87G Generator Differential Protection Relay

Description

The 87G Generator Differential Protection Relay is a digital protection device designed for the primary differential protection of synchronous generators. It compares the currents measured at the generator terminal and neutral ends to detect internal stator faults and provides fast trip protection when differential current exceeds the configured operating threshold.

The AGP-441H is applicable to generators up to 100 MW and supports differential high-set protection, percentage differential protection, CT open-circuit detection, fault recording and multiple communication interfaces.

What Is an 87G Generator Differential Protection Relay?

87G is the ANSI device number for generator differential protection, designed to detect phase-to-phase, ground, and turn-to-turn faults within the generator stator winding.

The relay compares currents from CTs at the generator terminals and neutral point. Under normal conditions, the currents are balanced. When an internal fault occurs, the differential current triggers an instantaneous trip.

Unlike 87T or 87GT, 87G focuses specifically on the generator stator winding, with a protection zone covering the stator winding and leads between the two CT sets.

As the primary protection for the generator, 87G provides high sensitivity and fast operation (typically 20–40 ms), ensuring instantaneous tripping to minimize damage and enhance safety.

How Does 87G Generator Differential Protection Work?

The 87G relay continuously compares currents from CTs at the generator terminals and neutral point. It calculates differential current and restraining current to determine if an internal fault exists.

Normal Generator Operation

Terminal current ≈ Neutral current → Differential current ≈ 0 → No trip

External Fault

High through-fault current flows through both CTs. Restraining current increases, trip threshold rises via percentage bias → 87G remains stable → No trip

Internal Stator Fault

Terminal current ≠ Neutral current → Differential current increases → Exceeds threshold → 87G operates → Generator breaker trips

Summary

Percentage Differential Protection for Generators

Percentage restraint (or biased) differential protection is the most widely used protection scheme for generators. It improves security during external faults and CT saturation while maintaining high sensitivity to internal stator faults. Unlike simple differential protection, which operates on a fixed current threshold, percentage differential protection uses a variable trip characteristic based on the magnitude of through-current.

Key Concepts

Differential Current (I_diff)

The differential current is the vector difference between the currents measured at the generator terminals and the neutral point.

I_diff = |I_terminal – I_neutral|

Under normal conditions and external faults, I_diff is near zero (or very small). When an internal stator fault occurs, I_diff increases significantly and triggers the relay operation.

Restraining Current (I_bias)

The restraining current (also called bias current or through-current) represents the current flowing through the generator. It is typically calculated as the average (or maximum) of the terminal and neutral currents.

I_bias = (I_terminal + I_neutral) / 2 (or max)

During external faults, a large fault current flows through the generator from the terminal side to the neutral side, increasing I_bias. The relay uses this I_bias to raise the trip threshold, ensuring stability.

Operating Characteristic

The relay operates when the differential current exceeds a threshold that increases with the restraining current. This creates a percentage bias characteristic with two or more slope segments:

Why Percentage Differential Protection Is Preferred for Generators

• High sensitivity for internal faults – Low-magnitude winding faults are reliably detected, even at low load

• Security during external faults – The bias characteristic prevents false tripping during high through-faults

• CT saturation tolerance – The biased characteristic compensates for CT errors and transient saturation

• Stability under heavy load – No risk of misoperation during normal loading or system disturbances

87G Relay Setting Considerations

When setting the 87G generator differential relay, the following parameters must be carefully selected:

87G vs 87GT Generator Protection

What Does 87G Generator Differential Protection Protect?

87G generator differential protection is designed to protect the generator stator winding against internal faults. Its protection zone is defined by the location of the current transformers (CTs) installed on both sides of the generator stator.

Protection Zone of 87G

The protected zone includes:

• Generator stator winding – the primary protected asset

• Generator leads – the connections between the stator winding and the terminal CTs

• Connections between the terminal CTs and the neutral CT – all current-carrying paths within the generator circuit

Any fault occurring within this CT-defined zone is detected as an internal fault by the 87G relay.

What Faults Does 87G Detect?

As primary protection for the generator stator, 87G is primarily intended to detect faults within the CT-defined generator stator protection zone, including:

• Phase-to-phase faults – short circuits between different phase windings

• Phase-to-ground faults – winding insulation failures to earth

• Turn-to-turn faults – short circuits between turns within the same phase winding

These faults typically produce an unbalanced differential current that triggers the relay to issue a fast trip command.

What Is Not Protected by 87G?

The 87G protection zone does not extend beyond the CT locations:

Faults occurring outside the CT-defined zone are handled by other protection functions such as 87T, 87GT, or backup overcurrent protection.

87G Generator Differential Protection Functions

The 87G generator differential protection relay integrates multiple protection functions to provide comprehensive coverage for the generator stator winding. The key functions are listed below, with the primary differential protection presented first.

87G Generator Differential Protection

This is the primary protection function for the generator stator winding. It continuously compares currents measured at the generator terminals and the neutral point. Under normal conditions and external faults, these currents remain balanced. When an internal stator fault occurs (phase-to-phase, phase-to-ground, or turn-to-turn fault), the balance is broken and the differential current exceeds the preset threshold, triggering an instantaneous trip command to isolate the generator from the system.

87GH Differential High-Set Protection

The high-set differential protection (87GH) is a faster, higher-sensitivity element that operates on a higher differential current threshold. It is designed to detect severe internal faults with large fault current magnitudes, providing even faster tripping for high-magnitude faults. This function ensures that critical faults are cleared with minimal delay, reducing the risk of extensive generator damage.

CT Open-Circuit Detection

Current transformers may experience open-circuit conditions due to wiring faults, loose connections, or CT failure. CT open-circuit detection continuously monitors the CT secondary circuits for open-circuit conditions. When detected, the relay issues an alarm to alert operators, allowing timely maintenance and preventing potential hazards such as high voltages across open CT circuits.

Differential Overcurrent Alarm

The differential overcurrent alarm function monitors the differential current and issues an alarm when it exceeds a preset threshold but remains below the trip setting. This provides early warning of developing faults, CT imbalance, or other abnormalities before they reach critical levels, enabling proactive maintenance and reducing the risk of unplanned tripping.

CT Broken-Circuit Blocking

When a CT broken circuit is detected, the CT broken-circuit blocking function temporarily blocks the differential protection to prevent false tripping caused by the unbalanced currents resulting from the open circuit. This ensures that the relay does not misoperate due to a CT circuit failure, while still raising an alarm to alert operators to the issue.

Summary of Functions

87G Generator Differential Protection Relay Specifications

How to Select a Generator Differential Protection Relay

How to Select a Generator Differential Protection Relay

Selecting the right 87G relay requires careful evaluation of generator parameters, CT characteristics, protection requirements, and installation conditions. Key selection factors are as follows:

1. Generator Rated Capacity
Determines protection sensitivity and fault current levels. Our 87G relay supports units up to ≤100 MW, suitable for small to medium hydro, thermal, and industrial generators.

2. Generator Rated Current
Affects CT ratio selection and relay range. Settings should be based on the rated current.

3. CT Ratio
Must be compatible and compensated within the relay to balance terminal and neutral currents.

4. CT Class and Anti-Saturation Performance
Higher-class CTs (e.g., C400, 5P20) improve stability during external faults. Our relay remains stable even under CT saturation.

5. Differential Protection Slope
Set minimum pickup, Slope 1, and Slope 2 to balance internal fault sensitivity and external fault stability.

6. High-Set Differential Protection (87GH)
Provides faster tripping for severe high-current faults. Recommended for critical units.

7. Communication Protocol
Supports IEC 61850, Modbus, IEC 60870-5-103/104 for SCADA integration.

8. Fault Recording
Captures fault waveforms for post-fault analysis and protection verification.

9. Installation Type
Supports both cabinet integrated and distributed installation.

Selection Summary

CTA: Contact us for 87G relay selection and protection setting support. Our engineering team will provide tailored configuration for your application.

87G Relay Communication and SCADA Integration

The 87G relay exchanges protection status, measurements, alarms, and event data with SCADA systems via RS-485, CAN, and Ethernet, supporting IEC 60870-5-103, Modbus, and IEC 61850 protocols.

This enables seamless integration into both legacy substations and modern digital power plants, allowing operators to monitor generator insulation health, track differential current trends, and respond to faults promptly from a central control room.

The relay also supports time synchronization via SNTP or IRIG-B, ensuring accurate event sequencing across the entire protection system.

With its flexible communication architecture, the 87G relay reduces wiring complexity, simplifies system expansion, and lays a solid foundation for future smart grid upgrades.

FAQ

Q1. What is an 87G generator differential protection relay?

Primary protection device for generator stator windings. Compares terminal and neutral currents to detect internal faults and issue instantaneous trip.

Q2. What does 87G protect on a generator?

Protects the stator winding and connections between terminal CTs and neutral CTs. Does not cover the transformer or external busbars.

Q3. How does generator differential protection work?

Compares terminal and neutral currents. Normal/external faults = balanced; internal fault = unbalanced differential current → trip.

Q4. What is the difference between 87G and 87GT?

87G protects the generator; 87GT covers generator-transformer unit.

Q5. Why is percentage restraint used in 87G protection?

Raises trip threshold as through-current increases, preventing false tripping during external faults and CT saturation while maintaining internal fault sensitivity.

Q6. How does 87G protection handle CT saturation?

Percentage bias and harmonic restraint maintain stability during CT saturation by raising the trip threshold with increased through-current.

Q7. What generator capacity can this 87G relay protect?

Supports generators up to ≤100 MW, suitable for small to medium hydro, thermal, and industrial applications.

Q8. Can the 87G relay be integrated with SCADA?

Yes. Supports IEC 60870-5-103, Modbus, IEC 61850 via RS-485, CAN, Ethernet for remote monitoring, alarms, and fault data retrieval.

Q9. How is an 87G generator differential relay tested?

Secondary injection testing verifies differential slope, pickup, high-set, and CT open-circuit detection. See our related test article for detailed guidance.

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