Technology Literature

What Tests Must be Done before the Transformer Leaves the Factory?

Before the transformer leaves the factory, the transformer manufacturer will conduct certain tests on the quality, function and service life of the transformer. General transformer experiments include two types, factory test and type test. A representative test carried out by the transformer type laboratory to verify whether the transformer conforms to the specified technical conditions. So, what tests are performed on the transformer before it leaves the factory?

Transformer ratio, group, direct resistance, no-load, load, capacity, power frequency withstand voltage, inductive withstand voltage, insulation resistance, dielectric loss, partial discharge, impulse voltage, etc.

1. Transformer winding insulation resistance testing

Insulation resistance can reflect the insulation state of electrical equipment to a certain extent (such as the dampness of the insulating medium and the degree of surface cleanliness)

High to low and ground: (primary winding to secondary winding and casing) insulation resistance of high voltage winding to low voltage winding and casing;

Low to high and ground: (secondary winding to primary winding and casing) insulation resistance of low-voltage winding to high-voltage winding and casing

2. On-load tap-changer testing

The on-load tap-changer is the only moving part connected to the transformer circuit, so the detection of the on-load tap-changer is very important. The test requires checking the action sequence of the on-load tap-changer, measuring the switching time, and the measurement content includes the on-load tap-changer The transition waveform, transition time, instantaneous transition resistance value, three-phase synchronicity, etc.

3. DC winding resistance testing

DC resistance is the resistance exhibited by the component when DC is applied, that is, the inherent static resistance of the component. For example, when the coil is connected to direct current and alternating current, its resistance is different. When connected to alternating current, in addition to DC resistance, the coil also has reactance, which reflects the synergistic effect of resistance and reactance, which is called impedance.

4. Short circuit resistance testing

The short-circuit impedance of the transformer means that when the secondary side is short-circuited, a voltage is added to the primary side to make the current reach the rated value. The ratio of this voltage to the rated voltage is the short-circuit impedance.

5. Withstand voltage testing

It can effectively find the overall defects of insulation, such as moisture and dirt, and local defects of insulation can be found through the relationship between current and leakage current. Since the voltage is divided by the insulation resistance at DC voltage, end insulation defects are found more efficiently than at AC voltage.

6. No-load/load (power factor) testing

The no-load test of the transformer is to apply the rated voltage from any set of coils of the transformer, and measure the no-load loss and no-load current of the transformer when the other coils are open. The no-load current is expressed as a percentage of the rated current.

The load of the transformer is mainly to ensure that the transformer can still operate normally under the condition of exceeding the expected load.

7. Transformer Turns Ratio testing

There is a polarity and transformation ratio relationship between the windings of the transformer. When several windings need to be connected to each other, the polarity must be known to make the connection correctly. If the transformer ratio and wiring group are inconsistent, there will be unacceptable circulating currents. Therefore, when the transformer is tested at the factory, the purpose of checking the transformation ratio, polarity and wiring group of the transformer is to check the correctness of the winding, the number of turns, the connection of the lead and the tap lead, the position of the tap changer and the marking of each outgoing terminal. For the installed transformer, it is mainly to check whether the position of the tap changer and the marking of each outgoing terminal are correct compared with the nameplate of the transformer; when the transformer fails, check whether there is an inter-turn short circuit in the transformer, etc.

The voltage difference of each tap position of the winding is not more than ±0.5% (rated tap position), and the voltage difference is not more than ±1.0% compared to the initial value.

8. Temperature rise (heat-run)testing

This test is mainly to check the structural performance of the transformer, specifically to check whether the transformer can be cooled quickly, that is, whether the heat generated by the total loss during the operation of the transformer can be quickly dissipated, and whether it meets the requirements of the IEC standard. Temperature rise, as well as the limit value of winding temperature rise, also check whether some other components have local overheating phenomenon, such as iron core, fuel tank and structural parts, etc. Generally, after insulation, loss, voltage ratio and DC resistance tests, according to the nameplate data or relevant regulations.

9. Transformer oil breakdown voltage test(oil BDV testing)

Transformer oil breakdown voltage test(oil BDV test) is one of the chemical analysis methods for transformer oil. It is measured by AC high voltage breakdown method, which is a necessary measurement item for transformers in new installation, overhaul or preventive test requirements.

10. Transformer Oil Tan Delta Test(oil dielectric loss testing)

Dielectric loss refers to the sum of polarization loss and conductivity loss caused by transformer oil under the action of alternating electric field

Through the dielectric loss factor test, it can reflect the operating status of the transformer oil.

The test items are required for transformer manufacture (with corresponding tester types):

Test Object

Testing Item

Tester Type

Tester Link


Insulation Resistance of Winding



DC Leakage Testing in 1min



Winding Resistance Testing



Winding Deformation Testing



Heat run Testing



Turns Ratio Testing



Tap Changer Dynamic Testing



Short Circuit Impedance Testing



No-load Power Factor Testing



Dan Delta Testing



Oil Breakdown Voltage (BDV)Testing



Oil Tan Delta Testing



Degaussing analyzer