Differential Relay for Transformer Protection: How to Select an 87T Relay

Selecting a differential relay for transformer protection requires more than checking the ANSI 87T function. Engineers and EPC contractors need to consider transformer rating, voltage ratio, vector group, CT characteristics, differential protection algorithms, harmonic restraint, communication requirements and project-specific protection standards.

A modern numerical 87T relay for transformer protection can integrate transformer differential protection with additional functions such as restricted earth fault protection, overcurrent protection, event recording and substation communication. The correct relay therefore depends on both the transformer application and the required protection scheme.

This guide explains the key technical criteria for transformer differential relay selection and shows what information should be confirmed before requesting a quotation or approving a relay for an EPC project.

For transformer protection selection, please refer to the transformer selection section.

What Is a Differential Relay for Transformer Protection?

What Is an 87T Differential Relay?

An 87T differential relay monitors the balance of current entering and leaving a transformer. Under normal and external fault conditions, currents remain balanced. When an internal fault occurs, the balance is disrupted and the relay trips. The “87T” designation refers to transformer differential protection under ANSI/IEC standards.

What Faults Does It Detect?

The relay detects internal faults within the transformer protection zone, including:

• Phase-to-phase and phase-to-ground faults
• Winding faults (including turn-to-turn faults)
• Internal lead and terminal faults

Detection speed and sensitivity depend on relay settings, CT performance, and protection scheme configuration.

Where Is the Differential Protection Zone?

The zone is defined by CT locations on each side of the transformer, covering windings and internal connections. Faults inside the zone cause tripping; faults outside are restrained. This provides fast, selective protection for the power transformer differential relay system.

For a detailed explanation, see our Transformer Differential Protection Working Principle guide.

Key Selection Criteria for a Transformer Differential Relay

1. Transformer Rated Power

When selecting an 87T relay, the following transformer rating information is required:

• MVA rating
• HV voltage
• LV voltage
• Frequency
• Number of windings (two-winding or three-winding)

These parameters determine the protection range, CT requirements, and relay configuration, which are core to transformer protection relay selection.

2. Transformer Voltage Ratio

The relay must accommodate the HV/LV current relationship and CT ratios. The voltage ratio defines the current relationship between both sides and influences CT selection and relay compensation settings.

For detailed calculation methods, see our Differential Protection Relay Setting Calculation guide.

3. Transformer Vector Group

The vector group defines the phase displacement between HV and LV windings. The relay must support the required vector group compensation for correct differential current calculation of the numerical transformer differential relay.

Common vector groups include:

• Dyn11
• Yd1
• Yd11
• Yy0

For detailed application guidance, see our Star-Delta Transformer Differential Protection and Star-Star Transformer Differential Protection guides.

4. CT Parameters

CT selection is critical for correct differential protection operation. The following CT parameters must be considered:

• CT ratio – Must match the transformer primary currents on each side
• CT secondary rating – Typically 1A or 5A, must be compatible with relay inputs
• CT accuracy class – Affects protection reliability during high fault currents
• CT knee-point voltage – Determines CT performance under fault conditions and saturation resistance
• CT burden – Must be compatible with relay input impedance and wiring length

5. Protection Functions Required

In addition to 87T differential protection, the relay may need to support:

• Restricted earth fault (REF) – For enhanced ground fault protection
• Overcurrent protection (50/51) – Backup protection for phase and ground faults
• Thermal overload protection – For transformer thermal monitoring
• Breaker failure protection – For backup tripping if the circuit breaker fails to open

The required functions depend on the application, project specifications, and protection philosophy for transformer differential protection relay deployment.

6. Harmonic Restraint Requirements

Harmonic restraint is essential for preventing false tripping during transformer energization. When selecting a relay, consider:

• Second harmonic restraint – Primary method for inrush blocking
• Fifth harmonic restraint – Optional for overexcitation detection (model-dependent)
• Cross-blocking capability – Restrains all phases when inrush is detected on any phase (model-dependent)

Ensure the relay provides the harmonic restraint features required for your application.

7. Communication and Integration

For substation automation and remote monitoring, the relay should support:

• IEC 61850 – Including GOOSE and MMS for digital substation integration
• Modbus RTU/TCP – For SCADA integration and legacy system compatibility
• IEC 60870-5-103 – For protection device communication (where applicable)
• Time synchronization – IRIG-B or SNTP for accurate event time-stamping

Select protocols based on project requirements and existing substation infrastructure.

8. Environmental and Mechanical Requirements

The relay must be suitable for the installation environment:

• Operating temperature range – Must match site ambient conditions
• Mounting type – Panel mounting, rack mounting, or DIN-rail (depending on relay form factor)
• Protection class – IP rating for dust and moisture protection
• Power supply – DC or AC voltage compatible with station battery or auxiliary supply
• Certifications – Compliance with utility, industry, or EPC project standards

9. Event Recording and Diagnostics

For post-event analysis and maintenance, consider:

• Fault recording – Waveform capture in COMTRADE format
• Sequence of events (SOE) – Time-stamped event logging
• Disturbance records – Pre- and post-fault data for fault analysis
• Self-diagnostics – Relay health monitoring and alarm functions

These features simplify troubleshooting and reduce outage time.

Essential Protection Functions to Consider

Function	Purpose
87T	Transformer differential protection
64REF / 87N	Restricted earth fault, where applicable
50/51	Overcurrent backup
50N/51N	Earth-fault backup
24	Overexcitation
49	Thermal overload
50BF	Breaker failure
86	Lockout logic
Fault recorder	Post-fault analysis

Differential Characteristic and Percentage Bias

Percentage bias raises the relay’s operating threshold as through-current increases, ensuring stability during external faults while maintaining sensitivity for internal faults, which is a core performance indicator for transformer 87T relay selection.

A single-slope characteristic applies one constant bias across the entire range, while dual-slope uses a lower slope for the low-current region to preserve sensitivity and a higher slope for the high-current region to improve stability during severe external faults and CT saturation.

The percentage bias characteristic ensures the relay remains stable during external faults and CT saturation by raising the operating threshold proportionally to fault current.

Actual slope and pickup settings should be determined during the protection study based on CT characteristics, transformer impedance, fault levels, and relay type.

Actual slope and pickup settings should be determined during the protection study.

Harmonic Restraint Requirements

When selecting a transformer differential relay, the harmonic restraint function should be reviewed to ensure it meets the application requirements. Key aspects to confirm include:

2nd harmonic restraint/blocking – The primary method used to prevent false tripping during transformer energization. The relay must detect inrush conditions and apply restraint based on the second harmonic content.

Overexcitation detection – Some relays provide fifth harmonic detection or other algorithms to identify overexcitation conditions and apply additional restraint. Availability depends on the relay model and should be confirmed for applications where overexcitation is a concern.

Harmonic measurement – The relay should measure harmonic content accurately from the digitized current waveforms. This forms the basis for all harmonic-based protection decisions.

Inrush discrimination – The relay must reliably distinguish inrush from internal faults. This is achieved through harmonic analysis and, in some relays, additional logic such as cross-blocking.

Configurable thresholds – The harmonic restraint thresholds should be adjustable to suit different transformer types, system conditions, and application requirements. Fixed thresholds may not be suitable for all installations.

Important Note:

The exact algorithm and settings depend on the relay manufacturer and protection scheme. Always refer to the specific relay documentation for detailed harmonic restraint implementation and setting guidance.

Communication and Substation Integration

Requirement	What to Check
IEC 61850	MMS / GOOSE support
Modbus	SCADA integration
IEC 60870-5-103	Legacy substation communication
Ethernet	Engineering / communication
GOOSE	Fast interlocking/tripping
Fault records	COMTRADE or equivalent
Time synchronization	IRIG-B / SNTP / PTP, if required

Two-Winding vs Three-Winding Transformer Differential Relay Comparison

Item	Two-Winding	Three-Winding
CT inputs	HV + LV	HV + MV + LV
Differential elements	Two-side	Multi-side
Vector compensation	Required	Required for each winding
Application	Standard power transformer	Three-winding transformer
Relay selection	2-winding 87T	3-winding 87T

Special Transformer Applications

Differential protection settings and relay configuration vary depending on the transformer type and application. The following sections describe key considerations for

Two-Winding Transformer

A two-winding transformer has one primary and one secondary winding. Differential protection compares currents from CTs installed on both sides. This is the simplest configuration and the most common for differential relay application. Setting calculations are straightforward, requiring compensation for CT ratio, voltage ratio, and vector group.

Three-Winding Transformer

A three-winding transformer has three separate windings, typically a primary and two secondary windings, or two primary windings and one secondary. Differential protection for three-winding transformers requires CTs on all three sides. The relay must compare currents from three sets of CTs, requiring more complex compensation and setting calculations. The differential current is the vector sum of all three winding currents. Some relays offer dedicated three-winding differential protection functions.

Auto-Transformer

An auto-transformer has a single continuous winding with a tap, providing a galvanic connection between HV and LV circuits. Differential protection for auto-transformers must account for the electrical connection between windings and the distribution of currents across the series and common windings. CT placement and compensation settings are more complex than for two-winding transformers. Some relays provide specific auto-transformer compensation algorithms.

Generator Step-Up Transformer

Generator step-up transformers connect generators to the transmission system. Differential protection must coordinate with generator protection and consider the high fault current contribution from the generator. Settings may require additional considerations for CT saturation and high through-fault currents. Some applications require transient monitoring for short-circuit stress or high inrush effects.

Industrial Transformer

Industrial transformers supply power to manufacturing plants, processing facilities, and large loads. Differential protection for industrial transformers may require integration with process protection, coordination with motor protection, and trip settings for high inrush applications. Space constraints and panel layout considerations may also influence relay selection. Communication interfaces should be compatible with plant SCADA or DCS systems.

High-Impedance vs Low-Impedance Differential Protection

Feature	High Impedance	Low Impedance
CT matching	Strict	More flexible
Configuration	Hardware-oriented	Digital / configurable
Application	Traditional schemes	Modern numerical schemes
CT saturation stability	Strong stability	Managed by restraint / algorithms
Engineering flexibility	Lower	Higher

High-impedance differential protection uses a high-value series resistor in the relay circuit to force current through the CTs during external faults, ensuring stability even when CT saturation occurs. It requires careful CT matching and is typically applied for busbar or restricted earth fault protection.

Low-impedance differential protection is commonly used in numerical relays. It relies on software-based percentage bias characteristics and digital signal processing to maintain stability. This approach offers greater flexibility, simpler CT requirements, and easier integration with other protection functions.

Important Note:

For modern numerical transformer differential relays, low-impedance percentage-biased schemes are commonly used, while high-impedance schemes remain relevant for specific protection applications. The selection between the two depends on the application requirements, CT performance, and protection philosophy.

Interoperability and Retrofit Considerations

When replacing or upgrading a transformer differential relay, practical compatibility must be checked to ensure smooth integration.

• CT Input Compatibility – Confirm CT secondary rating (1A or 5A) matches the new relay inputs.
• Binary Inputs and Outputs – Verify number and type of I/O meet existing scheme requirements.
• Communication – Ensure supported protocols and physical interfaces align with SCADA or substation automation systems.
• IEC 61850 – Confirm GOOSE and MMS support for digital substation integration.
• Existing Protection Scheme – Review coordination with backup protection, interlocking, and scheme-specific logic.
• Panel Dimensions – Check that the new relay fits within the available panel space.
• Wiring – Review terminal assignments and wiring compatibility; update documentation as needed.
• Settings Migration – Translate existing settings to the new relay format using manual or manufacturer tools.
• FAT and SAT – Plan factory and site acceptance testing to verify configuration, installation, and system integration.

Testing Requirements Before Commissioning

Before a transformer differential relay is placed into service, a series of tests should be performed to confirm correct installation, configuration, and operation. The following items should be verified:

• CT polarity – Confirm correct phase relationship between primary and secondary currents
• CT ratio – Verify that CT ratios match nameplate ratings and relay settings
• Relay input – Check that analog inputs are correctly connected and scaled
• Differential characteristic – Verify pickup and slope settings using secondary injection
• Trip logic – Confirm trip contacts and alarm outputs operate as configured
• Binary I/O – Verify correct mapping and response of all binary inputs and outputs
• Communication – Check protocol settings and data exchange with SCADA or substation automation systems
• Event recording – Confirm fault recording and waveform capture functions operate correctly
• Protection settings – Review all settings against the approved calculation and design documentation

These tests should be documented as part of the commissioning records. Both secondary injection and primary injection tests are typically required for full validation.

See our complete Transformer Differential Protection Testing guide.

Transformer Differential Relay RFQ Checklist

Information	Required
Transformer capacity	MVA
HV voltage	kV
LV voltage	kV
Transformer vector group	e.g. Dyn11
Number of windings	2 / 3
CT ratios	HV / LV / MV
CT secondary	1A / 5A
Frequency	50 / 60 Hz
Differential function	87T
REF	Required / Optional
Communication	IEC 61850 / Modbus / etc.
Binary I/O	Project requirement
Fault recording	Required / Optional
Installation	Panel / rack
Auxiliary supply	DC voltage
Applicable standards	Project specification

Questions to Ask Before Buying an 87T Differential Relay

Before selecting an 87T differential relay for your project, consider asking the following questions to ensure the relay meets the technical and project requirements:

• Does the relay support my transformer vector group (e.g., Yd11, Dyn11, Yy0)?
• What CT ratios are supported, and does the relay provide internal CT ratio compensation?
• Does the relay provide internal vector compensation, or are external CT connections required?
• Does it support both 1 A and 5 A CT secondary inputs?
• What differential characteristic is used (single slope, dual slope, or adaptive)?
• How is transformer inrush detected and restrained (e.g., second harmonic, cross-blocking)?
• Does the relay support IEC 61850 communication protocols?
• Does it provide GOOSE messaging for fast peer-to-peer communication?
• Can it record COMTRADE fault files for post-event analysis?
• What FAT (Factory Acceptance Test) documents are provided with the relay?
• Is factory testing available before shipment?
• Can the relay be customized to meet specific EPC project requirements (e.g., labelling, logic, protocol configuration)?

Recommended 87T Transformer Differential Protection Relay

If you are looking for a numerical 87T relay for a two-winding or three-winding transformer, review our transformer differential protection relay specifications, including CT inputs, vector group compensation, protection functions and communication options.

Key Features to Consider:

• CT Inputs – Supports both 1A and 5A CT secondary inputs with configurable ratio compensation
• Vector Group Compensation – Internal software compensation for common vector groups including Yd11, Dyn11, Yy0 and others
• Protection Functions – Percentage-biased differential protection with single or dual-slope characteristic, harmonic restraint for inrush blocking, and optional backup functions
• Communication Options – IEC 61850 with GOOSE and MMS, Modbus RTU/TCP, IEC 60870-5-103
• Event Recording – COMTRADE fault recording and sequence of events for post-event analysis
• Configuration Flexibility – Adjustable settings to match project-specific transformer and CT parameters

87T Differential Relay Selection – Q&A

Q1. What is a differential relay for transformer protection?

A differential relay for transformer protection is a device that compares currents entering and leaving a transformer. Under normal and external fault conditions, currents are balanced. When an internal fault occurs, an imbalance is created and the relay issues a trip signal.

Q2. What is an 87T differential relay?

The 87T differential relay is the ANSI/IEC designated protection function for transformer differential protection. “87” denotes differential protection, and “T” indicates application to a transformer. Modern 87T relays are numerical devices with advanced compensation, communication and recording features.

Q3. What information is required to select a transformer differential relay?

You will need transformer MVA rating, HV and LV voltages, vector group, number of windings, CT ratios and secondary ratings, required protection functions, communication protocols, and project standards.

Q4. Does an 87T relay support Dyn11 transformers?

Yes. Numerical 87T relays typically provide configurable vector group compensation, including support for Dyn11 and other common vector groups such as Yd11, Yd1, and Yy0.

Q5. What CT parameters should be checked before selecting an 87T relay?

Check CT ratio, secondary current rating (1A or 5A), accuracy class, knee-point voltage, and burden compatibility with relay inputs and wiring.

Q6. Should I choose a two-winding or three-winding differential relay?

This depends on your transformer configuration. Two-winding relays are suitable for two-winding transformers. Three-winding relays are required for three-winding transformers and provide compensation for currents from all three sides.

Q7. Does a numerical 87T relay provide internal vector compensation?

Yes. Numerical 87T relays perform vector group compensation internally through software configuration, eliminating the need for external CT connection adjustments.

Q8. What communication protocols should a transformer differential relay support?

Common protocols include IEC 61850 (GOOSE and MMS), Modbus RTU/TCP, and IEC 60870-5-103. Protocol selection depends on substation automation and SCADA integration requirements.

Q9. What additional protection functions should an 87T relay provide?

Optional functions may include restricted earth fault (REF), overcurrent protection (50/51), thermal overload protection, and breaker failure protection, depending on application requirements.

Q10. What testing is required before commissioning an 87T relay?

Testing includes CT polarity and ratio checks, secondary injection tests for pickup and slope, harmonic restraint verification, trip logic checks, binary I/O verification, and primary injection tests for full system validation.

Q11. What documents should be requested from the relay supplier?

Request FAT (Factory Acceptance Test) reports, test certificates, configuration files, wiring diagrams, communication setup guides, and the relay manual with setting instructions.

Q12. Can an 87T relay be used for transformer retrofit projects?

Yes. Numerical 87T relays are suitable for retrofit projects. Compatibility should be checked for CT inputs, binary I/O, panel dimensions, wiring, communication, and protection scheme integration.