To know what is the criterion for judging the transformer oil tanδ, we first need to clarify several definitions, that is, what is the dielectric, dielectric loss and dielectric loss angle.
Insulating materials are often referred to as insulating media, which can be divided into three types of insulating media: gas, liquid and solid. Insulating oil is a liquid insulating medium, a liquid dielectric, that is, an insulator that can withstand electrical stress.
Insulating oil has energy loss under the action of alternating electric field. One is the loss caused by electrical polarization, and the other is the loss caused by conductance. The so-called dielectric loss of insulating oil essentially refers to the sum of these two energy losses.
Due to the existence of dielectric loss, the phase difference between the current passing through the insulating oil and the voltage across it is not 90°, but a delta angle smaller than 90°. This delta angle is called the dielectric loss angle of the insulating oil. Usually the dielectric loss characteristic of insulating oil is expressed by the tangent value tanδ of its medium δ.
What is the significance of monitoring the tanδ of insulating oil?
(1) It can sensitively reflect the insulation characteristics of transformer oil. The increase of tanδ of the oil will seriously cause the deterioration of the overall insulation characteristics of the transformer. The dielectric loss causes heat to be generated inside the insulating oil. The greater the dielectric loss, the more heat is generated, which in turn further increases the dielectric loss. Such a vicious cycle will form a breakdown at the insulation defect, resulting in equipment failure.
(2) The degree of oil quality deterioration and pollution can be judged. After the insulating oil deteriorates and is polluted, the tanδ of the new oil can increase by more than ten times. It can be seen that the tanδ of the insulating oil is a very sensitive indicator for judging the degree of pollution and deterioration. Especially when the insulating oil is damp, the active current generated by some polar impurities in the oil increases under the action of an external alternating voltage, and the tanδ increases more obviously.
(3) The degree of refining of the transformer oil can be judged. For thoroughly refined insulating oil, when the temperature rises, tanδ does not increase significantly, while for over-refined or incompletely refined insulating oil, when the temperature rises, tanδ increases rapidly.
In short, the tanδ of transformer oil can reflect the impurity content and moisture content in the oil, and can further test the insulation, deterioration and refining degree of the oil. Therefore, whether it is new oil, regenerated oil, or operating oil, the tanδ should be tested regularly according to regulations.
What are the factors and precautions that affect the measurement of insulating oil dielectric loss?
(1) Applied voltage. Generally, when tanδ is measured at a low voltage, the voltage has no obvious effect on the dielectric loss angle. However, when the test voltage is increased, the tanδ value will increase significantly due to the dipole transfer of the medium under the action of high voltage, which causes power loss, so the dielectric loss factor increases with the increase of voltage. When measuring, it should be added to the rated voltage according to the regulations. According to the information, the voltage when measuring the dielectric loss factor must be added to 3~10kV to measure the real value, otherwise the error will be large.
The national standard is more accurate with the electric field strength: the test voltage has little influence on the test value. However, within the commonly used test voltage range of general power frequency bridges (equivalent to electric field strength 100~1000V/mm), the voltage has already affected the dielectric loss factor test value. When the electric field intensity is too high, the test value will increase significantly due to the secondary effect of the electrode, the discharge of the sample, and other reasons. Therefore, the measurement should be carried out under the specified electric field strength.
(2) Frequency. tanδ is related to the frequency at which the voltage is applied because the change in tanδ is a function of frequency, ie tanδ changes with frequency. Therefore, the appropriate test voltage frequency should be selected according to the regulations. Generally, 50Hz AC voltage is used to measure tand, because the insulation weakness of the exposed medium is more obvious than other frequencies, and this is also in line with the actual use of electrical equipment.
(3) The influence of temperature. The conductance of the medium changes with the change of temperature, so when the temperature rises, the conductance of the medium increases, the leakage current also increases, and the dielectric loss angle also increases. Therefore, the temperature has a great influence on the measurement results of tanδ. If the temperature difference is a few degrees during the measurement, the results of parallel experiments will not be consistent. Practice has proved that: the higher the temperature, the clearer the difference between good oil and bad oil. For example, at 60°C, there is almost no difference between good and bad oil, but at 100°C, their difference is very large. It is obviously exposed, but the temperature should not be too high indefinitely, because when the temperature is too high, it will promote the deterioration of oil quality and affect the test results. Therefore, the measurement of tanδ should be carried out at 80~100°C. The current national standard has been changed from 70°C to 90°C.
(4) The influence of moisture and humidity. Because the polarity of water is extremely strong, it is easy to dissociate under the action of electric field, and the conduction current of insulating oil is increased, which makes the tanδ of oil significantly increase. Increased humidity will increase the dissolved water in the oil sample (caused by oil moisture absorption), thereby increasing tanδ. Therefore, the measurement should be carried out under the specified relative humidity.
(5) It is related to external interference and the condition of measuring instruments. If the place where the instrument is placed has strong electromagnetic interference and is affected by mechanical vibration; or the position of the instrument is changed casually; the working surface of the electrode is dark, the smoothness does not meet the requirements, and the distance between the electrodes is uneven; the oil cup has not been dried or If it is not clean, there are air bubbles and impurities in the test oil; the insulation between each core wire and the shield is poor, etc., which will affect the measurement results. For this reason, care must be taken when measuring:
1) There should be no electromagnetic field and mechanical vibration interference around the test site;
2) The position of the equipment should not be changed casually;
3) The smoothness of the electrode working surface should reach 79. If the surface is found to be dark, it must be re-polished;
4) Each electrode should be kept concentric, and the distance between the gaps should be uniform;
5) The insulation resistance between the measuring electrode and the protective electrode should be more than 100 times the insulation resistance of the measuring equipment. The insulation resistance between each core wire and the shield is generally not greater than 50 ~ 100MΩ;
6) Measuring instruments must be cleaned and adjusted as required;
7) The oil sample injected into the oil cup should be free of air bubbles and other impurities;
8) The contact of each connection point of the line should be good, without open circuit or leakage;
9) When the voltage is applied to the oil sample to a certain value, there should be no discharge phenomenon during the boosting process.
For UHV transformers, the requirements for insulating oil are relatively high, so the oil treatment process should be very strict. When the test value is greater than the specified value, it should be retested twice at first. If the retested value is still stable at the initial value, it reflects that the batch of oil is abnormal. Most of the reasons are damp, or mixed with mechanical impurities, or mixed other liquids. Among them, the latter cannot be treated with oil, and the former can use vacuum oil filtration method, that is, use vacuum transformer Oil Purifier or Transformer Oil Regeneration plant to purify and dehydrate transformer oil, so as to remove moisture and impurities and achieve the purpose of purification. If the tanδ of the oil cannot be qualified by the vacuum oil filtering method, it means that other non-insulating liquids have been mixed in the oil.
When the transformer oil is at 90°C, its dielectric loss angle tanδ value should meet the requirements in the table below.
Transformer Oil before putting into operation
Running transformer oil
330KV and below： ≤1
330KV and below： ≤4
If you need vacuum Transformer Oil Purifier to reduce Tanδ of Transformer Oil (dissipation factor of transformer oil,tan delta of transformer oil), as well as improve other parameters of used transformer oil, such as dielectric strength, water content, gas content, etc. please don't hesitate to contact us PureTech, we are professional manufacturer of Transformer Oil Purifiers with various customized options, and many other oil filtration machines and power equipment. We believe that our profession can help you to solve the oil problems you are facing now.
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