This article examines the need for careful preparation of the vessels used in the testing of transformer dielectric fluids to ensure that reliable results are achieved. It discusses the factors that most commonly lead to unexpectedly low values for dielectric breakdown and gives practical information about techniques that will help eliminate moisture and other forms of contamination.
Sources of unexpected results
When unexpected results are obtained while testing the dielectric breakdown voltage of transformer dielectric fluids, the cause can often be traced to inadequate preparation of the test vessel. In particular, it is likely that insufficient attention has been given to one or more of the following key elements of vessel preparation:
Neglect of any of these elements has the potential to cause an unexpected drop in the measured breakdown voltage. It is essential, therefore, to ensure that all of these factors have been carefully considered and correctly implemented. To help users of dielectric fluid test sets to achieve this, each of the factors will now be examined in more detail.
Vessel storage and cleaning
IEC 60156 recommends that a separate test vessel assembly (Figure 1) be used for each type of dielectric fluid that is to be tested. This standard requires that test vessels are filled with dry dielectric fluid of the appropriate type, then covered and stored in a dry place. ASTM proposes the alternative option of storing the empty vessels in a dust-free cabinet. Immediately prior to testing, vessels that have been stored full of fluid must be drained and all internal surfaces, including electrodes, rinsed with fluid taken from the sample to be tested. The vessel must then be drained again, and carefully filled with the test sample, taking particular care to avoid the formation of bubbles. Vessels that have been stored empty, and those that are to be used for testing a different type of fluid from that which they were filled with during storage must be cleaned with an appropriate type of solvent before they are rinsed and filled.
ASTM D1816 specifies the use of a dry hydrocarbon solvent, such as kerosene (paraffin) that meets the requirements of D235. Solvents with a low boiling point should not be used as these evaporate rapidly, cooling the vessel and increasing the risk of condensation. Solvents commonly used include acetone and, in the USA, toluene. The use of toluene is, however, banned in Europe. Only lint-free clean-room wipes should be used for cleaning the vessel. Paper towels are not an acceptable substitute as they may introduce particles that hold moisture, causing the breakdown voltage of the fluid to be dramatically reduced. Touching the electrodes or the inside of the vessel should be avoided during cleaning, and electrodes should be checked for pitting or scratches that may result in the measured breakdown voltage being reduced. Keep in mind that these cleaning guidelines apply to all parts that will come into contact with the fluid sample during testing.
Setting the electrode gap
Setting the electrode gap accurately is very important; results are only valid if the gap is set correctly. A common problem is movement of the electrodes after the gap has been set and, for this reason, many users of dielectric fluid test sets check the electrode gap frequently - in some cases before each test. While effective, this procedure can be inconvenient and time consuming.
A better approach is to use a test set where the electrodes can be locked in place after setting. For setting the electrodes, the use of flat, smooth gap gauge is recommended (Figure 2). The best gauges have a black anodized coating that not only provide very smooth surface but also show when the gauge is worn, as the base metal of the gauge starts to become visible through the coating.
Rinsing and filling the test vessel
Before filling the test vessel, it is essential to rinse it with clean fluid or with fluid taken from the sample to be tested. Rinsing should always be carried out before each test, even when repetitive testing is being carried out in a laboratory. When rinsing the vessel, attention must be given not only to the vessel itself, but also to the impeller, magnetic bead, baffle plate, lid and electrodes. Rinsing should be applied to all surfaces that will come into contact with the fluid sample during testing. After the test vessel and associated components have been rinsed, it is absolutely essential for the vessel to be filled immediately with the fluid sample that is to be tested. In fact, ASTM D1816 specifies that the test vessel must be filled with the sample within 30 seconds of rinsing. This is because any significant delay - even a few minutes - can result in the film of fluid on the vessel walls absorbing water from the air. Since the vessel walls have a comparatively large surface area, this is likely to contaminate the fluid sample and reduce its breakdown voltage. In this context, it is worth noting that just 30 parts per million of water in the fluid will halve its breakdown voltage. When filling the test vessel, the fluid sample should be poured into the vessel swiftly while taking care to minimise turbulence, as this will entrap air. After filling, the vessel should be allowed to stand for a few minutes before testing to give time for any air bubbles to clear. It is, however, important not to extend the standing time unduly as, if this is done, the sample may absorb water from the air in the headspace above it, which once again will reduce the breakdown voltage. As soon as a visual inspection shows that the bubbles have cleared, the baffle or lid should be fitted to prevent further contact between the sample and the air.
Protection of the sample against undue contact with the air
When using an impeller-type stirrer that employs a baffle plate to protect the fluid sample against contact with the air, it is essential to ensure that the sample does not pass over the upper surface of the baffle plate. It is, however, also important that the fluid sample is in full contact with the underside of the baffle plate (Figure 3). As specified in the test standards, this will prevent moisture being absorbed from contact between the circulating fluid and the air.
60156 is recommended whenever possible, as it will circulate the fluid in the lower part of the test vessel, whereas an impeller will circulate all of the fluid in the vessel. The magnetic bead therefore has the advantage that any moisture absorbed by contact between the surface of the fluid and the air is not stirred into the sample, which reduces the risk of contamination. If an impeller must be used, it is vital that air is prevented from coming into contact with the surface of the fluid by fitting the baffle plate correctly and ensuring that the fluid level is sufficient to make contact with the bottom of the plate without flowing over the top surface.
When testing to ASTM D1816
ASTM D1816 specifies that the fluid must be stirred throughout the test, using a two-bladed motor-driven impeller. The standard prescribes the impeller dimensions and pitch as well as its operating speed, which must be between 200 and 300 revolutions per minute. In meeting these requirements, it is essential to take precautions against the sample coming into contact with the air, as described in the section above that deals with testing to IEC 60156.
When testing to ASTM D877
ASTM D877 does not specify stirring of the fluid sample.
Many of the impellers supplied with dielectric fluid breakdown test sets adopt a compromise design that is intended to allow testing with both the IEC and ASTM test standards.
A better option, however, is to use separate impellers for each test standard, as the design of the impellers can then be fully optimised to suit each standard (Figure 4). Special impellers are also available for use with heavily contaminated dielectric fluids (Figure 5).
These have larger blades to help ensure the effective circulation of particulates between the electrodes during the test so that their full effect on breakdown voltage can be assessed. When these special impellers are used, it is important to bear in mind that vigorous stirring can exacerbate the incorporation of moisture captured on the surface film of fluid left behind after rinsing at the vessel preparation stage. This makes the need for rapid filling even more critical if accurate breakdown results are to be obtained.
Small vesselsBreakdown voltage testing of the dielectric fluids used in transformers is usually carried out with test vessels of 400 ml capacity. However, some users of this type of testing now find that, in certain applications, it is advantageous to work with smaller test vessels, typically of 150 ml capacity. This is a particular benefit, for example, with tap changers that only use a small volume of dielectric fluid, which means that only small samples are available for testing. When these small vessels are used, the sample is not stirred during testing. This corresponds to the operating conditions for applications where small fluid volumes are used, as these do not involve fluid circulation.
Breakdown voltage testing of the dielectric fluids used in transformers quickly provides an invaluable first-line indication of the condition of the fluid. However, cleanliness and good preparation are essential if accurate and reliable test results are to be obtained.
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