Experimental Research for MW Wind Turbine Blade

INTRODUCTION

Wind power is an important component of renewable energy, It will to bear the burden of energy saving and emission reduction and optimization energy structure. Grid-connected wind turbine is developing rapidly in our country. The most basic components of the wind turbine is blade, the good design, reliable quality and superior performance of the blade is the keypoint of the normal and stable operation. This year China is on track to pass the United States as the world’s largest market for wind turbines,but the design techniques also rely on foreign sources (BP Group, 2012).

In practical working conditions, the wind flow past wind turbine blade with wind shear and instantaneous changes (Burton et al., 2001). By this the load of blade is ever-changing also. For safety, all factors were considered at loads calculate include wind shear, centrifugal force, pitch control delay (Lesihman, 2002; Jie et al., 2011). The variation features of blade loads were discussed based on strip theory. The effects of gravity, wind shear and pitch control delaying were analyzed. A method of the equivalent coefficient for wind rotor loads considered turbine practical efficiency was present. The results showed the calculation precision was high enough to meet the demand of project design.

The objective of this study is having a detailed understanding of turbine blade state of stress and strain. The load is firstly calculated by a method proposed by authors, then get the information by full-scale blade experiment.

MODEL OF LOAD CALCULATION

Strip theory: Strip theory starts with the four equations derived from momentum and blade element theories. In this analysis, it is assumed that the chord and twist distributions of the blade are known. The angle of attack is not known, but additional relationships can be used to solve for the angle of attack and performance of the blade. The forces and moments derived from momentum theory and blade element theory must be equal. Equating these, one can derive the flow conditions for a turbine design (Jonkman and Butterfield, 2009; Spera, 2009).

From axial momentum:

From angular momentum:

From blade element theory:

where, the thrust, dT, is the same force as the normal force, dF_N. Thus, equation from blade element theory can be written as:

The force of blade at different wind speed can be calculate by this theory. On the other hand, combine with the equivalent coefficient method, we determine the scheme of loading at the test (Jie et al., 2013).

FULL-SCALE BLADE TEST

Loading modes: In the test, there are five loading point, 15 m (F1), 21 m (F2), 25.5 m (F3), 30 m (F4) and 34.4 m (F5) distance from blade root. Direction of loading is Flap edge of the blade. The method of loading is multi-stage loading from 30, 60, 80, 100, 106 and 110% of limit load. Figure 1 depicts the distribution of loading point.

Figure 2 is the photo of experimental field. In the photo, the tested equipment is full-scale wind turbine blade. The total length of blade is 37.5 m, the power of turbine is 1.5 MW.

Location of detecting: In order to effectively observation the blade structural behavior with loading, chose 22 point to set up observation point at five different sections (0.8, 12, 23, 28 and 32 m). The specific situation see Fig. 3.

Test result
Tsection’s strain of wielding positive under different loads: When the load is up to 110%, the strain diagram of the position measuring point on the both sides of the main beam of three directions (-45, 0, +45°) is as follows.

Fig. 1:	Sketch of loading point

It can be obviously observed from the Fig. 4 and 5 that the strain of the blade under the condition of wielding forward in the direction of a 0° angle (along the direction of main beam) is larger. Due to the change in angle of the ply, the strain of other two directions is relatively slightly smaller. On the whole, we can find that SS surface is under tension strain and PS surface is under compressive strain when leaves waving the positive.

Test section’s principal strain of wielding positive under different loads: The principal strain of each measuring point on main beam SS surface under load at all level is shown in Fig. 6.

It can be shown in Fig. 6 the SS surface’s main strain of the main beam on the blade under the condition of wielding positive concentrate on the place at the range of 7 and 28 M. In the place of 7 M, the pressure strain at maximum is - 3163.80 με when the tensile strain at maximum is 3359.51με at 18 M. The strain of two places grows linearly.

Fig. 2:	Photo of experimental field

Fig. 3:	Sketch of observation position

Fig. 4:	Strain distribute of blade’s SS surface at full loading

Fig. 5:	Strain distribute of blade’s PS surface at full loading

Fig. 6:	Main strains of blade’s SS surface at different loading

It is mean that the increase ratio of strain and the reapportion of growth under the external load are basically the same. It shows that all test points of the SS surface are within the range of linear elastic.

Fig. 7:	Main strains of blade’s PS surface at different loading

Fig. 8:	Blade displacements at different loads

All measuring points’ principal strain of the main beam’s PS surface at all levels of load is shown in Fig. 7.

It can be shown in Fig. 7 that PS surface’s strain of the main beam concentrate on the place at the range of 7 and 28 M when leaves is on the static load experiment of wielding positive. Among them, the compressive strain at maximum is -5737.35με at 12 M and the tensile strain at maximum is 2169.80με at 7 M. And the strain at 12 M increases linearly. Due to Leaves are shell structure, when leaf surface appeared buckling phenomenon in the previous experiments, so the leading edge of blade section and the trailing edge of the two test interface (at 7 and at 18 M) of the blade section of larger are also monitored on this test.

All load points’ displacement under different loads of Waving positive.

All load sections when leaves is in waving a positive conditions appear the phenomenon which the displacement of the leaves’ roots is smaller and the end is larger. Based on field observations and the actual measurement, it can be known that the displacement of all sections is nearly linear, shown on Fig. 8. The displacement at maximum at the end of blade is 4.03 M when the section of 33.8 M is at full load.