Comprehensive Microprocessor-Based Protection Solution for Large Generators

1. Overview

With power systems evolving toward higher parameters, larger capacities, and more complex grid structures, the safe and stable operation of generating units is critical to overall grid reliability. Traditional relay protection devices face challenges such as blind zones and insufficient sensitivity when dealing with complex internal generator faults. This solution leverages advanced microprocessor-based protection technology, integrating multi-source information and intelligent algorithms to provide a fast, reliable, and comprehensive protection system for large generators (e.g., thermal, nuclear, and hydroelectric units). It aims to completely eliminate protection blind spots and ensure the security of power generation assets.

2. Core Challenges

Large generators face multiple internal fault threats during operation, including:

• ​Stator Winding Faults: Phase-to-phase short circuits, inter-turn short circuits, and ground faults. Inter-turn short circuits, in particular, exhibit low initial fault currents, making them difficult to detect with traditional transverse differential protection due to inherent blind zones.
• ​Rotor Circuit Faults: Single-point ground faults, double-point ground faults, and open or short circuits in the excitation circuit. While a single-point ground fault may allow continued operation, its progression to a double-point ground fault can cause magnetic asymmetry and severe unit vibration.
• ​Abnormal Operating Conditions: Reverse power, loss of excitation, over-excitation, overvoltage, and frequency abnormalities. Although not instantaneous faults, these conditions can severely damage the generator or threaten grid stability.

3. Detailed Solution

Our microprocessor-based protection solution adopts a hierarchical distributed architecture. The core protection relay integrates a robust hardware processing platform with mature protection algorithms, as detailed below:

3.1 For Stator Inter-Turn Short Circuits: Multi-Criteria Composite Protection

To address the insensitivity of traditional transverse differential protection to inter-turn short circuits within the same phase, this solution employs a multi-criteria fusion decision algorithm, significantly improving detection reliability and sensitivity.

• ​Technical Principles:
• Negative-Sequence Power Direction Criterion: Monitors negative-sequence current and voltage at the generator terminals to calculate the direction of negative-sequence power. Internal asymmetric faults (e.g., inter-turn short circuits) generate a negative-sequence source, with power direction flowing from the generator to the system, enabling accurate internal fault detection.
• Third Harmonic Voltage Variation Criterion: Tracks the amplitude ratio and phase difference between the neutral and terminal third harmonic voltages. Inter-turn short circuits disrupt the inherent distribution pattern of third harmonic voltages, to which this criterion is highly sensitive.
• Neutral Point Displacement Voltage Criterion: Serves as an auxiliary enhancement to improve reliability.
• ​Performance Advantages:
• High Sensitivity: Capable of detecting minor inter-turn short circuits as low as 0.5%.
• Rapid Operation: Full operation time under 20 ms, significantly limiting fault damage.
• High Reliability: Multiple criteria interlock or operate in parallel to prevent maloperation and avoid failure to operate.
• ​Case Study: After implementation in a 500MW coal-fired generator, the solution achieved 98% sensitivity in detecting inter-turn short circuits, successfully preventing major burn-out accidents caused by minor insulation defects.

3.2 For 100% Stator Ground Fault Protection: Dual-Technology Fusion Positioning

Traditional fundamental zero-sequence voltage protection exhibits blind zones near the neutral point. This solution combines two mature technologies to achieve 100% protection coverage from the terminals to the neutral point.

• ​Technical Principles:
• Conventional Zone (85–95%): Uses the third harmonic voltage ratio method to protect most of the stator winding from the neutral point toward the terminals.
• Blind Zone Compensation (Near Neutral Point, 5–15%): Employs injection-based stator ground fault protection. A low-frequency (20Hz or 12.5Hz) voltage signal is injected into the rotor circuit, and changes in the injection current are monitored to accurately calculate insulation resistance and fault location, completely eliminating blind zones near the neutral point.
• ​Performance Advantages:
• 100% Coverage: No blind zones, ensuring full stator winding protection.
• Precise Localization: Accurately pinpoints ground fault locations for targeted maintenance.
• ​Case Study: At a nuclear power plant, the solution successfully located a ground fault at only 3% from the neutral point, with less than 1% error, enabling planned maintenance and avoiding unplanned outages.

3.3 For Rotor Circuit Health: Dynamic Monitoring and Early Warning

Rotor circuit faults, particularly open rotating diodes, are common hidden dangers. This solution shifts from "post-fault protection" to "pre-fault warning" through real-time monitoring.

• ​Technical Principles:
• High-frequency current transformers (CTs) or dedicated monitoring modules installed at slip rings collect real-time excitation current waveforms.
• Built-in algorithms perform Fast Fourier Transform (FFT) harmonic analysis on the current.
• Open rotating diodes cause severe distortion of the excitation current waveform, significantly increasing characteristic harmonics (e.g., the fifth harmonic).
• ​Performance Advantages:
• Early Warning: Issues alerts based on harmonic content exceeding thresholds (e.g., fifth harmonic exceeding 8%), prompting maintenance checks on the rotating rectifier bridge before faults occur.
• Prevention of Escalation: Timely warnings prevent恶性 accidents such as insulation damage due to excitation current loss and rotor overheating.
• Condition-Based Maintenance: Provides critical data for predictive maintenance.

4. Summary and Value

This microprocessor-based protection solution integrates advanced sensing technology, signal processing algorithms, and multi-criteria intelligent decision-making to address traditional pain points in generator protection:

• Eliminates protection blind zones, achieving 100% coverage for stator inter-turn short circuits and ground faults.
• Transforms post-fault protection into pre-fault warning, effectively preventing faults through dynamic rotor monitoring.
• Validated by real-world cases, the solution offers high sensitivity, speed, and reliability, meeting the safety requirements of large and mega generators (500MW and above).