What Saturation Is
A current transformer has a magnetic core that can only carry a limited flux. When primary current is very high, or the burden on the secondary is too large, the core reaches its limit and the secondary current no longer follows the primary accurately. That is CT saturation.
Why It Matters
During a heavy fault, a saturated CT can deliver a reduced or distorted current to the relay. The relay may operate slower than intended, or in some schemes not operate correctly at all. That is exactly the moment protection must be reliable.
What Influences Saturation
- Fault current magnitude relative to the CT rated primary current.
- Secondary burden - relay input, lead resistance, and any other connected devices.
- DC offset in the fault current, which drives the core harder.
- Remanent flux left in the core from an earlier event.
Specifying Protection Class CTs
Protection CTs are specified by accuracy class and an accuracy limit factor or knee-point voltage, chosen so the CT stays accurate up to the highest fault current the relay must handle. Keep lead lengths and burden in mind. See CT and PT selection.
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