Electrical Test for Mineral Insulating Oil
Dielectric breakdown voltage - ASTM D877 and ASTM D1816
The dielectric breakdown voltage of insulating oil is a measure of its ability to withstand voltage stress without failure. It is the voltage at which breakdown occurs between two electrodes under prescribed test conditions. The test serves primarily to indicate the presence of electrically conductive contaminants in the oil, such as water, dirt, moist cellulosic fibers, or particulate matter. A high dielectric breakdown voltage does not indicate the absence of all contaminants, however.
The electrodes described in ASTM D877 are thin flat disks, which are not representative of the electrodes in transformers. Although the rounded electrodes described in ASTM D1816 do not duplicate the characteristics of insulated electrodes in transformers, they more closely approximate transformer applications. However, the electrodes described in ASTM D1816 are more responsive to particles and dissolved water in oil, both of which are detrimental to the electrical strength of oil in transformers. Therefore, test results in ASTM D1816 furnish a better evaluation of changes that may occur in the oil from transformers.
Two methods are recognized for measuring the dielectric breakdown voltage of insulating oils, as follows:
a) ASTM D877 is recommended for the routine acceptance of new, unprocessed oil from a supplier for use in circuit breakers. This test method uses thin flat-faced cylindrical electrodes with a 2.5 mm gap. The sensitivity of this method, to the general population of contaminates present in a liquid sample, decreases as applied test voltages used in this method become greater than 25 kV rms.
b) ASTM D18l6 is recommended for testing fluid that is being processed into transformers or contained in transformers and load tap changers. This method uses spherically shaped electrodes. The fluid sample is circulated continuously in the test cell throughout the test. The gap distance standard settings are 1 mm and 2 mm.
Dielectric breakdown impulse voltage - ASTM D3300
This test method is most commonly performed using a negative polarity point opposing a grounded sphere (NPS). The NPS breakdown voltage of fresh unused oils measured in the highly divergent field in this configuration depends on oil composition; decreasing with increasing concentration of aromatic, particularly polyaromatic, hydrocarbon molecules.
This test method may be used to evaluate the continuity of composition of oil from shipment to shipment. The NPS impulse breakdown voltage of oil can also be substantially lowered by contact with materials of construction, service aging, and other impurities. Test results lower than those expected for a given fresh oil may also indicate use or contamination of that oil.
Although polarity of the voltage wave has little or no effect on the breakdown strength of oil in uniform fields, polarity does have a marked effect on the breakdown voltage of oil in nonuniform electric fields.
Transient voltages may also vary over a wide range in both the time to reach crest value and the time to decay to half crest or to zero magnitude. The standard lightning impulse test described in IEEE Std C57.12.90-1999 [B4] specifies a 1.2 x 50 µs negative polarity wave.
Dissipation factor (power factor) - ASTM D924
The dissipation factor is a measure of the power lost when an electrical insulating liquid is subjected to an ac field. The power is dissipated as heat within the fluid. A low-value dissipation factor means that the fluid will cause little of the applied power to be lost. The test is used as a check on the deterioration and contamination of insulating oil because of its sensitivity to ionic contaminants.
Gassing of insulating oils under electrical stress and ionization - ASTM D2300
This test measures whether insulating oils are gas absorbing or gas evolving when subjected to electrical voltage. For certain applications, when insulating oils are stressed at high voltage gradients, it is desirable to know the rate at which gas is absorbed or evolved from the oil. The absorption or evolution of gas by a liquid under electrical stress is a function of the aromatic character of the liquid molecules. Liquids that are significantly aromatic in character will absorb gas as they are electrically stressed. Liquids that have little or no aromatic character will evolve hydrogen gas upon application of an electrical voltage. Currently, however, correlation of these test results with equipment performance is limited. Numerical results obtained in different laboratories or by using two different procedures may differ significantly in magnitude, and the results of this method should be considered qualitative in nature.
