AN909
TESTING OF TRANSFORMER
(Using DPATT-3Bi)
ILLU STR A TE D B Y E XA M PLE
1
©Uma Electronics Enterprise, 2012
http://www.umaelectronics.com
Version 3.0
28 Aug 2012
AN909
.
TESTING OF DISTRIBUTION
Transformer
Step BY Step
These tests are known by various names.
Short Circuit Test
OR
Full Load Test
OR
Copper Loss Test
OR
Winding Loss Test
Open Circuit Test
OR
No Load Test
OR
Iron Loss Test
OR
Core Loss Test
Example of Transformer Name Plate
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©Uma Electronics Enterprise, 2012
http://www.umaelectronics.com
Version 3.0
28 Aug 2012
AN909
.
SHORT CIRCUIT TEST OF TRANSFORMER
The test is conducted on the high voltage side of the transformer. Where
the secondary winding (usually of low-Voltage Side) of the transformer is short
circuited. The full load current is applied on the primary side of transformer. The core
losses are very small because applied voltage is only a few percentage of the nominal
voltage and hence can be neglected. Thus the wattmeter reading measures only the full
load copper loss.
Figure 1 SHORT CIRCUIT TEST
STEP1 -Before starting the test, Calculate the full load current from the name plate rating.
It’s Simple!
For the name plate shown above, we can easily see thatTransformer Rating:
HT Voltage=11KV
LT Voltage=415V (P-P)
KVA Rating=200 KVA
Take a calculator and solve√
√
3
.
©Uma Electronics Enterprise, 2012
http://www.umaelectronics.com
Version 3.0
28 Aug 2012
AN909
.
PREPARE TO SETUP
STEP2- Short the LT side of Transformer and apply voltage at HT side of Transformer
until the full load current achieved.
STEP3- Find out the Impedance Voltage.
Note down the voltage show on DPATT-3Bi
STEP4- Suppose that 10.49A comes on 463.1 V
Impedance Voltage=463.1V
% !"#$%&'#
1 .
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. 100
-%+#$ ()*+%,#
.
The SC Test of the transformer is displayed after pressing the F2
button as shown below:
STEP5- Copper losses are shown on W/PF page of DAPTT-3Bi.
Typical Values for Transformers of different values:EXAMPLE: - 11KV/415V
Rating of Transformer
100KVA
200KVA
300KVA
400KVA
500KVA
600KVA
Full Load Current
5.24A
10.48A
15.72A
20.96A
26.2A
31.44A
Impedance Voltage: That voltage at which full load current is achieved is
called impedance voltage.
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©Uma Electronics Enterprise, 2012
http://www.umaelectronics.com
Version 3.0
28 Aug 2012
AN909
.
OPEN CIRCUIT TEST OF TRANSFORMERThis test is carried out to determine iron losses in the
transformer. This test is important because whenever the transformer is ON it
consumes electricity in the form of iron loss. This loss occurs round the clock and it
should be minimized through better design of iron core.
Figure 2 OPEN CIRCUIT TEST
The core loss and magnetizing current depends on applied voltage and frequency.
It is recommended, that since we have no control on variation of mains frequency the
applied voltage is varied to keep the V/f ratio constant.
For example, in the figure above,
Standard voltage 240V P-N or 415V P-P is applied on LT side through variac keeping
HT side open.
STEP1 - Find the value of Rated Voltage:According to name plate:HT Voltage=11KV
LT Voltage=415V (P-P)
KVA Rating=200 KVA
Rated Voltage =11KV (Primary Side)
Rated Voltage =415V (Secondary Side)
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©Uma Electronics Enterprise, 2012
http://www.umaelectronics.com
Version 3.0
28 Aug 2012
AN909
.
STEP 2-
HT Side of Transformer is kept open and apply voltage through variac on LT
side until the rated voltage achieved.
In case Supply frequency is different from rated frequency, we have to calculate voltage to be
applied according to Supply frequency.
Example- Rated LT Voltage, VRT = 415 V
Rated Frequency, FRT = 50 Hz
The OC Test of the transformer is
displayed after pressing the F1 button as
shown below:
If, Supply frequency, FS =49.50Hz
Voltage to be applied,
2
5 67
8
5
34
39
:
:
;7
.:
. <: V
STEP 3Example-
Solve:
Find the value of No Load Current.
Transformer Rating
HT Voltage= 11KV
LT Voltage=415V
Rating of T/F= 200KVA
Rated Voltage=415V
Full Load Current =10.49A
No Load Current= 2.5% of Full Load Current
(According to BEE)
=>.?@ A B.C
=>>
STEP4-
6
0.26A
Iron losses are shown on W/PF page of DAPTT-3Bi .
©Uma Electronics Enterprise, 2012
http://www.umaelectronics.com
Version 3.0
28 Aug 2012
AN909
.
The maximum allowable losses at rated voltage and rated frequency permitted at 75 C
for 11/o.433 KV transformer can be chosen by the utility from the value of 3 star, 4 star
or 5 star rating for transformer up to rating of 200 KVA as indicated below:
Voltage
Ratio
Rating
(KVA)
3 Star
4Star
5Star
+11000/433-250V
16
Max.
Losses at
50%
Loading
(Watt)
150
11000/433-250V
25
210
695
190
635
175
595
11000/433-250V
63
380
1250
340
1140
300
1050
11000/433-250V
100
520
1800
475
1650
435
1500
11000/433-250V
160
770
2200
670
1950
570
1700
11000/433-250V
200
890
2700
780
2300
670
2100
11000/433-250V
Max.
Losses at
100%
Loading
(Watt)
480
Max.
Losses at
50%
Loading
(Watt)
135
Max.
Losses at
100%
Loading
(Watt)
440
Max.
Losses at
50%
Loading
(Watt)
120
Max.
Losses at
100%
Loading
(Watt)
400
The value of impedance of transformers at 75 0C shall be 4.5% for transformers
up to and 200 kVA and for ratings above 200 kVA shall be in accordance with IS
2026.(BEE)
Typical value of impedance voltage for transformer with two separate winding (At rated
current, given as a percentage of the rated voltage of the winding to which the voltage is
applied) I S 2026
Rated Power (KVA)
Impedance voltage(percent)
4.0
Up to 630
5.0
631 to 1 250
6.25
1 251 to 3 150
7.15
3 151 to 6 300
8.35
6 301 to 1 2500
10.0
12 501 to 2 5000
12.5
25 001 to 200 000
References:
1. I S 2026-1977-Specification for Power Transformer.
2. BEE CODE Transformer-2006
3. Electrical Machines (J.B. Gupta)
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©Uma Electronics Enterprise, 2012
http://www.umaelectronics.com
Version 3.0
28 Aug 2012
AN909
.