GOVERNMENT POLYTECHNIC BHUJ · Electrical Engineering Department

CO4 Virtual Lab: Transformer Differential Protection

Percentage-biased differential relay, CT mismatch, external faults, internal faults and inrush restraint

Switchgear and Protection Course Code: DI05009011 Semester 5 Academic Year 2026–27 Offline Browser Lab
CO4: Apply suitable protection schemes for generators, transformers, and induction motors to ensure safe and reliable operation under fault conditions.

Experiment

To study percentage-biased differential protection of a power transformer under healthy, external-fault, internal-fault and magnetizing-inrush conditions.

Aim

Simulate the currents supplied by CTs on both sides of a transformer, calculate differential and bias currents, and determine whether the numerical differential relay remains stable or issues a trip.

Learning outcomes

  • Differentiate internal and external transformer faults.
  • Explain the need for percentage bias.
  • Study the effects of CT error, CT saturation and tap mismatch.
  • Use second-harmonic restraint during magnetizing inrush.

Stability

The relay should remain restrained for load current and external faults.

Sensitivity

The relay should trip rapidly for internal winding and earth faults.

Security

Second-harmonic restraint should prevent unwanted tripping during inrush.

Educational model: The calculations are simplified for diploma-level learning. Actual transformer protection requires manufacturer relay logic, vector-group compensation, CT excitation data, transient studies and validated system parameters.

Theory

Differential principle

CTs are installed on the high-voltage and low-voltage sides. After ratio and phase compensation, the relay compares the two secondary currents.

Differential current: Iop = |I1 − I2| for through current.

Bias current: Ibias = (|I1| + |I2|)/2.

Percentage bias

The operating threshold increases with bias current. This improves stability when high external-fault current causes unequal CT errors or CT saturation.

Operate when: Iop > Ipickup + Slope × Ibias.

Internal fault: currents entering the protected zone do not balance, producing large differential current. The relay should trip both transformer-side circuit breakers.
External fault: the same through current enters and leaves the protected zone. The relay should remain restrained even if bias current is large.

Second-harmonic inrush restraint

Transformer energization may produce a large differential current without an internal fault. Magnetizing inrush typically contains significant second-harmonic content. A numerical relay can block differential tripping when the second-harmonic percentage exceeds a selected threshold.

Model basis and assumptions

  • Transformer rated currents are calculated from S/(√3V), and the entered CT ratios convert primary currents to 5 A relay-secondary quantities.
  • With numerical compensation enabled, both winding currents are referred to a common 5 A per-unit base and the entered tap mismatch is corrected.
  • Healthy and external-fault cases use equal through-current on both transformer sides; internal faults use user-selected in-zone source contributions that add in the differential element.
  • CT saturation is represented as a reduction in one CT's fundamental-current magnitude. It is a teaching approximation, not a transient CT waveform model.
  • The relay decision uses a continuous dual-slope percentage-bias characteristic and optional second-harmonic blocking. Manufacturer-specific operating time is intentionally not predicted.

Pre-Test

1. Transformer differential protection mainly protects:

2. Bias current is used mainly to improve:

3. An external fault should normally cause the differential relay to:

4. Second-harmonic restraint is associated with:

5. CT saturation during an external fault may create:

Transformer Differential Zone

HV BUS LV BUS CB-HV Closed CT1 HV side 1000 kVA | 11/0.415 kV Rated currents and CT ratios POWER TRANSFORMER HV LV CT2 LV side CB-LV Closed 87T RELAY Restrained
CT1 current
—
relay secondary
CT2 current
—
relay secondary
Differential current
—
Iop
Bias current
—
restraint quantity
Operate threshold
—
pickup + slope
Second harmonic
—
inrush discriminator
Operating margin
—
Iop − threshold
Relay decision
—
87T output
Select a preset or parameters, then run the simulation.
Percentage-bias characteristic
Fundamental and second-harmonic content

Procedure

1
Select Healthy, run the simulation and verify that Iop is below the operating threshold.
2
Select External fault. Observe the high bias current and restrained relay operation.
3
Select CT saturation. Increase LV CT saturation and determine whether the percentage bias prevents unwanted operation.
4
Select Internal fault. Verify that the operating point enters the trip region and both transformer circuit breakers open.
5
Select Inrush restrained. Verify that high second-harmonic content blocks the trip output.
6
Select Inrush without restraint. Observe the risk of unwanted tripping.
7
Change tap position, CT errors and bias slopes. Record each important case using Add Observation.

Observation Table

CaseTransformerCT ratiosScenarioI1 (A)I2 (A)Iop (A)Ibias (A)Threshold (A)Margin (A)H2 (%)CompensationDecisionReason

Post-Test

1. A high bias current with small differential current normally indicates:

2. Percentage bias helps prevent false operation due to:

3. During a true internal fault, the relay should:

4. If second harmonic is above the blocking threshold during inrush:

5. Numerical ratio/tap compensation reduces:

Result and Conclusion

No conclusion generated yet.

Suggested result statement

The percentage-biased differential relay remained stable during healthy and moderate external-fault conditions, operated for internal transformer faults, and was prevented from unwanted operation during magnetizing inrush by second-harmonic restraint. Numerical ratio and tap compensation reduced false differential current caused by CT-ratio and tap mismatch.

Methodological boundary: This laboratory is appropriate for concept demonstration and relative comparison. It is not a short-circuit program, electromagnetic-transient model, CT saturation waveform model, or manufacturer relay test set, and it must not be used to commission an actual transformer protection scheme.