The measurement deviation of the dielectric loss factor of the transformer oil depends to a large extent on the test conditions, especially the temperature and the applied frequency. At the power frequency and the recommended temperature of 90°C in the GB5654-85 standard, the loss mainly depends on the conductance of the transformer oil, that is, the existence of free carrier fluid in the transformer insulating oil. The deviation of the measured value is related to the following factors.
1. Impurities
The transformer oil sample contains traces of ionizable dissolved impurities or colloidal particles, which will strongly affect the value of the dielectric loss factor.
2. Sampling and storage of samples
Sampling in windy weather and improper sampling operation methods caused by transformer oil sample contamination, dirty electrodes or incomplete drying, etc., will make the test results unreliable.
Long-term storage of samples or exposure to strong light will cause qualitative changes; long-term exposure to humid environments will increase the water content of the transformer oil, which will affect the measured value.
Therefore, the sample must be stored in a dry, dark place. Strictly follow the sampling rules to prevent contamination of samples. In order to obtain the correct test value, the maximum storage period of the sample should be specified.
3. Temperature
The dielectric loss factor changes exponentially with the reciprocal of the absolute temperature. Therefore, the measurement needs to be performed under fairly accurate temperature conditions. However, at higher temperatures, the value of the dielectric loss factor will vary with the sample heating and constant temperature time. It is generally believed that the initial value can better represent the actual state of the transformer oil, so it is required to measure the loss factor as soon as the temperature reaches the test temperature.
4. Electric field strength
Generally speaking, the test voltage has less influence on the test value. However, in the test voltage range commonly used for general power frequency bridges (equivalent to electric field strength of 1000 V/mm), the voltage has already affected the test value of the dielectric loss factor. When the electric field intensity is too high, the test value will increase significantly due to the secondary effect of the electrode and the discharge of the sample. Therefore, the measurement should be performed under the specified electric field strength.