Differential protection is excellent, but it has a blind spot: a ground fault near the neutral point of a star-connected winding produces a tiny current that's barely detectable. This is where REF comes in — a specialized protection that sees these faint faults clearly.
The Problem REF Solves
In a winding connected in star with a grounded neutral, the voltage at any point in the winding is proportional to its distance from the neutral. A ground fault near the neutral occurs at a very low voltage, so a small fault current flows, which may not exceed the differential protection's pickup threshold, typically set above 20-30% to avoid false trips. The result: a significant portion of the winding near the neutral is effectively outside the differential protection's coverage.
How Does REF Work?
- It compares the residual current of the three-phase CTs (their vector sum) with the current from the CT at the neutral point.
- External ground fault: the two currents are balanced → no trip (hence the name "restricted" — to a defined zone).
- Ground fault inside the winding: the balance is disturbed → immediate trip.
- Because it monitors only ground faults within a narrow zone, it can be set to a very high sensitivity (a small percentage of rated current) without risk of false tripping.
A Quick Comparison with Differential Protection
| Differential 87T | REF (64REF) | |
|---|---|---|
| Covers | All fault types between the CTs on both sides | Ground faults only within a specific winding |
| Sensitivity to faults near the neutral | Weak | Very high — this is its purpose |
| Affected by inrush current | Yes, requires restraint | Less affected (inrush is generally balanced with respect to ground) |
Sample answer: Because differential protection has weak sensitivity to ground faults near the neutral point in grounded star windings: the voltage at the fault location there is low, so the fault current is smaller than the differential's threshold. REF protection compares the residual current of the phases with the neutral current within a restricted zone, detecting these faint faults with high sensitivity and tripping immediately, covering the differential protection's blind spot.
Assuming that differential protection covers 100% of the winding. In practice it may leave a noticeable portion of the winding near the neutral without effective protection, and neglecting REF on important transformers is a real protection gap.
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