In short. Restricted earth-fault protection compares the neutral current with the sum of the phase currents. It trips only for earth faults inside the protected winding, is fast and sensitive, and stays stable for faults outside. It needs matched CTs and careful setting.
Principle
For an external fault, current entering the winding through the phase CTs equals the current leaving through the neutral CT, so the relay sees almost nothing. For an internal earth fault, the currents do not balance, and the relay operates. Because only earth faults in the winding are covered, it is “restricted”.
Where it is used
- Star-connected generator stator windings with a neutral earthing point.
- Transformer star windings with earthed neutral.
It is added to differential protection because it detects low-level winding earth faults that differential may miss, especially when the neutral is resistance earthed. See differential protection.
Schemes
| Scheme | Features |
|---|---|
| High impedance | Voltage-operated relay with stabilising resistor; needs matching CTs of the same ratio and good knee-point voltage |
| Low impedance (biased) | Numerical relay with bias; tolerates CT differences, more flexible |
Setting and design points
- CTs on phases and neutral must be suitable, with adequate knee-point voltage for the through-fault current.
- For high impedance schemes, calculate the stabilising voltage setting and resistor so that the relay stays stable for the worst external fault, with CT saturation.
- The effective primary sensitivity depends on relay setting, magnetising currents of the CTs and the resistor. Check that it detects the minimum internal earth fault.
- Where the neutral is resistance earthed, fault current is limited, so sensitivity is critical.
Commissioning checks
- Verify CT ratios, polarity and wiring.
- Measure CT magnetising curves for high-impedance schemes.
- Do primary injection to confirm operation and stability. See injection testing.
- Record settings.
Illustrative example
A 1,500 kVA generator with its neutral earthed through a resistor that limits earth-fault current to a few hundred amps is protected with REF. The relay is set to detect a small percentage of the limited current. Primary injection through the neutral confirms it operates, and an external-fault test confirms that it remains stable.
How restricted earth fault works
REF compares the current entering a star-connected winding on the phase conductors with the current flowing in the neutral-to-earth connection. For a fault outside the winding, the two balance and the relay does not operate. For a fault inside the winding, current flows in the neutral that is not balanced by the phase currents, and the relay operates.
Operating quantity = | I(phases, residual) + I(neutral) | with CT polarity arranged so external faults cancel
Why use it
- A fault near the neutral end of a winding produces little voltage across the fault and low current in the line terminals, so differential protection may not see it.
- REF is very sensitive to such earth faults inside its zone.
- It is fast and clearly restricted to the winding, so it does not need to grade with downstream devices.
Applications
| Equipment | Role of REF |
|---|---|
| Transformer star winding | Detects winding earth faults, complements differential |
| Generator stator | Detects earth faults in the stator winding where earthing limits current |
Implementation points
- CTs on the phases and neutral should be of the same ratio and matched characteristics.
- High-impedance REF uses a stabilising resistor and a voltage-setting; low-impedance numerical REF uses algorithms. Choose one approach and design for it.
- CT knee-point voltage must be adequate for the stability setting.
- Wiring and polarity errors cause wrong operation; confirm by primary injection.
See differential protection and CTs and VTs.
Commissioning tests
- Stability test with through current.
- Operation test with injection into the zone.
- Check of CT polarity and ratio.
Common mistakes
- CTs of different types in the same scheme.
- Wrong CT polarity.
- Setting without knee-point checks.
What to send us
Send generator or transformer data and earthing arrangement. Send your application details. See also earth-fault protection.
Questions engineers ask
Why use REF as well as differential?
REF is more sensitive to low-level earth faults near the neutral, which differential may not detect.
What is a stabilising resistor?
A resistor in a high-impedance scheme that keeps the relay stable for external faults.
Can REF protect a delta winding?
No. It applies to earthed star windings.
Does it work with a solidly earthed neutral?
Yes, and it is still useful, though differential covers more.
This guide gives general principles. Scheme design needs CT data and a study.