Transformer Protection Relay: Types, Functions and Selection Guide

What Is a Transformer Protection Relay?

A Transformer Protection Relay is a critical protection device used to detect electrical faults and abnormal operating conditions in power transformers and initiate the appropriate protection action. By continuously monitoring transformer currents, voltages and other electrical parameters, a transformer protection relay helps protect transformers from internal faults, short circuits, earth faults, overload, overvoltage and other abnormal conditions.

Modern Transformer Protection Relays can integrate multiple protection functions into a single numerical relay, including transformer differential protection (87T), overcurrent protection (50/51), earth fault protection (50N/51N), overvoltage and undervoltage protection (59/27), overexcitation protection (24), and thermal overload protection (49).

The appropriate Transformer Protection Relay depends on the transformer rating, voltage level, winding configuration, CT characteristics, required protection functions and overall substation or power system protection scheme. This guide explains the main types and functions of transformer protection relays, their applications, and the key factors to consider when selecting a relay for a transformer protection project.

What Does a Transformer Protection Relay Protect Against?

A transformer protection relay guards against:

• Internal faults: Winding short-circuits (phase-to-phase, phase-to-ground), inter-turn faults, and core faults.
• Electrical abnormalities: Overcurrent, external short-circuits, overvoltage, undervoltage, and overexcitation.
• Thermal issues: Sustained overloads and cooling system failures leading to overheating.
• Mechanical/chemical issues: Low oil level, internal arcing, and decomposition of insulating oil (detected via gas accumulation).
• System disturbances: Through-faults (external faults that stress the transformer) and ferroresonance.

Types of Transformer Protection Relays

Transformer Differential Protection Relay — 87T

This is the primary protection for large transformers. It compares the current entering the transformer (primary side) with the current leaving (secondary side). Under normal conditions, these are equal (minus excitation current). During an internal fault, the difference exceeds a set threshold, and the relay trips instantly. It features restraint coils to prevent misoperation during inrush currents or through-faults.

Transformer Overcurrent Protection Relay — 50/51

50 (Instantaneous Overcurrent): Provides fast protection for severe internal faults. It operates without intentional time delay.

51 (Time-Delay Overcurrent): Acts as backup protection for the transformer and downstream feeders. It uses inverse-time characteristics (e.g., IEC or IEEE curves) to coordinate with downstream protective devices.

Transformer Earth Fault Protection Relay — 50N/51N

50N (Instantaneous Earth Fault): Detects high-magnitude ground faults.

51N (Time-Delay Earth Fault): Detects low-magnitude ground faults (e.g., high-resistance ground faults) using residual current measurement (vector sum of three-phase currents). It is critical for sensitive grounding protection.

Transformer Overvoltage and Undervoltage Relay — 59/27

59 (Overvoltage): Protects the transformer insulation from sustained high voltages caused by load rejection, system faults, or ferroresonance.

27 (Undervoltage): Protects against voltage sags and can be used to initiate load shedding or alarm systems to prevent motor stalling and instability.

Transformer Overexcitation Protection Relay — 24

Operates on the Volts/Hertz (V/Hz) ratio. When the transformer is overexcited (due to overvoltage or underfrequency), the core saturates, causing excessive magnetizing current and severe heating. The 24 relay trips or alarms based on a V/Hz characteristic curve, protecting the core and winding insulation.

Transformer Thermal Overload Protection — 49

Monitors the transformer’s hot-spot temperature using a thermal model. It calculates the winding temperature based on load current, ambient temperature, and cooling mode. It provides alarm and trip stages to prevent insulation degradation due to prolonged overloads.

Transformer Gas / Buchholz Protection

A mechanical (but electrically connected) relay installed in the oil pipe between the main tank and the conservator tank. It detects:

Slow gas accumulation (alarm): Indicates minor internal arcing or insulation breakdown.

Sudden oil surge (trip): Indicates a severe internal fault (e.g., short-circuit) that produces a pressure wave. It is mandatory for oil-immersed transformers with conservator tanks.

Transformer Protection Relay Functions and ANSI Codes

ANSI Code	Function	Description
87T	Transformer Differential	Main protection for internal phase/ground faults.
50	Instantaneous Overcurrent	Fast fault clearing for severe short-circuits.
51	Time-Delay Overcurrent	Backup overcurrent protection with coordination.
50N/51N	Earth Fault	Detects ground faults on the neutral/phase.
59	Overvoltage	Prevents insulation stress from overvoltage.
27	Undervoltage	Detects voltage sags and initiates load shedding.
24	Overexcitation	V/Hz protection against core saturation.
49	Thermal Overload	Calculates winding hot-spot temperature.
63	Buchholz (Gas)	Gas accumulation/oil surge detection.
26	Temperature (WTI/OIL)	Winding/oil temperature monitoring (RTD input).

Transformer Protection Relay Application by Transformer Type

Distribution Transformers

Rating: Usually < 10 MVA.

Protection: Typically use fuses or 51 overcurrent relays. Differential (87T) is rare due to cost. Earth fault (51N) and thermal (49) are common.

Power Transformers

Rating: > 10 MVA (substation level).

Protection: Full suite: 87T as primary, 51 as backup, 51N, 49, 24, and 63 (Buchholz). Voltage protection (59/27) is added depending on system conditions.

Step-Up Transformers (Generator Step-Up – GSU)

Location: Between generator and transmission grid.

Protection: Requires 87T (must handle CT saturation), 51 backup, 24 (overexcitation due to generator over-fluxing), and 63. Often requires a sudden pressure relay (63) due to high fault energy.

Two-Winding Transformers

Most common type. Protection is straightforward: one differential zone (87T) with one set of CTs on each side.

Three-Winding Transformers

Used for tertiary connections (e.g., capacitor banks, auxiliary loads).

Protection: 87T must have three restraint inputs (one per winding). Requires careful CT ratio matching and compensation for phase shifts between each winding.

Three-Winding Transformer Protection Solutions — In-Depth Overview

Generator Transformers

Critical protection: Differential (87T) must be extremely sensitive to detect low-level inter-turn faults. It often includes 87G (generator differential) coordination. Overexcitation (24) is mandatory due to generator startup/shutdown V/Hz variations. Negative sequence (46) may also be added to protect against unbalanced loads.

Generator-Transformer Differential Protection (87GT) — In-Depth Overview

How to Select a Transformer Protection Relay

Step 1 — Transformer Rating

Parameter	Why It Matters
kVA / MVA	Determines if 87T is needed (≥ 5 MVA → yes).
HV voltage	Defines CT insulation & grounding type.
LV voltage	Defines breaker capacity & need for 27 protection.
%Z (impedance)	Basis for short-circuit calc & overcurrent curve selection.

Step 2 — Transformer Type

Type	Relay Requirement
Two-winding	2 sets of 3-phase CT inputs.
Three-winding	3-restraint differential relay required (e.g., SEL-487E, P643).
Auto-transformer	87TN (zero-sequence differential) mandatory.
Generator step-up	24 (V/Hz) mandatory + underfrequency blocking.
Distribution	87T optional; 50/51 + 49 + temp monitoring suffice.

Step 3 — Protection Functions (Checklist for RFQ)

ANSI Code	Function	Decision Rule
87T	Differential	Required for ≥ 5 MVA.
50/51	Phase overcurrent	Always required.
50N/51N	Earth fault	Required for grounded neutral; 51N for isolated neutral.
27/59	Under/Overvoltage	59 for load rejection; 27 for bus-tie transfer schemes.
24	V/Hz overexcitation	Mandatory for generator step-up; optional for others.
49	Thermal overload	Strongly recommended for all.
46	Negative sequence	Optional unless significant unbalance exists.

Step 4 — CT Inputs (Critical Hardware Specs)

Parameter	Requirement / Decision
Secondary rating	1A (IEC) / 5A (ANSI) — must match breaker CT.
CT ratio	Ensure HV & LV secondary currents are close (compensation < 15%).
Accuracy class	87T → 5P20 or TPY; 50/51 → 10P10 minimum.
Burden (VA)	CT VA > relay input + cable burden. Use 1A for long cable runs (>100m).
Knee point voltage	Must exceed max fault secondary voltage; else → specify TPY class.

Step 5 — Communication Protocol

Protocol	When to Choose
IEC 61850	New digital substations (mandatory).
Modbus TCP	Most common, low-cost, PLC integration.
Modbus RTU	Simple 2-wire RS485 installations.
IEC 60870-5-103	European legacy / power plant internal.
DNP3	North American projects (mandatory).

Recommendation: IEC 61850 + Modbus TCP covers most projects.

Step 6 — Installation & Mounting

Location	Key Constraints
Relay panel (control room)	19″ rack or 4U/6U chassis; confirm DC supply (110V/125V or 220V).
Prefabricated shelter	Depth ≤ 220mm (shallow type).
Switchgear cubicle	Flush-mount, height ≤ 200mm, wide-temp (-25°C ~ +70°C).
Station automation	Support IRIG-B or NTP/SNTP for time sync.

Final RFQ Checklist — Fill in Before Sending to Suppliers

#	Item	Your Specification
1	Rating / Voltages	_____ MVA, _____ kV / _____ kV
2	Winding type	□ 2-wind □ 3-wind □ Auto
3	87T required?	□ Yes □ No
4	CT secondary	□ 1A □ 5A
5	CT class	□ 5P20 □ TPY □ 10P10
6	Protocol	□ IEC 61850 □ Modbus □ DNP3
7	Mounting	□ Rack □ Switchgear □ Wall
8	DC supply	_____ V DC

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Use this checklist to avoid specification gaps, prevent CT mismatch rework, and get accurate quotations on the first RFQ round.

Transformer Protection Relay Configuration

The configuration process involves:

• Setting CT Ratios and Polarity: Ensure correct connection (cross-connection for delta-wye compensation).
• Matching Magnetizing Inrush: Enable second-harmonic restraint (typically 15-20% of fundamental) to prevent 87T misoperation on energization.
• Setting Overcurrent Curves: Select appropriate inverse-time curves (e.g., Extremely Inverse for transformers) to coordinate with downstream fuses/breakers.
• Thermal Model Settings: Input transformer cooling classes (OA, FA, FOA), time constants, and ambient temperature.
• V/Hz Curve: Set multiple alarm/trip points based on the transformer’s design V/Hz capability.

Transformer Protection Relay for Substations and Power Plants

Substations: Emphasis is on selectivity and coordination with upstream and downstream devices. Use IEC 61850 GOOSE messages for fast inter-tripping. Often require dual redundant relays for high reliability.

Power Plants: Must withstand high fault currents and frequent generator start/stop cycles. Protection must interface with generator protection relays (e.g., 87G, 40) and excitation systems for coordinated overexcitation protection. Require robust anti-pumping and lockout features.

Transformer Protection Relay Product Options

Manufacturer	Popular Models	Key Features
Siemens	7UT85, 7SJ85	Multi-functional (differential + backup), IEC 61850, extensive thermal modeling.
ABB	RET670, REC670	High-speed differential, advanced CT saturation detection, flexible I/O.
GE	GET60, F60	Multilin series, easy-to-use interface, comprehensive breaker monitoring.
Schneider Electric	MiCOM P642, P643	Dedicated transformer relays with 3-winding capability, robust V/Hz protection.
Eaton	EMR-3000	Cost-effective for distribution, integrated 87T + 51.
SEL (Schweitzer)	SEL-387, SEL-487E	387 (2-winding), 487E (3-winding), highly customizable logic, advanced event recording.

FAQ

1. What is a transformer protection relay?

A transformer protection relay is an Intelligent Electronic Device (IED) that continuously monitors transformer electrical parameters (current, voltage, frequency, temperature, and gas pressure). Its primary function is to detect internal faults, abnormal conditions, and system disturbances, then send a trip signal to isolate the transformer before catastrophic damage occurs. It serves as the brain of the transformer’s protection system.

What are the main types of transformer protection relays?

The main types are categorized by their protection principle:

• Differential Relay (87T) — primary protection for internal faults.
• Overcurrent Relays (50/51) — backup and phase fault protection.
• Earth Fault Relays (50N/51N) — ground fault detection.
• Voltage Relays (27/59) — under/overvoltage protection.
• Overexcitation Relay (24) — V/Hz protection against core saturation.
• Thermal Relay (49) — winding hot-spot temperature monitoring.
• Buchholz Relay (63) — gas and oil surge detection for oil-immersed transformers.

What protection functions are used for power transformers?

A typical power transformer protection scheme includes:

• 87T (Differential) as primary protection.
• 51 (Time-delay overcurrent) and 50 (Instantaneous overcurrent) as backup.
• 51N/50N (Earth fault) for ground faults.
• 49 (Thermal overload) to prevent insulation aging.
• 24 (Overexcitation) to prevent core saturation.
• 63 (Buchholz) for incipient fault detection in oil-filled units.
• 59/27 (Over/Undervoltage) depending on system requirements.

What is the ANSI code for transformer differential protection?

The ANSI code for transformer differential protection is 87T. The “87” denotes the differential protection function, and the “T” specifically indicates it is applied to a transformer (as opposed to 87G for generators or 87B for buses).

When is 87T differential protection required for a transformer?

87T is generally required for transformers rated ≥ 5 MVA or when the transformer is considered critical to the power system (e.g., generator step-up units, substation main power transformers). For smaller distribution transformers (< 5 MVA), overcurrent protection plus fuses are often used instead due to cost considerations.

Can one transformer protection relay provide 87T, 50/51 and earth fault protection?

Yes. Modern numerical transformer protection relays are multi-functional IEDs that integrate differential protection (87T), phase overcurrent (50/51), earth fault (50N/51N), thermal (49), and overexcitation (24) into a single device. Examples include SEL-487E, Siemens 7UT85, and MiCOM P643. This reduces panel space, wiring, and overall cost.

How do I select a transformer protection relay?

Selection follows a 6-step process:

1. Transformer Rating — MVA, HV/LV voltages, and %Z.
2. Transformer Type — two-winding, three-winding, or auto-transformer.
3. Protection Functions — define required ANSI codes (87T, 50/51, etc.).
4. CT Inputs — specify 1A/5A, ratio, accuracy class (5P20/TPY), burden, and knee point voltage.
5. Communication — choose protocol (IEC 61850, Modbus, DNP3, etc.).
6. Installation — confirm mounting type, dimensions, temperature rating, and auxiliary supply.

What CT parameters are required for transformer protection relay selection?

Five CT parameters must be specified:

• Secondary rating: 1A (IEC) or 5A (ANSI) — must match breaker CTs.
• CT ratio: Selected so HV and LV secondary currents are closely matched (compensation factor < 15%).
• Accuracy class: 5P20 or TPY for differential (87T); 10P10 minimum for overcurrent (50/51).
• Burden (VA): CT rated burden must exceed relay input plus cable burden.
• Knee point voltage (Vk): Must be higher than the maximum secondary voltage under fault conditions; otherwise, TPY-class anti-saturation CTs are required.

What communication protocols are supported by numerical transformer protection relays?

Most modern relays support multiple protocols, with common options including:

• IEC 61850 (GOOSE/SV) — for digital substations.
• Modbus TCP / RTU — for general SCADA and PLC integration.
• DNP3 — mandatory for North American projects.
• IEC 60870-5-103/104 — common in European and utility applications.
  Most relays offer multiple protocol ports (Ethernet, RS-485, fiber) simultaneously.

What is the difference between transformer main protection and backup protection?

• Main protection (e.g., 87T differential) is designed to operate instantaneously for faults within the transformer zone. It is the primary line of defense and clears internal faults with minimal delay.
• Backup protection (e.g., 51 time-delay overcurrent) operates with a time delay and is intended to clear faults if the main protection fails, or to protect against external faults that persist. It provides redundancy and coordination with downstream devices.

CTA

Send us your transformer rating, voltage ratio, winding configuration, CT ratio and required protection functions. Our technical team can recommend a suitable transformer protection relay configuration.