ASTM D1816 Test Method for Dielectric Breakdown Voltage of Insulating Oils
ASTM D1816 Standard Test Method for Dielectric Breakdown Voltage of Insulating Oils of Petroleum Origin Using VDE Electrodes
4. Electrical Apparatus
4.1 In addition to this section, use IEEE Standard 4 to determine other requirements necesary for conducting test measurements, and maintaining error limits using alternating voltages. Procedures to ensure accuracy should follow the requirements of IEEE Standard 4. Calibration(s) shall be traceable to national standards and calibration should be verified annually or more often to ensure accuracy requirements. IEEE Standard 4 is required during the manufacturing of the test apparatus and utilized during calibration of the equipment.
4.1.1 Test Voltage - The test voltage shall be an alternating voltage having a frequency in the range from 45 to 65 Hz, normally referred to as power-frequency voltage. The voltage wave shape should approximate a sinusoid with both half cycles closely alike, and it should have a ratio of peak-to-rms values equal to the square root of 2 within more or less 5 %.
4.1.2 Generation of the Test Voltage - The test voltage is generally supplied by a transformer or resonant circuit. The voltage in the test circuit should be stable enough to be unaffected by varying current flowing in the capacitive and resistive paths of the test circuit. Non-disruptive discharges in the test circuit should not reduce the test voltage to such an extent, and for such a time, that the disruptive discharge (breakdown) voltage of the test specimen is significantly affected. In the case of a transformer, the short-circuit current delivered by the transformer should be sufficient to maintain the test voltage within 3 % during transient current pulses or discharges, and a short circuit current of 0.1 A may suffice.
4.1.3 Disruptive Voltage Measurement - Design the measurement circuit so the voltage recorded at the breakdown is the maximum voltage across the test specimen immediately prior to the disruptive breakdown, with an error no greater than 3 %.
4.2 Circuit-Interrupting Equipment - Design the circuit used to interrupt the disruptive discharge through the specimen to operate when the voltage across the specimen has collapsed to less than 100 V. It is recommended that the circuit design limit the disruptive current duration and magnitude to low values that will minimize damage to the electrodes and limit formation of non-soluble materials resulting from the breakdown, but consistent with the requirements of 4.1.2, but in no case should the short-circuit current exceed 1 mA/kV of applied voltage.
4.3 Votage Control Equipment - Use a rate of voltage rise of 0.5 kV/s. The tolerance of the rate of rise should be 5 % for any new equipment manufactured after the year 2000. Automatic equipment should be used to control the voltage rate of rise because of the difficulty of maintaining a uniform voltage rise manually. The equipment should produce a straight-line voltage-time curve over the operating range of the equipment. Calibrate and label automatic controls in terms of rate-of-rise.
4.4 Measuring Systems - The voltage shall be measured by a method that fufills the requirements of IEEE Standard No. 4, giving rms values.
4.5 Connect the electrode such that the voltage measured from each electrode with respect to ground during the test is equal within 5 %.
4.6 Accuracy - The combined accuracy of the voltmeter and voltage divider circuit shall be such that measurement error does not exceed 3 % at the rate-of-voltage rise specified in 4.3. For equipment manufactured prior to 1995 the maximum allowable error is 5 %.
5. Electrodes
5.1 The electrodes shall be polished brass spherically-capped electrodes of the VDE (Verband Deutscher Elektrotechniker, Specification 0370) type having the dimensions shown in Fig. 1 more or less 1 %, mounted with axes horizontal and coincident within more or less 1 mm.
6. Test Cell
6.1 Construct the test cell as a cube. The test cell shall be designed to permit easy removal of the electrodes for cleaning and polishing, verification that the shape is within the specified tolerance, and to permit easy adjustment of the gap spacing. The vector sum of the resistive and capacitive current of the cup, when filled with oil meeting the requirements of Specification D3487, shall be less than 200 µA at 20 kV, at power frequency. A test cell having a capacity of 0.95 L more or less 5 %, has been found to be satisfactory for an electrode spacing of 2 mm. A cell having a capacity of 0.5 L more or less 5 % has been found to be satisfactory for an electrode spacing of 1 mm. Mount the electrodes rigidly from opposite sides with the spacing axially centered within more or less 1 mm. Clearance from the electrodes to all sides, bottom, cover or baffle, and any part of the stirring device is at least 12.7 mm (1/2 in.). Provide the test cell with a motor-driven two-bladed impeller and drive shaft, constructed of a material having high dielectric strength. The two-bladed impeller is 35 mm (1 3/8 in.) more or less 5 % between the blade extremities, having a pitch of 40 mm (1.57 in.) more or less 5 % (blade angle of twenty degrees (20°) more or less 5 %), operating at a speed between 200 and 300 rpm. The impeller, located below the lower edge of the electrodes, rotates in such a direction that the resulting liquid flow is directed downward against the bottom of the test cell. Construct the test cell of a material of high dielectric strength, that is not soluble in or attacked by any of the cleaning or test liquids used, and is nonabsorbent to moisture and the cleaning and test liquids. So that the breakdown may be observed, transparent materials are desirable, but not essential. In order to preclude stirring air with the sample, provide the cell with a cover or baffle that will effectively prevent air from contacting the circulating liquid.
7. Adjustment and Care of Electrodes and Test Cell
7.1 Electrode Spacing - With the electrodes firmly locked in position, check the electrodes with a standard round gage for 2 more or less 0.03-mm (0.079-in.) spacing. If a dielectric breakdown does not occur during any of the consecutive breakdown tests using the 2 mm spacing or the sample is not adequate for the 2 mm spacing test cell a 1 more or less 0.03-mm (0.039-in.) spacing should be used to determine the breakdown voltage and the spacing reported. Flat "go" and "no-go" gages may be substituted having thicknesses of the specified value more or less 0.03 mm for electrode spacing of 1 or 2 mm. If it is necessary to readjust the electrodes, lock the electrodes and check the spacing. For referee tests or tests that will be used for close comparisons, the laboratories shall agree in advance on the spacing for the tests and ensure that all other requirements of this test method are met. The spacing agreed upon shall be measured with the gage that corresponds exactly to the selected spacing within tolerance stated above for the gage.
7.2 Cleaning - Wipe the electrodes and cell clean with dry, lint-free tissue paper, or a clean dry chamois. It is important to avoid touching the electrodes or the cleaned gage with the fingers or with portions of the tissue paper or chamois that have been in contact with the hands. After adjustment of the spacing, rinse the cell with a dry hydrocarbon solvent, such as kerosine or solvents of Specification D235. Do not use a low boiling point solvent, as its rapid evaporation may cool the cell, causing moisture condensation. If this occurs, before using, warm the cell to evaporate the moisture. Avoid touching the electrodes or the inside of the cell after cleaning. After thorough cleaning, flush the cell with new oil of the type to be tested that is filtered through a 5-micron filter and containing less than 25 ppm moisture. Conduct a voltage breakdown test on a specimen of this oil in the manner specified in this test method. If the breakdown voltage is in the expected range for this conditioned oil, the cell is considered properly prepared for testing other samples. A lower than anticipated value is considered as evidence of cell contamination; then repeat the cleaning and the breakdown test with clean dry oil.
7.3 Daily Use - At the beginning of each day's testing, the electrodes shall be examined for pitting and carbon accumulation, and the spacing checked. If the test of any sample is below the breakdown value being used by the operator as a minimum satisfactory value, drain the cell and flush the cell with new oil of the type to be tested that is filtered through a 5-micron filter and containing less than 25 ppm moisture before testing the next specimen. When not in use, keep the cell filled with oil that meets the requirements of Specification D3487 of the type normally tested. Alternatively, the cell may be stored empty in a dust-free cabinet. At the beginning of each days testing, clean according to 7.2.
7.4 Polishing of Electrodes - When electrodes show slight etching, scratching, pitting, or carbon accumulation, they should be removed from the test cup and polished by buffing with jeweler's rouge using a soft cloth or soft buffing wheel. The residue from the buffing should be removed by repeated wiping with lint-free tissue paper saturated with a suitable solvent, followed by solvent rinsing or ultrasonic cleaning. After careful inspection, any electrodes from which pitting cannot be removed by light buffing should be discarded, as more refinishing would destroy the electrode contour and dimensions shown in Fig. 1. Reinstall the electrodes in the test cup and adjust spacing and clean in accordance with 7.1 and 7.2.
8. Sampling
8.1 Obtain a sample of the oil to be tested using appropriate ASTM sampling apparatus. Oil sampling procedures are detailed in Practice D923. Particular reference should be made to the general precaution statement of this test method. The sample shall be taken in a dry, clean, non-permeable bottle. Tightly seal and shield from light until ready to be tested. Plastic bottles are permeable and moisture content of the sample may change resulting in a measurable difference when compared to samples collected in non-permeable containers.
