Requerimientos Para La Certificacion de Palas

7/31/2019 Requerimientos Para La Certificacion de Palas

1/14

REQUIREMENTS FOR THE CERTIFICATION

OF ROTOR BLADES

Gerd Wacker

Germanischer Lloyd WindEnergie GmbH

Steinhoeft 9

20459 Hamburg, Germany

ABSTRACT

In Germany, Denmark and India a wind turbine has to have a valid certification to get a

building permission. In The Netherlands and most of the other European countries, in USA,

Canada, Australia etc. investment groups and banks require a certification of the turbines.

And this includes the certification of the rotor blades.

The blades are highly stressed and are to be designed to withstand extreme loads, mainly the

50-year gust and fatigue (service live 20 years). In the following an overview is given of the

necessary analyses and documentation as well as a comparison of the relevant standards and

regulations.

KEY WORDS: Certification, Applications-Energy, Structural Analysis

1. INTRODUCTION

For the certification of the rotor blades of wind turbines the stiffness, strength, stability and

design life of the blade shall be proofed by analysis and verified by full-scale tests of the

blade. In addition tests of special details may be required.

The calculations and test results shall be sufficiently well documented to demonstrate the

strength, stiffness and durability of the blade. Special investigations shall be performed for theload introduction parts, e.g. adhesively jointed thread inserts, tip brakes.

The most important regulations and standards for certification on the international level are

the Regulations for the Certification of Wind Energy Conversion Systems of Germanischer

Lloyd (1) and IEC WT 01 IEC System for Conformity Testing and Certification of Wind

Turbines Rules and Procedures(2), International Electrotechnical Commission (IEC). IEC

WT 01 refers to IEC 61400 series of standards for wind turbines. For the certification of rotor

blades the most important standards herein are IEC 61400-1 Wind turbine generator systems

Part 1: Safety requirements (3) for the load assumptions and safety, IEC TS 61400-23

Wind turbine generator systems Part 23: Full-scale structural testing of rotor blades (4)

and IEC TR 61400-24 Wind turbine generator systems Part 24: Lightning protection (5).


2/14

To get a building permission for Germany, the biggest market for wind turbines with an

installed capacity of over 11,000 MW by end of November 2002, the Regulations for Wind

Energy Convertors (in German), German Institute for Civil Construction (6) shall be

fulfilled. This set of regulations covers the civil structures only and not the machinery,

therefore it is common to use the GL-regulations for the blades.

In Denmark the turbine shall have a valid certification according to the Technical Criteria for

the Danish Approval Scheme for Wind Turbines (7). In this approval scheme the general

requirements are stated and for details the document refers to other standards and regulations.

The Danish Standard DS 472 Loads and Safety of Wind Turbine Construction (8) describes

the requirements for the load analysis and for the safety system. Recommendations for design

documentation and the testing of wind turbine blades are given in document (9) with the same

name. Further the Recommendations for Fulfilling Requirements in the Technical Criteria

(in Danish) (10) is also valid and in some parts more detailed than (9).

Last but not least the Dutch pre-standard NVN 11400-0 Wind Turbines Part 0: Criteria for

Type-certification Technical Criteria (11) shall be mentioned. This standard contains theload assumptions, safety aspects, lightning and the requirements for the design analysis based

on IEC 61400-1 and the former NEN 6096 (19) and shall be used for wind turbines which are

erected in The Netherlands.

2. STRENGTH CALCULATIONS

In general the strength of rotor blades is to be proven by ultimate and fatigue analysis.

Components or parts of the rotor blade which are loaded under compression are to be checked

for stability (buckling and wrinkling). The strength analysis is to be carried out at least at an

adequate number of cross-sections of the rotor blade. The number of cross sections to be

examined depends on the type and the size of the blade, but as a typical number for a 30m

pitch blade (rotor blade without a tip brake) 8 cross sections are to be analyzed. Additional

verification of cross sections could be required in case there are some discontinuities in the

lay-up or in the thickness of the core material used.

The strength verification shall be based on characteristic values. The characteristic values

shall correspond to a survival probability of 95% and a confidence level of 95% (GL (1, 12),

IEC (2), NVN (11), DS (8)). It is to be proven that the stresses from the design loads do not

exceed the design strength of the material. The design strength is the characteristic value

divided by the partial safety factor of the material. For fiber reinforced plastics (FRP) thepartial safety factor of the material is a product of a safety factor and factors which consider:

aging,

temperature,

attack of moisture and

production method.

The strength verification can be performed by a stress or a strain analysis.

2.1 Ultimate strength analysis The actual safety shall be documented by stress or strain

analysis and by stability analysis using failure hypothesis for anisotropic materials that is

recognized in literature. In strength analysis according to GL (1, 12), a separate verification isrequired for rupture in and between the fibers. Within the ultimate strength analysis at least


3/14

the simultaneous acting of the edge*- and flatwise

**moments together with the shear force

acting in flatwise direction shall be applied.

In table 1 the results from the different regulations and standards are compared. The loads are

multiplied with the safety factors for the different codes and normalized to the GL/DIBt

values. For the material factor m according to DS (8) the normal safety class which iscommonly used, has been chosen. Further it was assumed that the laminate will be produced

by prepregs with unidirectional and continuous fibers and the blade will be post-cured. All

material factors are valid for a stress or strain analysis in fiber direction. To get a more

realistic overview the comparison includes the loads. The loads were calculated by the author

for a common 2 MW wind turbine and type class II or equivalent. The maximum wind speed

was between 55,6 m/s (DS) and 59,5 m/s (NVN IIA and IEC IIA).

TABLE 1 - Comparison of results from the different regulations and standards at blade root

applying flatwise loadsExtreme loads GL-II/DIBt III NVN IIA IEC IIA DS class 0 to 3

blade root flatwise (1) (11) (2) (8)4092 5173 4306 3755

1,50 1,35 1,35 1,30

1,00 1,00 1,00 1,00

2,45 1,75 1,101)

1,70

15038 12221 142422)

8299

1,00 0,81 0,95 0,55

Mx * f* n*m [kNm]

Relative

Mx [kNm]

f

n

m

1) Not accepted by the major certification bodies for FRP

2) For this comparison m as in GL (1) has been chosen

TABLE 2 - Comparison of results from the different regulations and standards at blademiddle section applying flatwise loadsExtreme loads GL-II/DIBt III NVN IIA IEC IIA DS class 0 to 3

blade middle flatwise (1) (11) (2) (8)

899 918 800 753

1,50 1,35 1,35 1,30

1,00 1,00 1,00 1,00

2,45 1,75 1,101)

1,70

3304 2169 26462)

1664

1,00 0,66 0,80 0,50

Mx *f*n* m [kNm]

Relative

Mx [kNm]

f

n

m



Edgewise: direction that is parallel to the local chord (4)Flatwise: direction that is perpendicular to the local chord (4)


4/14


applying edgewise loadsExtreme loads GL-II/DIBt III NVN IIA IEC IIA DS class 0 to 3

blade root edgewise (1) (11) (2) (8)

1249 1901 1901 1712

1,50 1,35 1,35 1,30

1,00 1,00 1,00 1,00

2,45 1,75 1,101)

1,70

4590 4491 62882)

3784

1,00 0,98 1,37 0,82

My *f* n*m [kNm]

Relative

My [kNm]

f

n

m



TABLE 4 - Comparison of results from the different regulations and standards at blade

middle section applying edgewise loadsExtreme loads GL-II/DIBt III NVN IIA IEC IIA DS class 0 to 3

blade middle edgewise (1) (11) (2) (8)

238 315 315 269

1,50 1,35 1,35 1,30

1,00 1,00 1,00 1,00

2,67 1,75 1,101)

1,70

954 744 11352)

594

1,00 0,78 1,19 0,62

My * f*n* m [kNm]

Relative

My [kNm]

f

n

m



Tables 1 to 4 show that GL requires the highest safety factors for ultimate stress or strain

analysis. The IEC standard does not differentiate between metal and FRP for m and thesafety factor is obviously too low for FRP. Because of this and because of a lack of further

detailed requirements for the certification of structures and components of wind turbines most

of the IEC certifications will be a mixture between IEC 61400-1 (for loads and safety system)

and GL-regulations (for structures and components).

In flatwise direction, the requirements according to GL (1) are the highest in tables 1 and 2.

Remarkable is the big difference between GL/IEC and the Danish Standard with a factor of

1,8. This difference results from the required wind speeds and yaw errors but also from the

different safety factors. The NVN standard is in between. The relations of the safety margins

between the different codes change over the blade length.

In edgewise direction tables 3 and 4 show that the IEC standard together with the GL material

factor leads to the highest values. These values are 37% higher at the blade root and 19% at

the blade middle section when compared to GL (1). The lowest values were found for the

Danish Standard. The NVN standard is in between and the relations of the safety margins

between the different codes change over the blade length, too.

In the new edition of the GL-Regulations (12) the loads will be harmonized with the IEC-

standard (3) in combination with the European common modifications (20) and the partial

safety factor for the ultimate strength analysis will be reduced by 10%. This reduction is

possible due to the better knowledge and understanding of the modern blade materials.


5/14

With exception of the GL (1, 12) regulations there are no other regulations or standards where

in the analysis for inter fiber failure is required. Depending on the failure hypothesis used, the

safety factor for the material m is the same as in fiber direction or less. The most favorable

failure hypothesis at the moment is the thesis according to Puck (13). This is because the

different failure modes, tension perpendicular to the fiber, combination between tension and

shear, compression perpendicular to the fiber and a combination between compression andshear can be treated differently.

For failure modes A and B (figure 1) the safety factor for inter fiber failure (IFF) IFF is

between 1,35 and 1,98 and for failure mode C IFF is between 1,98 and 2,64. The lower safety

factors can be used in case only a single layer failed. In case more than one layer fails and the

stiffness of the element decreases more than 5% due to this failure the higher safety factors

have to be used.

FIGURE 1 IFF according to Puck (13)

Other failure criteria are acceptable, too. E.g. the Tsai-Wu criterion (14) could be used

together with the maximum stress criterion. This combination is necessary because the results

for some loading conditions the Tsai-Wu criterion is not conservative. (15).

2.2 Stability The stability (wrinkling and buckling) of parts subjected to compression and/or

shear shall be verified on the basis of the design loads. According to GL (1, 12) a partial

safety factor of 1,633 shall be applied in stability analysis to the mean values of the material

stiffnesses. According to Danish Standard (9) the safety factor for stability can be individually

selected and shall cover following aspects:

Type of instability,

Material stiffness,

Geometric imperfections,

Fiber misalignment,

Workmanship and Calculation method.


6/14


7/14

m

Df

1

)()(

1

=

Equation 2

with

D() = cumulative damage sum andm = slope parameter of the S/N curve.

1,0

1,2

1,4

1,6(suction side)

60

30

0

(trailing edge)

330

300

270

(pressure side)

240

210

180

(leading edge)

150

120

FIGURE 2 Residual safety factor f() at the blade root

Figure 3 shows the residual safety factors for four different sections of the rotor blade. Atradius 3,5 m the blade has a transition section between the cylindrical root section and the

aerodynamic profile. There is only a minor recess at 0 (the trailing edge). The curve of the

residual safety at radius 10,8 m is quite different compared to radius 3,5 m. At 0 the curve

has a notch und there is a difference of approx. 20 % between the residual safety factor at 90

(pure flatwise loading) and under 140.

FIGURE 3 Residual safety factors for four sections of a rotor blade


8/14

For radius 15,6 m the lowest safety could be found. In the outer part of the blade the highest

residual safety factors were established in edgewise direction. All these analyses were

performed with GL-loads. In case that the residual safety factor for a separate flatwise

calculation is larger than 1,25 no further investigations are necessary at the blade section (16).

Tables 5 to 8 compare the results from the different regulations and standards. Forcomparison it shall be sufficient to use so called damage equivalent fatigue loads. The load

spectrum is transformed into one block with 10 million cycles and an amplitude resulting in

the same damage as the spectrum.

The loads are multiplied with the safety factors from the different codes in the way as in

ultimate strength analysis. Afterwards the results are normalized with the GL/DIBt values.

It is very interesting that for flatwise loads GL, NVN and IEC reveal nearly the same strength

level (table 5 and 6). As in the static case the Danish Standard is more benign compared to the

other codes. The relation of the results of the different codes changes over the blade length

only in a minor way.


applying flatwise fatigue loadsFatigue loads GL-II/DIBt III NVN IIA IEC IIA DS class 0 to 3

blade root flatwise (1) (11) (2) (8)

2514 2345 2345 1842

1,00 1,00 1,00 1,00

1,00 1,25 1,15 1,00

1,63 1,50 1,101)

1,70

4098 4397 43952)

3131

1,00 1,07 1,07 0,76

Mx * f*n* m [kNm]

Relative

Mx [kNm]

f

n

m




middle section applying flatwise fatigue loadsFatigue loads GL-II/DIBt III NVN IIA IEC IIA DS class 0 to 3

blade middle flatwise (1) (11) (2) (8)

629 581 581 473

1,00 1,00 1,00 1,00

1,00 1,25 1,15 1,00

1,63 1,50 1,101)

1,701025 1089 1089

2)804

1,00 1,06 1,06 0,78

Mx * f*n* m [kNm]

Relative

Mx [kNm]

f

n

m



In edgewise direction GL and Danish Standard are on the same level at the blade root (table

7). The IEC and NVN standards are 16% higher than GL. For the blade middle section GL

increases about 4% relative to the root section in comparison to the other standards. An

explanation for this can not be given at this time. IEC and NVN are approx. 12% higher and

the Danish Standard 9% lower when compared to GL (table 8).


9/14


applying edgewise fatigue loadsFatigue loads GL-II/DIBt III NVN IIA IEC IIA DS class 0 to 3

blade root edgewise (1) (11) (2) (8)

2429 2456 2456 2248

1,00 1,00 1,00 1,00

1,00 1,25 1,15 1,00

1,63 1,50 1,11)

1,70

3959 4605 46032)

3822

1,00 1,16 1,16 0,97

My * f*n*m [kNm]

Relative

My [kNm]

f

n

m




middle section applying edgewise fatigue loadsFatigue loads GL-II/DIBt III NVN IIA IEC IIA DS class 0 to 3

blade middle edgewise (1) (11) (2) (8)441 428 428 385

1,00 1,00 1,00 1,00

1,00 1,25 1,15 1,00

1,63 1,50 1,101)

1,70

719 803 8022)

655

1,00 1,12 1,11 0,91

My * f*n* m [kNm]

Relative

My [kNm]

f

n

m



3. FULL-SCALE TESTING

A full-scale rotor blade test shall be performed on the basis of the design loads by a

recognized testing body or under the supervision of the certification body. In these tests, the

areas with the maximum loading determined during the analysis shall be verified.

Furthermore the first and the second natural frequencies in flapwise*

and the first natural

frequency in lead-lag**

direction shall be measured. For lager rotor blades the first torsional

frequency may be of interest, too. For identification of the blade it is necessary to note blade

type and serial number and to measure the mass and the center of gravity, GL (1, 12), DS (9),

IEC (2, 4).

The blade shall withstand the static and fatigue tests without showing any damages ofsignificance for safety or the blades function (10).

NVN standard (11) does not require a full-scale blade test.

3.1 Static testing The blade shall be tested with a load, which is sufficiently higher than the

design load. This shall be done in order to take into account influences from temperature,

humidity and production (blade to blade variations) (9).

Flapwise: direction that is perpendicular to the surface swept by the undeformed rotor blade axis (4)** Lead-lag: direction that is parallel to the plane of the swept surface and perpendicular to the longitudinal axis

of the undeformed rotor blade (4)


10/14


11/14

After each new load step, the blade shall remain in the load situation for a defined time. This

time depends on the regulations or standards used and is in the range from 10 s. according to

GL (12) up to 5 min. acc. to DS (10).

3.2 Fatigue testing The full-scale rotor blade fatigue test is required in DS (7) and IEC (2)

only and details regarding the test procedure may be found in (4, 9, 10). According to GL (1,12) and NVN (11) only S/N curves of details, e.g. load introduction in the blade root, joint

connection of the spar are required. The S/N-curves shall have at least 4 load levels and the

samples shall fail between 10.000 cycles and 10 million cycles. In case different stress ratios

occur in the blade the most unfavorable has to be used or S/N curves for the different stress

rations have to be applied.

The blade shall be tested up to a load, which is sufficiently higher than the design load.

According to IEC (4) and Danish Standard (9) the design load shall be multiplied with a test

partial safety factor of 1,328. This load is called the test load. According to both documents

the safety factor includes the partial factor for consequence of failure (nf =1,15) and a blade

to blade variation factor (sf=1,1) and a factor for errors in the fatigue formulation (ef=1,05):

Test load = 1,328 x Design load Equation 4

The testing time has to be shortened for practical reasons, because no company could test 20

years to place a new type of rotor blade on the market. Therefore the load is increased and the

number of load cycles is decreased. The increase of the load depends on the decrease of the

number of load cycles and the slope parameter of the fatigue curve for the details and

materials tested. The slope parameter of the S/N curve m varies for the different materials

used in the blade (carbon and glass fiber reinforced epoxy or polyester) and for the details

(laminate, adhesive joints, bolt root connection) from m=3 to m=20. Because of this a single

representative slope has to be defined for the blade design for full-scale fatigue testing. The

number of load cycles is neither clearly stated in IEC (4) nor in Danish Standard (9) but at

least several million load cycles shall be tested in leadlag and in flapwise direction. 10

million load cycles in both directions are required in Danish Standard (10), which is valid

until 1st

March 2003 (17).

4. TIP-TO-TOWER CLEARANCE

To ensure a minimum clearance between the rotor blade tip and the tower surface a deflection

analysis has to be performed. It is state of the art to perform a dynamic and aero elasticdeformation analysis. For this kind of analysis the maximum blade deflection in the tower

sector (approximately 10) has to be multiplied with the safety factor stated in the table

below. The value of the tip-to-tower clearance has to be greater than the blade deflection

multiplied with the safety factor.

TABLE 10 - Partial safety factor for the blade deflection

Regulation/Standard GL (1, 12) IEC (3) NVN (11) DS (8)

Safety factor 1,428 1,485 1.51)

Sufficient clearance2)

1)

safety factor on the load, not on the deflection2)

no value stated in the regulations


12/14

5. LIGHTNING PROTECTION

Even for rotor blades made from non-conducting materials, a lightening protection is

necessary and required for certification (GL, IEC, NVN). The materials used for lightning

protection shall be able to withstand the electric, thermal and electrodynamic stresses by the

lightning current. The rotor blade shall have at least one receptor at the tip in both shells. Forlager blades (30 m and more) more than one receptor for each shell is required. The receptors

shall be connected with a conductive wire, tape or solid round material with the hub. In case

of carbon fiber reinforced blades the areas with carbon fibers shall be protected with a metal

mesh in addition to the receptor. The minimum cross section for the mesh for each shell is the

same as for the down conductor.

Minimum dimensions for materials used for down conductors are listed in table 11.

TABLE 11 Minimum dimensions of down conductors

Material Required in GL (1),

IEC (2, 5), DS (9)1) , NVN (11)

Required in GL (12);

Recommended in IEC (5)

Copper 16 mm 50 mm

Aluminium (alloy) 25 mm 50 mm

Steel solid tape

Steel solid round

50mm

50mm

60 mm

78 mm1)

According to Danish Standard (9) only recommended

The reason to increase the required minimum cross section for down conductors is that in case

of lightning the temperature of the material raises to 300C and more within one or two

seconds (18). This temperature could damage the fixing points and the core material.

6. CONCLUSION

The GL-Regulations (1) cover all details for the certification of rotor blades for wind turbines

in a comprehensive form. The GL-Regulations contain the highest requirements for ultimate

strength analysis and are on a similar safety level as the NVN standard (11). The Danish

Standard (7) has the lowest requirements for ultimate strength analysis. The IEC standard can

only be compared with the other codes in case that a realistic material safety factor is chosen.

The comparison of the codes for the fatigue loads reveals a higher strength level for the NVN

standard compared to GL-Regulations. As in the ultimate case the requirements in the Danish

Standard are more benign than the other codes.

A full-scale static test of the rotor blade is required in GL (1), IEC (2) and the Danish

Standard (7). A full-scale dynamic test is required in IEC (2) and Danish Standard only.

Because no wind turbine manufacturer has applied for a certification of a wind turbine

according to IEC WT 01 (2) yet, all dynamic blade tests were performed to get a Danish

certificate or the blade manufacturer carry out these tests for their own purposes.


13/14

7. REFERENCES

1. Regulations for the Certification of Wind Energy Conversion Systems,Germanischer Lloyd, Edition 1999

2. IEC WT 01 IEC System for Conformity Testing and Certification of WindTurbines, Rules and Procedures, Edition 2001

3. IEC 61400-1 Wind turbine generator systems, Part 1: Safety requirements, Edition1999

4. IEC TS 61400-23 Wind turbine generator systems Part 23: Full-scale structuraltesting of rotor blades, Edition 2001

5. IEC TR 61400-24 Wind turbine generator systems Part 24: Lightning protection,Edition 2002

6. Richtlinie fr Windkraftanlagen Einwirkungen und Standsicherheitsnachweisefr Turm und Grndung, Deutsches Institut fr Bautechnik, Edition June 1993,

revised 1995.

7. Technical Criteria for the Danisch Approval Scheme for Wind Turbines,Energistyrelsens godkendelsesordnung for vindmller Sekretariatet VEA-118,

Denmark, Edition 2000 with Corrections of 25.11.2002

8. Loads and Safety of Wind Turbine Construction, Danish Standard DS 472,Edition 1992

9. Recommendation for Design Documentation and Test of Wind Turbine BladesDanish Energy Agency, Type Approval Scheme for Wind Turbines, Edition 2002

10. Rekommandation til Opfyldelse af Krav 1 Teknisk Grundlag forTypegodkendelse og Certificering af Vindmller I Danmark, Energistyrelsens

godkendelsesordnung for vindmller Sekretariatet VEA-118, Denmark, Edition

1992

11. Dutch Prestandard NVN 11400-0 Wind Turbines Part 0: Criteria for type certification Technical criteria, Edition 1999

12. Regulations for the Certification of Wind Turbines, Germanischer Lloyd, Edition2003, Final Draft

13. A. Puck, Festigkeitsanalyse von Faser-Matrix-Laminaten, Carl Hanser VerlagMnchen Wien, 1996, ISBN 3-446-18194-6, pp. 59-80

14. S. W. Tsai and M. E. Wu, A General Theory of Strength for AnisotropicMaterials, Journal of Composite Materials,

15. A. Puck, in ref. 13, pp. 55

16. G. Wacker and A. Anders, Abschattungseffekte beeinflussen Restsicherheiten,Erneuerbare Energien 9/2002, pp. 27-30


14/14

17. , Danish Secretariat for Type Approval, dated 03.01.2003

Information from Prof. Wiesinger, Universitt der Bundeswehr, Munich, Germany,FGW-meeting, dated 08.07.1999

NEN 6096/2 Regulations for the Type-Certification of Wind Turbines: TechnicalCriteria, Edition 1994

IEC prEN 61400-1 Wind turbine generator systems Part 1: Safety requirements;Common Modifications, Edition 7/2002

Requerimientos Para La Certificacion de Palas

Documents

Transcript of Requerimientos Para La Certificacion de Palas