What are the consequences of opening the secondary side of a current transformer?
May 04, 2026| 1. High voltage of thousands to tens of thousands of volts induced on the secondary side.
During normal operation, the secondary current cancels out the magnetic flux of the primary current, resulting in a relatively small magnetic flux in the core. Once open-circuited, the secondary current becomes zero, the demagnetizing effect disappears, and the primary current becomes entirely the excitation current, leading to deep core saturation. According to the law of electromagnetic induction, this drastic change in magnetic flux will induce extremely high voltage (up to thousands or even tens of thousands of volts) in the secondary winding with a large number of turns, potentially breaking down the insulation and causing electric shock or arcing accidents.
2. Core overheating, potentially burning out the transformer.
After the core saturates, eddy current losses and hysteresis losses increase dramatically, causing the transformer to overheat severely, emitting a "humming" sound or a burning smell. Prolonged operation may lead to aging or burning of the winding insulation, causing permanent damage to the equipment.
3. Inaccurate measuring instruments, resulting in malfunctions or failures to operate relay protection.
After an open circuit, the secondary current is zero, causing ammeter and energy meter readings to drop to zero, making accurate measurement impossible. Simultaneously, the relay protection device cannot sample the current signal, potentially leading to malfunctions in differential protection or failure to operate during faults, affecting the safe and stable operation of the power system.
4. Residual magnetism, reducing measurement accuracy.
The open circuit process generates residual magnetism in the iron core, altering its magnetization characteristics and increasing the ratio and phase difference of the instrument transformer. Even after the circuit is closed, it is difficult to immediately restore normal accuracy, affecting the accuracy of subsequent measurements.
5. Fire and explosion risks.
High-voltage arcs may ignite surrounding combustibles, especially in confined spaces or oil-immersed instrument transformers, posing a potential risk of fire or explosion.
