Generator Transformer Unit Protection Scheme for Power Plants

Generator transformer unit protection is a comprehensive protection scheme designed for generator-transformer blocks in power plants. It protects the generator, step-up transformer and connection equipment against internal faults, abnormal operating conditions and external system disturbances.

A generator-transformer unit typically requires coordinated protection functions including differential protection (87GT), overcurrent backup protection, earth fault protection, overexcitation protection and abnormal operation protection.

What Is Generator Transformer Unit Protection?

Generator Transformer Unit Protection is an integrated protection scheme designed for a generator and its directly connected step-up transformer, treating them as a single electrical unit.

In most power plants, the generator and transformer are linked without a circuit breaker between them, meaning they share a common electrical zone where a fault on either machine can rapidly affect the other. For example, an internal fault in the transformer will cause the generator to continue feeding current into the fault, while a generator fault can stress the transformer windings and damage its core. Additionally, the connection zone between the two machines—such as busducts or cables—is not covered by standalone generator or transformer protection.

By applying coordinated unit protection, typically through an 87GT relay that combines generator differential (87G) and transformer differential (87T) functions, any fault within this interconnected system can be detected and cleared with a single, simultaneous trip command. This unified approach ensures comprehensive protection, prevents cascading damage, reduces plant downtime, and safeguards the substantial capital investment represented by both critical assets.

Generator Transformer Protection Functions

ANSI Code	Protection Function	Purpose
87GT	Generator Transformer Differential	Internal fault protection
50/51	Overcurrent Protection	Backup protection
64	Earth Fault Protection	Ground fault detection
24	Overexcitation	V/Hz protection
49	Thermal Protection	Overheating protection
32	Reverse Power	Generator motoring
40	Loss of Excitation	Field failure

Brief Function Descriptions

ANSI Code	Function	Description
87GT	Generator Transformer Differential	Compares currents on both sides of the generator and transformer windings. Any significant difference indicates an internal fault, triggering an instantaneous trip. This is the primary protection for the generator-transformer unit.
50/51	Overcurrent Protection	Provides backup protection for external faults and overloads. 50 is instantaneous for severe short circuits; 51 is time-delayed for coordination with downstream protections.
64	Earth Fault Protection	Detects ground faults in the generator stator or transformer windings. Early detection prevents escalation to phase-to-phase faults and major equipment damage.
24	Overexcitation	Monitors the V/Hz ratio to prevent core saturation in both the generator and transformer. Excessive V/Hz causes rapid heating and can damage the iron core.
49	Thermal Protection	Monitors winding and core temperatures using thermal models or direct RTD inputs. Prevents insulation degradation and extends equipment life by tripping or alarming before overheating occurs.
32	Reverse Power	Detects power flow reversal, indicating the generator is motoring due to prime mover failure. Protects turbines and gearboxes from mechanical damage.
40	Loss of Excitation	Detects excitation system failure. Loss of field causes the generator to draw reactive power from the grid, leading to voltage instability and rotor overheating.

Protection Tiers for Generator Transformer Units

Protection Level	Functions Included	Best For
Basic	50/51 + 64	Small or non-critical units
Standard	50/51 + 64 + 87GT	Most industrial applications
Comprehensive	87GT + 50/51 + 64 + 24 + 49 + 32 + 40	Large, utility-critical units

How Generator Transformer Unit Protection Works

Generator Transformer Unit Protection operates on the current balance principle. The relay continuously compares currents measured by CTs on both sides of the protected zone to determine whether the system is healthy or faulted.

Normal Condition

Aspect	Status
Generator side CT	Measures normal current
Transformer side CT	Measures equal current
Differential current	≈ 0
Relay action	No trip – normal operation continues

Under normal conditions, the current entering the transformer equals the current leaving the generator. The differential current is near zero, and the relay remains inactive.

Internal Fault

When an internal fault occurs within the protected zone (generator windings, transformer windings, or connection zone), the current balance is broken.

Aspect	Status
Generator side CT	High fault current
Transformer side CT	Mismatched current
Differential current	Exceeds threshold
Relay action	Trip circuit breakers

Common Internal Faults:

• Winding short circuit
• Phase fault (phase-to-phase or three-phase)
• Insulation failure (ground or phase faults)

Relay Response:

The fault current flows only within the protected zone, creating a significant differential current. The relay detects this imbalance and issues a simultaneous trip command to both the generator breaker and the transformer high-voltage breaker, isolating the entire unit from the grid within milliseconds.

Summary Table

Condition	Generator CT	Transformer CT	Differential Current	Relay Action
Normal	Normal	Equal	≈ 0	No trip
Internal Fault	High fault	Mismatched	> Threshold	Trip both breakers

This simple yet effective principle—comparing currents at both ends of the protected zone—ensures fast fault detection, minimizes damage, and protects both generator and transformer assets.

Applications of Generator Transformer Unit Protection

Generator Transformer Unit Protection is essential across all power generation applications where generators are directly connected to step-up transformers without a breaker between them.

Hydropower Plant

Aspect	Description
Typical Capacity	1MW – 800MW+
Key Protection Focus	87GT, 32, 81, 40, 24
Why Needed	Frequent start/stop cycles, over-speeding risks, water flow variations affect both generator and transformer

Thermal Power Plant

Aspect	Description
Typical Capacity	100MW – 1000MW+
Key Protection Focus	87GT, 50/51, 64, 49, 24, 40, 32
Why Needed	Continuous high-load operation, large fault currents, critical grid interconnection requiring high reliability

Gas Power Plant

Aspect	Description
Typical Capacity	10MW – 400MW
Key Protection Focus	87GT, 50/51, 81, 24, 40, 32
Why Needed	Rapid start/stop cycles, high sensitivity to frequency deviations, fast fault clearance required

Industrial Power Generation

Aspect	Description
Typical Capacity	1MW – 100MW
Key Protection Focus	87GT, 50/51, 64, 32, 27/59, 81
Why Needed	Continuous production depends on reliable power; faults can halt entire production lines

Summary Table

Application	Capacity	Voltage	Key Functions
Hydropower	1–800MW	400V–13.8kV	87GT, 32, 81, 40, 24
Thermal	100–1000MW+	11kV–24kV	87GT, 50/51, 64, 49, 24, 40, 32
Gas	10–400MW	6.6kV–13.8kV	87GT, 50/51, 81, 24, 40, 32
Industrial	1–100MW	400V–11kV	87GT, 50/51, 64, 32, 27/59, 81

Why Unit Protection is Critical Across All Applications

• Protects generator, transformer, and connection zone
• Single trip command clears faults fast
• Prevents fault propagation between machines
• Reduces downtime and protects capital investment

Difference Between 87G, 87GT and 87T Protection

Although generator protection and generator transformer protection sound similar in name, they differ fundamentally in protection scope and protected equipment.

Protection	Protected Equipment	Scope of Protection
87G	Generator stator	Internal stator winding faults only
87T	Transformer	Transformer winding faults only
87GT	Generator-transformer unit	Generator + step-up transformer + connection zone

87G – Generator Protection

87G (Generator Differential Protection) is the primary protection for the generator. It compares currents measured by CTs on the neutral side and line side of the generator stator windings to detect internal phase-to-phase faults. Its protection scope is limited to the generator itself, covering the stator winding zone between the neutral CT and the line CT. When an internal fault is detected, 87G issues an immediate trip command, disconnecting only the generator from the system.

Typical Application: Standalone generators, or as the primary protection for the generator within a generator-transformer unit.

👉 [Learn more about 87G Generator Differential Protection Relay →]87G

87T – Transformer Protection

87T (Transformer Differential Protection) is specifically designed to protect the transformer itself. It compares currents measured by CTs on the high-voltage and low-voltage sides of the transformer to detect internal winding faults. Its protection scope is limited to the transformer tank interior and does not cover the generator or the connection zone between them.

👉 [Learn more about 87T Transformer Differential Protection Relay →]87T

87GT – Generator Transformer Unit Protection

87GT (Generator Transformer Differential Protection) adopts a holistic protection approach, treating the generator, generator step-up transformer (GSU), and the connection zone between them as a single unified protection unit. Since there is typically no circuit breaker between the generator and the step-up transformer, they are electrically directly connected and share a common electrical zone. A fault in either machine will rapidly affect the other, requiring coordinated and unified protection.

87GT protection compares currents from CTs on the generator line side, the transformer high-voltage side, and the transformer neutral point. This provides comprehensive coverage of the generator, transformer, and the connection zone between them. Any differential current indicates an internal fault within the unit, triggering a simultaneous trip command that disconnects the entire generator-transformer unit from the grid. This coordinated approach forms the core of an effective Generator Transformer Protection System, ensuring that all components within the unit are safeguarded against internal faults.

Typical Application: Generator-transformer units in hydro, thermal, and gas power plants.

👉 [Learn more about 87GT Generator Transformer Protection Relay →]87GT

Why Do We Need to Distinguish Between Them?

Protection Type	Protection Scope	Fault Response
87G	Generator only	Trips only the generator
87T	Transformer only	Trips only the transformer
87GT	Generator + Step-up transformer + Connection zone	Trips both generator and transformer simultaneously

The key difference is: 87G protects an isolated piece of equipment, while 87GT protects the complete generator-transformer unit. Because there is no circuit breaker separating the generator and transformer within the unit, any internal fault requires simultaneous isolation of both machines—and this is precisely the core value of 87GT protection. For plant operators, implementing dedicated Generator Step Up Transformer Protection is essential to secure the critical link between generation and transmission.

Related Generator Transformer Protection Products

For comprehensive protection of generator-transformer units, we offer the following key products:

87GT Generator Transformer Protection Relay

The 87GT Generator Transformer Protection Relay is specifically designed for generator-transformer units, providing integrated differential protection that covers the generator, step-up transformer, and the connection zone between them. This relay combines generator differential (87G) and transformer differential (87T) functions with harmonic restraint to prevent tripping on transformer inrush current, making it the ideal choice for hydro, thermal, and gas power plants.

Generator Protection Relay

For standalone generator applications or as the generator-specific component within a unit protection scheme, our Generator Protection Relays deliver comprehensive protection including 87G differential, overcurrent (50/51), loss of excitation (40), reverse power (32), and negative sequence protection (46), suitable for a wide range of generator applications.

👉 Learn more about Generator Protection Relay →

Generator Protection Relay Solution

From small diesel generators to large utility-scale turbines, our Generator Protection Relay Solutions deliver end-to-end engineering services including relay setting assistance, protection scheme design, and SCADA communication configuration, ensuring your critical generation assets remain reliably protected.

👉 Explore Generator Protection Relay Solutions →

Commissioning & Testing Complete Guide

Standard commissioning and testing are the guarantee of long-term stable operation of the relay. The whole process covers pre-operation inspection and professional testing:

Pre-Commissioning Checklist

• Check CT/VT wiring polarity and ratio consistency
• Verify the accuracy of all protection parameter settings
• Test communication connection and alarm signal output

Core Testing Items

• Secondary Injection Test: Simulate various fault signals to verify relay action accuracy
• Primary Injection Test: On-load test to check actual protection operation effect
• End-to-End Test: Verify linkage between relay and breaker trip circuit

FAT (Factory Acceptance Test) and SAT (Site Acceptance Test) must be completed before project delivery to ensure zero fault operation.

Generator Transformer Unit Protection – FAQ

Q1: What is generator transformer unit protection?

A: An integrated protection scheme that treats the generator, step-up transformer, and connection zone as a single unit. Since no circuit breaker separates them, a fault anywhere requires simultaneous tripping of both machines, typically via an 87GT relay.

Q2: What is the purpose of 87GT protection?

A: To provide primary high-speed protection for the entire generator-transformer unit by detecting internal faults and issuing a simultaneous trip command to both breakers, preventing fault propagation and minimizing damage.

Q3: What is the difference between 87G and 87GT?

	87G	87GT
Protects	Generator only	Generator + Transformer + Connection zone
Trip	Generator only	Both generator and transformer

Q4: What protection functions are required for generator transformer units?

A: Key functions include:

Function	ANSI	Purpose
Differential	87GT	Primary internal fault protection
Overcurrent	50/51	Backup protection
Earth Fault	64	Ground fault detection
Overexcitation	24	V/Hz protection
Thermal	49	Overheating protection
Reverse Power	32	Prevents motoring
Loss of Excitation	40	Excitation failure detection

Q5: Where is generator transformer unit protection used?

A: In hydro, thermal, gas, and industrial power plants where generators are directly connected to step-up transformers.

Q6: How does Generator Step Up Transformer Protection work?

A: Operates on the current balance principle:

Condition	CT Readings	Differential Current	Relay Action
Normal	Equal	≈ 0	No trip
Internal Fault	Unequal	> Threshold	Trip both breakers

Internal faults include winding short circuits, phase faults, and insulation failure.

Conclusion

The safe operation of power generation equipment relies on accurate fault prediction and rapid response to various risks. 87GT Protection enables sensitive detection of internal faults by comparing currents on both sides of the generator-transformer unit. A well-designed generator transformer protection scheme not only safeguards the security of critical assets but also upholds the long-term value of reliable and continuous power delivery.

For the selection of transformer protection relays, please refer to the relevant selection content.