Very high levels for furan test results may indicate substantial damage to the insulating system. The calculations that we perform to estimate the condition of the solid insulation by using the furans analysis results is performed in a two-step approach to estimate a DP and calculate insulation life remaining.
There are oil processing and transformer maintenance procedures that will significantly change the content of furans. If the unit is subjected to reclamation/regeneration using an adsorbent system such as Fuller’s Earth, alumina, or one of the other reclaiming adsorbents that can be reactivated, the degree to which furans will be removed will depend on how well the reclaiming job was performed.
There are three methods for dielectric breakdown voltage testing. The oldest, the flat disk method, is not very sensitive to the presence of moisture and not sensitive to changes in moisture content, unless the percent saturation of the oil is greater than 60 percent. The method is also not sensitive to the aging and oxidation of the oil. In spite of the fact that the application of this method may be limited in mineral oil filled transformers with regard to moisture increases and oil aging, D877 dielectric breakdown voltage determinations continue to yield important information for a wide variety of equipment types and insulating liquid types – including oil filled transformers. Therefore, SDMyers continues to recommend the test for purposes of providing that information.
The ASTM D1816 standard method of measuring dielectric breakdown voltage uses spherical VDE electrodes. This method is run at one of two gap setting: 1 mm or 2 mm. Because of the greater sensitivity, the rate of voltage rise is lower. Also, the D1816 test cell has a motor driven agitator that runs during the test to cause the oil to flow between the electrodes, carrying suspended particles into the gap between the VDE spheres where they can affect the breakdown voltage.
The D1816 dielectric breakdown voltage is a more sensitive method of dielectric breakdown voltage testing and is generally more useful than the D877 testing method under many circumstances. This method is sensitive to dissolved gases in the oil. As a result, high dissolved gas content in the insulating oil sample may depress the D1816 value to the point where it is outside the acceptable range. In short, oil that is acceptable in every respect that affects its performance in electrical equipment may still "fail" the D1816 determination because of dissolved gas content.
When D1816 test results are outside of the acceptable range, the first consideration is to cross reference the results to moisture, liquid power factor, and liquid screen tests to identify possible causes.
D1816 Dielectric Breakdown Voltage Uses VDE Electrodes to evaluate new oil. The minimum D1816 values that are acceptable are 20 kV for the 1 mm gap setting and 36kV for the 2 mm gap setting. D1816 oil results that do not meet the minimum for the selected gap should be rejected.
The IEC method from Standard 60156 including values used in IEC standards for unused and in-service mineral oil uses electrodes that are similar geometrically to the VDE electrodes used in ASTM D1816. The spherical electrodes are spaced 2.5 mm apart, and the rate of voltage increase is 2,000 volts per second. The method in Standard 60156 allows the optional use of an impeller, operating in similar fashion to the one described for D1816, except that it operates at 250 to 300 rpm. The IEC method also allows use of a magnetic stirrer operating at a similar rate, if there is no significant chance that magnetic particles will be removed from the oil. The presence of magnetic particles would affect dielectric breakdown in the transformer, so removal of those particles by the stirrer during the analysis would yield unrepresentative values.
In Part 1 of a two-part series on PCB testing, we will present some background information on PCB's, address when PCB testing is appropriate, and describe how a typical laboratory may perform PCB testing.
In Part 2 of the PCB series, PCB regulations, and the classification of electrical equipment based on PCB test results is discussed.