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:

  • A reference transformer with adjustable ratio from 0 - 130
  • A source of excitation of alternating current
  • A voltmeter
  • An ammeter
  • A galvanometer current detector null
  • As well as a set of terminals for your connection.

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:

  • ASTM D3487-88, Specification for mineral insulating oil used in electrical appliances.
  • ASTM D 923-91, Test method for the sampling of electrical insulating liquids.
  • ASTM D3612-93, Test method for the analysis of gases dissolved in electrical insulating oil by gas chromatography.
  • ASTM D1816-97, Standard test method for the dielectric breakdown voltage of insulating oils of petroleum origin using VDE electrodes.
  • ASTM D924-92 (b), Test method for the dissipation factor (or power factor) and the relative permeability (dielectric constant) of electrical insulating liquids.
  • ASTM D971-91, Test method for the interfacial tension of the oil against water by the ring method.
  • ASTM D974-92, Test method for the neutralization number by titration of the color indicator.
  • ASTM D1500-91, Test method for ASTM Color of Petroleum Products (ASTM color scale).
  • ASTM D 1298-85, Practice for density, relative density (specific gravity) or API gravity of crude oil and liquid petroleum products by hydrometer method.
  • ASTM D1524-84, Method for the visual examination of used electric insulating oils of petroleum origin in the field.
  • ASTM D 2285-85, Test method for the interfacial tension of electrical insulating oils of petroleum origin against water by the method of weight loss.
  • ASTM D1533-88, Test method for water in insulating liquids (Karl Fischer Method).
  • ASTM D 3612-93, Test method for the analysis of gases dissolved in electrical insulating oil by gas chromatography.
  • ASTM D 3613-92, Sampling test methods for electrical insulating oils for gas analysis and determination of water content.
  • ASTM D 5837-99 (2005), Standard test methods for furan compounds in electrical insulating liquids by high performance liquid chromatography (HPLC).
  • Measurement of the transformation ratio, polarity verification and phase relation according to ICONTEC Standard No. 471.
  • Measurement of ohmic resistance of windings in accordance with ICONTEC Standard No. 375.
  • Measurement of the earthing resistance of each transformer (RETIE Chapter 15.)
  • Check and measure lightning conductor earthing - RETIE Chapter 15.
  • IEEE-400.1- 2001 Field tests to measure the insulation of isolated cables.
  • IEC - International Electrothecnical Commission.


The following are the standards for testing Power Transformers:

  • Visual Examination Standard ASTM D-1524.
  • Dielectric Strength Standard ASTM D-877.
  • Colorimetric Index ASTM D-1500 standard.
  • Neutralization Number ASTM D-974.
  • Interfacial Tension ASTM D-971 Standard.
  • Water Content ASTM D-1533 Standard.
  • Specific Gravity ASTM D-1298 Standard.
  • Chromatographic analysis of the oil according to ASTM D-3612-93.
  • Transformation ratio and Polarity; ANSI / IEEE C57.12.91.
  • Winding resistance; ANSI / IEEE Std. 62-1995.
  • Excitation Current; ANSI / IEEE Std. 62-1995.
  • Impedance; ANSI / IEEE Std. 62-1995.
  • Power factor and capacitance of the windings; ANSI / IEEE Std. 62-1995.
  • Insulation resistance; ANSI / IEEE C57.12.91.
  • Sweep frequency response. (FRA); IEEE C57-159 / D5.
  • Physical - Chemical (According to IEEE C57.106-2006).
  • Furan compounds (According to ASTM D5837 standard, IEC 61198).
  • Gases Dissolved by the Chromatography process (According to IEC 60599).
  • Analysis of PCBs (According to ASTM-D4059 standard) and Inhibitor Content (According to ASTM-D2668 standard).
  • Dielectric tests. (IEC-60076-3).
  • Measurement of noise level. (IEC-60076-10).

The standards for tests for power switches are:

  • High voltage switches - Part 1; Switches for nominal voltages greater than 1 kV and lower than 52 kV "- IEC 62271-103 ed1.0 (2011-06).
  • High voltage switches - Part 2; High voltage switches for nominal voltages of 52 kV and above. "- IEC - 62271-104 ed1.0 (2009-04)
  • Specification of technical grade sulfur hexafluoride (nf6) for use in electrical equipment. - IEC - IEC - 60376 - 2006
  • Switch High voltage distribution and control device - Part 101: Synthetic tests. - IEC 62271 - 101 - 2006 15.3

The rules for testing power disconnectors are:

  • AC disconnectors (disconnectors) and earthing switches - IEC 62271-102
  • High voltage switches. - IEC 62271-103 ed1.0 (2011-06).


The standards for tests on current transformers are:

  • Measuring transformers "part 6:" Requirements for protection current transformers for transient performance. "- IEC 61869-2 ed1.0 (2012-09).


Standards for tests on voltage transformers:

  • "Instrument transformers", part 3: "Inductive voltage transformers". - IEC 61869-3 ed1.0 (2011-07)
  • "Instrument transformers", part 5: "Capacitor voltage transformers". IEC 61869-5 ed1.0 (2011-07)


Standards for tests on surge arresters:

  • Surge arresters - Part 4: Metal oxide surge arresters without spaces for a.c. systems. - IEC 60099- 2007.


Standards for relay tests and control:

  • Electrical relays- IEC 60255- 2006
  • Telecontrol equipment and systems- IEC 60870 - 2011
  • Electromagnetic compatibility (ECM) - IEC 61000-2003