Description
Product Overview
The 87GT Generator Transformer Protection Relay is a dedicated numerical protection device designed for generator-transformer unit applications.
It integrates generator transformer differential protection, backup overcurrent protection, voltage protection and monitoring functions to provide fast and reliable protection for medium and small power generation systems.
The relay is widely used in hydroelectric power plants, thermal power plants and industrial generator-transformer units requiring high-performance protection solutions.
Key Features of 87GT Generator Transformer Protection Relay
| Feature | Benefit |
| 87GT Differential Protection | Fast detection of internal generator-transformer faults |
| Multi-function Protection | Reduce additional relay requirements |
| CT Ratio Compensation | Improve differential protection reliability |
| SCADA Communication | Easy integration with substation automation |
| Flexible Configuration | Suitable for different generator ratings |
Protection Functions
| ANSI Code | Function |
| 87GT | Generator Transformer Differential Protection |
| 50/51 | Phase Overcurrent Protection |
| 51N | Earth Fault Protection |
| 46 | Negative Sequence Protection |
| 49 | Thermal Overload Protection |
| 59/27 | Over/Under Voltage Protection |
| 40 | Loss of Excitation Protection |
| 32 | Reverse Power Protection |
More configurable functions available according to project requirements.
Monitoring and Control Functions
Communication Functions
Time Synchronization Function
Printing Function
Recording Functions
Applications
| Application | Example |
| Hydropower Plant | Generator-transformer protection |
| Thermal Power Plant | GSU protection |
| Industrial Power Generation | Medium generator units |
| Distributed Generation | Grid-connected generators |
How Does 87GT Differential Protection Work?
The 87GT differential protection operates on the current balance principle to protect the entire generator-transformer unit, including the connection zone between them.
Normal Condition
Under normal conditions, the current flowing out of the generator equals the current flowing into the transformer (adjusted for CT ratios). CTs on both sides measure these currents, and the differential current is zero. The relay remains inactive.
Fault Condition
When an internal fault occurs (winding short circuit, phase fault, or insulation failure), the current balance is broken. The currents on the generator side and transformer side become unequal, producing a differential current. The relay detects this imbalance when it exceeds the preset threshold.
Relay Action
The relay issues a trip command to both the generator and transformer breakers simultaneously, isolating the entire unit from the grid within milliseconds.
Summary
In short, equal currents = normal operation; unequal currents = internal fault → instantaneous trip.
87GT vs 87G vs 87T – Key Differences
| 87G | 87T | 87GT |
| Full Name | Generator Differential | Transformer Differential | Generator Transformer Differential |
| Protected Equipment | Generator stator only | Transformer only | Generator + Transformer + Connection zone |
| Protection Scope | Between generator neutral CT and line CT | Between transformer primary and secondary CTs | Entire generator-transformer unit |
| Fault Response | Trips generator breaker only | Trips transformer breakers only | Trips both breakers simultaneously |
| Best For | Standalone generators | Standalone transformers | Generator-transformer units in power plants |
Why Choose Our Relay
In modern power systems, selecting the right protection relay is critical for ensuring generator safety and operational continuity. Our generator protection relays stand out for the following key reasons:
Key takeaway: 87G and 87T protect individual pieces of equipment, while 87GT protects the entire unit—essential when no breaker separates the generator and transformer.
Need a 87GT Protection Relay Solution?
FAQ
Q1. What is a Generator Transformer Protection Relay?
A Generator Transformer Protection Relay is a dedicated protection device that safeguards the generator step-up transformer against internal faults, external disturbances, and abnormal operating conditions. It typically integrates differential protection (87GT) as the primary element, along with backup functions such as overcurrent, earth fault, and thermal protection, ensuring fast fault clearance and reliable transformer operation.
Q2. What is 87GT protection?
87GT is the ANSI device number commonly used for generator-transformer differential protection (also referred to as overall differential protection). It compares currents on the generator side and the transformer side to detect internal faults within the protected zone. When the differential current exceeds the preset threshold, the relay issues a trip command to isolate the transformer from the system.
Q3. What does 87GT protect?
87GT protects the generator-transformer unit, covering the zone from the generator terminals to the transformer high-voltage terminals. The protected zone is defined by CT locations on both sides and typically includes the generator bus duct, the step-up transformer, and associated connections. The exact boundary may vary based on system configuration and CT placement.
Q4. How does generator transformer differential protection work?
The relay continuously monitors currents on both sides of the generator transformer via CTs. Under normal conditions, the currents are balanced and the differential current is near zero. When an internal fault occurs, the balance is broken and the differential current rises. Once it exceeds the threshold, the relay trips. During external faults or inrush, the relay restrains via percentage bias or harmonic blocking logic.
Q5. What is the difference between 87GT, 87T and 87G?
87GT provides combined protection for the entire generator-transformer unit, while 87G and 87T cover only the generator or the transformer individually.
Q6. Why does 87GT differential protection require CT compensation?
CT compensation is required to balance the currents measured on the generator side and the transformer side during normal operation. Compensation addresses:
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CT ratio differences (due to different CT ratings)
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Transformer turns ratio differences
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Phase shift caused by the transformer vector group (e.g., YNd1, YNd11)
Without proper compensation, the relay would see a false differential current under normal load and may misoperate.
Q7. Can an 87GT relay integrate with SCADA systems?
Yes. Modern 87GT relays support standard communication protocols for seamless SCADA integration, including IEC 61850, Modbus, IEC 60870-5-103, and IEC 60870-5-104. This enables remote monitoring, event logging, alarm management, and control functions, making them suitable for modern digital substations and automated power plants.
Other Generator Protection Equipment
64R Rotor Earth Fault Protection Relay | Voltage Injection
87G/50/51 Generator Protective Relay
Digital generator differential protection relay
Ping-Pong Type Rotor Earth Fault Protection Relay
Generator Protection Relay Panel
Jack –
The product performs excellently with complete functions and reliable quality, maintaining stable operation in projects.