Teleprotection

A scheme where a Zone 1 under-reaching element at the local terminal sends an immediate, unconditional trip signal to the remote terminal upon fault detection.

• Key Mechanism: The under-reaching element covers only 80-90% of the line, ensuring it never overreaches beyond the remote bus.
• Channel Requirement: Requires a high-speed, secure communication channel; no local decision logic is needed at the receiving end.
• Weak Infeed Logic: Requires supplementary logic to handle conditions where the remote terminal has insufficient fault current to detect the fault independently.
• Application: Used on critical transmission corridors where absolute speed is paramount and channel security can be guaranteed.

A scheme where a Zone 2 over-reaching element detects a fault and sends a permissive signal to the remote terminal. Tripping occurs only when the local relay sees the fault AND receives a permissive signal from the remote end.

• Security vs. Speed: More secure than DUTT because it requires confirmation from both ends, preventing false trips from channel noise.
• Over-Reaching Logic: Zone 2 elements are set to reach 120-150% of the protected line, ensuring coverage of the entire line including the remote bus.
• Current Reversal Guard: Includes logic to prevent misoperation during current reversals on parallel lines when a fault is cleared sequentially.
• Echo Logic: Retransmits the received permissive signal back to the sending terminal to cover conditions where a weak terminal cannot detect the fault.

A hybrid scheme combining elements of both DUTT and POTT. A Zone 1 under-reaching element sends a direct trip, while a Zone 2 over-reaching element sends a permissive signal.

• Dual Logic Paths: The under-reaching element provides fast tripping for close-in faults; the over-reaching element provides coverage for end-of-line faults with remote confirmation.
• Channel Degradation Resilience: If the communication channel fails, the scheme gracefully degrades to Zone 2 time-delayed backup protection rather than failing completely.
• Coordination Complexity: Requires careful coordination of Zone 1 and Zone 2 reach settings to avoid race conditions between the direct and permissive paths.

A scheme where a reverse-looking directional element sends a block signal to prevent the remote terminal from tripping for external faults. Tripping is permitted unless a block signal is received.

• Fail-Safe Philosophy: The default state is to trip; the channel must actively block tripping. A channel failure during an internal fault does not prevent tripping.
• Carrier-Based Origins: Historically implemented with power line carrier (PLC) using ON/OFF keying, where carrier signal presence indicates a block.
• Reverse Zone Setting: The reverse-looking element must be set to see all external faults behind the terminal, typically with a reach of 150-200% in the reverse direction.
• Transient Block Logic: Includes timers to extend the block signal briefly after the reverse element drops out, preventing false tripping during sequential fault clearing.

A scheme designed for frequency-shift keying (FSK) channels where the receiver distinguishes between guard, trip, and noise conditions. Provides a short permissive window when channel quality degrades.

• Three-State Channel: Guard (no fault), Trip (permissive), and Noise (channel failure). The receiver can differentiate between a lost channel and an intentional trip signal.
• Unblocking Window: If the channel transitions from guard to noise, the receiver asserts a short (typically 150-300 ms) permissive output, allowing tripping if local protection elements pick up.
• Security Timer: After the unblocking window expires, the scheme locks out to prevent tripping on sustained channel noise, reverting to time-delayed backup protection.
• Modern Relevance: Largely superseded by direct fiber and multiplexed digital channels but still found in legacy FSK power line carrier installations.

A unit protection scheme that continuously transmits synchronized current phasor data between line terminals via fiber optic channels. Tripping occurs when the vector sum of currents entering the zone exceeds a restraint threshold.

• Phasor Synchronization: Uses GPS-based time synchronization or echo timing to align current samples from both ends within microseconds.
• Alpha Plane Characteristic: The restraint characteristic is defined on a complex ratio plane, providing sensitivity to high-resistance faults while remaining secure for CT saturation.
• Channel Asymmetry Compensation: Algorithms compensate for unequal transmit and receive path delays on multiplexed SONET/SDH networks.
• Multi-Terminal Capability: Modern 87L relays support three or more terminals, enabling differential protection of tapped lines and breaker-and-a-half substation configurations.

The intelligent electronic device that executes the teleprotection logic. A protection relay continuously samples voltage and current inputs, applies distance, differential, or directional algorithms, and issues a trip command when a fault is detected. In teleprotection schemes, the relay also manages the communication interface, sending and receiving permissive, blocking, or direct transfer trip signals to remote terminals via fiber optic or power line carrier channels.

A specific protection element critical to transmission line teleprotection. The relay calculates the apparent impedance (Z = V/I) to the fault point. Zone 1 typically covers 80-85% of the line and trips instantaneously. Zone 2 extends to 120% with a time delay. Teleprotection schemes like Permissive Overreach Transfer Trip (POTT) use communication channels to allow instantaneous Zone 2 tripping for end-of-line faults, eliminating the coordination time delay.

A unit protection method that compares the vector sum of currents entering and leaving a protected zone. Under normal conditions, Kirchhoff's current law dictates the sum is zero. An internal fault creates a differential current. Teleprotection channels transmit digitized current samples between line terminals for line current differential (87L) schemes, enabling absolute selectivity and instantaneous tripping for the entire line length without stepped distance zones.

The automated sequence that restores a circuit breaker after a trip. Transient faults (e.g., lightning strikes) clear after de-energization, so auto-reclosing restores service without operator intervention. Teleprotection coordinates reclosing at both line terminals, using check synchronism and line dead/live voltage checks to prevent out-of-phase reconnection. Typical sequences include single-shot (one reclose attempt) or multi-shot logic with programmable dead times.

A device that inserts high impedance during a fault to reduce prospective short-circuit current. As grids densify with distributed generation, fault levels can exceed breaker interrupting ratings. FCLs protect downstream equipment from excessive electromechanical stress. Teleprotection schemes must account for the FCL's dynamic impedance insertion, as it alters the apparent impedance seen by distance relays and can affect zone reach accuracy.