Transformer testing & maintenance – full fact list.

Transformer testing is done as per IS-2026, should be carried out on each job as per recommendation by the manufacturer. The test voltage, injected current, insulation resistance measuring tools etc – all must be graded as per the recommendation by the transformer manufacturer.

Transformer being an efficient and vital machine in electrical system, require special care and attention for testing and maintenance.
Though as it does not have any rotating object, just a static machine, transformer is almost over 95% efficient.

Thus transformer does not need frequent testing and maintenance. Only require six month or yearly testing and maintenance..

01. Measurement of winding resistance

This test measures the resistance of the HV & LV winding. The values of resistance should be balance for all three phases and should match the designed values.
Equipment used : Digital resistance meter

02. Measurement of insulation resistance

Measures the insulation resistance of HV & LV windings with respect to earth (body) and between LV & HV winding.

03. Separate source voltage withstand test (High Voltage tests on HV & LV)

This test checks the insulation property between Primary to earth, Secondary to earth and between Primary & Secondary.

HV high voltage test : LV winding connected together and earthed. HV winding connected together and given 28 KV ( for 11KV transformer) for 1 minute.

LV high Voltage test : HV winding connected together and earthed. LV winding connected together and given 3 KV for 1 minute.

Equipment used : High Voltage tester ( 100KV & 3KV)

04. Induced Over voltage Withstand test (DVDF test)

This test checks the inter turn insulation. For a 11KV/433V transformer,866 Volts are applied at the 433V winding with the help of a Generator for 1 minute. This induces 22KV on 11KV side. The frequency of the 866V supply is also increased to 100HZ.
Equipment used : MOTOR GENERATOR SET

05. Measurement of voltage ratio

This test measures the voltage ratio as per the customer’s requirement.V1/V2 = N1/N2. The voltage ratio is equal to the turns ratio in a transformer. Using this principle, the turns ratio is measured with the help of a turns ratio meter. If it is correct , then the voltage ratio is assumed to be correct.
Equipment used : Turns Ratiometer

06. Measurement of NO LOAD LOSS & current.

The iron losses and no load current are measured in this test. The 433V winding is charged at 433V supply & the 11KV winding is left open .The power consumed by the transformer at no load is the no load loss in the transformer. Effect of actual frequency must be taken into account.
Equipment used : Wattmeters or power analyser.


This test measures the power consumed by the transformer when the 433V winding is short circuited and The rated current is passed through the 11KV winding.
Equipment used : Wattmeters or power analyser.

08. Vector Group Verification

This test verifies the Dyn-11 vector group of a distribution transformer.
Equipment used : voltmeter.

09. Oil BDV test.

Oil breakdown voltage is checked as per IS-335.
100 mm L X 70 mm B X 80 mm Ht. glass pot.500ml Oil sample.Spherical electrodes with gap of 2.5 mm.Recommended value : 60KV
Equipment used : OIL BDV TEST SET.

10. Tests on OLTC – on load tap changer(if Attached)

All the dielectric tests check the insulation level of the job.
Impulse generator is used to produce the specified voltage impulse wave of 1.2/50 micro seconds wave
One impulse of a reduced voltage between 50 to 75% of the full test voltage and subsequent three impulses at full voltage.

For a three phase transformer, impulse is carried out on all three phases in succession.
The voltage is applied on each of the line terminal in succession, keeping the other terminals earthed.
The current and voltage wave shapes are recorded on the oscilloscope and any distortion in the wave shape is the criteria for failure.

electrical power transformer
Typical Electrical power transformer – plays the vital role in electrical system

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