A maior rede de estudos do Brasil

Grátis
987 pág.
Beeman

Pré-visualização | Página 50 de 50

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