In short. Differential protection trips when the current entering a protected zone differs from the current leaving it by more than a bias-adjusted threshold. It is fast, selective and independent of other devices’ timing, which is why it is used on larger generators, transformers and buses.
Principle
Under normal load and external faults, current in equals current out, and the difference is near zero. An internal fault creates a difference, which operates the relay. The relay uses a percentage bias, raising the threshold as through-current increases, to allow for CT errors.
Applications
| Application | Notes |
|---|---|
| Generator (stator) | Phase-to-phase and winding faults; CTs at neutral and terminal ends |
| Transformer | Needs vector group and ratio compensation, inrush and overexcitation restraint |
| Busbar | Multiple circuits, strong CT performance needed |
| Cable or motor | For critical or large items |
Key design points
- CT matching: ratios, class and burden. Poor CT performance causes false operation.
- Saturation: external faults with large DC offset can saturate CTs and create false differential current.
- Inrush restraint: transformer energisation draws current on one side only; harmonic restraint avoids tripping.
- Bias slope: chosen from CT errors and tap range.
- Zone boundaries: CT locations define the zone. Overlap with adjacent zones.
Settings steps
- Collect the CT ratios, classes and winding data.
- Set the minimum pickup above standing errors.
- Choose slope(s) for CT errors and tap changes.
- Enable harmonic restraint and unrestrained element as appropriate.
- Test with injection and through-fault checks.
Illustrative example
A 2.5 MVA generator uses a differential relay with CTs at the neutral and the terminals. With a minimum pickup of 10 percent of rated current and a slope of 25 percent, the relay stays stable for an external fault but trips on an internal phase fault within a few cycles. The same CT classes are used on both ends to avoid mismatch.
Principle in one line
Current entering a protected zone equals current leaving it in normal conditions. If they differ by more than a set amount, a fault must be inside the zone. The relay compares CT secondary currents from both ends.
Differential current = | I1 + I2 |; Restraint current = (| I1 | + | I2 |) ÷ 2, operate if differential exceeds slope × restraint + minimum pickup
The biased characteristic
CT errors and tap changers produce small spurious differential current during heavy through-faults. A biased (percentage) characteristic raises the threshold as through current grows, which keeps the relay stable on external faults but sensitive to internal ones.
Transformer-specific issues
| Issue | Treatment |
|---|---|
| Magnetising inrush on energising | Second-harmonic restraint or blocking |
| Phase shift of vector group | Vector compensation in the relay or CT connection |
| Ratio mismatch | Ratio correction in a numerical relay |
| Overexcitation | Fifth-harmonic restraint |
Generators and busbars
Generator differential covers the stator winding between neutral-side and terminal CTs. Busbar differential uses CTs on every connection to the bar. Each application has special issues, such as CT saturation on bus faults.
CT requirements
- Equal or well-matched CT classes at both ends.
- Adequate knee-point voltage to avoid saturation on through faults.
- Correct polarity.
- Low-burden wiring.
Commissioning checks
- Verify CT ratios and polarity.
- Inject through current and confirm low differential.
- Inject in-zone and confirm operation.
- For transformers, check the energisation behaviour.
See transformer protection and busbar protection.
Common mistakes
- CTs of different classes on each side.
- No inrush restraint on a transformer.
- Wrong vector group compensation.
What to send us
Send the equipment data and CT details. Send your application details. See also transformer protection and CTs and VTs.
Questions engineers ask
Why is differential faster?
It needs no time grading because it trips only for faults inside its zone.
What causes false differential trips?
CT saturation, mismatch, wiring errors and transformer inrush without restraint.
Is it needed on small generators?
Often not. It is common above a size or criticality threshold set by the owner or standards.
How is it tested?
By secondary and primary injection, plus checking stability during energisation.
This guide gives general principles. Relay settings need specific equipment data.