Thermal Power Plant TSI Case: Turbine Mechanical Measurement and Monitoring for Safe Unit Operation

Customer Background

The turbine-generator is the heart of a thermal power plant, and mechanical quantity instruments are the electrocardiogram that watches it beat. In the thermal instrumentation of a 600 MW-class power plant we support, the Turbine Supervisory Instrumentation (TSI) system continuously monitors shaft relative vibration, bearing absolute vibration, axial displacement, differential expansion, casing expansion, rotor eccentricity, speed / zero speed and key phasor. However, frequent false TSI alarms, drifting readings and unclear maintenance standards troubled the instrumentation team, so they worked with our engineers to rebuild a reliable measurement and maintenance system.

Challenge

• Frequent false TSI alarms that disturbed operators and risked unit trips;
• Unstable or drifting signals on shaft vibration, axial displacement and expansion channels;
• Gap voltage moving out of the linear zone, signal loss caused by open or short circuits, and inaccurate readings from contamination, loose brackets and aging sensors;
• No clear standards-based procedure for installation, overhaul and acceptance of mechanical measurement loops.

Solution: A Complete Measurement and Maintenance System

Our solution covers the six parameter families of turbine mechanical condition monitoring - vibration, displacement, expansion, speed, eccentricity and key phasor - built on three core sensing technologies:

• Eddy-current sensors - the workhorse of TSI measurement. Non-contact measurement based on the eddy-current effect with a 0-10 kHz frequency range, resolution down to 0.01 mm and strong resistance to oil contamination. Typical output is -4 V to -20 V DC, with the gap voltage set at the linear midpoint (about -10 V DC for a 200 mV/um, 8 mm probe). They cover shaft vibration, axial displacement, differential expansion, eccentricity, key phasor and speed measurement.
• LVDT sensors - contact-type displacement sensors for large travels up to 0-80 mm with linearity within ±1% F.S., mainly used for casing expansion and part of differential expansion. TD-2 series thermal expansion sensors provide 4-20 mA retransmission plus local indication.
• Magnetoelectric velocity sensors - the traditional choice for bearing absolute vibration, rigidly mounted on machined bearing pedestals; modern units may use piezoelectric accelerometers with double integration.

Typical 600 MW Unit TSI Configuration

For sensor selection, the probe coil diameter determines the measuring range: 8 mm probes cover about ±1 mm for shaft vibration, eccentricity and key phasor; 11 mm probes ±2 mm; 14 mm probes ±4 mm for medium axial displacement and differential expansion; 25 mm probes ±6 mm; and 50 mm probes ±12 mm for large expansion measurement.

Fault Handling and Maintenance Highlights

Following GB/T 34578-2017 and DL/T 1056-2019, we helped the plant establish daily inspection and troubleshooting mechanisms for the five most common fault families: unstable output or drift, abnormal gap voltage jumps, signal loss or constant output, inaccurate readings, and frequent false alarms. Key engineering practices include:

• Single-end grounding of the shielded cable at the TSI cabinet side only, with the probe side floating to eliminate ground loops;
• Checking probe coil resistance (about 7.5 Ω ± 0.5 Ω for typical series) to identify open or short circuits;
• Sealing high-frequency joints with heat-shrink tubing and keeping them insulated and floating as required by DL 5190.4-2019;
• Setting the zero reference with the thrust disc seated against the thrust bearing, and verifying protection set-points by simulated wear tests;
• Calibrating sensors every 6-12 months, keeping insulation resistance above 1 MΩ, and maintaining complete technical archives.

Outcome

After the retrofit, false alarms were eliminated, measured values returned to the trustworthy linear zone, and the plant gained a clear standards-based workflow covering installation, routine inspection, overhaul acceptance and protection logic testing (DL/T 1012-2006, GB/T 11348.2-2012, GB/T 6075). Shaft vibration evaluation is now performed against the A/B zones of the national standards, and every measurement loop truly acts as the sentry of long-term, safe turbine operation.

For OEM supply of eddy-current probes, preprocessors, LVDT and velocity sensors, or TSI retrofit and commissioning support, contact our engineering team.