Description

Table of Contents

Overview

The ASL-441H Digital Line Protection and Monitoring Unit, as one of the professional transmission line protection relays and digital relays for transmission line protection, is a high-performance line protection relay suitable for low-current grounding systems with ungrounded neutral points or neutral points grounded via arc-suppression coils at voltage levels of 66 kV and below. It serves as bay-level equipment for line units.

Protection Functions and Corresponding ANSI Codes

Three-stage Definite-time Overcurrent Protection

Function Description: Includes instantaneous overcurrent trip, delayed instantaneous trip and regular overcurrent protection. All three stages support undervoltage blocking and directional blocking, with independent enable and disable functions.
ANSI Code: 27, 50, 51, 67

Auto-reclosing Protection

Function Description: Supports three-phase single-shot auto-reclosing, reclosing post-acceleration, reclosing synchronism check, dead-line voltage check and current blocking reclosing functions.
ANSI Code: 25, 79

Under-frequency Load Shedding Protection

Function Description: Equipped with slip blocking, undercurrent blocking and undervoltage blocking, which can be enabled or disabled independently. The output supports configurable trip (reclosing blocking) or alarm mode.
ANSI Code: 27, 46, 81

Zero-sequence Overcurrent Protection

Function Description: Supports configurable trip or alarm output.
ANSI Code: 50N / 51N

Zero-sequence Overvoltage Protection

Function Description: Supports configurable trip or alarm output.
ANSI Code: 59N

Inverse-time Overcurrent Protection

Function Description: Provides inverse-time overcurrent protection characteristic.
ANSI Code: 51I

Small-current Grounding Line Selection Function

Function Description: Realizes fault line selection for small-current grounding system.
ANSI Code: 51NG

Overload Protection

Function Description: Prevents equipment damage caused by continuous overload operation.
ANSI Code: 49

Bus Charging Protection

Function Description: Provides fast protection during busbar charging operation.
ANSI Code: 62

Loss of Voltage Protection

Function Description: Detects system undervoltage and loss of voltage fault.
ANSI Code: 27

CT Disconnection Fault Detection

Function Description: Monitors and identifies current transformer circuit disconnection fault.
ANSI Code: 60

Manual Closing and Reclosing Post-acceleration Protection

Function Description: Triggers accelerated tripping during manual closing and auto-reclosing operation.
ANSI Code: 50HS

Control Circuit Disconnection Detection

Function Description: Detects disconnection fault of device control circuit.
ANSI Code: 74

PT Disconnection Fault Detection

Function Description: Monitors and identifies voltage transformer circuit disconnection fault.
ANSI Code: 60

Communication Mode

Optional: RS-485, CAN bus, Ethernet, IEC 60870-5-103 (IEC-103)

Main Functional Specifications

Protection response time (including the response time of the output relay): Average error in response time: not more than 35 ms;
Precision operating range: Minimum precision current: 0.08 In, Maximum precision current: 20 In
Current setting error: When the current exceeds 1.00 A, the error shall not exceed ±2.5%; Voltage setting error: shall not exceed ±3%
Measurement accuracy: Current (I)/Voltage (U): Class 0.2; Power (P)/Reactive power (Q): Class 0.5
Remote signal resolution: no more than 2 ms

Event Logging, Fault Recording

The recorded events include protective action events and device malfunctions;
Record the type of protection trip, the time of the trip, and the operating parameters at the time of the trip;
The fault recording data includes: the time of the protection operation, the type of protection operation, and the rms values before and after the recording was initiated;
Data length of the fault recording: 2 cycles before the fault and 4 cycles after the fault;
The SOE can retain 48 sets of data after a power loss, and the fault report can retain 16 sets of data after a power loss.

Line Protection Device — Outline and Installation Dimensions

FAQ

Q：what is line level device protection?

A:Line Level Device Protection refers to protection functions and device configurations deployed at the individual line level in a power distribution/substation system.

Q：How to Select Relay Protection for Power Transmission Lines?

A:Transmission line protection devices commonly adopt line protection with overcurrent relays for most transmission line protection systems, specifically three-stage current protection. For higher voltage scenarios, transmission line protection using distance relay together with optical fiber differential protection is applied to safeguard transmission lines.

Q：What is the effect of nonlinear loads on the operation of protective devices?

A:The resulting effects include harmonic distortion, voltage stability issues, and power factor problems, among others

Q What is a line protection relay?

A protection device for transmission & distribution lines. It monitors electrical parameters, detects faults and trips breakers to isolate faulty sections and secure power grid operation.

Q Main ANSI core functions

21 Distance protection, 50/51 Phase overcurrent, 51G Zero-sequence earth fault, 67 Directional protection, 87L Line differential, 79 Auto-reclosing, 46 Negative sequence unbalance, PSB power swing blocking, VT/CT supervision.

Q Zone 1 / 2 / 3 meaning

Zone1: Fast trip, covers 80%–85% of the line
Zone2: Full line plus short adjacent section with short delay
Zone3: Remote backup protection with long time delay

Q 87L Line differential vs distance protection

Differential compares currents at both line ends via communication, with high selectivity and resistance to arc resistance; needs fiber links. Distance protection works independently and serves as backup when differential communication fails.

Q False tripping under heavy load

Causes: Improper pickup setting, load encroachment, wrong CT ratio, missing power swing blocking. Fix: Adjust parameters, enable blocking functions, correct transformer ratio.

Q Protection fails to trip during faults

Check function enable status, CT/VT polarity, pickup thresholds, trip circuit integrity and device blocking alarms.

Q VT fuse failure hazard

Loss of voltage disables distance protection and risks maloperation. The relay will block distance elements and send an alarm.

Q CT secondary rule

Never open CT secondary circuit while energized, otherwise dangerous high voltage will occur. Short terminals first before rewiring.

Q SOTF protection

Switch Onto Fault: Instant trip when closing breaker onto a pre-existing line fault.

Q Single-phase auto-reclose

Most overhead faults are temporary single-phase earth faults. Single-phase reclosing retains two healthy phases to maintain system stability, better than three-phase reclosing.

Q Auto-reclose lock conditions

Locked out after repeated reclosing on permanent faults, manual trip, bus fault trip, VT/CT abnormality or hardware failure.

Q How to select main protection

Short/medium lines: Distance protection as main protection. Long EHV lines: Fiber 87L differential as primary, distance as backup.

Q Standard communication protocols

IEC 61850, IEC 104, Modbus, DNP3.

Q Working environment

-25℃ ~ +70℃, non-condensing humidity, meets IEC 60255 EMC anti-interference standards.

Q Regular inspection items

Daily SCADA status check; annual protection injection & trip loop test; periodic wiring and insulation inspection; firmware upgrade only during power outage.

3 reviews for ASL-441H line protection relay

xiao zhang – June 3, 2026

This multi-function line protection relay integrates distance(21), overcurrent(51), earth fault(51N), under/over voltage(27/59) and auto-reclosing(79) functions. Strong anti-interference, flexible grading setting and diversified communication options suit MV & HV overhead/cable lines for overseas power projects.
Rated 5 out of 5

Jack – June 4, 2026

Fast response of digital I/Os, works smoothly with automation systems.
Rated 5 out of 5

Jack – June 4, 2026

Intuitive panel interface — field commissioning engineers love it.

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ASL-441H line protection relay