Oil and Other Field Tests

What other field tests are performed on a transformer?

Oil Tests

In addition to the DGA test, several other tests can be performed on the insulating oil. Each of these tests can be used to reinforce the results from the other tests including the DGA. Some of these tests include:

Dielectric Test – This test measures the overall insulating characteristics of the oil. It is primarily used to detect amounts of free water and particulate contamination in the oil. The results of the dielectric test are reported in KV

Interfacial Tension Test (IFT) – This test measures the ability of oil to resist the absorption of moisture into the oil molecule. The presence of oxidation by-products such as acids and sludge will be indicated with a low IFT. The IFT is reported on dynes per cubic centimeter ( dynes.cm³ )

Neutralization Number ( Acid Number ) – This tests measures the amount of acids  that are contained in the oil. These acids can cause further breakdown of the oil if allowed to remain untreated. The acid number is reported in milligrams of potassium hydroxide needed to neutralized the acids contoined in 1 gram of oil ( mg/KOH/gm )

Color Index – The color index test is a purely visible test. It compares the actual color of the oil to a set of standards. The darker the oil, the more contamination is present. The color index test is reported as a numerical value between .5 and 5.0 with 5.0 being the worst.

Karl Fischer Test ( Moisture-in-Oil ) The Karl Fisher test measures the exact amount of free and dissolved water molecules contained in the oil sample. The Karl Fisher test is reported in a numerical value.

Electrical Tests

In some cases, after performing the standard series of oil tests it is desirable to perform electrical tests on the transformer to substantiate the findings during the oil tests. Plesae note that the following tests require that the unit be taken off line and isolated for a brief of time.

Transformer Turn Ratio Test ( TTR ) –  The TTR test measure the ratio of the turns in the high voltage winding to the low voltage winding. It is used to determine if a turn to turn fault exists that would eventually render the unit useless.

Megger Test – The Megger test verifies the overall condition of the different types of insulation in the windings. The presence of moisture in the transformer oil will significantly degrade the overall insulation capabilities. A low megger test result usually dictates that the unit be removed from service and dried out.

Polarization Index Test ( PI ) – The PI test measures the ability of the insulation to absorb  voltage over a period of time. This gives and indication of the overall insulation quality of the individual pieces of insulation in the transformer. This test is usually performed in conjunction with the Megger Test.

Resistance Measurements - Actual conductor resistances can be measured by the use of an instrument called a Digital Low Resistance Ohmmeter or DLRO. This instrument is a highly accurate measuring device and is capable of measurements as low as .000001 ohm (one millionth of an ohm). The DLRO test is extremely useful in the locating of a bad connection. For example, if a high resistance reading is found while measuring a high voltage winding, it may prove to be that the tap changer is defective or dirty, causing a bad connection. If this condition is not corrected, extreme heat will build up during operation of the transformer and cause one or more components to fail prematurely, in addition to adding to the operating costs of the unit from the added losses.

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