Modal properties of a vertical axis wind turbine in operating and parked conditions

•Full scale structural behavior of a vertical-axis wind turbine (VAWT) is shown.•For the first time the damping coefficients of a real VAWT turbine are deeply analyzed.•In operating conditions damping ratios are much lower than for large-sized HAWT.•Dynamics of secondary elements can be read on the supporting shaft.•Effect of a thunderstorm on a real VAWT is presented.•Results obtained from over 50 response time-histories selected over 4 years of data.