What tests are mandatory after installing a current transformer?
Feb 20, 2026| I. Insulation Resistance Test – The First Line of Defense for Safe Operation
This is a fundamental test before commissioning any electrical equipment, used to determine the quality of insulation between windings and to ground.
The test includes: insulation resistance between the primary winding and secondary winding, between the primary winding and the casing, and between the secondary windings and the casing.
Use a megohmmeter: 2500V range for the primary side, 1000V range for the secondary side.
Judgment Criteria:
The insulation resistance of the primary winding should not be less than 1000MΩ;
The insulation resistance of the secondary winding should not be less than 500MΩ;
For capacitive current transformers, the insulation resistance to ground of the final shield must also be measured, and it should generally not be less than 1000MΩ.
Precautions: The test should be conducted in a dry environment, and the temperature and humidity should be recorded; the windings not under test should be reliably grounded, and a shielding ring should be added if necessary to eliminate the influence of surface leakage.
II. AC Withstand Voltage Test – A "Pressure Test" of Insulation Strength
This test directly verifies the insulation system's ability to withstand high voltage. It is a destructive test and must be performed only after the insulation resistance has passed the test.
Test Method:
Short-circuit and ground all secondary windings;
Apply a power frequency AC voltage (45–65Hz) to the primary side for 1 minute;
The test voltage is typically 85% of the factory value.
Pass Criteria:
No flashover, no breakdown, no abnormal noise during the test;
No significant decrease in insulation resistance value before and after the withstand voltage test.
Special Notes:
Oil-filled or gas-filled instrument transformers must be allowed to stand for at least 24 hours after installation before testing to ensure internal air bubbles are expelled;
SF₆ insulated instrument transformers should be tested under rated air pressure;
The test voltage should be measured on the high-voltage side, and the voltage peak value should be monitored to ensure accuracy.
III. Turns Ratio and Polarity Check – The Fundamental Guarantee of Measurement and Protection Accuracy
These two tests directly relate to the accuracy of energy metering and the correct operation of relay protection.
Transformer Ratio Test: Apply approximately 150A of current (or 20%~100% of the rated current) to the primary winding and measure the output current of each secondary winding. The measured transformer ratio should be consistent with the nameplate value, and the error should not exceed the specifications in the equipment's technical documents (generally ±1% is allowed for protection-grade CTs, and stricter for metering-grade CTs). Current or voltage methods can be used, and a comprehensive transformer tester is commonly used on-site for rapid completion.
Polarity Check: Use the DC induction method or a transformer calibrator for testing. The standard is "reduced polarity," meaning P1 and S1 are the same polarity terminals. Incorrect polarity will cause phase sequence disorder in the three-phase system, reverse energy meter operation, and malfunction or failure of protection devices; 100% confirmation is required.
IV. Recommended Supplementary Key Characteristic Tests: Although not mandatory, these tests are strongly recommended in practical engineering, especially for protection-grade or critical circuit transformers.
Excitation Characteristic (Voltage-Ampere Characteristic) Test: The purpose is to determine whether the core is saturated and to verify the linear response capability of the CT under large currents such as short circuits. By applying pressure to the secondary side and recording the voltage-current relationship curve, the result is considered (qualified) if there is no significant difference compared to the factory data. This test is of great significance for relay protection setting.
DC Resistance Measurement: Used to check the quality of winding conductor connections and detect problems such as poor contact and broken strands. Use a double-arm bridge or DC resistance tester to measure the primary and secondary windings. The measured value should not deviate from the factory value by more than ±2%.
V. Specialized Tests as Required: For specific types of current transformers, the following tests are also required:
Dielectric Loss Factor (tanδ) Test: Applicable to capacitive current transformers, used to determine insulation aging or moisture absorption. The tanδ of the final screen should not exceed 2%.
Gas Humidity and Leakage Test: For SF₆ insulated current transformers, to ensure that gas insulation performance meets standards.
Partial Discharge Test: Used for high-voltage or critical equipment to detect minor insulation defects and prevent breakdown during long-term operation.
