Download TRANSFORMER CONNECTIONS

EE 280
Experiment #5 – Fall 1999
TRANSFORMER CONNECTIONS
The purpose of this experiment is to investigate the characteristics of a transformer and to demonstrate the different
types of three phase transformer connections.
Laboratory Equipment:
This lab will make use of:
1) Three single phase Hampden 1.0kVA transformers.
2) A three phase variac
3) The resistive load cart
4) The TekMeter as an oscilloscope
4) Various instruments
Proceedure:
A) Part 1 - Single Phase Transformer Exciting Current
1) Construct the circuit in figure 1. The current probe will be used to display the current on the scope. Use a
voltmeter to measure the voltage on the transformer. Turn the variac fully counterclockwise (minimum voltage).
Have your instructor examine your circuit.
120/ 208 V
3 phase
input
A
B
C
N
out put
A
B
C
N
t r ansf o r m er
3 phase
v ar iac
t o scope
Figure 1
2) Energize the circuit. Slowly turn on the breaker of the variac. Adjust the voltage to about 110% of rated voltage
(about 230 V).
3) Display the current waveform on the scope. Make a sketch the current, noting any distortion. Record the peak
value and the RMS value of the current and the voltmeter reading.
4) Repeat Part 3 for 100% (208 V) and 75% (about 155 V) of rated voltage.
(over)
B) Part 2 - Wye - Wye (Y-Y) connection
1) Construct the circuit in figure 2. Be sure to parallel the two low voltage windings. Have your instructor examine
your circuit.
3 φ INPUT
208/120 V
Figure 2
2) Energize the circuit. Measure and record a primary L-L voltage and a primary phase (L-N) voltage.
3) Measure and record a secondary L-L voltage and a secondary phase (L-N) voltage. (Use the same secondary
voltages as the primary voltages that you used in Part 2.)
4) Connect the phase B Line connection of the transformer secondary to the common terminal of the scope.
Using B as the reference for voltage measurements, display the secondary line A-B voltage on channel 1 and the
secondary phase N-B voltage on channel 2 of the scope. Invert the channel 2 waveform so that you are looking at
B-N voltage. Sketch the waveforms. Measure any phase displacement in the two voltages. Disconnect the scope.
Ask your lab TA to take the same measurements with the phase angle meter.
The purpose of this is:
i) to see the difference (if any) of the turns ratios of an individual transformer and the turns ratio of a transformer
bank, and
ii) to check for any phase shift of the voltage.
C) Part 3 - Wye - Delta (Y-∆
∆) connection
1) Have your Lab TA give a brief lecture on the precautions that must be taken when connecting up a wye-delta
transformer. Then construct the circuit in figure 3. Be sure to parallel the two low voltage windings. Pay attention
to the neutral connection! Make sure that the load switches on the cart are off. Have your instructor examine
your circuit.
Figure 3
2) Energize the circuit. Measure the RMS magnitudes of a primary L-L voltage and a primary phase voltage.
3) Measure a secondary L-L voltage (which is also the secondary phase voltage).
The purpose of this is:
i) to see the difference (if any) of the turns ratios of an individual transformer and the turns ratio of a transformer
bank.
4) Display the voltage across the open connection of the delta on the scope. Sketch the waveform. Be sure to note
the time scale and measure the frequency. Deenergize the circuit. Disconnect the scope.
The purpose of this is to check for a third harmonic voltage. Think about why this might be present.
5) Close the delta connection. Energize the circuit. Load each of the phases so that the load cart reads 900W.
Measure a phase and L-L voltage on the primary side. Note any differences from Part (2) above. Measure a L-L
voltage on the secondary. Note any differences from Part (3). Using the current probe, measure one of the phase
currents in the delta and one of the secondary line currents. Next, use the Tek meter to measure the following
waveforms: line current going toward load, current “circulating” in delta winding of transformer, L-N voltage at load,
and the L-L voltage at the load. Sketch each waveform, making note of any harmonic distortion.
6) Ask for your Lab TA’s help on this step, as there is only one phase angle meter. (Don’t use the Tek Meter for this
step!) You will be measuring the phase angle shift between primary and secondary of the transformer. Connect
VAN of the primary to input 1, and then use channel 2 to measure the phase angle of VBN and VCN on the primary
and VAB, VBC, VCA on the secondary. Make notes on figure 3 as to how it is connected. Consider whether the
transformer is connected corrected for positive sequence on both sides, and if you are able to actually recreate the
standard IEEE 30° phase shift.
7) Deenergize the circuit. Clean up your work bench.
Report Questions
1) Comment on the current waveforms measured in part (A3) and (A4). Explain how saturation causes this
waveshape. What harmonics are present?
2) Calculate the voltage ratio of the transformer bank in setups B and C. Compare this to the turns ratio of the
individual transformers. Is this what you expected?
3) Comment on the sketch of the primary and secondary voltage in part (B4).
4) Comment on the sketch of the voltage made in part (C4). What causes this voltage?
5) For the Y-∆ transformer in Part 3, comment on the phase and line voltages in the Y as compared to the expected
values and comment on the phase and line currents in the ∆ as compared to the expected values.
6) In Part 3, what could happen if you connect the delta incorrectly? Use closed voltage phasor diagrams to explain
how you as an engineer could determine the correct the connection without resorting to trial and error.
7) If you’d like prevent third harmonic voltages from reaching the load, would recommend the wye-wye or wye-delta
connection? Why?
8) For the wye-delta transformer of part 3, sketch out a circuit diagram and voltage phasor diagram which show the
correct winding connections for the IEEE standard 30° phase shift. Explain what you’ve done and how this works.
Congratulations at this point – you’ve mastered what is one of the most common (and least understood)
transformer connections in industrial/factory distribution systems.