Distance Protection Configuration in SEL Relays
Key Takeaway
How to configure distance protection (21) in SEL relays — zone reach, mho vs quadrilateral, pilot schemes (POTT, DCB), and load encroachment settings.
Quick Answer
Configure distance protection by setting CT and VT ratios, zone reaches (Z1 at 80-85% of line impedance, Z2 at 120-150%, Z3 for backup), zone time delays, characteristic type (mho or quadrilateral), and zero sequence compensation factor (K0) for ground distance.
Zones
- Zone 1 — Instantaneous, 80-85% of line impedance
- Zone 2 — Time-delayed, 120-150% of line, backup for remote bus
- Zone 3 — Long delay, backup for adjacent zones
Characteristics
- Mho — Circular on R-X plane, simpler, good for bolted faults
- Quadrilateral — Independent resistive/reactive reach, better for high-resistance ground faults
Pilot Protection
POTT (Permissive Overreach Transfer Trip) and DCB (Directional Comparison Blocking) use MIRRORED BITS or communications channels between line-end relays for simultaneous high-speed tripping.
K0 Compensation
Zero sequence impedance differs from positive sequence (~3× higher). K0 factor compensates ground distance elements for accurate ground fault impedance measurement.
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Frequently Asked Questions
Instantaneous reach set to 80-85% of line impedance. Below 100% to avoid overreaching due to CT/VT errors.
Mho is circular on the R-X plane, simpler. Quadrilateral has independent resistive/reactive reach, better for high-resistance ground faults.
Zero sequence impedance is ~3× positive sequence. K0 adjusts ground distance measurement to correctly reflect fault impedance.
Still have questions about your setup? Talk to an engineer or call (210) 405-4248.