Dielectric Breakdown Voltage Testing - Part 1, Standard MethodsTechnical Briefs
What causes bad D1816 dielectric breakdown voltage values? The first article in this series will discuss the three standard methods that SD Myers is equipped to perform, and why we perform them for our customers.
There are two standard methods from ASTM International: D877, Standard Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using Disk Electrodes, and D1816, Standard Test Method for Dielectric Breakdown Voltage of Insulating Oils of Petroleum Origin Using VDE Electrodes. VDE stands for Verband Deutscher Electrotechniker – the Association for Electrical, Electronic, and Information Technologies – a German standards organization similar in function to the IEEE in the United States. The D1816 method uses electrodes specified to conform to the design of a VDE standard, Specification 0370. The third standard method with which we are concerned is IEC Standard 60156, Insulating Liquids – Determination of the breakdown voltage at power frequency – Test method. Standard 60156 uses electrodes that are similar geometrically to the VDE electrodes.
In all three test methods, the electrodes are mounted in a test cell that is filled with the insulating liquid being tested, so that the electrodes are completely covered by the insulating liquid. The test cell is then installed in a dielectric breakdown voltage apparatus so that the electrodes can be charged with an appropriate AC voltage. The AC voltage is increased until breakdown occurs, signified by the operation of automatic circuit interruption equipment included in the dielectric voltage breakdown apparatus. Breakdown is a disruptive discharge from one electrode to the other.
The methods vary with regard to the geometry of the electrodes themselves, as noted above, and also with regard to the gap spacing of the electrodes and rate of rise for the applied AC voltage. A D1816 test cell will be equipped with a two bladed impeller and shaft to provide agitation of the test specimen. A 60156 test cell may also be equipped with a similar impeller, but it is optional for this method.
ASTM D877 is an older method. The electrodes resemble flat coins, and they are spaced 0.1 inches (2.54 mm) apart. Prior to testing using the D877 device, it is imperative to inspect the electrodes for cleanliness, pits, or signs of corrosion, clean and polish them as needed in accordance with the standard method, check the edges of electrodes with a gauge to ensure that the edges are sharp and have not become rounded off, and confirm the gap setting using either round or flat gauges according to the standard method. The rate of AC voltage rise across the electrodes during the determination of dielectric breakdown voltage by the D877 method is 3,000 volts per second.
Electrodes for the D1816 and IEC 60156 methods are similar to each other. The electrodes resemble mushroom caps, each being a segment of a 25 mm radius sphere with the edges rounded to a radius of 4 mm. In the D1816 method, there are two gap settings that may be used, either 1 mm (0.039 inches) or 2 mm (0.079 inches). Similar to the D877 method, the electrodes are inspected prior to the filling of the test cell, and any cleaning or polishing that is needed, along with mandatory confirmation of the gap setting, is also performed prior to the filling of the test cell.
For a D1816 determination, the rate of rise for the AC voltage is 500 volts per second. The impeller is located below the electrodes and operates at 200 to 300 rpm in such a way as to direct the oil flow to the bottom of the test cell. This agitates the sample gently during the dielectric breakdown voltage, and causes particles that may be present to be directed into the gap between the electrodes.
For a dielectric breakdown voltage using the IEC method from standard 60156, 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 above for D1816 except with a rate of 250 to 300 rpm. This 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.
Dielectric breakdown voltage determinations are performed to identify the presence of contaminants in the oil. These include contamination by moisture, particles (including conductive particles, dirt and debris, and insulation particles), and oil aging and oxidation byproducts. Sensitivity of a given method to each of these general classes of contamination as well as interferences and the limitations of the method are considered when evaluating the possible application of a given method and the results obtained.