Vacuum drying by current transformer short circuit method

- Feb 10, 2020-

The current transformer is usually dry without oil on site, but some manufacturers think that the product should be electrified and heated when it is full of oil, so as to observe the temperature rise of the product. This is particularly important for current transformers of 220kV and above, because the insulation of these products is relatively thick. In the oil-free state, the conduction effect is poor, even if the temperature is low from the outside, and the internal insulation may be aged or even burnt due to overheating. Therefore, it is best to heat the product with electricity under the condition of oil. After the temperature rises and tends to be stable, drain the oil inside the product for oil-free drying. Under the condition of oil-free vacuum, the current passing through the primary winding shall not be higher than 70% of the rated current.
The key points of operation are as follows:
(1) Remove the metal expander and install the temporary cover plate with the joints or valves for vacuumizing, oiling and measuring
(2) The primary winding is connected in series, and it is reliably short circuited with copper conductor of equal cross section.
(3) I open the secondary junction box, remove the external wiring, measure the DC resistance of the secondary winding with a double arm bridge, record the corresponding temperature, and use the data as the reference value for calculating the average temperature of the secondary winding in the drying process.
(4) measure the insulation resistance of the primary and secondary windings to the ground with an insulation resistance meter, and record the corresponding temperature
(5) Under the condition of primary winding short circuit, measure the volt ampere characteristic. At this time, apply AC voltage to all parallel wells of the protection winding until the secondary winding < or primary winding) ammeter reaches the rated current of the transformer. Record the applied AC voltage.
(6) Thermocouples shall be set at the exposed part of the primary winding lead, the short circuit connection, etc., and thermometers shall be set at the outer surface, middle, lower and top cover of the porcelain bushing, etc., so as to measure and control the drying temperature.
(7) Connect the heating circuit and vacuum line.