ISO 10816-21:2015 specifies the measurement and evaluation of mechanical vibration of wind turbines and their components by taking measurements on non-rotating parts. It applies to horizontal axis wind turbines with mechanical gearbox and rated generator output exceeding 200 kW and the following design and operational characteristics:
a) installation on supporting systems (tower and foundation) made of steel and/or concrete,
b) horizontal axis rotor with several rotor blades,
c) rotor bearing separate from or integrated into the gearbox,
d) generators driven via gearbox,
e) generators of the synchronous or asynchronous type (mostly equipped with 4-pole generator),
f) generators with only a fixed pole number or which are pole-changeable for speed adjustment,
g) output control by rotor blades (pitch or stall wind turbines),
h) generator coupled to the power grid via converter or directly.