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.
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.
line differential protection relay working principle
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.
