27190100 lamp flicker on power systems(1)

8/2/2019 27190100 Lamp Flicker on Power Systems(1)

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CHAPTER 22LAMP FLICKER ON POWER SYSTEMS

Original Author:S. B. Griscom

V OLTAGE regulation has been one of the most im-portan t problems of the electric indu stry since itsinception. The sizes of man y p arts of a pow ersystem are determined largely by this one considerationalone. A large propor tion of the selling p rice of electricalpow er is the interest and other fixed charges on prod uctionand distribution facilities, so that any improvement in regu-lation is ultimately reflected in higher rat es. Similarly,types of load imposing exceptionally severe regulation re-quirements will also increase the cost of supp lying energy.In the early d ays of the industry, a relatively wide rangeof voltage variation was permissible, because the publicwas at that time unaccustomed to uniform lighting in-tensity. Today , there is a greater consciousness as towhether the voltage level is about right, as indicated bythe whiteness of the light and by lamp life. While, how-ever, a narrow er v oltage band is required than formerly,this is not always the limiting factor in voltage regulation.Nu merou s new devices hav e been added to power linesin the last few years, which impose rapid and frequentchanges of load, with correspond ingly rapid voltagechanges. Repeated observations have shown that rapidchanges of voltage are mu ch more annoying than slowones, so tha t flicker effects m ay limit th e usefu l load -carrying ability of individual circuits long before maximu msteady-state regulation or heating is reached.

Consequently, the voltage regulation pr oblem must nowbe considered from two angles: the norm al d rop in voltagefrom light load to full load, and th e superimp osed flickersdu e to motor-starting and to various pu lsating and ir-regular loads. The differences in voltage betw een lightand full load affect th e per forma nce, efficiency, and life ofelectrical equipm ent, and are treated in Chap. 10. Thepresent chapter considers only the flicker comp onent ofvoltage regulation, and deals primarily with the reactionof the hum an eye to variations in electric light intensity.

I . PE RMISSIBLE FLICKERThe permissible amou nt of flicker voltage cann ot bestated concisely for several reasons. There is first the

hu man element; one individual may think objectionablea flicker not percep tible to anot her. The lighting fixtureused is of considerable importan ce. Smaller w attage in-candescent lamps change illum ination more quickly u pona change of voltage than lamps with heavier filaments.The character of the voltage chan ge is also importan t.Cyclic or r a p i d l y recurring voltage changes ar e generallymor e objectionable tha n non -cyclic. On non -cyclic chan gesthe ann oyance du e to the flicker is affected by the rate of

Revised by:S. B. Griscom

change, dur ation of change, a n d fr e q u e n c yo f oc c u r r e n c e fthe flicker. These and other factors greatly com plicate t h eproblem of assigning limits to perm issible flicker vol tages .Nu merou s investigators have stud ied the flicker prob-lem. The most complete analysis is found in the reportThe Visual Perception and Tolerance of Flicker, pre-pared by Utilities Coordinated Research, Inc. and printedin 1937, from wh ich Figs. 1 to 4 of this cha pter are re-produced.Figure 1 show s the cyclic pu lsation of voltage a t w h i c hflicker of 115 volt tungsten -filamen t lamp is just percep-

Fig . lCycl ic pu lsa t ion o f vo l tage a t wh ich f licke r o f 115-vo l tt u n g s t e n f il a m e n t l a m p i s j u s t p e r c e p t i b le -d e r i v ed f ro m 1 1 0 4observa t ions by 95 pe rson s in fie ld te s t s o f 25 -wat t , 40 -wat t ,a n d 6 0 - wa t t l a m p s c o n d u c t e d b y C o m m o n w e a l t h E d i s onC o m p a n y . F i gu r e s o n c u r v e s d e n o t e p e r c e n t a g e s o f o b s e rv e r sexpec t ed to pe rce ive f licke r when cyc l ic vo l tage pu lsa t ion s o fi n d i c a t e d v a lu e s a n d fr e q u e n c i e s a r e i m p r e s s e d o n l ig h t i n gc i r c u it s . P lo t t e d p o i n t s d e n o t e m e d i a n s o f o b s e rv a t i o n - a tva r ious f requenc ies , num ber o f obse rva t ions in each case

be ing ind ica t ed by ad jacen t f igu res .

tible. Flickers as low as g volt were percep tible in 10percent of the observations, when the rate of variationwas 8 cycles per second. In order for the variations to beperceptible in 90 percent of the observations, however,the voltage change h ad to be over one volt at the samefrequency. The rang e between 6 and 12 cycles per secondwas the most critical.Figure 2 shows the minimu m abru pt voltage dip to causeperceptible flicker in a 60-wat t, 120-volt t un gsten-filamen tlamp, as a function of intensity of illum ination. Curvesare shown for 5 and 15 cycles (60 cycles p er second basis)du rations of voltage dip. It should be noted that abr up tvoltage dip s of 1.5 to 2.0 volts were perceptible.

7 1 9


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Chapter 22 Lamp Flicker on Power Syst ems 721the frequen cy of occurrence and the class of service. Hereagain, judgm ent is an imp ortant factor as well as technicalfacts. The maximu m allowable fluctuations practiced byone operating compan y are shown in Table I.

This is a very comprehensive set of standard s and h asproved satisfactory in practice.

II. ORIGIN OF FLICKER VOLTAGESFlicker voltages m ay originate in the power system, bu tmost frequently in the equipm ent connected to it.

1. Generating Equipmen tPrime Mowers-Engine driven generators are prob-ably r espon sible for most of the rare cases of flicker or igi-

nating du e to the power system itself. Curve (a) of Fig. 5shows the variation in tangential force of a four cylinder

Fig . 5Curves f rom a four -cy l inder 300 rpm Diese l eng ine a tfu l l load d r iving a genera t o r . The va r ia t ion in ve loc i ty causeda c o r r e s p o n d i n g v a r ia t i o n i n t h e g e n e r a t e d v o lt a g e .

300 rpm Diesel engine at full load, and Curve (b) showsthe corresponding percent change in angu lar velocity ofthe rotating parts. With all other factors constan t, thisnon-un iform rate of rotation prod uces a fluctuation inamp litude of the generator voltage The total variationin voltage is the same as the total variation in speed; inthis examp le 0.7 percent. The frequency of the variationis equal to the rpm times the num ber of pow er strokesper revolution ; in this case 300x2 = 600 per minu te or10 per second.

Referring to Fig. 1, it is seen that 0.7 percent change involtage is read ily perceived by most individ uals. Fig. 4indicates that most operators regard th is as too mu chflicker to be tolerable. Abou t the only practicable remed iesare increasing the flywh eel effect, or chang ing the speedto get with in a less objectionable frequency range. In thisactual case, the flicker of the original installation causedman y comp laints and it was satisfactorily corrected byincreasing the flywheel effect.

When tw o or more engine-driven generators are in con-tinuou s operation at the same station, the amp litude ofthe fluctuation can frequently be lowered, and the fre-

quency dou bled to get it out of the objectionable range,by synchronizing the generators so that the power strokesof the two engines alternate rather than occur simu ltane-ously. This can be don e because there are usually m orepoles on the generators than cylinders on the engine, par -ticularly in those en gines w here the flicker is in the ob-jectionable range. A stroboscope or similar d evice usedwith the regular synchroscope perm its such synchronizing.It has sometimes been though t that it should be possibleto correct flicker o f this typ e by the use of special v oltageregulators of unu sually fast response. In practically everycase this is completely out of the question because thefrequen cy of the flicker is too high for the time consta ntof the generator field. For example, the field time constantof a typical mod erate-sized engine-type generator is be-tween 0.5 and 1.5 seconds, whereas the range of mostobjectionable flicker is between 1/ 4 nd $ second per cycle.Even electronic excitation systems a re un able to regu latevoltage at such a high rate.Generators-A symmetrical generator w ith constantload, excitation and angular velocity prod uces a constantterminal voltage. If any of these quan tities varies, how-ever, the terminal voltage also varies.It is possible to have a sufficient degree of non-un iform-ity in the generator air gap to cause pulsating terminalvoltage. However, the commercial m anufacturing toler-ances are sufficiently close that no case of flicker d ue tothis cause is known to have occurred. To prod uce flickerin this mann er, both the rotor and stator mu st be eccentric.Stator bores of all bu t the smallest size machines will in-heren tly have a certain degree of eccentricity, because theymu st be built up with segmental laminations. In spite ofthis built-up cons tru ction, quite close tolerances are heldby the use of accurate dies and assembly keys and dow els.Further attempts at improv ement wou ld be very difficultas it wou ld require bor ing or grinding the inner bore ofthe stator punchings. This is quite und esirable from thestandp oint of accum ulation of iron chippings and filingsbetween laminations and into the slots, which might resultin a condition of insulation breakdow n and localized heat-ing of the stator The roto r eccentricity is, because o f thenecessity of dyn amic balancing, held norm ally to quiteclose tolerances. Since no voltage fluctuation s can be pro-du ced if the rotor is concentric with the shaft, no modifi-cation of standard manu facturing procedu res has ever beennecessary from the standpoint of flicker voltages.Abrup t changes of load on generators prod uce corre-spond ing changes in the terminal voltages. This voltagefluctuation is the result of two factors: the change in speed,and the regulation of the machine. In central stationpractice it is very un usu al for change in speed to be asignificant factor. Sudd en load increments are usu ally toosmall as compared with the total generating capacity tochange the speed materially. Even if the speed changes,however, the rate at which the voltage drop s is ordinarilyso slow, that the effect is imp erceptible to the eye (seeFig. 3).

A typical voltage-time regulation curve of a large tur-bine generator, following sud den app lication of load isshown in Fig. 6. Speed and excitation voltage ar e assumedconstan t. Three po ints on this curve are of especial in-


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722 Lamp Flicker on Power Systems Chapter 22

Fig . 6Voltage- t ime regu la t ion o f a la rge tu r bo-genera t o rfo llowing sudde n app l ica t ion o f load .

terest. Point (a) is the voltage imm ediately following theapp lication of load; point (b) is the voltage after thevoltage h as settled; point (c) is an extrapolation of thecurve from (b) back to zero time. Each of these pointsmay be determined closely by the use of the app ropr iategenerator reactance. In fact, the standard definition ofthe various reactances has been made for this particularuse. For a fuller discussion of machin e characteristicssee Chap. 6.Point (a) is determined by the use of the machine sub-transient reactance xd. In the case of an initially un -loaded machine, the voltage (O-a) is the vector differencebetween the no-load voltage an d the prod uct of the loadcurrent times the subtransient reactance. That is,

O-a = E, - IxdThe voltage rapidly falls further to a point (2) and at

a much lower rate to point (b).The reason may be described app roximately as follows.At the instant of load application, the magnetic flux inthe air gap remains su bstantially constant, and the initialdrop in voltage is principally that du e to reactance of thearmature winding. How ever, the armatu re currents setup a demagn etizing effect to buck the field flux. Thedecreasing field flux generates voltages and currents in thefield structure, which resist or delay the ultimate change.The ind uced currents in some parts of the field structure,such as the eddy currents in the pole face, dam per wind -ings, or rivets, subside rapidly because of the high resist-ance of the path, and allow part of the flux to changequ ickly. In the average m achine, a bou t 0.1 second isrequired for this change. Most of the change of voltagebetween points (a) and (x) is du e to this cause. Themajority of the field flux is encircled by the field wind ingwh ich is of very low resistance, and , therefore, constitu tesan effective dam per to rapid changes of voltage. Thechange in voltage from (x) to (b), therefore, constitu tesan effective dam per to rapid changes of voltage. Thechange in voltage from (x) to (b) is, therefore, comp ara-tively slow, from 3 to 10 seconds being required for 90percent of the change to take p lace in large machines.

Point (x) is not directly calculable by using stand ardmachin e reactances alone. Point (c), how ever, can becalculated in the same mann er as point (a), except thattransient reactance is used . That isO-c=E,-Ixd

Similarly, po int (b) is calculatedactance using the relation:

from synchronous re-O-b= E, - Ixd

The transition from (a) to (x) and from Cx) to (b) maybe calculated by using the app ropr iate machine time con-stants. This procedure is more fully described in Chap. 6.From th e stand point of flicker voltage, the following p ointsare of interest.For single load app lications more than 10 cycles in du-ration (on a 60-cycle system), the voltage r egulation po int(c) of Fig. 6, calculated from the transient reactance, isthe determining quan tity. Fig. 2 shows that there is littledifference in perception lasting from 5 to 15 cycles o f volt-age drop . In average machines, th e subtransient dr op isusually about two-thirds of the transient drop . However,after abou t the first 5 cycles, the voltage drop s to the valuedetermined by transient reactance. A further d rop involtage takes p lace d ue to the decrement of the field,reaching point (b) on Fig. 6. Usually, this synchronou sreactance drop is not more than two or three times thetransient reactance drop. Automatic voltage regulatorsmay limit the drop to less than 1% times the transientdrop . Reference to Fig. 3 shows that for a transition timeof the ord er required (3 to 10 second s), the add itionalvoltage drop d ue to field decrement is not perceptibleFor load du rations less than 5 cycles, it is likely that theregulation as calculated from the subtransient reactancedeterm ines the perm issible flicker. While the voltage dr opat the end of 5 cycles is greater tha n initially, the tran sitionis gradu al and it is dou btful if the eye can d iscern sosmall a difference.For load d urations between 5 and 10 cycles, it is prob-able that an average between subtr ansient and transientreactances shou ld be used to calculate flicker voltages forcomparison with perception data similar to those given inFigs. 1 to 3.The pr oper reactan ce to be used to calculate the effectof cyclic variations dep ends up on the frequency of theiroccurrence. The following range is suggested for gener-ators 5000 kva and above.

In smaller machines the field time constant may be soshort that pu lsation frequencies below 2 cycles per secondmay r equire th e use of synchronou s reactance.Excitation Systems-Excitation systems are rarelythe cause of flicker voltages in central station pr actice. Inlarger generators, field time constants above 3 secondscause variation in armatu re voltage to be very gradualno matter how fast the excitation may change. Occasion-ally, hun ting of generator voltage regulators causes widevoltage fluctuations, but this is not a true flicker. On smallgenerators, continuously vibrating regulators occasionallycause a small pulsation of the armatur e voltage.


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724 Lamp Flicker on Power Sy stems Chapter 2Pub lication C 50-1943 American Stand ard RotatingElectrical Machinery of the American Standar ds Com-

mittee establishes the amount of pu lsations for synchro-nou s mot ors. Section 3-160 reads:Pulsating Armature Current: When the driven load such as

that of reciprocating type pumps, compressors, etc., requires avariable torque during each revolution, the combined installationshall have sufficient inertia in its rotating parts to limit thevariations in motor armature current to a value not exceeding66 percent of full load current.NOTE IThe basis of determining this variation shall beby oscillograph measurement and not by ammeter readings. Aline shall be drawn on the oscillogram through the consecutivepeaks of the current wave. This line is the envelope of the cur-rent wave. The variation is the difference between the maximumand minimum ordinates of this envelope. This variation shallnot exceed 66 per cent of the maximum value of the rated fullload current of the motor. (The maximum value of the motorarmature current to be assumed as 1.41 times the rated full loadcurrent.) Adopted Standard 6-13-1923.

The above excerpt provid es a basis for standard izationand gives a criterion for a design u nlikely to cause flicker.However, there are still possibilities that this amou nt ofpu lsation may at times result in flicker, p articularly if therate is between 6 and 12 cycles per second, and th e supp lyline impedan ce is high.An analysis of Fig. 7 shows that with an ind uction motorboth the current and power factor pulsate wh en the motorload varies, the power factor being highest when the loadis highest as shown in the tabulation below. Usually, thearmatu re time constant is high comp ared with the rate

of load fluctuation, and the steady-state performan ce omod erately-sized indu ction m otors as determined by tesor circle diagram may be used in calculating flicker du e tcyclic load variation of pow er factor with load, but specifidata should be used where obtainable.

The variation of pow er factor of a synchronou s motodu ring cyclic load fluctuations is a more comp licated phnomenon. The average power factor is, of course, greatlyinfluenced by the supply voltage and by the field excita-tion. The variations from this average power factor d uto load fluctuation is largely dep endent up on the rate the fluctuations as compared with the time constant of thfield. For examp le, if the field time constan t is 1 seconand the load fluctuates once every 2 seconds the synchronou s reactance of the machine determines the extent of thchange in power factor. If, however, the power fluctuations are, say, 8 cycles per second, th e transient reactanclargely determines the change in pow er factor b ecause thload swings are too rapid to dem agnetize the field.Since in flicker problems, the change in load is of greaterconcern than the magnitude of the load, the average powfactor is of no par ticular interest. The preferable pr oc

Fig . 7-Osc i l log ram of cu r ren t I , vo l tage E , and t h ree pha se F ig . 8Vecto r d iag rams i l lu s t ra t ing me th od o f ob ta in ingpower W of a 100-hp wound-ro to r induc t ion mot o r d r iv ing an m a g n i t u d e a n d p h a s e p o si t i o n o f s y n c h r o n o u s m o t o r c u r r e na i r c o m p r e s s o r . T h e vo l t a ge c h a n g e w h i c h c a n n o t b e m e a s - and m agn i tud e o f bus vo l tage wi th ch ange o f load . X,

u red f rom the osc i l log ram cause d ob jec t ionab le f licke r . s y s t e m r e a c t a n c e a n d X , i s m o t o r r e a c t a n c e .


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C ha p t e r 22 Lamp Flicker on Power Systems 725du re, if comp lete m otor da ta are available, is to calculatethe changes in the bus supply voltage to the motor due tochanges in the load on the motor. The m ethod is illus-trated in the vector diagrams on Fig. 8. Vector diagram(a) shows the vector relations for a synchronou s motoroperating at full load and 80 percent pow er factor lead.E,, Ebue and E, are respectively the system voltage, bussup ply voltage to the motor and the internal voltage o f themotor. IR, and IX, are the voltage d rops throu gh thesystem impedance. IX, is the drop through the motorwhere X, may be the synchronou s, transient or sub-transient reactance dep ending up on the rate of load fluctu-ation compar ed to the time constant of the machine.Using diagram (a) as the starting point wh ere the motorpower factor angle 4 is known along with the averageload, Ebue and all of the reactances, the change in busvoltage can be obtained as shown in vector diagram (b).For all sud den changes in load the system voltage, E,, an dthe internal voltage of the motor, E,, remain substantiallyconstant. To determine the sud den dip in bus voltage itis necessary to calculate a curve of bus voltage againstmotor load or motor load change. This requires for eachpoint on the curve that a magn itude of current be assumedand the voltage drop through the system and motor de-term ined. This will locate the interna l voltage E, withrespect to the system voltage E, (In Fig. 8 E, and alsoE, in the diagrams (a) and (b) have th e same magnitud e).

Fig . 9Charac t e r i s t ic s o f a typ ica l synchron ous mot o r a tn o r m a l r a t e d v o lt a g e . Curve A is fo r rap id ch anges in loadf rom in i t ia l va lue and cu rve B is fo r s low changes .

The position of the voltage drop s will then d etermine theposition of the curren t vector as well as the bu s voltagevector Ebus. Using the current, voltage (EbuB) and theangle between them the power can be found . With th ecurve of bus voltage against motor load change the voltagefor any desired change in motor load can be obtained.The variation in reactive kva with real pow er is shownin Fig. 9 for a typical synchronou s motor. These d ata are

Fig . 10Charac t e r i s t ic s o f a typ ica l indu c t ion m oto r .

for a pow er factor of 80 percent at full load, bu t for ordi-nary p urp oses the variations in reactive factor m ay besuperim posed on the initial reactive factor. Curve A isfor a rapid rate of fluctuation starting from full load 80percent pow er factor; Curves B are for a rate slow com-pared to the field time constant with fixed terminal voltage.Motor Driven Intermittent Loads-In this cate-gory fall motor drives w here the nature of the work callsfor heavy overloads, and for cyclic loads of long and ir-regular period. Saw mills and coal cutt ers are typicalexamples of app lications where heavy overloads, some-times to the stalling point, are common and difficult toprevent. The motor currents in such installations varyrapidly from light load, through pull-out at heavy currentand high pow er factor, to the high locked-rotor current atlow pow er factor. Pun ch pr esses and shears are examples ofapp lications where the load goes throu gh wide variations,but wh ere flywheels and other d esign features limit boththe rate of app lication and magnitud e of the load swings.Motors used to drive intermittent loads are likely tohave been designed with special characteristics. If possible,the fluctuation in current and power factor should be ob-tained b y test or from the man ufacturer. In the absenceof such sp ecific dat a, Cur ve B of Fig. 9 may be used forslow cycling in termittent loads, and the curve of Fig. 10may be used for app lications where pull-out and stallingoccur.Electric Furnaces-There are three general types ofelectric furn aces-resistance, induction, and arc. The re-sistance furnace usually causes no more flicker th an anyother resistance load of comparable size. Most indu ction


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726 Lamp Flicker on Power Systems Chapter 22ing the regulator settings or by a combination of severalof these procedures. Forcing the furn ace in this mannerincreases b oth the magnitud e and the violence of the loadswings. The typ e of the scrap being melted also affectsthe extent of the load swings, heavy scrap causing widerfluctuations than light scrap.

The oscillogram of Fig. 12 rep resents a shor t par t of amelting-down period of a 10 000-kva arc furnace, At times,

Fig . 11Three-phase me l t ing a rc fu rnace o f th e Herou l t type .

f u r n a c e s o p e r a t e a t h i g h f r e qu e n c y , a n d t h e r e f o r e , a r e

While the average load factor and power factor of elec-tric arc furnaces are as good or better than man y oth er

connected to the power line through a frequency changer

indu strial devices, the problem of sup plying them withpow er is usually m uch more difficult. During the meltingdow n period, pieces of steel scrap will at times, more or

and consequently represent a fairly steady load.

less, completely bridge the electrodes, ap proximating ashort circuit on the second ary side of the furnace tr ans-former. Consequently, the melting dow n period is char-

Three-phase steel melting ar c furnaces of the Heroult

acterized by violent fluctuations of current at low pow er

typ e, illustrated in Fig. 11, are being used to a considerab le

factors, sing le-phase. When the refining period is reached ,the steel has been melted down to a pool and arc lengths

extent to make high grade alloy steel, and frequently cause

can be maintained uniform by autom atic electrode regu-lators, so that stable arcs can be held on all three elec-trod es. The refining period is, therefore, characterized by

voltage flicker.

a steady three-phase load of high pow er factor.

the current variations occur at a periodicity app roximatingthe rate of the most objectionable flicker. A grap hic chart

Fig . 12Osci l log ram a t s ta r t o f hea t in a 1000 0 kva Herou l t

illustrating the variation of load over a longer period ofoper ation is show n in Fig. 13. These tw o figures are

t y p e t h r e e - p h a s e a r c f u r n a c e .

repr ints of figures from reference 10.

A s ing le -phas e a rc s t ru ck andr e s t r u c k 1 0 t i m e s i n t h e s p a c e o f 1 5 s e c o n d s b e fo r e a l l t h r e ep h a s e s s t r u c k . Af t e r t h i s i n i t i a l p e r i o d , a ll t h r e e p h a s e s s t r u c ka n d r e s t r u c k 1 0 t i m e s w i t h c u r r e n t s i n a l l t h r e e p h a s e s f a ir l ywel l ba lanced be fo re th e a rcs becam e genera l ly s tab le . A por -

t i o n o f t h i s p e r f o r m a n c e i s s h o w n o n t h i s f ig u r e lo .

Calculated curves in Fig. 14 show the electrical char-acteristics of a 10000-kva, three-phase arc furn ace. Thesecurves were prepared on the assumption that the maximumattainable current wou ld be app roximately twice norm alat 50 percent pow er factor. The effective imp edan ce ofthe arc (based on 11 500 volts in the pr imary) is plottedas the abscissa. For conven ience, zero ohm s, as plotted ,represents the minimu m arc resistance as determined bythe so-called short circuit cond ition. Actually, at thispoint there is app reciable voltage drop at the electrodetips, and considerable arc energy; the curves are plottedhe size of load fluctuations du ring the melting dow nperiod is influenced by a nu mber of factors, of which the in this manner only to show the working range. It is ofrate of melting is perhap s the most importan t. The fur- interest that the point of maximu m pow er is not that ofnace-supp ly tran sformers have winding taps for control of maximu m kva. The usual melt-down range is probablythe arc voltage and in the smaller sizes (abou t 6000 kva between the points correspond ing to 0 and 10 ohms, theand below) have separate built-in reactors to limit the arcs fluctuating du ring this period so that the heatingcurrent and stabilize the arc. The rat e of melting is subject effect is some sort of an average betw een these limits. Theto further control by means of electrode regulators. Some- refining range is probably above 10 ohm s.times the produ ction of the furnaces is stepped up by It is difficult to obtain definite figures on the values ofraising the arc voltage, reducing th e series reactance, rais- instantaneous swings in current and pow er factor for use


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hapter 22 Lamp Flicker on Power Syst ems

. 1 3 -G r a p h ic c h a r t s a t t i m e o f s a m e h e a t s h o w n o n o s c i l-F ig . 12 . Furnace swings occur approx imat e ly once

a second lO.flicker determinations, because an oscillograph mu st bethe maximum swings cannot always be caught.small furn aces, the curren t may reach a maximu m ofat full load, but the process of reaching this

usually throu gh a series of small increments, andnoted previously the annoyance to lighting customerslargely a matter of the rate of change rather than theThe kva swings given in Figure 15 are equivalent swings.

Fig. 14---Electrical charac t e r i s t ic s o f aa r c f u r n a c e .

1 0 0 0 0 k v a , t h r e e - p h a s e

These valu es will give app roxima tely the same flicker asthe single-phase swings given in references 14 and 15. Thecurve values are not-the maximu m possible swings for agiven furnace size but ar e good values to use in estimatingflicker. The frequen cy of occurrence of these swings cor-respond s to the Extremely Frequen t classification as given

Fig . 15 Equ iva len t kva swings in an e lec t r ic a rc fu rnace .

in Table 1. Load swings can occur m ore rapidly, but theirmagnitu des are less than those in Fig. 15. These curvescan be used in conjunction with the method suggested inSec. 5, to estimate the amou nt of flicker. The informationshown in Fig. 15, together with suitable system constantsshould give a fair app roximation of the flicker voltage tobe expected.Elec t r i c elders-This is a class of equip men t ofgreat imp ortance in pow er system flicker. Most weldershave a smaller ``on time than off time, and conse-quently, the total energy consumed is small comp ared withthe instantaneous dem and . Fortunately, most welders arelocated in factories, wh ere other p rocesses requ ire a largeamou nt of pow er, and where the supp ly facilities are suffi-ciently heavy , so tha t n o flicker trou ble is experienced .In isolated cases, but nonetheless impor tant, the weldermay be the major load in the area, and serious flickermay be imposed on distribution systems ad equate forordinary loads.


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728 Lamp Flicker on Power Systems C h a p t e r 2 2

The mor e comm on types of electric welders are:(1) Flash welders(2) Pressure butt welders(3) Projection welders(4) Resistance welder s

(a) Spot(b) SeamIn welders the source voltage, usu ally 230, 460 or 2300volts is stepped down to a few volts to send high current

throu gh th e parts to be welded. Practically all weldersin service are single-phase, although experimental three-phase welders show promise.With flash welders, on e piece is held rigidly, and theother is held in quasi-contact with it, with voltage app lied.An arc is formed, heating the metal to incandescence, andthe movable piece is mad e to follow to maintain the arc.The heating of the metal is partly by the passage of currentand partly by burn ing w ith the arc. After a sufficienttemperatu re and heat penetration has been obtained, thepieces are forced together und er great pr essure. In somecases, the p ower is cut off before this up set; in oth ers,the pow er is left on. The current, draw n du ring the flash-ing per iod, is irregu lar because of the instability of the arc,so tha t the flicker effect is obnoxious mor e than if thecurrent were steady at its maximu m value. The averagepow er factor du ring flashing may be as high as 60 percent.At up set, it is about 40 percent. The flashing may last upto 20 or 30 second s, but 10 seconds is more comm on. Thedu ration of pow er du ring up set is usu ally short; of theorder of 1/ 2 second . This type of welder may draw u pto 1000 kva during flashing and about twice this loadingat upset.Pressure butt welders are similar to flash welders, exceptfor the impor tant difference th at th e parts being weldedare kept continuou sly in contact by a following p ressure.The heating is prod uced p rimarily by contact resistance.From a power supp ly standpoint the butt welder is moredesirable th an the flash welder because the welding currentonce app lied, is pr actically steady and the only flickerprod uced is at the time p ower is app lied and removed .The range of currents and pow er factors is about thatfor flash welders.Projection welders are similar to pressure butt weldersexcept that the latter usually join pieces of about equalsize, and projection welders usually join small pieces tolarge ones. The curren t dem and is usually smaller, b utthe operations are likely t o be more frequent.In resistance welders curr ent is app lied th rough elec-trodes to the parts to be welded, usu ally thin sh eets ofsteel or aluminu m. The weld is accurately timed to bringthe metal just to the welding temperatu re. The pieces arefused together in a small spot. In the spot welder, one ora few such spots completes the weld. In a seam welder,a long succession of spots p rodu ces the equivalent of asingle continuous weld or seam. Resistance welders arecharacterized by large short-time currents. In spot weld-ers, the curr ent may be applied for only a few cycles (ona 60-cycle basis), with weld s following one anot her in afraction of a second u p to about a minu te. Thus, from aflicker stand point there are a succession of individual volt-

Fig. 16Ignitron t i m e r f or r e si s t a n c e w e ld e r .age dips occurr ing at objectionably frequent intervals.Seam welders have an on du ration of a few cycles followed by an off du ration also of only a few cycles. The


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C h a p t e r 2 2 Lamp Flicker on Power Systems 729

Fig . 17Typica l re s is tan ce welders---(a) spotwelder .

welder, (b) seam

process is a continuou s on e while a given piece is in themachine, and since the periodicity of the w elds is un iform,the flicker can be ann oying even for relatively small voltagedips. The essence of good spot and seam w elding is ac-curate control of the heat, consequently accurate magni-tud e and du ration of current are necessary. Vacuu m tubesare being used to a large extent for welder control functionsbecause there are no wearing parts, and close and con-sistent regu lation of the heat is possible. Fig. 16 show s aph otograp h of an ignitron electric timer and Fig. 17 showsa typ ical resistance welder.

Resistance welders draw ing energy from all three phasesgreatly minimize flicker. Electron ic devices are used toconvert from the GO-cycle, 3-phase sour ce to a single-ph aseoutp ut of lower frequency, say 10 cycles per second. Onsmall welders, the stored energy of capacitors or indu ctorscan often be used to minimize the peak dem and from thesource.MiscellaneousUnder this category come specialequ ipm ent as electric shovels, heavy rolling mills, andsimilar installat ions. Most of these mu st be consideredindividually as to special features and power supp ly.Strip mining shovels frequently cause severe voltagedips in pow er systems, principally because of their largesize and wide variation of their loads. The fast rate ofload application is usu ally injurious to the power systemprincipally by creating a wide band of voltage fluctuation,rather than flicker as it is commonly encountered. Thesite of mining op eration is often at out-of-the-way locationswhere the pow er requirements for general p urp oses aresmall and hence, the norm al p ower facilities are of lowcapacity, and very susceptible to flicker du e to loadchanges.

The large continuou s rolling mills now used extensivelyin prod ucing wide metal strip have imp osed a new problemon the pow er indu stry. Like the electric shovel, these loadsdo not necessarily prod uce flicker in the customary senseof the word. The power supply is usually through motor-generator sets withou t ad ded flywheel effect. The loadcomes on and d rops off in steps as the metal enters orleaves the rolls. The ind ividu al increm ents are not inthemselves abrup t, a fraction of a second to over a secondbeing required for the metal to enter a roll completely.A large hot strip mill and a typical load chart are shownin Figs. 18 and 19.

The pow er draw n by a large continu ous mill may buildup to 30 000 kw in a period of 8 seconds, stay nearly con-


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730 Lamp Flicker on Power Systems C h a p t e r 2 2

Fig . 19Load char t fo r a ho t s t r ip ro l l ing m i l l.

stant for a minu te, and then drop to almost zero in another8-second period, There may th en be an off period of aminu te followed by a repetition of the load cycle. Thepow er source is usually amp le so that no flicker is percep-tible to the eye, bu t there is neverth eless a tend ency forthe voltage to weave up and down. This is und esirablebecause it widens the band of voltage regulation and maycause excessive op eration of feeder voltage regulator s.Autom atic control of the excitation to the motor-generatorsets to conform to the load variations is effective inminimizing these voltage swings.

Fig . 20Power f low be tween a s te e l mi l l and a la rge in te rcon -n e c t e d p o w e r s y s t e m .

A heavy cycling load of this kind may p rod uce widefrequency variations on an isolated pow er sup ply systemand wide load swings on an interconnected system. Apower plant recording chart in Fig. 20 shows the powerflow between the steel mill pow er plant and a large powerpoo l. Fig. 21 (a) show s the hot strip mill load cycle and

Fig. 21(a) Hot st r ip mill load. (b) Effect on freque nc y ofl a r ge i n t e r c o n n e c t e d s y s t e m .

Fig. 21 (b) the results of calculations on how power surgesof this kind cause frequency disturban ces which travel aswaves between the local power company to which the steelplant is connected and a larger pow er pool.

I I I . LOCATION OF FLICKER VOLTAGESLoad equipmen t may create flicker conditions in oneor more of the following locations:(1) Secondary distribution(2) Primary lines(3) Substation busses(4) Generating stationsAny flicker in bus voltage of the generating station canbe expected to show up at practically all points served by

tha t station . Similarly if a substation bus flickers, all ofthe radial loads from that substation are affected. Primaryline flicker affects all custom ers remote from the sour ceof flicker, and t o a lesser extent, some of those n earer thesour ce of sup ply. Second ary circuit flicker is usu ally con-fined to an area immediately adjacent to the source ofthe disturbances.The location of flicker voltage, or the extent of the af-flicted area, ha s a considerab le influence on possible r em-edies. If the genera ting station busses are affected, thereare usu ally no commercially practical means of remedyingthe situation on the pow er system, and the correction mu stusually be mad e at the utilization point. If a substationis affected, but the generation stations are not, then m oretie lines or transmission at higher voltage can be employed,or a separate line run from the generating station to theaffected area. Sometimes the utilization equ ipm ent itselfcan be corrected. If a pr imary line is affected, improve-ments can be mad e in either th e pow er system or theutilization equipment. If the distribution-system alone isaffected the correction may be made either on the systemor the utilization device. If the utilization device is stand -ard equipm ent, it is usu ally best to correct the distribution


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Chapter 22 Lamp Flicker onsystem, and thus improve other loads as well. If theutilization device is special, it is pr obably mor e efficient tocorrect the device.

IV. REMEDIAL MEASURESA large variety of corrective equipm ent and procedures

can be used to minimize flicker. Those m ost common lyconsidered are :

1.2.3.4.5.6.7.8.9.10.11.12.

:rator setsdens

Motor genePhase convertersSynchronous conSeries capacitorsShunt capacitorsVoltage regulatojBooster transformMotor startersExcitation contraLoad controlFlywheelscc---L--- _I -- ---

l-sners21

i3ys r ;em c r langes3. Motor Generator Sets

A corrective scheme using m-g sets is illustrated in Fig.22. In general, it is probably true that a motor-generator

Fig . 22 -Motor -genera t o r se t .

set between the utilization device and the power systemgives the maximum possible reduction in flicker, becauseit is effective in minim izing three of the m ost undesirableload characteristics: single phase, low pow er factor, andsudden application Since the only tie between the motorand the generator is the shaft, th e disturbances du e tosingle-phase load or to low pow er factor are not transferredto the pow er system. The reactance of the driving motor,in conjunction with the flywheel effect of the motor andgenerator delay the transfer of a change in load to thepow er system. The rate at which the voltage d rops istherefore lessened and th e eye is less likely to perceivethis flicker.The motor-generator set is probably the costliest ar-rangemen t, heaviest, least efficient, and occupies morefloor space than any of the various corrective devices thatcan be used. But the m-g set has the advantage of con-sisting entirely of standard equipm ent, and is, therefore,reliable an d well und erstood apparatus. The motor endmay be synchronou s, squirrel-cage indu ction, or wou ndrotor indu ction, the latter u sually being provid ed with aflywheel and slip regulator. The generator end may be

Power Systems 731suitable for the sup ply of either single-phase or polyph aseloads.When a synchronous motor d raws additional power fromthe line it drop s back in phase position. This causes atemp orary dr op in speed, but th e flywheel effect of therotor tends to oppose this change and to give up tem-porarily part of its rotational energy. This results in acushioning of the rate of app lication of load to the powersystem, and a material redu ction in peak deman d can beeffected for loads of short du rations as compared with one-half of the natu ral per iod of electro-mechanical oscillation(see Chapter 13). The natural period usually ranges be-tween $$ and 1 second , so that for loads lasting about ?&second and less, substantial redu ctions in peak demandcan be expected. Thus, synchronou s-motor-driven m-gsets are quite su itable for spot and seam welders havingan on time o f 1 to 10 cycles (60-cycle basis). Similarly,sud den increases or decreases of load are shielded from thepow er system if the load factor is high, bu t the load issubject to shor t violent irregu larities. This is tru e of elec-tric furnaces, for example, where the overall load factor isgood, but there is considerable ``choppiness, sud den powerfactor changes and short-circuiting of individual phases.For this type of load, synchronous motor drives are nearlyas effective from the flicker standp oint as squ irrel-cageindu ction, and preferable for other reasons.When an indu ction motor draws add ed power from theline, it drop s in speed. Its outp ut, in the normal w orkingrange, is closely prop ortional to the slip, that is, to thedifference between synchron ous and actual speed. If loadis sud denly app lied to a generator driven by a squirrel-cageindu ction-motor, the system does not feel the full effectuntil the motor-generator set has slowed dow n from nearlysynchronou s speed to full-load speed. In the meanwhile,the inertia of the rotating parts supp lies the energy, andthus the rate at which pow er is draw n from the system ismaterially redu ced. Furthermor e, as in the case of syn-chronou s d riving motors, if the generator load consists ofa series of short pulses, the load is off before its full effectis transmitted to the power system, and the peak load onthe system is thereby decreased . Because an inductionmotor m ust actually slow dow n, whereas a synchronou smotor merely shifts in phase, the rate of load applicationto the power system is less for the ind uction than for thesynchronou s motor. On an average, it takes an indu ctionmotor-generator set about one second to transfer full loadto the source. In Fig. 3, it is show n that this delay aloneresults in dou bling the threshold of flicker perception, ascompared with the perception due to sudd en voltage d ipof equal magnitude.

If the load pulses last several seconds, the power drawnfrom the system levels off to the amoun t of generator loadplus losses for either a synchronou s or squirrel-cage motordrive. The voltage d rop in the power system d uring thissteady load period is usu ally about the same for eitherthe indu ction or synchronou s motor drive, assuming thatthe excitation of the synchronou s motor is constant. Byincreasing th e synchronou s-motor excitation with the loadthe final regulation of the system can be mad e very small.However, from the standp oint of flicker su ch excitationchanges are usually imp erceptible because of the time


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Chapter 22 Lamp Flicker on Power Systems 733du ced still further by the slower rate at which the voltagedips, particularly with the indu ction set. For loads of veryshort du ration such as ?& second and less the voltage dr opmay be reduced to l/ 10 or even l/ 20.Motor generator sets may be had with either single- orthree-phase generators. Even when the generator is singleph ase, it is customary to use a three-phase star statorwind ing using only two legs in series. The third phase iswou nd for possible futu re use, or to increase synchronizingpow er if paralleled with other u nits, or du mm y coils maybe placed in the slots. If single-phase loads are to becarried, the field mu st be built w ith low resistance dam perwind ings to minimize rotor heating. In the larger sizes,single-phase machines are mou nted on springs to minimizevibration du e to the pulsating. torque caused by single-phase operation.When more than on e utilization device causing flicker isinvolved, the question of a single m -g set versus an m-g setfor each such load must be answered. In these cases it isvery importan t to consider the regulation of the generatorof the set and how constant a voltage is required by theutilization devices. For example, it frequently happ ensthat a factory is using several electric w elders which pro -du ce 5 percent voltage dip s of very objectionab le fre-quency. This 5 percent dr op usu ally d oes not affect th eperforman ce of the welders, and they could be operatedat rand om on the power system. If a motor-generator setis to be used, however, the transient reactance of the gen-erator is apt to be as high as 35 percent based on its ratedcurrent, and , assuming that th e welder reactive currentequals th e generator rating, a 35 percent, dr op in voltagewou ld occur. If only one welder is operated at a time, th isis quite satisfactory, as the welder tap can be set on thebasis of closed circuit voltage, that is, the regu lation ofthe generator can be taken into accoun t. If, however,another welder is operated simultaneously, even thou ghon another phase, the add itional voltage d rop, un com-pensated by the welder tap , is enough to spoil the weld.In order to operate several chopp y loads simultaneouslyfrom the same m -g set, it is therefore necessary to use anoversize generator (from a thermal standp oint) to keepthe regulation w ithin required limits. Alternate solutionsare to interlock utilization devices so that they cannotoperate simu ltaneously or to provide separate m-g sets foreach device. Another alternative is to use one commondriving motor and several separate generators on the sameshaft. The separate m-g set plan has the advantage ofperm itting operation at partial capacity in case of dam ageto one set, but is costlier.4. Phase BalancersIn indu strial plants a large percentage of the potentialcauses of flicker are single-phase devices. A discussion ofph ase balancers is, therefore, of interest, although therehave been few commercially installed.

In a single-phase circuit the flow of pow er pu lsates ata frequency twice that of the alternating sup ply, whereasin a balanced polyph ase circuit the flow of pow er is uni-form. Therefore, in ord er to effect a conversion betweena single-phase and a polyph ase system, some energy stor-age is necessary. This stora ge m ay be mad e in static de-

vices such as indu ctances and capacitors, or in rotatingequip men t with mechan ical inertia. Except for small sizes,the static equipm ent has not yet been foun d commerciallypractical.

A lack of app reciation of this fund amental energy re-quirement has led to frequent prop osals of schemes at-tempting single-phase to polyph ase conversion by trans-former conn ection. Fig. 25 is typ ical of these schemes. It

F i g. 2 5 Un s o u n d a t t e m p t t o s u p p l y b a la n c e d t h r e e - p h a s epower t o a s ing le -phase load .

is not only completely ineffective for its intended pu rpose,but is also wasteful of transformer capacity. Although thetransformers are all loaded equally, the currents draw nfrom the source as shown by the current arrows, are stillsingle-phase, and a single-phase transformer is, therefore,preferable.

The most familiar type of ph ase converter is that shownin Fig. 26. It has been extensively used in railway electri-fications to convert single-phase pow er from the contactsystem to three-phase pow er for the locomotive motors;this is merely th e converse of the phase-balance. As shown ,a rotating two-ph ase m achine is connected to the three-

F i g. 2 6 S c h e m a t i c d i a gr a m f or p h a s e c o n v e r t e r u s e d e x -tens ive ly on ra i lway e lec t r i f ica t iona to conver t s ing le -phas ep o w e r fr o m t h e t r o l le y t o t h r e e - p h a s e p o w e r fo r t h e lo c o m o -t i v e m o t o r s . A r o t a t i n g t w o -p h a s e m a c h i n e i s c o n n e c t e dt h r o u g h t h e e q u i va l e n t o f a S c o t t - c o n n e c t e d t r a n s f o r m e r t o

t h e t h r e e - p h a s e p o w e r s y s t e m .


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734 Lamp Flicker on Power Systems Chapter 22phase pow er system th roug h the equivalent of a Scott-connected transformer, which also serves as the primaryfor the single-phase load wind ing. The two-phase machinemay be of the indu ction type and act as a phase converteronly, or it may be synchronou s and used for power factorcorrection as well. Because of the regulation of the ma-chine, the source curren ts are not balanced du ring variable-load conditions, unless the taps on the transformer windingare varied. From this point of view, it is not very su itablefor choppy loads. Where there are several separatesingle-phase loads to be served, th e capacity of a converterof this type must be equal to the sum of the individualloads.

The series type of ph ase converter is show n in Fig. 27.This is pr obably mos t efficient for conversion from three-

F i g. 2 7 S e r ie s t y p e o f p h a s e c o n v e r t e r f r om t h r e e p h a s e t os i n g l e p h a s e .

phase to single-phase, where the single-phase load is notexpected to grow, cannot be distributed between ph ases,and where no pow er factor correction is required. It con-sists of a counter-rotational indu ction-type series machine,connected throu gh transformers in such a mann er as tooffer a high imped ance t o negative-sequence current be-tween the single-phase load and the three-phase sup ply.

Fig . 28Ser ies impe dan ce type o f pha se ba lancer .

When a single-phase load is sud denly app lied, a magnetiz-ing transient results, so that p art of the negative-sequencecompon ent of load curr ent is passed on the source. Al-thou gh this transient subsides in about 0.1 second , itdetracts considerably from the value of the scheme foruse with choppy loads.The series imped ance balancer show n in Fig. 28 consistsof an auxiliary indu ction-type machine in series with thepolyphase supply and with the main shunt m achine.The single-phase load is draw n from between the two.The series machine rotates opp ositely to norm al directionfor positive-sequence app lied voltage, an d therefore, offershigh imped ance to negative-sequence currents an d lowimped ance to positive-sequence currents. The shun t ma-chine therefore takes the negative-sequence compon ent ofload current. The positive-sequence compon ent of loadcurrent is taken by the system if the shun t is an ind uctiontype unit. If a synchronou s type u nit is used for the shun tmachine, it can also take th e wattless compon ent of loadcurrent with suitable control of excitation. As with theseries phase converter, the series machine does not imme-diately resp ond to load changes, and temporarily (for about0.1 second) som e unbalanced current is draw n from thesour ce. The scheme, like the series ph ase balancer, is in-herent in its action, no regulators being required u nlesspow er factor correction is used. This method has oneimpor tant advan tage over the previous two schemes inthat the size of the shunt machine need only be enoughto take care of the maximu m un balance of load. For ex-amp le, if there are a num ber of individual single-phaseloads as illustrated in Fig. 29, they may be distributed

Fig . 29Effec t ive use o f a synch ronous conde nse r in conn ec-t ion wi th a f luc t ua t ing load .

between the phases, and the shunt machine need carryonly the unb alance componen t. The series machine mu st,however, have enou gh capacity to carry the total positivesequence current.

Phase balancers, as a class, are not particularly suitablefor flicker elimination except perh aps in bord erline caseswhere only a mod erate improv ement (perhaps a one-halfredu ction in voltage d ip) is required. In this case theymay be the cheapest and most efficient remedy.5. Synchronous Conden sers

The voltage dip on a pow er system resulting from asud denly app lied load is equal to the vector prod uct ofthe current and the system impedance giving proper con-sideration to vector positions. Consequently, one way ofredu cing flicker is to redu ce the system imp edance. Usu-


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736 Lamp Flicker on Power Systems Chapter 22for standar d capacitors). The voltage along the line isshown by the diagram at (c), Curve A showing the un-compensated voltage and B the compensated voltage. Thepoint of interest emph asized by (c) is that the comp ensat-ing voltage is introdu ced in one step while the voltagedr op along the line is uniform. For this simple case with

Fig. 3@---Typical applicat ion of series capa cit ors .(a) Layout ordinar ily favora ble to application of series capacitors(b) Location of series capacitor(c) (A) Without capacitors; (B) with capa citors.

no intermediate line loads, the voltage gradient along theline is un impor tant, and , subject to limitations outlinedlater, complete voltage-drop compensation at the distribu-tion substation may be secured .

The vector diagram s for series capacitors at variouspow er factors are shown in Fig. 31. These d iagrams showthat on ly the indu ctive compon ent of line imped ance iscompensated by the capacitor. However, if the pow erfactor of the load increment is low and constant, it ispossible to over-comp ensate for the system reactance, andthu s partly or completely nullify th e resistance compon entof line drop . With variable loads and power factors thisprocedu re can cause und esirable voltage-regulation char-acteristics and therefore each case of over-comp ensationmu st be considered on its own merits.

Where there are distributed loads along a line, it is nec-essary to consider the location of the capacitors. The ca-pacitor gives its full voltage boost at the point o f its in-stallation, and therefore loads immediately ahead andbehind th e capacitor differ in voltage by the amoun t ofboost in the capacitor. In general, th e best capa citor loca-Fig . 31The vec to r d iag rams show t he vo l tage d rop ac ross thes e r i e s c a p a c i t o r r e q u i r e d i f a c a p a c i t o r i s a d d e d s o t h a t t h es e n d i n g v o lt a g e w i l l b e t h e s a m e a s t h e l o a d c e n t e r v o l t a g ewhen th e load power fac to r i s (a ) 90 pe rcen t ; (b ) 75 pe rcen t ;

(c ) 60 pe rcen t .


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Chapter 22 Lamp Flicker on Power Syst ems 737tion is one-third the electrical distan ce betw een th e sour ceand the flicker-produ cing load, as shown by Fig. 32.In pr inciple series capa citors are effective in reducingflicker caused by practically all typ es of fluctuat ing loads.However, their effect is only beyond their p oint of instal-

Fig . 32Percen t vo l tage regu la t ion - in genera l , by p lac ingt h e s e r i e s c a p ac i t o r a b o u t l / 3 f t h e e le c t r i c a l d i s t a n c e b e t w e e nt h e s o u r c e a n d t h e l o a d , t h e v o lt a g e o n b o t h s i d e s o f i t a r ekep t wi th in p lus o r min us l imi ts in which f licke r i s no tob jec t ionab le .

lation; hence they d o not correct the system as a whole.For examp le, a series capacitor installed just ahead ofsubstation B in Fig. 33 may remove most of the voltagefluctuation on that bu s. However, at Station A, therema y still be considerab le voltage fluctuation , as the seriescapacitors do not correct the sup ply circuits. Anotherpoint to be noted from Fig. 33 is that the series capacitormust be large enough to carry all loads beyond its pointof installation. Consequ ently, if the flicker-pr oducing load

Fig . 33Ser ies capac i to r mu s t be la rge enou gh to ca r ry t o ta ls u b s t a t i o n l o a d .

is small as compared with norm al load, the cost of theseries capacitor is too high for the correction obtained.Series capacitors are therefore economical p rimar ily w herethe flicker load is a large portion of the total, where thecircuit resistance is equal or lower than the reactance,where the flicker-prod ucing load is of low pow er factor,and wh ere the supp ly circuits are fairly long.Und er certain circumstan ces series capacitors will pr o-du ce, in conjun ction with other app aratus, voltage or cur-rent surges in the line. The magnetizing inrush curr ent oftransformer banks, an d the self-excitation of synchronou sor indu ction motors are some of the factors causing thisphen omenon , which is too involved for treatment here, butis discussed in items 4 and 5 of the ta ble of references.

Fig . 34Ser ies capac i to r in s t a l led wi th a we ld ing load tor e d u c e k i lo v ol t a m p e r e d e m a n d a n d im p r o v e p o w e r fa c t o r .

Capacitors in Series with the Equipment-This ap -plication is limited to utilization equipm ent with a constantinductive reactance, for wh ich it is possible to comp ensatewith a series capacitor, so that the load drawn from thesup ply circuit is practically at unity p ower factor a t alltimes. Thus, although the pow er draw n from the line isstill fluctuatin g, the resultan t flicker voltage is greatlyredu ced. Figure 34 shows such comp ensation app lied to awelding transformer. Inasmu ch as the load itself is cor-rected, the benefits are felt all over the sup ply system.Several such app lications have been successfully mad e tospot and seam welders (see reference 3).7. Shunt CapacitorsContrary to frequent misconceptions, perm anently con-nected sh unt capacitors are of no benefit w hatever inminim izing flicker; in fact, they may ma ke it slightlyworse. An example show s the reason readily. A systemwith 10 percent indu ctive reactance in the sup ply leads,serving an intermittent load having an indu ctive reactanceof 100 percent is show n in Fig. 35 (a). Resistance in bothline and load will be neglected to simp lify the example,bu t the same general effect will be observed if resistance

Fig . 35Shunt capac i t o rs a re no t e ffec t ive in reduc in g vo l taged ips .

were present. When the switch is open E,=Es. When theswitch is closed, the voltage at EL= +jloo+j10+j100 Es = 91percent Es. Fig. 35 (b) show s a similar circuit except acapacitor having a reactance equal and opp osite to that ofthe load is perm anently connected in the circuit. Whenthe switch is open , the voltage EL= -do0 Es= 111+j10 -jlOOpercent Es. When the switch is closed, the net load im-pedance is c -jlOO) (+jloo ) = o.- j100+ j100 . This means that thecombination of the capacitor and reactor draw s no currentfrom the source, and E,=Es. Thus, comp aring the twocases, withou t the capacitor the voltage d rops from 1 0 0percent to 91 percent, a change o f 9 percent. With ca-


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738 Lamp Flicker on Power Systems Chapter 22pacitors, the voltage drop s from 111 percent to 100 percent,a change of 11 percent.Shunt capacitors connected to utilization equipm ent sothat they are switched in accordan ce with load, redu cevoltage d rop . To be effective, the utilization device mu stdraw a current that is substantially constant in magnitu deand pow er factor du ring the on p eriod, as, for example,some forms of resistance welders on which long runs aremad e withou t change of set-up. Motor starting is oneexample of an app lication to which shunt capacitors can-not be used effectively in this mann er for flicker redu ction.Motor inrush current app roximates six times full load. Ifthis is neutralized by a shun t capacitor, the initial voltagedip is greatly redu ced. However, when the motor comesup to speed, the voltage rises above th e initial voltage.8. Voltage Regulators

Voltage regulators are also totally un suited to correctingflicker. This statement app lies both to generator voltageregulators, or to step- or indu ction-type feeder regu lators.These devices operate only when the voltage changes;furtherm ore there is a time lag before voltage is restoredto normal. As shown in Fig. 3, abrup t changes in voltage,the ones that voltage regulators cannot eliminate, are thevery ones to which the hum an eye is most sensitive. Con-sequen tly, the flicker is perceived before the regu lator caneven start. It is sometimes thou ght that an electronicregulator and exciter can eliminate this difficulty and pre-vent voltage d ips. How ever, th e field time constant of thegenerator which in large u nits is as high as 10 second s andeven in very small machines may be one second , m akescorrection by this means imp ossible.9. Compensating Transformers

As illustrated in Fig. 36, a compensatin g tran sformer issimilar in effect to a line d rop compen sator used in voltageregulator control except th at the size of the elements isthat of a pow er device rather than that of an instrum ent.The curr ent d rawn by the flicker-produ cing load passesthrou gh a resistance and reactance branch, and the voltage

F i g. 3 6 C o m p e n s a t i n g t r a n s f o r m e r c a n b e u s e dc ia l cases to reduc e vo l tage d ips . in ve ry spe -

dr op thu s created is add ed to the lighting-load voltage bymeans o f a series transformer. By pr oper selection of theresistance, reactance, and series-transformer ratio, theflicker in the lighting circuit may be eliminated almost

completely. Satisfactory results can often be obtained byomitting the resistor, and in such cases, the apparatu sbecomes simply a transformer with an air gap in itsmagnetic circuit.Despite the technical simp licity of this schem e, it haspractical and econom ic limitations. It is app arent thatthe improv ement in the lighting circuit is obtained at theexpense of the flicker-produ cing load. This limits the ap-plication to cases where the lighting load is only a smallprop ortion of the total. In general, the equipm ent mu stbe individ ually designed for a specific set of cond itions,since th e prop ortions and size are affected by the line volt-age, line drop, total curr ent, and ratio of loads. Shouldsystem changes n ecessitate its removal, there is small like-lihood of being able to use the compen sating transformerelsewhere. The cost of the apparatu s is rather high becauseit is not standard.10. Motor Starters

As pointed out under Utilization Equipment, mostmotors can be started directly across the line because eventhe larger sizes are usually supplied from heavy feederscompared to the size of the motor. Where this is not thecase, a starter may be required if the starting is frequent.It is difficult to generalize on the question of motor start-ing, because individual cases vary with the motor size,type, and the starting torque of both m otor and load.Starting compensators are now being used mu ch lessthan form erly. This is du e largely to the acceptance ofacross-the-line starting , bu t also to the realization tha tthe two voltage d ips caused by the compensator may beas objectionable as one larger dip when starting across theline. In this respect reactor starting is sup erior, becausethe circuit is not opened at transition, and the reactor-short-circuiting operation may not result in a noticeablevoltage d ip if the motor is substantially up to speed. Areactor starter causes a greater initial voltage drop thana compensator, because the starting kva is decreased onlydirectly as the starting voltage an d not as the square ofthe voltage.When th e continuous-load rating of the feeder is thesame as of the m otor, the use of wou nd -rotor motors withstepped-resistance starters in the rotor circuits usuallyavoids annoying flicker. The cost of the motor and con-trol is greater, but where the m otor is near the end of along line and is started frequently, this may be the mosteconomical choice.Where motors are started infrequently, but where theresu ltant voltage d ip is still objectionable, some form ofincrement starter may be warran ted. In a starter of thistype, the stator current is increased in steps until the motorrotates, and the remaining imped ance is cut out of thecircuit after the motor h as reached full speed. There areno standard starters of this type on the market, and thefew that have been built have been specially designedfor the par ticular service. In general, they represen t acombination of auto-transformer and reactor starting, theswitching being done without opening the circuit duringthe entire sequence.Resistance starters in the stator circuits have been em-ployed. On small integral h orsepower motors the simp lest


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Chapter 22 Lamp Flicker on Power Systems 739

and cheap est of these is a single-step resistor wh ich is cut .out after the motor comes up to speed. As with reactorstarting used on larger motors, the short-circuiting of theresistor does not usually cause a noticeable voltage dip,and the initial d ip of course is considerably redu ced. Re-sistance starters should be adjustable for individual re-quirements; in extreme cond itions a variable resistor m aybe desirable. These starters are in general m ore expensiveand more difficult to maintain by unskilled attendan ts.11. Excitation Con trol

This involves single-step incremen ts of the field excita-tion of synchronou s motors by switches actuated by theequipm ent causing the flicker. This method is generallyineffective in eliminating flicker caused by abru pt voltagedips as explained und er Voltage Regulators. However,it can redu ce considerably the width of the band of voltageregulation, which annoys power-sup ply companies by caus-ing too frequent operation of feeder-voltage regulators asthey attempt to compensate for the voltage swings. Suchswings are caused by continuous strip r olling m ills, largeelectric shovels, etc., w here the variations of load are large,but wh ere the rates of app lication and removal are mod er-ate, say 10 to 30 percent per second.

Fig . 37Sys tem layou t .(a ) Fluctuating load on substa tion bus affected all loads fed from

bus.(b) Fluctuat ing load feeders separa ted from rest of the load.

at a time wh en the lighting load is low. Control of loadis not a very general solution to reduction of flicker, andit is employed in but few cases.

12. Load Control 13. FlywheelsIn som e cases it is possible to minimize lamp flicker by

controlling manu facturing processes. For example, in aplant operating two or three resistance welders, it may bepossible to provid e interlocks so that not more than on eis operated at the same instant. A remedy of this kind isonly possible if the on time is short compared to theoff time, otherwise the prod uction rate wou ld be slowedup considerably. Similarly in arc-furnace work the vio-lence of the current swings dur ing melting can be redu cedby lowering pr odu ction rate d uring th is phase of the cycle.It is also possible to perform flicker-prod ucing operations

A general discussion of the effect of flywheels is givenun der Motor-Generator Sets, but the same principlesapp ly to direct-driven app aratus. This method has con-siderable value for mechanical loads having short d urationswith long off periods, such as shears, p un ch presses, etc.14. System Changes

In practically all cases of flicker cau sed by utilizationequipm ent, there is a direct relationship between theamou nt of the flicker and the size of the pow er supp lysystem. For example, assume t h a t a w e ld e r c a u s e sa t h r e e


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740 Lamp Flicker on Power Systems Chapter 22percent voltage flicker on a residential substation, whereonly one percent is acceptable. Tripling the size of thesup ply to the substation wou ld redu ce the flicker to therequired level, and this wou ld constitute one way of elim-inating the flicker. If this were don e by mu ltiplying thenu mber of incoming lines and transformer banks by threeit wou ld probably be the most costly of all possible correc-tive measures. Usually more economical system changescan be made.A common form of substation supply with tw o or morefeeders from the generating station paralleled to a singlebus is show n in Fig. 37(a). With this arrang emen t, all loadsfed from the substation are subjected to any flicker prod ucedon the outgoing feeders. Figure 37 (b) shows a low voltagebus divided into tw o sections, one for residential and com-mercial loads, the other for indu strial loads. This layoutis based on the fact that voltage fluctuat ions objectionableto residential customers are acceptable to indu strial users.There is probably a greater flicker tolerance in shop workthan in residence lighting, and ind ustrial plants are usu -ally willing to accept flicker when it is caused by theirown operation.Other method s of stiffening the power system involvechanging the voltage of the supp ly line, tapp ing nearbyhigh-voltage, high-capacity lines, add ing more transformercapacity, or run ning a separate line to the flicker-prod ucingload. Local conditions determine wh at remedial measuresare most suitable in a par ticular case. Occasionally systemincreases are justified if the additiona l capacity may beneeded later anyway.15. Comparison Chart

A reference chart showing at a glance the remed ial meas-ures available and those m ost prom ising for a particulartype of flicker is shown in Table 2. Inasmuch as the besttechnical solution may n ot be the most economical, theremedies are compared from both p oints of view.

1.2.

3.

4.

5.

6.

7.

8.

9.

10.

11.12.

13.14.

15.

REFERENCESTh e Visua l Perception and Tolerance of Flicker, prepared byUtilities Coordin at ed Resear ch, Inc.--New York, 1937.Lamp F licker Awaits Ideal Motor S tar ter, by L. W. Clark,Electrical World, April 9, 1938.Power-Factor Correction of Resistan ce-Welding Machines bySeries Capacitors, by L. G. Levoy, Jr., A.I.E.E. Transactions,1940.Analysis of Series Capacitor Applicat ion Problems, by Con-cordia and Butler, A.I.E.E. Transactions, 1937. Vol. 56.Self-Excitat ion of Indu ction Motors with Series Capacitors, byC. F. Wagner, A.I.E.E. Pap er No. 41-139. Pr esent ed at Sum -mer Convention, Yellowstone Par k.A Lamp Flicker Slide-Rule, by C. P. Xenis a nd W. Perin e,Presented at E.E.I. Transmission and Distribution Committ eeMeeting, Chicago, May 5, 1937.Power Supply for Resista nce-Welding Machines, Commit teeon Electric Welding, A.I.E.E. Transactions, 1940. Vol. 59.Power Supply for Resista nce-Welding Machines-Fa ctoryWiring for Resistan ce Welders, Commit tee on Electric Welding,A.I.E.E. paper 41-82Contains a Num ber of Exam ples.Power Supply for Welding, by A. S. Douglass and L. W. Clark,The American Welding Society J ournal, October 1937.Large Electric Arc Furn aces-Performance and Power Supply,by B. M. Jones and C. M. Stearns, A.I.E.E. Transactions, 1941.Vol. 60.Arc Fur nace Loads on Long Trans mission Lines, by T. G.Le Clair, A.I.E.E. Transactions, 1940. Vol. 59.10 000 kva Ser ies Capa citor Impr oves Voltage in 66 Kv. lineSupplying Large Electric Fur nace Load, B. M. Jones, J. M.Arthu r, C. M. Stear ns, A. A. J ohnson, A.I.E.E. Transactions.Vol. 67, 1948.Voltage Translator Scheme Cuts Light Flicker due to Welders,R. 0. Askey, Electrical World, January 6, 1945, page 63.Electric Arc Furn aces and Equipment Producing H eavy Fluc-tuat ions, Par t II-the solutions, by B. M. Jones. Presentedbefore E.E.I. Electrical Equ ipment Committ ee, Old Point Com-fort, Va., October 10, 1950.Power Compan y Service to Arc Fur naces, by L. W. Clark,A.I.E.E. Transactions 1935.

27190100 lamp flicker on power systems(1)

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