How to judge whether the quality of Residual Current Transformer is qualified?

1. Appearance inspection
Shell integrity: Check whether the shell of the transformer has any signs of damage, cracks, deformation or burning. If such problems exist, it may indicate that the internal structure of the transformer has been damaged, affecting normal use.
Mark clarity: Check whether the markings on the surface of the transformer are clear, accurate and complete, including model, specification, rated current, rated voltage, accuracy level, manufacturer and other information. Unclear markings may lead to incorrect parameter selection during use.
Pin connection: Check whether the pins are firm, whether there are loose, cold soldering, breakage and other phenomena, and whether the copper wire at the root of the pin is open. Good pin connection is the basis for ensuring the normal operation of the transformer.
2. Insulation performance test
Insulation resistance measurement: Use the insulation resistance tester or the resistance range of the multimeter to measure the insulation resistance between the primary winding and the secondary winding, between the primary winding and the shell, and between the secondary winding and the shell. In general, the insulation resistance should be greater than several hundred megohms or even higher. The specific value can be referred to the product specification manual. If the insulation resistance value is too low, it means that the insulation performance is poor, which may cause leakage, short circuit and other faults, affecting the measurement accuracy and safety of use.
Insulation strength test: By applying a certain AC or DC high voltage, the insulation tolerance of the transformer is tested. The test voltage should be determined according to the rated voltage level of the transformer and relevant standards. Under the specified test voltage, the transformer should be able to withstand a certain time (such as 1 minute) of withstand voltage test without insulation breakdown or flashover, otherwise it means that the insulation strength is insufficient and the product is unqualified.
3. Transformation ratio test
Theoretical transformation ratio calculation: According to the nameplate of the transformer, determine the ratio of its primary rated current to the secondary rated current, that is, the theoretical transformation ratio. For example, the primary rated current of a Residual Current Transformer is 100A and the secondary rated current is 0.1A, then the theoretical transformation ratio is 1000:1.
Actual transformation ratio measurement: You can use professional equipment such as current transformer tester to pass a known current on the primary side, measure the induced current on the secondary side, and then calculate the actual transformation ratio. Compare the actual transformation ratio with the theoretical transformation ratio. The deviation between the two should be within the allowable range. Generally, the transformation ratio deviation of the transformer with a higher accuracy level is smaller, such as no more than ±0.5% or ±1%. If the deviation is too large, it means that the transformation ratio of the transformer is inaccurate, which will lead to a large error in the measurement result.
4. Accuracy test
Use standard current source: Use a high-precision standard current source to input currents of different sizes to the primary side of the transformer. The current value should cover the rated current range of the transformer and a certain overload current range.
Measure the secondary current: Use a high-precision ammeter to measure the current output from the secondary side of the transformer, and record the corresponding measured value.
Comparative analysis: Compare the measured value with the expected value calculated based on the theoretical transformation ratio to calculate the accuracy. The calculation formula for accuracy is: Accuracy = (measured value - expected value) / expected value × 100%. According to different accuracy level requirements, the accuracy of the Residual Current Transformer should meet the corresponding standards. For example, the accuracy of the 0.5 level transformer should be within ±0.5%.
5. Linearity test
Input different current values: Input a series of currents of different sizes on the primary side of the transformer, gradually increase from zero to the rated current and a certain overload current, and measure the corresponding output current on the secondary side at the same time.
Draw a curve: Use the primary input current as the horizontal coordinate and the secondary output current as the vertical coordinate to draw the input and output characteristic curve of the current transformer.
Evaluate linearity: Observe the shape of the curve. If the curve is approximately a straight line, it means that the linearity of the transformer is good; if the curve is obviously bent or distorted, it indicates poor linearity. Generally, the Residual Current Transformer is required to have good linearity within the rated current range to ensure the accuracy and reliability of the measurement results.
6. Residual magnetism coefficient test
Apply DC current: First, apply a certain amount of DC current to the primary winding of the transformer to magnetize the core to saturation.
Cut off the current measurement: Then cut off the DC current, put the transformer in an open circuit state, and use a magnetometer and other equipment to measure the residual magnetic flux density in the iron core.
Calculate the residual magnetic coefficient: Calculate the residual magnetic coefficient based on the measured residual magnetic flux density and saturation magnetic flux density. The residual magnetic coefficient should meet the requirements of product standards or relevant specifications. Excessive residual magnetic coefficient may cause errors in the normal operation of the transformer, affecting its performance and measurement accuracy.
7. Temperature characteristic test
Change the ambient temperature: Place the Residual Current Transformer in different temperature environments, such as low temperature (-25℃), normal temperature (25℃), high temperature (55℃), etc.
Measure related parameters: At each temperature point, measure the transformer's transformation ratio, accuracy, insulation resistance and other parameters, and record the corresponding data.
Analyze the influence of temperature: Observe the changes in parameters with temperature and evaluate the performance stability of the transformer at different temperatures. A qualified Residual Current Transformer should be within the specified temperature range, and the changes in its various performance parameters should be within the allowable error range to ensure reliable operation at different ambient temperatures.
8. Overload capacity test
Apply overload current: Apply an overload current that exceeds a certain multiple of the rated current to the primary side of the transformer, such as 1.2 times, 1.5 times, 2 times the rated current. times, etc., for a certain period of time.
Check performance changes: After the overload current is applied, immediately measure the transformer's ratio, accuracy, insulation resistance and other parameters, and compare them with the parameters before overload. At the same time, check whether the transformer has abnormal phenomena such as overheating, smoking, odor, etc.
Judge overload capacity: If the transformer's various performance parameters can still meet the requirements after overload, and there is no obvious sign of damage, it means that it has a good overload capacity; otherwise, the overload capacity is insufficient and there are problems with product quality.
9. Anti-interference ability test
Apply interference signal: Apply different types of interference signals on the primary or secondary side of the transformer, such as power frequency interference, high frequency interference, pulse interference, etc. The amplitude and frequency of the interference signal should be determined according to the interference situation in the actual use environment.
Observe the measurement results: While applying the interference signal, measure the output current of the transformer, and observe whether the output signal is stable, whether there are obvious fluctuations, distortion or malfunctions, etc.
Evaluate anti-interference capability: If the transformer can still accurately measure the residual current in the presence of interference signals and the output signal is stable and reliable, it means that it has strong anti-interference capability; otherwise, the anti-interference capability is poor and it is easy to be affected by external interference, affecting the measurement accuracy and protection performance.