Differential protection compares currents entering and leaving a defined zone. It is fast and selective for internal faults, but measurement errors, CT saturation and transformer phase shift must be stabilised.
Learning objectives
Draw the zone, form operate and restraint quantities and apply the special compensation needed for transformers, busbars and machines.
Core engineering principles
The zone is fixed by CT locations
Anything between the CTs is internal; anything outside should remain stable. Breaker and cable stubs must not be unintentionally left between zones.
Restraint stabilises measurement error
Percentage differential characteristics increase required operate current as through current grows, helping remain secure during CT error and external faults.
CT saturation is critical on external faults
One saturated CT can create false spill current. Bus differential may use dedicated algorithms, check zones and rapid saturation detection.
Transformers require ratio and vector compensation
Winding ratio, tap position, phase shift and zero-sequence removal must be represented. Magnetising inrush and overexcitation require harmonic or waveform-based restraint.
Function suffix states the protected object
87T, 87B, 87M and 87G are application identifiers; settings and algorithms differ substantially.
Engineering application method
- Step 1: Place CTs and draw the protected zone.
- Step 2: Calculate ratio, phase and zero-sequence compensation.
- Step 3: Set pickup and restraint slopes from CT and fault studies.
- Step 4: Configure inrush, overexcitation or saturation stability.
- Step 5: Test internal, external, CT-saturation and energisation cases.
Practical example
A transformer differential relay can trip on normal energisation if phase compensation is correct but inrush restraint is disabled. Conversely, excessive restraint can delay a genuine turn-to-turn fault.
Common mistakes
- Matching CT ratios without vector-group compensation.
- Ignoring tap-range mismatch.
- Testing only balanced internal faults.
- Using harmonic block without security review.
- Assuming all 87 functions use the same settings.
Design and review checklist
- Are CT locations and zone boundaries clear?
- Is compensation correct?
- Are external faults stable under CT saturation?
- Is inrush handled?
- Are trip destinations and lockout defined?
Standards basis and official sources
- IEC 60255-1:2022 — Common requirements for measuring relays and protection equipment.
- IEC 61869-1:2023 — General requirements for instrument transformers and low-power instrument transformers.
- IEEE C37.2-2022 — Device function numbers, acronyms and contact designations.
Engineering note: Verify the contracted standard edition, amendments, manufacturer evidence and project-specific studies before applying these principles to a supplied assembly.