Volume 2 wind energy 2 12 – testing standardization, certification in wind energy

Volume 2 wind energy 2 12 – testing, standardization, certification in wind energy Volume 2 wind energy 2 12 – testing, standardization, certification in wind energy Volume 2 wind energy 2 12 – testing, standardization, certification in wind energy Volume 2 wind energy 2 12 – testing, standardization, certification in wind energy Volume 2 wind energy 2 12 – testing, standardization, certification in wind energy Volume 2 wind energy 2 12 – testing, standardization, certification in wind energy

F Van Hulle, XP Wind Consultancy, Leuven, Belgium

© 2012 Elsevier Ltd All rights reserved

2.12.1.1 Brief History of Standardization in Wind Energy

2.12.1.2 Wind Energy Technology-Specific Issues

2.12.1.3 Overview and Status of International Wind Energy Standards

2.12.2 Standards with Design Requirements for Wind Turbines

2.12.2.1 Wind Turbine Design-Related IEC Standards

2.12.2.1.1 IEC 61400-1: Wind turbines – Part 1 Design requirements

2.12.2.1.2 IEC 61400-2: Wind turbines – Part 2 Design requirements for small wind turbines

2.12.2.1.3 IEC 61400-3: Wind turbines – Part 3 Design requirements for offshore wind turbines

2.12.2.1.4 IEC 61400-4: Wind turbines – Part 4 Design and specification of gearboxes

2.12.2.1.5 IEC 61400-24: Wind turbines – Part 24 Lightning protection

2.12.2.1.6 IEC 61400-25: Wind turbines

power plants (six standards) 2.12.2.2 Other Standards Related to Wind Turbine-Specific Design Aspects

2.12.2.2.1 Personnel safety: EN 50308

2.12.2.2.2 Offshore wind turbine support structures: DNV-OS-J101

2.12.2.2.3 Small wind turbine performance and safety standards (United Kingdom and United States)

2.12.3 Testing Methods for Wind Turbines and Wind Plants

2.12.3.6 Electrical Characteristics and Power Quality Measurements

2.12.3.8 Safety and Function Testing

2.12.4 Certification in the Wind Industry

2.12.4.1 General Aspects of Certification in Wind Energy

2.12.4.2 Certification Systems in Wind Energy

2.12.4.2.1 Wind turbine type certification

2.12.4.2.2 Wind energy project certification

2.12.4.2.3 Other certification systems for wind energy

References

IEC Standards (to be purchased via IEC or the National Standardization Institutes)

Relevant Websites

2.12.1 Introduction

At the end of 2010, the global wind industry had produced and installed wind power plants at an annual rate in the order of 36 GW, representing a capex value of approximately 65 billion USD At the same time, the globally operating wind power capacity amounted close to 200 GW This power source comes in the form of wind plants consisting of up to hundreds of wind turbines dispersedly installed over land and sea The wind plants are designed for unattended operation during 20 years in all possible weather situations The increasing penetration demands for increasing reliability and power plant capabilities This chapter describes the approach that has been developed and implemented by the wind power industry together with the research community for ensuring quality and safe and economic operation during the lifetime of the wind plant, notably testing and certification and the related standardization

2.12.1.1 Brief History of Standardization in Wind Energy

Testing and certification in the field of wind power has evolved already from the early days in parallel with the industrial development, and was mainly promoted by specialized research institutes In the 1980s, methods for functional wind turbine

tests (e.g., power performance, acoustic noise) and certification processes using simplified design criteria and rules were set up in various countries (Denmark, The Netherlands, United States, Canada, and Germany) to be used in local spatial approval procedures and national incentive schemes and to support the early industrial development These methods were drawing upon the experience with the testing of large ‘national’ wind turbine prototypes that were built in the 1970s and 1980s in various countries (Denmark, Sweden, Germany, United Kingdom, and Italy)

Wind turbine test stations in Europe and North America established platforms for exchange of testing practices in both formal and informal circuits This resulted, for example, in recommended practices for testing issued under International Energy Agency (IEA) in areas like power performance testing, acoustic measurements, power quality, and anemometry The development of harmonized practices in certification took considerable advantage from international research projects developing common wind turbine design methods and benchmarking wind turbine design tools in Europe and the United States Since the end of the 1980s, the International Electrotechnical Commission (IEC) took up the results of the European and American standardization efforts in wind energy under the Technical Committee TC88, establishing the 61 400 series of wind energy standards, in all fields deemed relevant (design, testing, and certification) This process was assisted by various dedicated research efforts, for example, by the European Wind Turbine Standards project (EWTS), which elaborated aspects not yet covered or insufficiently covered by IEC

As a result, there is today a well-established practice of testing and certification of most aspects relevant to wind turbine projects, which leads to worldwide recognition of the results and products by the relevant market parties Major future challenges arise from the continuous upscaling of the technology and the expected massive expansion of offshore wind energy bringing the need for new measurement and assessment methods

2.12.1.2 Wind Energy Technology-Specific Issues

The characteristics of wind energy technology and the constraints arising from the site-specific external conditions lead to specific issues related to testing and certification Some important ones are highlighted here:

– Heavy external design conditions: Wind turbine standards and certification have to take account of the fact that wind power plants need to operate unattended and remain within their design limits for 20 years during a wide range of design conditions, including extreme events Moreover, these conditions are very site specific

– Short product cycle – fast developing product size: The high rate of introduction of new and larger wind turbine types puts pressure on the speed of certification The modular approach to certification is a practical solution to that

– Wind turbine siting: Because of the closeness of wind turbines to habitation and other risk-sensitive areas, sufficiently rigorous safety approach has to be followed in safety requirements to limit the risk below a set level, but at the same time being rational and workable

– Large wind turbine sizes: The average wind turbine sizes have been increased considerably, and are such that they involve severe constraints on locations of testing and on the size of testing facilities for blades and other major components (e.g., offshore structures) The large sizes also pose challenges for calibration methods in mechanical load measurements

– Wind variability: Most of the wind turbine testing involves measurement of the wind conditions Specific testing methods for accurate, traceable measurements are not straightforward because of the spatial and temporal variability of the wind vector Increasing wind turbine sizes make this issue more complex

– Workers risk: Being large structures involving working at great heights in environments with electrical and mechanical risks poses specific requirements for personnel and labor safety

– New electrical functionalities: With increasing penetration of wind power in power systems, new network requirements and wind power plant functionalities are developed bringing about the need for new corresponding test and verification procedures Moreover, certain aspects such as electrical characteristics at wind farm level cannot be tested physically, bringing the need for combination of physical testing and modeling

Many of the above issues are in principle very challenging; however, several have been solved satisfactorily This chapter intends to demonstrate how these issues have been addressed with the development and implementation of proper standards, resulting in a comprehensive approach in the sector to deal with quality and safety

2.12.1.3 Overview and Status of International Wind Energy Standards

Testing and certification according to a set of dedicated standards is a major element of ensuring safety and adequate functionality of design and operation of wind turbines according to a traceable quality In this respect, the approach in wind energy is not different from the approach in any other power generation technology

This chapter intends to present an overview and status of internationally available wind energy standards This is done in regard

to the relevant stages in the wind turbine life cycle Broadly speaking, a distinction should be made in

Table 1 Availability of international standards (mainly IEC wind energy standards of the series IEC 61400) for different aspects in the various stages in turbine life

Design

Certification (type certification, project certifcation, component certifcation)

Stage in wind

turbine life

Wind turbines Small wind turbines Offshore wind t

Gearboxes Protectiv

Power performance Acoustics Loads and components (blades) Electri

Design and

Installation and

Network

Operation and

maintenance

Decommissioning

– design requirements and the related standards;

– testing methods and the related standards; and

– certification methods and related procedures and standards

The wind energy standards that have been developed within the IEC are covering all these three categories Table 1 presents an overview of the fields covered by the specific wind energy standards developed in IEC and other international wind energy specific standards (e.g., European Standards (EN standards)) The gray-shaded cells indicate cases that are not applicable The mark up in the cell indicates where an international wind energy standard is available In the next sections, for the particular areas mentioned in the table, the applicable standards will be described, and where relevant ongoing work will be mentioned Moreover, other relevant national or regional standards will be indicated, where appropriate

Thus, in general, it can be concluded that the majority of relevant aspects pertinent to the important stages in the lifetime of wind turbines can be covered with international standards This is not surprising as engineering characteristics and their method of measurement or specification in principle are globally applicable In the more detailed discussion in the next sections, it will be indicated where it is more appropriate to also take account of local (national) standards This is specifically applicable in the electrotechnical area where local codes are applied

The mutual recognition of testing and certification practices is very important in the international wind energy market This recognition has been a continuous concern during the development of the international approach Formally, the recognition of test results and certification is ensured by the process of accreditation (according to the International Organization for Standardization (ISO) 17025 for testing and ISO 45011 for certification) and international agreements between accreditation bodies This is a system that functions pretty well in wind energy In addition to this, specific networks have been set up in the industry to establish a high-quality profile in measurements Examples in the area of testing are the Measnet network in Europe and the Regional Test Center Initiative in the United States Examples in the field of certification include the Microgeneration Certification Scheme (MCS) developed by the Department of Energy and Climate Change (DECC) in the United Kingdom and the Small Wind Certification Council (SWCC) initiative in the United States, both in the area of small wind turbines

2.12.2 Standards with Design Requirements for Wind Turbines

Wind power technology has diverse engineering aspects (structural, mechanical, electrical, control systems) and for each of them a range of engineering standards apply However, in this section, only standards that are developed exclusively for wind turbine systems are discussed, mainly IEC standards, complemented by specific other standards

2.12.2.1 Wind Turbine Design-Related IEC Standards

The most comprehensive documents laying down design requirements for wind turbines are the standards for wind turbines developed under the technical committee TC88 (Wind Turbines) The wind energy standards are of the series IEC 61400 The available published standards include (situation end of 2010)

– Part 1: Design requirements (for wind turbines in general)

– Part 2: Design requirements for small wind turbines

– Part 3: Design requirements for offshore wind turbines

– Part 4: Design and specification of gearboxes

– Part 24: Lightning protection

– Part 25: Communication for monitoring and control of wind power plants (six standards)

Other aspects that are presently work in progress under IEC TC88 include

– Part 5: Rotor blades

– Part 26: Time-based availability for wind turbines

– Part 27: Electrical simulation models for wind power generation (this topic could be considered partly design related, partly testing related)

The available standards as listed above are explained briefly in this section

The standard, now in its third edition, specifies essential design requirements to ensure the engineering integrity of wind turbines It

is relevant for all wind turbine subsystems such as structural parts, control and protection mechanisms, electrical systems, and mechanical systems In principle, it applies to wind turbines of all sizes, but states that for small wind turbines IEC 61400-2 may be applied Equally, it is mainly geared toward horizontal axis turbines but in principle does not preclude its applicability to vertical axis machines

Some basic features of the standard are discussed below:

– The structural safety in general is based on probabilistic design methods as described in the ISO standard 2394 General principles

on reliability for structures, including the use of partial safety factors for loads and materials

– For the calculation of the loads, a system of load cases is defined as a combination of design situations (operational states of the wind turbine) and external conditions (e.g., wind conditions) For the various load cases, the standard indicates how to evaluate the different limit states: ultimate, fatigue, accidental, serviceability, also depending on the type of loads considered (aerody­ namic, inertia, other) The design life is assumed to be 20 years

– The standard contains descriptions for the wind and turbulence model It gives a system of wind turbine classes that specify the average wind speed, reference wind speed, and turbulence values for four normal classes and one special class (see Table 2) – For the design of control and protection systems, the principle is that the wind turbine should remain safe with a single failure in the protection system

– Electrical safety refers mainly to existing standards

2.12.2.1.2 IEC 61400-2: Wind turbines – Part 2 Design requirements for small wind turbines

Small wind turbines may require a different approach; therefore, a specific standard was set up that is applicable to wind turbines (of all concepts) with a rotor swept area smaller than 200 m2 The structure of the standard is similar to IEC 61400-1, but it is geared

Table 2 Basic parameters for wind turbine classes

Wind turbine

Vref

A

B

C

(m −1s)

Iref (−)

Iref (−)

Iref (−)

0.16 0.14 0.12 37.5 values specified by the designer

The parameter values apply at hub height Vref average wind speed in the standard classes is 20% of the value of Vref turbulence categories The value of the turbulence (I ) applies at 15 m s−1

toward more simplicity in order to be applicable to small turbines Presently, an update of the standard is being prepared by a maintenance team (MT2) under TC88

2.12.2.1.3 IEC 61400-3: Wind turbines – Part 3 Design requirements for offshore wind turbines

This standard is specifically focused on the design of offshore wind turbines In principle this is for bottom-mounted offshore wind turbines, not for floating constructions, for which a separate standard will be prepared (new work item is proposed) For many aspects, the requirements of IEC 61400-1 apply; however, because of the application offshore, striking differences with respect to IEC 61400-1 include

– The standard prescribes how the marine conditions should be taken into account This relates to wave conditions and the correlation between wind and waves, which is decisive for the loading on the structure A further elaboration of wave conditions

is made in various types of sea states (normal, severe, and extreme), sea currents (subsurface and near-surface currents, currents induced by breaking waves), water level, sea ice, marine growth, seabed movement, and scour The wind conditions are like in Part 1, but the inclination angle has to be assumed zero

– The standard gives guidance for the assessment of external conditions at an offshore site

– The types of loads to be analyzed include gravitational/inertial loads, aerodynamic loads, actuation loads (like, e.g., yawing), hydrodynamic loads, and sea ice loads

– The system of load cases is much more complex than for the onshore wind turbines, because of all possible sea states and ice states The standard indicates the relevant wind models that have to be taken into account, depending on the wind turbine design situation (power production, occurrence of fault, etc.) and the corresponding wave, directionality, sea current, water level, type of analysis, and partial safety factors

– In the analysis, a basic division is made between rotor–nacelle assembly (RNA) and support structure

– For foundation design, the standard makes reference to ISO 19900/01/02/03 series

2.12.2.1.4 IEC 61400-4: Wind turbines – Part 4 Design and specification of gearboxes

In fact, the standard for gearboxes is published as an ISO document being the result of a common IEC/ISO project The document was drafted by a combined committee of the American Wind Energy Association (AWEA) and American Gear Manufacturers Association (AGMA), with members representing international wind turbine manufacturers, operators, researchers, consultants, and gear, bearing, and lubricant manufacturers Based on data from field experience, the standard actually describes the differences

in operation and loading of gearboxes between use in wind turbines and in other gear applications The standard is instrumental for developing clear specifications of the information needs between wind turbine and gearbox manufacturers for wind turbine applications and discusses issues specific to wind turbine applications and gear design

2.12.2.1.5 IEC 61400-24: Wind turbines – Part 24 Lightning protection

This standard was developed to inform designers, purchasers, operators, certification agencies, and installers of wind turbines on the state-of-the-art of lightning protection of wind turbines Wind turbines pose a lightning protection problem different from other devices due to their physical size and nature, and due to extensive use of insulating composite materials, such as glassfibre reinforced plastic The standard prescribes how the lightning protection system should be fully integrated into the different parts of the wind turbines to ensure that all parts likely to be lightning attachment points are able to withstand the impact of the lightning and that the lightning current may be conducted safely from the attachment points to the ground without unacceptable damage or disturbances

to the systems

2.12.2.1.6 IEC 61400-25: Wind turbines – Part 25 Communication for monitoring and control of wind

power plants (six standards)

This standard number represents a series of six standards relevant for the operational stage, focusing on the communications between wind power plant components and actors, such as wind turbines, a supervisory control and data acquisition (SCADA) system, or a condition monitoring system (CMS) It defines wind power plant information models and an information exchange model, and deals with the mapping of these two models to a standard communication profile The standard enables connectivity between a heterogeneous combination of client and servers from different manufacturers and suppliers The structure of the above-described approach is reflected in the division of the different parts of IEC 61400-25:

– Part 25-1: Overall description of principles and models

– Part 25-2: Information models

– Part 25-3: Information exchange models

– Part 25-4: Mapping to communication profile

– Part 25-5: Conformance testing

– Part 25-6: Logical node classes and data classes for condition monitoring

2.12.2.2 Other Standards Related to Wind Turbine-Specific Design Aspects

Other standards of international relevance specifying design requirements for wind turbines exist The most relevant to mention here are related to the following aspects

2.12.2.2.1 Personnel safety: EN 50308

This European Standard EN 50308 specifies requirements for protective measures relating to the health and safety of personnel, relevant in the stages of commissioning, operation, and maintenance of wind turbines The standard specifies requirements regarding

– hardware provisions being a part of the turbine such as platforms, ladders, and lighting; and

– manuals and warning signs to accommodate safe and quick operation, inspection, and maintenance

The requirements and/or measures specified account for many types of hazards: hazards of mechanical, thermal, or electrical origin, noise hazards, or hazards caused by neglecting ergonomic principles in machine design The document in principle is related to onshore wind turbines For offshore applications, the document only draws attention to additional provisions and procedures that may be necessary for offshore turbines Also, the document does not include requirements for provisions and procedures for lifts and suspended access equipment (SAE) in wind turbine towers

2.12.2.2.2 Offshore wind turbine support structures: DNV-OS-J101

This standard issued by Det Norske Veritas (DNV; guideline on the design of offshore wind turbine structures) is applicable for the design of offshore wind turbine structures and also for meteorological masts As such it is a complete stand-alone specification and provides principles, technical requirements, and guidance for design, construction, and in-service inspection of offshore wind turbine structures However, it is not applicable for support structures and foundations for transformer stations for wind farms (there the DNV-OS-C101 applies)

2.12.2.2.3 Small wind turbine performance and safety standards (United Kingdom and United States)

For completeness sake, two national standards are mentioned here, drafted specifically for small wind turbines, namely one for the United Kingdom issued by the British Wind Energy Association (BWEA) (nowadays by Renewable Energy UK) and one for the United States However, unlike the document titles suggest, these standards are not specifying design requirements (for which they make a reference to the IEC standard) They mainly deal with specification of performance, acoustic, and duration tests, and are rather a guideline in certification of small wind turbines Therefore, it is not further discussed in this section; some of the content is discussed in Section 2.12.4

2.12.3 Testing Methods for Wind Turbines and Wind Plants

2.12.3.1 Introduction

This section deals with current state-of-the-art methods and related standards for testing of wind turbines and wind plants (wind farms) In principle, any new wind turbine type will at least undergo all the tests performed at a representative prototype or specimen as described here It is clear that for new wind turbine types, primary functional aspects such as functioning of control and safety systems, power performance, and mechanical loads are essential elements that without doubt need an experimental verification, from the point of view of both the wind turbine manufacturer and the future wind owner As such, the described tests are applied for new wind turbine types, for example, in the so-called prototype testing and type characteristic measurements, but also apply for existing wind turbines and wind energy projects, for example, related to regular check for approval or performance verification The methods are applicable both for development testing by manufacturers or for third-party testing, for example, in the frame of type or project certification In principle, the described methods apply for wind turbines irrespective of the size Small wind turbines however may require a different approach Where relevant, this will be discussed below in the description of the individual test

Table 3 gives an overview of the main fields covered by testing and the related standards’ main standardized tests

Although the described tests are covering fairly well the potential scope of aspects of wind turbines that could be tested, other tests may be of relevance, for example, specific tests of individual components and construction details

Per specific test, the following sections will explain

– what exactly is tested and the purpose of the test;

– existing test procedure and/or standard; and

– brief description of testing methodology

Wind speed measurement is an element that is common to most of the below described tests As indicated earlier, the method of measurement of the wind speed is far from obvious Therefore, it was considered useful before discussing the individual testing

Table 3 Overview of state-of-the-art wind turbine tests, applicable standards, and link with wind turbine certification (according to IEC 61400-22)

Testing method Applicable IEC standard Link with certification Safety and function

Power performance Mechanical loads Rotor blade tests Acoustic noise emission Power quality

IEC 61400-22 (Annex) IEC 61400-12-1 IEC 61400-13 IEC 61400-24 IEC 61400-11 IEC 61400-21

Type testing, mandatory Type testing, mandatory Type testing, mandatory Type testing, mandatory Type characteristic measurement, optional Type characteristic measurement, optional

methods to devote a specific section to the discussion of some critical aspects of wind turbine measurements and the related technique (anemometry)

2.12.3.2 Wind Speed Measurement

Most of the testing in the area of wind energy makes use of measurement techniques (sensors, data acquisition) according to normal engineering practice also used in other disciplines An important exception to this is the measurement of the wind speed Important physical reasons related to turbine size bring about significant challenges, which increase with wind turbine size Classical sensors such as anemometers measure the wind vector in a point whereas the parameters to be characterized (power, loading) are related to

a large surface (rotor swept area) and atmospheric wind has very low spatial coherence The consequences on accuracy are exacerbated by the strong dependence of the parameters on wind (power depends on the cube of the wind speed) Also, with increasing wind turbine sizes, the positions for measuring wind speed become higher and higher, with corresponding increased costs for the measurement mast Remote sensing techniques (like, e.g., light detection and ranging (LIDAR)) bring significant advantages in this respect, as they avoid the need for construction of high meteorological masts

Despite the limitations, the current accepted practice is to measure the wind speed with a cup anemometer, as the best available technical compromise Alternative wind measurement techniques are being used in various applications, each with its specific pros and cons (sonic anemometer, propeller anemometer, sound detection and ranging (SODAR), LIDAR) However, there is not yet an internationally accepted agreement for the use of these types of anemometers in the industrial commercial standardized testing of wind turbines Hence, current standards recommend the use of the cup anemometer for measuring the wind speed, complemented

by a separate wind vane for measuring the wind direction A lot of research has been done during the past 30 years to understand the problems of the cup anemometer and to develop optimal solutions to go around its limitations [1] In this respect, the research work done in the frame of the European AccuWind project may be mentioned, which developed a methodology for classifying cup anemometers according to the sensitivity of their accuracy to various influences such as inflow angle, temperature, and air density [2] The method provides guidance to the user on the choice of the right instrument for a specific application (e.g., in complex terrain) Hence it could be concluded that the physical limitations of the instrument are largely solved by

– objective methods and minimum requirements for calibration;

– requirements on the mounting of the cup anemometer; and

– classification of cup anemometers

2.12.3.3 Power Performance Testing

In power performance testing, the relationship between the electrical power output of the wind turbine and the wind speed is established, also called the power curve of the wind turbine

The power curve is a unique and essential performance characteristic of the wind turbine as generation unit For the wind turbine manufacturer, it is a basic performance measure of the product and therefore is used in the process of turbine development, for example, for optimizing control settings For the developers and owners, a measured power curve enables an unambiguous statement (by calculation) of the expected annual energy output of the wind turbine at a given site and wind regime, plus a quantitative value of the uncertainty in power output Therefore, it is a critical element in any energy output assessment of wind energy projects, and is a basis for commercial agreements in wind energy development As a consequence, an accurate uncertainty assessment in the measurement is of high importance Furthermore, power performance is a mandatory test in the type certification according to IEC 61400-22 and most other certification schemes (see Section 2.12.4) It is also used for product verification in order

to allow statement of conformity of actual product with the documented wind turbine type

The test procedure for power performance testing is described in the IEC 61400-12-1 Edition 1.0 An update of the document (Edition 2.0) is under preparation Additional requirements for the measurement of the power curve are described by Measnet, the network of qualified measurement institutes Additional specifications for small wind turbines are given in the earlier-mentioned AWEA and BWEA standards The method in these documents is largely based on IEC; however, it makes adaptations to cater for the

non-grid-connected aspects, exclude the effects of potentially fluctuating battery voltage, and enable a fast determination of the power curve (1 min averages)

Conceptually, the power performance test procedure is relatively simple At the wind turbine electrical system, current and voltage are measured as well as a number of additional signals (status, blade pitch angle, rotational speed, control settings where relevant) The wind speed and wind direction are measured on a separate meteorological mast at hub height and some other heights

in order to establish the wind shear Furthermore, the atmospheric pressure and temperature are measured to enable data normal­ ization to standard air density

The standard procedure prescribes the scanning frequency and averaging time and statistical values that shall be kept by the data acquisition system The data flow is processed with the so-called method of bins The standard also states the minimum required number of values of power and wind speed in all the wind speed bins over the whole relevant measurement range (between cut-in and cut-out wind speed) in order to ensure that there are representative values of power over the whole range of relevant wind speeds This often is an issue for the high wind speeds (above 20 m s−1) that rarely occur during test campaigns Therefore, a special procedure is described in the standard on how to extrapolate measured curves to high wind speeds

The measurement involves some special issues:

– It has to be ensured that the measured wind speed is uniquely defined It is recognized that the method is a compromise because the wind is only measured at a point, whereas the effective output of the wind turbine is determined by the wind field seen by the entire rotor disk The anemometer has to be located close to the wind turbine but not so close that the measurement is affected Therefore, the measurement procedure includes detailed rules for the distance between the mast and the wind turbine Furthermore, there are strict rules on angular measurement sectors around the mast from which data have to be rejected when wind is blowing from these directions, because of potential disturbing effects of the wind turbine on the wind measurement – Measuring the wind at the wind turbine nacelle would be practical, but is problematic because of the disturbing wake effects A new IEC standard is in preparation to prescribe how to carry out power performance testing with nacelle anemometers – Disturbing influences of the terrain should be avoided, and the standard prescribes the requirements for the test site If the terrain deviates from these (e.g., in complex terrain), a so-called site calibration shall be carried out to establish correction factors – Turbulence influences the power output; therefore, rules for data filtering are prescribed

– A number of applications would welcome wind farm power curves (e.g., project verification, short-term forecasting) To cater for that need, the IEC TC88 intends to develop a specific standard for wind farm power curve measurements

The output of the test is typically a graph of power output versus wind speed and a corresponding table with prescribed format The values to be reported also include the uncertainty on the measured power per wind speed bin

2.12.3.4 Mechanical Load Measurements

In a mechanical load campaign, the principal loads (forces and moments) on a wind turbine are measured, which essentially consist

of blade root loads, rotor loads, and tower loads The selection of the locations for measuring these forces and moments ensures that the design mechanical stresses can be determined in any part and location of the wind turbine

The measured loads are used for different purposes The actual measurement of loads on a wind turbine (proto)type is of high value, because it gives a real-world check and benchmarking for the output of analytical design methods (aeroelastic models and codes) that have to deal with a large number of parameters often with high uncertainties These uncertainties increase with the scale of the machine Thus, measurements enable verification/validation of the outputs of the design software tools that calculate the ultimate and fatigue loading and stresses and the corresponding strength and stress reserve margins in the structural components of the wind turbine In principle, loads measured according to the standard can also be used as a reference for direct determination of design loads

in specific conditions Therefore, mechanical load measurements are mandatory type tests in the process of wind turbine certification Furthermore, dedicated load measurement campaigns are helpful in the process of prototype development testing

The procedure for measuring the mechanical loads of wind turbines is described in the standard IEC 61400-13, which in its present version dating from 2001 is a technical specification A new version which will be a full standard is presently under development The document is aimed at the test engineer who will design and implement the test program to meet the specific design or certification needs The specification provides specific guidance on load measurements on key structural components and load paths

A mechanical load measurement campaign is quite complex and involves accurate measurement of a large number of signals The loads – basically forces and moments – are measured with the help of strain gauges and accelerometers applied on selected locations The standard sets a minimum set of fundamental load quantities to be measured:

– On at least one blade, the flap and lead-lag bending moments are measured in the blade root

– The rotor loads include the tilt moment, the yaw moment, and the rotor torque (moments around three perpendicular axes) – The tower loads include tower bottom bending in two perpendicular directions (fore-and-aft and lateral)

In principle, from such a set of measured loads, the loading in any part of the wind turbine structure can be derived by calculation, with the help of the exact knowledge of the load path geometry from the design, in other words, the exact dimensions within the

wind turbine structure For meteorological quantities, the standard prescribes as a minimum the measurement of wind speed and wind direction at hub height, as well as air temperature and air density Furthermore, a number of wind turbine operation quantities shall be measured including electrical power, rotor speed, pitch angle, yaw position, and rotor azimuth Other status signals relevant for the operation are recommended

Before and during the measurement campaign, all individual load measurement chains – sensor, amplifier, cable, data acquisition system – have to be calibrated, to establish a unique and traceable relationship between the measured signal and the actual physical force or moment This is not always possible or practical by applying calibrated external forces – for example, by pulling at the blade or tower with a known force – because of the large size of the components of modern wind turbines Therefore, blade root loads are mostly calibrated by measuring the effect of the blade’s own weight during slow rotation of the rotor, which is then showing a sinusoidal variation of the signal from the blade root strain gauge Tower loads, on the other hand, are calibrated with the help of shunt calibration, which is measuring the effect of the eccentric overhanging mass of the nacelle on the tower bottom strain gauges during a full rotation of the nacelle with the yawing system

The load measurement campaign is structured in such a way that it reflects the design load cases of the wind turbine design standard (IEC 61400-1) Therefore, measurement load cases are defined corresponding to relevant design load cases, namely, the combinations of wind turbine operational state and corresponding external (wind) conditions and both in steady-state operation and during transients (braking, yawing, start-up, and shutdown)

The so-called capture matrix is used to organize the measured time series The capture matrix has two objectives: it can be used as a guideline for programming the data acquisition system for automatic and unattended operation of the measurement system and it can

be used as a tool to decide when the measurement requirements are fulfilled, in other words when there are sufficient load data at different wind speed and turbulence values in order to reach The standard prescribes minimum amounts of data to be collected per wind speed and turbulence bin

Test reports of mechanical load campaigns are very extensive because of the complexity of the system and the large amount of data The standard gives guidance for such reports In general, the practice is to divide the report into two parts The first part describes the measurement setup, including site, instrumentation, and the details about the calibration of all measurement chains The second part describes the data and the results of their processing and analysis, including typical time histories, load statistics, frequency spectra, fatigue load spectra, and equivalent loads An uncertainty analysis has to be included as well

2.12.3.5 Acoustic Noise Measurements

In a sound measurement campaign for wind turbines, the apparent A-weighted sound power levels, spectra, and tonality at integer wind speeds from 6 to 10 m s−1 of an individual wind turbine are determined, possibly including the directivity The wind speed range is chosen such that it is considered representative of wind conditions under which potential nuisance can be caused by the wind turbine

The measured acoustical emissions by wind turbines are important in various stages of project development and project implementation and for different stakeholders, wind turbine manufacturers, project developers, wind farm operators, planners, and regulators The measurement of acoustic noise emission is a type characteristic measurement in wind turbine certification systems (see Section 2.12.4), thus is optional in the certification process

The test procedure for acoustic noise emission measurements is laid down in the IEC 61400-11 Edition 2.1 Besides, there is a standard IEC 61400-14 prescribing the method for making a declaration of acoustic noise emission for particular wind turbine type based on a series of measurements at the similar type at various locations Specific aspects of acoustic measurements are specified in the BWEA standard for small wind turbines

A measurement setup in general consists of acoustic measurements, wind measurements, and wind turbine power measure­ ments Performing the measurements at positions within a prescribed reference distance to the wind turbine minimizes the influence of terrain effects, atmospheric conditions, or wind-induced noise In order to reduce the wind noise generated at the microphone and avoid the influence of different ground types, the microphone has to be positioned on a board placed on the ground There are a number of options for determining the wind speed corresponding to the acoustic measurements The preferred method is to derive the wind speed from the measured power output of the wind turbine, using its (known) power curve The limitation of this method of course is the fact that it does not work at wind speeds above the rated speed (because in that range the power does not change any more) The standard gives rules on how to deal with power rating situations that could interfere with this wind speed range The standard also gives recommendations on how to determine the relevant wind speed values with the help

of a meteorological mast or by using the nacelle anemometer as a reference

The measurement campaign consists of taking simultaneous measurements of sound pressure levels and wind speeds over short periods of time and over a wide range of wind speeds The measured wind speeds are converted to corresponding wind speeds at a reference height of 10 m and a reference roughness length of 0.05 m The sound levels at standardized wind speeds of 6, 7, 8, 9, and

10 m s−1 are determined and used for calculating the apparent A-weighted sound power levels The directivity is determined by comparing the A-weighted sound pressure levels at three additional positions around the turbine with those measured at the reference position For each of the representative measurement cases, the background noise is measured with stopped wind turbine The presence of audible tones in the noise at different wind speeds has to be determined on the basis of a narrowband analysis, and the standard gives procedures on how to determine tonality taking account of background noise, masking noise levels, and audibility of the tones

Information to be provided in a measurement report includes characterization of the wind turbine and the site, description of the instrumentation, and details about the acoustic and nonacoustic data and an uncertainty analysis

2.12.3.6 Electrical Characteristics and Power Quality Measurements

Electrical tests measure essential characteristics of the power output of wind turbines such as power variations, voltage variations, harmonic contents (read definition), and fault-ride-through capabilities

Electrical power quality measurements are mainly intended to determine the impact and mutual influences of the connection

of the wind turbine on electrical characteristics of the local network, in other words to characterize the grid-friendliness of a wind turbine or wind plant The measurement of the power quality is considered as optional type characteristic measurement in the frame of wind turbine type and project certification according to IEC 61400-22 (see Section 2.12.4) The output of the tests is used in the process of grid connection, for example, to obtain the formal authorization to connect and feed-in power into the network The test results (together with other relevant information) enable the grid operator to assess whether the generation unit/facility complies with the requirements of the local network connection code (grid code) With increasing wind penetration

in networks these requirements are becoming extensive and increasingly complex In some cases, it will be necessary to complement tests at wind turbine level with simulations in order to enable one to produce the relevant answers at wind farm level for aspects that cannot be tested at wind farm level, such as fault-ride-through capability

The international standard for the measurement of power quality is the IEC 61400-21 (Edition 2) Measurements according to this standard address most of the relevant power quality parameters, both in normal operation and during network faults The standard defines and specifies these parameters, and provides measurement procedures for measuring and quantifying the characteristics as well as for assessing compliance with power quality requirements, including estimation of the power quality expected from the wind turbine type when deployed at a specific site, whether or not in wind farms In addition, network operators may prescribe test methods, which in general can be very different depending on country, power system, and so on For example, network operators in Spain and Germany established specific procedures for assessing wind power plant capabilities in the frame of a system of additional incentives (bonus) for wind plants equipped with functionalities enabling grid services (reactive power, fault-ride-through, etc.) It goes beyond the scope of this chapter to discuss such local requirements Finally, it can be mentioned that a standard is under development under IEC for dynamic models of wind turbine to be used in grid integration processes and integration studies The provisional title is IEC 61400-27 Edition 1.0 Electrical simulation models for wind power generation

The measurements aim in general to verify the characteristic power quality parameters for a substantial part of the operational range of the assessed wind turbine, at least up to a wind speed of 15 m s−1 This enables a practical approach and is expected to enable sufficient scope of verification of the characteristic power quality parameters of the assessed wind turbine The power quality characteristics to be measured include

– voltage fluctuations during continuous operation and during switching operations;

– current harmonics, interharmonics, and higher frequency components;

– response to voltage dips;

– characteristics of active power (maximum measured power, ramp rate limitation, set-point control);

– reactive power (reactive power capability, set-point control);

– grid protection (protection levels and disconnection times for over- and undervoltage and frequency in the grid); and

– reconnection time

The standard contains a detailed format for reporting the characteristic measured values

2.12.3.7 Rotor Blade Testing

In full-scale rotor blade testing, an entire rotor blade is mounted on a test rig – most often in a specialized laboratory – and is subjected to a series of static and fatigue loads representative of the design loads The test objective is to experimentally verify that a specific blade type possesses the strength (fatigue and ultimate) and service life as foreseen in the design

The blades of a wind turbine rotor are among the most critical components of the wind turbine Blade tests are therefore considered as mandatory tests in wind turbine type certification (see Section 2.12.4) The need for specific tests will depend on the level of uncertainty in the design assessment due to the use of new materials, new design concepts, new production processes, and so

on, and the possible impact on the structural integrity Furthermore, rotor blade testing is helpful in the process of prototype development, and therefore rotor blade manufacturers usually have their own testing facilities

The procedure for testing rotor blades is documented in the IEC/TS 61400-23 Wind turbine generator systems – Part 23: Full-scale structural testing of rotor blades The document recognizes the wide range of methods (dictated by the test system hardware) that were developed over time at various testing laboratories Therefore, it did not intend to be a restrictive standard that favors one method to the exclusion of all others The primary emphasis is to identify and describe commonly accepted practices among the various laboratories and to give guidance in establishing blade test criteria Therefore, rather than being a full-scope