Conditions and Methods for High Voltage Testing of Power Transformers

Electricity is an important energy source for production and daily life in modern society. Ensuring its safe and normal operation is one of the most important tasks of the power system. At present, there are still certain drawbacks and problems in the high-voltage testing of power transformers in China, especially the accuracy and reliability of test results that still need to be further improved. This article only discusses relevant issues.

1. Methods for high voltage testing of power transformers

The method of high voltage test of power transformer is:

(1) Connect the leads according to the wiring schematic diagram of the power transformer, and ensure the safety and reliability of the grounding between the transformer and the control box;

(2) Before the high-voltage test of the power transformer, carefully check whether the connections of each part of the wiring are in good condition, 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 lights up, you can press the start button; after the red indicator light lights up, wait for the voltage to increase;

(4) The tester rotates the handle of the voltage regulator in the control box clockwise and at a constant speed to slowly increase the voltage, and closely observes the changes in the indication of the instrument and the operation of the test sample;

(5) After the high-voltage test of the power transformer is completed, quickly adjust the voltage to zero, press the stop button to cut off the power supply, and untie the leads connected during the test.

2. Contents of high voltage test of power transformer

In order to ensure the accuracy and authenticity of the high-voltage test results of power transformers, the test content must be reasonably selected in strict accordance with relevant regulations. The content of power transformer high voltage test mainly includes: insulation resistance measurement, leakage current measurement, dielectric loss factor test, AC withstand voltage test, etc.

2.1 Measurement of insulation resistance

In the high-voltage test of power transformers, insulation resistance measurement is the most convenient and simple preventive test. In the insulation resistance measurement of the transformer, the overall moisture degree of the insulation, overheating aging degree, contamination, etc. can all be reflected with the insulation resistance. Taking the insulation resistance measurement of a high-voltage side voltage of 110kV and a capacity of 31 500kVA transformer as an example, the absorption ratio of insulation is closely related to temperature changes. When the temperature reaches above 35°C, the absorption ratio of dry insulation reaches the limit and begins to decrease. The absorption ratio of damp insulation will change irregularly. Therefore, during the insulation resistance measurement of the transformer, the temperature of the laboratory must be reasonably controlled to ensure the authenticity of the actual measured value of the insulation absorption ratio.

2.2 Measurement of leakage current

In the measurement of leakage current of power transformers, digital leakage current testers are mainly used for measurement. Its rated working voltage is generally below 2.5kV, which is significantly lower than the rated working voltage of the transformer. If the DC megohmmeter cannot meet the voltage requirements in the test, a test method of adding DC high voltage can be used to ensure the accuracy of the transformer leakage current measurement results. Under high-voltage conditions, if the leakage current of the transformer is significantly higher than the current under low-voltage conditions, 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 usage requirements.

2.3 Partial discharge test

The partial discharge test of power transformer is a common "non-destructive" test item. The main test methods are: (1) Use the power frequency withstand voltage as the pre-excitation voltage, reduce it to the partial discharge test voltage, and measure after a duration of 10 to 15 minutes. Partial discharge amount; (2) Use the overvoltage during simulated operation as the pre-excitation voltage, reduce it to the partial discharge test voltage, continue for 1 to 1.2 hours, and measure the partial discharge amount. The second test method can measure whether partial discharge occurs in the transformer under long-term operating voltage to ensure the safe operation of the power transformer in applications. In addition, in the partial discharge test of power transformers, the design of the insulation structure, the field strength of the insulating medium, the surface field of the charged and grounded electrodes, the processing and processing of insulating parts, etc. must all be considered to ensure 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 the power transformer, 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 transformer The insulation performance is not ideal and may cause varying degrees of destructive damage. When the overvoltage during simulated operation is used as the pre-excitation voltage, the standard voltage duration requirement for the partial discharge test is 1 hour. 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 a power transformer, the amount of partial discharge is usually related to the field strength on the surface of the live and grounded electrodes, but has nothing to do with the frequency of the power supply. Therefore, the noise at the test site should be controlled as much as possible, and the amount of partial discharge of the power supply should also be controlled. Go into isolation.

2.4 Transformation ratio measurement

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. limitations; high accuracy and sensitivity; the 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 wiring group is one of the basic conditions for parallel operation of transformers. Therefore, judging the wiring group of power transformers is also an indispensable item in high-voltage testing. Commonly used test methods include: AC voltmeter method, phase meter method, transformation ratio bridge method, DC induction method, group table 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.

2.5 Dielectric loss factor test

In the high-voltage test of power transformers, the dielectric loss factor test is one of the basic insulation preventive test items. Its main test purpose is to determine the insulation performance of the transformer based on the size of the dielectric loss factor. Under normal operating conditions of the transformer, the change in dielectric loss factor is closely related to the size of the insulation loss. During the test process, testers can use relevant results to understand the overall degree of moisture and deterioration of the transformer insulation, thereby obtaining accurate test results. In the dielectric loss factor test of power transformers, the results are significantly better than the insulation resistance measurement and leakage current test, mainly because during the test process, there is little correlation with factors such as test voltage and equipment size, and the tester can accurately judge Transformer insulation changes.

2.6 AC withstand voltage test

The AC withstand voltage test of a power transformer is mainly used to identify its insulation strength. Using this test method can directly reflect the concentrated performance defects of the transformer, thereby ensuring the improvement of the insulation performance of the transformer and avoiding serious damage caused by insulation aging. safety incident. 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. Only after obtaining the relevant test results can the AC withstand voltage test be organized. If the statistics and calculations of relevant test results are unreasonable, it will directly affect the accuracy of the AC withstand voltage test results.

3. Safety design methods for high voltage testing of power transformers

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 be directly related to the accuracy of the 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 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 to prevent induced voltage must be taken between the test equipment and other equipment. Usually, the test equipment and other instruments and equipment are short-circuited and reliably grounded. In the high-voltage test room, special short-circuit grounding wells and grounding systems must be set up according to test requirements. Various capacitive equipment idle in the test room must also be short-circuited and grounded as required. Since the high-voltage test of the power transformer is conducted in a closed six-sided shielding environment, instantaneous discharge may occur during the test. Therefore, the high-voltage cables in the test room must be protected by metal tubes and laid underground. Under normal circumstances, the length of the metal protection tube should be >15m, and it should be connected to the ground electrode every 5m to strictly control the probability of discharge counterattack.

3.2 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 to ensure the safety of test equipment and test personnel. In the case of good grounding conditions, the test room should also be regarded as a special equipotential body. The shells of all metal instruments and equipment in the test room must be well grounded, especially between the transformer and the test equipment. There is a reliable, safe and stable metallic connection. In the high-voltage test room, the location of the grounding point should be clearly marked to prevent personnel from getting electric shock during the test.

3.3 Fire protection and explosion protection

During the high-voltage test of power transformers, it is necessary to strictly prevent overload or short circuit during operation of the transformer. Special attention should be paid to the decomposition and expansion of insulating materials, insulating oil, etc. due to factors such as high temperature and spark action, resulting in gasification and causing the transformer to vaporize. The internal pressure increases sharply, which may cause the transformer shell to explode and cause a large amount of insulating oil to spray out and burn. The oil flow will further increase the risk of fire. Therefore, during the high-voltage test of power transformers, attention must be paid to the prevention of safety issues to ensure the safety of the test.

The high-voltage test of power equipment is a high-tech and complex project. In the high-voltage test of power transformers, reasonable test conditions, methods and content must be selected, and safety design during the test process must be paid attention to to ensure the smooth progress of the test operation and obtain Corresponding test data can be used to scientifically determine the comprehensive performance of the transformer.

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