Investigation of potential extreme load reduction for a two‐bladed upwind turbine with partial pitch

Abstract

This paper presents a wind turbine concept with an innovative design combining partial pitch with a two-bladed (PP-2B) turbine configuration. Special emphasis is on extreme load reduction during storm situations at standstill, but operational loads are also investigated. In order to compare the loads and dynamics of the PP-2B turbine, a partial pitch three-bladed (PP-3B) turbine and a normal pitch regulated three-bladed (3B) turbine are introduced on the basis of solidity similarity scaling. From the dynamic comparisons between two-and three-bladed turbines, it has been observed that the blade vibrations are transferred differently from the rotor to the tower. For a three-bladed turbine, blade vibrations seen in a fixed frame of reference are split with ±1P only. A twobladed turbine has a similar split of ±1P but also includes contributions on higher harmonics (±2P, ±3P, … etc.). Further on, frequency split is also seen for the tower vibrations, where an additional ±2P contribution has been observed for the two-bladed turbine. Regarding load comparisons, the PP-2B turbine produces larger tower load variations because of 2P excitation during the operational cases. However, extreme loads are reduced by approximately 20% for the PP-2B and 18% for the PP-3B compared with the 3B turbine for the parked condition in a storm situation. Moreover, a huge potential of 60% is observed for the reduction of the extreme tower bottom bending moment for the PP-2B turbine, when the wind direction is from ±90°to the turbine, but this also requires that the turbine is parked in a T-configuration.

Aero‐structural dynamics of a flexible hub connection for load reduction on two‐bladed wind turbines

“…Being dismissed for their complex dynamic behavior and unfavorable visual and noise impact since the late 1980s, several innovative two‐bladed turbine designs are currently being tested for the offshore market. Each of these concepts makes use of the characteristic shape of the two‐bladed rotor to address offshore specific challenges, such as accessibility, installation and maintenance . Most recently, the Skywind 3.4 MW prototype has been erected on an onshore site.…”

“…Because two‐bladed turbines are operated at higher tip speed ratios, lower gear ratios and compact transmission components are possible. Furthermore, it was found that two‐bladed turbines possess favorable load behavior in extreme wind conditions . While these properties may imply smaller component dimensions and lower cost, two‐bladed turbines experience higher fatigue loading.…”

Modal dynamics of structures with bladed isotropic rotors and its complexity for two-bladed rotors

“…Recent studies of two-bladed turbines have focused on their aeroservo-elastic control (Solingen, 2015;Solingen et al, 2016a, b;Wang and Wright, 2016) and on their design loads . In the latter study, Kim et al (2015) plots the spectrogram of the tower top signal obtained from nonlinear time simulations of a two-bladed turbine and compares it to the spectrogram of a three-bladed version of the same turbine. There are similarities between these spectrograms which lead the authors to conclude that two-bladed turbines have similar modes as three-bladed turbines.…”

Fatigue and extreme load reduction on two‐bladed wind turbines using the flexible hub connection

“…Regarding time‐variance, it was pointed out by Skjoldan and Hansen as well as Cacciola that due to the rotation of the rotor with respect to a flexible but stationary support structure, every principal eigenmode possesses an infinite number of associated “fan” modes shifted in frequency by . It was shown for two‐bladed turbines by numerical analysis (Hansen and Kim et al) that these fan modes can have a nonneglectable contribution compared with the principal mode. The identification of the principal and the fan modes of time‐variant systems requires the use of periodic analysis methods, such as the implicit Floquet analysis or Hill's method, which can be computationally expensive for high‐fidelity models.…”