Wind turbines are subject to vibration – some more than others. Insiders know which types of turbines run stably, which were only built in small numbers because of vibration issues, and which –such as variable speed turbines – are at risk of vibration when they rotate close to the tower’s natural frequency. Often these vibrations can be minimized by balancing the rotor blades.
Field balancing of rotors, machines and fans is common practice in machine and system engineering. It is worth considering having the rotor blades of a wind turbine balanced, especially since there are wind turbines with relatively strong, low frequency vibrations and because rotor blades can be balanced using one-plane balancing. Preliminary trials have shown that PRUFTECHNIK measuring equipment is sensitive enough to perform linear measurements and balancing at 0.1 Hz and higher.
In the example blow, an operator reported frequent shutdowns triggered by a vibration monitor integrated in the nacelle. The initial response would be to assume that the vibration monitor was not adjusted properly and to attempt to correctly adjust the device. The next step might be to search for a new, less vibration prone location in the nacelle for mounting the device. But not in this particular case. Here, PRUFTECHNIK was immediately contracted to measure the vibrations and to reduce their magnitude by means of field balancing. Figures 1–3 show various stages of this work – from attaching the balancing belt to measuring the imbalance and fixing the additional masses.
One-plane field balancing was performed in four steps:
- Diagnosis measurements
The first step was to measure and evaluate the low frequency amplitude spectra of the vibration velocity. The results showed that there was a rotational speed range within the rated operating range of the turbine in which the tower’s natural frequency underwent particularly strong excitation. This made it clear that even small imbalance masses or other disturbances would cause the rotor/nacelle/tower system to vibrate.
- In the imbalance run
In the imbalance run, a foreign mass is applied to the system in order to “get to know the system”. Additional masses are mounted on a rotor blade and measurements are made of the rotational vibrations by means of accelerometers. Also, rotational speed and phase are determined using optical sensors. Because the system described here exhibited resonance excitations, only small imbalance masses could be attached. To reduce aerodynamic imbalance influences, the wind turbine had to be run in a non-critical state and without pitching.
- Balancing runs
The balancing runs themselves were performed using the balancing program in the VIBXPERT measuring device. The measuring program was configured so that only three fixed positions existed for applying the additional masses –namely, the three rotor blades. The suggested balancing masses were attached to the premounted balancing belt and the balancing run was repeated.
- Repeat diagnosis measurements
Following the balancing runs, the diagnosis measurement is repeated and compared to the initial condition. In this particular case, a reduction in the vibration load of about 40% could be achieved in the resonating state. The diagnosis measurements also revealed that the natural resonance of the tower was excited by aerodynamic imbalances. The operator was advised to electrically block the critical speed ranges to prevent the aerodynamic imbalances from causing resonance.
Other causes for very low frequency imbalances and disturbance excitations in wind turbines are shown below.