High voltage test of power transformer - a full set of knowledge in detail!


The method of high voltage test of power transformer is as follows:
(1) Connect the lead wires according to the wiring diagram of the power transformer, and ensure the safety and reliability of the grounding of the transformer and the control box;
(2) Before the high-voltage test of the power transformer, carefully check whether the wiring of each part is in good contact, and check whether the voltage regulator in the control box is adjusted to the "zero" position;
(3) After the power transformer is connected to the power supply and the green indicator light is on, you can press the start button; after the red indicator light is on, wait for the boost;
(4) The test personnel rotate the handle of the pressure regulator in the control box clockwise and at a constant speed to slowly increase the pressure, and closely observe the indication changes of the instrument and the operation of the test product;
(5) After the high-voltage test of the power transformer is completed, quickly adjust the voltage to zero, press the stop button and cut off the power supply, and untie the lead wires connected in the test.
In order to ensure the accuracy and authenticity of the high-voltage test results of power transformers, the test content must be selected reasonably in strict accordance with relevant regulations. The content of high voltage test of power transformer mainly includes: measurement of insulation resistance, measurement of leakage current, dielectric loss factor test, AC withstand voltage test, etc., the following is a specific introduction. In the high-voltage test of power transformers, insulation resistance measurement is the most convenient and simple preventive test. In the measurement of the insulation resistance of transformers, the overall degree of dampness of the insulation, the degree of overheating and aging, and the pollution can all be reflected with the size of the insulation resistance. Taking the insulation resistance measurement of a transformer with a high-voltage side voltage of 110 kV and a capacity of 31500 kVA as an example, the absorption ratio of the insulation is closely related to the temperature change. When the temperature reaches above 35 °C, the absorption ratio of the dry insulation reaches the limit and starts Decrease, and the absorption ratio of damp insulation will change irregularly. Therefore, in the measurement of the insulation resistance of the transformer, the temperature of the laboratory must be reasonably controlled to ensure the authenticity of the measured value of the insulation absorption ratio.
In the measurement of power transformer leakage current, digital display leakage current tester is mainly used for measurement, and its rated working voltage is generally below 2.5kV, which is obviously lower than the rated working voltage of transformer. If the DC megohmmeter cannot meet the voltage requirements in the test, the test method of adding DC high voltage can be adopted to ensure the accuracy of the measurement results of the transformer leakage current. In the case of high voltage, if the leakage current of the transformer is significantly higher than that of the low voltage, it indicates that the high voltage insulation resistance of the transformer is smaller than the low voltage insulation resistance, that is, the transformer itself has quality defects, and the anti-leakage function cannot meet the use requirements.
The partial discharge test of power transformers is a common "non-destructive" test item. The test methods mainly include: (1) take the power frequency withstand voltage as the pre-excitation voltage, reduce it to the partial discharge test voltage, and after the duration of 10 to 15 minutes, Measure the partial discharge; (2) take the overvoltage in the simulated operation as the pre-excitation voltage, drop it to the partial discharge test voltage, and measure the partial discharge for 1 to 1.2 hours.
The second test method can measure whether the partial discharge phenomenon occurs in the transformer under the long-term working voltage, so as to ensure the safe operation of the power transformer in the application. In addition, in the partial discharge test of power transformers, the insulation structure design, the withstand field strength of the insulating medium, the surface field of the charged and grounded electrodes, the processing and process of the insulating parts, etc. must be considered so that the partial discharge amount is less than the specified value, rather than The main basis is whether the main and longitudinal insulation is discharged.
In the partial discharge test of power transformers, when the power frequency withstand voltage is used as the pre-excitation voltage, the duration of the test voltage is about 15 minutes. Properly extending the voltage duration of the partial discharge test has a certain effect on the insulation performance test. If The insulation performance of the transformer is not ideal, which may cause different degrees of destructive damage. When the overvoltage in the simulated operation is used as the pre-excitation voltage, the voltage duration standard of the partial discharge test is required to be 1h. How long the transformer can withstand the pre-excitation voltage is closely related to the volt-second characteristics of the insulation structure.
In the partial discharge test of power transformers, the partial discharge is usually related to the field strength on the surface of the charged and grounded electrodes, but not to the frequency of the power supply. Therefore, the noise at the test site should be controlled as much as possible, and the partial discharge of the power supply should also be Quarantine.
The transformation ratio measurement methods of power transformers mainly include: double voltmeter method, transformation ratio bridge method, etc. Among them, the transformation ratio bridge method is a commonly used method in field tests. It mainly has the following advantages: it is not affected by the stability of the power supply. High accuracy and sensitivity; error can be read directly: the test voltage can be adjusted, which is relatively safe. In the transformation ratio test of power transformers, continuous group tests can also be completed simultaneously, and the same connection group is one of the basic conditions for parallel operation of transformers. Therefore, judging the connection group of power transformers is also a high-voltage test an indispensable item. Commonly used test methods are: AC voltmeter method, phase meter method, transformer ratio bridge method, DC induction method, group meter method, etc. The group meter is a common special instrument for testing the group, phase sequence, and polarity of power transformers. The meter has the advantages of easy use, intuitive reflection, and correct indication.
In the high-voltage test of power transformers, the dielectric loss factor test is one of the basic insulation preventive test items. The main purpose of the test is to determine the insulation performance of the transformer according to the dielectric loss factor. In the normal operation state of the transformer, the change of the dielectric loss factor is closely related to the size of the insulation loss. During the test, the test personnel can grasp the overall degree of moisture and deterioration of the transformer insulation through the relevant results, so as to obtain accurate test results.
In the dielectric loss factor test of power transformers, the results are obviously better than the insulation resistance measurement and leakage current test, mainly because the correlation with the test voltage and equipment size and other factors is small during the test process, and the test personnel can accurately judge Transformer insulation changes.
The AC voltage withstand test of power transformers is mainly used to identify the size of its insulation strength. This test method can directly reflect the concentrated performance defects of the transformer, so as to ensure the improvement of the insulation performance of the transformer and avoid serious damage caused by insulation aging. security incident. Before carrying out the AC withstand voltage test of the power transformer, the insulation resistance, leakage current, dielectric loss factor, etc. of the voltage transformer must be carefully measured, and the AC withstand voltage test can only be organized after obtaining the relevant test results. If the statistics and calculation of relevant test results are unreasonable, it will directly affect the accuracy of AC withstand voltage test results.In the high-voltage test of power transformers, due to the large test voltage required, if effective safety design methods cannot be adopted, it will directly affect the accuracy of test results and the safety of test personnel. Therefore, during the high-voltage test of power transformers, we must pay attention to the research and application of safety design methods, so as to ensure the smooth development and progress of the test work.
In the high-voltage test of power transformers, effective measures to prevent induced voltage must be taken between the test equipment and other equipment. Usually, the test equipment is short-circuited with other instruments and equipment, and grounded reliably. In the high-voltage laboratory, special short-circuit grounding wells and grounding systems should be set up according to the test requirements, and short-circuit grounding should be carried out for various capacitive devices that are idle in the laboratory.
Since the high-voltage test of the power transformer is carried out in a closed six-sided shielding environment, instantaneous discharge may occur during the test. Therefore, the high-voltage cables in the laboratory must be protected by metal tubes and laid underground. In general, the length of the metal protection tube should be >15 m, and it should be connected to the ground electrode every 5 m, so as to strictly control the occurrence probability of the discharge counterattack phenomenon.
In the high-voltage test of power transformers, it is necessary to ensure that the grounding system of the laboratory is good, and the grounding resistance generally needs to be below 0.5Ω, so as to ensure the safety of test equipment and test personnel. In the case of good grounding conditions, the laboratory should also be regarded as a special equipotential body, and the shells of all metal instruments and equipment in the laboratory must be well grounded, especially between the transformer and the test equipment. There are reliable, safe and stable metal connections. In the high-voltage test room, the position of the grounding point should be clearly marked to prevent personnel from getting an electric shock during the test.
In the high-voltage test of power transformers, it is necessary to strictly prevent the transformer from being overloaded or short-circuited during operation. Special attention must be paid to the decomposition, expansion, and gasification of insulating materials and insulating oil due to factors such as high temperature and electric sparks, which will cause the transformer to The internal pressure increases sharply, which may cause the explosion of the transformer casing and cause a large amount of insulating oil to spray out and burn, and the oil flow will further expand the danger of fire. Therefore, during the high-voltage test of power transformers, it is necessary to pay attention to the prevention of safety problems to ensure the safety of the test.

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