In the testing process of electrical insulating oil, an adjustable high-voltage voltage needs to be applied to both ends of the insulating oil cup. The voltage rises gradually from zero. When the voltage reaches a certain value, the insulating oil will be destroyed and lose its resistance. Conduction occurs with the generation of an arc, and this voltage is called the breakdown voltage of the insulating oil.
The breakdown voltage is closely related to the test conditions, which include the waveform of the applied voltage, frequency, crest factor, short-circuit current of the test transformer, shape of the electrodes, distance between the electrodes, condition of the electrode surface, volume of the oil cup, boosting speed, The temperature and humidity of the laboratory are related. Due to the large dispersion of parallel experiments, several experiments are usually done, and the average value of all results is taken. In this way, the time from pouring the insulating oil into the oil cup to the first breakdown, the time interval between each breakdown, and whether the oil sample is stirred during the interval also affect the test results, and become the test conditions that must be strictly controlled.
Among all the conditions, the shape of the electrode, the distance between the electrodes and the condition of the surface of the electrode have the most obvious influence on the test results. It can be said that it is meaningless to not specify the breakdown voltage of the test method and not to test the breakdown voltage strictly according to the specified conditions. Strictly speaking, the breakdown of pure insulating oil free of moisture, dust, fibers and other impurities begins with the polarization and ionization of individual oil molecules in the electric field. Its chemical composition has little effect on the breakdown voltage. Oils of different grades should have approximately the same breakdown voltage and the dispersion of parallel test results for the same oil sample should not be too large, and the breakdown voltage values should be high. However, such pure oils do not exist in industrial applications. That is to say, the oil always contains a certain amount of non-oil impurities, and these impurities are often polarized between the electrodes before the oil molecules are ionized, arranged along the direction of the electric field, and then form tiny pathways, which are connected and run through the two poles , leading to rapid breakdown of the insulating oil. The more impurities in the oil, the lower the breakdown voltage. Therefore, it can be said that the breakdown voltage of oil is a test item that reflects the pollution of oil, especially new oil, and the cleanliness of treated oil through the ability of oil to breakdown electric field.
1. Problems that are easy to encounter when testing the breakdown voltage of insulating oil
(1) The test reliability problem caused by the unevenness of the oil sample. Because the insulating oil has dissolved some metal ions, water molecules and other impurities during long-term operation, there are some tiny bubbles attached to the cup wall and the surface of the electrode when it is poured into the oil cup. During the high-voltage test, due to the effect of strong electric field, the unevenness of the oil sample will result in some more complex insulation breakdown phenomena such as electric branch breakdown and tip discharge. Therefore, the test method stipulates that the same cup sample should be tested more than 5 times, and then the average value should be taken to ensure the reliability of the test.
(2) Volume rise phenomenon in high pressure test. Generally, the breakdown voltage value of insulating oil is in the range of 10,000 to 100,000 V. Even if calculated according to the breakdown voltage value of 0.55 times the test voltage, the real voltage value at both ends of the electrodes can be as high as tens of thousands of V, which produces the capacity rise phenomenon called in the high voltage test technology, that is, the voltage at the secondary ends of the step-up transformer The voltage value is higher than the voltage multiplication ratio across the primary.
(3) Detection of breakdown current. The object of the breakdown voltage test is actually to detect whether there is a large current in the test circuit. However, the capacitance effect of the secondary side of the step-up transformer obviously causes a large charge and discharge current in the test circuit at a higher voltage, which will bring certain difficulties to the detection.
(4) Electromagnetic compatibility design. During the high-voltage test, the electromagnetic radiation produces strong electromagnetic interference to the outside, which is another problem that should be considered in the circuit design. Due to the limitation of volume and weight, portable instruments should be considered from the aspects of circuit design and installation position of each component.
(5) Protection of the circuit. After the insulating oil breaks down, the high-voltage circuit should be cut off in time to avoid obvious changes in the oil quality in the test oil cup. Therefore, the contact of the relay that cuts off the high-voltage circuit should be effectively protected, otherwise it will seriously affect the test accuracy and the maintenance of the instrument.