What are the consequences of excessive secondary load on current transformers?
Sep 05, 2025| I. Core Impacts
1. Excessive Measurement Error
Core saturation causes secondary current waveform distortion, resulting in meter errors exceeding 20% (when the load on a 5A system exceeds 0.8Ω).
Energy meters may experience negative errors, undercounting energy by over 30%.
2. Failure of Protection Devices
Insufficient secondary current during a short circuit causes relay protection to fail to operate or malfunction (e.g., false tripping for out-of-zone faults).
Differential protection errors occur due to impedance imbalance between the CTs on both sides.
II. Risk of Equipment Damage
1. Accelerated Insulation Aging
Overcurrent causes winding temperature rise to exceed the design value, shortening insulation life by five times (after a continuous overload of 30% for six months).
2. Mechanical Structural Deformation
High current electromagnetic forces increase stress on winding conductors, and long-term overload may cause structural deformation.
3. Burnout of Secondary Terminals
Continuous high temperatures may damage wiring. Terminals, causing open circuit faults.
III. Typical Case Analysis
220kV Substation A Case Study: When the short-circuit current exceeded 20kA, the secondary voltage reached 186V (exceeding the knee voltage of 125V), causing protection malfunction.
Metering Circuit Case Study: Excessive transformation ratios led to significant meter errors at low loads, distorting power factor measurements.
IV. Preventive Measures
1. Load Verification
Regularly test the secondary circuit impedance (quarterly recommended). Use the volt-ampere method to verify whether the actual load exceeds the limit.
2. Equipment Selection Optimization
Select rated capacity based on 60% of the actual load to avoid core saturation.
In differential protection circuits, ensure impedance matching between the CTs on both sides (error ≤ 0.5%).
3. Operation Monitoring
Install CT secondary overvoltage protectors to prevent open circuits. Monitor secondary circuit temperature and abnormal instrument indications.
