lightning protection tgel
TRANSCRIPT
29th
International Conference on
Lightning Protection
23rd
– 26th
June 2008 – Uppsala, Sweden
10-16-1
THE UPDATE OF IEC 61400-24 LIGHTNING PROTECTION OF
WIND TURBINES
Troels S. Sorensen DONG Energy
[email protected] DONG Energy, A.C. Meyersvaenge 9, DK-2450 Copenhagen SV, Denmark.
J.A. Plumer Lightning Technologies Inc.
Joan Montanyà Tech. Uni. of Catalonia [email protected]
Thomas Holm Krogh Siemens Wind Power
Blas Hermoso Uni. Public, Navarra
Josef Birkl Dehn + Söhne
Tobias Gehlhaar Germanischer Lloyd
Brian McNiff McNiff Light Industies [email protected]
Kim Bertelsen Electricon
Vidyadhar Peesapati, Uni. of Manchester
Dan Brown Culham Lightning
Lars Bo Hansen LM Glasfiber
Wolfgang Zischank, Uni. Fed. Armed Forces Munich [email protected]
Hans V. Erichsen Vestas Wind Systems A/S
Ruben Rodriguez Sola Gamesa
Yarú Méndez Hernández GE Global Research
Ian Cotton Uni. of Manchester
Shigeru Yokoyama Kyushu University / CRIEP
Yoh Yasuda, Kansai University
Joachim Holbøll Technical Uni. of Denmark
Soren Find Madsen
Highvoltage.dk [email protected]
Zafiris Politis Raycap Corporation [email protected]
Abstract – The first edition of the IEC 61400 Wind Generator Systems – Part 24 Lightning Protection [1] was published as
a technical report (TR) in July 2002, and as such its scope was to present lightning and lightning protection to a relatively
young industry. It presented background statistical information on lightning damage to wind turbines and it gave guidance to
lightning protection best practices. Since then the wind power industry has developed rapidly towards even larger wind
turbines and into a booming and more mature industry in need of an industry standard for lightning protection. This is the
background for the update of the IEC 61400-24 under preparation, which transforms the TR into a full standard based on the
general lightning protection standards of the recent IEC 62305 series [2], on the general standards for EMC of the IEC 61000
series [3], the specific standards for electrical systems on machinery and the general standards for electrical systems, and with
regard to the blades, on both the latest research and on the air craft industry standards issued by SAE / EUROCAE [4-5].
© ICLP2008 Local Organisation Committee
10-16-2
1 INTRODUCTION
Wind turbines are the fastest growing source of electrical energy with annual growth rates of about 30 per cent in
recent years and totalling 94 GW generator capacity installed world-wide by the end of 2007. More than 20 GW of wind
power was installed in year 2007, which was also the year when the USA became the biggest market for wind power
with 5,2 GW of new wind power generation capacity (26% of the world market), followed by Spain 3,5 GW; China 3,4
GW; India and Germany both with 1,7 GW. Germany is still the world leading wind power nation with 22,2 GW of
installed wind power generation capacity [6].
In terms of lightning protection the numbers stated above would translate into tens of thousands of tall structures,
each of an average height of more than one hundred metres, placed at windy locations and therefore very exposed to
lightning; structures that also contain relatively complex electrical and control systems and that have rotating composite
blades up to 60 metres long. Given the frequency of lightning occurrences in the regions of the world where this new
expansion is taking place, all these new wind turbines will be hit several times by lightning during the 20 years in service
life. This makes lightning protection an important challenge, and it is obvious that given the numbers of wind turbines
now being installed it cannot be done on an individual wind turbine basis, but has to be met by the wind turbine industry
by implementing standardized lightning protection in their series-produced machines.
The IEC TC88 Project Team 24 preparing this new revision of the IEC 61400-24 has much stronger industry
participation compared with the working group who prepared the first edition. This is a clear indication of the
importance attributed to lightning protection by the wind turbine industry itself, which is also reflected by the fact that
today wind turbine manufactures employ their own lightning protection specialists, whereas previously lightning
protection of wind turbines was handled on behalf of the manufacturers by external consultants. One point to make is
that whilst the IEC 61400-24 is limited to horizontal axis wind turbines it is recommended that manufacturers of vertical
axis wind turbines to observe similar good practices as contained in the standard.
Fortunately, both research and experience accumulated over the last few decades have shown that wind turbines can
be effectively protected against lightning by applying the well-known and proven lightning protection techniques which
are described in the lightning protection standards and in the lightning protection literature. This is the case for the
electrical and control systems, and also for most of the wind turbine structure. The exceptions are the blades for which
new protection systems have had to be developed, and the large bearings of the mechanical drive train; these pose
special problems as they are in the direct down conduction path for the current when lightning attaches to the blades. As
indicated above , lightning protection of blades fabricated from composite materials represent a special challenge which
has been addressed in different ways by blade manufacturers, firstly on a trial and error basis, and over the last decade or
so using more dedicated research and development programs including field tests, laboratory tests and analytical work
[e.g. 7-9].
The update of the IEC 61400-24 focuses on how to apply existing standards for lightning protection, EMC, electrical
systems etc. to wind turbines in order to achieve effective lightning protection of electrical and control systems and the
general wind turbine structure. The update emphasizes testing as key to proving the validity of the lightning protection
system design. An effort has been made to describe a range of high voltage and high current tests for testing of blades,
originally developed and used successfully for qualification air craft structures [4], which have in recent years been
successfully adapted to testing of wind turbine blades and discrete components in the lightning down conduction system
of the wind turbine [e.g. 7-9].
In this paper the IEC TC88 project team 24 will take the reader on a brief tour of the new update of the IEC 61400-
24 while briefly familiarizing the reader with relevant background information. The reader should not use this as a
substitute for referring to the full standard when designing lightning protection for wind turbines.
2 A BRIEF INTRODUCTION TO FUNDAMENTAL WIND TURBINE ANATOMY
As the reader may not be familiar with the lingua used for wind turbines, a visual guide is included in Fig. 1 and Fig.
2, which may be helpful to the non-specialist. There are many variations between wind turbine designs for example
turbines with three blades, two blades and even only one, wind turbines without gearboxes, and different arrangements
of the nacelle. However these figures will serve nicely as a general introduction.
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Fig. 1 - Three bladed horizontal axis wind turbine with tubular tower
Fig. 2 - Generalised wind turbine nacelle design (Example of a wind turbine with gear box)
3 Structure of the new IEC 61400-24 Lightning Protection of Wind Turbines
As with most other international standards, IEC 61400-24 is organized with a main normative part, which defines the
specific issues for wind turbines and references other standards to be considered when designing lightning protection for
wind turbines, and with details of more instructive nature placed in informative Annexes. An outline of the structure and
contents is given in Table 1.
Table 1: Outline of the contents of the new IEC 61400-24
Subjects in the main normative part are:
• Definition of the lightning environment for wind turbines
• Procedure for lightning exposure assessment
• Requirements for lightning protection of subcomponents o - Blades o - Nacelle and other structural components o - Mechanical drive train and yaw system o - Electrical low voltage systems and electronic systems and
installations o - Electrical High Voltage (HV) power systems
• Requirements for earthing of wind turbines and wind farms
• Requirements with regards to personnel safety
• Requirements for documentation of lightning protection system
• Requirements for inspection of lightning protection system
•
Subjects placed in the informative annexes are:
• Description of the lightning phenomenon in relation to wind turbines
• Guide to lightning exposure assessment
• Description of protection methods for blades
• Test specifications for blades and components
• Guide to application of Lightning Protection Zones (LPZ) concept to a wind turbine
• Guide to selection and installation of a coordinated SPD protection in a wind turbine
• Additional information on bonding and shielding and installation technique
• Guide to earth termination systems
• Guide to defining measurement points for field tests of lightning protection
• A typical lightning damage questionnaire
• Guide to lightning monitoring systems
• Guidelines for small wind turbines - Microgeneration
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4 Review of main subjects treated in the standard
A. Definition of the lightning environment for wind turbines
The standard uses the lightning current parameters defined in IEC 62305-1 for wind turbine lightning protection system design and for lightning protection component dimensioning, selection and testing. The lightning current parameter values defined in IEC 62305-1 are generally considered adequate for lightning protection of wind turbines.
For wind turbines placed in certain geographical area where exposed to high numbers of upward lightning, and particularly where winter lightning occur, it may be relevant to increase the required durability of wear parts to more than Lightning Protection Level I with regards to the parameter for total transferred charge i.e. Qflash = 300 C. The wear parts include air terminals (e.g. receptors on the blades, provided that this lightning protection concept is chosen), air terminals or air termination systems, sliding contacts, spark gaps and Surge Protection Devices (SPDs). The parameter of total flash charge transfer is decisive in determining the wear (melting) of materials and therefore influences the need for maintenance. Alternatively shorter maintenance intervals for the lightning protection system components subject to wear may be necessary for wind turbines in such areas or perhaps the robustness of these components should be re-engineered in order to withstand the complete wear and tear caused by lightning strikes and environmental conditions over the whole lifetime of the wind turbine.
B. Lightning exposure assessment
The standard follows the procedures for lightning exposure and risk assessment defined in IEC 62305-2, while
adapting it to wind turbine application.
It is advisable always to obtain information about local lightning occurrence from authorities such as national
weather bureaus, and to consult other operators of wind turbines in the area or operators of other installations such as
local power companies to obtain information about local conditions, and if relevant particularly about occurrence of
upward lightning and winter lightning.
It is recommended that for calculation of collection area all wind turbines are modelled as tall masts with height
equal to the hub height plus one rotor radius. This applies to wind turbines with any types of blades including blades
made solely from non-conductive material such as glass fibre reinforced plastic (GFRP). It is recommended to include
local terrain variations to get an effective height for the wind turbine as illustrated in Fig. 3.
Fig. 3 - Effective height, H, of wind turbine exposed on a hill
It is also recommended to consider the calculation of the collection areas of connected structures as illustrated in Fig.
4 for a wind turbine of height Ha and another structure of height Hb connected by underground cable of length Lc. (i.e.
according to IEC 62305-2 Annex A). Lightning flashes inside the narrow area AI along the cable route may penetrate to
and affect the cable directly, while lightning flashes inside the wider area Ai may induce transients and may cause pin-
hole punctures of the cable insulation. This approach can be extended to wind farms with many wind turbines, in which
case overlapping collection areas of neighbouring wind turbines should be divided between the wind turbines along the
line defined by the intersection of the 1:3 gradients.
H
1:3
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Fig. 3 – Collections area of an installation with a wind turbine connected by a service line cable to another structure
(ρ is the resistivity of the soil).
C. Lightning protection of wind turbine sub components
The standard recommends application of the lightning protection procedures defined in IEC 62305 to wind turbines, and recommends that all subcomponents should be protected according to LPL-I unless it is shown and demonstrated by a risk analysis that a lower level is adequate.
Application of the rolling sphere method to a wind turbine is shown in Fig. 4 by which can be identified the parts of
the structure exposed to direct lightning flash attachment i.e. LPZ 0A according to the Lightning Protection Zoning concept, as well as the areas protected by the structure itself LPZ 0B (e.g. the area on the ground close to the tower). Furthermore in Fig. 5 is shown an example of application of the Lightning Protection Zoning Concept defining internal LPZ 1 and LPZ 2 i.e. areas of the wind turbine with higher protection level and lower lightning parameter levels.
2
2
2
1
1
Fig. 4 –Application of rolling sphere to wind turbine Fig. 5 –Example of application of the Lightning Protection
Zoning concept to wind turbine
1:3 gradient
3 × Ha
area
Wind turbine position
1:3 gradient
Other structure
Cable connection of length Lc
25 √ρ
3 × Hb
√ρ
Ai is the collection area of lightning flashes near the service line
Ai = 25 Lc √ρ
AI is the collection area of lightning flashes to the service line
AI = (Lc – 3(Ha + Hb )) √ρ
Lc
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C.1 Blades
The blades represent one of the two special lightning protection challenges unique to wind turbines. The blades are complex in terms of their geometry and construction and up to more than 60 m long, made from fibre reinforced composite materials, placed on up to more than 100 m high towers and rotating in a vertical plane (horizontal axis wind turbines) – while exposed to direct lightning attachment. Wind turbine blades are the most exposed parts of the turbine, as is clear when applying the rolling sphere method, which identifies most of the blade surfaces as Zone 0A (c.f. Fig. 4), and experience the full electromagnetic and mechanical (pressure wave) impact and energy content from the lightning current, the electric field and the magnetic field associated with lightning strikes. The blades therefore have to be protected accordingly.
The criteria for adequacy of protection for blades are to show that the design and positioning of the lightning air termination system on the blade ensure efficient lightning interception, and that the down conductor system can sustain the effects of lightning current corresponding to the lightning protection level I (unless show by risk analysis that LPL-II or LPL-III is sufficient as shown in table 2).
Protection
Level
Peak Current
[kA]
Specific Energy
Content
[kJ/Ohm]
Average Rate of Current Rise
[kA/µs]
Total Charge Transfer
[C]
I 200 10000 200 300
II 150 5600 150 225
III/IV 100 2500 100 150
Table 2 – Lightning Protection Levels Although the rolling sphere method indicates that lightning may attach anywhere on most of the blade surfaces, it is
clear from field experience that the majority of lightning attachments are located at the blade tip, and that only a minority attaches elsewhere on the blade. It is therefore concluded in the standard that the air termination system positioning tools (rolling sphere, protective angle etc.) in IEC 62305-3 do not apply to wind turbine blades, and the standard therefore requires that the ability of the air termination system and down conductor system to intercept lightning strikes and conduct lightning currents must be verified by either of the following methods:
1. High Voltage and High Current tests (discussed in section H below) 2. Demonstration of similarity of the blade type (design) with a previously certified blade type, or a blade type
with documented successful lightning protection in service for a long period under lightning strike conditions. 3. By using analysis tools previously verified by comparison with test results or with blade protection designs that
have had successful service experience.
Furthermore, the standard describes known lightning protection methods for blades (e.g. the concepts shown in Fig.
6), how to consider the effects of electrically conducting components and parts, such as tip shafts, carbon fibre
composites and wiring for sensors in the blades in the lightning protection system design and how to conduct
appropriate testing to verify the design.
C.2 Nacelle and other structural components
Lightning protection of the nacelle and other structural components of the wind turbine (i.e. hub, nacelle and tower – c.f. Fig. 1 and Fig. 2) should be made using the large metal structures themselves as much as possible for lightning air termination, equipotentialization, shielding and conduction of lightning current to the earthing system. Additional lightning protection components such as air terminals and rods for protection of meteorological instruments and obstacle lights on the nacelle, down conductors and bonding connections shall be made and dimensioned according to IEC 62305–3.
In general, lightning protection of the nacelle and other structural components of the wind turbine is straight forward and should be done according to the methods described in the IEC 62305 standard series. The wind turbine should be divided into lightning protection zones, LPZ, as exemplified in Fig. 5. For each LPZ the lightning protection designer should evaluate the lightning threat level, and should design the lightning protection based on equipotential bonding, electromagnetic shielding and application of surge protection devices (SPDs).
Details of how to apply lightning protection to the nacelle and other structural components are included in the
standard.
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A B C DReceptor
Downconductor
Steel wire
Downconductor
Metal mesh
IEC 1864/02 Fig. 6: Lightning protection concepts for large modern composite material wind turbine blades
C.3 Mechanical drive train and yaw system
The mechanical drive train represents the other significant lightning protection challenge that is unique to wind
turbines. This is because the mechanical drive train, with the large rotating bearings, shafts, gears and associated
hydraulic and electrical actuator systems, are in the direct path of the lightning current when lightning attaches to the
blades.
The standard recommends that all parts of the mechanical drive train that are subject to damage due to lightning
currents or lightning arcs between moving parts, for example bearings and actuators be protected by sliding contacts or
spark gaps. These components are designed to divert the lightning current away from the component to be protected or
reduce the lightning current flowing through the component to a level that the component can sustain and withstand. The
standard requires that the efficiency of such protection systems be validated by testing (see section H) and/or analysis,
and that the expected lifetime of wear parts such as sliding contacts and spark gaps shall be documented.
C.4 Electrical systems and electronic systems and installations
Electrical systems and electronic systems and installations of a wind turbine are subject to damage from the Lightning ElectroMagnetic imPulse, LEMP, originating from the lightning impulse current. In fact damage statistics show that most lightning related damages on wind turbines affect the electric and electronic systems.
The standard requires that LEMP Protection Measures (LPMS) be provided to protect against damages and to avoid
failure of these systems. It is required that the protection is designed using the systematic approach of the Lightning Protection Zones (LPZ c.f. Fig. 7) concept according to IEC 62305- 4 and using the appropriate methods including:
• Earthing
• Bonding
• Magnetic and electrical shielding and line routing (system installation)
• Coordinated SPD protection
• Ensuring adequate EMC immunity levels for systems and apparatus
• Isolation, circuit design, balanced circuits, series impedances, etc.
This systematic approach requires that the need for protection be determined for every circuit crossing a LPZ
boundary, and also be evaluated for long circuits within one zone (i.e. longer than 10 metres). The protection can be
achieved by using SPDs, by using shielded cables, by using shielding cable routes or combinations thereof – as indicated
in Fig. 7.
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In general, the standard refers to the relevant electrical standards for electrical low voltage systems and installations incl. IEC 60204-1, IEC 60204-11 [10] concerning low and high voltage electrical systems for machinery respectively, and IEC 60364 concerning electrical installations of buildings, and to IEC 61000-5-2 concerning (EMC) installation and mitigation guidelines on earthing and cabling and of course to IEC 62305-4 Protection against lightning – Part 4: Electrical and electronic systems within structures.
The standard requires that SPDs comply with IEC 61643-1 for low voltage power systems and with IEC 61643-21
for telecommunication and signalling systems, and that SPDs are selected and installed according to IEC 60364—4-44,
IEC 60364—5-53 and IEC 61643-12 for the protection of power systems, and IEC 61643-22 for the protection of the
control and communication systems [11-15]. Furthermore the standard describes the additional requirements for the
selection and installation of SPDs in wind turbine applications
The standard provides guidance on how to ensure coordination of SPDs, coordinate with withstand capabilities of
the components to be protected, and defines appropriate tests to verify the selection and design.
The standard recommends that metal oxide arresters without air gap according to IEC 60099-4 be used for protection of high voltage power systems, and should be selected and applied in accordance with IEC 60099-5 [16-17], unless a high voltage insulation coordination study is made to show that high voltage arresters are not needed.
Fig. 7 – Example of LPMS division of electrical system into protection zones with indication of where circuits cross
LPZ boundaries and showing the long cables running between tower base and nacelle. Protection may be achieved by
using coordinated SPDs, by using shielded cables, by using shielding cable routes, or combinations thereof as needed.
D. Earthing and bonding for wind turbines
The earthing system serves to disperse lightning currents and to prevent damage to a wind turbine. The earthing system is also intended to protect personnel and livestock against electric shock. When faults occur in the electrical systems, the earthing system serves to keep the touch and step voltages as well as the overall earth potential rise to a safe level, until protection devices have tripped and safely interrupted the flow of fault current. These issues are usually covered by requirements in the Electrical Codes, and therefore establishment of earthing systems is mandatory for wind turbines.
For lightning protection, the earthing system serves to disperse and conduct the high frequency and high energy lightning current into the earth without causing dangerous thermal and/or electrodynamic effects.
The standard describes the application of the general types of earthing systems included in IEC 62305-3, and strongly recommends to include metal parts in the foundation structures in the earthing system, as using the metal parts of the large foundation structures will result in the lowest possible earthing resistance, and as attempting to separate an earthing system from the metal parts of the foundation would represent a structural hazard particularly for concrete foundations.
It is furthermore recommended to interconnect the earthing systems of individual turbines with horizontal earthing conductors along the underground cable routes between the wind turbines and to the grid connection station. This serves
G3~
V
VSPD
Powerelectronics
Power supplyControl
Equipment
Power supplyControl
Equipment
LPZ 2LPZ 2 LPZ 2
LPZ 2
LPZ 1
LPZ 1
LPZ 2
Highvoltage
Operationbuilding
Low voltageswitch gear
Tower base Tower Nacelle
Top box
Shielded cable /
Shielding cable route
Generator
Hub
LPZ 2
Shielded cable /
Shielding cable route
Shielded cable /
Shielding cable route
SPD
SPD
SPD
SPD
SPD
SPD
SPD
SPD
SPD
SPD
Shielded cable /
Shielding cable
route
Shielded cable /
Shielding cable
route
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to achieve a low overall earthing resistance for the wind farm, to reduce potential differences between the different structures of the farm, and to reduce the probability of direct lightning flashes to cable routes in the ground.
The standard discusses details of design of earthing systems for different types of wind turbine foundations, and gives guidance on maintenance and inspection of earthing systems.
E. Personnel safety
Erection of large wind turbines on land takes several days when including the time it takes to assemble and disassemble the very large cranes that are used. Offshore wind turbines on the other hand may be erected within less than a day by the use of special vessels or jack ups. In addition, there is usually up to a few weeks of post erection completion work before the wind turbine is commissioned. During this time many people are at work in, on and around the wind turbine, and they are at considerable risk of being affected if lightning strikes the wind turbine.
Therefore the standard states that safety procedures with regards to lightning should be established, which should include:
• Checking of local weather forecasts regularly (e.g. every morning)
• Consider using a lightning warning system or lightning warning service
• First aid education for personnel on lightning injuries and injuries due to electrical accidents
• Application of intermediate earthing system connections as soon as possible
• Identification of safe locations
• Making signal for lightning warning known to everybody on the site
• Instructing personnel to o keep look out for developing thunderclouds, audible thunder and visible lightning o be aware of signs of high electrical fields from thunder clouds, such as hair standing on end, crackling
sounds or light glow from pointed extremities such as air terminals o interrupt work and go to nearest safe location when lightning threat has been realised or lightning
warning signal is received
Different types of automatic warning systems are available for monitoring lightning activity in a warning area (WA) around a construction site, which can be used to trigger an alarm when lightning activity is identified within the WA. Such systems are typically based on detection of electrostatic fields from thunderclouds, detection of electromagnetic impulses from lightning activity or combinations thereof. Lightning warning systems may not provide warning of all lightning flashes, especially not of the first flash in a developing storm. Therefore it is essential that all personnel be made aware of the signs of developing thunderstorms and the risk of lightning to their personal safety.
During construction work connections of cranes, generators etc. to the earthing system should be made as soon as possible.
Platforms inside tubular towers are in general considered safe locations, as the tower is a near to perfect Faraday cage. People in the wind turbine should be instructed to stop work and go to the closest platform inside the tower and stay there until the thunderstorm has passed. Other safe places are inside metal roof vehicles, metal containers etc.
F. Documentation of lightning protection systems
The standard lists the general documentation necessary for assessment of the design of the lightning protection for a
wind turbine.
The general documentation including:
• arrangement drawings of the wind turbine showing the components of the lightning protection system and the
lightning protections zoning
• equipotential bonding plans
• earthing system plans
• single line diagrams showing positions of SPDs in the electrical systems and control systems
• selection of the lightning protection level, LPL
• dimensioning of materials used for lightning protection system conductors etc. (corresponding to selected
LPL)
• selection and coordination of SPDs power system insulation coordination studies
• verification of the blade protection
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• verification of the protection of the mechanical drive train
• tests made to lightning protection components
• validation tests made on SPD protection of systems and apparatus
• installation, inspection and maintenance instructions
• etc.
Wind turbine project specific documentation including:
• information about local lightning occurrence
• enhancement factors such as winter lightning, local terrain variations etc.
• information about earthing conditions / soil resistivity
• information about electrical power grid connection
• wind turbine and wind farm earthing system plans
• installation, inspection and maintenance instructions
• checklists
• etc.
G. Inspection of lightning protection systems
The standard requires that inspections of the lightning protection system should at least be performed during the following processes:
• production of the wind turbine
• during installation of the wind turbine
• during commissioning of the wind turbine
• periodically at such intervals that are reasonable with regard to the location of the wind turbine
• after situations where parts of the wind turbine has been dismounted or repaired (i.e. blades, main components, controls systems etc.)
• etc.
Inspections should be made according to plans provided by the wind turbine manufacturer as part of his quality assurance systems, and in the case of specific wind turbine projects provided by the wind turbine manufacturer in cooperation with the owner/operator of the wind farm.
H. Test specifications for blades and components
Test specifications included in the standard should be used in the development of new blades and their lightning protection systems, and for verifying designs with respect to their capability of handling lightning flashes. The items to be tested would be specimens of the blade, including the tip and sufficient portions of the blade inboard of the tip to represent the complete lightning protection design incl. down conductor systems, connecting components and other components of the lightning protection design. The tests include both High Voltage Strike Attachment tests and High Current Physical Damage tests.
The test specifications include: discussion of purpose of test, detailed instructions for each test setup, test specimen selection, test impulse waveforms, measurements and data recordings, data interpretation, and step-by-step test procedures.
H.1 High Voltage Strike Attachment tests
The High Voltage Strike Attachment tests are applied to determine specific lightning strike attachment points and breakdown paths across or through non-conducting materials, such as wind turbine blades and nacelles. Since the currents that flow during these tests are representative only of lightning leader currents with low peak values, and not the much more intense stroke currents, the attachment tests are intended only to show the path(s) that may be taken by lightning strikes. The damage caused by these tests is not comparable to possible damage from the lightning currents.
The High Current Physical Damage tests are used to assess actual damage from lightning currents. The test methods presented are applicable to both complete tip designs, and to smaller sections of the down conductor like connection components etc.
The High Voltage Strike Attachment tests are intended for wind turbine blades, but may be applied to nacelles fabricated of fibreglass or other non-conducting materials. The tests can be used to assess:
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• Location of possible leader attachment points and flashover or puncture paths on blades and other non conducting structures,
• Optimization of the location of protection devices (air terminals, receptors),
• Flashover or puncture paths, along or through dielectric surfaces, and/or
• Performance of protection devices
There are three High Voltage Strike Attachment test arrangements, designated Test Setup A (Fig. 8 and Fig. 9), Test Setup B (Fig. 10) and Test Setup C (Fig. 11). Test Setups A and B are most appropriate for tests of blade specimens during design development and verification. Test Setup C is most appropriate for developmental tests to evaluate skin panel construction and possible diverter strip configurations.
Each test arrangement is intended to result in initiation of electrical activity, such as corona, streamers and leaders, at the test specimen (and not at the external electrode) as occurs at a wind turbine blade just before a lightning strike attachment. Once ionization of the air at the test specimen is initiated, the streamer will progress toward the other electrode, which has a large geometry and is intended to represent an electric field equipotential surface some distance from a blade extremity. In this way the influence of the external test electrode on test results is minimized.
Overviews of the test arrangements showing the high voltage generator, test specimen, and external electrode in Test
Setups A, B and C are illustrated in Fig. 8, Fig. 9, Fig. 10 and Fig. 11.
The high voltage waveform used should be a double exponential switching type impulse voltage with rise times in
the order of 50-250µs and decay times in excess of 2000µs. This voltage waveform is selected since it is the most
representative of the electric field in the vicinity of a structure during an initial leader attachment. Test Setup A is the most desirable arrangement, since it usually allows a larger dimension external electrode (i.e. a
conductive surface on the laboratory floor) and a more realistic electric field environment during the test around the blade specimen to be provided.
Blade
specimen
Voltage divider
HV generator
Fig. 8 - Initial Leader Attachment Test Setup A (Specimen should be tested in several positions representing different
directions of the approaching leader)
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Blade
specimen
Fig. 9 - Leader Connection Point must be away from test specimen
Test Setup B is intended to create a similar electric field arrangement about the test specimen as in Test Setup A
while allowing larger or heavier test specimens and support structures to be placed on the laboratory floor. In this arrangement a large diameter electrode must be suspended above the test specimen. A large diameter is essential to avoid non-realistic field intensifications due to the edges of the suspended electrode.
Test Setup C is most appropriate for developmental tests to evaluate or compare dielectric strengths of candidate
skin materials and/or local protection designs. However, tests of panels should not be employed for verification of
complete protection designs, since the panel specimens do not represent all significant features of the non-conducting
structures being verified.
Fig. 10 - Initial Leader Attachment Test Setup B (Specimen should be tested in several positions representing
different directions of the approaching leader)
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Fig. 11 - for Local Protection Device (e.g. diverter) Evaluations Test Setup C
A High Voltage Swept Channel test is defined which is be applicable to surfaces of a wind turbine blade that are exposed to initial leader attachment when the blade is rotating, so that a leader may “sweep” along the surface a short distance prior to first stroke arrival. The test, which is illustrated in Fig. 12 can be used to assess:
• Possible puncture locations on non-conducting (i.e. dielectric) composite surfaces,
• Flashover paths over non-conducting surfaces, or
• Performance of protection devices, such as diverter strips, provided that they are implemented.
Blade motion
Blade cross section
HV electrode
Fig. 12 - Swept leader test arrangement
Electrode Electrode
DielectricPanel
Ground Wire
aDiverter
D
d
Determining Distance 'D' as a Function of Proximity 'd' to an Internal Conductor
HardwareMockup
HardwareMockup
Distance 'a' is the shorter dimension of the panel's width or height
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H.2 High Current Physical Damage tests
High Current Physical Damage tests are used to determine the effects due to a lightning attachment to a blade or nacelle surface and current flow away from such an attachment. These effects can be evaluated at the points of attachment and along the path taken by the lightning current.
These tests, which are illustrated in Fig. 13 and Fig. 14, are applicable to structures such as wind turbine blades and nacelles that are exposed to direct strikes or conducted lightning currents.
The test currents to be applied include the First Short Stroke and the Long Stroke, with parameter values correspon-ding to the Lightning Protection Level (LPL) that has been assigned to the part of the blade or other wind turbine structure that is being tested.
The tests are used to determine the direct (physical damage) effects that may result at the locations of possible lightning channel attachment to a blade or where high current and energy densities may flow away from a point of entry during a lightning strike. Examples are blade air terminal systems and associated electrical conductors, metal foils, diverter strips, and fittings and connectors in the lightning current path. The test can be used to assess:
• Arc attachment damage.
• Hot spot formation.
• Metal erosion at receptors.
• Adequacy of protection materials and devices.
• Magnetic force effects
• Blast and shock wave effects
• Behaviour of joints and hardware assemblies.
• Voltages and currents at points of interest throughout a lightning protection system.
Fig. 13 - High Current test arrangement for non-conductive surfaces
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Fig. 14 – Generator arrangement for High Current tests
I. Test specifications for system level immunity
Test specifications are included in the standard to be used for verifying system level immunity for equipment such as cabinets containing control system components (Fig. 15), and for verifying system immunity to induced effects. The equipment to be protected is tested under service conditions, i.e. the device is activated and connected to its nominal supply voltage and stressed with the nominal discharge current parameters of the installed SPDs. Where applicable additional circuits, such as communication lines, sensors, motors shall be connected during the test.
Fig. 15 – Example circuit of a SPD discharge current test of a distribution board under service conditions.
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5 CONCLUSIONS
This paper has presented the update of the IEC 61400 Wind Turbine Generator Systems – Part 24: Lightning
Protection, which is currently being prepared by the IEC TC 88 Project Team 24. The update is a full standard based on
the general lightning protection standards of the recent IEC 62305 series, on the general standards for EMC of the IEC
61000 series, the specific standards for electrical systems on machinery and the general standards for electrical systems,
and with regards to the blades on the latest research and on the air craft industry standards issued by SAE / EUROCAE
6 REFERENCES
[1] IEC TR 61400-24, "Wind Turbine Generator Systems – Part 24: Lightning Protection", 2002, [Online]. Available: http://www.iec.ch [2] IEC 62305, "Lightning Protection", 2006, [Online]. Available: http://www.iec.ch [3] IEC 61000, “Electromagnetic compatibility (EMC)”, [Online]. Available: http://www.iec.ch [4] SAE ARP 5412 / EUROCAE ED-84 “Aircraft Lightning Environment and Related Test Waveforms”, February 2005 [5] .SAE ARP 5416 / EUROCAE ED-105 “Aircraft Lightning Test Methods”, Section 5: Direct Effects Test Methods, 2005-3. [6] Global Wind Energy Council, [Online]. Available: http://www.gwec.net. [7] Larsen, F.M and Sorensen, T., “New lightning qualification test procedure for large wind turbine blades”. Proceedings of International
Conference on Lightning and Static Electricity, Blackpool, UK, 2003 [8] Madsen, S.F., “Interaction between electrical discharges and materials for wind turbine blades particularly related to lightning protection”.
Ørsted-DTU, The Technical University of Denmark, Ph.D. Thesis, March 2006 [9] Bertelsen, K., Erichsen, H.V., Madsen, S.F., ”New high current test principle for wind turbine blades simulating the life time impact from
lightning discharges”, Proceedings of International Conference on Lightning and Static Electricity, Paris, France, August 2007 [10] IEC 60204, “Safety of machinery, Electrical Equipment of machines”, [Online]. Available: http://www.iec.ch [11] IEC 61643-1 “Surge protective devices connected to low-voltage power distribution systems – Part 1: Performance requirements and testing
methods”, [Online]. Available: http://www.iec.ch [12] IEC 61643-12 “Surge protective devices connected to low-voltage power distribution systems – Part 12: Selection and application
principles”, [Online]. Available: http://www.iec.ch [13] IEC 61643-21 “Low voltage surge protective devices - Part 21: Surge protective devices connected to telecommunications and signalling
networks - Performance requirements and testing methods”, [Online]. Available: http://www.iec.ch [14] IEC 61643-22 “Low-voltage surge protective devices – Part 22: Surge protective devices connected to telecommunications and signalling
networks – Selection and application principles”, [Online]. Available: http://www.iec.ch [15] IEC 60364 “Electrical installations of buildings”, [Online]. Available: http://www.iec.ch [16] IEC 60099-4 “Surge arresters - Part 4: Metal-oxide surge arresters without gaps for a.c. systems”, [Online]. Available: http://www.iec.ch [17] IEC 60099-5 “Surge arresters - Part 5: Selection and application recommendations”, [Online]. Available: http://www.iec.ch
29th International Conference on Lightning Protection