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Electromagnetic voltage transformers can be classified into unearthed voltage transformers (fully insulated) and earthed voltage transformers (graded insulation) based on the insulation level across the primary winding terminals. Unearthed voltage transformers refer to those where all parts of the primary winding, including terminals, are insulated to earth at the insulation level. Their AC withstand voltage test includes external applied power frequency withstand voltage test and induced overvoltage test. Earthed voltage transformers refer to single-phase voltage transformers with one end of the primary winding directly earthed, or three-phase voltage transformers with the star connection point of the primary winding directly earthed, such as cascade voltage transformers. Their AC withstand voltage test is conducted through the multiple frequency induced overvoltage test, as the power frequency withstand voltage can only assess the insulation level of the earthed end (N).

2. Explain the work content, personnel division, live parts, and on-site safety measures to the work team members, inform them of danger points, and start work after confirming the procedures;

3. Prepare test equipment, instruments, meters, and tools. Ensure that all instruments and meters are in good condition and within the calibration period;

4. Review the test records of the transformer under test, including oil test results, which should be qualified. The oil level indicator of the transformer should be normal.

6. Check that the transformer casing (if any), base, and core (if grounding is required) are reliably grounded, and the bushing surface should be clean.

7. Disconnect all external connections on each side of the transformer, and ensure that external leads maintain sufficient safe distance from the terminals and are securely fixed.

The AC withstand voltage test is the most direct method to assess the insulation strength of electrical equipment. It is decisive in determining whether the equipment can be put into operation and is an important means to ensure the insulation level of equipment and avoid insulation accidents. The AC withstand voltage test is a destructive test. Only after routine insulation tests such as insulation resistance of the test object pass can the AC withstand voltage test be conducted. If poor insulation conditions (such as moisture or local defects) are found, they should be treated and qualified before conducting the AC withstand voltage test to avoid unnecessary insulation breakdown.

2. Selection of test equipment and instruments: For the external applied power frequency withstand voltage test, a power frequency test transformer is selected. The capacity of the test transformer is chosen based on the capacitance of the test object, and the voltage is selected according to the test voltage. Generally, the voltage of the test transformer should be more than 1.2 times the test voltage. The capacity of the test transformer P > CxU2 ×10-3 (kVA), where CX is the capacitance of the test object (F), U is the test voltage (kV), and =2f.

The high-voltage output terminal of the test transformer should be connected in series with a protective resistor to reduce the overvoltage at the outlet of the transformer's high-voltage winding when the test object flashes over or breaks down, and to limit the short-circuit current. The value of this protective resistor is generally 0.1/V-0.5/V, and it should have sufficient thermal capacity and length. The resistance value should not exceed 30k. The protective resistor in series with the protective sphere gap usually has a resistance value of 1/V. For the induced overvoltage test, a triple-frequency induced overvoltage test device or a variable-frequency power supply series resonance device is usually selected, exciting from the secondary winding or applying voltage directly to the primary winding.

3.1 Wiring and inspection: ① Short-circuit both ends of the primary winding, short-circuit and ground the other secondary windings, ground the casing (if any) and base. The schematic wiring diagram is as follows.

② Connect the voltage regulator, test transformer, AC voltage divider, peak voltmeter, sphere gap, and protective resistor according to the diagram. Connect the power supply to the voltage regulator. After checking that all test equipment, instruments, and meters are correctly wired and indicate correctly, with the permission of the test leader, close the switch, and perform a no-load voltage increase on the pressurizing circuit. If no abnormality is found, reduce the voltage to zero, disconnect the power supply, and connect the high-voltage output of the test transformer to the primary winding of the transformer under test. Ensure sufficient insulation distance between the pressurizing lead and surrounding objects. If necessary, use an insulating rod to perform the withstand voltage test on the lead.

① Check that all test equipment, instruments, and meters are correctly wired and indicate correctly. With the permission of the test leader, close the power switch and prepare to start increasing the voltage. The voltage increase must start from zero (or near zero) and must not be a sudden closing. The rate of voltage increase can be arbitrary before 75% of the test voltage. After 75% of the test voltage, increase the voltage uniformly at a rate of about 2% per second until the test voltage is reached. After timing for 1 minute, quickly and uniformly reduce the voltage to zero (or below 1/3 of the test voltage), then disconnect the power supply. Use a discharge rod to ground the test winding and the high-voltage part of the test transformer and fully discharge.

③ The test process should be supervised and announced verbally. Test personnel should concentrate highly during the test and be alert to any abnormal situations at all times.

The induced overvoltage test involves either applying sufficient excitation voltage to the secondary winding to induce the specified test voltage in the primary winding, or directly exciting the primary winding with the specified test voltage. Regardless of the method, the test voltage should be measured on the high-voltage side.

① For unearthed voltage transformers, first ground one end of the primary winding. For earthed voltage transformers, ground the earthed end (N) of the primary winding. Ground one terminal of each secondary winding, and ground the casing (if any) and base. The schematic wiring diagrams for exciting from the secondary winding and applying voltage directly to the primary winding are as follows:

② Connect the triple-frequency test device (or variable-frequency series resonance test set). Connect the test power supply to the test device. After checking that all test equipment, instruments, and meters are correctly wired and indicate correctly, with the permission of the test leader, close the switch, and perform a no-load voltage increase on the pressurizing circuit. If no abnormality is found, reduce the voltage to zero, disconnect the power supply, and connect the output of the triple-frequency induced overvoltage test device to the secondary winding terminals a and n for excitation. For those with large capacitive current, a compensation reactor can be connected to the auxiliary winding terminals da and dn. The compensation reactor is connected to the secondary winding terminals a and n. It is advisable to choose a secondary winding with larger capacity for excitation. The high-voltage terminal of the voltage divider is connected to terminal A of the primary winding. When using a variable-frequency series resonance device to apply voltage directly to the primary winding, connect the primary winding terminal to the high-voltage terminal of the reactor, ground one end of each secondary winding, and leave the other end floating.

③ Ensure sufficient insulation distance between the pressurizing lead and surrounding objects. If necessary, use an insulating rod to perform the induced overvoltage test on the lead.

① Check that all test equipment, instruments, and meters are correctly wired and indicate correctly. With the permission of the test leader, close the power switch and prepare to start increasing the excitation voltage. The voltage increase must start from zero (or near zero) and must not be a sudden closing. The rate of voltage increase can be arbitrary before 75% of the test voltage. After 75% of the test voltage, increase the voltage uniformly at a rate of about 2% per second until the test voltage is reached. After the withstand test, quickly and uniformly reduce the voltage to zero (or below 1/3 of the test voltage), then disconnect the power supply.

② If a variable-frequency series resonance device is used, after closing the power supply, the tuning of the test circuit must be performed at a very low excitation voltage. Change the frequency of the power supply to maximize the voltage at the test object terminal. At this point, the circuit reaches resonance. Then increase the excitation voltage at the specified rate to reach the test voltage on the high-voltage side. After the withstand test, uniformly and quickly reduce the voltage, then disconnect the power supply.

③ When the test voltage frequency is equal to or less than twice the rated frequency, the test duration is 60 seconds. When the test voltage frequency is greater than twice the rated frequency, the test duration is: test frequency/rated frequency 120 (seconds), but not less than 15 seconds. For unearthed voltage transformers, the test voltage should be applied to each line terminal, with the duration being half of the specified time, but at least 15 seconds.

④ The test process should be supervised and announced verbally. Test personnel should concentrate highly during the test and be alert to any abnormal situations at all times. Keep observing during the test. If any abnormality occurs, stop the test.

② The power frequency withstand test voltage between secondary windings and to the casing is 2000V. A 2500V megger can be used to measure insulation resistance instead.

⑤ When the core magnetic flux density of voltage transformers (including the electromagnetic unit of CVT) is high, the induced overvoltage test should be conducted according to the following regulations:

3) Before and after the induced overvoltage test, a no-load current measurement at rated voltage should be conducted once each. The two measured values should not show significant differences;

4) For oil-immersed transformers of 110kV and above, before and after the induced overvoltage test, a chromatographic analysis of the insulating oil should be conducted once each. The two measured values should not show significant differences;

6) When conducting the induced overvoltage test on the intermediate transformer of a capacitor voltage transformer, the divider capacitor should be disconnected. If it is impossible to disconnect on site due to product structure, the induced overvoltage test may not be conducted.