Generators run continuously under harsh conditions with high voltage, heavy current and high rotating speed, prone to faults such as stator winding short circuit, rotor grounding and loss of excitation. Severe faults will damage the generator and even cause power system collapse and massive economic losses. Generator Protection Relay effectively prevents such risks and ensures stable operation of generator units.
Hence, deploying complete electrical protection relays is a must to secure normal generator operation.
This article elaborates on core functions, application scenarios and selection tips of protection relays for generator, together with setting value rules and test verification approaches. It also summarizes major domestic manufacturers in China and the unique strengths of their products.
Common Generator Protection Functions
| Category | Fault / Abnormal Condition | Description |
|---|---|---|
| Electrical Faults (Severe consequences, require immediate tripping) | Stator winding inter-phase short circuit | Most hazardous fault, may burn out windings |
| Stator winding inter-turn short circuit | Short circuit between different turns of the same phase | |
| Stator winding single-phase earth fault | Risk of generator core burnout | |
| Rotor winding single-point / two-point earth fault | Causes abnormal excitation and intensified vibration | |
| Loss of excitation fault | Absorbs reactive power from grid and leads to voltage collapse | |
| Abnormal Operating Conditions (Alarm first, then delayed treatment) | Overload, Overvoltage, Abnormal frequency | General abnormal operating parameters |
| Reverse power | Risk of overheating for turbine blades | |
| Out-of-step operation, Excessive excitation current, etc. | Abnormal unit operating status |
87G Protection Relay
Principle
Based on Kirchhoff’s Current Law, Generator Differential Protection 87G compares the current of the same phase at the generator terminal and neutral point.
Under normal operation, the currents on both sides are equal in magnitude and identical in phase, resulting in zero differential current. When an inter-phase short circuit occurs inside the stator winding, the currents differ and the differential current rises, triggering an instantaneous protection trip.
Features
It operates rapidly within 20 to 40 milliseconds with excellent selectivity. Generator Differential Protection acts as the main protection for generators with a capacity above 1MW.
Generator Differential Protection calculation
Take a 10 MW, 10.5 kV generator (rated current: 687 A) as an example. Key settings for generator differential protection are listed below:
Minimum operating current: Set to 0.1~0.3Ie to evade normal unbalanced current. Here: 0.2×687 ≈ 137.4 A (primary), 1.15 A (secondary, CT ratio 600/5).
Percentage restraint coefficient: 0.4~0.5, preventing maloperation under external faults.
Differential high-speed trip: 4~6Ie for quick clearance of severe internal faults. Here: 5×687 ≈ 3435 A (primary), 28.6 A (secondary).
Second harmonic restraint ratio: 15%~20%, blocking maloperation from magnetizing inrush current.
64G Protection Relay
Protection Requirements
Codes require 100% stator earth protection for generators ≥100 MW with no blind zone. It shall detect earth faults near the neutral point of large generators directly coupled with main transformers.
Earth Fault Classification
Duration: Transient, intermittent, permanent earth fault Location: Internal, external earth fault Nature: Metallic, arcing, resistive earth fault Cause: Real, false earth fault
Neutral Grounding Modes
Common modes in China: grounding via single-phase VT, high-resistance grounding via distribution transformer, and arc suppression coil grounding (under-compensation).
64R Protection Relay
Monitors excitation winding insulation.
Single-point earth fault: No immediate danger, but risks evolving into two-point fault.
Two-point earth fault: Triggers partial short circuit, field distortion, heavy vibration, rotor burnout and shaft magnetization. Immediate tripping required.
Fault Classification
Single / two-point fault; Metallic (low resistance) / non-metallic (high resistance, arcing); Stable / unstable fault.
Working Principles of Rotor Earth Fault Protection
Ping-pong type: Non-injection, uses excitation voltage. Works in normal operation only.
Square wave / low-frequency injection type: External power supply, full-range operation including start & stop. Measures resistance and fault location, widely used for large generators.
Bridge type: Simple and low-cost. Large blind zone at midpoint, rarely used now.
46 Protection Relay
This protection detects negative sequence current caused by system unbalanced faults, as excessive negative sequence current will overheat and damage the generator rotor.
Fault Types
Unbalanced three-phase load, phase-to-phase short circuit, open-phase operation.
Features
Adopts inverse time characteristics. It issues alarms for minor overloads and trips rapidly under severe faults.
32 Relay Protection
Reverse Power Protection (ANSI 32R)
It monitors the direction of active power. When power reverses (the unit absorbs active power from the grid, P<0) and exceeds the setting value, the protection will trip the circuit breaker or issue alarm signals after a time delay to avoid equipment damage and grid disturbance.
For generator applications
The generator operates as a motor and draws power from the grid, which will damage the prime mover (steam turbine / gas turbine).
For new energy grid connection
Prevent excess power from photovoltaic and wind power systems from feeding back to the grid.
Generator Protection Relay solutions
We provide complete generator protection relay products.It should be noted that the generator relay protection scheme varies slightly for different projects, subject to actual requirements.
Generator Differential Protection Device
As the generator’s primary protection, this device monitors winding differential current to detect internal faults such as phase, inter-turn and earthing short circuits. It trips immediately to isolate faults and safeguard the generator and grid, featuring fast response, high sensitivity and high reliability.
Generator Backup Protection Device
It serves as backup protection for generators. It detects external faults and abnormal operating conditions, trips with a time delay when main protection fails, and protects the unit and power system.
Generator Rotor Earth Fault Protection Device
It guards against rotor winding earth faults. It triggers tripping or alarm timely to prevent rotor insulation damage and ensure unit safe operation.
Generator Comprehensive Protection Device
Suitable for small generator projects. Combining differential and backup protection functions, the device supports panel mounting and installation on high-voltage switch cabinets.
Chinese Relay Protection Products
Our Generator Protection Relay and other relay protection devices boast high reliability and flexible configuration, which can be tailored to diverse project demands. Built for durability, they work steadily in harsh field environments. As professional Generator Protection Relay solutions, they are manufactured in line with IEC and IEEE international standards and enjoy wide recognition across global markets.
