Transformer protection — 87T / 87REF / 63
ANSI 87T differential primary, 87REF restricted earth fault, percentage-restraint bias + 2nd/5th harmonic blocking, sudden-pressure (63), Buchholz, thermal (26/49). One modern numerical relay (SEL-787 / GE T60 / RET670 / 7UT85).
Step 1 — Transformer protection: ANSI 87T primary + backups
Reference notes
Power transformers are protected by a layered set of ANSI functions implemented in a modern microprocessor relay (SEL-787, GE T60, ABB RET670, Siemens 7UT85). Use Next → to walk through 87T differential (primary), 87REF restricted earth fault, percentage-restraint biasing, inrush blocking, and the mechanical / thermal protections that complete the package.
ANSI function set
| ANSI | Function |
|---|---|
| 87T | Transformer differential — primary internal-fault detection |
| 87REF | Restricted Earth Fault — wye-winding ground-fault protection |
| 50 / 51 | Phase overcurrent — backup + downstream coordination |
| 50G / 51G | Ground overcurrent |
| 63 | Sudden Pressure relay — rapid oil-pressure rise from arcing |
| Buchholz | Gas-collection relay (oil-immersed conservator-tank transformers) |
| 26 | Hot-spot thermal (RTD) |
| 49 | Thermal overload (IEEE C57 thermal model) |
| 71 | Oil-level alarm |
| Tank GF | Tank ground-fault (insulation failure to ground) |
| 23 | Cooling-fan / pump control |
87T transformer differential — the cornerstone
- CT on each winding terminal.
- Currents converted to per-unit on common MVA base with software vector-group compensation (e.g., 30° for Dy11).
- Healthy operation: per-unit input = per-unit output → I_diff = 0.
- Internal fault disturbs KCL → I_diff > 0 → trip.
- Pickup typically 20-50 % rated; operates in 1-2 cycles.
Percentage restraint (bias)
External (through) faults create apparent differential from CT errors — without restraint, the relay would falsely trip on big external faults.
Where I_restraint = (I_pri + I_sec) / 2 or max(I_pri, I_sec). Typical: slope 25 % at low currents, slope 50 %+ at high currents (dual-slope) to ride through CT saturation.
Harmonic blocking
- 2nd-harmonic block: magnetizing inrush at energization has rich 2nd-harmonic (typically 15-30 %). Relay blocks trip when 2nd-harmonic ratio > threshold (~15 %).
- 5th-harmonic block: over-excitation (V/f > 110 %) produces rich 5th-harmonic. Relay blocks trip to avoid false operation during V/f events.
87REF restricted earth fault
87T is desensitized to ground faults near the wye neutral (small fault current limited by neutral grounding impedance, plus relay slope characteristic). 87REF fills this gap:
- 3 phase CTs in residual connection (output = 3·I_0) + 1 neutral CT.
- Healthy: residual = neutral current → differential = 0.
- Ground fault inside wye winding: differential ≠ 0 → trip.
- Pickup 5-10 % rated; operates < 1 cycle.
- Covers lower 70-90 % of wye winding from neutral.
- 87T + 87REF together cover ~100 % of winding.
Sudden pressure (63) and Buchholz
- ANSI 63 — rapid oil-pressure rise relay. Detects gas evolution from internal arcing before 87T might clearly pick up. Operating time 50-200 ms. Critical for incipient turn-to-turn faults.
- Buchholz — gas-collection relay in the pipe between main tank and conservator (oil-immersed transformers with conservators). Slow gas accumulation = alarm (incipient fault). Rapid oil surge = trip (major fault). Standard in Europe, India, Asia.
Thermal protections
- 26 — hot-spot RTD reading. Alarm at IEEE C57 hot-spot limit (110-120 °C), trip at destructive limit (140-150 °C).
- 49 — IEEE C57 thermal model. Computes winding temperature from load currents using the transformer's thermal time constants. Gives early warning of sustained overload.
- 71 — oil-level alarm via float / pressure transducer.
Redundancy for large units
Large transformers (typically > 100 MVA) use TWO complete protection relays from different manufacturers, each with its own CT sets and trip path. Diverse design protects against manufacturing defects, design bugs, and setting errors. Trip outputs combined via OR. NERC PRC standards require this on bulk-electric-system transformers.
Modern numerical relay implementation
- Single device handles 87T + 87REF + 50/51 + 50G/51G + 26 + 49 + 71 + cooling control.
- Plus oscillographic recording on every trip (sub-cycle resolution waveforms for forensics).
- IEC 61850 / DNP3 / Modbus communication to SCADA.
- Programmable logic for custom schemes.
- Annual or biannual secondary-injection testing per IEEE C37.230.