Generator Differential Protection Relay 87G

Description

Overview

The AGP-441H is a digital generator differential protection relay designed for generators up to 100 MW. It provides phase-separated 87G and 87GH differential protection, fast internal fault tripping, CT open-circuit detection and flexible communication interfaces for power plant protection and control systems.

Key Features

• Generator Capacity: ≤100 MW
• Protection: 87G / 87GH
• Protection Type: Phase-separated differential protection
• High-Speed Trip: ≤25 ms for differential high-set protection
• CT Protection: CT open-circuit detection and blocking
• Communication: RS-485 / CAN / Ethernet / IEC 60870-5-103
• Installation: Panel integrated or distributed installation

Function Configuration

Protection Configuration

Monitoring and Control Functions

Communication Functions

Time Synchronization Function

Printing Function

Protection Information Functions

 

Ordering Instructions

• Product model, name, and order quantity;

• Rated AC current of current transformer: 1A/5A;

• Device operating power supply voltage level: AC/DC220V, DC110V, DC48V;

• Digital input module voltage level: AC/DC220V, DC110V, DC48V;

• Whether the DC input/output function is required (optional);

• Delivery address and schedule;

• Important notes / special requirements.

What Does the 87G Generator Differential Relay Protect?

The 87G differential relay protects the generator stator zone defined by the terminal-side and neutral-side CTs.

Applications of Generator Differential Protection Relay 87G

Generator differential protection relay 87G is the primary protection for internal stator faults, widely applied across various power generation scenarios. Actual configuration should be selected based on generator capacity, voltage level, CT arrangement, and protection requirements.

Hydropower Generators

Thermal Power Generators

Gas Turbine Generators

Industrial Generators

Diesel Generator Sets

Note: Not all diesel generators require 87G protection—small standby units are typically protected by overcurrent protection alone.

Configuration Selection Summary

Important Reminders

• No universal solution—each generator must be designed individually

• Small units may not require 87G—overcurrent protection may be sufficient

• Grid-connected generators typically require differential protection

• 87G must coordinate with backup protections (50/51, 32, etc.)

Final configuration should be designed by qualified protection engineers based on specific project parameters.

87G Generator Differential Protection – FAQ

1. What is a generator differential protection relay 87G?

The 87G generator differential protection relay is the primary high-speed protection device for detecting internal faults within a generator’s stator windings. It continuously compares currents measured by CTs on the generator terminal side and neutral side. Under normal conditions or external faults, the currents are equal and the differential current is near zero. When an internal stator fault occurs (such as a phase-to-phase short circuit, turn-to-turn fault, or insulation failure), the currents become unequal, generating a differential current that triggers an instantaneous trip command to isolate the generator from the system.

2. What does an 87G generator differential relay protect?

The 87G relay protects the generator stator zone defined by the terminal-side and neutral-side CTs, including:

The 87G relay does not protect: external system faults, transformer faults, rotor faults, loss of excitation, reverse power, or overload conditions—these require separate protection functions.

3. What is the difference between 87G and 87GH protection?

How they work together: For minor internal faults, only 87G operates. For severe faults (e.g., high-magnitude short circuits), both 87G and 87GH operate simultaneously. During external faults with CT saturation, 87G remains stable due to its restraint characteristic, while 87GH remains stable due to its high pickup setting.

4. What CTs are required for generator differential protection?

Generator differential protection requires two sets of three-phase CTs:

Key CT requirements:

• CT ratio must be properly matched between both sides

• CT polarity must be correctly verified during commissioning

• CT accuracy class must be suitable for protection applications

• CT saturation performance must be considered for external fault stability

• Modern relays can compensate for ratio mismatches using internal ratio matching

5. How does the 87G relay handle CT open-circuit conditions?

Modern 87G relays include dedicated CT open-circuit detection to prevent relay maloperation and protect personnel.

When a CT open circuit occurs:

• The relay detects the abnormal condition by monitoring current imbalance and voltage presence

• It issues an alarm to alert operators, rather than a trip command

• The affected phase is typically blocked from differential protection operation to prevent false tripping

• The remaining healthy phases continue to provide protection

Typical detection logic:

• One phase current is near zero while voltage is present

• Other phase currents are normal

• The relay determines this is a CT circuit failure rather than an internal generator fault

6. What generators can the 87G relay protect?

The 87G relay is suitable for a wide range of generator types and capacities:

Selection guidance:

• Generators >1MW: Recommended to configure 87G protection

• Grid-connected generators: Mandatory regardless of capacity

• Generators <500kW: Typically protected by overcurrent (50/51) without 87G

7. What communication protocols does the generator differential protection relay support?

Modern generator differential protection relays support multiple communication protocols for seamless SCADA and plant control system integration:

Supported data exchange:

• Analog measurements (voltage, current, power, frequency)

• Status indications (breaker position, relay health, alarms)

• Protection events and trip records

• Control commands (remote breaker open/close, reset)

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