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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Pilot wire used in
Non unit type of protection
Unit type of protection
Both A and B
None of above
Non unit type of protection
Quick Summary: Pilot wire protection is classified as a unit type protection because it protects a specific zone (the protected section of the transmission line) by comparing the currents entering and leaving that section. It operates on the principle of circulating current or opposed voltage, ensuring the relay trips only for faults within the defined protected zone, ignoring faults outside it.
Pilot wire protection is classified as a unit type protection because it protects a specific zone (the protected section of the transmission line) by comparing the currents entering and leaving that section. It operates on the principle of circulating current or opposed voltage, ensuring the relay trips only for faults within the defined protected zone, ignoring faults outside it.
Idiff=∣Iin−Iout∣ — Differential current principle
IR=IA−IB=0 — Balanced condition in healthy state
The system compares the currents at both ends of the protected line using pilot wires. Under normal or external fault conditions, the vector sum of currents entering and leaving the zone is zero, resulting in no current through the relay. If an internal fault occurs, the balance is disturbed, and the resulting differential current operates the relay.
Pilot wire protection is inherently a unit protection scheme.
It provides high-speed, selective clearing of faults.
It is primarily used for feeder and transmission line protection.
The sensitivity is limited by the length of the pilot wires due to capacitance effects.
Highly selective; operates only for internal faults.
Fast operating speed.
Independent of the load current magnitude.
High installation cost due to auxiliary pilot wires.
Limited by the maximum length of the pilot circuits.
Prone to interference and induced voltages.
Short transmission lines
Underground cable feeders
Busbar protection
| Feature | Non-Unit | Unit |
|---|---|---|
Protection Category | Non-unit (e.g., Distance) | Unit (e.g., Pilot Wire) |
Non-unit types include overcurrent and distance relays, which cover zones that overlap rather than being precisely defined by physical boundaries.
The term 'Unit Protection' refers to schemes that protect only a specific physical section of the system.
B is correct — Pilot wire protection is a classic example of unit protection because it covers only a specifically defined zone between the current transformers.
Always remember that 'Unit Protection' schemes (like Differential or Pilot Wire) have a distinct boundary, whereas 'Non-Unit' schemes (like Distance relays) have zones defined by time-impedance characteristics.