Description

Table of Contents

Product Overview

The AST-441H Transformer Differential Protection Relay is a numerical ANSI 87T protection device designed for power transformer protection in substations and industrial power systems. It provides high-speed differential protection for internal transformer faults, with CT ratio and phase compensation, percentage-biased differential protection, harmonic restraint, event recording and fault recording functions.

The relay supports 1A or 5A CT inputs and can be integrated into transformer protection and control systems through standard communication interfaces. It is suitable for transformer protection applications up to 110 kV, subject to the project configuration and relay specification.

Key Features of the 87T Transformer Differential Protection Relay

1. ANSI 87T Differential Protection

High-speed differential protection for internal transformer faults within the protected zone.

2. Percentage-Biased Differential Protection

Bias/restraint characteristics improve stability during external faults and CT errors.

3. CT Ratio and Phase Compensation

Supports transformer-side current compensation according to CT configuration and transformer vector group.

4. Harmonic Restraint

Helps prevent unwanted differential operation during transformer energization and magnetizing inrush.

5. Fault and Event Recording

Records protection operations, events and fault information for commissioning and post-fault analysis.

6. Flexible CT Input

Supports 1 A and 5 A rated CT secondary inputs according to the selected configuration.

7. Communication Interfaces

Supports project-dependent interfaces and protocols such as RS-485, Ethernet and IEC 60870-5-103.

8. Protection and Control Integration

Can be integrated into transformer protection, measurement, control and substation automation systems.

Transformer Protection Functions and ANSI Codes

Protection Function	ANSI Code	Purpose
Transformer Differential Protection	87T	Main protection for internal transformer faults
High-Speed Differential Protection	87T-HS	Fast operation for severe internal faults
CT Open-Circuit Detection	—	Detects abnormal CT secondary conditions
Differential Current Alarm	—	Warns of abnormal differential current
Harmonic Restraint	—	Prevents unwanted operation during inrush

General Technical Data

No.	Parameter Category	Core Technical Specification
1	Power Supply & Rated Electrical Parameters	1. Rated control voltage: AC/DC 220 V 2. Power supply ripple coefficient: ≤ 5% 3. Voltage tolerance: -20% to +15% 4. Rated AC current In: 1 A/5 A (specify when ordering) 5. Rated AC voltage: 100 V 6. Rated frequency: 50 Hz (customizable per project requirements)
2	Power Consumption	1. DC power circuit: ≤ 25 VA (normal operation), ≤ 30 VA (device activated) 2. AC voltage circuit: ≤ 1 VA/phase (at 100 V rated voltage) 3. AC current circuit: ≤ 0.5 VA/phase (1 A In), ≤ 1 VA/phase (5 A In)
3	Output Contact Performance	1. DC circuit: 50 W breaking capacity (voltage ≤ 250 V, current ≤ 0.5 A, time constant 5 ± 0.75 ms); allowable make current ≤ 5 A 2. AC circuit: 50 W breaking capacity (voltage ≤ 250 V); maximum allowable make current 5 A
4	Insulation Performance	Meets IEC standard requirements for insulation resistance, dielectric strength, and impulse voltage

What Is an 87T Relay?

An 87T relay is a transformer differential protection relay that compares the currents entering and leaving the protected transformer zone to detect internal electrical faults.

87 = Differential Protection
T = Transformer
87T = Transformer Differential Protection

How Does an 87T Relay Work?

transformer differential protection relay 87T operating principle

Normal Operation:I₁ ≈ I₂ → Low differential current → No trip

Internal Fault:I₁ ≠ I₂ → High differential current → 87T operates → Transformer breaker trips

87T Relay Settings

No.	Display Name	Range	Step	Remark
1	High-speed Differential Setting
1.1	High-speed Differential Protection	1/0	—	Enable(1) / Disable(0)
1.2	High-speed Differential Setting Value	0.40～99.99A	0.01A
2	Differential Protection Setting
2.1	Differential Protection Enable	1/0	—	Enable(1) / Disable(0)
2.2	CT Broken Circuit Blocking Differential	1/0	—	CT break blocking function: Enable(1) / Disable(0)
2.3	Differential Current Setting	0.50～99.99A	0.01A
2.4	Restraint Current Setting	0.50～99.99A	0.01A
2.5	Differential Bias Current Setting	0.50～99.99A	0.01A
2.6	CT Broken Circuit Threshold	0.50～99.99A	0.01A
2.7	Harmonic Restraint Coefficient	0.10～0.70	0.01	Second harmonic restraint ratio
2.8	Slope / Ratio Restraint Coefficient	0.30～0.70	0.01
2.9	Balance Coefficient	0.100～1.999	0.001
3	Overcurrent Fan Control
3.1	Overcurrent Fan Start	1/0	—	Enable(1) / Disable(0)
3.2	Ventilation Time Delay	0.00～999.99s	0.01s
3.3	Fan Start Current Setting	0.5～99.99A	0.01A
4	General Setting
4.1	Motor Differential Mode Selection	1/0	—
4.2	HV Side Y/△-11 Connection	1/0	—
4.3	HV Side Y/△-01 Connection	1/0	—
4.4	Secondary Wiring Mode	1/0	—	Delta/Y(1) / Y/Y(0)
4.5	Control Circuit Break Alarm	1/0	—	Alarm enable(1) / disable(0)
4.6	Current Angle Display	1/0	—
4.7	Secondary Current Rated	1/0	—	1A input(1) / 5A input(0)
4.8	Inter-tripping C6C7	1/0	—	High-speed & differential trip C06C07(1)
4.9	Rated Secondary Current	0.50～99.99A	0.01A	HV side CT secondary rated current

87T Transformer Differential Protection Zone

87T relay transformer differential protection zone diagram

87T vs 50/51 Transformer Protection

Feature	87T Relay	50/51 Relay
Protection principle	Differential current	Overcurrent
Main protection area	Transformer internal zone	Backup / overcurrent
ANSI code	87T	50/51
CT requirement	CTs on transformer sides	Current measurement CT
Internal winding faults	Highly sensitive	Depends on fault current
External fault stability	Bias/restraint	Time-current coordination
Typical role	Main transformer protection	Backup protection

How to Test an 87T Relay?

Before commissioning the transformer differential protection, technicians shall check the relay wiring, verify the CT ratio and CT polarity, and confirm the correctness of transformer vector group compensation; subsequently, perform a comprehensive secondary injection test, including differential pickup test, bias and slope characteristic test, harmonic restraint test, and trip output test, and finally record and verify all test results to ensure protection performance complies with design requirements.

Transformer Differential Protection Relay Applications

Power Substations

Transformer main protection for substation applications.

Industrial Power Systems

Protection for transformers supplying industrial plants and large electrical loads.

Power Plants

Transformer protection for generator step-up and auxiliary transformers where applicable.

Distribution Transformers

Suitable relay configurations for medium-voltage transformer protection.

EPC Substation Projects

Suitable for protection and control panel integration, commissioning and project-specific relay configuration.

How to Select a Transformer Differential Protection Relay

Selection Factor	What to Check
Transformer Rating	MVA and voltage levels
Transformer Vector Group	YNd11, Dyn11, Yy, etc.
CT Ratio	HV/LV CT ratios
CT Secondary	1 A or 5 A
Protection Functions	87T, backup protection, alarms
Communication	Required substation protocol
Fault Recording	Number of channels and record capacity
Installation	Panel or rack mounting
Project Standard	IEC / utility / EPC specification

Why Choose Our 87T Transformer Differential Protection Relay?

Project-Based Configuration

Relay parameters and functions can be configured according to transformer ratings and project protection requirements.

Technical Documentation

Provide technical specifications, wiring diagrams, configuration information and supporting documentation for project review.

Factory Testing / Pre-Shipment Verification

Provide functional verification and configuration checks before shipment according to the agreed project requirements.

EPC Support

Support technical communication during relay selection, panel integration and commissioning.

Replacement and Alternative Applications

Suitable for projects requiring an alternative transformer differential protection relay, subject to technical compatibility and project approval.

Outline and Installation Dimensions

AST-441H 87T transformer differential protection relay front view Dimension Drawing of the Rear Side of the Transformer Differential Protection Device Side Dimension Drawing of the Transformer Differential Protection Device AST-441H transformer differential protection relay dimensions

Request a Quote for 87T Transformer Differential Protection Relay

Send us your transformer rating, voltage ratio, vector group, CT ratios, required protection functions and communication protocol. Our technical team can review the application and recommend a suitable relay configuration.

CTA：

Request 87T Relay Technical Information

FAQ

Q1. What is a transformer differential protection relay?

A primary protection device that detects internal transformer faults (e.g., turn-to-turn and phase-to-phase faults) by comparing currents on both sides, and trips to isolate the transformer upon fault detection.

Q2. What is ANSI 87T protection?

ANSI 87T is the standard function code for transformer differential protection. “87” denotes differential protection, and “T” stands for transformer.

Q3. What transformer faults does an 87T relay protect against?

Internal faults including:

Turn-to-turn faults
Phase-to-phase short circuits
Ground faults
Winding open circuits/breaks
Core faults (e.g., overheating from multiple ground points)

Q4. Does the relay support 1A and 5A CT inputs?

Yes. Both 1A and 5A CT secondary inputs are supported and configurable via relay settings.

Q5. How does the relay compensate for transformer vector group?

It applies software-based amplitude and phase correction to match different vector groups (e.g., Ynd11, Dyn1). Users only need to enter the vector group and connection type; the relay calculates the differential current automatically.

Q6. How does 87T protection prevent transformer inrush tripping?

It uses harmonic restraint (mainly second-harmonic braking). During inrush, the high second-harmonic content raises the operating threshold or blocks the protection, preventing false trips.

Q7. What communication interfaces are available?

Common options include:

RS485 (Modbus RTU)
Fiber optic Ethernet (IEC 61850, Modbus TCP)
Fiber optic serial ports
Optional: Profibus, DNP 3.0 (select models)

Q8. What information is required to select an 87T relay?

Key parameters needed:

Rated capacity (MVA/kVA)
Voltage ratio (e.g., 110kV/10kV)
Vector group (e.g., Ynd11)
CT ratios and connection types (both sides)
Required protection functions
Communication protocol
Power supply and installation environment

Q9. Can the relay be integrated into a transformer protection and control panel?

Yes. It is available in 19-inch chassis or drawout modules, supporting panel or DIN-rail mounting, and can be integrated with other protection and control devices in a cubicle.

Q10. How can I request the technical datasheet and quotation?

Sales email (with project requirements and transformer parameters)
Technical support hotline
Online inquiry form on our website

Our team will recommend the right model and provide a quotation and datasheet accordingly.

2 reviews for Transformer Differential Protection Relay | ANSI 87T

xiao zhang – June 3, 2026

Transformer differential relay (87T) adopts percentage restraint algorithm, strong anti-CT-saturation capability and CT break lockout. Instant trip for internal winding faults, flexible parameter setting and multi-communication options for all types of distribution & power transformers.
Rated 5 out of 5

Jack – June 4, 2026

Backed by good technical support and documentation — overall a solid choice.

Add a review

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