Why is the secondary side of a current transformer not allowed to be open-circuited, while a transformer can operate at no-load?

Secondary Open-Circuit Condition (Danger!):

When the secondary circuit is open, the secondary current I₂ instantly drops to zero.

However, the primary current I₁ is determined by the main line load and remains unchanged.

According to Ampere's circuit law, without the counteracting effect of I₂N₂, the enormous magnetic potential I₁N₁ on the primary side is entirely used to excite the core. This magnetic potential is hundreds or even thousands of times greater than during normal operation.

The consequences are catastrophic:

Extremely high spike voltages are generated: The rapidly increasing magnetic flux causes the core to saturate quickly. According to Faraday's law of electromagnetic induction (E = -N dΦ/dt), an extremely high voltage (peaking at thousands or even tens of thousands of volts) is induced across the secondary winding terminals. This is highly dangerous, capable of breaking down insulation and endangering equipment and personnel safety.

Severe core overheating:

High saturation induces massive hysteresis and eddy current losses, causing the core to heat intensely and potentially burn the winding insulation.

Generation of residual magnetism, affecting accuracy:

Core saturation produces significant residual magnetism, increasing the current transformer's ratio error and phase angle error, leading to permanent degradation in measurement accuracy.

Threat to personal safety: High voltage may generate arc discharges, posing electric shock hazards to operators.

Conclusion: CTs operate based on the principle of "magnetic potential balance," with minimal magnetic flux during normal operation. An open circuit disrupts this balance, directing immense energy into the core and causing high voltage and overheating.

Impact on Measurement Accuracy: Even if there is no immediate damage after opening the circuit, severe saturation can cause residual magnetism in the core, increasing the ratio and angle errors of the CT and permanently reducing measurement accuracy.

To summarize: energy demand is determined by the load, and demand is extremely low when no load is applied.

During normal operation:

During no-load operation (secondary side open circuit), the main magnetic flux Φ_m remains constant.

To maintain this constant magnetic flux, the primary side draws only a small no-load current (excitation current) from the grid, typically only 2%-8% of the rated current.

This no-load current is used only to establish the magnetic field (generate magnetic flux) and compensate for iron losses. Energy consumption is minimal and poses no risk to the transformer. This is a normal and safe design operating state for the transformer.

Current transformers are sensitive to open circuits because their constant current source characteristics can cause magnetic flux saturation, leading to high voltage and overheating.

Transformers are sensitive to short circuits, but no-load operation is completely normal because their constant voltage source characteristics ensure stable magnetic flux, requiring only a small excitation current when no-load.

LZZBJ4-35 current transformer poduced by Shaanxi West Power Tongzhong Electrical Co., Ltd. is a kind of sensor used to measure large current, mainly used in substations, power systems and electrical equipment, etc. The transformer adopts magnetic core type DC current transformer, which is characterized by high linearity, strong anti-interference ability, small volume, simple structure, etc., and it is a kind of very high cost-effective current transformer.

For more information ,pls contact us

Contact: Ms.Grace Liu (Director of Sales Department)

Email:xdtz04@westpowerelectric.com

Mobile: +86 18091765882(WhatsApp/Wechat/facebook )

Website:https://www.xdtzelectrical.com