Test on power transformers

In this article we define the tests that are performed on power transformers. And we will start by highlighting two indices that are the basic factors for judging the conditions of the transformer insulation: "the absorption index" and "the polarization index" (the higher these indexes, the better the insulation condition results). Let's analyze the following tests:

a) Power factor test: This factor is recommended for the detection of humidity and is the main criterion for judging the isolation of windings of a transformer. Amperis has a wide range of equipment related to the maintenance of transformers.
The power factor will always be the ratio between the losses (milliwatts) and the load (millivoltamperes); and the value obtained will depend only on the humidity and the temperature of the equipment under the conditions in which it is operating (regardless of the thickness or the area of ​​the insulation). Since the temperature of the unit under test, the results vary, the readings must be corrected to a reference Tº of 20ºc, that is why this is an alternating current test. The criteria to be followed to consider an acceptable value of power factor are the following: 0.5% of a new transformer up to 2.0% for a transformer contaminated or degraded in its insulation, always referred to 20ºc. If the value is higher than 2%, an investigation of the cause of the weak insulation is required (for example, penetration of water into the insulating oil).

b) Insulation resistance test: It is the resistance (in Megohms) that offers insulation by applying a direct current voltage for a given time. This test is measured from the application of a constant voltage for the duration of the test. From this test it can be said that a small leakage current results through the insulation of the equipment under test, which is reduced to a constant minimum value.
The minimum values of insulation resistance (at 20ºc) are: for 15.0 KV: 410 MEGOHMS, for 34.5 KV: 930 MEGOHMS and for 230/115 KV: 3100 MEGOHMS.

c)Transformation ratio test. The transformation ratio is defined as the ratio of turns or voltages between the windings of the transformer (primary and secondary). The TTR is the most used method to perform the transformation ratio test. The TTR team consists basically of:

Thanks to the realization of this test, short circuits are detected between turns, polarity, phase sequence, open circuits, etc. Amperis has leading teams in the market to obtain the transformation ratio such as:








Here you can analyze a comparison of the transformation information meters.


d) Ohmic resistance test to windings. This test is performed to calculate the total losses of a transformer, identifying false contacts or points of high resistance in the windings. The test can be applied to power transformers, distribution transformers, instruments, autotransformers, voltage regulators, etc. The device used for this measurement is an ohmmeter with ranges from 10 micro-ohms up to 1999 ohms.
The results of the measurements with this test must be similar between the 3 phases of the windings, if there are divergences, it means that there is a false internal contact of the phase that presents the greatest value, which heats the equipment and eventually produces damage that will force the equipment to be removed from service for repair.


e) High potential or dielectric strength test. Through this test we know the service conditions of the insulating oil. The dielectric strength of the oil is defined as the minimum voltage at which an arc between two metallic electrodes is produced, which gives us information about the capacity of the oil to withstand electrical stresses without failure. We know that there is contamination with water or other contaminants if the dielectric strength is low. The apparatus used to perform the dielectric strength test consists of:

  1. A transformer
  2. A voltage regulator (0-60 kV)
  3. A switch,
  4. A voltmeter
  5. A test cup with two flat electrodes

As a significant fact, the minimum allowed value of dielectric strength for an oil in operation is 25 kV. The resistivity test is basic when investigating a transformer with an abnormally low and stagnant insulation resistance, which may be the cause of low oil resistivity.



Below we list the rules for conducting maintenance tests on substation equipment:


The following are the standards for testing Power Transformers:

The standards for tests for power switches are:

The rules for testing power disconnectors are:


The standards for tests on current transformers are:


Standards for tests on voltage transformers:


Standards for tests on surge arresters:


Standards for relay tests and control: