Common Causes of Excessive Secondary Load on Current Transformers
Sep 17, 2025| I. Design and Selection Defects
1. Inadequate Capacity Matching
The total secondary circuit impedance exceeds the CT's rated load (e.g., >0.8Ω for a 5A system), resulting in errors exceeding the required accuracy level.
Failure to consider the load correction factor for harmonic environments (multiply by 1.2-1.5 times).
2. Improper Cable Configuration
Long-distance, small-section cables (e.g., 2.5mm² copper cable with a resistance >0.7Ω per 100 meters) significantly increase impedance.
Failure to provide margin (actual load should be ≤60% of the rated value).
II. Operation and Maintenance Issues
1. Excessive Parallel Equipment
Each additional Class 0.5 instrument increases the load by 0.2Ω. Overload: Sharing CTs for protection and metering can easily lead to overcapacitance. Excessive neutral point CT load (e.g., a 250m cable resulting in a 3Ω resistance in this example) can cause differential protection to malfunction.
2. Poor contact: Oxidation or loosening of terminals can cause contact resistance greater than 0.1Ω, and multiple grounding points in the secondary circuit can generate circulating current.
III. Abnormal system conditions:
1. Short-circuit current surge: During a fault, a sudden increase in primary current causes core saturation, increasing the equivalent impedance on the secondary side. Transient processes can cause the secondary induced electromotive force to exceed the limit (e.g., Es1 = 530V < Es = 640V in this example).
2. Harmonic pollution: High-order harmonics (e.g., third harmonic > 15%) increase core losses and increase the equivalent load.
