Relay Protection Academy Module 08 of 25
Module 08 Intermediate

Protection Signalling
and Intertripping

⌛ ~2 hours 📚 IEC 60834 / IEEE C37.94 📑 12 slides

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Module 08 / 8.1

Why Teleprotection Is Required

Weak or zero infeed
One end sees negligible fault current - its distance relay cannot measure impedance. The strong-infeed end must intertrip the weak-infeed breaker.
Transformer-feeder units
A fault between HV breaker and transformer cannot be seen by HV-side OC protection. Intertripping is the only fast clearance path.
Unit protection zones
Differential zones must trip all bounding breakers simultaneously. Remote breakers need a trip command via teleprotection.
Auto-reclose coordination
Multi-ended feeders need synchronised reclose sequences to prevent out-of-phase reconnection.
Teleprotection vs. telecontrol Teleprotection carries time-critical trip/block commands. Telecontrol carries non-urgent SCADA data. Both share physical infrastructure; protection always takes priority.

Module 08 / 8.2

Three Teleprotection Modes

Direct Intertripping
Remote breaker trips on signal alone - no local condition required. Fastest. Any spurious signal causes outage. Needs highest channel security.
Permissive Tripping
Remote command only acts if local relay has also detected a fault. Spurious noise alone cannot trip. Balanced security and dependability.
Blocking Scheme
Signal sent to block for external faults. Loss of channel does not prevent tripping for in-zone faults. Highest dependability - trips by default.
PUTT vs. POTT PUTT: local relay must see fault in Zone 1 before sending permissive. POTT: both ends send permissive if they see fault in overreaching Zone 2 - mutual confirmation required.

Module 08 / 8.3

Security, Dependability, Speed

Security (Puc)
Probability of an unwanted command (false trip). More security requires complex coding - adds delay.
Target: <1 incorrect trip per 500 equipment-years.
Dependability (Pmc)
Probability of a missing command (failure to trip). More dependability reduces security.
Target: <1 failure per 1000 transmissions.
Speed
Modern digital terminals: 4-40 ms. Faster = fewer code bits checked = less secure.
⎓
Diagram needed

Trade-off triangle: vertices Security, Dependability, Speed. Arrows on edges show improving one degrades adjacent. Mark Direct, Permissive, Blocking scheme positions. Dark background, accent colour labels.

Module 08 / 8.4

Transmission Media

Private pilot wires
Dedicated copper pair. Low cost, short distances (<few km). Isolation transformers mandatory against earth potential rise.
Rented pilot wires
Leased telecoms pair. Lower CAPEX but utility has no circuit control. A linesman's test oscillator can inject a false intertrip tone.
Power Line Carrier (PLCC)
HF signal (300-3400 Hz band) on the power conductor. Utility owns both paths. Signal is attenuated by the fault itself. Ice triples attenuation.
Optical fibre (OPGW)
Fibre in the earth wire. Full EMI immunity, very high bandwidth. Standard for new EHV. On/off modulation avoids source non-linearity.
Microwave radio
Line-of-sight, 2-12 GHz. No line infrastructure needed. Susceptible to antenna icing and licence constraints.

Module 08 / 8.5

PLCC Coupling Equipment

Line Trap (Wave Trap)
Parallel LC in series with line. High impedance at carrier frequency - prevents signal escaping to station busbar.
Coupling Capacitor
HV capacitor in series with signal path. Low impedance at carrier; blocks power frequency from electronics. Rated for full line voltage.
Drainage Coil
Passes power-frequency current safely to earth; presents high impedance to the carrier signal.
⎓
Diagram needed

PLCC single-line: HV conductor, Line Trap in series, branch down via HV Coupling Capacitor, Coupling Filter+Drainage Coil to earth, HF cable to PLCC transceiver. Label all. Dark background.

Module 08 / 8.6

Signalling Methods

DC voltage step
Voltage level change on pilot wire. Simple and fast. Vulnerable to external voltage during fault.
Plain tone
Tone present = trip. Fast, low bandwidth. Poor security - any noise at that frequency mimics a command.
Frequency Shift Keying (FSK)
Guard tone during healthy; command tone for trip. Channel failure (loss of guard) detected immediately and raises alarm. More secure than plain tone.
Digital PCM
Multi-bit codes with checksums and error correction. Highest security/dependability. Compatible with SDH/PDH and optical fibre.
FSK advantage A plain tone scheme cannot distinguish channel failure from a "trip" command. FSK can, because the guard tone proves channel integrity continuously.

Module 08 / 8.7

Fault Clearance Time Budget

Formula
Common omission The auxiliary trip relay (5-15 ms) is frequently excluded from the budget. Include every series element in the trip chain.
Typical times
Signalling4-40 ms
Relay10-40 ms
Trip relay5-15 ms
Circuit breaker40-80 ms
Total (fast scheme)60-175 ms

Module 08 / 8.8

PLCC Attenuation Budget

Line attenuation
0.03-0.2 dB/km (fair weather). Ice or hoar-frost: up to 3x the fair-weather value.
Fault attenuation
Phase-to-earth fault short-circuits the signal path. Additional 20-30 dB for permissive schemes where signal must pass through the fault.
Terminal loss
Coupling filter + wave trap + HF cable, both ends. Typically 11 dB total.
Signal boost
Transmitter power raised 10-20 dB on relay "start". Not applicable to direct intertrip (signal does not pass through fault).

Module 08 / 8.9

Pilot Wire Hazards

Earth potential rise (EPR) During a phase-earth fault, the station earth grid rises to high voltage. Metallic pilots carry this into the telecoms network - injuring personnel at the far end and damaging relay inputs. Isolation transformers rated for EPR (5-15 kV) are mandatory.
  • Rented pilots: linesman's test oscillator can inject a false intertrip tone
  • Rented pilots are unsuitable for direct intertripping
OPGW optical fibre: inherent advantages
  • No electric current - EPR cannot appear on fibre
  • Complete galvanic isolation
  • Immune to electromagnetic induction
  • On/off modulation avoids light-source non-linearity

Module 08 / Worked Example

Fault Clearance Time Budget

Given Target: 100 ms. CB: 40 ms. Trip relay: 10 ms. Relay: 20 ms.

Digital optical fibre: 4-10 ms. PLCC: 8-20 ms. Both within budget.

Trip relay omission error If 10 ms trip relay is excluded, engineer incorrectly allows 40 ms. Selects PLCC rated 35 ms. Actual clearance = 40 + 20 + 35 + 10 = 105 ms - exceeds limit.
PLCC attenuation check (150 km line) (150 x 0.2 x 3) + 11 + 30 = 131 dB, exceeds an 85 dB budget by 46 dB. Migrate to optical fibre or add signal boost.

Module 08

Knowledge Check