Protection must cover electrical and mechanical fault evidence
Transformer protection is layered because no single function is sensitive and secure for every fault. Phase differential is fast for many internal faults; restricted earth fault improves sensitivity near a grounded neutral; overcurrent provides backup; thermal and overexcitation functions cover abnormal operation; Buchholz, sudden-pressure and temperature devices detect conditions not immediately visible in terminal currents.
1. Required transformer data
Collect MVA, all winding voltages, vector group, neutral grounding, impedance at each tap, tap range, cooling/rating stages, inrush information, through-fault withstand, CT ratios/classes/locations, breaker locations, on-load tap changer, mechanical protection contacts and system min/max faults. Confirm whether the MV relay protects only the feeder cable/primary winding or a complete transformer zone with CTs on both sides.
2. Function selection
| Function | Purpose | Application note |
|---|---|---|
| 87T restrained differential | Fast internal phase/earth fault protection within CT zone. | Requires ratio/vector compensation, zero-sequence treatment, inrush and CT saturation security. |
| REF / 64 or 87N | Sensitive earth faults on grounded-wye/zigzag winding between phase CTs and neutral CT. | Zone/polarity critical; low- or high-impedance implementation. |
| 50/51 phase overcurrent | Primary/secondary backup and feeder short-circuit protection. | Coordinate with downstream LV/MV devices and transformer damage/inrush. |
| 50N/51N neutral/earth | Ground-fault backup and NGR duty protection. | Measurement source and grounding determine sensitivity. |
| 49 thermal | Winding/transformer thermal overload model. | Use manufacturer loading/cooling data and temperature inputs where available. |
| 24 V/Hz overexcitation | Protect core from excessive flux due to high voltage/low frequency. | Coordinate with excitation and system operating limits. |
| 63/Buchholz/pressure, 26/temperature | Mechanical/gas/pressure and temperature protection. | Trip/alarm contacts, latching and test method defined by transformer design. |
| 50BF, TCS, 86 | Ensure isolation and lockout after transformer trip. | Trip all source sides and block automatic re-energization. |
3. Differential compensation
A healthy transformer has different current magnitudes and phase angles on its windings. Numerical relays scale CT ratios, compensate vector-group phase shift and remove zero sequence where required before applying Kirchhoff balance. Enter actual vector group and CT polarity exactly. Incorrect zero-sequence removal can cause operation for an external earth fault on a grounded-wye winding.
For a 10 MVA, 20/6.3 kV Dyn11 transformer, rated currents are about 289 A on 20 kV and 916 A on 6.3 kV. CTs such as 300/1 and 1000/1 give near-rated secondary currents, but the relay must perform exact tap/ratio and 30° compensation. CT performance must be checked for maximum external through fault.
4. Percentage restraint
87T compares operate current with restraint derived from winding currents. A low-slope region provides sensitivity near load; higher slope provides stability for CT mismatch and saturation at high through current. An unrestrained high-set may trip very fast for severe internal faults, but must remain secure for inrush and CT saturation according to relay design.
Do not copy slope and pickup from a similar MVA transformer. Evaluate CT error, tap range, OLTC mismatch, relay compensation and maximum through-fault duty.
5. Inrush and overexcitation security
Transformer energization can produce several times rated current on one or more phases with strong asymmetry. Traditional relays use second-harmonic blocking/restraint; modern algorithms may also use fourth harmonic or waveform/dwell-time recognition. Fifth-harmonic criteria can provide overexcitation security. Set thresholds from the relay application guide and transformer/system studies.
Inrush restraint can delay an internal-fault trip if applied too broadly. Modern relays include cross-blocking choices and unblocking/high-set logic. Test internal fault during inrush if the application is critical, using manufacturer or simulated waveforms.
6. Restricted earth fault
REF compares neutral current with residual current from the phase CTs on the same grounded winding. It is especially sensitive to faults near the neutral where phase differential current can be small. The protected zone is between phase CTs and neutral CT. Correct polarity is essential: currents should restrain for external faults and operate for internal faults.
Low-impedance numerical REF uses biased/directional calculation. High-impedance REF requires matched CTs, knee-point and stabilizing-resistor calculations. REF is not a universal solution for a high-impedance-grounded winding; verify available ground-fault current and the relay principle.
7. Overcurrent backup coordination
Primary 51 pickup must carry emergency transformer load and inrush/cold load while detecting the minimum downstream fault reflected to the primary. Plot transformer through-fault withstand and downstream device curves. A high-set 50 stage must remain secure for maximum secondary through fault and inrush or be supervised by differential/inrush logic.
Coordinate backup clearing with transformer short-time withstand, switchgear ratings, NGR rating and breaker failure. Protection of the feeder cable between switchgear and transformer may require a sensitive/high-speed zone not delayed by the complete downstream network.
8. Trip matrix and re-energization
87T, REF, Buchholz trip and sudden pressure commonly trip and lock out all source breakers, initiate breaker failure and block ATS/autoreclose. Temperature alarms may stage cooling, alarm then trip. Distinguish OLTC protection if it has a separate compartment/isolating device. Re-energization after a transformer differential or mechanical protection trip should require investigation and authorized reset.
9. Commissioning tests
- Primary CT ratio/polarity and transformer vector group/phase rotation.
- Through-current stability across windings and differential operate test.
- External earth fault to prove zero-sequence compensation and REF restraint.
- Internal REF simulation and sensitivity.
- Inrush restraint using waveform playback or validated method.
- All mechanical protection contacts, alarm/trip stages and DC circuits.
- Trip every source breaker, 86 lockout, breaker failure and SCADA/event recording.
- Verify settings at extreme OLTC positions and each cooling rating.
Dedicated REF design guide
For a complete REF application workflow—including 64REF/87N terminology, winding coverage, high- versus low-impedance implementation, CT stability and sensitivity calculations, trip logic and commissioning—see Restricted Earth-Fault Protection Explained.
Related protection guides
Engineering limitation
This guide explains a defensible engineering workflow; it is not a project setting calculation. Final protection functions, settings, wiring and trip logic must be based on the approved single-line diagram, short-circuit and coordination studies, equipment data, grid code, relay manual, and verified commissioning results. Changes require formal protection-management control.
References and further reading
- IEEE C37.91-2021 — practical transformer-protection guidance
- IEC 60255-187-1:2021 — differential protection requirements and tests
- ABB RET615 transformer protection — function and application overview
- SEL — Fundamentals of Short-Circuit Protection for Transformers — differential, REF and negative-sequence protection
- SEL — A Call to Action for REF Protection — REF principles and setting sensitivity
Standards must be applied using the edition required by the project, utility and local law. Standards summaries on public pages are not substitutes for the controlled documents.