5.55TRANSFORMERS

SINGLE-PHASE CONNECTIONS

76. Connections for standard distribution transformers are shown in Figs.5.45 and 5.46. Distribution transformers of medium and small capacity are almostinvariably arranged with two secondary coils, which can be connected in series orparallel with each other so that the same transformer can be used to supply eitherof two different secondary voltages or a three-wire system. In transformers of thel20/240-V type their secondary windings can be so connected as to deliver 120 or240 V or for a 120/240- V three-wire circuit. The connections of the secondarycoils are made, either by splicing the secondary leads or with connectors, outsidethe transformer case. Some transformers are constructed with two primary coils asshown in Fig. 5.45. If the necessary primary-coil connections are made, they canbe used on primary circuits of either of two voltages. Standard practice with respectto number of coils and voltage ratings is given in Secs. 38 and 39.

-Primary circuit

2400Volts

4800Volts

--4800-. Volts

,. :~~L ~ .

I~~II I1--'\Nv.- I

I)

>-C'~c0.-E"O.- C ,-~'--'

Cl.~

I

}

-~~~~~~J---'--:'=': °, [~~~~t: =-~ iL.: oJ I L J I

~ I I

I .[; ...~- I I I -vvv- . ;f I I

~ I I00' I IC , / ,

]~ ~~. .oE-I2OV-TI20V-1~~ Secondory circuit ~ 240 Volts

Primory coils in series Primorycoils in series Primory coils in porollelSecondory coils Secondory coils Secondory coils in series

in parollel In series Three wire connection

FIGURE 5.45 Connections of standard distribution transfomlers.

5.56 DIVISION FIVE

77. Transformer connections for three-wire secondary service are shownin Figs. 5.45, 5.46, and 5.47. In the arrangement in Figs. 5.45 and 5.46, one trans-fonDer only is used. Its secondary windings are connected in series, and a tap ismade to the point of connection between the two windings, providing 240 V be-tween the two outside wires and 120 V on each of the side circuits. The transfonnershould have a capacity equal to the load to be supplied, and the three-wire circuitsshould be carefully balanced. If the three-wire circuits are decidedly unbalanced,the transfonner should have a capacity equal to twice the load on the more heavilyloaded of the two side circuits.

SinQle-ohase. prImary circuit

2400 V

", t ~

l~ j ,0Lamps,

~ ~VY===:~Yf\J 666(

~

-OO( ~

~Lood -0..

~

Fuses Lamps-120 V ~ ;;--o-6C;- ("

i-660-0(Three - wi re. secondory circuit

FIGURE 5.47 TWo transfomlers serving a three-wire circuit.

In Fig. 5.47, two transformers are shown connected to serve a three-wire circuit.The three-wire load should be balanced as nearly as possible, and if it is very nearly

TRANSFORMERS 5.57

balanced, each transformer should have a capacity equal to one-half of the totalload. If the load is badly unbalanced,~ach transformer should have a capacity equalto the load on its side of the circuit. See "Parallel Operation" shown in Fig. 5.47.

TWO-PHASE CONNECTIONS

78. Transformers connected to four-wire two-phase circuits are shown inFig. 5.48. As a rule, two-phase primary lines are four-wire as shown, and to sucha four-wire line the transformers are connected to each of the side circuits, as ifeach side circuit were a single-phase circuit not having any connection with theother. The total load should be so divided between the phases that the loads oneach will be as nearly equal as possible. Each transformer should be designed forline voltage and will carry line current. Each transformer should have a kilovolt-ampere capacity equal to one-half of the kilovolt-ampere load that is served by thetwo transformers.

Two- phose, primary circuit

r1 .-I ,~ovvy.J:J\M,-.IV\I'.--I

IJ

---I---t i I

~:I~'Ii 1 J L

Two - phose, secondory circuit ~

FIGURE 5.48 Transformers with two-phase to two-phase four-wireconnection.

79. Transformers connected to three-wire two-phase circuits are shown inFig. 5.49. The current in the center line wire AA for balanced load is 1.41 timesthe current in either of the outer wires. Each transformer has line voltage impressedon it and carries one-half the total load. A General Electric Co. publication com-ments thus: "Considerable unbalancing of voltage at the end of a transmission lineor cable is experienced with the three-wire, two-phase system owing to the mutualinduction between phases. Where the power factor is low, a still worse regulationis obtained, making satisfactory operation difficult. Very few systems now operateon this plan, and practically all of them could be improved by the use of someother system."

5.58 DIVISION FIVE

80. Mixed connections are sometimes made with two-phase transform-ers, as shown in Fig. 5.50. With improper connections such as those shown,difficulty will be experienced in the operation of motors, and they may not run atall.

Two - phose. crimorv ci rcuit

-

~

~oil;,

~MJ

'rv"'"1.;p 'fU-

urorf .., ~

L-I

I~" T..,"'

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..."'c0

.=

. . cMixed primory Mixed secondory Proper connections

FIGURE 5.50 Correct and incorrect connections for transformers serving two-phase motors.

THREE-PHASE CONNECTIONS

81. Comparison of one three-phase transformer with a group of single-phase transformers (Standard Handbook) that can be employed for obtainingthe same service has been summed up by J. S. Peck as follows. The advantages ofthe three-phase transformer are (I) lower cost, (2) higher efficiency, (3) less floorspace and less weight, (4) simplification in outside wiring, and (5) reduced trans-

5.68 DIVISION FIVE

Three - phase, primary circuit

TIT mA

B

C

2200 V Tr

rrEe

1

f).-

it- w E "-

Q)d

,~~"'"~E j'-"N\I--"'VV'v4

01

b1

all

bIl

N 220 V

~

220V ~ 1 Phose Ii ~,/ :m. -~CA CA,.""Anru ,.;r,..,;+j/ :2D:

Two- phose, secondary circuit

FIGURE 5.59 Transformers connected for three-phase to two-phase transformation.

93. Explanation of the transformation from three-phase to two-phase(Standard Handbook). Assume the simple case of a total power of 30,000 W, 1.0pf, at 100 V, three-phase to be transformed (without loss) to 30,000 W, 100 V, two-phase (see Fig. 5.60). If the load is balanced on the two-phase side, there will be15,000 W per phase, or 150 A at 100 V. Since the three-phase power is representedas V3/E = 30,000, where / is the current per line wire and E is the emf betweenline wires, / must equal 173.2 A, because E has been taken as 100 V.

As shown in Fig. 5.60, the three-phase150 A 173.2 Amp. coils of one transformer must be de-Amp fiOO signed for 100 V and 173.2 A, while the

t -:::. three-phase coils of the other transformer

""~ must be designed for 86.6 V and 173.2

~ A. The current through the coil CD di--- 86.6 vides equally. A part (86.6 A) goes

100 0 C 173.2 Amp throu gh DA and an equal Part (86 6 A )Volts ...T' 86.6 Volts passes differentially through D[J; thus the

86.6- mmf of these two currents has a resultantt ~ of zero, and it has no effect upon the core

!')'Y T flux insofar as transformer TI is con-~ ~ ~rv-Yv~ cerned. The coil ADB carries a total value

B 1732 A <11--100 volts-l of current of 173.2 A throughout all its..I . mp'~t ~ turns, but the current in one half is 60

time degrees out of phase with that in theFIGURE .5.60 Three-phase to two-phase other half. That is to say the 173.2 A in

transformation. .'one half is made up of a load current of

TRANSFORMERS 5.69

150 A, in leading time quadrature with which is 86.6 A, while that in the otherhalf is made up of a load current of 150 A, in lagging time quadrature with whichis a superposed current of 86.6 A. The magnetizing effect of the 173.2 A is, there-fore, 150 A, and the current in the two-phase side of transformer T1 is 150 A. Inthe Tl transformer the mmf of 173.2 A in 86.6 percent turns is just equal to thatof 150 A in 100 percent turns; these two currents are directly in time-phase op-position, and the apparatus operates in all respects like a one-phase transformer.The phase relations and the relative values of the several components of currentsare shown in the vector diagram of Fig. 5.60.

94. Kilovolt-ampere ratings of transformers for Scott connection (three-phase to two-phase) for serving a given horsepower load. The accompa-nying table gives the ratings recommended by the Westinghouse Electric Corp. fortransformers serving squirrel-cage induction motors and indicates the efficiency ofthe installation. The temperature guarantee with performance as shown is a 50°Crise.

95. T-connected transformers for transforming from three-phase to three-phase (Standard Handbook). A method of employing two transfonners in three-phase transfonnation which practically overcomes the disadvantages of the V con-