Differential Protection Configuration in SEL Relays
Key Takeaway
How to configure current differential protection (87) in SEL relays for transformers and lines — CT compensation, slope settings, harmonic restraint, and commissioning.
Quick Answer
Configure transformer differential (87T) in SEL-387/487E by setting winding configuration, CT compensation for ratio and phase angle, dual-slope percentage characteristic (SLP1 25%, SLP2 50-70%), and harmonic restraint (2nd harmonic 15-20% for inrush, 5th harmonic 35% for overexcitation).
Transformer Challenges
- CT ratio mismatch (different kV sides)
- Phase shift (delta-wye = 30° shift)
- Magnetizing inrush (false differential on energization)
- Overexcitation (false differential on overvoltage)
Settings
- Slope 1 (SLP1) — 25% minimum slope at low current
- Slope 2 (SLP2) — 50-70% high slope for CT saturation security
- O87P — Minimum pickup (differential current to operate)
- PCT2 — 2nd harmonic restraint (15-20%, blocks on inrush)
- PCT5 — 5th harmonic restraint (35%, blocks on overexcitation)
Commissioning
- Verify CT polarity using load current
- Check differential current near zero on load
- Verify restraint current is correct
- Test operate with secondary injection
Frequently Asked Questions
Magnetizing inrush (addressed by 2nd harmonic restraint), CT saturation during external faults (addressed by dual-slope characteristic), and incorrect CT polarity or compensation.
During inrush, differential current is rich in 2nd harmonic. If 2nd harmonic exceeds PCT2 threshold (15-20%), the 87T element is blocked, preventing misoperation.
The ratio of operate current to restraint current required for operation. Higher slope = less sensitive but more secure against CT errors.