Being Familiar with Operating Characteristics of Three Phase Transformers

• Objectives:

-Being familiar with operating characteristics of three phase transformers.

-Being able connect transformer windings in wye and delta configurations, and verify that windings are connected with the proper phase relationships.

-Voltage and current measurements will be used to study transformer operation and working characteristics.

• Introduction:

The main features of three-phase circuits that are important to recall are that thereare two types of connections, wye and delta. Furthermore, in wye-connected threephasecircuits, the line voltages are greater than the phase voltages by the factor √3and the line and phase currents are equal. On the other hand, in delta-connectedthree-phase circuits, the line currents are greater than the phase currents by thefactor √3 and the line and phase voltages are equal.

A three-phase transformer can be a single unit or three single-phase units, and theprimary and secondary windings can be connected in either wye or delta to give fourtypes of connections, delta-delta, wye-wye, delta-wye, and wye-delta. Usually,three-phase power systems has a line voltage of 208 V (380 V or 415 V in somecountries), and standard 120-V voltage (220 V or 240 V in some countries) can beobtained between a line wire and the neutral wire as shown in Figure 9-1. Thewyeconnectedsecondary provides three-phase 120/208-V power using 4 wires asshown, and the primary side of the transformer may be connected in delta like in thefigure, or in wye. One big advantage of using a delta configuration for the primary isthat only three wires are needed to distribute the three phases.

Figure 9-1. Commercial Three-Phase 120/208-V Power System

An advantage of a delta-delta connection is that two single-phase transformers(instead of three) can be operated in what is known as the open-delta or "V"configuration if one of the three transformers becomes damaged or is removed fromservice. The open-delta transformer bank still delivers phase voltages and currentsin the correct relationship, but the capacity of the bank is reduced to 57.7% (1/√3)of the total nominal capacity available with three transformers in service.

In the delta-delta and wye-wye configurations, the line voltage at the secondary isequal to the line voltage at the primary times the inverse of the turns ratio. In thedelta-wye configuration, the line voltage at the secondary is equal to the line voltageat the primary times the inverse of the turn ratio times √3. In the wye-deltaconfiguration, the line voltage at the secondary is equal to the line voltage at theprimary times the inverse of the turn ratio times 1/√3.

Regardless of how the windings in a three-phase transformer are connected,precautions must be taken to ensure that the secondaries are connected with theproper phase relationships. For a wye configuration, this means that the voltagemeasured across any two secondary windings (line voltage) must be √3 timesgreater than the voltage across either winding (phase voltage). If not, theconnections must be reversed before continuing.

With a delta configuration, the voltage measured between the ends of twoseriesconnectedsecondary windings must equal the voltage across either winding. If not,the connections must be reversed. When one end of the third winding is connected,the voltage measured across all three series-connected windings must equal zerobefore connecting them together to close the delta. It is extremely important to verifythat the voltage within the delta equals zero before the delta isclosed. If not, theresulting current will be very high and damage the windings.

• Objectives:

When you have completed this exercise, you will be able to connect three-phasetransformers in delta-delta and wye-wye configurations. You will measure windingvoltages to verify that the secondary windings are connected with the proper phaserelationships, and you will verify that the voltage within a delta equals zero before thedelta is closed.

• Discussion

As mentioned earlier, four common ways of connecting transformer windings to forma three-phase transformer are: delta-delta, wye-wye, delta-wye, and wye-delta, asshown in Figures 9-2 and 9-3. In order to set up a wye connection,first connect the three components (windings) together at a common point forinterconnection with the neutral wire, then connect the other end of each componentin turn to the three line wires. To set up a delta connection, connect the firstcomponent in series with the second, the second in series with the third, and thethird in series with the first to close the delta loop. The three line wires are thenseparately connected to each of the junction nodes in the delta loop.

Figure 9-2. Delta-Delta and Wye-Wye Connections

Figure 9-3. Delta-Wye and Wye-Delta Connections

Before a three-phase transformer is put into service, the phase relationships mustbe verified. For a wye configuration, the line voltages at the secondary windings mustall be √3 times greater than the corresponding phase voltages. If not, windingconnections must be reversed. To verify that phase relationships are correct for awye configuration, the voltage between two windings (EAB) is measured as shown inFigure 9-4 (a) to confirm that it is √3 times greater than the line-to-neutral voltageacross either winding (for example EAN). The voltages between the third windingandthe others (EBC and ECA) are then measured to confirm that they are also √3 timesgreater than the phase voltage (EAN), as shown in Figure 9-4 (b).

Figure 9-4. Confirming Phase Relationships in a Wye-Connected Secondary.

For a delta configuration, the line voltages at the secondary windings must all beequal. If not, winding connections must be reversed. To verify that phaserelationships are correct for a delta configuration, the voltage across two seriesconnectedwindings (ECA) is measured as shown in Figure 9-5 (a) to confirm that itequals the voltage across either winding (EAB and EBC). The third winding is thenconnected in series, and the voltage across the series combination of the threewindings is measured to confirm that it is zero before the delta is closed, as shownin Figure 9-5 (b). This is extremely important for a delta configuration; because avery high short-circuit current will flow if the voltage within the delta is not equal tozero when it is closed.

Figure 9-5. Confirming that the Delta Voltage Equals Zero

• Procedure

1. Connect the Three-Phase Transformer module in the delta-deltaconfiguration shown in Figure 9-6.

Figure 9-6. Three-Phase Transformer Connected in Delta-Delta

1. Turn on the power and adjust the voltage control to obtain the line-to-linevoltage ESequals 200V . Use E1 to measure the winding voltages andrecord the results.

E1-2 = ----- V, E1-7= ----- V, E1-12 = ----- V

E3-5 = ----- V, E3-10 = ----- V, E3-15 = ---- V

1. Do the measurements confirm that the secondary windings are connectedwith the proper phase relationships?

----- Yes ----- No

1. Are the voltages within the secondary delta equal to zero, thus confirmingthat it is safe to close the delta?

----- Yes ----- No

Note: The value of E3-15 will not be exactly zero volts because ofsmall imbalances in the three-phase line voltages. If it is morethan 5 V, the winding connections have to be checked carefully.

1. When the winding connections are confirmed to be correct, close the deltaon the secondary side of the transformer. Connect E1, E2, and E3 tomeasure the line voltages at the secondary.Turn on the power and adjust the voltage control to obtain ESequals 200V. Note that the transformer is connectedusing the 1:1 ratio, so the primary and secondary voltages should be equal.
2. Observe the voltage phasors on the Phasor Analyzer. Does the displayconfirm they are equal with a 120E phase shift between each of them?

----- Yes ----- No

1. Turn off the power. Connect E2 to measure line voltage E1-2 on the primaryside. Turn on the power and adjust the voltage control voltage to obtain ESequals 200V. Compare the voltage phasor of E1-2 on theprimary side with that of E3-5 on the secondary side. Does the PhasorAnalyzer display show that the voltages are equal and in phase, except forpossibly a small difference due to transformer reactance?

----- Yes ----- No

1. Turn off the power and connect the Three-Phase Transformer module in thewye-wye configuration shown in Figure 9-7.

Figure 9-7. Three-Phase Transformer Connected in Wye-Wye

1. Turn on the power and adjust the voltage control to obtain ES equals 200V. Use E1 tomeasure the winding voltages and record the results.

E1-6 = ----- V, E1-11 = ----- V, E6-11 = ----- V

E1-2 = ----- V, E6-7 = ----- V, E11-12 = ----- V

E3-8 = ----- V, E3-13 = ----- V, E8-13 = -----V

E3-5 = -----V, E8-10 = -----V, E13-15 = -----V

1. Do the measurements confirm that the secondary windings are connectedwith the proper phase relationships?

----- Yes ----- No

1. Are the line-to-line voltages on the primary and secondary sides of thetransformer √3 times greater than the line-to-neutral values?

----- Yes ----- No

1. Connect E1, E2, and E3 to measure phase voltages E3-5, E8-10, and E13-15 atthe secondary. Turn on the powerand adjust ES at about the same value as used previously.
2. Observe the voltage phasors on the Phasor Analyzer. Does the displayconfirm they are equal with a 120o phase shift between each of them?

----- Yes ----- No

1. Turn off the power without modifying the setting of the voltage control.
2. Connect E2 to measure phase voltage E1-2 on the primary side. Turn on thepower and compare the voltage phasor of E1-2 on the primary side with thatof E3-5 on the secondary side. Does the Phasor Analyzer display show thatthe voltages are equal and in phase, except for possibly a small differencedue to transformer reactance?

----- Yes ----- No

• Conclusion

You connected transformer windings in three-phase delta-delta and wye-wyeconfigurations, and measured winding voltages to ensure that secondary windingswere connected with the proper phase relationships. You confirmed that the voltagewithin a delta was zero before closing the delta, and that the delta-delta and wye-wyeconfigurations produced no phase shift between the incoming primary voltages andthe outgoing secondary voltages.

• Objectives

When you have completed this exercise, you will be familiar with the voltage andcurrent ratios of three-phase transformers connected in delta-wye and wye-deltaconfigurations. Measurements of primary and secondary voltages will demonstratethat these configurations create a phase shift between the incoming and outgoingvoltages.

• Discussion

As seen in the previous exercise, primary and secondary voltages in delta-delta andwye-wye connections are in phase and the voltage at the secondary is equal to thevoltage at the primary times the inverse of the turns ratio. In delta-wye and wye-deltaconnections however, there will be a 30o phase difference between the primary andsecondary voltages. Also, in the delta-wye configuration, the line voltage at thesecondary is equal to the line voltage at the primary times the inverse of the turnratio times √3. On the other hand, in the wye-delta configuration, the line voltage atthe secondary is equal to the line voltage at the primary times the inverse of the turnratio times 1/√3.

The 30o phase shift between the primary and secondary does not create anyproblems for isolated groups of loads connected to the outgoing lines from thesecondary. However, if the outgoing lines from the secondary of a three-phasetransformer have to be connected in parallel with another source, the phase shiftmight make such a parallel connection impossible, even if the line voltages are thesame. Recall that in order for three-phase circuits and sources to be connected inparallel, line voltages must be equal, have the same phase sequence, and be inphase when the parallel connection is made.

Figure 9-8 shows a three-phase transformer, with a turns ratio equal to 1:1,connected in the delta-wye configuration and feeding a three-phase load. Thevoltage across each primary winding EPRI equals the incoming line voltage, but theoutgoing line voltage ESEC is √3 times that voltage because the voltage across anytwo secondary windings is √3 times greater than the voltage across a singlesecondary winding. Note that if the three-phase transformer had a turns ratio of 1:10,the line voltage at the secondary would be 10 x √3 times greater the line voltage atthe primary, because the inverse of the turns ratio is multiplied by the √3 factor. Theline current in the secondary is the same as the phase current, but the line currentin the primary is √3 times greater than the corresponding phase current.

Figure 9-8. Three-Phase Delta-Wye Configuration

• Procedure:

1. Connect the Three-Phase Transformer module in the wye-delta configurationshown in Figure 9-9. Make sure that the voltage within the delta is zerobefore closing the delta.

Figure 9-9. Three-Phase Transformer Connected in Wye-Delta

1. Turn on the power and adjust the voltage control to obtain the line-to-linevoltage ES equals 200V. Connect E1, E2, and E3 to measure the linevoltages at the primary and record the results. Find also the averagevalue of the line voltage given .

E1-6 = ----- V, E11-1 = ----- V, E6-11 = ----- V

AVG (E1,E2,E3) = ----- V

1. Observe the voltage phasors on the Phasor Analyzer. Are they approximatelyequal with a 120o phase shift between each of them?

----- Yes ----- No

1. Turn off the power without modifying the setting of the voltage control.Connect E1, E2, and E3 to now measure the line voltages at the secondary.Turn on the power and record theline voltages as well as the average value of the line voltages.

E3-5 = ----- V, E8-10 = ----- V, E13-15 = ----- V

AVG (E1,E2,E3) = ----- V

1. Observe the voltage phasors on the Phasor Analyzer. Does the displayconfirm they are equal with a 120ophase shift between each of them?

----- Yes ----- No

1. Turn off the power without modifying the setting of the voltage control.Connect E2 to measure line voltage E1-6 on the primary side. Turn on the power and compare the voltagephasor of E1-6 on the primary side with that of E3-5 on the secondary side.Does the Phasor Analyzer display confirm a phase shift of around 30obetween the two?

----- Yes ----- No

1. Calculate the ratio AVG ESEC / AVG EPRI using the values recorded in steps6 and 9. Is it approximately equal to 1/√3?

----- Yes ----- No

1. Turn off the power and connect the Three-Phase Transformer module in thedelta-wye configuration shown in Figure 9-10. Set the Resistive Loadmodule thus R equals 1100Ω, and connect I1, I2, and I3 to measure thethree line currents to the load.

1. Connect E1, E2, and E3 to measure the line voltages at the primary, turn onthe power, and adjust the voltage control to obtain the line-to-line voltage ofESequals 200V. Recordthe value of the line voltages, as well as the average value.

E1-2 = ----- V, E6-7 = ----- V, E11-12 = ----- V

AVG (E1,E2,E3) = ----- V

Figure 9-10. Three-Phase Transformer Connected in Delta-Wye

1. Observe the voltage and current phasors on the Phasor Analyzer. Does thedisplay confirm that the voltage and current phasors are in phase?

----- Yes ----- No

1. Turn off the power without modifying the setting of the voltage control.Connect E1, E2, and E3 to now measure the line voltages E3-8, E8-13, andE13-3 on the secondary side. Turnon the power. Does the Phasor Analyzer display show that the voltagephasors lead the current phasors by 30o?

----- Yes ----- No

Note: Since the currents in the secondary are in phase with thevoltages in the primary, the Phasor Analyzer display is equivalentto observing all voltage phasors at the same time, except for thedifference in scale between the parameters.

1. Return to the Metering application and record the measured values for theline voltages at the secondary, and also the average value.

E3-8 = ----- V, E8-13 = ----- V, E13-3 = -----V

AVG (E1,E2,E3) = ----- V

1. Calculate the ratio AVG ESEC / AVG EPRI using the values recorded in steps9 and 12. Is it approximately equal to √3?

----- Yes ----- No

1. Turn off the power and connect I1 and I2 to measure the line and phasecurrents on the primary side of the delta-wye configuration by opening thecircuit at points X and Y shown in Figure 9-10. Remember to reconnect theload resistors at the secondary when I1 and I2 are disconnected.
2. Turn on the power and calculatethe ratio ILINE / IPHASE for the primary circuit using the measured currents. Isthe ratio approximately equal to √3?

----- Yes ----- No

1. Is the line current on the primary side approximately equal to the line currenton the secondary side?

----- Yes ----- No

• CONCLUSION

You connected a 1:1 three-phase transformer in wye-delta and delta-wyeconfigurations, and saw that the line voltage between primary and secondary eitherincreased or decreased by a √3 factor. You also confirmed that the outgoing linevoltages at the secondary were shifted 30o with respect to the incoming linevoltagesat the primary.

• Objective

When you have completed this exercise, you will be able to connect twotransformers in an open-delta configuration to supply a balanced three-phase load.You will also be able to demonstrate that the maximum power in the open-deltaconfiguration is 57.7% (1/√3) the capacity of a normal delta-delta configuration.

• Discussion

The open-delta connection allows three-phase balanced loads to be supplied usingonly two transformers. This configuration is useful if the amount of load power is notexcessive, or when one of the three transformers must be taken out of servicebecause of damage or some other reason. The most important thing to note is thatthe power capacity in the open-delta configuration is 57.7% of the total capacity ofthe normal delta-delta configuration, or 86.6% of the capacity of the two remainingtransformers. The reason for this is quite simple, and Figure 9-11 will be used toillustrate the explanation.