ECE 449 - Power Systems Laboratory

# Experiment 4: Three-Phase Transformer Connections

## Introduction

When it is necessary to raise or to lower the voltage of a three-phase power supply, by means of three transformers, any one of the four combinations of connections shown in Table 1 may be used:

Table 1
Low-tension side High-tension side
Y
Y
Δ
Δ
Y
Δ
Y
Δ

Theoretically, all four combinations may be made to give correct results; but in practice, in a given case, one particular combination usually has some advantages.
When the transformer primaries are connected in Y and the secondaries are connected in
Δ, or vice versa, the line voltages in the two circuits become displaced by 30 electrical degrees.

## Objectives

The purpose of this experiment is to learn the principal characteristics of the four combinations of transformer winding connections presented in the introduction.

## Prelab Assignments

Review the theory of 3-phase transformer connections. Make a chart listing the advantages and disadvantages of each combination in Table 1. Include all of the performance properties that differ between the combinations even if you think they may not be an advantage or disadvantage. Decide which of the various characteristics you will measure. Prepare tables with suitable headings and captions to enter your results. Draw circuit diagrams for the four configurations showing how the transformers are to be connected to each other and to the source, and how many multimeters are needed to perform the experiment. Bring a USB memory stick to capture waveforms from the instruments for your report.

## Equipment

1. Three single-phase transformers
2. Multimeters(DVM)
4. Fluke Power Quality Meter (PQM) – from stockroom (SR)
5. Optically Coupled USB Cable for Fluke PQM from SR
6. High Sensitivity, AC-DC Current Clamps Probe from SR
7. Six additional long Sheathed and stackable banana cables from SR
8. 3 sets of sheathed non-retractable banana cables from SR

## Procedure

Warnings:

1. When connecting three-phase transformers, the secondary Δ side should never be closed until a test is first made to determine that the voltage within the Δ is zero (or close to zero). If not, and the Δ is closed on itself, the resulting current will be of short-circuit magnitude, with resulting damage to the transformers. To determine the proper phase relationships in a Δ connected secondary, the voltage is measured across two windings.  This voltage should be equal to the voltage across either winding. If not, one of the windings must be reversed. One side of the third winding is then connected and the voltage is measured across the three windings. This voltage must be very close to zero. If not, the third winding connection must be reversed.
2. Different transformers used in this lab may have different ratings – Make a note of the rating of the transformer(s) you are using and don’t exceed the rating. This is easy to do when switching loads or unbalancing the circuit. If you aren’t sure if your change will cause the transformer to become overloaded, then you should raise the voltage slowly and watch the current/power flow carefully. You may not be able to run all parts of this experiment at 120 Volts. Overloading the transformer may either blow the internal fuse if there is one or burn out the transformer.

Two of the key differences discussed in textbooks that you should explore are the ability to suppress third harmonics and the phase shift created by some of the configurations.  Measuring harmonics can be done easily with the Fluke meters.  Measuring voltage shifts is not as easy since one side of a probe is grounded by the oscilloscope as indicated Figure 1 below.  One method you can consider is to place a resistor across the voltage to be viewed on the oscilloscope.  Assuming that the current through the resistance is in phase with the voltage a current clamp probe or a current transformer will output a voltage proportional to the current which is floating and can have one side safely grounded by connection to the oscilloscope.  Then the time delay between the two signals can be measured and converted to an angle.  The Fluke meter itself can also be used since it can view one voltage and one current at the same time.  By reading the calculated power factor one can find the angle.  However you need to look at the display on the fluke showing the two signals like an oscilloscope so that you can determine which is signal leading and which is lagging.

1. Label the three live wires of the three-phase supply A, B, and C.  Adjust the AC Variac to 20 Vrms line to neutral (L-n) and measure the three line to neutral voltages and the three line to line voltages.  Test if the neutral is grounded or not.
2. Each single-phase transformer has four windings. Create a 1:1 (240v:240v) ratio transformer by connecting the two primary windings in series and the two secondary windings in series. Check with a DMM the continuity of these two windings.
3. You will now connect three single-phase transformers in succession in the four possible ways listed in the introduction, and measure all the voltages and characteristics of the three-phase transformers that you create.  Read the rest of this procedure before doing this step to save yourself some time.

## Y - Y

1. Connect three identical single-phase transformers in Y-Y configuration with a Y connected load on the secondary side. Include in your circuit the DVMs necessary to measure the primary and secondary voltages.
2. Apply 60 Vrms L-n to your circuit and with no load on secondary side; measure and record all three line-to-neutral (L-n) and line-to-line (L-L) voltages on the primary and secondary side.
3. Use the optically coupled USB cable to connect your meter to the computer. Start the Flukeview software on the computer and make certain that it connects with your meter. If not, look at the device manager to determine the port that it is connected to and then choose that port for the Flukeview software.
4. With primary side neutral disconnected measure and record all three line-to-line (L-L) voltages on the primary and secondary side. Check primary side voltage harmonics with the Fluke PQM. Use the Flukeview software to capture this harmonics waveform for your report.
5. Connect the primary side neutral and apply a balanced three phase load to secondary side of 150W from resistive load cart. Measure and record all three line-to-neutral (L-n) and line-to-line (L-L) voltages on the primary and secondary side. Measure and record all three line currents on the primary and secondary side using the AC Current Clamp of Fluke PQM.
6. Display and record the voltage phase shift waveform between primary and secondary sides on a two channel oscilloscope. To ensure this, you should use two AC Current Clamp Probes each one connected to a two channel oscilloscope and measuring the primary and secondary line current of the same line (A, B, or C). Measure and record with the oscilloscope the time delay between the two signals.
7. Apply an unbalanced three phase load to secondary side by decreasing load of one rack from resistive load cart to 50 or 100W. Measure and record all three line-to-neutral (L-n) and line-to-line (L-L) voltages on the primary and secondary side. Measure and record all three line currents on the primary and secondary side using the AC Current Clamp of Fluke PQM.
8. Disconnect secondary side neutral and measure and record all three line-to-neutral (L-n) and line-to-line (L-L) voltages on the primary and secondary side. Measure and record all three line currents on the primary and secondary side using the AC Current Clamp of Fluke PQM.
9. With primary side neutral disconnected measure and record all three line-to-neutral (L-n) and line-to-line (L-L) voltages on the primary and secondary side. Measure and record all three line currents on the primary and secondary side using the AC Current Clamp of Fluke PQM. Check primary side voltage harmonics with the Fluke PQM. Use the Flukeview software to capture this harmonics waveform for your report.
10. Reconnect primary and secondary side neutral.
11. Replace a transformer in the bank with a different one with the same 1:1 (240v:240v) ratio. Record new transformer rate.
12. Monitoring appropriate voltages and currents to ensure that you don’t exceed the ratings of any of the transformers. Repeat steps 2 to 9.
13. Return the circuit to initial conditions of three identical single-phase transformers in Y-Y configuration with a Y connected load on the secondary side by changing the transformer of different rate.

## Y - Δ

1. Change the secondary side of the Y-Y configuration to form a D-connected secondary side with the primary side neutral connected to the source neutral. Do not close the final side of the D to perform test indicated in warning section.
2. Apply 10 Vrms L-n to your circuit to do the D test to see that there is close to zero voltage across this connection. If result from the D test is right, measure and record the voltage and frequency appearing across this open terminals with and without the primary side neutral connected to the source neutral.
3. Reconnect primary side neutral to the source and close secondary side connection to create a three identical single-phase transformers in Y-D configuration with a D connected load on the secondary side.
4. Apply 60 Vrms L-n to your circuit and with no load on secondary side; measure and record all three line-to-neutral (L-n) and line-to-line (L-L) voltages on the primary and line-to-line (L-L) voltage secondary side.
5. Apply a balanced three phase load to secondary side of 150W from resistive load cart. Measure and record all three line-to-line (L-L) voltages on the primary and secondary side. Measure and record the three line currents on the primary side and all three line/phase currents on the secondary side using the AC Current Clamp of Fluke PQM.
6. Display and record the voltage phase shift waveform between primary and secondary sides on a two channel oscilloscope. To ensure this, you should use two AC Current Clamp Probes each one connected to a two channel oscilloscope and measuring the primary and secondary line current of the same line (A, B, or C). Measure and record with the oscilloscope the time delay between the two signals.
7. Measure and record the power factor created between line-to-neutral (L-n) primary voltage with secondary line current of the same line (A, B, or C) and line-to-line (L-L) primary voltage with primary line current of the same line (A, B, or C).

## Δ - Y

1. Create a D -Y configuration with a Y connected load on the secondary side. To accomplish this, you should switch the three live wires of the three-phase supply A, B, and C of your preceding Y-D configuration from Y primary side to D secondary side. Connect the Y configured load to the now Y secondary side. Include in your circuit the DVMs necessary to measure the primary and secondary voltages.
2. Apply 60 Vrms L-L to your circuit and with no load on secondary side; measure and record the three line-to-line (L-L) primary voltages and all three line-to-neutral (L-n) and line-to-line (L-L) voltages on the secondary side.
3. Apply a balanced three phase load to secondary side of 150W from resistive load cart. Measure and record all three line-to-line (L-L) voltages on the primary and secondary side. Similarly, measure and record the three line currents on the secondary side and all three line and phase currents on the primary side using the AC Current Clamp of Fluke PQM.
4. Display and record the voltage phase shift waveform between primary and secondary sides on a two channel oscilloscope. To ensure this, you should use two AC Current Clamp Probes each one connected to a two channel oscilloscope and measuring the primary and secondary line current of the same line (A, B, or C). Measure and record with the oscilloscope the time delay between the two signals.
5. Measure and record the power factor created between line-to-line (L-L) primary voltage with secondary line current of the same line (A, B, or C) and line-to-line (L-L) primary voltage with primary line current of the same line (A, B, or C).

## Δ - Δ

1. Create a D -D configuration with a D connected load on the secondary side. To complete this, you should change the secondary side of the D-Y configuration to form a D-connected secondary.
2. Apply 60 Vrms L-L to your circuit and with no load on secondary side; measure and record the three line-to-line (L-L) primary and secondary voltages.
3. Apply a balanced three phase load to secondary side of 100W from resistive load cart. Measure and record all three line-to-line (L-L) voltages on the primary and secondary side. Also, measure and record all three line and phase currents on the primary and secondary side using the AC Current Clamp of Fluke PQM.
4. Display and record the voltage phase shift waveform between primary and secondary sides on a two channel oscilloscope. To ensure this, you should use two AC Current Clamp Probes each one connected to a two channel oscilloscope and measuring the primary and secondary line current of the same line (A, B, or C). Measure and record with the oscilloscope the time delay between the two signals.
5. Convert your D -D configuration to “Open Delta” (Ⅴ-Ⅴ) by removing one of your transformers from configuration. Keep the D connected load on the secondary side.
6. Repeat steps 2 to 4.

## Discussion

1. Discuss the advantages and disadvantages of the connections used in the laboratory.  Specifically point out those that you observed.
2. Discuss any other results that you think are interesting or confirm or disprove your expectations.
3. Explain any unbalance observed and difference in your data/readings due to the effect of the neutral in Y-Y configuration.
4. Explain any unbalance observed and difference in your data/readings due to the effect of the different rate transformer in Y-Y configuration.
5. Explain the voltage and frequency appearing across the open terminals of the D test from Y- D configuration with and without the primary side neutral connected to the source neutral.
6. Calculate phase shift angle between primary and secondary line-to-line (L-L) voltages of Y-D and D -Y configurations.
7. Where is the “Open Delta” (Ⅴ-Ⅴ) used and for what reason?
8. What precautions must be observed when paralleling 3-phase transformer banks?
9. What is meant by transformer magnetizing inrush current? Did you see any signs of its presence while performing the experiment?

## References

1. The A.I.E.E., Transactions, of 1914, Vol. 33, has several interesting papers and discussions about practical considerations of transformers connections.