Transformer Testing | Type Tests and Routine Tests of Transformer

Type tests, routine tests and special tests confirm the specification and performance of an electrical power transformer. Some of those tests are done at the manufacturer’s premises before the transformer is delivered.

Transformer manufacturers run two main factory groups: type tests on a representative unit and routine tests on every unit.

Some transformer tests are also done at the user’s site before commissioning, and later on a regular or emergency basis through the service life of the unit.

Type of Transformer Testing

Tests done at factory

1. Type tests
2. Routine tests
3. Special tests

Tests done at site

1. Pre-commissioning tests
2. Periodic/condition monitoring tests
3. Emergency tests

Type Test of Transformer

To show that the transformer meets the customer’s specification and design, the unit is tested at the manufacturer’s premises. Some tests confirm the basic design of that transformer. Those tests are done mainly on a prototype unit, not on every unit in a lot. A Type test of transformer confirms the main design of a production lot. IEC 60076-1 treats type tests as tests on a representative transformer of the type.

Routine Tests of Transformer

Routine tests of transformer confirm the operational performance of each individual unit in a production lot. Routine tests are carried out on every unit manufactured, as in IEC 60076-1.

Special Tests of Transformer

Special tests on transformers are agreed with the purchaser to obtain extra information useful during operation or maintenance of the transformer.

Pre Commissioning Test of Transformer

The transformer also goes through further tests after it is installed and before it is put into service. Those tests are the pre-commissioning tests of the transformer. They check the condition of the transformer after installation and compare the low voltage results with the factory test reports.

Type tests of transformer include:

1. Winding resistance test of transformer
2. Transformer ratio test
3. Transformer vector group test
4. 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. Measurement of insulation resistance
7. Dielectric tests of transformer
8. Temperature rise test of transformer
9. Tests on on-load tap-changer
10. Vacuum tests on tank and radiators

Routine tests of transformer include

1. Winding resistance test of transformer
2. Transformer ratio test
3. Transformer vector group test
4. 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. Measurement of insulation resistance
7. Dielectric tests of transformer.
8. Tests on on-load tap-changer.
9. Oil pressure test on transformer to check against leakages past joints and gaskets

On this page, routine tests include all the type tests listed above except the temperature-rise and vacuum tests. An oil-pressure test is also done to check for leaks at joints and gaskets. IEC 60076-1 lists temperature-rise as a type test and does not treat every dielectric test the same way on every unit.

Special Tests of transformer include

1. Dielectric tests.
2. Measurement of zero-sequence impedance of three-phase transformers
3. Short-circuit test
4. Measurement of acoustic noise level
5. Measurement of the harmonics of the no-load current.
6. 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 Winding Resistance Measurement

Transformer winding resistance measurement gives I²R losses and the winding temperature at the end of a temperature-rise test. Factories run it as a type test and as a routine test. Site crews also run it to check the winding circuit for loose connections, broken strands, high contact resistance in tap changers, and high voltage leads and bushings.

There are different methods for measuring transformer winding resistance, as follows:

• 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.

Winding resistance also depends on tap position, so it is measured at every tap.

Transformer Ratio Test

Transformer performance depends on the designed turns or voltage ratio. The transformer ratio test is therefore a type test of transformer and is also done as a routine test. For an electrical power transformer, the voltage and turns-ratio test checks that the ratio is correct at each tap.

A common site method is to apply a three-phase 415 V supply to the HV winding with the LV winding open. Induced voltages at the HV and LV terminals give the actual voltage ratio. The test is repeated at every tap. Factory ratio tests may use a dedicated ratio meter instead of a 415 V supply.

Magnetic Balance Test of Transformer

A magnetic balance test of a transformer is done only on three-phase transformers to check imbalance in the magnetic circuit.

Procedure of Magnetic Balance Test of Transformer

1. Keep the tap changer of transformer in normal position.
2. Now disconnect the transformer neutral from ground.
3. 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.

For an autotransformer, the magnetic balance test should be repeated on the LV winding as well.

A three-phase core has three limbs side by side. One phase winding is wound on each limb. The voltage induced in each phase depends on the position of that limb in the core. Induced voltages of the phases with respect to the neutral are shown in the table below.

Magnetizing Current Test of Transformer

The Magnetizing current test of transformer is used to find defects in the magnetic core, shifted windings, failed turn-to-turn insulation or tap-changer problems. Those conditions change the effective reluctance of the magnetic circuit and so change the current needed to establish flux in the core.

1. Keep the tap changer in the lowest position and open all IV and LV terminals
2. 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
3. Measure the supply voltage and current in each phase
4. Now repeat the magnetizing current test of transformer test with keeping tap changer in normal position
5. Repeat the test while keeping the tap at highest position

On a three-phase transformer the two outer limbs usually give two similar higher readings and the centre limb a lower reading.

Agreement to within 30% of a previous exciting-current measurement is a common site rule of thumb. If the measured exciting current is about 50 times the factory value, a winding fault is likely and needs further analysis. Those bands are not IEC 60076 pass/fail limits.

Caution: This magnetizing current test of a transformer is to be carried out before DC resistance measurement.

Vector Group Test of Transformer

On a 3 phase transformer, a vector group test is required. Matching vector groups is a condition for parallel operation of transformers.

Three-phase transformers are built with several internal connections. Those connections give different secondary voltage magnitudes and phase displacements. Magnitude can be matched for parallel operation by the turns ratio, but a phase displacement cannot be cancelled by ratio alone.

So a transformer must be chosen whose vector group has the same phase sequence and phase displacement as the existing system. Transformers of the same vector group have the same phase sequence and the same phase displacement between primary and secondary.

Before buying an electrical power transformer, check that its vector group matches the existing system. The vector group test confirms that match.

Insulation Resistance Test or Megger Test of Transformer

The Insulation resistance test of transformer is a type test and is also used in service. It checks the condition of the overall insulation of an electrical power transformer.

Procedure of Insulation Resistance Test of Transformer

1. Disconnect all the line and neutral terminals of the transformer
   
2. Megger leads to be connected to LV and HV bushing studs to measure insulation resistance IR value in between the LV and HV windings
3. 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
4. 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.

Take the measurements as follows:

• For autotransformer: 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 transformers: 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 the 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 a transformer check insulation strength. Several tests cover insulation quality; the dielectric test is one of them. Dielectric tests are done in two steps.

The first is the separate-source voltage withstand test, where a single-phase power-frequency voltage of the prescribed level is applied to the winding under test for 60 seconds while the other windings and the tank are earthed, and the insulation is watched for failure.

The second is the induced-voltage test of the transformer, where a three-phase voltage of twice rated secondary voltage is applied to the secondary winding for 60 seconds with the primary open-circuited. IEC 60076-3 also raises the frequency so the core is not overfluxed; duration then depends on that frequency.

The frequency of the applied voltage is also typically double power frequency in this teaching method. If the insulation does not fail, the test is successful.

Other insulation tests on a transformer include the lightning-impulse test, the switching-impulse test and the partial-discharge test.

Induced Voltage Test of Transformer

The induced-voltage test of the transformer is intended to check the inter-turn and line-end insulation as well as main insulation to earth and between windings:

1. Keep the primary winding of transformer open circuited.
2. Apply three-phase voltage to the secondary winding. The applied voltage should be twice of the rated voltage of secondary winding in magnitude and frequency.
3. The duration of the test shall be 60 seconds.
4. The test shall start with a voltage lower than 1/3 the full test voltage, and it shall be quickly increased up to the desired value.

The test passes if no breakdown occurs at full test voltage.

Temperature Rise Test of Transformer

The Temperature rise test of transformer is a type test of transformer. It checks whether the temperature-rise limits of the winding and oil meet the specification. Both oil temperature rise and winding temperature rise are measured.