Restricted earth fault (REF, 87N or 64REF) is a differential protection that compares the neutral current of a wye winding with the sum of its phase currents. It detects ground faults inside the winding, even close to the neutral, where transformer differential protection has little sensitivity.
What 87N/64REF (restricted earth fault) protection is
Restricted earth fault, REF for short, is a differential protection limited to ground faults on a single winding. It compares two measurements:
- The winding neutral current, measured with a CT on the neutral-to-ground connection.
- The residual current of the phases of the same winding (3I0), measured with the phase CTs.
If the ground fault is outside the zone, both currents are equal and cancel inside the relay. If it is inside the winding, only one of them sees it, or both see it in the same direction, and a differential current appears.
In ANSI/IEEE C37.2 nomenclature it is identified as 87N (neutral differential). In IEC practice it is called 64REF or simply REF. In IEC 61850 each manufacturer models it differently, often with a differential protection node; check your relay’s data model.
It is called “restricted” because its zone is limited to what lies between the phase CTs and the neutral CT. It does not see faults outside that zone, which allows it to be very sensitive and fast.
Where it is applied
- Power transformers with a grounded wye winding, solidly or resistance grounded, at high and medium voltage.
- Generators with low-impedance neutral grounding.
- Grounding transformers (zigzag or wye-delta), where REF protects the grounding transformer itself.
- Autotransformers, with a high-impedance scheme covering the series and common windings.
- Transformers in solar and industrial plants, on the grounded winding.
The typical case is a resistance-grounded wye winding. The resistor limits the ground fault current to a low value, and for a fault close to the neutral the current reflected on the line side is so small that 87T differential does not see it. REF, on the other hand, measures directly at the neutral and keeps its sensitivity over most of the winding.
How it works
There are two principles.
High-impedance REF. The phase CTs and the neutral CT are connected in parallel and feed a measuring element in series with a stabilizing resistor. For an external fault, current circulates between the CTs and hardly passes through the relay. If one CT saturates, its magnetizing impedance collapses and the other CT drives current through it; the stabilizing resistor makes the voltage needed to operate the relay higher than the voltage that appears in that case. For an internal fault, the CTs have nowhere to discharge the current except through the relay, the voltage rises and the relay operates. A non-linear resistor (varistor) is installed in parallel to limit that voltage.
The classic stability criterion is that the relay setting voltage must be higher than the voltage that appears when one CT fully saturates at the maximum external fault:
V_s ≥ I_f · (R_CT + 2·R_L)
where I_f is the secondary current of the maximum external fault, R_CT the secondary winding resistance of the saturated CT and R_L the resistance of one conductor of the loop. The CT knee-point voltage must be sufficiently higher than V_s, according to the manufacturer’s recommendation. This scheme requires CTs of equal ratio and defined characteristics, for example class PX per IEC 61869-2 (class C with known excitation curves in IEEE C57.13 practice).
Low-impedance REF. It is a numerical differential. The relay calculates the residual phase current, compares it with the neutral measurement and applies a bias characteristic, like any percentage differential. It accepts CTs with different ratios, because it compensates in software, and can share CTs with other functions. Many manufacturers add a directional criterion that compares the phase of the neutral current with the residual current to improve stability.
| Aspect | High impedance | Low impedance |
|---|---|---|
| Principle | Phase and neutral CTs in parallel on a measuring element with stabilizing resistor | Numerical differential between residual phase current and neutral, with bias |
| Stability under saturation | Setting voltage higher than that of a saturated CT at the maximum external fault | Bias characteristic and, in many relays, a directional criterion |
| CTs | Equal ratio and defined characteristics, for example class PX | Accepts different ratios and can share CTs with other functions |
Setting criteria
- High impedance: stabilizing voltage calculated with the maximum external fault from the short-circuit study, CT and wiring resistances measured in the field, and verification of the primary operating current, which includes the magnetizing currents of all CTs at that voltage.
- Low impedance: minimum differential pickup below the smallest internal fault current to be detected, and bias slopes according to CT errors and expected saturation.
- Sensitivity: define what percentage of the winding must be covered, based on the current limited by the neutral resistor.
- CTs: ratios, class and knee point according to IEEE C37.110 or IEC 61869-2. See CT excitation test and knee point.
Everything depends on the study and on the relay manual. There is no universal setting.
How each manufacturer implements it
- Siemens SIPROTEC (DIGSI) offers biased low-impedance REF and, depending on the model, the high-impedance variant with external components.
- SEL (AcSELerator QuickSet) includes low-impedance REF in its transformer relays, with a directional criterion.
- Relion (Hitachi Energy 670/650; ABB 615/620) (PCM600) has separate functions for low-impedance REF and for high-impedance differential.
- GE Vernova (Multilin, EnerVista) includes biased low-impedance REF in its transformer relays.
- Schneider MiCOM/Easergy (MiCOM S1 or Easergy Studio) offers both types depending on the model.
The most relevant differences are how the minimum pickup is defined, how the neutral CT is scaled against the phase CTs and what additional stability criterion each one uses.
Why it trips or blocks
- Reversed neutral CT polarity. It is the most common cause of trips on external faults. It is found with a polarity test or, in service, with the oscillography of the first external fault.
- Neutral CT ratio entered wrong in the low-impedance relay.
- CTs with different ratios in a high-impedance scheme, or one CT of a different class.
- CT saturation on severe external faults or during energization, if the bias or the stabilizing voltage is not enough.
- Open CT in a high-impedance scheme: very high voltages appear, the varistor operates and may be damaged.
- Wiring shared with other functions that leaves a double ground in the secondary circuit.
The first thing to check is the oscillography: compare the neutral current with the residual current calculated from the phases. The guide to current transformer terminals and secondaries helps to check polarities and grounding.
How it is tested
With secondary injection, for example with an Omicron CMC 356:
- High impedance: verify the relay operating voltage, the stabilizing resistor and the varistor, and calculate the primary operating current with the CT excitation curves.
- Low impedance: verify the minimum pickup, the bias slopes and stability by simulating an external fault, with neutral and phase currents in opposition; then an internal fault.
- Polarity and stability: during commissioning, primary injection confirms that a simulated external fault produces no differential current.
Did your relay trip or block on this function and it is not clear why? At Tiza Group we review the events and settings remotely. See our remote relay configuration service or contact us.
References
- IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
- IEEE C37.91, Guide for Protecting Power Transformers.
- IEEE C37.102, Guide for AC Generator Protection.
- IEEE C37.110-2023, Guide for the Application of Current Transformers Used for Protective Relaying Purposes.
- IEC 61869-2, Additional requirements for current transformers.
- J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.
Frequently asked questions
Why did the transformer REF trip for a ground fault on the line?
Almost always it is the polarity or ratio of the neutral CT. With the neutral CT reversed, an external fault looks internal. Check the oscillography: for an external fault, the neutral current and the residual phase current must be in opposition inside the relay.
What is the difference between high- and low-impedance REF?
High-impedance REF uses a stabilizing resistor and requires CTs with equal ratio and suitable characteristics. Low-impedance REF is a numerical biased differential that accepts different ratios and compensates in software.
If I already have 87T, do I need REF?
In resistance-grounded windings or for faults close to the neutral, 87T may not have enough sensitivity, and REF covers that region. IEEE C37.91 treats it as a complement to transformer differential protection.