Transformer Testing | Type Test

and Routine Test of Transformer

Under Electrical Transformer
For confirming the specifications and performances of an
electrical power transformer it has to go through numbers of
testing procedures. Some tests are done at manufacturer
premises before delivering the transformer. Mainly two types of
transformer testing are done at manufacturer premises- type
test of transformer and routine test of transformer. In
addition to that some transformer tests are also carried out at
the consumer site before commissioning and also periodically in
regular & emergency basis through out its service life.

Type of Transformer Testing

Tests done at factory
1.
2.

Type tests
Routine tests

3. Special tests
Tests done at site
1. Pre-commissioning tests
2.
3.

Periodic/condition monitoring tests

Emergency tests

Type Test of Transformer

To prove that the transformer meets customers specifications and
design expectations, the transformer has to go through different
testing procedures in manufacturer premises. Some transformer
tests are carried out for confirming the basic design expectation
of that transformer. These tests are done mainly in a prototype
unit not in all manufactured units in a lot. Type test of
transformer confirms main and basic design criteria of a
production lot.

Routine Tests of Transformer

Routine tests of transformer is mainly for confirming
operational performance of individual unit in a production lot.
Routine tests are carried out on every unit manufactured.

Special Tests of Transformer

Special tests of transformer is done as per customer

requirement to obtain information useful to the user during
operation or maintenance of the transformer.
Pre Commissioning Test of Transformer
In addition to these, the transformer also goes through some
other tests, performed on it, before actual commissioning of the
transformer at site. The transformer testing performed before
commissioning the transformer at site is called precommissioning test of transformer. These tests are done to assess
the condition of transformer after installation and compare the
test results of all the low voltage tests with the factory test
reports.
Type tests of transformer includes
1.
2.

Transformer winding resistance measurement

Transformer ratio test.

3.
4.

Transformer vector group test.

Measurement of impedance voltage/short circuit impedance

(principal tap) and load loss (Short circuit test).

5. Measurement of no load loss and current (Open circuit test).
6.
7.

Measurement of insulation resistance.

Dielectric tests of transformer.

8.
9.

Temperature rise test of transformer.

Tests on on-load tap-changer.

10. Vacuum tests on tank and radiators.

Routine tests of transformer include
1. Transformer winding resistance measurement.
2.
3.

Transformer ratio test.

Transformer vector group test.

4.

Measurement of impedance voltage/short circuit impedance

(principal tap) and load loss (Short circuit test).

5.
6.

Measurement of no load loss and current (Open circuit test)

Measurement of insulation resistance.

7.
8.

Dielectric tests of transformer.

Tests on on-load tap-changer.

9.

Oil pressure test on transformer to check against leakages

past joints and gaskets.

That means Routine tests of transformer include all the type tests
except temperature rise and vacuum tests. The oil pressure test
on transformer to check against leakages past joints and gaskets
is included.
Special Tests of transformer include
1. Dielectric tests.
2.

Measurement of zero-sequence impedance of three-phase

transformers

3.
4.

Short-circuit test.
Measurement of acoustic noise level.

5.
6.

Measurement of the harmonics of the no-load current.

Measurement of the power taken by the fans and oil pumps.

7.

Tests on bought out components / accessories such as

buchhloz relay, temperature indicators, pressure relief devices,
oil preservation system etc.

Transformer
Measurement

Winding

Resistance

Transformer winding resistance measurement is carried out to

calculate the I2R losses and to calculate winding temperature at
the end of a temperature rise test. It is carried out as a type test
as well as routine test. It is also done at site to ensure
healthiness of a transformer that is to check loose connections,
broken strands of conductor, high contact resistance in tap
changers, high voltage leads and bushings etc.
There are different methods for measuring of transformer
winding, likewise
Current voltage method of measurement of winding resistance.
Bridge method of measurement of winding resistance.

Kelvin bridge method of Measuring Winding Resistance.

Measuring winding resistance by Automatic Winding Resistance

Measurement Kit.
NB:- Transformer winding resistance measurement shall be
carried out at each tap.

Transformer Ratio Test

The performance of a transformer largely depends upon
perfection of specific turns or voltage ratio of transformer. So
transformer ratio test is an essential type test of transformer.
This test also performed as routine test of transformer. So for
ensuring proper performance of electrical power transformer,
voltage and turn ratio test of transformer one of the vital tests.
The procedure of transformer ratio test is simple. We just apply
three phase 415 V supply to HV winding, with keeping LV winding
open. The we measure the induced voltages at HV and LV
terminals of transformer to find out actual voltage ratio of
transformer. We repeat the test for all tap position separately.

Magnetic Balance Test of Transformer

Magnetic balance test of transformer is conducted only on three
phase transformers to check the imbalance in the magnetic
circuit.
Procedure of Magnetic Balance Test of Transformer
1.

First keep the tap changer of transformer in normal

position.

2.
3.

Now disconnect the transformer neutral from ground.

Then apply single phase 230 V AC supply across one of

the HV winding terminals and neutral terminal.

4.
Measure the voltage in two other HV terminals in
respect of neutral terminal.
5.
Repeat the test for each of the three phases.
In case of auto transformer, magnetic balance test of transformer
should be repeated for LV winding also.
There are three limbs side by side in a core of transformer. One
phase winding is wound in one limb. The voltage induced in
different phases depends upon the respective position of the limb
in the core. The voltage induced in different phases of

transformer in respect to neutral terminals given in the table

below.
Left side phase

Central phase

Right side ph

AN

BN

CN

Voltage applied at left side phase

230 V

180 V

50 V

Voltage applied at central phase

115 V

230 V

115 V

Voltage applied at right side phase

50 V

180 V

230 V

Magnetizing Current Test of Transformer

Magnetizing current test of transformer is performed to
locate defects in the magnetic core structure, shifting of windings,
failure in turn to turn insulation or problem in tap changers.
These conditions change the effective reluctance of the magnetic
circuit, thus affecting the current required to establish flux in the
core.
6.

First of all keep the tap changer in the lowest position

and open all IV & LV terminals.

7.
Then apply three phase 415 V supply on the line
terminals for three phase transformers and single phase 230 V
supply on single phase transformers.
8.
9.

Measure the supply voltage and current in each phase.

Now repeat the magnetizing current test of

transformertest with keeping tap changer in normal position.

10.
And repeat the test with keeping the tap at highest
position.
Generally there are two similar higher readings on two outer limb
phases on transformer core and one lower reading on the centre
limb phase, in case of three phase transformers. An agreement to
within 30% of the measured exciting current with the previous
test is usually considered satisfactory. If the measured exciting
current value is 50 times higher than the value measured during
factory test, there is likelihood of a fault in the winding which
needs further analysis.
Caution: This magnetizing current test of transformer is to
be carried out before DC resistance measurement.

Vector Group Test of Transformer

In three phase transformer, it is essential to carry out a vector
group test of transformer. Proper vector grouping in a transformer
is an essential criteria for parallel operation of transformers.
There are several internal connection of three phase transformer
are available in market. These several connections gives various
magnitudes and phase of the secondary voltage; the magnitude
can be adjusted for parallel operation by suitable choice of turn
ratio, but the phase divergence can not be compensated. So we
have to choose those transformer for parallel operation whose
phase sequence and phase divergence are same. All the
transformers with same vector ground have same phase
sequence and phase divergence between primary and secondary.
So before procuring one electrical power transformer, one should
ensure the vector group of the transformer, whether it will be
matched with his or her existing system or not. The vector group
test of transformer confirms his or her requirements.

Insulation Resistance Test or Megger Test

of Transformer
Insulation resistance test of transformer is essential type
test. This test is carried out to ensure the healthiness of over all
insulation system of an electrical power transformer.
Procedure of Insulation Resistance Test of Transformer

11.
First disconnect all the line and neutral terminals of the
transformer.
12.
Megger leads to be connected to LV and HV bushing
studs to measure insulation resistance IR value in between the
LV and HV windings.
13.
Megger leads to be connected to HV bushing studs and
transformer tank earth point to measure insulation resistance
IR value in between the HV windings and earth.
14.
Megger leads to be connected to LV bushing studs and
transformer tank earth point to measure insulation resistance
IR value in between the LV windings and earth.
NB : It is unnecessary to perform insulation resistance test
of transformer per phase wise in three phase transformer.

IR values are taken between the windings collectively as

because all the windings on HV side are internally
connected together to form either star or delta and also all
the windings on LV side are internally connected together
to form either star or delta.
Measurements are to be taken as follows:
For auto transformer: HV-IV to LV, HV-IV to E, LV to E. For two
winding transformer: HV to LV, HV to E, LV to E. Three winding
transformer: HV to IV, HV to LV, IV to LV, HV to E, IV to E, LV to
E.
Oil temperature should be noted at the time of insulation
resistance test of transformer. Since the IR value of
transformer insulating oil may vary with temperature. IR values
to be recorded at intervals of 15 seconds, 1 minute and 10
minutes.
With the duration of application of voltage, IR value increases.
The increase in IR is an indication of dryness of insulation.
Absorption coefficient = 1 minute value/ 15 secs. value.
Polarization index = 10 minutes value / 1 minute value.

Dielectric Tests of Transformer

Dielectric tests of transformer is one kind of insulation test. This
test is performed to ensure the expected over all insulation
strength of transformer. There are several test performed to
ensure the required quality of transformer insulation, dielectric
test is one of them. Dielectric tests of transformer is performed in
two different steps, first one called Separate source voltage
withstand test of transformer, where a single phase power
frequency voltage of prescribed level, is applied on transformer
winding under test for 60 seconds while the other windings and
tank are connected to the earth and it is observed that whether
any failure of insulation occurs or not during the test. Second one
is induced voltage test of Transformer where, three phase
voltage, twice of rated secondary voltage is applied to the
secondary winding for 60 second by keeping the primary of the
transformer open circuited. The frequency of the applied voltage
should be double of power frequency too. Here also if no failure of
insulation, the test is successful. In addition to dielectric tests of

transformer there are other type test for checking insulation of

transformer, such as lightning impulse test, switching impulse test
and partial discharge test.
Induced Voltage Test of Transformer
The induced voltage test of transformer is intended to check the
inter turn and line end insulation as well as main insulation to
earth and between windings15.
Keep the primary

winding

of

transformer

open

circuited.
16.
Apply three phase voltage to the secondary winding.
The applied voltage should be twice of rated voltage of
secondary winding in magnitude and frequency.
17.
18.

The duration of the test shall be 60 second.

The test shall start with a voltage lower than 1/3 the

full test voltage, and it shall be quickly increased up to desired

value.
The test is successful if no break down occurs at full test voltage
during test.

Temperature Rise Test of Transformer

Temperature rise test of transformer is included in type test of
transformer. In this test we check whether the temperature
rising limit of the transformer winding and oil as per specification
or not.
In this type test of transformer, we have to check oil temperature
rise as well as winding temperature rise limits of an electrical
transformer.