How to test whether a current transformer is functioning properly?

I. Basic Inspection
1. Visual Inspection
Check the casing for cracks, burn marks, or rust, and check for loose terminals.
Confirm that the primary and secondary windings are clearly labeled and that there is no insulation damage.
2. Insulation Resistance Test
Use a 2500V megohmmeter to measure the insulation resistance of the primary winding to the secondary winding and the casing. The normal value should be ≥1000MΩ.
For low-voltage transformers (e.g., 0.66kV), a 1000V megohmmeter can be used, requiring a resistance of ≥100MΩ.
II. Functional Test
1. Transformation Ratio Verification
Clamp Meter Method: Clamp the primary conductor directly to measure current I1 and the secondary conductor to measure current I2. Calculate the transformation ratio (I1/I2) and compare it to the nominal value. The error should be ≤5% (for protection level) or ≤2% (for measurement level). Impressed Current Source Method: Apply a known current (e.g., 10A) to the primary side and measure the output current on the secondary side with a multimeter to verify the accuracy of the turns ratio.

2. Polarity Test: Use a DC method or a turns ratio tester to verify the polarity of the terminals to prevent measurement errors or misoperation of the protection.

Simple Verification: Connect a light bulb to the secondary side. If the bulb lights up when power is applied to the primary side, the polarity is correct.

III. Advanced Tests

1. Excitation Characteristics Test: Draw a volt-ampere characteristic curve to check for core saturation (the slope of the curve should be above the rated voltage).

Calculate a 10% error curve to ensure the accuracy of the protection device.

2. Partial Discharge Detection: Transformers of 35kV and above require a high-frequency signal to detect partial discharge (≤50pC).

Abnormal heating or noise may indicate internal discharge.

IV. Simple Methods

1. Light Bulb Test: Connect a light bulb to the secondary side. If the bulb lights up when power is applied to the primary side, the winding is normal (only applicable to low-voltage, low-current scenarios). Note: This method cannot verify the transformation ratio and accuracy.
2. Multimeter Resistance Range
Primary resistance close to 0Ω, secondary resistance tens to hundreds of ohms; open circuit or abnormal resistance indicates winding damage.
Use the beep range to test the S1-S2 terminals; normal continuity should be achieved.
Precautions
Secondary side open circuits are strictly prohibited: Open circuits during operation may generate dangerous high voltages. Ensure S2 is reliably grounded.
Power-off operation: Before testing the high-voltage transformer, power must be disconnected and discharged.
Tool Selection: A high-precision digital multimeter or dedicated clamp meter is preferred. Errors exceeding 5% require repair.