Power Systems Lab · 18 modules · all computed in your browser
CT saturation
DC offset, burden, knee and the C37.110 criterion.
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Power Systems Lab · 18 modules · all computed in your browser
DC offset, burden, knee and the C37.110 criterion.
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CT saturation with DC offset
If secondary
125.0 A
C37.110 need Vs ≥ If·Zb·(1+X/R)
5250 V
Saturates?
Yes
Symmetrical check If·Zb
250 V
Simplified ideal-core model: once the secondary flux reaches the knee flux, the core stops transferring current until the flux reverses. Real cores (the Jiles-Atherton or EMTP Type-96 models) saturate gradually.
Learn it
A CT can only produce secondary current while its core flux stays below saturation. The flux is the time-integral of the secondary voltage (current × burden). A DC offset in the fault current keeps pushing flux one way, so even a CT that is fine for symmetrical current can saturate a few cycles into an asymmetrical fault. Remanence left from an earlier fault makes it worse.
Saturated CTs under-report current. That makes overcurrent relays slow and can give differential relays false operate current on external faults.
Quiz · 0/3
1. What makes a CT saturate even though the symmetrical If·Zb is below its knee?
2. The quickest field fix to reduce saturation is…
3. Why test polarity and ratio during commissioning, as on your primary-injection jobs?