Line protection — 87L + 21 + 67 + pilot schemes
Transmission-line protection: 87L line differential (100% coverage via comms), 21 distance multi-zone (Z1≤80% / Z2 120% / Z3 220%), 67 directional OC, pilot schemes POTT / DCB / DUTT / DTT. SEL-411L / 421, GE D60, REL670, 7SA8.
Step 1 — Line protection: 87L + 21 + 67 + pilot schemes
Reference notes
Transmission-line protection combines unit protection (87L line differential) with non-unit protection (21 distance and 67 directional overcurrent) plus communication-assisted pilot schemes. Use Next → to walk through each protection type, the multi-zone distance characteristic, the four major pilot schemes (POTT, DCB, DUTT, DTT), and modern integrated platforms.
Two philosophies
- Unit protection (87L) — compares per-phase currents at both line ends via communications. 100 % line coverage, 1-cycle operation. Requires reliable comms.
- Non-unit (21 distance) — calculates Z = V / I locally. Multi-zone graded characteristic. No comms dependence at the basic level.
ANSI 87L line current differential
- CTs at both terminals. Communications channel: fiber, microwave, or pilot wire.
- Standards: IEEE C37.94 (electrical), IEC 60834 (teleprotection over digital).
- Time sync via GPS pulse-per-second or echo-delay compensation.
- Each relay digitizes currents at synchronized instants, exchanges data, computes differential locally.
- 1-cycle operation, 100 % line coverage, no underreach/overreach.
- Ideal for cable circuits where Z-based methods are unreliable.
- Comms-channel failure forces fallback to distance protection.
ANSI 21 distance protection
Calculates apparent impedance Z = V / I at the relay location → approximates distance to fault. Multi-zone graded scheme:
| Zone | Reach | Delay | Purpose |
|---|---|---|---|
| Z1 | 80-85 % of line | No intentional delay (~1 cycle) | Primary instantaneous trip |
| Z2 | ~120 % of line | 15-25 cycles | Covers Z1 gap + backup adjacent line |
| Z3 | ~220 % | 60-90 cycles | Backs up multiple adjacent lines |
Why Z1 < 100 %: avoid overreaching past the remote bus due to CT/PT errors, fault arc impedance, and infeed effects. The 15-20 % gap is covered by Z2 (slow) or pilot-accelerated to Z1 speed.
Impedance characteristics
- MHO — circle through origin in R-X plane. Common in early distance relays.
- QUADRILATERAL — rectangle bounded by reactance + resistance + directional lines. Modern numerical relay standard.
- REACTANCE — line in X-axis only + separate directional element.
Communication-assisted pilot schemes
| Scheme | Logic |
|---|---|
| POTT | Permissive Overreaching Transfer Trip — Z2 overreaches at both ends; permissive signal sent. Local trips when local Z2 pickup + permissive received. |
| DCB | Directional Comparison Blocking — reverse element sends a BLOCK signal to remote. No block + local Z2 pickup = trip. |
| DUTT | Direct Underreaching Transfer Trip — Z1 set short; remote Z1 pickup directly sends a transfer trip signal to local. |
| DTT | Direct Transfer Trip — pure remote trip command (also used for breaker failure backup). |
Trip times: ~2 cycles total (1 cycle local + 1 cycle comms). Comms channel monitored continuously; failure forces revert to time-delayed Z2.
ANSI 67 directional overcurrent
- 50/51 overcurrent + directional element (V-polarized).
- Forward direction defined by current-voltage angle relative to the relay's max-torque angle.
- Trip only when current > pickup AND direction = forward.
- 67P (phase) uses phase voltage as reference; 67G (ground) uses 3·V_0.
- Standard for distribution and sub-transmission with parallel sources or ring topology where plain 50/51 is non-selective.
Redundancy for critical lines
765 kV, generator interconnections, key tie lines per NERC PRC standards: TWO complete independent protection schemes, each with its own relay from a different manufacturer, separate CT cores, separate trip paths to two breaker trip coils (TC1 / TC2), separate DC supplies. Typical combination: 87L (primary A) + 21 with pilot scheme (primary B). Trip output via OR.
Modern integrated relays
- SEL-411L — pilot-trip line current differential, includes 87L + 21 + 67 + 50/51 + 79 + 25 + 50BF.
- SEL-421 — distance protection with all pilot schemes; 87L optional.
- GE D60 — comparable multifunction platform.
- ABB REL670 — IED 670 series, configurable function blocks.
- Siemens SIPROTEC 7SA8 / 7SD8 — distance and differential platforms.
All support IEC 61850 / DNP3 / Modbus, IRIG-B time sync, oscillographic recording, programmable logic.