Knee Point Voltage
The voltage at which a current transformer core begins to saturate, critical for protection CT performance.
Knee point voltage is critical for protection CTs—the voltage at which 10% increase causes 50% increase in magnetizing current. Marks where CT saturates and accuracy degrades.
For protection, CT must remain linear during maximum faults for accurate relay signals. Depends on fault magnitude, CT ratio, burden, ALF.
IEC defines for class PX/X; IEEE C57.13 uses accuracy class. Proper selection ensures knee point exceeds worst-case fault voltage.
QDTB supplies protection CTs with verified knee points integrated into switchgear and substations.
Related Terms
Current Transformer
An instrument transformer producing secondary current proportional to primary current, for measurement and protection.
Impedance (%Z)
Total opposition to current flow in a transformer as a percentage, determining fault current and voltage regulation.
Fault Current
The extremely high current during a short circuit that transformers must withstand mechanically.
More Terms Starting with “K”
K-Factor
A transformer rating indicating ability to supply non-linear harmonic loads without exceeding temperature limits.
kVA Rating
The apparent power rating of a transformer in kilovolt-amperes, determining load-carrying capacity.
Kidney Loop Filtration
An online oil purification system continuously filtering transformer oil through a side-loop circuit to remove moisture and particles.
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