Current transformers are the core equipment in the power system that perform current measurement and relay protection functions. The stability and reliability of their insulation directly determine the safety and stability of the overall power grid operation. The withstand voltage test is a core and effective test method for verifying the insulation strength of current transformers and identifying internal insulation defects. This test is a destructive test, and its core principle is to apply a test voltage higher than the rated working voltage on the insulation structure of the equipment to assess the equipment’s ability to withstand abnormal overvoltages, thereby determining whether the equipment’s insulation performance meets the standards and whether it is ready for parallel operation.
Due to the potential damage risks caused by the withstand voltage test to the equipment insulation, the execution of the test must adhere to strict sequence: all non-destructive test items such as insulation resistance measurement, dielectric loss factor and capacitance measurement must be completed first, and only after all test results are qualified, the withstand voltage test can be carried out. This operational principle can effectively avoid irreversible damage caused by directly applying high voltage when the equipment insulation itself has serious defects, reducing unnecessary equipment scrapping and loss.
I. Test Classification and Technical Standards
Based on the structural characteristics of current transformers and the purpose of test assessment, the withstand voltage test can be divided into multiple categories. Each category of test has distinct voltage levels, application areas, and qualification criteria. The specific test requirements are as follows:
1.1 Withstand voltage test for primary winding to ground and to secondary winding
This project is the core test item for evaluating the main insulation strength of current transformers. It is conducted using rated frequency industrial frequency alternating current voltage. The test voltage value is determined based on the rated voltage level of the equipment and the corresponding insulation level. For the equipment’s commissioning test scenario, the test voltage value is 80% of the factory test value, and the continuous voltage application duration is 60 seconds. High-voltage level current transformers have extremely high requirements for the capacity of test equipment and on-site safety protection measures. The operation must be carried out strictly in accordance with the test conditions.
1.2 Insulation withstand voltage test for secondary winding
The insulation withstand voltage test for secondary windings between each other and to ground is a routine routine test item. According to the current test procedures, the standard test voltage is 3000V, and the voltage application duration is 1 minute. In on-site operations, if there is no condition for industrial frequency withstand voltage test, 2500V megohmmeter can be used to measure insulation resistance as a preliminary alternative judgment basis, but this alternative method is only suitable for simple screening and cannot fully equivalent the insulation assessment effect of industrial frequency withstand voltage test, and cannot be used as the final qualification judgment basis.
1.3 Insulation withstand voltage test for winding segments
For current transformers with segmented structures, the insulation withstand voltage test for winding segments is a mandatory test item, used to evaluate the insulation reliability of the winding segmentation structure. The test voltage parameters for this project are based on the equipment’s factory technical documents and the technical conditions marked on the nameplate. On-site operations must strictly follow the established parameters and must not arbitrarily change the test standards.
1.4 Withstand voltage test for end screen to ground
For current transformers with voltages of 110kV and above, the insulation performance of the end screen lead-out terminal (ground terminal N) needs to be carefully checked. The standard industrial frequency withstand voltage for this area is 2000V. When there is no industrial frequency test condition on-site, a 2500V megohmmeter can be used for alternative insulation screening, assisting in judging the insulation status.
II. Power Frequency Withstand Voltage Test Equipment and Preparations Before the Test
Complete pre-test preparations are the basis for ensuring accurate test data and avoiding test safety risks. They mainly include environmental condition confirmation, initial insulation measurement of equipment, and verification of test circuit wiring.
2.1 Environmental condition requirements
The test site must be kept dry and tidy, with the relative humidity of the environment not exceeding 80%. The surface of the tested equipment should have no condensation or obvious dirt and impurities. If the environmental humidity exceeds the standard or the equipment surface gets damp, it will significantly reduce the accuracy of the measurement data, easily cause surface flashover on the equipment, lead to incorrect test results, and affect the judgment of insulation status.
2.2 Preliminary Insulation Test
Before the formal pressure test is carried out, the preliminary insulation resistance test of the equipment must be completed. For the primary winding, a 2500V megohmmeter is used to measure the insulation resistance between it and the secondary winding as well as the equipment casing; for the secondary winding, a 1000V or 2500V megohmmeter is used to measure the insulation resistance between the windings and to ground, as well as the insulation resistance between the windings themselves. The measured insulation resistance values must meet the lower limit requirements of the regulations, and there should be no obvious downward trend when compared with the historical test data of the equipment, before entering the subsequent pressure test stage.
2.3 Wiring Specifications for the Test Circuit
The test wiring must strictly follow standardized norms. The high-voltage leads are firmly connected to the tested windings, and the connection parts should have no burrs or sharp corners to avoid electric field distortion and local discharge or breakdown. All non-test parts of the equipment, including the secondary winding terminals, the equipment casing, the core, the clamping parts, etc., need to be reliably grounded. The voltage on the high-voltage side should be directly measured through a capacitor divider or a voltage transformer, with the measurement error controlled within 3%, and it is strictly prohibited to indirectly calculate the high-voltage side voltage by converting the value from the low-voltage side, to ensure the accuracy of the voltage data.
III. Test Operation Procedures
The power frequency withstand voltage test should be operated strictly in accordance with the standardized voltage rise, stabilization, and voltage drop procedures, and no illegal operations should be carried out. The specific operation requirements are as follows:
The test voltage should be raised steadily from zero, with the rise speed controlled at 3% to 5% of the rated voltage per second to avoid additional impact on the equipment insulation from the overvoltage during the closing operation. After the voltage reaches the specified test value, the voltage should be stabilized for 60 seconds.
During the stabilization period, the test personnel should closely observe the equipment status and instrument data throughout the process, and carefully check for any abnormalities such as贯穿性 breakdown discharge of the tested equipment， surface flashover along the surface， abnormal sounds and vibrations， and significant fluctuations in the pointer of the test instrument. If any abnormality is detected， it is determined that the insulation of the equipment is defective， and the test should be immediately stopped for problem verification.
After the test duration exceeds the standard， the test voltage should be smoothly reduced to zero first， then the test power supply should be cut off， and finally， a dedicated discharge rod should be used to fully discharge the tested equipment. The discharge should follow the process of first grounding through a limiting resistor and then directly grounding， to completely discharge the residual charges of the equipment and ensure the safety of the operators.
IV. Test Result Analysis and Determination
Based on the phenomena during the test and the test data before and after the test， the withstand voltage test results can be classified into two categories： qualified and unqualified. The specific determination criteria are as follows：
Qualified determination： Within the specified test voltage and stabilization time range， the tested equipment does not experience breakdown， flashover， abnormal discharge， etc.， and the insulation resistance values before and after the test do not show significant fluctuations or decreases， and it is determined that the insulation strength of the equipment meets the operation requirements.
Unqualified determination： When the test voltage suddenly drops， the test current increases significantly， the equipment emits cracking sounds， or produces obvious sparks during the test， it is determined that the insulation of the equipment is unqualified. The test should be immediately stopped， the defect location and cause of breakdown should be accurately identified， and a comprehensive analysis should be conducted based on multiple test data such as partial discharge detection and dielectric loss measurement， and after completing the defect rectification， the test can be conducted again.
For equipment of different insulation types， subtle abnormalities should be differentiated for determination： During the test of oil-immersed current transformers， slight and controllable partial discharge sounds are within the allowable range， continuous squeaking sounds and cracking sounds are considered serious abnormalities； SF6 gas-insulated current transformers should focus on monitoring whether there are gas leaks and abnormal fluctuations in equipment pressure during the test.
V. Safety Technical Requirements
The withstand voltage test is a high-voltage and high-risk operation， safety protection is the primary criterion for the operation， and all safety technical norms must be strictly implemented throughout the process： The test area should be equipped with sturdy safety fences and conspicuous warning signs, and dedicated supervisors should be assigned. Unrelated personnel are strictly prohibited from entering the high-voltage operation area. The test operators must wear complete personal protective equipment such as insulating gloves and insulating shoes in a standardized manner.
The test circuit should be equipped with complete overcurrent protection devices. When a breakdown or flashover fault occurs in the equipment, the protection device should act quickly to cut off the power supply and limit the damage caused by the fault current to the test equipment and the equipment under test. The high-voltage side of the test transformer should be connected with a protection resistor to effectively suppress high-frequency oscillation overvoltage and avoid overvoltage damage to the equipment insulation.
Throughout the entire operation process, strict adherence to the core safety principles of the current transformer operation test must be maintained: during equipment operation and all types of test operations, the secondary side must not be open-circuited. When the secondary side is open-circuited, the current on the primary side will all be converted into excitation current, causing severe saturation of the core, resulting in an extremely high peak voltage being induced in the secondary winding. This not only will cause the insulation of the secondary side to be broken down but also will cause fatal electric shock to the operators. Before changing the test wiring or switching the range, the secondary side must be short-circuited and reliably grounded, and the short-circuit device must be removed after the operation is completed.
Post time: Jul-07-2026