenger wire.

An aerial cable was selected, see Figure 12, since extreme reliability was wanted for this relatively long run. The aerial cable is, of course, not subject to attack by the chemical atmosphere as long as the bronze armor and rubber hose are intact. In addition, aerial cable is less vulnerable to lightning than open overhead lines.

UNDERGROUND DUCT LINE

At Midland there is a large amount of underground piping which requires fre­quent excavation with power shovels. The relatively high water table might pre­sent a problem in keeping manholes from flooding. There is a feeling on the part of some of the personnel that the soil in the plant is more or less saturated with chem­icals which might have a deleterious ef­fect on the cable. These factors, and the higher cost of underground cable have limited its application to the very few situations where there is no room over­head, where it is desirable to eliminate ex­posure to lightning, or mechanical dam­age, or where appearance requirements dictate.

COST FACTORS

It is estimated that a 13.8-kv open overhead line of 3 500,000-circular-mil triple-braided-weatherproof wire costs about $7.00 per foot installed on 50-foot poles with spans averaging 100 feet or less. For aerial cable, 3-con­ductor 500,000-circular-mil cable alone costs about $6.00 to $7.00 per foot, de­pending on the type of aerial cable used, and installed costs are estimated at $12.00 to $13.00 per foot. These figures are for a pole line carrying one circuit only. Where there is more than one cir­cuit on a pole, the cost per foot, per cir­cuit goes down.

A 500,000-circular-mil aerial cable does not, of coin-se, have the same ciu-rent carrying capacity as three open triple-braided-weatherproof wires. Therefore the cost per kilovolt-ampere transmitted is still greater with aerial cable.

For an underground circuit of one 500,000-circular-mil 3-conductor cable it is estimated the cost would be about $15.00 per foot installed. A comparison of cost of the three types of construction is shown in Table 1.

Another advantage of aerial cable is saving in space. Figure 11 shows the space required to handle nine 2,400-volt open overhead circuits. Compare this with the space required to get eight cir­cuits away from South Power House, Figure 13. Incidentally, all of the 13.8-kv circuits taking power away from South Power House are 3-conductor, varnished cambric, interlocked bronze armored aerial cable.

A few words on the reliability of aerial cable may be of interest. In the plant there are now approximately 25,000 feet of aerial cable. On this cable there have been three failtnes which gives a record 0.185 failure per mile per year. There is no similar record for open overhead lines but it can be reported that the number of failiu-es for the open overhead lines per foot per year is considerably in excess of that for aerial cable.

Figure 14 shows the 3-conductor 600,000-circular-mil self-supporting aerial cable in which one of the three aerial cable failures occurred. The contractor had some diffi­culty getting this cable to hang properly from its messenger since the messenger apparently had a tendency to kink and twist. The installation was difficult be­cause of the somewhat tortuous route the cable had to follow. The route is ap­proximately 800 feet long and has about eight right angle bends as well as a few off­sets. Although not noticed at the time of installation, it now appears that this cable had been manhandled somewhat dtning installation.

Dow has since been advised by two dif­ferent cable manufacturers that an in­stallation over this route would be con­sidered fairly difficult for any self-sup­porting type of aerial cable of this size. It is believed that the principal difficulty lies in the tendency of the messenger to twist and turn as the cable comes off the reel. It is passed along to the reader as one experience with larger size self-sup­porting aerial cable, in the belief that any­one contemplating such an installation should make proper allowances in their planning for it.

Experience With Corrosion of Aluminum Alloys

Pin-ely aside from discussion of the Midland generation and distribution sys­tem, the following is a discussion of some experience Dow Chemical Company has

had with corrosion of aluminum alloys. In 1946 it was noticed that the metal

rings with which the circuit breaker bush­ings were held in place at the outdoor sub­station were severely corroded, some to the extent that a bushing could be ex­pected to pop out of the circuit breaker with resulting fireworks any time one was closed.

An analysis of the metal showed it to be an aluminum alloy having about 4.3 per cent copper. Dow was advised that such corrosion could be expected with this high copper content; and that there were several other alloys with only a trace of copper which would reduce considerably the susceptibility of the alloy to corrosion. The rings in question were replaced with rings of low-copper content and no further trouble has been experienced.

Conclusions

In summary, there are several things that have become apparent from Dow's experience at Midland, and they are passed along to other engineers who may have similar problems in planning their power distribution systems.

1. A purely radial system is simple, easy to operate, and dependable.

2. The total name plate rating of genera­tion connected to a bus section should not exceed 1/16 the short circuit interrupting capacity of that bus.

3. Where several bus sections are found necessary, each section should be tied to the system synchronizing bus by reactors of suitable capacity.

4. A system having generation and loads spread over a considerable area needs a "stiff backbone." Dow is going to build one by means of tie lines between station synchronizing busses.

5. The normal load connected to a bus section should approximately equal the generation and other sources on that sec­tion, to avoid carrying power through re­actors and tie lines during normal conditions. This keeps tie capacity available for han­dling emergencies. 6. In laying out a station, provide for enough feeders to take away the power generation connected to that bus.

7. Where the atmosphere bears chemicals and fly ash, build indoor substations, metal enclose, over insulate liberally and use aluminum w ith low copper content.

8. Geography and availability of equip­ment are important, but for best results following the preceeding rules for good clean electrical system design is usually more im­portant in planning system changes.

N o Discussion

JANUARY 1 9 5 2 Friel, Smith—Electric Power Supply for a Large Chemical Plant 8 9