Preparing the test vessel for BDV testing
Successful dielectric breakdown voltage testing depends not only on obtaining a good sample, as discussed in the previous section, but also on ensuring that the test vessel is properly prepared. The preparation of the test vessel can be divided into two key elements - the first is storing, cleaning and filling, and the second is setting the electrode gap.
9.1 Storing and cleaning test vessels
IEC 60156 recommends that a separate test vessel assembly is used for each type of insulating fluid that it is required to test. This standard requires that the test vessels are filled with dry insulating fluid of the type that they will be used to test, then covered and stored in a dry place. ASTM offers an alternative option of storing the vessels empty in dust-free cabinet.
Immediately prior to testing, vessels stored full must be drained and then all internal surfaces, including the electrodes, rinsed with fluid taken from the sample to be tested. The vessel should then be drained again, and carefully filled with the test sample, taking particular care to avoid the formation of bubbles.
If the vessel was stored empty, or if it is to be used for a different type of fluid from that with which it was filled during storage, it should be cleaned with an appropriate solvent before the rinsing and filling procedures described above are followed. ASTM D1816 specifies the use of a dry hydrocarbon solvent such as kerosene, which meets the requirements of D235. Solvents with a low boiling point should not be used as these evaporate rapidly, cooling the vessel and giving rise to the risk of condensation. Solvents commonly used include acetone and, in the USA, toluene. Toluene is banned in Europe.
Use lint-free clean-room wipes to clean the vessel. Do not use paper towels as they may introduce particles that hold moisture, causing breakdown values to be dramatically reduced. Touching the electrodes or the inside of the vessel should be avoided and during cleaning, the electrodes should be checked for pitting or scratches that may cause breakdown voltage values to be decreased.
9.2 Setting the electrode gap
Setting the electrode gap accurately is very important, as the results obtained are only valid if the gap is correct. A big problem is movement of the electrodes after the gap has been set and for this reason, many users of oil test sets check the electrode gap frequently - sometimes before every test. A better solution is to use test sets where the electrodes can be locked in position.
It recommends the use of flat, smooth gap gauges. Our latest gauges have a black anodized coating, which not only provides a smooth surface but also shows when the gauge is worn, as the shiny aluminium starts to show through the coating.
9.3 Hints and tips for vessel preparation
(1) If rinsing the test vessel with the sample oil before testing, it is most important to immediately fill the test vessel with the oil sample to be tested. Any significant delay will result in the oil film on the vessel's walls absorbing water from the air, and since the walls have a large surface area, this will contaminate the oil sample and reduce the breakdown voltage once it has been mixed with the sample.
(2) Pour the oil into the vessel swiftly with minimum turbulence so as not to entrap air.
(3) Allow the sample to stand for a few minutes before the testing to allow air bubbles to clear.
(4) Do not leave the sample in the vessel to stand for too long before testing as it will absorb water from the air in the headspace above it. This will reduce the breakdown voltage.
(5) If you are using an impeller stirrer that utilises a baffle plate to exclude air from the oil sample ensure that:
- Oil does not pass over the upper surface of the baffle plate
- Oil is in full contact with the underside of the baffle plate
(6) The use of a magnetic bead for IEC 60156 will circulate oil in the lower portion of the test vessel, whereas the impeller will circulate all of the oil in the test vessel. The magnetic bead therefore has the advantage that moisture absorbed by oil in contact with air is not stirred into the sample, avoiding unwanted contamination.
(7) Remember that the rules of cleaning and preparing the vessel also apply to the magnetic bead, impeller, baffle plate and electrodes, not just the vessel walls.
(8) When performing continuous testing of many oil samples, such as in laboratory environments it is important to clean or rinse the test vessel between every sample tested.
(9) Always refer to the appropriate test standard to ensure the preparation is performed as specified.
