Lab 6: Transformers in parallel and 3

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ELEN 3441 – Fundamentals of Power Engineering Lab # 6 Spring 2008 Lab 6: Transformers in parallel and 3-phase transformers.
Objective: to learn how to connect transformers in parallel; to determine the efficiency of parallel
connected transformers; to connect transformers in delta and Y configurations; to study the voltage
and current relationships.
Equipment: Power Supply, DAI, two Transformer modules (8341), 3-phase transformer (8348),
Variable resistance (8311)
Theory:
In an ideal transformer, the power in the secondary windings is exactly equal to the power in the
primary windings. This is true for transformers with a coefficient of coupling of 1.0 (complete
coupling) and no internal losses. In real transformers, however, losses lead to secondary power
being less than the primary power. The degree to which a real transformer approaches the ideal
conditions is called the efficiency of the transformer:
Efficiency (%) =
Pout
⋅100%
Pin
where Pout and Pin are the real output and the input powers. Apparent and reactive powers are not
used in efficiency calculations.
Transformers may be connected in parallel to supply currents greater than rated for each
transformer. Two requirements must be satisfied:
1) The windings to be connected in parallel must have identical output ratings;
2) The windings to be connected in parallel must have identical polarities.
Severe damage may be made to circuitry if these requirements are not satisfied.
Single-phase transformers can be connected to form 3-phase transformer banks for 3-phase
power systems. Four common methods of connecting three transformers for 3-phase circuits are ∆∆, Y-Y, Y-∆, and ∆-Y connections. An advantage of ∆-∆ connection is that if one of the
transformers fails or is removed from the circuit, the remaining two can operate in the open-∆ or V
connection. This way, the bank still delivers 3-phase currents and voltages in their correct phase
relationship. However, the capacity of the bank is reduced to 57.7 % ( 1
3 ) of its original value.
In the Y-Y connection, only 57.7% of the line voltage is applied to each winding but full line
current flows in each winding. The Y-Y connection is rarely used.
The ∆-Y connection is used for stepping up voltages since the voltage is increased by the
transformer ratio multiplied by 3 . The Y-∆ connection may be used for stepping down voltages.
More on the advantages and disadvantages of these connection types can be found in the
lecture notes.
Page | 1 ELEN 3441 – Fundamentals of Power Engineering Lab # 6 Spring 2008 The four connection types are shown in Figure 06-1.
Figure 06-1.
Regardless of the connection method, the windings must be connected in the proper phase
relationships. To determine these in a Y-connected secondary winding, the voltage is measured
across two windings as shown in Figure 06-2. The voltage A to B should be equal to 3 times the
voltage across either winding. If the voltage is equal to that across either winding, then one of the
windings must be reversed. The third winding c is then connected and the voltage C to A or B
should also equal
reversed.
3 times the voltage across any one winding. If not, the winding c must be
Figure 06-2.
To determine the proper phase relationships in a ∆-connected secondary winding, the voltage
is again measured across two windings as shown in Figure 06-3. The voltage A to C should equal
the voltage across either winding. If not, one of the windings must be reversed. The winding c is
then connected as shown, and the voltage C1 to C should equal zero. If not, winding c must be
reversed and open ends can be joined after that to form the ∆-connection.
Page | 2 ELEN 3441 – Fundamentals of Power Engineering Lab # 6 Spring 2008 Figure 06-3.
Note: the ∆-connection should never be closed until the test is first made to verify that the voltage
within the ∆ is zero. Otherwise, severe damage may be made!
Experiment:
1) Using two identical Transformer modules and a Variable resistance module, construct the
circuit shown in Figure 06-4. Start the Metering window and make active the channels E1,
E2, I1, I2, and I3. Note: all meters must be in the AC mode.
Figure 06-4.
Page | 3 ELEN 3441 – Fundamentals of Power Engineering Lab # 6 Spring 2008 While wiring your transformers, pay specific attention to the polarity of the windings. Tip:
use the DAI channel E1 as the input voltmeter. Place all the resistance switches to their OFF
positions first for zero load current. Before turning ON the power, have your instructor to
verify the correct wiring.
2) Once the correct wiring has been verified, turn ON the PS and slowly adjust the input
voltage to approximately 30-40 V. If the windings are properly wired, load and secondary
currents must be approximately zero. By closing the appropriate switch on the Variable
resistance module, make the load resistance to be 1200 Ω. Observe that the ammeters I1 and
I2 read approximately equal currents. If not, consult with your instructor.
3) Turn OFF the PS and modify your circuit as shown in Figure 06-5. Tip: use the DAI channel
I1 to measure the input current. Caution: high voltages will be present in the circuit since
step up transformers are used!
Figure 06-5.
Turn ON the wattmeter P1 and set it to measure the real input power (for meters E1 and I1).
Turn ON the PS and slowly increase the input voltage to 120 V. Set the load resistance to
400 Ω (1200 Ω and 600 Ω connected in parallel). The load current should be approximately
0.5 A. Record (as quickly as possible!) the values read by the meters to a Data table. Without
readjusting the input voltage, turn OFF the PS.
4) Rewire your circuit to make it as indicated by Figure 06-4 again. Turn ON the PS. Note: the
input voltage must be the same as in Part 3. If it slightly differs, adjust the input voltage to
make it equal to one in Part 3. For the same load resistance as in Part 3, record (as quickly as
possible!) the voltages and currents to your Data table.
5) Using the 3-phase transformer module, construct the 3-phase transformer shown in Figure
06-7.
Page | 4 ELEN 3441 – Fundamentals of Power Engineering Lab # 6 Spring 2008 Figure 06-6.
Figure 06-7.
Turn ON the PS and adjust the input voltage to approximately 40 V. Measure and record to a
new Data table the input voltages E1, E2, E3, E4, E5, and E6 and the output voltages E7,
E8, E9, E10, E11, and E12. Tip: in order to save results of 4 measurements, your Data table
should contain 4 records. Make sure to remember the order in which the voltages are saved.
Note: since the voltages are relatively low, you may connect voltmeters (one wire at a time)
while power is ON.
Start the Harmonic analyzer and observe harmonics of voltage across any secondary
winding. Save the Harmonic analyzer’s screen by printing it to a pdf-file.
Start the oscilloscope and observe the voltage across any secondary winding. Save the
oscilloscope data to a txt-file.
Page | 5 ELEN 3441 – Fundamentals of Power Engineering Lab # 6 Spring 2008 Ground the primary winding’s neutral: i.e. connect the transformer’s terminal 2 to the PS
terminal N. Save the Harmonic analyzer’s screen by printing it to another pdf-file. Save the
oscilloscope data to another txt-file.
Turn OFF the PS.
6) Construct the 3-phase transformer shown in Figure 06-8.
Figure 06-8.
Turn ON the PS and adjust the input voltage to approximately 40 V. Measure and record to
the Data table the input voltages E1, E2, E3, E4, E5, and E6 and the output voltages E7, E8,
and E9. Make sure to remember the order in which the voltages are saved. Note: you may
connect voltmeters (one wire at a time) while power is ON.
Observe harmonics of voltage across any secondary winding. Save the Harmonic analyzer’s
screen by printing it to another pdf-file.
Turn OFF the PS.
7) Construct the 3-phase transformer shown in Figure 06-9.
Page | 6 ELEN 3441 – Fundamentals of Power Engineering Lab # 6 Spring 2008 Figure 06-9.
Caution: before finalizing the connection on the secondary windings, you need to verify that
the voltage within ∆ is indeed zero as discussed in the “Theory”! Open the circuit at the
point “A” and place a voltmeter across the opening. Turn On the PS and slowly increase the
voltage to approximately 40 V. the voltmeter at the point “A” should read approximately
zero volts. Return the input voltage to zero and turn OFF the PS. Remove the voltmeter and
finalize the connection at the point “A”. Turn ON the PS and adjust the input voltage to
approximately 40 V. Measure and record to the Data table the input voltages E1, E2, and E3
and the output voltages E4, E5, and E6. Make sure to remember the order in which the
voltages are saved. Note: you may connect voltmeters (one wire at a time) while power is
ON.
Observe harmonics of voltage across any secondary winding. Save the Harmonic analyzer’s
screen by printing it to another pdf-file.
8) Construct the 3-phase transformer shown in Figure 06-10.
Figure 06-10.
Turn ON the PS and adjust the input voltage to approximately 40 V. Measure and record to
the Data table the input voltages E1, E2, and E3 and the output voltages E4, E5, and E6.
Make sure to remember the order in which the voltages are saved. Note: you may connect
voltmeters (one wire at a time) while power is ON.
Turn OFF the PS and disassemble your circuit.
Page | 7 ELEN 3441 – Fundamentals of Power Engineering Lab # 6 Spring 2008 In your report:
1) For the voltages, currents, and the input power you have measured in Part 3 and Part 4,
determine and report:
1. The load power;
2. The circuit efficiency;
3. The transformer losses;
4. The power delivered by the transformer 1;
5. The power delivered by the transformer 2.
2) Is the load reasonably distributed between the two transformers? Explain. Discuss the
possible advantages of parallel connection of transformers.
3) For the experiment in Part 5, determine the type of transformers’ connection first. For the
input voltage of 40 V, predict the values for the voltages E1 through E12. Explain your
computations. Are these values compatible with the values you have measured? Discuss
possible sources of discrepancy.
4) For the experiment in Part 6, determine the type of transformers’ connection first. For the
input voltage of 40 V, predict the values for the voltages E1 through E9. Explain your
computations. Are these values compatible with the values you have measured? Discuss
possible sources of discrepancy.
5) For the experiment in Part 7, determine the type of transformers’ connection first. For the
input voltage of 40 V, predict the values for the voltages E1 through E6. Explain your
computations. Are these values compatible with the values you have measured? Discuss
possible sources of discrepancy.
6) For the experiment in Part 8, determine the type of transformers’ connection first. For the
input voltage of 40 V, predict the values for the voltages E1 through E6. Explain your
computations. Are these values compatible with the values you have measured? Discuss
possible sources of discrepancy. What do you conclude regarding the turn ratio of the 3phase transformer you used?
7) Report the screenshots you collected by Harmonic analyzer in Parts 5, 6, and 7. Describe
your observations. Did you observe any effects of grounding of the primary neutral in Part
5? Import to Matlab the oscilloscope data recorded in Part 5 with and without grounded
neutral. On the same axes, plot these two voltages as functions of time. Describe the
differences between two plots. Based on your observations, which connection type(s) would
you recommend as most advantageous?
Page | 8 
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