Lab No. 3
BAHRIA UNIVERSITY KARACHI CAMPUS
Department of Electrical Engineering
POWER DISTRIBUTION AND UTILIZATION
LAB EXPERIMENT # 3
ANALYSIS OF DISTRIBUTION TRANSFORMER ON
ASYMMETRICAL LOAD
OBJECTIVE:

Examine the load capacity of secondary side of distribution transformer using asymmetrical load
with different connection modes on primary side.
Investigate the zero impedance of distribution transformer with various connection modes.

EQUIPMENT:
1.
2.
3.
4.
5.
3.1.
EST 1013T1 Three-phase power supply
EST 1080TT Three-phase transformer
EST 1017R Resistive load
EST 2109T2A5 Digital Ammeter
EST 2109T3PV Digital Voltmeter
THEORY:
Normally only symmetrical loads occur in systems operating at extra-high voltage, high-voltage and
medium voltage levels. Consequently, the same balanced magnetic relationships prevail in the
transformers used there as in single-phase models. However, in low-voltage systems severe
asymmetries can arise due to the possibility of single-phase consumers being connected there. We
should now examine the most extreme case of a single-phase load, whose connection to the lower
voltage side of a distribution transformer with accessible star point is very plausible.
Our considerations here are based on a winding which is connected once in delta configuration and
once in star point configuration on the primary side as shown in the Figure 3.1 and 3.2.
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Lab No. 3
Fig. 3.1 Delta connection on the primary side
When the primary side is in delta connection and there is a single-phase load, magnetic equilibrium
settles in both sides, because the current in the loaded winding on the primary side can flow back and
forth via two outer conductors. The other two phase windings remain current less. The circuit can be
loaded to the nominal current.
Fig. 3.2 Star connection on the primary side
This is not the case for star connection on the primary side. The primary current flowing in the
relevant phase winding here can only fed via the other two phase windings. Here fluxes arise which
cannot be neutralized on the secondary side. This results in magnetic leakage fluxes, which have to
link up via the transformer housing and through the air.
This brings about severe voltage drops and asymmetries on the secondary side. Therefore,
transformers in this circuit configuration may only be operated in single-phase with approx 10% of
their nominal power. A possible remedy would be connecting a neutral point conductor on the
primary side. This would result in the two other primary windings being current less and thus
reestablishing a balanced magneto motive force. However, due to economic considerations, this
measure is discarded in practice as it would necessitate a four-conductor system on the primary
voltage side (generally in the medium voltage network).
In motors and generators (components with moving parts) the positive-phase and negative-phase
impedances vary in magnitude; whereas in transformers and transmission lines (static components)
they are identical. The zero impedance always deviates from the positive- and negative-phase
impedance and is determined by measuring, whereby the same ac voltage is applied to all three phase
windings of the components and then the current is measured.
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Lab No. 3
TASK 1: Δ-Y Distribution Transformer with Asymmetrical Load Behavior
High voltages are present in this Laboratory Experiment. Do not make any
connections with the power on. The power should be turned off after completing
each individual measurement.
PROCEDURE:




Assemble the circuit in accordance with the foregoing connections diagram in Figure 3.3(6.1 Dy).
Set the secondary side of the transformer in star connection at tap UN = 220V and retain this
during the entire experiment.
First we will investigate a delta-star connection: set the primary-side in delta connection
380V.
Set the single-phase resistive load to R1 value and adjust the supply voltage in order to obtain the
nominal voltage 380V (phase-to-neutral voltage 220V) on the primary side of the three- phase
transformer: this value must be kept constant for all the measurements.
CIRCUIT DIAGRAM:
Figure 3.3 Asymmetrical Load Connections Diagram
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Lab No. 3
OBSERVATIONS AND MEASUREMENTS:


Beginning from the R1 value reduce the resistive load in steps till the R7 value. For each step
measure the load current I2 as well as the three phase voltages Uu, Uv and Uw on the secondary
side.
Enter the measured value into the following table.
Table 3.1 Δ-Y Connections Resistive Load Measurement Reading
R
R1
R2
R3
R4
R5
R6
R7
I2 (A)
Uu (V)
Uv (V)
Uw (V)
TASK 2: Y-Y Distribution Transformer with Asymmetrical Load Behavior
PROCEDURE:
 Change the primary-side so that we can investigate a star-star connection in accordance with
the foregoing connection diagram in Figure 3.3 (6.2 Yy) primary star connection 220 V.
OBSERVATIONS AND MEASUREMENTS:
 Repeat the above measurements.
Table 3.2 Y-Y Connections Resistive Load Measurement Reading
R
R1
R2
R3
R4
R5
R6
R7
I2 (A)
Uu (V)
Uv (V)
Uw (V)
OBSERVATIONS:
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Lab No. 3
TASK 3: Determine the zero impedance of Star-Star (Yy) connected Distribution Transformer
PROCEDURE:


Assemble the circuit in accordance with the foregoing connections diagram in Figure 3.4
(5.1:YY).
First we will investigate a star-star connection: set the primary-side in star connection 380 V
and the secondary-side in star connection UN = 220 V.
CIRCUIT DIAGRAM:
Figure 3.4 Zero Impedance Connections Diagram
OBSERVATIONS AND MEASUREMENTS:

Adjust the supply voltage until a current of approximately 0.6 A flows and measure the
corresponding voltage:
Io = ……… A
Uo = ……… V
Determine the zero-impedance:
𝒁𝟎𝒀𝒚 =
𝐔𝐨
𝐈𝐨
=…………Ω
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Lab No. 3
Task 4: Determine the Zero Impedance of Star-Delta (Yd) connected Distribution Transformer
PROCEDURE:

To investigate a star-delta connection in accordance with the foregoingconnections
diagram in Figure 3.4 (5.2:Y-D).
OBSERVATIONS AND MEASUREMENTS:

Adjust again the supply voltage until a current of approximately 2A flows and measure the
corresponding voltage.
Io = ……… A
Uo = ……… V
Determine the zero-impedance:
𝒁𝟎𝒀𝒚𝒅 =
𝐔𝐨
𝐈𝐨
=………… Ω
OBSERVATIONS:
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CONCLUSION:
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