ASL-446H Line Differential Protection Relay

Description

ASL-446H Line Differential Protection Relay Overview

The ASL-446H is a high-speed numerical line differential protection relay specifically designed for the protection of short overhead lines, cable feeders, and mixed line systems where distance protection may face limitations. It operates on the well-established current differential principle—continuously comparing the magnitude and phase of currents measured at both terminals of the protected line via a communication channel.

Under normal load or external fault conditions, the currents at both ends remain substantially balanced, resulting in a differential current close to zero. In this state, the relay remains stable and does not trip. However, when an internal fault occurs—such as a phase-to-phase short circuit, three-phase fault, or earth fault within the protected zone—the current balance is broken. The relay detects a significant differential current and issues an instantaneous trip command to clear the fault with high speed and full selectivity.

The ASL-446H supports fiber-optic communication between line terminals, ensuring secure and reliable data exchange. It is an ideal primary protection solution for distribution networks, industrial power systems, and renewable energy collector lines requiring dependable high-speed protection.

ASL-446H ANSI 87L Protection Functions

Transmission Line Differential Protection (ANSI 87L)

Continuously compares current magnitude and phase at both ends of the line. The protected zone is defined by the CTs at each terminal. Under normal load or external fault conditions, currents are balanced and differential current ≈ 0. During internal faults, the balance is broken, differential current increases, and the relay trips instantaneously. The restraint characteristic ensures stability during external faults.

Differential Instantaneous Protection

When differential current exceeds the high-set threshold, the relay trips instantaneously without intentional delay. This element is independent of the percentage restraint, ensuring fast clearing of severe internal faults.

CT Disconnection Protection

Detects open circuits in CT secondary circuits, issues an alarm, and blocks differential protection when configured—preventing false tripping caused by spurious differential currents from CT circuit faults. Blocking logic is configured via the CT Broken Line Blocking Differential setting.

How Does Line Differential Protection Work?

Line differential protection (ANSI 87L) compares currents at both ends of the protected line. Any difference indicates an internal fault.

Normal Operation

Current flows in from one end (I₁) and out from the other (I₂). The two currents are approximately equal, the differential current approaches zero, and the relay remains inactive.

External Fault

During an external fault, fault current flows through the line, but both CTs see the same through-current. The current balance is maintained, differential current ≈ 0, and the relay remains stable—no trip.

Internal Line Fault

During an internal fault, the current balance is broken. The currents at the two ends become unequal, differential current exceeds the threshold, and the relay issues an instantaneous trip command—opening breakers at both ends simultaneously.

CT Disconnection

A CT secondary circuit open circuit creates a false current imbalance, simulating an internal fault. The relay’s CT disconnection detection issues an alarm and, when configured, blocks differential protection to prevent false tripping. Protection remains active on healthy phases.

Summary

Line Differential Protection Working Principle

Communication for Line Differential Protection

Because line differential protection requires current information from both ends of the protected line, reliable communication between line terminals is an essential part of the protection scheme.

Communication	Application
RS-485	Serial communication
CAN	Device communication
Ethernet	Network communication
IEC 60870-5-103	Substation automation

Line Differential Protection Communication Requirements

Line differential protection (87L) relies on continuous, reliable end-to-end communication between the relays at each terminal of the protected line to exchange current data and status information. The availability of the communication link directly influences the performance and security of the protection system. The practical requirements for its implementation are typically considered in terms of:

• Data Exchange – Each relay needs to transmit sampled current values to the opposite terminal in real-time, enabling the comparison calculation.

• Synchronization – The current measurements from both ends must be time-aligned to ensure accurate differential calculation. This is often achieved through channel-specific techniques.

• Communication Reliability – The communication path (fiber optic, pilot wire, or microwave) must be highly reliable, as any disturbance can affect protection performance.

Communication Failure is a key consideration for system engineers. If the communication link is interrupted, the relays can no longer compare currents, and the differential protection function is typically blocked to prevent unwanted tripping. In this scenario, the system depends on backup protection schemes, such as distance protection (ANSI 21) or overcurrent protection (ANSI 50/51), to clear faults with a time delay. A communication failure alarm is usually generated to alert operators to restore the channel as soon as possible. Detailed logic may depend on the specific relay model and project configuration.

Technical Specifications

Parameter	ASL-446H
Relay Type	Numerical line differential protection relay
Main Protection	Line differential protection
Differential Instantaneous Protection	Supported
CT Disconnection Blocking	Supported
Response Time	≤35 ms*
Precision Current Range	0.08–20 In
Current Setting Error	≤±2.5%*
Voltage Setting Error	≤±3%*
Current / Voltage Accuracy	Class 0.2
Power / Reactive Power	Class 0.5
Remote Signal Resolution	≤2 ms
Communication	RS-485 / CAN / Ethernet
Protocol	IEC 60870-5-103

Setting List of Transmission Line Differential Protection

No.	Display Name	Range	Step	Remarks
1	Differential Instantaneous Trip Setting
1.1	Differential Instantaneous Trip Protection	1/0	—	Differential instantaneous trip: Enable(1) / Disable(0)
1.2	Instantaneous Trip Setting Value	0.40～99.99A	0.01A
2	Differential Protection Setting
2.1	Differential Protection Enable/Disable	1/0	—	Differential protection: Enable(1) / Disable(0)
2.2	CT Broken Line Blocking Differential	1/0	—	CT broken line blocking differential: Enable(1) / Disable(0)
2.3	Differential Setting Value	0.50～99.99A	0.01A
2.4	Restraint Setting Value	0.50～99.99A	0.01A
2.5	Differential Current Setting Value	0.50～99.99A	0.01A
2.6	CT Broken Line Setting	0.50～99.99A	0.01A
2.7	Slope Coefficient	0.30～0.70	0.01
2.8	Balance Coefficient	0.100～1.999	0.001
3	General Setting
3.1	Control Circuit Broken Line	1/0	—	Control circuit broken line alarm: Enable(1) / Disable(0)
3.2	Display Current Angle	1/0	—	Display current angle: Enable(1) / Disable(0)
3.3	Current Corresponding to 1A	1/0	—	Set (1) as 1A / Set (0) as 5A
3.4	Interlock C6-C7	1/0	—	Enable(1) inter-tripping / Disable(0) inter-tripping
3.5	Rated Current	0.50～99.99A	—	Secondary rated current of local line side

ASL-446H Applications

The ASL-446H is a high-speed numerical line differential protection relay designed for short transmission lines and feeders up to 110 kV. It is particularly suited for applications where distance protection may face limitations due to short line lengths, and where fast and selective fault clearing is required.

Application	Key Benefit	Typical Users
Short Transmission Lines	Full-line high-speed tripping without reach limitations	Utilities, industrial plants
Overhead Lines	Instantaneous fault clearing regardless of fault current	Utilities, renewable projects
Cable Lines	High-speed isolation minimises cable damage	Industrial, commercial networks
Substation Interconnections	Definitive zone boundaries for selective fault clearing	Utilities, system operators
MV/HV Line Protection (≤110kV)	Comprehensive differential + backup protection	Utilities, EPC contractors

Line Differential Protection vs Distance Protection

Outline and Installation Dimensions

   

FAQ

1. What is a line differential protection relay?

A high-speed relay that compares currents at both ends of a line and trips when a difference indicates an internal fault.

2. How does line differential protection work?

Under normal or external fault conditions, currents at both ends are balanced, and differential current ≈ 0. During an internal fault, the balance is broken, differential current exceeds the threshold, and the relay trips instantaneously.

3. What is ANSI 87L protection?

ANSI 87L is the standard designation for line differential protection—a high-speed primary scheme that uses current comparison between line terminals for sensitive and selective fault detection.

4. Difference between line differential and distance protection?

5. Why is communication required?

Real-time current data exchange between line terminals is needed to compare currents and determine whether a fault is internal or external.

6. What happens during a CT secondary disconnection?

The relay detects the CT disconnection and issues an alarm. If configured via the CT Broken Line Blocking Differential setting, it blocks differential protection for the affected phase to prevent false tripping.

7. What line types can the ASL-446H protect?

Suitable for short transmission lines and feeders up to 110 kV, including: short overhead lines, cable feeders, substation interconnections, medium-voltage (10–35kV) and high-voltage (66–110kV) lines.

8. What information is required to configure the ASL-446H?

Required information includes: system voltage (≤110 kV), CT ratios at both ends, line length and impedance, communication channel parameters, protection settings (differential pickup, restraint slope, etc.), and communication protocol. Settings must be performed by qualified protection engineers.