400

ABSTRACT

IEEE Transactions on Power Delivery, Vol. 8, No. 1, January 1993 ARC FURNACE FLICKER MEASUREMENTS AND CONTROL

Bharat Bhargava Southern C a l i f o r n i a Edison Company

The paper p resents t h e r e s u l t s o f a r c f l i c k e r i n v e s t i g a t i o n s and harmonic measurements taken on a 55 MW a r c fu rnace i n Southern C a l i f o r n i a Ed ison 's area. The a r c fu rnace has been i n o p e r a t i o n s ince 1976 and has a 65 MVAR S t a t i c Var System (SVS) i n s t a l l e d t o improve t h e customer 's power f a c t o r , reduce t h e v o l t a g e f l u c t u a t i o n s and a r c fu rnace f l i c k e r . Al though the SVS improved t h e power f a c t o r and reduced the v o l t a g e f l u c t u a t i o n s and incandescent lamp f l i c k e r , i t had caused cons iderab le f l o u r e s c e n t f l i c k e r which was s p e c i a l l y n o t i c e a b l e a t some remote l o c a t i o n s about t e n m i l e s away. Because o f t h e excessive f l o u r e s c e n t f l i c k e r , t h e customer was asked t o l i m i t t h e a r c fu rnace load t o 40 MW. However, because o f t h e growing demand f o r s t e e l and t h e customer 's need f o r a d d i t i o n a l power, s tud ies and measurements i n t h e f i e l d and l a b o r a t o r y were conducted t o i n v e s t i g a t e t h e reasons f o r f l o u r e s c e n t f l i c k e r and ways t o c o n t r o l t h e f l i c k e r a t increased load so t h a t a d d i t i o n a l power cou ld be prov ided t o the customer. The paper p resents t h e r e s u l t s o f these i n v e s t i g a t i o n s and f i e l d measurements.

INTRODUCTION

A 55 MW a r c furnace has been i n o p e r a t i o n i n Southern C a l i f o r n i a Ed ison 's (SCE) Etiwanda 66 kV system s ince 1976. When t h i s l a r g e a r c furnace was i n s t a l l e d , a 65 MVAR S t a t i c Var System (SVS) was a l s o i n s t a l l e d by t h e customer t o improve t h e furnace power f a c t o r and t o minimize t h e v o l t a g e f l i c k e r on t h e ad jacent 66 kV subs ta t ions . Al though i n s t a l l a t i o n o f the SVS reduced t h e incandescent f l i c k e r which was be ing observed whenever t h e a r c furnace operated above 40 MW load, t h e o p e r a t i o n o f t h e SVS caused cons iderab le f l u o r e s c e n t l i g h t f l i c k e r and complaints f rom o t h e r customers. Th is f l u o r e s c e n t l i g h t f l i c k e r was more pronounced i n some l o c a t i o n s 10 t o 15 m i l e s away. On account o f t h e v i s i b l e f l i c k e r and complaints f rom o t h e r customers, t h e immediate i n t e r i m s o l u t i o n was t o r e s t r i c t t h e power usage by t h e a r c furnace t o l e s s than 40 MW (70%) load by d i s a l l o w i n g o p e r a t i o n on h igher fu rnace t rans former taps. The customer operated the a r c fu rnace f o r several years a t the reduced furnace load. This o b v i o u s l y increased t h e charge m e l t i n g t ime and o p e r a t i n g c o s t . Because o f t h e tough f o r e i g n compet i t ion faced by t h e s t e e l manufacturer i n t h e market i n 1985, t h e customer dpproached SCE t o p e r m i t o p e r a t i o n a t h igher loads and furnace t rans former taps t o improve the e f f i c i e n c y o f the furnace o p e r a t i o n .

Ex tens ive d iscuss ions w i t h t h e SVS manufacturer and a study conducted by him t o r e s o l v e t h e f l u o r e s c e n t l i g h t f l i c k e r problems r e s u l t e d i n the manufacturer a d v i s i n g t h a t t h e SVS c a p a c i t y cou ld be increased from

92 WM 195-8 PWRD the IEEE Transmission and Distribution Committee of the IEEE Power Engineering Society for presentation at the IEEE/PES 1992 Winter Meeting, New York, New York, January 26 - 30, 1992. Manuscript submitted September 16, 1991; made available for printing January 9, 1992.

A paper recommended and approved by

65 MVAR t o 100 MVAR and t h a t t h e a d d i t i o n a l f i l t e r s should be i n s t a l l e d . However, SCE and t h e customer d i d n o t agree w i t h these recommendations.

Extensive s t u d i e s and f i e l d measurements were t h e r e f o r e conducted by SCE t o understand t h e f 1 uorescent f 1 i cker problem and t o determi ne ways t o p r o v i d e f u l l 60 MW power t o t h e customer w i t h o u t adverse ly a f f e c t i n g t h e o t h e r customers on t h e SCE system. Th is paper p resents t h e measurement r e s u l t s , and t h e understanding gained f rom these a r c fu rnace measurements.

SYSTEM DESCRIPTION

F igure 1 shows t h e c i r c u i t diagram of t h e a r c fu rnace and t h e method o f s e r v i c e which e x i s t e d f rom 1976 t o March, 1987. The 60 MVA a r c fu rnace i s supp l ied by a 66 kV t ransmiss ion l i n e f rom t h e Etiwanda 66 kV bus about 1.1 m i l e away f rom t h e customer. The 66 kV bus i t s e l f i s f e d f rom a 230 kV bus by two 280 MVA, 230/66 kV t rans formers . The s h o r t c i r c u i t du ty on t h e 230 kV bus v a r i e s between 6000 MVA t o 15,000 MVA depending upon the number o f t ransmiss ion l i n e s and genera t ing u n i t s on t h e system. The normal ly expected s h o r t c i r c u i t d u t y i sabout 10,300 MVA w i t h a l l 230 kV l i n e s i n s e r v i c e b u t w i t h t h e genera t ing u n i t s o f f . The s h o r t c i r c u i t du ty on t h e 66 kV bus v a r i e s between 1900 MVA t o 2360 MVA. The normal expected s h o r t c i r c u i t du ty i s about 36 t imes t h e furnace t rans former MVA. The 66 kV l i n e feeds t h e o t h e r customers as w e l l as t h e P o i n t o f Common Coupl ing (PCC) a t t h e Ameron Substa t ion which feeds t h e a r c fu rnace.

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F i g . 1. System arrangement f o r serv ing 40 MW TAMCO

F igure 2 shows t h e d e t a i l s o f t h e s t a t i c var system i n s t a l l e d by t h e customer. The 65 MVAR SVS i s i n s t a l l e d on t h e 33 kV bus i n p a r a l l e l t o t h e 63 MVA furnace t rans former . Based on the recommendations o f t h e SVS manufacturer, t h i r d , f i f t h , seventh and h igher harmonic f i l t e r s were i n s t a l l e d as p a r t o f t h e SVS. The 33 kV bus i s f e d by t h r e e 63/33 kV t rans formers .

The purpose o f i n s t a l l a t i o n o f the SVS was t o improve t h e customer 's power f a c t o r and vo l tage on t h e 33 kV bus and t o reduce f l i c k e r . The SVS has improved t h e power f a c t o r and reduced incandescent lamp f l i c k e r b u t increased f l i c k e r on the f l u o r e s c e n t l i g h t s cons iderab ly . The f l u o r e s c e n t lamp f l i c k e r was n o t i c e a b l e on almost a l l o f t h e 66 kV system be ing fed by the SCE's Etiwanda 66 kV bus. On account o f t h i s

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flicker, the power level of the furnace was limited to less than 40 MW. The SVS manufacturer was approached several times since 1977 by the customer and SCE to examine the cause of the fluorescent light flicker and advise if it could be improved by SVS modification. The manufacturer, however, did not agree that the SVS could cause any such phenomenon.

Details of the static var system

I I I l I I l I l

REQUEST FOR ADDITIONAL W E R

On account of the extreme competition faced by the steel manufacturer in 1985, the manufacturer approached SCE again to provide full 6 0 MW power. The least expensive alternative was expected to be to increase and modify SVS. Therefore, the problem of flicker was once again referred to the SVS manufacturer. ' As recommended by the SVS manufacturer, a detailed study was contracted to him by SCE to examine if the flicker could be reduced and the allowable power increased by some simple modifications to the SVS. The SVS manufacturer recommended that additional power could be given to the customer once the SVS capacity were increased from 65 MVAR to 100 MVAR and an additional second and fourth harmonic filters were installed with adequate resistive damping added to the SVS. The SVS manufacturer also advised that some additional cooling of the thyristors could enable them to handle this increased power. The SVS manufacturer further expressed that the flicker could be caused by the pole frequencies being created by the third harmonic filter of the SVS. The resistive damping of. the new harmonic filters could help to reduce these pole frequencies and could reduce f 1 uorescent f 1 i cker. The manufacturer a1 so recommended resistive damping of the existing third and fifth harmonic filters of the SVS.

SVS DESIGN CRITERION

Discussions with the SVS manufacturer indicated that the SVS wax designed according to the CGEB criterion (Reference 1 ) which limits t h e short circuit

401 voltage depression (s.c.v.d.) to 2% maximum and 0.25% variation in the voltage for t20% fluctuations in the furnace current but suggested that this criterion may not have been adequate to eliminate fluorescent light flicker. A literature search for fluorescent flicker indicated that there are no standards set for f 1 uorescent 1 i ght f 1 i cker.

In view of the questions raised regarding the actual variations in voltage caused by the arc furnace and frequencies generated by the SVS, field measurements were conducted by SCE. The actual measurements taken by SCE showed more than 6% voltage variations at some locations when compared from cycle to cycle even when the SVS was in operation. These variations were considerably more than predicted in the study. The harmonic measurements at the furnace PCC also showed low levels of second and fourth harmonics at the furnace, questioning the need of the second and fourth harmonic filters. Further. the operating experience and the SVS sizing criterion did not provide sufficient evidence that the larger size SVS would solve the flicker problem; as measurements indicated that even at the present load level full SVS capacity is not being used.

The study recommendations given by the SVS manufacturer were therefore not accepted by SCE or the customer. However, other alternative ways to provide additional power were investigated as the customer once again approached SCE to release full power to improve his arc furnace operating eff,iciency.

FIELD MEASUREMENTS

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Fig. 3. Spectrum analysis of line current at TAMCO with furnace off ( 1 ) and on ( 2 )

402 f requenc ies and i n j e c t as much as 30% o f load c u r r e n t a t these f requenc ies i n t o the system.

B . That t h e v o l t a g e harmonics a t the furnace l o c a t i o n were r e l a t i v e l y low. F igure 4 shows t h e v o l t a g e frequency spec t ra w i t h furnace o f f and w i t h t h e furnace on, r e s p e c t i v e l y . A s a percentage o f the 60 Hz vo l tage, the vo l tage harmonics a r e much lower than t h e c u r r e n t harmoni cs.

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F i g . 4. Spectrum a n a l y s i s o f vo l tage a t TAMCO w i t h

C. A h i g h component o f non-60 Her tz c u r r e n t f requenc ies was n o t i c e d a t a remote subs ta t ion . The h igher f requencies were a t tenuated , b u t subsynchronous f requenc ies were s t i 11 q u i t e h igh . F igure 5 shows the frequency spec t ra f o r t h e c u r r e n t w i t h t h e fu rnace o f f , and the furnace on. Comparison o f t h e two curves shows t h a t a s u b s t a n t i a l amount o f c u r r e n t a t non-60 Her tz f requenc ies i s i n j e c t e d by the a r c furnace and propagate t o o t h e r subs ta t ions as w e l l .

fu rnace o f f ( 1 ) and on ( 2 ) .

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F i g . 5. Spectrum a n a l y s i s o f l i n e c u r r e n t a t A r c h l i n e Substa t ion w i t h furnace o f f (1 ) and on ( 2 ) .

D. The v o l t a g e harmonics a t a nearby Substa t ion were again q u i t e low. F igure 6 shows the vo l tage frequency spec t ra w i t h t h e furnace o f f and w i t h the fu rnace on i n d i c a t i n g t h a t even a small subsynchronous v o l t a g e from a r c furnace i s i n t r o d u c i n g l a r g e subsynchronous c u r r e n t s i n t h e system. A 2.5% vo l tage v a r i a t i o n was recorded a t t h i s Subs ta t ion . The vo l tage v a r i a t i o n s were much h igher a t a remote l o c a t i o n compared t o c l o s e r l o c a t i o n s i n d i c a t i n g t h a t t h e t ransmiss ion l i n e s were ampl i f y ing some frequencies and i n c r e a s i n g f l i c k e r a t remote l o c a t i o n s .

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F i g . 6. Spectrum a n a l y s i s o f vo l tage a t Arch Substa t ion w i t h furnace o f f ( 1 ) and on ( 2 )

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F igure 7 shows t h e h a l f wave r e c t i f i e d vo l age p l o t w i t h a r c furnace on taken a t a remote l o c a t i o n . Vol tage f l u c t u a t i o n s up t o 6.6% were in t roduced by t h e a r c furnace even w i t h the SVS i n o p e r a t i o n . F igure 8 shows t h e frequency spec t ra measured i n two d i f f e r e n t t e s t s a t t h i s l o c a t i o n .

6 6 % I 2 OF 2

F i g . 7 . O s c i l l o g r a p h i c r e c o r d o f hal fwave r e c t i f i e d vo l tage a t Eastern Customer Serv ice D i v i s i o n .

E. Var ious vo l tage frequencies due t o a r c furnace were n o t i c e a b l e a t SCE's Customer Serv ice Fontana o f f i c e about f i v e m i l e s away and even though q u i t e low, were causing v i s i b l e f l i c k e r .

F igure 9 shows t h e vo l tage frequency spec t ra a t t h i s o f f i c e (a ) w i t h the furnace o f f , (b) w i t h the furnace on b u t w i t h no n o t i c e a b l e f l i c k e r and ( c ) w i t h observed f l i c k e r . The t h r e s h o l d o f f l i c k e r was est imated t o be somewhere between the two measurements. The o s c i l l o g r a p h i c records o f t h e vo l tage f o r these measurements on a 120 v o l t o u t l e t showed t h a t a t 2.7% vo l tage f l u c t u a t i o n t h e f l i c k e r was n o t i c e a b l e b u t was n o t n o t i c e a b k when t h e vo l tage f l u c t u a t i o n s were below 2.0%.

403

PHOTO E L t C l R l C CELL

Fig . 8. Spectrum a n a l y s i s o f vo l tage a t Eastern Customer Serv ice D i v i s i o n i n two separate t e s t s .

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F i g . 9. Spectrum a n a l y s i s of v o l t a g e a t SCE's Fontana o f f i c e . The Curve (1 ) shows t h e harmonics w i t h t h e fu rnace o f f , t h e Curve ( 2 ) w i t h furnace on b u t f l i c k e r n o t observable and Curve (3) w i t h t h e furnace on and observable f l i c k e r .

LAB MEASUREMENTS

Measurements were conducted i n the e l e c t r i c a l apparatus Test Laboratory t o understand f luorescent l i g h t f l i c k e r . Several lessons were learned d u r i n g these l a b measurements.

The c i r c u i t shown i n F igure 10 was s e t up i n t h e Test Lab t o enable i n j e c t i o n o f v a r i a b l e f requencies and t o c r e a t e f l i c k e r a r t i f i c i a l l y . The vo l tage f l u c t u a t i o n magnitude which r e s u l t e d i n p e r c e p t i b l e and o b j e c t i o n a b l e f l i c k e r a t var ious f requencies was measured. The composite waveform c o n t a i n i n g the two frequencies was f e d t o a f l u o r e s c e n t l i g h t (20 w a t t s ) . The l i g h t o u t p u t o f t h e lamp was measured by a p h o t o v o l t a i c c e l l p laced under the f l u o r e s c e n t l i g h t . The vo l tage waveform and t h e l i g h t o u t p u t were recorded on an o s c i l l o s c o p e . The vo l tage waveform was a l s o recorded on an HP DSA which enabled accura te measurement o f the f requency conten t o f t h e v o l t a g e waveform.

F ig . 10. C i r c u i t f o r f l u o r e s c e n t f l i c k e r measurement i n t h e t e s t l a b .

Measurements recorded i n the l a b i n d i c a t e d t h a t . the f l u o r e s c e n t l i g h t f l i c k e r i s caused by a wide range o f f requencies. I n f a c t , f requencies which were i n the 220 Hz range o f t h e fundamental o r t h e harmonic o f 60 Hz f requency were observed t o beat w i t h t h e fundamental o r t h e harmonic f requency and generate f l u o r e s c e n t lamp f l i c k e r . The f l u c t u a t i o n o f vo l tage r e q u i r e d t o cause o b j e c t i o n a b l e f l i c k e r , however, depended upon the beat f requency. Even a low l e v e l , l e s s than 0.5% i n the 5-15 Hz beat f requency range, cou ld cause p e r c e p t i b l e and o b j e c t i o n a b l e f l u o r e s c e n t f l i c k e r .

F igure 11 shows t h e curve p l o t t e d as a f u n c t i o n o f v o l t a g e magnitude and i n j e c t i o n f requency and beat f requency f o r t h e 0-90 Hz range, f o r t h e f l u o r e s c e n t lamp. The measurements showed t h a t a t a beat f requency o f 5 t o 8 Hz, even a 0.3% (-50 dB) s i g n a l causes p e r c e p t i b l e and o b j e c t i o n a b l e f l i c k e r . These curves f o l l o w t h e SCE Vol tage F l u c t u a t i o n Design Curve shown i n F igure 12 v e r y c l o s e l y f o r beat f requency ranges between 5-20 Hz. I n f a c t , the present design curve i s more conserva t ive than t h e one developed by these measurements. Since t h e p e r c e p t i b i l i t y and o b j e c t i o n a b i l i t y o f f l i c k e r changes from i n d i v i d u a l t o i n d i v i d u a l , t h e e x i s t i n g curve was used f o r d e f i n i n g f l i c k e r l e v e l s .

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F i g . 11. The measured p e r c e p t i b l e and i r r i t a b l e v o l t a g e f l i c k e r l e v e l s f o r f l u o r e s c e n t lamp.

Measurements i n t h e l a b f u r t h e r showed t h a t t h e f l u o r e s c e n t l i g h t o u t p u t changes cons iderab ly w i t h change i n t h e i n p u t vo l tage t o the f l u o r e s c e n t l i g h t . F igure 13 shows t h e i n p u t v o l t a g e and t h e f l u o r e s c e n t l i g h t o u t p u t w i t h a 70 Hz i n j e c t e d vo l tage a t observable f l i c k e r l e v e l . The 70 Hz vo l tage which r e s u l t s i n observable f l i c k e r i s about 0.8% o n l y , b u t r e s u l t e d i n more than 3% f l u o r e s c e n t l i g h t o u t p u t measured by t h e p h o t o v o l t a i c c e l l . The 10 Hz beat f requency envelope can be seen c l e a r l y and appears t o

404

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F i g . 12. SCE v o l t a g e f l i c k e r design curve.

1

cause f l i c k e r . F igure 14 shows t h e frequency spectrum measured by t h e DSA f o r t h i s i n j e c t e d vo l tage a t 70 Hz r e s u l t e d i n o b j e c t i o n a b l e f l i c k e r .

F igure 15 shows t h e v o l t a g e and t h e f l u o r e s c e n t l i g h t o u t p u t recorded a t 80 Hz i n j e c t e d frequency. A t 80 Hz, t h e o b j e c t i o n a b l e f l i c k e r occur red a t a vo l tage v a r i a t i o n o f about 3%. The f l u o r e s c e n t l i g h t o u t p u t aga in changes much more as compared t o t h e i n p u t vo l tage. The frequency spectrum f o r t h i s measurement i s shown i n F igure 16 and shows the 80 Hz and the 20 Hz present i n t h e waveform.

L l W T INTENSITY VS VOCTAQE

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L o VAC POWER. , -L IGHT INTENSITY REFERENCE

F i g . 13. The v o l t a g e and f l u o r e s c e n t l i g h t i n t e n s i t y waveform w i t h 0.8 v o l t , 70 Hz i n j e c t e d vo l tage.

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F i g . 14. Spectrum a n a l y s i s o f t h e composite vo l tage waveform w i t h 0.8 v o l t , 70 Hz i n j e c t e d v o l t a g e shown i n F igure 13.

LIGHT INTENSITY V S VOLTAGE

FLOURESCENT LIGHT INTENSITY f WlTW P E R C E P T I I L E FLICKER

" LIGHT INTLMSITY REFERENCE

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F i g . 15. The vo l tage and f l u o r e s c e n t l i g h t i n t e n s i t y waveforms w i t h 3.2 v o l t , 80 Hz i n j e c t e d vo l tage.

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F i g . 16. Spectrum a n a l y s i s o f t h e composite vo l tage waveform w i t h 3.2 v o l t , 80 Hz i n j e c t e d v o l t a g e waveform shown i n F igure 15.

PROPOSED SERVICE ALTERNATIVES

The t h r e e a l t e r n a t i v e s considered f o r p r o v i d i n g a d d i t i o n a l power t o TAMCO a r e shown i n F igure 17. The a l t e r n a t i v e A , which had t h e minimum cos t , cons is ted o f upgrading the e x i s t i n g SVS a t TAMCO f rom 65 MVAR t o 100 MVAR and i n c r e a s i n g t h e 66/33 kV t r a n s f o r m a t i o n c a p a c i t y by adding a 15/20 MVA 66/33 kV t rans former . This a l t e r n a t i v e was i n v e s t i g a t e d e x t e n s i v e l y by conduct ing a study and by f i e l d measurements. From t h e r e s u l t s o f t h e study and the f i e l d measurements, i t was concluded t h a t t h i s a l t e r n a t i v e would no t work s a t i s f a c t o r i l y f o r t h e increased load.

The o t h e r two methods i n v o l v e d serv ing TAMCO f rom t h e 230 kV system. The purpose o f p r o v i d i n g power a t 230 kV was t o inc rease the s h o r t c i r c u i t du ty a t t h e PCC and add the t rans former impedance between the a r c furnace and o t h e r system loads t o f i l t e r t h e f l i c k e r s e n s i t i v e f requenc ies .

The a l t e r n a t i v e B inc luded serv ing t h e TAMCO load f rom an i s o l a t e d 66 kV bus by p r o v i d i n g an a d d i t i o n a l 230166 kV t rans former a t Etiwanda f o r TAMCO and i n c r e a s i n g the Ameron (now TAMCO) 66/33 kV t r a n s f o r m a t i o n c a p a c i t y by adding a 15/20 MVA, 66/33 kV t rans former . The p o i n t o f common coup l ing w i t h o t h e r Edison load would be t h e 230 kV bus a t E t i Wanda.

The t h i r d a l t e r n a t i v e C inc luded s e r v i n g TAMCO by a 230 kV l i n e f rom Etiwanda and b u i l d i n g a 230133 kV s u b s t a t i o n a t Ameron Substa t ion . Th is a l t e r n a t i v e would p r o v i d e a much h igher s h o r t c i r c u i t du ty by reduc ing one l e v e l o f t rans format ion .

405

E71W4N04 220KV TR4NY

& E R N A T E A-PUEFE- EDIIO* I T A M

54 ' 'TR4! !F 64 220Kv9us --@ v2,12

i ALTERNATE I

I Y E INSTALL 3rd 22WCSKV W K . 011 WY I T R . M F o I Y R a E A I I U U I E SSKV LINES

I ALTERNATE C

F i g . 17 . A l t e r n a t i v e s f o r g iven 60 MW power t o TAMCO.

TAMCO FIELD TESTS

I n view o f t h e l a s t two 230 kV a l t e r n a t i v e s o f f e r e d by TAMCO, i t was f e l t necessary t o demonstrate t h a t TAMCO's o p e r a t i o n a t f u l l load l e v e l w i t h these a l t e r n a t i v e s would n o t c r e a t e o b j e c t i o n a b l e f l i c k e r t h a t migh t be v i s i b l e throughout t h e SCE Etiwanda system. Therefore, f i e l d t e s t s were conducted i n which t h e TAMCO load was f e d from the SCE's 230 kV system.

Tests f o r the proposed s e r v i c e a l t e r n a t i v e s t o TAMCO a l l o w i n g an increased a r c furnace load o f 60 MW were conducted s u c c e s s f u l l y . The t e s t s i n v o l v e d s i m u l a t i n g these s e r v i c e a l t e r n a t i v e s by i s o l a t i n g t h e 66 kV TAMCO a r c fu rnace load c i r c u i t on a separate 230/66 kV t rans former bank f rom the r e s t o f t h e load a t Etiwanda S t a t i o n . By i s o l a t i n g the 66 kV bus a t Etiwanda, the p o i n t where t h e a r c fu rnace i s coupled t o o t h e r SCE loads i s s h i f t e d f rom the 66 kV system t o t h e 230 kV system. F igure 18 shows t h e s i m p l i f i e d one l i n e diagram o f t h e system arrangement f o r these t e s t s . Separa t ing t h e 66 kV buses, however, reduced t h e s h o r t c i r c u i t du ty on t h e 33 kV bus below t h e minimum l e v e l r e q u i r e d f o r sa fe o p e r a t i o n o f t h e SVS a t Etiwanda Generat ing. S t a t i o n on 66 kV bus and i t s f i l t e r s . A 150 MW peaker was t h e r e f o r e r u n d u r i n g t h e t e s t s t o keep t h e s h o r t c i r c u i t du ty on t h e 33 kV bus above the minimum r e q u i r e d l e v e l .

Measuring ins t ruments , c o n s i s t i n g o f HP Dynamic Signal Analyzers and o s c i l l o g r a p h s , were i n s t a l l e d a t Etiwanda and a remote Customer Serv ice D i v i s i o n O f f i c e . The Customer Serv ice Eastern D i v i s i o n O f f i c e (Eastern D i v i s i o n O f f i c e ) was t e m p o r a r i l y switched t o t h e Etiwanda 66 kV system and served f rom t h e Etiwanda 230/66 kV No. 1 (5A) bank. This remote D i v i s i o n O f f i c e l o c a t i o n wa5 se lec ted as a meter ing l o c a t i o n because a number o f f l i c k e r s tud ies made i n t h e pas t had shown t h a t t h e f l i c k e r a t t h i s l o c a t i o n was the h i g h e s t observed a t any l o c a t i o n when f e d from the Etiwanda 66 kV bus.

220KV MlR4 LOU4 - 2 2 0 I V 54N BERNAROIN)

2 2 0 Y V VISTA

- 220KVPADUA

2 2 0 I V CHINO

I

W & 6 6 K V BUS

40 uw TO 6OMW

F i g . 18. Meter ing l o c a t i o n s and system arrangements f o r TAMCO f i e l d t e s t s .

The frequency spectrum a n a l y s i s was extremely h e l p f u l i n t h e a n a l y s i s o f the measurement r e s u l t s . These frequency spectrum measurements were taken i n t h e peak mode and thus recorded t h e maximum v a r i a t i o n s t h a t occur red d u r i n g t h e e n t i r e t e s t p e r i o d . The measurements recorded d u r i n g d i f f e r e n t t e s t s w i t h t h e s i m i l a r system c o n f i g u r a t i o n s matched f a i r l y c l o s e l y , i n d i c a t i n g t h a t even though the a r c furnace load changes randomly and r a p i d l y , i t has s i m i l a r f requency spectrum c h a r a c t e r i s t i c s when averaged over a t e s t charge p e r i o d .

Furnace Operat ion Without Peaker and SVS. The base case was es tab l i shed w i t h no peaker and no SVS on the i s o l a t e d 66 kV svstem. TAMCO was asked t o draw maximum power f rom t h e SCE system. The s h o r t c i r c u i t d u t y a t t h e 33 kV bus w i t h t h i s system arrangement was about 400 MVA. The low s h o r t - c i r c u i t du ty and the poor furnace power f a c t o r l i m i t e d t h e maximum power drawing by TAMCO a t t h e i r h i g h e s t t a p t o between 40 and 45 MW.

F igure 19 shows t h e vo l tage and c u r r e n t f requency spec t ra recorded d u r i n g t h i s t e s t on t h e Ameron- A r c h l i n e 66 kV l i n e which feeds t h e TAMCO load. The vo l tage frequency spectrum shows t h e t h i r d harmonic t o be much h igher b u t s t i l l around 1%. The l a r g e second harmonic c u r r e n t was observed t o be g e n e r a l l y c o n t r i b u t e d by t h e t rans former e n e r g i z a t i o n t r a n s i e n t which was tu rned on a t the s t a r t o f furnace opera t ion . The c u r r e n t f requency spectrum a l s o shows l a r g e low frequency c u r r e n t s even though t h e vo l tage a t these f requenc ies i s q u i t e low i n d i c a t i n g a low system impedance t o these f requenc ies .

POWER SPEC 1 497 aro o%owlr ~ o n n or, POWER SPEC 2 502 aro O % O r ~ p ~ o n n

CURRENT VOLTAGE

- - I 0 1 1 , C O % 1 1

F i g . 19. Vol tage and c u r r e n t f requency Spectra measured a t 66 kV Etiwanda 6A bus feed ing TAMCO f o r Test w i t h the peaker and SVS " o f f " .

406 Figure 20 shows t h e 66 kV s i d e and t h e 230 kV

s i d e vo l tage v a r i a t i o n s on t h e 230/66 kV No. 2 bank (6A). Comparison o f the two curves shows t h a t the vo l tage f l u c t u a t i o n s were a t tenuated cons iderab ly on t h e 230 kV s i d e w i t h t h i s arrangement.

8 d 0 V I

VOLTIGE

I % d 8

rm. " 0 I %

-BO 0 F x l V 0 01 %

F i g . 20. Comparison o f vo l tage frequency spec t ra a t 66 kV 6A and 220 kV Etiwanda busses w i t h the peaker and SVS " o f f " .

F igure 21 shows the frequency spectrum a n a l y s i s o f v o l t a g e f o r t h i s t e s t a t t h e Eastern D i v i s i o n O f f i c e , which i s served f rom t h e 66 kV bus s i d e of t h e 230/66 kV 5A bank as shown i n F igure 18. No v i s i b l e f l i c k e r was observed a t t h e Eastern D i v i s i o n O f f i c e and the frequency spectrum shows t h a t a l l o f t h e f requenc ies a t t h e D i v i s i o n O f f i c e were below 0.5% l e v e l . The 60 Hz harmonics a r e c o n t r i b u t e d by t h e l o c a l load and as such a r e h i g h e r . The frequencies o t h e r than t h e harmonics were much lower, g e n e r a l l y be ing below -60 dB (0.1%).

IO dB

/ o w

By 15111 STOP 400 Hz -79

x:60 HZ Y: 0 92 d 0 V START o nz

I %

0 I %

0 0 I %

F i g . 21 . Vol tage frequency spectrum measured a t Eastern D i v i s i o n O f f i c e f o r Test w i t h peaker and SVS " o f f " .

Furnace Opera t ion With Peaker bu t Without SVS. F igure 22 shows t h e measurements taken w i t h the peaker u n i t i n oDera t ion and t h e SVS o f f . This t e s t was conducted ' t o determine the a t t e n u a t i o n o f var ious f requenc ies t h a t occurs because o f the peaker. Under the normal o p e r a t i o n w i t h t h e proposed s e r v i c e arrangements, t h e peaker would n o t be i n o p e r a t i o n when t h e 60 MW power i s g iven t o TAMCO. I t was t h e r e f o r e impor tan t t h a t t h e f l i c k e r l e v e l s be c a l c u l a t e d , keeping t h i s o p e r a t i o n a l d i f f e r e n c e i n mind and account ing f o r t h e shunt ing e f f e c t o f t h e peaker d u r i n g t h e t e s t s .

F igure 23 shows the frequency spectrum f o r t h i s t e s t and compares i t w i t h the above case f o r measurements taken on t h e 66 kV 6A t rans former bus. A

d i f f e r e n c e o f 6 dB, which t r a n s l a t e d i n t o a r p d u c t i o n l e v e l o f about 50%. can be observed. A s i m i l a r r e d u c t i o n was seen on the measurements taken a t t h e remote D i v i s i o n O f f i c e and a l s o on the V i s t a 230 kV l i n e .

CURRENT VOLTAGE POWER SPEC I 569hrp O%Orlp Wonn Or1

loo% -WWER SPEC 2 63(ll"* O%O*lP Wan"

- . .

- , 5 0 0

10 0 /Om. IO k

d 0 I %

rm. V

0 , %

0 01 % - 9 5 -no o Fsd I O Hz ...- ,YERO*-ARCH 6 6 K V E 400

- 6 1 BANK 2 2 0 K V I

x " 0

€a "I 60.%.ZmdBVlm.

x "b

-60 HI -22.004 dsvrn.

F i g . 22. Vol tage and c u r r e n t f requency spec t ra measured a t Etiwanda 66 kV bus 6A w i t h t h e peaker "on" and SVS " o f f " .

VOLTAGE POWER SPEC I 3 3 9 4 O%OwiD Hmmn or I

-POWER SPEC I 491 Alp O%Ovlp Ham 0 0

IO 0 /Or"

d 8

,111. V

-80 0 F i d I 0 Mz AMERON-DRCH 6 6 K V E

X V 60 HI - 5 5341mdBVrm - 60 HZ 8 6 4 0 3 6 n d B V m s

F i g . 23. Comparison o f vo l tage frequency spectrum measured a t Etiwanda 66 kV bus 6A w i t h peaker " o f f " and "on" r e s p e c t i v e l y . The f l i c k e r s e n s i t i v e f requenc ies are about 6 dB lower i n w i t h peaker "on" when compared t o curve w i t h peaker " o f f " .

STATUS PAUSED VOLIAGE PEAK

I m v

10

OB I O l V

-79 START

800%

I O %

I 0 %

0 , %

0 01 %

F i g . 24. Comparison o f vo l tage frequency spectrum measured a t Eastern D i v i s i o n O f f i c e f o r Test w i t h peaker " o f f " and "on", r e s p e c t i v e l y . The f i g u r e shows lower f l i c k e r s e n s i t i v e f requencies i n 40 t o 240 Hz range w i t h peaker "on".

Furnace Opera t ion Wi th SVS and With Peaker. Dur ing t h i s t e s t , bo th t h e SVS and t h e peaker were i n o p e r a t i o n and TAMCO was aga in al lowed t o draw maximum power. F igure 25 shows t h e megawatts recorded f o r these t e s t s . With t h e SVS i n opera t ion , t h e a r c furnace was a b l e t o inc rease i t s maximum power f rom 48 MW t o up t o 59 MW, i n d i c a t i n g t h a t t h e SVS i s a b l e t o inc rease TAMCO's produc t ion s u b s t a n t i a l l y by reduc ing TAMCO's var demand a t t h i s s h o r t c i r c u i t du ty l e v e l (450 MVA) .

HEW XATTS

MEGA i b R S

F i g . 25. Comparison o f Megawatts, Megavars and Vol tage recorded by a Brush recorder f o r t e s t s w i t h and w i t h o u t SVS. The SVS enables TAMCO t o inc rease MW drawn w i t h t h i s arrangement by about 25 t o 30% and reduces average MVAR by about 35 MVAR. The 65 MVAR SVS c a p a b i l i t y i s used r a r e l y . The SVS reduces vo l tage v a r i a t i o n s s u b s t a n t i a l l y .

F igure 26 shows t h e v o l t a g e and c u r r e n t f requency spec t ra taken on t h e Ameron A r c h l i n e 66 kV l i n e f o r t h i s t e s t . By comparing t h i s w i t h F igure 19, i t can be n o t i c e d t h a t t h e v o l t a g e and c u r r e n t f requency spec t ra have been a l t e r e d s i g n i f i c a n t l y by the SVS. Al though t h e SVS, which has the t h i r d and f i f t h harmonic f i l t e r s , reduces these harmonics, i t a l s o generates s i d e band f requenc ies o f 147 t o 220 Hz. These s i d e band p o l e f requenc ies were i n f a c t observed t o be much h iaher than t h e o r i a i n a l harmonics l e v e l ~~

gene:if;'fe$ PxGE-arc furnace w i t h o t t t h e SVS. P a r t o f WWER SPEC I 3?7Arp O X o r l p nonn - ... . .- . . . . .

loo% -PO-WER SPEC 2 3 7 7 4 ~ ~ O%ovlp "(ann ""2

- 1 0 0 I / T I I I I I 1 I 1 I

- 85

10.0 / O b

dB

rm. V

-80 0

F.4 Y - bYERON-ARCH 6 6 K V i

X VO

x Yb

60 HI - 1 v . 2 a s m a ~ v r n ~

60 HI -8.0423 a w r m t

... ...

-

10%

I %

0 I %

0 01 %

F ig .26 . Comparison o f v o l t a g e frequency spec t ra a t Etiwanda 66 kV bus w i t h t h e SVS "on" and " o f f " . Three peaks a t f requencies 147, 220 and 380 Hz, c a l l e d po les , a r e generated by t h e SVS f i l t e r s . Some o t h e r f requenc ies i n t h e range o f 40 t o 280 Hz a l s o inc rease as a r e s u l t o f the SVS.

407 t h i s inc rease i n f requenc ies cou ld be a t t r i b u t e d t o an inc rease i n t h e power l e v e l f rom 45 MW t o 60 MW, b u t t h e major p a r t o f t h e s ide band frequencies i s a t t r i b u t a b l e t o t h e SVS harmonic f i l t e r c h a r a c t e r i s t i c s .

F igure 27 compares the frequency spectrum measurements taken on t h e 66 kV bus and on t h e 230 kV bus, r e s p e c t i v e l y . These measurements were recorded a t a maximum load o f 60 MW. Al though t h e p o l e f requencies generated by SVS are s t i l l n o t i c e a b l e on the 230 kV s ide , they are much lower when compared t o t h e 66 kV s ide . The 147 Hz p o l e f requency reduced from about -43 dB t o about -65 dB i n d i c a t i n g a r e d u c t i o n o f about 22 dB. The 220 Hz p o l e f requency i s l i k e w i s e a t tenuated f rom -42 dB t o about -68 dB, a r e d u c t i o n o f more than 26 dB. These measurements i n d i c a t e t h a t i s o l a t i n g t h e 66 kV bus a t tenuates t h e f l i c k e r s e n s i t i v e f requencies by over 90% and g r e a t l y helps t o e l i m i n a t e f l i c k e r .

.... .. - 0 .0

10.0 l D l V

4n

r m V

-.on F d .I - AYEIK)N-ARCH U M V E

100%

10%

1%

0.1%

aoi U

X m @O MI ?I W3wBVmi - so U. 102973nUV.m.

..

F i g . 27. Comparison o f vo l tage frequency spec t ra a t Etiwanda 66 kV (6A) and 220 kV bus Test w i t h peaker and SVS i n opera t ion . By i s o l a t i n g t h e 66 kV bus, t h e po le f requencies are a t tenuated f rom -43 dB t o -65 dB (147 Hz) and from -42 dB t o -68 dB (220 Hz) .

F igure 28 compares t h e frequency spec t ra taken a t t h e Etiwanda 230 kV bus w i t h the measurements taken a t t h e Eastern D i v i s i o n O f f i c e . The purpose o f these measurements was t o examine the propagat ion c h a r a c t e r i s t i c s o f these p o l e f requencies on t h e 66 kV subtransmission system. The presence o f l a r g e t h i r d and f i f t h harmonics i n the measurements taken a t t h e Eastern D i v i s i o n O f f i c e i s f rom the l o c a l load. The SVS f i l t e r s absorb t h e harmonics near t h e a r c furnace area b u t cannot reduce harmonics a t remote l o c a t i o n s . The p o l e f requencies f rom SVS a r e t r a n s m i t t e d and a m p l i f i e d and i n t e r a c t w i t h harmonics generated by l o c a l loads a t these remote l o c a t i o n s . The 220 Hz p o l e f requency i s a m p l i f i e d by about 9 dB by t h e t ransmiss ion system.

COMPARISON OF 40 MW AND 60 MW FURNACE OPERATION

F igure 29 compares the measurements taken a t t h e 40 MW and 60 MW a r c furnace load l e v e l on t h e Ameron- A r c h l i n e feeder . Dur ing these t e s t s , t h e SVS was on, and t h e peaker was i n o p e r a t i o n . These measurements i n d i c a t e d t h a t on a l l the measurements, t h e r e was about a 3 dB t o 4 dB increase when the a r c fu rnace l e v e l was increased from 40 MW t o 60 MW. The increase a l s o occurs on the 147 and 220 Hz p o l e f requenc ies . This inc rease i n d i c a t e s t h a t a t the 60 MW l e v e l , the SVS was a b l e t o c o n t r i b u t e more dynamic vars and i s be ing u t i l i z e d more e f f e c t i v e l y . The maximum l e v e l recorded a t 60 MW was -58 dB a t 220 Hz. Th is

408 CONCLUSIONS

V 0 L T 4 0 E

. _. POWER SPEC I 6 4 8 4 ~ 0 O % O v l p "ann o r l - A.STORED RAN0E.-dBV STATUS: MUSED PEAK 745 f

10 0 > loox

IO %

I x

0 I %

0 01 %

F i g . 28. Comparison o f v o l t a g e frequency spec t ra a t Etiwanda and Eastern D i v i s i o n O f f i c e f o r t e s t w i t h peaker and SVS i n opera t ion . Al though t h e 147 Hz p o l e f requency i s s t i l l a t about t h e same l e v e l (-65 dB) i n t h e f i g u r e , t h e 220 Hz p o l e f requency i s a m p l i f i e d by t h e 66 kV t ransmiss ion system by about 9 dB f rom -68 dB t o -59 dB.

measurement i s w i t h t h e peaker i n opera t ion . With the peaker o f f , i t was est imated t h a t i t would be 6 dB h igher a t about -52 dB and was est imated t o be about 2 dB l e v e l below t h e est imated percept ion l e v e l . The f l i c k e r s e n s i t i v e f requenc ies cou ld approach t h e est imated percept ion l e v e l w i t h l a r g e r s i z e furnace. The measurements a l s o showed t h a t t h e f l i c k e r s e n s i t i v e f requenc ies would inc rease i n about t h e same p r o p o r t i o n as t h e furnace l e v e l as i n d i c a t e d by t h i s t e s t . The SVS p o l e f requenc ies which were n o t i c e d t o be t h e h i g h e s t i n these measurements cou ld have, however, been reduced by a p p r o p r i a t e f i 1 t e r and SVS design.

VOLTAGE

PWEb SPEC I 210 h o%orir non" 0"l - COYER SPEC I 37TAvg O%Orlp Hem 0.0

10.0 /DIv

4B

rma V

I I I IPfI -10.0

AYROWARCH. 66UV E

F i g . 29. Comparison o f v o l t a g e frequency spectrums measured a t Etiwanda 66 kV 6A bus a t 40 MW and 60 MW power l e v e l s . The p o l e f requenc ies a r e about 3 t o 4 dB h igher a t 60 MW l e v e l .

No a t tempt was made t o modify t h e SVS f i l t e r s as t h e equipment i s owned by t h e customer. Reducing the magnitude o f p o l e f requenc ies by a p p r o p r i a t e l y des ign ing t h e f i l t e r cou ld have, however reduced the f l i c k e r problems.

The l a b measurements and the f i e l d t e s t s show t h a t :

1.

2.

3.

4.

5.

6.

7 .

8.

9.

The f l u o r e s c e n t f l i c k e r can be caused by f requencies which are i n t h e *20 Hz range o f the 60 Hz fundamental o r i t s harmonic f requency by genera t ing beat f requencies i n 220 Hz range. The f l i c k e r curves should be expanded t o consider these frequencies as w e l l . Even a small change o f 0.8% i n t h e vo l tage magnitude r e s u l t s i n about t h r e e t imes t h e change i n t h e l i g h t o u t p u t o f t h e f l u o r e s c e n t l i g h t s .

The eyes are extremely s e n s i t i v e t o t h e 5 Hz t o 15 Hz beat f requency range and even a small change o f 0.3% (50 dB) can be de tec ted . The f i e l d measurements a l s o c o n f i r m t h e b o r d e r l i n e o f i r r i t a t i o n t o be i n t h i s range, and conf i rmed t h e p r e v i o u s l y r e p o r t e d measurements. These l e v e l s a r e cons iderab ly l e s s than t h e present standards i n IEEE-519 which permi ts up t o 5% THD.

The f l i c k e r l e v e l s a r e reduced s i g n i f i c a n t l y (by a f a c t o r o f e i g h t approximately) when the a r c furnace i s f e d by an i s o l a t e d 66 kV bus and t h e PCC i s moved t o t h e 230 kV system. The f l i c k e r l e v e l s were w i t h i n the acceptable range a t 60 MW power w i t h i s o l a t e d busses.

The power drawn by an a r c furnace can be increased s i g n i f i c a n t l y by the SVS. The l a r g e var load o f t h e a r c fu rnace l i m i t s t h e c a p a b i l i t y t o draw power w i t h o u t t h e SVS. The SVS enabled the customer t o draw the 60 MW power and t o c o r r e c t t h e power f a c t o r . The SVS a l s o reduced t h e v o l t a g e ampl i tude f l u c t u a t i o n s a t t h e Etiwanda 66 kV bus.

The e x i s t i n g standards o r t h e C.G.E.B. f l i c k e r c r i t e r i o n may n o t ensure f l u o r e s c e n t f l i c k e r f r e e fu rnace opera t ion .

Fluorescent l i g h t f l i c k e r can be caused by SVS and o t h e r f i l t e r s together w i t h t h e a r c furnace i f t h e f i l t e r s a r e n o t designed p r o p e r l y . The SVS f i l t e r s reduce the t h i r d , f i f t h and h igher harmonics be ing generated by the a r c fu rnace, b u t generate sideband o r p o l e f requenc ies . The peak l e v e l s o f these sideband frequencies a t t h e m o n i t o r i n g l o c a t i o n were much h igher than t h e o r i g i n a l harmonics. They were, however, lower f o r t h e 230 kV s e r v i c e a l t e r n a t i v e s than f o r t h e 66 kV s e r v i c e and they d i d n o t cause any n o t i c e a b l e f l i c k e r a t 230 kV simulated serv ice .

The SVS p o l e f requencies which a r e n o t i c e d t o be the h i g h e s t o f t h e f l i c k e r s e n s i t i v e f requencies cou ld have been reduced by a p p r o p r i a t e f i l t e r and SVS design.

The t e s t s showed t h a t some frequencies do g e t a m p l i f i e d s i g n i f i c a n t l y by up t o 9 dB when they propagate through the 66 kV t ransmiss ion system t o t h e remote l o c a t i o n s and r e s u l t i n a h igher f l i c k e r l e v e l a t these l o c a t i o n s .

F l i c k e r s e n s i t i v e f requencies inc rease i n p r o p o r t i o n t o the a r c f u r n a c e l f i l t e r s izes and cou ld approach p e r c e p t i b l e f l i c k e r l e v e l s (-50 dB) w i t h l a r g e r a r c furnace s i z e , even w i t h the proposed system arrangements.

References

(1) Engineer ing Recommendations P. 7/2, F i f t h Ch ie f Eng ineer 's Conference, Ju ly , 1970, o f t he E l e c t r i c i t y Counci l , CGEB, England.

(2 ) C y c l i c F l i c k e r o f F luorescent Lamps by Mr . W. R. Weise, t he C i n c i n n a t i Gas & E l e c t r i c Co., C i n c i n n a t i , Ohio.

Biography

Bharat Bhargava was born on November 8, 1939. He r e c e i v e d a B.E. ( E l e c t r i c a l ) degree from the U n i v e r s i t y o f De lh i , I nd ia , i n 1961 and a M.S. ( E l e c t r i c a l Power) degree from Rensselaer Po ly techn ic I n s t i t u t e , Troy, New York, i n 1976. M r . Bhargava i s p r e s e n t l y working w i t h the Southern C a l i f o r n i a Edison Company, Rosemead, C a l i f o r n i a . H is f i e l d s o f i n t e r e s t a re E l e c t r i c a l T rans ien ts , Subsynchronous Resonance and System Dynamics. He i s a member o f IEEE and CIGRE.

DISCUSSION

MARK R. WILHELM, Siemens Energy and Automation, A t1 an ta , Georgia.

Both a rc furnaces and t h y r i s t o r c o n t r o l l e d reac to rs produce v a r i a t i o n s i n the power system vo l tage t h a t can r e s u l t i n f l icker . Both a l s o produce harmonics. It i s impor tan t t h a t these two separate phenomena n o t be confused w i t h each o ther .

Any network vo l tage v a r i a t i o n produces changes i n the o u t p u t o f lamps connected t o the network. When these changes occur r a p i d l y enough ( i .e . , i n a f requency range f rom a few cyc les p e r second up t o 20 Hz) they a re r e f e r r e d t o as f l i c k e r and can be very annoying. The r a p i d vo l tage v a r i a t i o n t h a t produces f l i c k e r i s a modulation o f the power system vo l tage.

1.5 ,

10 Hz. Modulation of 60 Hz. -1.5 I

Any dev ice connected t o the power system t h a t draws non-s inuso ida l c u r r e n t o r produces non-sinusoidal v o l t a g e i s a source o f hannonics. Devices t h a t produce harmonics can o f t e n be considered as c u r r e n t sources t h a t i n j e c t c u r r e n t s a t f requenc ies o t h e r than the fundamental (50 o r 60 Hz). For most harmonic sources, these c u r r e n t s a re a t f requenc ies t h a t a re i n t e g r a l m u l t i p l e s o f the fundamental. I n the case o f an arc furnace, a broad band o f f requenc ies i s i n jec ted .

Modulat ion o f a 60 Hz fundamental a t 'X I Hz w i l l show up i n a f requency spectrum a n a l y s i s as 60 Hz, 60 - x Hz, and 60 + x Hz. That i s ,

[1+ AxSin(2xxt)l x Sin(2x60t) = Sin(2~60t) - A/2xSin(2r{60+ x}t) + A/2x Sin(2x{60-x}tl

409 However, t he non-fundamental f requencies found i n the above equat ion are not caused by non-sinusoidal sources and are not hannonic distortion.

I n the s u b j e c t paper i t i s suggested t h a t t he upper frequency l i m i t of e x i s t i n g f l i c k e r curves be r a i s e d t o accommodate f requenc ies i n the range o f 60 +/- 20 Hz. F l i c k e r curves p l o t acceptable vo l tage modulation l e v e l s vs. modulation f requencies. Because the human eye becomes i n s e n s i t i v e t o i l l u m i n a t i o n v a r i a t i o n s a t f requenc ies h i g h e r than 20 Hz, these curves extend o n l y t h i s f a r . Extending these curves above 20 Hz would serve no purpose.

The paper a l s o suggests l o w e r i n g l e v e l s i n e x i s t i n g harmonic standards t o accommodate f l i c k e r produced by a r t i f i c i a l l y "beat ing" an added vo l tage w i t h the fundamental vo l tage. As was p o i n t e d ou t above, t he sidebands which r e s u l t f rom fundamental f requency vo l tage modu la t ion a r e the r e s u l t o f modulat ion, n o t t he cause. Harmonic standards are designed t o cover harmonic d i s t o r t i o n o f t he fundamental and/or i n j e c t i o n o f non-fundamental c u r r e n t s , no t v o l t a g e modulat ion.

I n the s u b j e c t paper, t e s t s were conducted where v o l t a g e v a r i a t i o n (and t h e r e f o r e f l i c k e r ) was synthesized by adding another vo l tage t o the fundamental. Th is produced beat ing o f these two vol tages. I d e a l l y , t he fundamental should have been modulated d i r e c t l y . I t i s p o s s i b l e t h a t the confus ion between harmonic d i s t o r t i o n and f l i c k e r r e s u l t e d because o f t h i s i n d i r e c t method o f producing f l i c k e r .

The au thor i s t o be commended f o r t a c k l i n g a t i m e l y sub jec t . With the present p o p u l a r i t y o f s t e e l " m i n i - m i l l s " more and more a rc furnaces w i l l be showing up- -o f ten connected t o a u t i l i t y having l i t t l e exper ience w i t h t h i s k i n d o f load. Care fu l a t t e n t i o n must be p a i d t o the impact o f these loads on the u t i l i t y system--both i n terms o f f l i c k e r and vo l tage d i s t o r t i o n .

I t i s a l s o good t o see a paper t h a t c o r r e l a t e s vo l tage v a r i a t i o n and f l u o r e s c e n t lamp f l i c k e r . E x i s t i n g f l i c k e r curves are based on incandescent lamp response--which i s s i g n i f i c a n t l y d i f f e r e n t f rom f l u o r e s c e n t response.

Tests descr ibed i n the paper p o i n t o u t t he s e n s i t i v i t y o f f l u o r e s c e n t l i g h t s t o v o l t a g e modulat ion. Because o f t he n o n l i n e a r na ture o f f l u o r e s c e n t lamps, i t i s a l s o p o s s i b l e t h a t r a p i d changes i n the fundamental wave shape ( i .e . , changes i n o r modulat ion o f t he system harmonics) may produce lamp f l i c k e r . Th is i s a s u b j e c t worthy o f cons iderab le f u r t h e r i n v e s t i g a t i o n .

CLOSURE

BHARAT BHARGAVA

I would l i k e t o thank M r . Wilhelm f o r h i s d i s c u s s i o n on t h i s paper. Mr . Wi lhelm has r a i s e d t h r e e p o i n t s i n h i s d iscuss ion . The f i r s t p o i n t i s t h a t t he f l i c k e r i s caused by the modulat ion o f vo l tage wave and n o t by harmonics. The second p o i n t i s t h a t t he f l i c k e r curve represents the modulat ion l e v e l versus the modulat ion frequency, hence the f requenc ies above 20 Hz need n o t be i n c l u d e d i n t h e f l i c k e r curve as recommended i n the paper. The t h i r d p o i n t i s t h a t the f l i c k e r o r t h e ampl i tude modulat ion o f t he system vo l tage i s n o t caused by harmonics, t h a t i t has t o be t r e a t e d d i f f e r e n t l y , and t h e r e f o r e the acceptable harmonic standards need no t be reduced as recommended i n the paper.

410 With respec t t o the f i r s t p o i n t , i t i s agreed t h a t incandescent lamp f l i c k e r i s caused by ampl i tude modulat ion o f t he v o l t a g e wave. Since the SVS reduced the v o l t a g e ampl i tude modulat ion f o r low frequencies, t he SVS a t t h i s l o c a t i o n was ab le t o e l i m i n a t e the incandescent lamp f l i c k e r . However, t he SVS in t roduced po le f requenc ies a t 147 and 220 Hz, which caused f l u o r e s c e n t l i g h t f l i c k e r a t remote l oca t i ons . S t r i c t l y speaking, these f requenc ies are no t harmonics bu t cause modulat ion o f t he v o l t a g e wave, propagate a t remote l o c a t i o n s and cause f l u o r e s c e n t l i g h t f 1 i c ker.

With respec t t o the second p o i n t , i t i s b e l i e v e d t h a t l i m i t i n g the f requenc ies t o 20 Hz i n the f l i c k e r curve g ives an impression t o the user t h a t t he f l i c k e r i s caused by f requenc ies below 20 Hz on ly . Our measurements i n the f i e l d and the s imu la ted l a b o r a t o r y t e s t s c l e a r l y showed t h a t t he f requenc ies above 20 Hz which are n o t m u l t i p l e s o f 60 Hz can beat w i t h the fundamental 60 Hz frequency o r i t s harmonics and can r e s u l t i n f l u o r e s c e n t l i g h t f l i c k e r . Th is was the main r e s u l t o f our i n v e s t i g a t i o n and t h e r e f o r e i t has been recommended t h a t t he h igher f requenc ies be i nc luded i n the f l i c k e r curve. A l t e r n a t i v e l y , as i n d i c a t e d by M r . Wilhelm, a p p r o p r i a t e c l a r i f i c a t i o n c o u l d be added t o the f l i c k e r curves t o i n d i c a t e t h a t t he f l i c k e r curves show the modulat ion l e v e l f o r d i f f e r e n t modu la t ion f requencies.

With respec t t o the t h i r d p o i n t , I agree w i t h the exp lanat ions g iven by Mr . Wilhelm f o r f l i c k e r , harmonics and ampl i tude modulat ion i n h i s d iscuss ion . Regarding t h i s vo l tage ampl i tude modulat ion, t he f requency spectrum measurements taken i n the f i e l d i n d i c a t e d the presence o f m u l t i p l e f requenc ies a t remote l oca t i ons . These f requenc ies were be ing generated by t h e a rc fu rnace and t h e SVS. The two main f requenc ies which stood ou t c l e a r l y i n the measurements were t h e SVS f i l t e r po le f requencies. These f requenc ies propagated a t remote l o c a t i o n s by way o f system frequency modulat ion o f vo l tage waveforms. The beat f requenc ies r e s u l t i n g f rom these f requenc ies and the harmonics a t remote l o c a t i o n s were n o t i c e d t o cause f l u o r e s c e n t l i g h t f l i c k e r . S i m i l a r f requency i n j e c t i o n was t h e r e f o r e at tempted i n the l a b o r a t o r y t o determine the cause o f f l u o r e s c e n t f l i c k e r . The r e s u l t s o f t he f requency i n j e c t i o n i n the l a b o r a t o r y were a l s o v e r i f i e d by conduct ing f requency spectrum a n a l y s i s i n the l abo ra to ry . Figures 14 and 16 i n t h e paper show the frequency spectrum analyses. The sideband f requenc ies i n these f i g u r e s show s i m i l a r f requency sidebands as were measured i n the f i e l d .

To my knowledge, the non-60 Hz, non-harmonic f requenc ies a l though present i n the system a re no t accounted f o r i n the To ta l Harmonic D i s t o r t i o n (THD). Even though these f requenc ies are n o t harmonics, they do have adverse e f f e c t s on the system power q u a l i t y and should be covered by the standards i n some way.

F i n a l l y , I agree w i t h M r . Wilhelm t h a t f u r t h e r understanding and i n v e s t i g a t i o n o f t he f l u o r e s c e n t l i g h t f l i c k e r phenomenon and the e f f e c t o f non-60 Hz f requenc ies i s requ i red . However, we have r e p o r t e d the i n f o r m a t i o n t h a t was ob ta ined by us i n i n v e s t i g a t i n g the a rc f l i c k e r problem w i t h f l u o r e s c e n t l i g h t s f o r t he b e n e f i t o f t he Indus t r y . m Manuscript received April 10, 1992.