Transformer Inspection

Condition assessment or life assessment of a transformer should use a variety of techniques. These techniques encompass electrical testing, a physical assessment, review of historical design and test data, review of future load and use requirements, oil analysis, and others. No one technique will be able to accurately assess the condition, and they all must be used in concert to provide a good evaluation.



Our services focus on electrical testing as follows:

A. Test on Insulation - Power Factor / Tangen Delta

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 volt-amperes 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 C (Doble Standard). 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. In conjunction with tangent delta testing is usually perform some testing as follows:

A.1 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.

A.2 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; one is “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 milliampere (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.

A.3 Bushing Test

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.


B. Turn Ratio Test

Measurements are made by applying a known low voltage across one winding and measuring the induced voltage on the corresponding winding. The low voltage is normally applied across a high voltage winding so that the induced voltage is lower, reducing hazards while performing the test. The voltage ratio obtained by the test is compared to the nameplate voltage ratio. The ratio obtained from the field test should agree with the factory within 0.5%. New transformers of good quality normally compare to the nameplate within 0.1%.
For three-phase delta/wye or wye/delta connected transformers a three-phase equivalency test should be performed. The test is performed and calculated across corresponding single windings. Look at the nameplate phasor diagram to find out what winding on the primary corresponds to a particular winding on the secondary. Calculate the ratio of each three-phase winding based on the line to neutral voltage of the wye winding. Divide the line-to-line winding voltage by 1.732 to obtain the correct line-to-neutral voltage. Check the tap changer position to make sure it is set at the position on which the nameplate voltage is based. Otherwise, the turns ratio test information cannot be compared with the nameplate. Nameplate information for Reclamation transformers is normally based on the tap 3 position of the tap changer. See the manufacturer’s instruction manual for the specific turns ratio tester for details.


C. Winding Resistance Test

Winding resistances are measured using a Wheatstone Bridge for values 1 ohm or above and using a micro-ohmmeter or Kelvin Bridge for values under 1 ohm. An instrument made by AVO is available for loan to facilities. Take readings from the top of each bushing to neutral for wye connected windings and across each pair of bushings for delta connected windings. If the neutral bushing is not available on wye 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 good 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.


D. Sweep Frequency Response Analysis Test

These tests show, in trace form, the winding transfer function of the transformer and are valuable to determine if any damage has occurred during shipping or during a through fault. Core grounds, core displacement, and other core and winding problems can be revealed by this test. These tests should be conducted before and after the transformer has been moved or after experiencing a through fault. Results should be compared to baseline tests performed at the factory or as soon as possible after receiving the transformer.