What factors affect the temperature rise of a current transformer?
Sep 01, 2025| I. Electrical Parameter Factors
1. Primary Current and Overload
The square of the current is directly proportional to the temperature rise (Joule's Law). A 10% overload may increase the temperature rise by 15%-20%.
When the secondary is open, the core magnetic flux saturates, and the local temperature rise can reach over 200°C.
2. Secondary Load Impedance
When the load exceeds the rated load (e.g., 5VA for a Class 0.2 CT), each 1% increase in error corresponds to a 3-5K increase in temperature rise.
II. Material and Structural Factors
1. Core Material
Silicon Hysteresis loss in steel sheets causes temperature rise; nanocrystalline materials can reduce this loss by 30%.
The resistivity of copper windings (1.72 μΩ·cm) significantly affects temperature rise, while aluminum windings increase temperature by 65%.
2. Heat Dissipation Design
Natural convection heat dissipation efficiency is affected by cabinet height (recommended > 2 meters).
Sealed compartments (such as SF6 insulation) experience a 15%-20% higher temperature rise than open structures.
III. Environmental and Installation Factors
1. Ambient Temperature
For every 1°C increase in ambient temperature, the temperature rise increases by 0.5°C. -1K (Baseline value adjusted according to UL891)
For every 100-meter increase in altitude, the dry-type CT temperature rise limit decreases by 0.5%.
2. Installation Process
A 10μΩ increase in contact resistance (e.g., M10 bolt torque <25N·m) results in a 3-5K increase in temperature.
When the distance between cabinets is <1m, the temperature rise increases by 5%-10%.
IV. Operational and Maintenance Factors
1. Cooling System
Insufficient oil level or contamination (moisture >50ppm) in oil-immersed CTs can cause temperature rise. Increase by 15%-20%
Heat dissipation efficiency decreases by 20% when dust accumulation >2mm is present.
2. Insulation aging: The thermal conductivity of insulation materials decreases after long-term operation (e.g., 0.2W/(m·K) for epoxy resin).
V. Impact of Special Operating Conditions
1. Harmonic Current: High-frequency harmonics increase eddy current losses in the core, potentially causing the temperature rise to exceed the limit by 30%.
2. DC Component: The DC component of the short-circuit current (time constant 100ms) accelerates the core's saturation temperature rise.
