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After checking the polarity of a current transformer using the DC method, or when the power is cut off under AC current and the secondary winding is accidentally opened, residual magnetism may develop in the core. This can affect the accuracy and performance of the transformer. To ensure accurate error testing, it's important to demagnetize each core before conducting tests. Demagnetization can be done using either an open-circuit or closed-circuit method, depending on the situation.
Open-Circuit Demagnetization
In this method, one winding (either primary or secondary) is left open while a power frequency current is applied to the other. The current is gradually increased from zero to about 10% of the rated current, then slowly reduced back to zero. This process is repeated 2 to 3 times. If the induced voltage across the open winding exceeds 75% of the value specified for the inter-turn insulation test at 10% of the rated current, the current should be reduced to prevent damage. It’s generally recommended to apply current through the primary winding. For transformers with multiple secondaries, the unused windings should remain open during the process.
Closed-Circuit Demagnetization
This method involves connecting a resistor equivalent to 10 to 20 times the rated load to the secondary winding. Then, the primary winding is supplied with a current that is gradually increased from zero to 1.2 times the rated current, followed by a slow decrease back to zero. This helps eliminate residual magnetism effectively. When dealing with a transformer that has multiple secondary windings, only one of them should be connected to the load, while the others are left open.
For high-precision current transformers, such as those rated at class 0.2 or higher, it is advisable to use the closed-circuit demagnetization method. This ensures better accuracy and reliability during testing. Proper demagnetization not only improves measurement precision but also extends the life of the transformer by reducing magnetic saturation and related losses.
Always follow manufacturer guidelines and safety procedures when performing demagnetization. Ensuring correct methods are used will help maintain the integrity and performance of your electrical equipment over time.
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