Diesel Generator Speed Measurement and Protection System

The speed measurement and protection system of a diesel generator plays a critical role throughout the entire operational process. Speed not only reflects the load state of the diesel generator but also directly affects ignition, operational stability, and overspeed protection. Therefore, a well-designed speed measurement system offers comprehensive monitoring and protection, effectively preventing equipment failure and extending its service life. This article will explore the system's structure, working principles, advantages, common issues, and improvement measures.

Structure and Working Principle of the Speed Measurement System

In the speed measurement system of a diesel generator, speed monitoring is typically conducted for both the main engine and the turbocharger. The main engine's speed measurement system usually includes four sensors: two for speed measurement that display readings on the CMR or OCW panel via a speed transmitter, and two for the governor. These sensors operate using non-contact proximity technology, measuring speed without physical interaction, significantly reducing wear.

Working Principle

Each sensor contains an oscillator, trigger, and amplifier. The oscillator's coil generates an alternating electromagnetic field. When a metallic conductor, like a flywheel tooth, approaches, the magnetic field changes, causing fluctuations in the oscillator's output voltage and frequency. The speed transmitter processes this frequency signal, converting it into a standard 4-20 mA current signal, which is then relayed to the speed gauge for display and monitoring. This process provides high-precision speed detection, offering real-time operational information for the diesel generator.

System Advantages and Multi-layered Protection Features

The speed measurement system of diesel generators provides several benefits, especially in terms of multi-layered protection, which ensures operational safety.

Non-contact Measurement: Sensors operate without physical contact with rotating parts, preventing mechanical wear. This design not only extends sensor life but also reduces maintenance requirements related to wear.
Online Maintenance: The system's design allows for easy sensor removal and installation during operation, greatly improving maintenance convenience.
Multi-level Protection: The speed transmitter of the main engine offers several protective functions:
Pre-lubrication Pump Control: Ensures full lubrication of generator bearings, reducing wear.
Excitation and Synchronization Locking: Ensures synchronized operation during grid connection, preventing accidents due to asynchronous conditions.
Overspeed Trip Protection: If speed exceeds safe limits (electrical overspeed at 565 rpm, governor overspeed at 575 rpm), the system automatically triggers shutdown to prevent damage.
Mechanical Overspeed Protection: An independent mechanical overspeed device (>595 rpm) on the diesel engine provides physical security against runaway incidents due to overspeed.

Common Issues and Improvement Measures

While the system performs well overall, some issues may arise during operation, requiring targeted improvements to enhance stability and reliability.

1. Flywheel Runout Causing Probe Damage

Flywheel runout can lead to radial or axial movement. If the sensor is too close, it can wear down or be damaged, causing unstable or lost speed readings and triggering false electrical overspeed alarms. Ideally, the sensor should be 2.5 ± 0.3 mm from the flywheel, with a recommended 2 mm gap maintained through feeler gauge measurement.

2. Bracket Vibration Affecting Measurement Accuracy

Vibrations from the diesel engine may destabilize the sensor signal, leading to fluctuating speed readings. Reinforcing the bracket, and if necessary, welding it directly to the engine can reduce vibration effects on the sensor.

3. Oil Contamination on Flywheel Interfering with Measurement

High-speed flywheel rotation may cause lubricating oil to splash onto the sensor, affecting accuracy. Installing an oil shield on the flywheel can reduce oil contamination and ensure consistent sensor performance.

4. Speed Transmitter Malfunction

Faults in the speed transmitter can cause unstable output signals, leading to fluctuating or absent speed readings. Unstable output may be due to loose terminals or internal circuitry faults. Verifying the transmitter using a frequency generator and securing terminal connections can stabilize signal transmission. If necessary, adjust or replace the transmitter.

5. Sensor Failure and Diagnostics

Sensor issues can be initially diagnosed via the red indicator light. When flywheel teeth pass by, the light should brighten; if it remains unchanged or off, the sensor may be faulty and should be replaced promptly to ensure accurate speed measurement.

Optimization Suggestions and Conclusion

The speed measurement and protection system of a diesel generator provides essential monitoring and safeguarding. By optimizing sensor installation distance, reinforcing brackets, keeping sensors free of oil, regularly calibrating transmitters, and checking connections, system reliability and measurement accuracy can be significantly enhanced. The system's multi-layered overspeed protection effectively ensures stable operation under high loads and complex conditions.

Overall, the design of the speed measurement and protection system is integral to the safety and performance of diesel generators. It enables real-time monitoring of speed conditions and quickly responds to anomalies to prevent damage from issues such as overspeed. With regular maintenance and proper management, the system can achieve its full potential, providing strong support for the long-term, stable operation of diesel generator units.