When conducting transformer oil dielectric strength test (breakdown voltage test), why are there strict requirements on electrode shape, gap between electrodes, voltage raising speed and voltage characteristics?
The test proves that the same transformer oil sample uses three different shapes of electrodes under the same conditions, and the measurement results are different. Generally, the measured dielectric strength value of the spherical electrode (or called as mushroom shape) is the highest, followed by the semisphere shape, and the flat plate shape is the lowest. When the breakdown voltage of the oil is above 30kV, if the measured value of the flat plate electrode is accurate, the spherical electrode is roughly 4~6kV higher, and the semisphere electrode is roughly 1~2kV higher. When the breakdown voltage of the oil is below 30kV, the above difference has a tendency to shorten or is basically the same. In the past, the electrodes used most was a flat electrode, which complies with the standards of the electric power industry. It cannot be eliminated all at once. It can only be slowly changed to the national standard. The use of spherical electrodes or semisphere electrodes is in line with the reality.
The reason for this phenomenon is that the uniformity of the electric field strength between electrodes of different shapes is different. The electric field distribution between the flat plate electrodes is uniform, the impurities in the transformer oil are easy to connect into small conductive bridges, and the electric field strength between the semisphere shape and the spherical electrode is uneven, and the impurities have more chances to hit the spherical surface, and they are repelled by the electric field after receiving the charge. The conductive bridge is not easy to form, so that the breakdown voltage value is higher.
Obviously, the distance between the electrodes directly affects the value of the dielectric strength. The smaller the distance between the electrodes, the easier it is for the moisture and impurities in the transformer oil to form small conductive bridges, and the lower the insulation dielectric strength (transformer oil bdv). On the contrary, it will be higher. China stipulates that the distance between the two electrodes is 2.5mm.
The voltage raising speed also has an influence on the breakdown voltage value but not much. For example, if the breakdown voltage test is performed at 3kV/s and 2kV/s boost speeds, the average value is generally about ±2~3kV, which is less than the average value of the standard deviation of each group of measured values. The national standard stipulates 3kV/s. The boost mode is continuous boost.
The characteristics of the applied voltage are different, which also affects the size of the dielectric strength. When other conditions are the same for the same oil sample, the dielectric strength measured by DC voltage is lower than that measured by AC voltage.
Therefore, in the process of measuring the insulation strength, it must be carried out in strict accordance with the specified requirements, otherwise the measurement results will not be comparable and will cause errors.
PureTech's transformer oil insulation strength tester (transformer oil bdv tester) uses a microprocessor to automatically complete boost, keep stirring, static discharge, calculate, printer and other operations, it is a necessary measure in the conventional test equipment.
In addition, PureTech has been committed to the treatment of transformer oil for many years. It has developed and produced transformer oil filtration equipment, vacuum dehydration equipment, transformer oil regeneration equipment, transformer oil injection equipment, etc., which can remove moisture and impurities in transformer oil, therefore the good insulation performance of the transformer oil is guaranteed.
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