Transmission Line Protective Relay | Working Principle and Selection Guide 50/51/87L/21

As the most widely used product in the field of relay and protection, Transmission Line Protective Relay is commonly adopted in substations, distribution stations, hydropower stations, pumping stations and photovoltaic power plants.

Core Functions & Types of Transmission Line Protection

Line protection can be classified by fault detection principles into current protection, zero-sequence current protection, impedance (distance) protection and current differential protection. It can also be divided into two categories based on whether communication channels for data exchange are required.

Protection Type	Principle & Features	Typical Application	Operating Speed
Stepped Current Protection	Operates solely with local electrical quantities, coordinated by setting values and time delays	10kV ~ 35kV distribution lines	Possible time delay
Distance Protection	Determines fault location by measuring fault impedance with time grading	Main protection for 110kV transmission lines	Instant tripping for near-end faults
Pilot Differential Protection	Exchanges current data between two line ends and compares current differences directly	Main protection for 110kV and above transmission lines	Instant tripping along the entire line

Conventional Transmission Line Protection Relay

50 51 Relay Protection

As a common relay protection device, three-stage current protection for transmission lines combines instantaneous overcurrent protection (Stage I), short-time delayed overcurrent protection (Stage II) and long-time delayed definite-time overcurrent protection (Stage III). This integrated scheme provides full-line protection and backup protection for adjacent power components.

67 Relay Protection

As an electrical protection relay, directional current protection for transmission lines adds power direction detection elements to conventional overcurrent protection.

It operates only when fault current flows from the busbar to the line (forward direction), eliminating mal-operation triggered by reverse faults in double-source systems or loop networks.

79 Relay Protection

Protective relay systems are equipped with the automatic reclosing function (ANSI 79). After the line trips due to faults, the circuit breaker will reclose automatically after a short preset delay (0.5–1 second) to resume power supply interrupted by transient faults such as lightning strikes and wind-borne debris.

In case of reclosing onto a permanent fault, the device will trip once more and block subsequent reclosing operations.

81U Relay Protection

As an under frequency protection relay, a digital rate-of-change underfrequency protective relay for power systems performs the under-frequency load shedding function (ANSI 81U). When system frequency drops to the setting value due to active power deficiency, it automatically cuts off non-essential loads in preset steps to stop frequency deterioration, avoid system collapse and restore frequency to a safe range.

50 51n Protection Relay

Feeder protection relays are equipped with zero sequence protection relay. The zero-sequence overcurrent protection (ANSI 50N/51N) utilizes zero-sequence current generated in earth faults to identify faults selectively.

It can trip single-phase earth faults and work as main or backup protection against earth short circuits in high-current earthing systems.

Overload Relay Protection

Digital multifunction protective relays integrate overload protection relays with thermal overload protection (ANSI 49) for line protection. This function activates when the line current exceeds its continuous safe carrying capacity for a certain period.

It issues alarm signals or trips loads after a long delay following inverse-time or definite-time characteristics, preventing accelerated aging and damage of line insulation caused by prolonged overheating.

What Is an 87L Protection Relay

As a typical line differential protection relay, fiber-optic current differential protection (ANSI 87L) exchanges real-time three-phase current sampling data between two line terminals via optical fiber channels.

It calculates the phasor sum, namely the differential current, of currents at both ends in accordance with Kirchhoff’s Current Law.

When the differential current exceeds the setting value, an internal fault is detected and the whole line trips instantly.

This function is not affected by system oscillation, load variation or open-phase operation.

How Does an 87L Protection Relay Work?

The 87L protection relay carries out accurate current measurement at both line ends, transmits synchronized current information through communication between line terminals, conducts continuous differential current calculation, detects internal line faults reliably, and sends valid trip command output to corresponding circuit breakers.

87L Protection Zone

The 87L protection zone is defined between the current transformers installed at the terminals of the protected transmission line.

87L Relay Communication

87L Relay Communication enables data exchange between line differential relays at both line terminals. Supported communication modes include fiber optic communication, Ethernet communication, dedicated protection communication channel, direct fiber connection and multiplexer-based communication.

Key parameters cover communication latency, while channel failure supervision is implemented to monitor the operating status of the communication path continuously.

87L Relay Settings

87L Protection Relay Settings define key operating parameters such as differential current pickup and restraint setting, CT configuration, communication channel parameters, internal fault trip settings and external fault stability criteria for line differential protection.

Common 87L Setting Mistakes

Problem	Possible Result
Wrong CT ratio	False differential current
Reversed CT polarity	Relay may operate incorrectly
Communication failure	87L blocking/alarm
Incorrect differential pickup	Sensitivity or security problem
Incorrect restraint setting	Stability problem during external faults

87L vs 21 Distance Protection

Feature	87L	21
Protection principle	Current differential	Impedance measurement
Main protection zone	Defined line section	Distance zones
Communication	Normally required	Not always required
Internal fault selectivity	Very high	Zone-based
Long-line application	Excellent	Excellent
Weak infeed application	Strong	Requires additional schemes
Typical role	Main line protection	Main/backup line protection

87L vs 50/51

Feature	87L	50/51
Principle	Differential current	Overcurrent
Main protection	Yes	Usually backup / staged
Communication	Required for line differential scheme	Not necessarily
Internal line faults	Very sensitive	Depends on fault current
External fault selectivity	High	Time/current coordination
Application	Transmission lines	Distribution/transmission backup

87L Applications

87L Applications cover overhead lines and underground cables in medium and high voltage systems. Line differential protection offers high-speed fault identification for internal faults and is commonly used for utility transmission circuits, industrial feeders and renewable energy collection lines. Multiple communication options are available to support data exchange between remote 87L relays.

How to Test an 87L Protection Relay?

• Verify relay wiring
• Check CT ratio and polarity
• Inspect communication channel
• Carry out secondary injection test
• Test differential pickup threshold
• Examine restraint characteristic curve
• Simulate internal fault conditions
• Verify trip output circuits
• Test communication failure alarm function
• Document all test results

21 Protection Relay

As a core function of power systems protection and relaying, line distance protection (ANSI 21) is integrated into the digital protective relay.

It judges the distance of fault points by measuring the impedance, which is the ratio of voltage to current at the protection installation position.

When the measured impedance value falls within the preset impedance setting range (protection zone), the protection will operate and trip the circuit.

This implementation realizes the selective removal of faulty lines in power grids.

Protection Relay configuration guide

Application Scenario	Voltage Level	Core Protection Functions	Recommended Protection Type	Application Notes
Low-voltage distribution line	0.4kV	Overcurrent, overload, leakage current, phase loss protection	Low-voltage integrated protector, MCCB with protection	Factory, civil power distribution and terminal circuits
Medium-voltage short line	6~10kV (≤1km)	Instantaneous overcurrent, definite time overcurrent, residual overcurrent	Conventional feeder protection device	Urban distribution network, short-distance outgoing lines in plant areas
Medium-voltage long line	6~10kV (＞1km)	Distance protection, residual current protection, auto-reclosing	Intelligent line protection device	Suburban & rural power grid, long-distance overhead lines
High-voltage transmission line	35kV/110kV	Pilot protection, distance protection, residual current protection, auto-reclosing, remote tripping	Complete set of high-voltage line protection	Regional tie lines and main transmission lines
Dedicated cable line	6~35kV cable	Residual current protection, overcurrent protection, cable fault location	Special protection for cable lines	Dedicated lines for power plants and key users, prone to earth faults
Mixed overhead & cable line	6~35kV	Overcurrent protection, residual current protection, low-current earth fault feeder selection	General line protection with feeder selection function	Urban-rural hybrid lines with complex fault conditions
Low-current earthing system	6~10kV (Ungrounded / Arc suppression coil earthed)	Low-current earth fault feeder selection, residual current alarm	Line protector with earth fault selection function	Traditional distribution network; no immediate tripping for single-phase earth fault

The standard installation position for the Transmission Line Protective Relay is on protection panels in the secondary equipment room of substations and distribution stations.

For nearby outgoing circuits, the Transmission Line Protective Relay can also be arranged inside secondary compartments of switchgears, ring main units and outdoor terminal boxes.

Conclusion of the Transmission Line Protective Relay

The protection relay market continues to expand amid the upgrading of power grids worldwide. As an important hub of the industry, China protection relay sector boasts a large number of competitive protection relay companies that keep advancing product technologies and market layout.

To ensure proper equipment application and safe power grid operation, professional substation relay protection training has become indispensable. It helps frontline staff master operational skills, standardize maintenance work, and further support the sustainable and high-quality development of the entire relay protection industry in China.