Differential Protection Configuration in SEL Relays
Key Takeaway
How to configure transformer current differential (87T) in SEL-387/487E — winding/CT compensation, dual-slope SLP1/SLP2, harmonic blocking (PCT2/PCT4/PCT5), and commissioning. Line differential (87L) is out of scope here.
Quick Answer
Configure transformer differential (87T) in SEL-387/487E by entering winding connections and CT data (ratio and phase compensation), setting the dual-slope percentage characteristic, and enabling harmonic blocking/restraint. On the SEL-487E, factory defaults commonly cited in SEL documentation are about SLP1 = 35%, SLP2 = 75%, PCT2 = 15%, PCT4 = 15%, and PCT5 = 35% — always confirm in the model-specific instruction manual and application studies. This article covers 87T transformer differential, not line current differential (87L).
Transformer Challenges
- CT ratio mismatch (different kV sides / taps)
- Phase shift (e.g., delta-wye often 30° depending on vector group)
- Magnetizing inrush (false operate current on energization)
- Overexcitation (false differential under high V/Hz)
- CT saturation on external faults (security vs sensitivity tradeoff)
Key Settings (SEL-487E framing)
- O87P — Minimum operate (differential) current pickup in per-unit of tap
- Slope 1 (SLP1) — Normal-security slope; SEL-487E default commonly 35% (not a universal “25%” rule)
- Slope 2 (SLP2) — Higher slope used when external-fault/high-security logic asserts; SEL-487E default commonly 75%
- PCT2 / PCT4 — Second- and fourth-harmonic percentages used for inrush security (blocking and/or restraint, depending on E87HB/E87HR). Defaults commonly 15% each. Modern SEL-487E documentation emphasizes 2nd and 4th harmonics, plus waveshape-based inrush detection for low-harmonic inrush
- PCT5 — Fifth-harmonic percentage (default commonly 35%) used for overexcitation-related blocking/monitoring; with voltage inputs, V/Hz elements also apply
- U87P — Unrestrained (instantaneous) differential pickup — set high enough to ride through inrush because it is not harmonic-blocked
Commissioning
- Verify CT polarity and winding compensation with load or injection
- On load, confirm operate current near zero and restraint current as expected
- Validate slope/pickup with secondary injection (including SLP2/high-security path if used)
- Confirm harmonic blocking/restraint asserts during simulated inrush / known energization tests
- Document final settings against the SEL instruction manual for that firmware
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Frequently Asked Questions
Magnetizing inrush (addressed by 2nd/4th harmonic blocking or restraint and waveshape logic), CT saturation on external faults (dual-slope / high-security SLP2), overexcitation (5th harmonic / V/Hz), and incorrect CT polarity or compensation settings.
On SEL-487E, harmonic security uses settings such as PCT2 (2nd), PCT4 (4th), and PCT5 (5th), with harmonic blocking (E87HB) and/or harmonic restraint (E87HR) enabled as required. Inrush is typically rich in 2nd (and often 4th) harmonic content; when harmonic quantities relative to fundamental exceed the configured percentages, the restrained differential path is blocked or restrained so the 87T element does not trip on energization. Exact logic and scaling are model-specific — use the instruction manual.
Slope is the operate-to-restraint ratio required to trip. Higher slope means less sensitive but more secure against CT ratio/saturation errors. SEL-487E uses SLP1 in normal conditions and SLP2 when external-fault/high-security logic is active; typical factory defaults are about 35% and 75%.
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