Teleprotection

A direct tripping scheme where a protection signal from the remote terminal unconditionally initiates a breaker trip at the local terminal. This is the simplest and fastest teleprotection logic.

• Security Risk: Highly susceptible to false trips from channel noise or interference.
• Application: Used for transformer faults, reactor protection, and breaker failure where a trip must occur regardless of local relay measurement.
• Channel Requirement: Demands a highly secure, often dual-channel, communication path.
• Key Distinction: No local fault detector supervision is required; the received signal alone commands the trip.

A permissive scheme where tripping is allowed only when a local overreaching Zone 2 fault detector operates AND a permissive signal is received from the remote end. This logic balances speed with high security.

• Operation: The remote relay sends a permissive signal when its own overreaching elements detect a forward fault.
• Security Logic: A trip cannot occur without local fault detection, preventing false trips from spurious channel signals.
• Channel Reversal: A guard signal is often used; loss of guard blocks the scheme to prevent maloperation during channel fade.
• Weak Infeed: Requires special logic (echo or weak-infeed trip) when one terminal lacks sufficient generation to detect the fault.

A permissive scheme using underreaching Zone 1 elements to key the communication signal. The remote terminal trips instantaneously if its own Zone 1 operates, or after a time delay if a permissive signal is received.

• Direct Trip Component: A local Zone 1 trip is executed immediately without waiting for a channel signal.
• Channel Independence: The scheme does not require a communications channel for the Zone 1 instantaneous trip portion.
• Limitation: The underreaching element may not see high-resistance faults at the remote end, requiring a complementary time-delayed backup.
• Modern Usage: Largely superseded by POTT in modern digital relays due to POTT's superior coverage for end-of-line faults.

A blocking scheme where a reverse-looking fault detector sends a block signal to prevent the remote terminal from tripping for external faults. Tripping is permitted only when a forward fault is detected and no block signal is received.

• Channel Logic: The absence of a signal permits tripping; the presence of a signal blocks it.
• Dependability Advantage: Channel failure during an internal fault does not prevent tripping, making it inherently dependable.
• Coordination: The blocking transmitter must operate faster than the remote forward-looking tripping element to prevent false trips for external faults.
• Application: Often used on power line carrier (PLC) channels where the signal is transmitted on the faulted phase and may be attenuated by the fault itself.

A hybrid scheme that combines permissive and blocking logic to overcome channel security issues during internal faults. A continuous guard signal is transmitted; its loss is interpreted as a potential fault condition.

• Guard Signal: A constant monitoring tone confirms channel integrity. Loss of guard unblocks the receiver for a short window.
• Unblocking Window: If the guard is lost and a local fault detector operates, the relay is permitted to trip for a limited time (typically 150-300 ms) before reverting to a blocked state.
• Noise Immunity: Provides high security against spurious noise bursts that could mimic a permissive trip signal.
• Legacy Context: Historically used with frequency-shift keying (FSK) power line carrier channels to distinguish between channel failure and a valid trip command.

A unit protection scheme that compares the magnitude and phase angle of currents entering and leaving the protected line segment using digital communications. It is not strictly a teleprotection scheme but a current-based differential relay that uses a communications channel.

• Kirchhoff's Law: Trips when the vector sum of currents exceeds a restraint threshold, indicating an internal fault.
• Channel: Requires a high-bandwidth, synchronized digital channel (typically IEEE C37.94 or direct fiber) to exchange current phasors.
• Immunity: Inherently immune to power swings, load encroachment, and series compensation effects that challenge distance-based teleprotection.
• Synchronization: Relies on GPS or IEEE 1588 PTP for aligning current samples from both line terminals.

A direct tripping scheme where a protection relay at one terminal sends an unconditional trip command to the remote terminal. Unlike permissive schemes, the receiving end does not require local fault detection to execute the trip.

• Application: Transformer fault protection, breaker failure
• Channel speed: Typically < 10 ms end-to-end
• Security: Requires high-reliability channels to prevent false trips

A permissive protection scheme where a relay sends a trip permission signal when its overreaching Zone 2 elements detect a fault. The remote terminal trips only when it receives this signal and its own Zone 2 elements operate.

• Overreach: Zone 2 typically set to 120-150% of protected line
• Security vs. Dependability: Balances false trip prevention with fault clearing certainty
• Channel: Can tolerate momentary signal loss better than DTT

A permissive scheme using Zone 1 underreaching elements (typically 80-85% of line) to send the transfer trip signal. The remote end trips on receipt of the signal without requiring local fault detection.

• Key advantage: High-speed clearing for faults beyond local Zone 1 reach
• Weak infeed logic: Essential for terminals with low fault current contribution
• Channel asymmetry: Must account for different fault detection times at each terminal

A reverse-looking protection scheme where a relay sends a blocking signal to prevent the remote terminal from tripping for external faults. The remote relay trips only if its forward-looking elements operate and no blocking signal is received.

• Channel requirement: Must be fast and reliable; failure causes false trips
• Coordination: Reverse Zone 3 elements typically initiate blocking
• Advantage: Inherently secure for external faults behind the relay

A fail-safe mechanism in permissive schemes that allows tripping for a short window (typically 150-300 ms) after a permissive channel is lost, provided local fault detectors remain picked up.

• Purpose: Maintains dependability during channel fade or interference
• Guard signal: Continuous monitoring tone confirms channel health
• Risk window: Limited duration prevents false trips during prolonged channel failure

Generic Object Oriented Substation Event protocol provides the high-speed, publisher-subscriber communication backbone for digital teleprotection over Ethernet. GOOSE messages replace traditional copper pilot wires.

• Latency: Class P2/3 performance requires < 3 ms transfer time
• Retransmission: Exponential backoff ensures delivery without TCP overhead
• VLAN tagging: IEEE 802.1Q priority ensures deterministic network behavior