1. Power transformer high voltage test method
(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 turned on, after 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 tester rotates the handle of the pressure regulator in the control box clockwise and at a constant speed, slowly increases the pressure, and closely observes the indication change of the instrument and the operation of the sample;
(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 leads connected in the test.
2, power transformer high voltage test content
in order to ensure the accuracy and authenticity of the high voltage test results of power transformers, the test contents must be selected reasonably in strict accordance with the relevant regulations.
The contents of the power transformer high-voltage test mainly include: insulation resistance measurement, leakage current measurement, media loss factor test, AC withstand voltage test, etc., the following specific introduction.
2.1 insulation resistance measurement
in the high voltage test of power transformer, insulation resistance measurement is the most convenient and simple preventive test. In the measurement of the insulation resistance of the transformer, the overall degree of moisture, overheating and aging, pollution and so on can 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 31 500 kVA as an example, the absorption ratio of insulation is closely related to the temperature change. When the temperature reaches above 35 ℃, the absorption ratio of dry insulation begins to decline after reaching the limit, while 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.
2.2 Leakage Current Measurement
in the measurement of power transformer leakage current, the main use of digital leakage current tester for measurement, the rated working voltage is generally below 2.5kV, significantly lower than the rated working voltage of the 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 the current in the case of low voltage, it indicates that the high-voltage insulation resistance of the transformer is less than the low-voltage insulation resistance, that is, the transformer itself has quality defects, and the leakage prevention function cannot meet the requirements of use.
2.3 partial discharge test
partial discharge test of power transformer is a common "non-destructive" test item. The test methods mainly include:(1) taking power frequency withstand voltage as pre-excitation voltage, reducing it to partial discharge test voltage, and measuring partial discharge after duration of 10~15 min;(2) Taking overvoltage in simulated operation as pre-excitation voltage, reducing it to partial discharge test voltage for 1~1.2h, and measuring partial discharge.
The second test method can measure whether the transformer has partial discharge 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 design of insulation structure, the withstand field strength of insulation medium, the surface field of charged and grounded electrodes, the processing and process treatment of insulation parts, etc. should make the local discharge less than the specified value to be considered, rather than whether the main and longitudinal insulation discharge is the main basis.
In the partial discharge test of power transformer, when the power frequency withstand voltage is used as the pre-excitation voltage, the duration of the test voltage is about 15 min. Proper extension of 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, it may cause different degrees of destructive damage. When the overvoltage in 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 transformer, the partial discharge quantity is usually related to the field strength of the charged and grounded electrode surface, but not related 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 quantity of the power supply should also be isolated.
2.4 transformation ratio measurement
power transformer transformation ratio measurement methods mainly include: double voltmeter method, transformation ratio bridge method, etc., in which the transformation ratio bridge method is commonly used in field tests, which mainly has the following advantages: not limited by the degree of power stability; high accuracy and sensitivity; error can be read directly: the test voltage can be adjusted, relatively safe.
In the transformation ratio test of power transformer, the test of continuous groups can also be completed synchronously, and the same group of junction lines is one of the basic conditions for parallel operation of transformers. Judging the junction group of power transformer is also an indispensable item in high voltage test.
The commonly used test methods are: AC voltmeter method, phase table method, transformation ratio bridge method, DC induction method, group table method, etc.
Group table is a common test power transformer group, phase sequence, polarity of the special instrument, the table has the advantages of easy to use, reflect the intuitive, correct instructions and so on.
2.5 dielectric loss factor test
in the high-voltage test of power transformer, the medium loss factor test is one of the basic insulation preventive test items, the main purpose of which is to determine the insulation performance of the transformer according to the size of the medium loss factor. Under the normal operation of the transformer, the change of the dielectric loss factor is closely related to the size of the insulation loss. During the test, the tester can master the overall moisture and deterioration degree of transformer insulation through the relevant results, so as to obtain accurate test results.
In the test of the dielectric loss factor of the power transformer, the result is obviously better than the insulation resistance measurement and leakage current test, mainly because the test process is less related to the test voltage and equipment size and other factors, the tester can accurately judge the insulation change of the transformer.
2.6 AC withstand voltage test
the AC withstand voltage test of power transformer is mainly used to identify the size of its insulation strength, and this test method can directly reflect the centralized performance defects of the transformer, so as to ensure the improvement of the insulation performance of the transformer and avoid serious safety accidents caused by insulation aging. Before conducting 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 be organized after obtaining the relevant test results. If the statistics and calculation of the relevant test results are unreasonable, it will directly affect the accuracy of the AC withstand voltage test results.
3, power transformer high voltage test safety design method
in the high-voltage test of power transformers, due to the large test voltage required, if an effective safety design method cannot be adopted, it will directly affect the accuracy of the test results and the safety of the test personnel. Therefore, in the process of high voltage test of power transformer, we must pay attention to the research and application of safety design method, so as to ensure the smooth development and progress of the test work.
3.1 prevent induced voltage and discharge counterattack
in the high-voltage test of power transformers, effective measures must be taken to prevent induced voltage between the test equipment and other equipment, usually short-circuiting the test equipment with other instruments and equipment, and reliable grounding.
In the high-voltage laboratory, a special short-circuit grounding well and grounding system shall be set up according to the test requirements, and the idle capacitance equipment in the laboratory shall also be short-circuit grounded according to the requirements.
Since the high-voltage test of power transformer is carried out in a closed six-sided shielding environment, there may be instantaneous discharge during the test, so the high-voltage cables in the laboratory must be protected by metal pipes and buried. Under normal circumstances, the length of the metal protection tube should be> 15 m, and every 5 m connected with the ground electrode, so as to strictly control the probability of discharge counterattack.
3.2 and reliable grounding
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. The shells of all metal instruments and equipment in the laboratory should be well grounded, especially between the transformer and the test equipment. There must be a reliable, safe and stable metallic connection. In the high-voltage laboratory, the position of the grounding point should be clearly marked to prevent the phenomenon of electric shock in the test.
3.3 fire prevention and explosion protection of
in the high-voltage test of power transformers, it is necessary to prevent overload or short circuit of the transformer during operation. Special attention should be paid to the decomposition and expansion of insulating materials, insulating oil and other factors due to high temperature, electric spark and other factors, resulting in gasification, resulting in a sharp increase in the pressure inside the transformer, which may cause the explosion of the transformer shell to cause a large amount of insulating oil to spray out and burn, and the oil flow will further expand the risk of fire. Therefore, in the process of high voltage test of power transformer, we must pay attention to the prevention of safety problems, in order to ensure the safety of the test.
4. Conclusion In short, the high-voltage test of power equipment is a high-tech complex project. In the high-voltage test of power transformers, reasonable test conditions, methods and contents must be selected, and safety design during the test should be paid attention to to ensure the smooth operation of the test, obtain corresponding test data, and then scientifically determine the comprehensive performance of the transformer.
The above is the introduction of the transformer high-voltage test by the AJE testing certification, which is for reference only. Some materials come from the Internet and do not represent the views of the AJE testing technology and are responsible for its authenticity. If you are involved in the content of the work, copyright and other issues, please contact us within 30 days, we will delete the content in the first time!
