Currently, the insulation condition of high-voltage test transformers can be inspected through many detection methods, but based on practical work experience, most tests cannot accurately reflect the insulation change patterns of the transformer.

　　There are many factors leading to these situations, among which the most important factor is that during testing, staff may overlook certain minor details, resulting in inaccurate final experimental data and thus erroneous conclusions, making actual operations increasingly difficult.

　　1. High-voltage test transformer related issues in high-voltage testing

　　1.1. Voltage polarity and leakage current during testing

　　High-voltage test transformer moisture mainly starts from its outer casing, and the most important factor causing insulation moisture in high-voltage test transformers is the outer casing.

　　According to the electroosmosis phenomenon, in an electric field, water molecules inside the transformer exist in the form of positive charges. However, once a positive polarity voltage is applied to the transformer winding, the water molecules in the insulation will no longer be compatible, so they are expelled and flow toward the casing, reducing the moisture content inside, which will cause the leakage current inside the transformer to gradually decrease.

　　The principle for the opposite situation is the same. If a negative polarity voltage is applied to the transformer winding, the water molecules inside the transformer will undergo similar changes, mainly reflected in:

　　The water molecules in the insulation will be attracted and will move through the insulation toward the transformer winding, causing the moisture in the high-field-strength areas of the insulation inside the high-voltage test transformer to gradually increase, making the leakage current continuously larger.

　　At the same time, special attention should be paid to the voltage: polarity does not affect all high-voltage test transformers. The most representative case here is new high-voltage test transformers, where voltage polarity cannot change the actual measurement data size.

　　Since the insulation of new high-voltage test transformers is generally not damp, the moisture content is usually negligible, and with the influence of the electric field, electroosmosis becomes less obvious, so during positive and negative polarity test voltages, the leakage current magnitudes obtained are basically the same. For old transformers, the test voltage polarity will seriously affect the accuracy of the test measurement data.

　　1.2. Influence of temperature on insulation resistance

　　As for insulation resistance itself, its sensitivity to temperature is very obvious. Many insulation resistances become very small due to an increase in temperature. Once the temperature rises, the molecules and ions inside the insulation resistance will collide more rapidly, aggravating the polarization problem of the resistance, and the conductance will increase, causing the insulation resistance value to become smaller and smaller.

　　Not only that, if the temperature continues to rise, the water molecules in the insulation layer decompose many impurities in some resistances, accelerating the decrease of the insulation resistance value. If the surface of the insulation resistance is dirty, the decrease speed will be even more obvious.

　　1.3. Influence of voltage rise rate on leakage current measurement

　　Generally speaking, leakage current is one of the attributes of a transformer, and it has no direct relationship with the voltage rise rate. However, during testing, we have learned that if a microammeter is used to read the current, the value read differs greatly from the actual leakage current value. It is a leakage current that does not actually exist because there is a small amount of composite current including a small absorption current.

　　Therefore, the voltage rise rate will seriously affect the reading of the leakage current value, especially for larger transformers, where the impact is even more obvious.

　　In addition, since large transformers have particularly strong absorption capacity, in order to make the leakage current reading more accurate and realistic, one should learn some effective skills and corresponding countermeasures. During actual measurement, it is generally not necessary to spend a lot of time.

　　The displayed data is only the current value one minute after voltage application. Obviously, this data contains some absorption current, and there is a certain relationship between the measured leakage current value and the voltage rise rate. If the voltage is continuously increased, absorption will occur during the voltage increase, and the measured current value will be smaller than the actual current value.

　　If the voltage is increased in a very short time or the increase rate is fast, absorption may not occur during the voltage rise, and the measured current will be much larger than the actual current, which may lead to erroneous analysis results.　　

　　In short, when conducting insulation tests on power transformers, it is essential to pay attention to minor details and not overlook them; otherwise, it will affect the measurement results, cause misjudgment, lead to wrong conclusions, and bring unnecessary troubles and difficulties to actual work.

　　In addition, the influence of factors such as site humidity, high-voltage wiring, and residual charge should also be considered, and the principles of their harmful effects should be understood. At the same time, one should master various testing techniques, accurately read measurement data, and ensure the effective, scientific, reasonable, and feasible operation of the equipment.