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lor the lights hut not high eiiimgh for t,he 220-volt motors. An autolrmisformer could he used l o step 208 volt,s up to 220 volts for the motors only. Where power is generated by the plant,’s oxvti geiir.ralors, the voltage on t,he powerhouse bns can be held constant or exwi varied with load to compensate for voltage drop as load comPs on. Problems of voltage rcgulat,ion where industrial generators are operalnd i r i parallel with utility systems are referred to in Chap. 15. Generator Voltage Regulators. REDUCING FLICKER (See Fig. 4.27) Reduction of flicker is often a much more difficult prohlem than the reduction of voltage spread previously referred to. Flicker due to reciprocating motor-driven loads such as compressors, purich presses, et,c., can often be reduced by increasing the inertia of the met:lranical system to smooth out the pulsations. Where this is not One is to separate flicker-producing load from the lights or critical load, i.e., use separate supply circuits. The nther is to use a voltage stabilizer, Fig. 4.24, to feed the critical load. Sometimes the critical load is fed through a motor-generator set to pro- vide good voltage for tliet load. This, lioxvevrr, is more expensive over all thaii voltage stabilizers and in gcmral offers no advantage in this ive, t,wo t,liings may be done. FIG. 4.24 Typicol voltage stabilizer. VOLTAGE-STANDARD RATINGS, VARIATIONS, CALCULATION OF DROPS 227 application. Voltage regulators previously discussed are not fast enough to correct flicker, but for single-phase circuits, and in small sizes, auto- matic voltage stabilizers are available to hold voltage mit,hin very close limits. A typical model is designed to maintain an output voltage of 115 volts with maximum variation of plus or minus 1 per cent, even though the supply volt,age may vary between 95 and 130 volts. The volt,age stabilizer has no moving parts and no electronic tubes; its opera- tion is obt,ained from the properly coordinated characteristics of reactors and caparitors. Series capacitors can be of value in reducing volt- age flicker. C:hapt,er 8 contains a complete discussion of the application of series capacitors. Series Capacitors. SELECTION OF TRANSFORMER TAPS All modern transformers in ratings above 100 kva and most or those helow that kva rating have taps in the windings to change the turn ratio. The taps do not materially affect the voltage drop through the trans- former; they merely change the turn ratio, hence the no-load voltage ratio. For example, a standard transformer rated 2400-480 volts may have four 2>5 per cent taps in the 2400-volt winding. The standard for these taps in transformers used in industrial systems is to have two 256 pcr cent, taps above 2400 volts and two 24i per cent taps below 2400volts. The no-load ratios of such a transformer would be as given in Table 4.11. TABLE 4.11 No-load Voltoge Ratios of Standard Transformer Rated 2400-480 Volts 2520-480 “0th 5% obove tap 2460-480 volts 236% obove top 2400-480 volts Norrnol rating top 2340-480 volts 2>P% below top 2280-480 volts 5% below tap These taps do not improve voltage regulation but are only for changing the general vokage level iq the plant. If a 2400-480-volt transformer is connected to a system whose maximum voltage is 2520 volts, then the 2520-480-volt tap could be used which would provide a maximum of 480 volts no load on the system, as shown by curve A , Fig. 4.25. If, for example, another system had a maximum no-load voltage of 2400 volts, then the 240&480-volt tap could be used to provide 480 volts no load in the plant. Similarly if a plant had a maximum voltage of 2280 vo!ts, then the 2280-480-volt tap could be used to provide a maximum of 480 volts no load in the plant, as shown in curve C , Fig. 4.25. It will be noted that in all cases the second- ary no-load voltage is 480 volts; so the secondary system does not know This would be as shown in curve B, Fig. 4.25. 2600- - - - - 480 VOLTS MAX - U c > - > U - (L a 2 2 4 0 0 - (r I - 2300- - 440 V 4 8 0 VOLTS MAX MIN ---- ---- - _ _ _ _ _ _ _ - - -? 40 VOLTS SPREAD 4 4 0 V B 480 VOLTS MAX MIN --------- _ _ _ _ _ _ _ _ VOLTAGkSTANDARD RATINGS, VARIATIONS, CALCULATION OF DROPS no-load voltage. By using the next tap up on the transformer, that is, the one rated 2460-480 volts, the turn ratio of the transformer has now been changed so that the no-load voltage is 472 volts, as shown in curve B , Fig. 4.26. The voltage spread will be substant,ially the same, i.e., 40 volts, so that the minimum voltage is now 432 volts, which is well above the recommended minimum for plant distribution systems. By judicious selection of the transformer tap t,he voltage within the plant can he kept Tyithin acceptable 1imit.s provided that the primary voltage does not vary more than about 5 per cent and that the plant dis- tribution system is designed along modern lines with the load-center sys- tem using short secondary feeders and transformers not larger than about 1500 kva a t 480 volts or proportional sizes a t other secondary volt,ages. Changing taps cannot, correct conditions where voltage spread is t,oo great. For example, suppose a plant suffered from low voltage at remote points and had a large volt,age spread. To be specific, suppose the spread was 80 volts and the minimum voltage at the remote end was 400 volts, then the maximum voltage would be 480 volts. If taps are changed to raise the general voltaga level, the spread will not change but the 400-volt 229 4 8 5 VOLTS MAX - -- -- - - - - - - - - - -_- 40 VOLTS SPREAD I 440 V MIN g 2400 a I- J 0 5 > a a I - LL P FIG. 4.26 excessive no-load voltage by proper election of taps on transformer. Voltage profile showing that rotisfactory voltages con be obtained without 230 VOLTAGE-STANDARD RATINGS, VARIATIONS, CALCULATION OF DROPS Volloqe Correction For TypicoI Feeder Circuits FIG. 4.27 Summary of methods of improving VOLTAGE-STANDARD RATINGS, VARIATIONS, CALCULATION OF DROPS 231 LOW LOAD VOLTAGE "No b o d " Except shunt <apa<itorl ore on1 Feeder Condition VOltoge 1 . tow Leoding Aulornotic switching of capoci- ot no lood tors Voltage regu1otor l if no peno1ty CIQYX for leoding power factor1 2. High leeden d r o p 3. High feeder drop. 4. High feeder drop. I . Normal drop. 2. Normol drop. 3. vo1t.g. rile Ci.<"iI Loading Normal Normal Normal Overload HIGH LOAD VOLTAGE iee 1A1 (81 ICI (81 ID1 ICI (El I81 IEI IF1 - Volt.ge regdotor Tronlformer ,ap setting i I NO load Voltage 'egulalor Tranrformer top 'elting IBI 1A1 101 (A1 IGI IF1 IVolt.ge regulator is Only P'"<tiC.l I0l"tiO"J IHJ LIGHTING FLICKER Lood Causing Flicker I . Rerirtan<e welders >POI or seom. 2. Flmh. rssislmnre welders. 3. Motor loads. such 01: sow mill^, Rubber milli. Grinders. 4. Arc furnorer. Correcl by m e o n r of Series rapodor with welder to reduce dernond by power.farlor correction .eO<ta"Ce Separote welder supply r i r 4 l Volloge stabilizer lor lighting circuit Separate welder supply c i r w i t Voltoge Ifobiliier for lighting circuil re.artance Voltage slmbilirer for lighting circuit Series c"Po<itor in line to ne",r.lize ,y,,em Series coparilor in line to "e",,.lize 'y'lem Sep..ate motor '"pply Ci.CYil Sante 0 s for lmotonl voltoge conditions in an indurlriol p lant 232 VOLTAGE-STANDARD RATINGS, VARIATIONS, CALCULATION OF DROPS minimum may he raised to 420 volts. At the same time
