THREE-PHASE TRANSFORMER CONNECTIONS
ECE 454/554: Power Systems Laboratory
Contributors: Dr. A.A. El-Keib
Dr. Tim A. Haskew
Mr. Clifton Black
Mr. Johnny Carlisle
Mr. Neil Hutchins
Objectives
 Learn how to appropriately model three-phase transformers and transformer banks.
 Understand the phase relationship in Y- connections
 Master the appropriate connection of three-phase transformer banks
References
Electromechanical Energy Devices and Power Systems, Zia A. Yamayee and Juan L.
Bala, Jr., John Wiley and Sons, Inc., New York, New York, 1989.
- Sections 3.5 and 4.7
Pre-Lab Assignment
1) Appropriately connect the low-voltage side of the transformer bank below in a connection. Be sure to draw an accurately scaled phasor diagram indicating all lineto-neutral and line-to-line voltages on both sides of the transformer bank.
A
B
C
2400/240 V
a
b
c
2) A bank of three single-phase, 60 Hz, 12,470/277 V, 1100 kVA transformers are
connected  on the high-voltage side and Y on the low-voltage side. Each
transformer has a reactive impedance of 6%. If the transformer supplies rated load at
rated voltage and 0.9 pf lagging on the low-voltage side, find the following:
(a) the high-side line voltage magnitude
(b) the input complex power
(c) the winding current magnitude on the high-voltage side
(d) the winding current magnitude on the low-voltage side
Equipment List
Quantity
1
1
1
1
1
Description
Three-Phase Transformer Bank
Three-Phase Ammeter
Three-Phase Watt/VAR Meter
Three-Phase Power Supply
Phase Angle Meter
Number
EMS 8348
EMS 8425
EMS 8446
EMS 8821
EMS 8451
Procedure
1) Connect one transformer from the three-phase transformer bank (EMS 8348) for
open-circuit testing in the 208/208 V configuration as illustrated below. Note that
each transformer is rated at 208/120-208 V and 250 VA. Use one of the ammeters
from the three-phase ammeter (EMS 8425) on the 0.5A range and the Watt/VAR
meter (EMS 8446). Use the 4-5 line voltage from the three-phase power supply
(EMS 8821) to supply an input voltage of approximately 208V. Record the input
voltage, current , and real power.
Watt/VAR meter
4
~
3
1
A
4
V
5
2
5
Quantity
V
I
P
ECE 454/554: Power Systems Laboratory
Three-Phase Transformer Connections
Value
Units
V
A
W
2) Connect the same transformer used in step 1 for short-circuit testing in the 208/208 V
configuration as illustrated below. Use one ammeter from the three-phase ammeter
(EMS 8425) on the 2.5 A range, the voltmeter onboard the three-phase power supply
(EMS 8821), and the Watt/VAR meter (EMS 8446). Gradually increase the input
voltage until the current reaches 1.2 A. Record the voltage, current, and real power.
Watt/VAR meter
4
~
3
1
A
4
V
5
2
5
Quantity
V
I
P
Value
Units
V
A
W
With the information from steps 1 and 2, compute the equivalent circuit of the
transformer of the form provided.
208/208 V
ECE 454/554: Power Systems Laboratory
Three-Phase Transformer Connections
3) Connect the same transformer used in step 1 for short-circuit testing in the 208/120 V
configuration as illustrated below. Use one ammeter from the three-phase ammeter
(EMS 8425) on the 2.5 A range, the voltmeter onboard the three-phase power supply
(EMS 8821), and the Watt/VAR meter (EMS 8446). Gradually increase the input
voltage until the current reaches 1.2 A. Record the voltage, current, and real power.
Watt/VAR meter
4
~
3
1
A
4
V
5
2
5
Quantity
V
I
P
Value
Units
V
A
W
With the information from steps 1 and 3, compute the equivalent circuit of the
transformer of the form provided.
208/120 V
For the remaining steps in this laboratory exercise, connection figures/circuits will not
be provided. You will be required to develop them on your own. Be sure to include the
detailed connection diagrams or circuits in your lab reports. Also, the term primary
will be used to indicate the source side of the transformer bank, and the term
secondary will be used to indicate the load side of the transformer bank.
ECE 454/554: Power Systems Laboratory
Three-Phase Transformer Connections
4) Connect the three-phase transformer bank (EMS 8348) Y-Y for 208/208V operation
(note that these values are line-to-line values). Even though you will be operating
with each transformer winding at a voltage of 120 V, you will have to use the 208/208
V windings. Using the three-phase resistive load (EMS 8311), apply a Y-connected
load of 300 . Measure and record the magnitudes and phase angles of all six
primary voltages and all six secondary voltages using the 4-N voltage as your
reference. Construct a complete phasor diagram. Does it turn out as expected?
Measure and record the line current magnitude (from one phase only) on the primary
and secondary. Do your measurements agree with the results produced by analysis of
your equivalent circuit assuming known load impedance and load voltage magnitude
(taken from the measurement)?
Quantity
Magnitude (V)
Primary
Phase Angle (deg.)
V4N
V5N
V6N
V45
V56
V64
Secondary
Van
Vbn
Vcn
Vab
Vbc
Vca
Quantity
Primary Line Current
Secondary Line Current
ECE 454/554: Power Systems Laboratory
Three-Phase Transformer Connections
Magnitude
Units
5) Connect the three-phase transformer bank (EMS 8348) - for 208/208V operation
(note that these values are line-to-line values). Using the three-phase resistive loads
(EMS 8311), apply a Y-connected load of 300 . Measure and record the magnitudes
and phase angles of all six primary voltages and all six secondary voltages using the
4-N voltage as your reference. Construct a complete phasor diagram. Does it turn out
as expected? Measure and record the line current magnitude (from one phase only)
on the primary and secondary. Do your measurements agree with the results produced
by analysis of your equivalent circuit assuming known load impedance and load
voltage magnitude (taken from the measurement)? Since the load is equivalent to
step 4, do your results agree? Explain why or why not.
Quantity
Magnitude (V)
Primary
Phase Angle (deg.)
V4N
V5N
V6N
V45
V56
V64
Secondary
Van
Vbn
Vcn
Vab
Vbc
Vca
Quantity
Primary Line Current
Secondary Line Current
ECE 454/554: Power Systems Laboratory
Three-Phase Transformer Connections
Magnitude
Units
6) Connect the three-phase transformer bank (EMS 8348) Y(primary)-(secondary) for
208 V (primary)/208 V (secondary) operation (note that these values are line-to-line
values). Your connection should be such that the secondary voltages lead the
corresponding primary voltages. Using a three-phase resistive load (EMS 8311) and a
three-phase inductive load (EMS 8321), apply a Y-connected load of 300+j300 .
Measure and record the magnitudes and phase angles of all six primary voltages and
all six secondary voltages using the 4-N voltage as your reference. Construct a
complete phasor diagram. Does it turn out as intended? Measure and record the line
current magnitude (from one phase only) on the primary and secondary. Do your
measurements agree with the results produced by analysis of your equivalent circuit
assuming known load impedance and voltage (taken from the measurement)?
Quantity
Magnitude (V)
Primary
Phase Angle (deg.)
V4N
V5N
V6N
V45
V56
V64
Secondary
Van
Vbn
Vcn
Vab
Vbc
Vca
Quantity
Primary Line Current
Secondary Line Current
ECE 454/554: Power Systems Laboratory
Three-Phase Transformer Connections
Magnitude
Units
Questions
1) Using bases of 750 VA and 208 V on the primary side of the transformers, draw the
appropriate per-phase, per-unit equivalent circuit for the system connected in step 4.
Solve the circuit and use your results to compute all primary and secondary voltages
in SI units. How do the results agree with the measurements?
2) Using bases of 750 VA and 208 V on the primary side of the transformers, draw the
appropriate per-phase, per-unit equivalent circuit for the system connected in step 5.
Solve the circuit and use your results to compute all primary and secondary voltages
in SI units. How do the results agree with the measurements?
3) Using bases of 750 VA and 208 V on the primary side of the transformers, draw the
appropriate per-phase, per-unit equivalent circuit for the system connected in step 6.
Solve the circuit and use your results to compute all primary and secondary voltages
in SI units. How do the results agree with the measurements?
ECE 454/554: Power Systems Laboratory
Three-Phase Transformer Connections