Transformer Testing

Transformer testing falls into three broad categories: factory testing when the transformer is new or has been refurbished, acceptance testing upon delivery, and field testing for maintenance and diagnostic purposes. Some tests at the factory are common to most power transformers, but many of the factory tests are transformer-specific. Not all of the tests are performed at the factory, and not all of them are performed in the field. Each transformer and each situation is different, requiring its own unique approach and tests.

DC Winding Resistance Measurement

If generation of ethylene, ethane, and perhaps methane in the DGAs indicates a poor connection, winding resistances should be checked. Turns ratio, sweep frequency response analysis (SFRA), Doble tests, or relay operations may give indications that dc testing is warranted. Winding resistances are tested in the field to check for loose connections on bushings or tap changers, broken strands, and high contact resistance in tap changers. Results are compared to other phases in wye-connected transformers or between pairs of terminals on a delta-connected winding to determine if a resistance is too high. Resistances can also be compared to the original factory measurements or to sister transformers. Agreement within 5% for any of the above comparisons is considered satisfactory. If winding resistances are to be compared to factory values, resistance measurements will have to be converted to the reference temperature used at the factory (usually 75 °C). To do this, use the following formula:

Rs = Rm * (Ts + Tk)/(Tm + Tk)

Rs = Resistance at the factory reference temperature (found in the transformer manual);
Rm = Resistance you actually measured;
Ts = Factory reference temperature (usually 75 ºC);
Tm = Temperature at which you took the measurements;
Tk = Constant for the particular metal the winding is made from: 234.5 ºC for copper, 225 ºC for aluminum.


It is very difficult to determine actual winding temperature in the field, and, normally, this is not needed. The above temperature corrections are necessary only if resistance is to be compared to factory values. Normally, phase resistances are compared to each other or to sister transformers at the same temperature, and actual winding temperatures and corrections are not needed. Compare winding resistances to factory values; change in these values can reveal serious problems. A suggested method to obtain an accurate temperature is outlined below. If a transformer has just been de-energized for testing, the winding will be cooler on the bottom than the top, and the winding hot spot will be hotter than the top oil temperature. The average winding temperature is needed, and it is important to get the temperature as accurate as possible for comparisons.

CAUTION: Do not attempt to run an excitation current test immediately after any direct current (DC) test. Energizing with dc will leave a residual magnetism in the core and will ruin the results of the excitation current test.

The most accurate method is to allow the transformer to sit deenergized until temperatures are equalized. This test can reveal serious problems, so it’s worth the effort. Winding resistances are measured using a Wheatstone Bridge for values of 1 ohm or above and using a micro-ohmmeter or Kelvin Bridge for values under 1 ohm. A multi-amp (now AVO) makes a good instrument for these measurements, which is quick and easy to use. Take readings from the top of each bushing to neutral for wye-connected windings and across each pair of bushings for deltaconnected windings. If the neutral bushing is not available on wyeconnected windings, take each one to ground (if the neutral is grounded) or take readings between pairs of bushings as if it were a delta winding. Be consistent each time so that a proper comparison can be made. The tap changer can also be changed from contact to contact, and the contact resistance can be checked. Make sure to take the first test with the tap changer “as found.” Keep accurate records and connection diagrams so that later measurements can be compared.

Core Insulation Resistance and Inadvertent Core Ground Test (Megger®)

Core insulation resistance and core ground test is used if an unintentional core ground is suspected; this may be indicated by the DGA. Key gases to look for are ethane and/or ethylene and possibly methane. These gases may also be present if there is a poor connection at the bottom of a bushing or a bad tap changer contact.

CAUTION: Do not attempt to run excitation or SFRA tests on a transformer immediately after using dc test equipment. Residual magnetism will remain in the core and ruin the excitation current and SFRA test results.

Therefore, this test is only necessary if the winding resistance test shows all connections and tap changer contacts in good condition. The intentional core ground must be disconnected. This may be difficult, and some oil may have to be drained to accomplish this. On some transformers, core grounds are brought outside through insulated bushings and are easily accessed. A standard dc Megger® (1,000-volt Megger® is recommended) is then attached between the core ground lead (or the top of the core itself and the tank [ground]). The Megger® is used to place a dc voltage between these points, and the resistance is measured. A new transformer should read greater than 1,000 megohms. A service-aged transformer should read greater than 100 megohms. Ten to one-hundred megohms is indicative of deteriorating insulation between the core and ground. Less than 10 megohms is sufficient to cause destructive circulating currents and must be further investigated. A solid, unintentional core ground may read zero ohms; this, of course, causes destructive circulating currents and must be corrected before energization. Some limited success has been obtained in “burning off” unintentional core grounds using a dc or ac current source. This is a risky operation, and the current may cause additional damage. The current source is normally limited to a maximum of 40 to 50 amps and should be increased slowly to use as little current as possible to accomplish the task. This should only be used as a last resort and then only with consultation from the manufacturer, if possible, and with others experienced in this task.

CAUTION: This will generate gases which will be dissolved in the oil and will show up in the DGA! Take a sample for DGA with in 72 hours after burning off the unintentional core ground and compare this DGA with the prior one to determine what gases were created by this task.

Doble Tests on Insulation

Doble testing is important to determine the condition of a transformer, because it can detect winding and bushing insulation integrity and problems in the winding and core. Doble tests are conducted in the field on de-energized transformers using special test equipment. Generally, a Doble M-4000 test set is used along with accompanying software. The software automatically performs analysis of test results and responds with a four letter code: G = Good, I = Investigate, D = Deteriorated, and B = Bad. These codes refer to insulation quality. If a “D” or “B” code is encountered, the insulation should be re-tested, carefully investigated, and the problem explained before re-energizing. Other tests may have to be performed; and, perhaps, an internal inspection should be considered before the unit is re-energized. The Doble Company should be consulted, along with the transformer manufacturer, and other transformer experts. If the problem is severe, the unit may have to be taken out of service.

Insulation Power Factor Test

The purpose of this test is to determine the state of dryness of the windings and insulation system and to determine a power factor for the overall insulation, including bushings, oil, and windings. It is a measure of the ratio of the power (I2R) losses to the voltamperes applied during the test. The power factor obtained is a measure of watts lost in the total transformer insulation system, including the bushings. The power factor should not exceed 0.5% at 20 EC. Temperature correction of test results can be performed automatically on the Doble test set. The watts lost should not exceed one-half of one percent of the total power input (voltamperes) from the test. The values obtained at each test are compared to previous tests and baseline factory tests, and a trend can be established as the insulation system ages.

Capacitance Test

This test measures and records the capacitance (including bushings) between the high and low-voltage windings, between the high-voltage winding and the tank (ground), and between the low-voltage winding and the tank (ground). Changes in these values as the transformer ages and events occur, such as nearby lightning strikes or through faults, indicate winding deformation and structural problems, such as displaced wedging and winding support.

Excitation Current Test

The purpose of this test is to detect short-circuited turns, poor electrical connections, core de-laminations, core lamination shorts, tap changer problems, and other possible core and winding problems. On three-phase transformers, results are also compared between phases. This test measures current needed to magnetize the core and generate the magnetic field in the windings. Doble software only gives two indications on this test: “G” for good and “Q” for questionable. On a three-phase, wye/delta or delta/wye transformer test, the excitation current pattern will be two phases higher than the remaining phase. Compare the two higher currents only. If the excitation current is less than 50 milliamps (mA), the difference between the two higher currents should be less than 10%. If the excitation current is more than 50 mA, the difference should be less than 5%. In general, if there is an internal problem, these differences will be greater. When this happens, other tests should also show abnormalities, and an internal inspection should be considered. The results, as with all others, should be compared with factory and prior field tests.

CAUTION: Perform the excitation test before any DC tests. Excitation current tests should never be conducted after a DC test has been performed on the transformer. Results will be incorrect because of residual magnetism of the core left from the DC tests.

Bushing Tests

For bushings that have a potential tap, both the capacitance between the top of the bushing and the bottom tap (normally called C1), and the capacitance between the tap and ground (normally called C2) are measured. To determine bushing losses, power factor tests are also performed. C2 capacitance is much greater than C1. Bushings without a potential tap are normally tested from the bushing top conductor to ground and Ahot collar@ tests. These test results are compared with factory tests and/or prior tests to determine deterioration. About 90% of bushing failures may be attributed to moisture ingress, evidenced by an increasing power factor from Doble testing on a scheduled basis.

Percent Impedance/Leakage Reactance Test

This is normally an acceptance test to see that nameplate percent impedance agrees with the measured percent impedance when the transformer arrives onsite. Normally, a 3% difference is considered acceptable. However, after the initial benchmark test, the percent impedance should not vary more than 2% from benchmark. As the transformer ages or suffers events such as through faults, nearby lightning strikes, and other surges, this test is used in the field to detect winding deformation. Winding deformation can lead to immediate transformer failure after a severe through fault, or a small deformation can lead to a failure years later.
Percent impedance/leakage reactance testing is performed by short circuiting the low-voltage winding and applying a test voltage to the high-voltage winding. Reluctance is resistance to lines of magnetic flux. Reluctance to the magnetic flux is very high in spaces between the high- and low-voltage windings and spaces between the windings and core. Reluctance is very low through the magnetic core so that the vast majority of total reluctance is in the spaces. When winding movement (distortion) occurs, these spaces change. Therefore, the reluctance changes, resulting in a change in the measured leakage reactance. Changes in leakage reactance and in capacitance tests serve as an excellent indicator of winding movement and structural problems (displaced wedging, etc.). This test does not replace excitation current tests or capacitance tests but complement them, and they are used together. The excitation current test relies on reluctance of the core while the leakage reactance test relies on reluctance of the spaces. See Doble’s Leakage Reactance Instrument Users Guide, and IEEE®, Guide for Diagnostic Field Testing of Electric Power Apparatus-Part 1: Oil-Filled Power Transformers, Regulators, and Reactors (IEEE 62-1995™).