ECE 420
Spring 2007 on-campus
Experiment o. 1
Three Phase Transformers
Experiment No. 1
Three Phase Transformers
Objective:
The objectives of this experiment are to find the approximate per phase equivalent circuit
parameters of a 3-φ transformer bank using short circuit and open circuit tests and to
determine the regulation and efficiency of the transformer bank through load test. The
regulation and efficiency of the transformer bank determined through load test will be
compared with those calculated using the transformer parameters.
Apparatus:
1. 1 3-φ variac
2. 2 Test tables
3. 4 multimeters
4. 2 wattmeters
5. 1 15 kVA 3- φ transformer Bank
6. 1 3-φ Resistive load bank with fan
7. 1 3-φ Capacitive Load Bank
Procedure:
1. Short Circuit Test:
a) Refer to Fig.1 for Short circuit test. Connect the high voltage windings of each 1-φ
transformer in parallel and then connect them in STAR configuration. Connect the
low voltage windings of each 1-φ transformer in parallel and then connect them in
DELTA configuration. Make sure that the transformer is connected according to
nameplate specifications.
b) Connect the three phases of the low voltage windings to test table 2. Connect the
three phases of the high voltage windings through the test table to the output
terminals of the variac. Make sure that all test table line switches are open. Connect
the input terminals of the variac to the 120V AC power panel. Do not turn on power.
Have the lab instructor check your set up.
c) With the test table's line switches still open, turn on power and adjust the variac until
its output voltage is zero.
1/11
ECE 420
Spring 2007 on-campus
Experiment o. 1
Three Phase Transformers
d) Calculate the rated current for the input windings. Check your value with the lab
instructor. Make sure that your current transformer is set up to handle this current.
e) Protecting the measuring instruments, close the test table line switches. Immediately
after closing the line switches check the current in lines 1,2 and 3. The current should
be close to zero. If the current is not close to zero de-energize your circuit and
recheck it.
f) Increase the output voltage of the variac until the ammeter in line 1 indicates rated
current. Measure and record VH12, VH23, VH31, IH1, IH2, IH3, IX1, IX2, IX3, PHWM1 and
PHWM2. Protect your measuring instruments and de-energize the circuit. Readings
should be taken with great precaution as you may see currents as high as 100A!
2. Open Circuit Test:
a) Refer to Fig.2 for Open circuit test. Connect the high voltage windings of each 1-φ
transformer in parallel and then connect them in STAR configuration. Connect the
low voltage windings of each 1-φ transformer in parallel and then connect them in
DELTA configuration. Make sure that the transformer is connected according to
nameplate specifications.
b) Connect the three phases of the high voltage windings to test table 1. Make sure that
the output connections are open circuited! Connect the three phases of the low
voltage windings through test table 2 to the output terminals of the 120V AC power
panel. Make sure that all test table line switches are open. Do not turn on power.
Have the lab instructor check your set up.
g) Measure and record VX12, VX23, VX31, IX1, IX2, IX3, PXWM1, PXWM2, VH12, VH23 and VH31.
Readings should be taken with great precaution as you may see voltages as high
as 420V!
3. Transformer with R Load:
a) Set up the circuit as shown in Fig.3.
b) Make sure that the resistance bank is connected to rated voltage. Open circuit the
secondary; i.e., resistance set to infinity. Turn ON the mains and then turn the
switches on the test table to ON position.
c) Now adjust the resistive load until IH1 = IH2 = IH3 = rated current. Measure and record
VX12, VX23, VX31, IX1N, IX2N, IX3N, PXWM1, PXWM2, VH12, VH23, VH31, IH1N, IH2N, IH3N,
PHWM1, PHWM2 and PHWM3 for rated current. .ote that a two-wattmeter method is
used on LV side and a three-wattmeter method on HV side for measurement of
three-phase power. So the selection switch on HV side should be used only
between positions 1-. and 3-..
2/11
ECE 420
Spring 2007 on-campus
Experiment o. 1
Three Phase Transformers
d) Now vary the load resistance such that IH1 = IH2 = IH3 equals ½ of rated current.
Measure and record the same quantities as in step “c” for each value of IH.
4. Transformer with RC Load:
a) Set up the circuit as shown in Fig.4.
b) Make sure that the resistance and capacitive bank are connected to rated voltage.
Open circuit the secondary by setting the resistance bank and capacitance bank to
infinity. Turn ON the mains and then turn the switches on the test table ON. Now
adjust the resistive and capacitive loads such that they draw equal currents and the
line currents IH1 = IH2 = IH3 = rated current. Measure and record VX12, VX23, VX31, IX1N,
IX2N, IX3N, PXWM1, PXWM2, VH12, VH23, VH31, IH1N, IH2N, IH3N, PHWM1, PHWM2 and PHWM3 for
rated current. .ote that a two-wattmeter method is used on LV side and a threewattmeter method on HV side for measurement of three-phase power. The
selection switch on HV side should be used only between positions 1-. and 3-..
c) Now vary the load resistance such that IH1 = IH2 = IH3 equals ½ of rated current.
Measure and record the same quantities as in step “c” for each value of IH.
Report:
1. Designate the parallel connected low voltage (X) windings the primary and the
parallel connected high voltage (H) windings the secondary.
2. Using the data found in the open circuit test calculate RC,X1 and XM,X1.
GCX1 is given by
Poc
GMX
Poc
3
 Voc 


 3
2
2
Voc
BM,X is given by
BMX
2
Where
Yoc
2
Yoc − GMX
Ioc
 Voc 


 3
3/11
ECE 420
Spring 2007 on-campus
Ioc
CT ⋅
Experiment o. 1
Three Phase Transformers
IX1 + IX2 + IX3
3
VX13 + VX23 + VX12
Voc
PT⋅
Poc
WMR⋅PT⋅CT ⋅ PXWM1 + PXWM2
3
(
)
Where WMR is the Wattmeter ratio, CT is the current transformer ratio and PT is the
Potential Transformer ratio (Treat PT = 1 for the whole experiment).
3. Using the data found in the short circuit test calculate Req,H and Xeq,H.
ReqH is given by
 Vsc 
ZeqH


3


Isc
Xeq,H is given by
XeqH
2
2
ZeqH − ReqH
Where
ReqH
 Psc 
 
 3 
2
Isc
Draw the approximate per phase transformer equivalent circuit with all parameters
referred to the primary (LV side) as shown in Fig 5. Show the calculated values of
each parameter. Remember to include units. Remember the short circuit test was
performed with the voltage applied to the high voltage side! Also make a note of
the transformation ratio “a”.
4. Efficiency from input and output power measurements:
The efficiency of a transformer is given by:
P out
%η
⋅ 100
P in
a) Using the data gathered in Part 3, Transformer with R Load, calculate the efficiency
(η) of the transformer for full, ½ and no load.
b) Using the data gathered in Part 4, Transformer with RC Load, calculate the efficiency
(η) of the transformer for full, ½ and no load.
4/11
ECE 420
Spring 2007 on-campus
Experiment o. 1
Three Phase Transformers
5. Voltage regulation from the voltage measurements:
Voltage regulation of a transformer is given by:
%Reg
VHLLnoload − VHLLload
VHLLload
Where VHLL = the average Line to Line voltage on HV side.
a) Using the data gathered in Part 3, Transformer with R Load, calculate the voltage
regulation of the transformer for full, ½ and no load.
b) Using the data gathered in Part 4, Transformer with RC Load, calculate the voltage
regulation of the transformer for full, ½ and no load.
6. Efficiency from the transformer equivalent circuit parameters:
a) Using the values of VXLN (LV side average line to neutral voltage), IX (LV side
average current), and PF (power factor), gathered in Part 3, Transformer with R Load,
and the equivalent circuit of your transformer, calculate the efficiency of the
transformer for full, ½ and no load.
b) Using the values of VXLN (LV side average line to neutral voltage), IX (LV side
average current), and PF (power factor), gathered in Part 4, Transformer with RC
Load, and the equivalent circuit of your transformer, calculate the efficiency for the
transformer for full, ½ and no load.
7. Voltage Regulation from the transformer equivalent circuit and its parameters
a) Using the values of VXLN (LV side average line to neutral voltage), IX (LV side
average current), and PF (power factor), gathered in Part 3, Transformer with R Load,
and the equivalent circuit of your transformer, calculate the voltage regulation of the
transformer for full, ½ and no load.
b) Using the values of VXLN (LV side average line to neutral voltage), IX (LV side
average current), and PF (power factor), gathered in Part 4, Transformer with RC
Load, and the equivalent circuit of your transformer, calculate the voltage regulation
for the transformer for full, ½ and no load.
8. Compare the results found in step 4 with 6 and step 5 with 7 of the report section.
Calculate percent differences. What conclusions can you draw?
5/11
ECE 420
Spring 2007 on-campus
Experiment o. 1
Three Phase Transformers
Table1: Short Circuit Test:
Quantity
Measured reading
HV Side
Readings
PHWM1
PHWM2
VH12
VH23
VH13
IH1
IH2
IH3
LV Side
Readings
IX1
IX2
IX3
Table 2: Open Circuit Test:
Quantity
Measured reading
LV Side
Readings
PXWM1
PXWM2
VX12
VX23
VX13
IX1
IX2
IX3
HV Side
Readings
VH1
VH2
VH3
6/11
ECE 420
Spring 2007 on-campus
Experiment o. 1
Three Phase Transformers
Table 3: R Load:
Quantity
Full Load
½ Full Load
LV Side
Readings
PXWM1
PXWM2
VX12
VX23
VX13
IX1
IX2
IX3
HV Side
Readings
PH1
PH2
PH3
VH1
VH2
VH3
IH1
IH2
IH3
7/11
ECE 420
Spring 2007 on-campus
Experiment o. 1
Three Phase Transformers
Table 4: RC Load:
Quantity
Full Load
½ Full Load
LV Side
Readings
PXWM1
PXWM2
VX12
VX23
VX13
IX1
IX2
IX3
HV Side
Readings
PH1
PH2
PH3
VH1
VH2
VH3
IH1
IH2
IH3
IHR1
IHR2
IHR3
IHC1
IHC2
IHC3
8/11
ECE 420
Spring 2007 on-campus
Experiment o. 1
Three Phase Transformers
3 VARIAC
A
TEST TABLE1
1H
TEST TABLE2
HV SIDE
LV SIDE
STAR
DELTA
H1 H3
1X
X1 X3
AC
N
AC
N
AC
N
B
C
2H
X2 X4
H2 H4
H1 H3
2X
X1 X3
N
N
X2 X4
H2 H4
3H
H1 H3
3X
X1 X3
N
N
X2 X4
H2 H4
LOW VOLTAGE SIDE
SHORT CIRCUITED
Fig.1. Short Circuit Test, Part 1
TEST TABLE 2
A
LV SIDE
DELTA
1X
HV SIDE
TEST TABLE 1
STAR
H1 H3
1H
X1 X3
AC
AC
N
120 V
240 V
X2 X4
AC
H2 H4
2X
C
H1 H3
B
2H
X1 X3
120 V
240 V
X2 X4
H2 H4
3X
X1 X3
120 V
H1 H3
3H
240 V
N
X2 X4
H2 H4
HIGH VOLTAGE SIDE
OPEN CIRCUITED
Fig.2. Open Circuit Test, Part 2
9/11
ECE 420
Spring 2007 on-campus
Experiment o. 1
Three Phase Transformers
TEST TABLE 2
A
1
LV SIDE
HV SIDE
DELTA
STAR
TEST TABLE 1
H1 H3
1
IHR1
X1 X3
AC
AC
N
120 V
240 V
X2 X4
AC
H2 H4
2
C
H1 H3
B
2
IHR2
X1 X3
120 V
R
240 V
R
X2 X4
H2 H4
3
X1 X3
120 V
H1 H3
3
240 V
N
X2 X4
H2 H4
Fig.3. Transformer with resistive load, Part 3
Fig.4. Transformer with resistive and capacitive load, Part 4
10/11
R
IHR3
ECE 420
Spring 2007 on-campus
Experiment o. 1
Three Phase Transformers
IH
Req,X
VXLNAC
Xeq,X
IX
VX
a=
a
GM,X
BM,X
NX
3⋅ N H
aVH
a2ZLOAD
Fig. 5. Per Phase Equivalent Circuit referred to LV side for the 3φ Transformer.
11/11