What are the effects of excessive secondary load on current transformers?

1. Measurement and metering errors
1. Increased current ratio error
Exceeding the rated secondary load impedance can cause core saturation, increase the excitation current, and cause the current ratio error to exceed the required accuracy level (e.g., the error for level 0.5 may rise to over 2%).
Phase error also increases, affecting power factor measurement accuracy (COSφ error may exceed 0.1).
2. Energy metering distortion
Errors increase significantly at low loads (e.g., the error may reach 1.8% at 20% of rated current).
Errors further worsen in harmonic environments (the error doubles when the third harmonic content exceeds 15%).
2. Protection failure
1. Relay protection misoperation/failure
Differential protection misoperation due to current imbalance (differential current exceeds 0.1). 0.5In)
Overcurrent protection may fail to operate due to current decay (operation time may be extended by more than 30%).
2. Abnormal fault response
The secondary voltage under short-circuit current may exceed the knee voltage (e.g., 186V measured > 125V), causing the protection device to fail to operate reliably.
III. Equipment Safety Risks
1. Accelerated insulation aging
Core overheating (temperature rise exceeding 70°C) may cause carbonization of the insulation material, increasing the risk of breakdown.
Oil-immersed CTs may experience excessive acetylene levels (>5μL/L).
2. Mechanical damage
Magnetic saturation may induce 50-100Hz low-frequency vibration, leading to loose or broken terminals. Poor secondary circuit contact may cause arcing (risk is significant when contact resistance > 0.1Ω).