Mission Brief
The Fault Nobody Sees
A 20 MVA transformer's 13.8 kV winding is resistance-grounded through a 20 Ω NGR. A ground fault near the neutral end of the winding makes so little current that the phase differential and the 51N both stay silent while the winding slowly cooks. Your job: commission the Restricted Earth Fault element so it catches that near-neutral fault — yet stays rock-stable when a heavy fault downstream saturates a CT.
- See REF as a current balance around a restricted zone (Kirchhoff)
- Understand why external faults cancel and internal faults don't
- Set the high-impedance stability voltage above a saturated CT (IEEE C37.110)
- Size the stabilising resistor and confirm CT knee-point ≥ 2·Vs
- Prove sensitivity to a near-neutral fault and check CT polarity
It's a balance around a bag
Forget the name for a second. REF is Kirchhoff's law around a zone: add up every current crossing the boundary — the three phase CTs and the neutral CT. Whatever flows in must flow out. If the sum isn't zero, current is escaping inside the zone.
balanced — in equals out
Now the name — “restricted”
The protected zone is restricted to the area between the phase CTs and the neutral CT. And it's a differential of the ground current — neutral CT versus the residual of the three phase CTs — not an overcurrent like 51N. That's the whole reason people confuse it.
External vs internal — the flip
External fault: the ground current leaves on a phase and returns through the neutral — the two CTs cancel, spill ≈ 0, stable. Internal fault: the current returns to ground inside the zone and never reaches the phase CTs — spill = the neutral current, operate. Toggle it above.
Why bother — it sees what nothing else can
A ground fault near the neutral end of the winding drives very little current. The 87T is slope-desensitised and the 51N sits above load, so both go blind down there. REF carries no load current, so it can be set far more sensitively and protect the near-neutral winding everyone else misses.
Winding Coverage — REF vs 51N
How far down the wye winding (toward the neutral) each element can still detect a ground fault. The near-neutral zone makes little current.
REF protects 95% of the winding versus 51N's 37% — the bottom 58% near the neutral is only covered by REF.
The only real enemy — through-fault CT saturation
On a heavy external fault one phase CT can saturate and under-read, creating a false spill. High-impedance REF fixes this with a stabilising resistor and a setting voltage above what a saturated CT can ever produce (and a CT knee-point ≥ 2·Vs). Slide the saturation up and watch the setting hold the line.
High-Impedance Stability (IEEE C37.110)
The modern alternative — biased REF
Numerical relays skip the resistor and use a percentage-slope differential on the residual instead — the same characteristic plane as the 87T, applied to the ground current.
Biased REF restrains with a single percentage slope (20%) above a minimum pickup (0.1× In) — tolerant of CT mismatch, no stabilising resistor. Drag the operating point: internal faults climb into operate, external imbalance stays restrained.
drag the point — operate above the dual-slope line (slope1 20% · slope2 20%)