Differential protection (87T) is the principal protection of power transformers. It compares the current entering and leaving the transformer; if the difference exceeds the permitted limit an internal fault (inter-turn, phase-to-phase or phase-to-earth short circuit) is declared and breakers trip within milliseconds.
Operating principle
Two quantities are calculated for each phase:
Ir = (|I1| + |I2|) / 2 (restraint current)
I1 and I2 are the CT secondary currents after ratio and vector-group compensation. Under normal conditions or external faults Id is close to zero; for internal faults Id is large. The relay operates when the point (Ir, Id) lies above the operating characteristic.
Dual-slope characteristic
- Base pickup (Idmin): typically 0.2 to 0.3 times rated current (pu), to cover CT error, tap changer and magnetising current.
- Slope 1: about 20 % to 30 %, for ratio and tap-changer errors.
- Breakpoint: about 1.5 to 2 pu.
- Slope 2: about 50 % to 80 %, to prevent mal-operation when CTs saturate on a heavy external fault.
- Unrestrained (high-set) differential: usually 8 to 10 times rated current, for instantaneous tripping on severe faults even when CTs saturate.
These values are general guidance; final values follow the relay manual and the study.
Vector-group and CT-ratio compensation
Older relays used auxiliary CTs and delta/star connections for compensation. Digital relays do this in software: CT ratios and the vector group (e.g. Dyn11) are entered and the relay compensates phase shift and magnitude and removes zero-sequence current so an external earth fault does not cause mal-operation.
Harmonic restraint
- Inrush current when a transformer is energised can be several times rated current and flows on one side only, so it looks like an internal fault. It contains a high second-harmonic content (typically above 15 %); the relay blocks the function to avoid mal-operation.
- Over-excitation caused by high voltage or low frequency produces fifth harmonic (common threshold 20 % to 35 %).
- Cross-blocking uses harmonic detection in one phase to block all three phases while keeping safe energisation onto a two-phase fault.
Complementary protection
- Restricted earth fault (REF, 64): high sensitivity for earth faults near the winding neutral.
- Buchholz relay (63) and oil temperature/pressure for internal faults that do not produce much current.
- Backup overcurrent (50/51) for external faults and as back-up to the differential.
Testing a transformer differential relay
- Check base data: rating, voltages, vector group, CT ratios, tap and settings.
- Pickup test: inject single-phase current into one side up to Idmin.
- Slope test: inject two currents simultaneously into the primary and secondary sides at several points of the characteristic and plot the operating boundary against the theoretical characteristic.
- Vector-group compensation test: inject balanced three-phase currents with angles matching the vector group; the differential current must be near zero.
- Harmonic restraint test: add second- and fifth-harmonic components to the base current and observe blocking at the set threshold.
- Unrestrained function and operating time.
- Logic and trip matrix: commands to both side breakers and breaker-failure initiation.
- Primary and on-load test: after energisation read the real currents on both sides; the differential current should stay within CT and tap-changer error.
Common mistakes
- Incomplete or wrong vector group and CT ratio entry
- Reversed CT polarity
- CT secondary circuit earthed at two points
- Ignoring the tap changer and not setting a suitable Slope 1
- Wrong harmonic thresholds, causing a trip at energisation
Testing and commissioning of transformer differential relays is a core ENP service, delivered with a full report and the settings file.