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ISO 20816-21 specifies vibration evaluation criteria for onshore and offshore wind turbines. It covers:

• Main bearing — slow-speed shaft bearing
• Gearbox — multi-stage speed increaser
• Generator — high-speed electrical machine
• Tower top (nacelle) — structural vibration

Both velocity (mm/s) and acceleration (g) measurements are addressed depending on the component.

Velocity (mm/s RMS) is used for gearbox, generator, and tower top where vibration frequencies are in the 10–1000 Hz range.

Acceleration (g RMS) is used for main bearings operating at very low speeds (10–20 RPM), where high-frequency bearing defect signals are better captured by accelerometers using envelope demodulation techniques.

Larger turbines (>3 MW) have:

• Lower rotational speeds
• Higher structural flexibility
• Greater rotor inertia
• More complex drivetrain dynamics

The zone boundaries are generally consistent across power classes as they relate to component severity. Power class provides context for interpretation and trending.

Tower top (nacelle) vibration measures the overall structural response to:

• Rotor mass and aerodynamic imbalance
• Yaw misalignment with wind direction
• Drivetrain forces transmitted through the nacelle
• Tower structural resonance
• Foundation issues

It is a key indicator of overall turbine health and can reveal issues not visible from individual component monitoring.

For Zone C on a wind turbine component:

• Review CMS (condition monitoring system) trends and SCADA data
• Correlate vibration with wind speed and power output
• For bearings: check envelope spectrum for defect frequencies
• For gearbox: analyze oil samples for metal particles
• Consider turbine curtailment to reduce loads
• Schedule maintenance during the next suitable weather window
• For tower: check foundation, yaw system, and rotor balance