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: Pout Efficiency (%) = 100% P where P out and P in 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. in Page 1
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 3 times the voltage across any one winding. If not, the winding c must be reversed. 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 C 1 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
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
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
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
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
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
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 3- phase 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