Problem 661: EE Board June 1990
What should be the kVA rating of each two transformers in an open-delta bank when the three- phase balanced load
is 290.4 kVA?
A. 155.60 kVA
B. 173.40 kVA
C. 150.25 kVA
D. 167.66 kVA
Problem 662: EE Board April 1981
A V or open delta connected transformer bank, composed of two identical units serves a balanced 3 phase load of 16
kVA at 230 volts. Solve for the minimum size in kVA of the transformers needed.
A. 8 kVA
B. 9.24 kVA
C. 16 kVA
D. 8.25 kVA
Problem 663: EE Board April 1986
AV (open delta)'connected transformer bank serves a balanced 3-phase load of 40 kVA at 230 V and 0.866 pf lagging.
Solve for the minimum kVA of each transformer needed to accommodate the load without overloading.
A. 25
B. 24
C.23
D. 20
Problem 664: EE Board April 1985
Two single-phase distribution transformers are banked in V (open delta). If each transformer is rated
37.5 kVA, solve for the maximum 3-phase load that the bank can carry without overloading, considering that the load
has a power factor of 0.85lagging.
A. 52.5 kW
B. 49.5 kW
C. 53.6 kW
D. 55.2 kW
Problem 665:
An open delta bank consisting of two single phase transformers is operating with a balance of three-phase load of
50kVA, 440V at 0.8pf lagging and a single-phase load of 10kW resistive connected across c-a leg. Determine the
minimum rating of two transformers. Assume a phase sequence of a-b-c
A. 38.80 kVA,34.06 kVA
B. 34.32 kVA, 33.40 kVA
C. 35.48 kVA, 36.34 kVA
D. 30.83 KVA,39.52 kVA
Problem 666: EE Board April 1980
Two (2) Single phase distribution transformers, connected in open delta will supply power to a 200 hp, 3-phase
induction motor, operating at 0.70 power factor and 0.90 efficiency. Solve for the minimum size in kVA of each
transformer needed to supply the power without being overloaded.
A. 136.73 kVA
B. 124.45 kVA
C. 130.38 kVA
D. 129.48 kVA
Problem 667: EE Board October 1986
The Utility Company supplies two single-phase transformers banked in open delta. If each transformer is rated 75-
kVA capacity, what is the maximum 3-phase load in kilowatts that the bank can carry without suffering overload
considering that the load has a lagging power factor of 0.80.
A. 104 KW
B. 112kW
C. 130 kW
D. 114 kW
Problem 668: EE Board October 1980
Two identical 1 -phase transformers connected in open delta will serve a 3-phase motor load of 250 hp, 0.70 pf
lagging, 0.90 efficiency. Solve for the minimum size in kVA of each transformer required to serve the load without
overloading.
A. 150.33
B. 170.91
C. 160.43
D. 185.13
Problem 669: EE Board October 1985, EE Board April 1986
An open-delta connected transformer bank serves a balanced 3-phase load of 40 kVA at 230.V and 0.866 pf lagging.
Solve for the real power and pf at which each transformer is operating.
A.11. 54 kW at 0.5 of, 23.09 kW at unity pf
B. 12.22 kW at 0.5 pf, 22.42 at unity pf
c. 11.54 kW at 0.866pf, 23.09 kW at 0.707 pf
D. 12.22 kW at 0.866 pf, 22.42 kW at 0.707 pf
Problem 670:
An open delta, three-phase transformer consisting of two single-phase transformers is operating with a balanced
three-phase load 50kVA, 440V at a lagging pf of 0.8. Determine the operating pf of each transformer.
A. 0.393 lagging, 0.993 lagging
B. 0.383 lagging, 0.887 lagging
C. 0.383 lagging, 0.993 lagging
D. 0.393 lagging, 0.887 lagging
Problem 671: EE Board April 1983
Two single-phase transformers are connected in V (open delta) and serving a delta connected impedance load, each
impedance is equal to 16<36.87°ohms.lf the transformer voltages impressed on the impedances are:
V ab=240<0 ° ,V ab =240<24 0 ° , V ab=240<12 0 ° volts
Solve for the kVA delivered by each transformer.
A. 7.28,5.20
B. 5.87,6.61
C. 6:84,5.64
D. 6.24,6.24
Problem 672: EE Board June 1990
Two single-phase transformers in an open-delta bank served a three-phase balanced load of 290.4 kVA. If a third
transformer is added for delta operation, what percent increase in load does this represent?
A. 73.2%
B. 84.6%
C. 80.5%
D. 79.4%
Problem 673: EE Board October 1983
Three-single phase transformers each rated 75 kVA are banked in delta and supplying a 3-phase load drawing
160 kVA at 0.8 lagging power factor. If one transformer is removed for repairs, solve for the amount of overloading
of the remaining units.
A. 30 kVA
B. 10 kVA
C. 20 kVA
D. 25 kVA
Problem 674: EE Board April 1992
A 6600-volt, 60 Hz, 2-phase network is connected to a three-phase, 4-wire system by a scott-connected transformer
combination. If there are 1500 turns on the 2-phase side, calculate the number of turns on the 3-phase side required
for the main and teaser transformers. Voltage per phase on the 3-phase side is 240 V.
A. 94 turns, 83 turns
B. 93 turns, 84 turns
C. 95 turns, 82 turns
D. 92 turns, 85 turns
Problem 675: EE Board April 1985
'A balanced 3-phase transformer bank has delta-connected primary and wye-connected secondary windings. The
primary to secondary winding voltage ratio is 2400/277. Solve for the primary line current if the low-side (secondary)
load draws a line current of 1000 A.
A. 200A
B. 115A
C. 150A
175A
Problem 676:
A three-phase 150 kVA, 4160 V, 0.8 lagging pf load is to be supplied by three single-phase transformers having a turns
ratio of 2. Calculate the current that flows in the primary windings if the transformers are connected delta-delta.
A. 10.41A
B. 6.01A
C. 20.82A
D. 12.02A
Problem 677: EE Board October 1991
What should be the turns ratio of a three phase transformer to transform 10,000kVA from 230 kV to 4160 V if the
transformer is to be connected Δ-Y?
A. 94
B. 98
C. 96
D. 97
Problem 678:
A bank of three 50:1 single-phase transformers connected Y-Δ is used to step down the HV transmission line rated
138 kV. Determine the magnitude of the step down voltage.
A. 126 kV
B. 6.90 kV
C. 138.55 kV
D. 159.35 kV
Problem 679: EE Board October 1998
The potential transformer of a line has a ratio 132,000/66. These are connected wye-delta. The voltmeter reads 64
volts. What is the voltage of the line?
A. 221.7 kV
B. 112.5 kV
C. 135.2kV
D. 132 kV
Problem 680: EE Board October 1997
A power transformer rated 50,000 kVA, 34.5 kV/13.8 kV is connected wye-grounded primary and delta on the
secondary. Determine the full load phase current on the secondary side.
A. 2092A
B. 1725A
C. 1449 A
D. 1208 A
Problem 681: EE Board October 1998
A 69/13.8 kV, 10 MVA three-phase transformer is connected wye-grounded delta. What is the kV rating of the
secondary winding?
A. 7.97
B. 15
C. 23.9
D. 13.8
Problem 682: EE Board October 1997
The PTs of a 220 kV transmission line have ratio of 132.76 kV/66.375 kV and are connected wye-wye. A voltmeter
connected line to ground reads 66 volts. What is the transmission line voltage?
A. 228.63 kV
B. 220.10 kV
C. 223.15 kV
D. 225.25 kV
Problem 683: EE Board October 1997
A power transformer rated 50,000 kVA, 34.5 kV/13.8 kV is connected wye-wye. What are the line currents at full
load?
A. 525.2 A, 1725.4A
B. 836.7 A,2092 A
C. 483.1 A, 1207.7 A
D. 1449.3 A,3623.2A
Problem 684: EE Board October 1985
Two transformers are connected in parallel to supply a common load of 175kVA.Transformer A is rated 100 kVA with
equivalent impedance of 9.6 ohms while transformer B is rated 75 kVA with equivalent impedance of 12.5 ohms.
Find the kVA load of each transformer.
A. 78,97
B.76,99
C. 80,95
D.75,100
Problem 685: EE Board October 1981
Two distribution transformers are connected in parallel supplying a load of 125kVA. If transformer A is rated 50 kVA,
with 14 ohms equivalent reactance and transformer B is rated 75 kVA with 11 ohms equivalent reactance. Solve for
the loading in kVA of each transformer.
A. 55 kVA, 70 kVA
B. 50 kVA, 75 kVA
C. 58 KVA,67 KVA
D. 52 kVA, 73 kVA
Problem
686:EE
Board
March
1998
A 125-kVA distribution transformer with 4% impedance is connected in parallel with another transformer rated 75-
kVA and with 3% impedance. Both have the same voltage ratio. Neglect resistance of each transformer. The total
load is 140 kVA at 85percent power factor. How much load does each carry
A. 80.5 kVA/59.5 kVA
B. 77.78 kVA/62.22 kVA
C. 80 kVA/ 60 kVA
D. 87.50 kVA/52.5 kVA
Problem 687:
Two parallel transformers have 8% reactance each. Oneis rated 25 MVA and the other at 15 MVA. For a given 30
MVA load with a lagging pf of 0.8, Determine the load carried by each transformer?
A. 18.50 MVA, 11.50 MVA
B. 18.75 MVA, 11.25 MVA
C. 15.25 MVA, 14.75 MVA
D. 16.45 MVA, 13.55 MVA
Problem 688:
A 500-kVA single-phase transformer "A" with percentage impedance of 0.010+j 0.05 is connected in parallel with a
250-kVA transformer “B” with a percentage impedance of 0.015 + j 0.04. If the transformers are serving a single-
phase load rated 800-kVA, 80% lagging pf, determine the operating pf of each transformer.Assume the transformers
have the same turns ratio.
A. A = 0.762, B = 0.803
B. A = 0.772, B = 0.856
C. A = 0.762, B = 0.856
D. A = 0.772, B = 0.803
Problem 789: EE Board April 1993
The MERALCO has two single-phase transformers with equal turns ratio and ratings and are operated in parallel to
supply a load of 280 kW at 0.80 lagging pf. Transformer A has a resistance of 2% and a reactance of 8%. Transformer
B has a resistance of 1% and a reactance of 6%. Determine the power delivered by the transformers A and B to the
load.
A. A = 123 kW, B = 157 kW
B. A = 125 kW, B = 155 kW
C. A = 122 kW, B = 158 kW
D. A = 127 kW, B = 153 kW
Problem 690: EE Board April 1994
Two single-phase transformers with equal ratings and turns ratio are operated in parallel to supply a load of 180 kW
at a lagging pf of 0.90. Transformer A has a resistance of 1% and a reactance of 6%. Transformer B has a resistance of
2% and a reactance of 5%. Calculate the power delivered by each transformer.
A. 83kW,97 kW
B. 79 kW,101kW
C. 88 kW,92 kW
D. 80 kW,100 kW
Problem 691:
Two single-phase transformers A and B are connected in parallel to serve a 600-A single- phase load at 0.80 pf lagging.
The equivalent impedance of the transformers are 2 + j 3 and 2.3 +j 5 ohms, respectively. Determine the ratio of the
kW output of transformer A to the kW output of transformer B. Assume the transformers have equal ratings and
turns ratio.
A. 1.56
B. 1.88
C. 1.68
D. 1.74
