IEC-61400-24-2010-Wind-turbines-Part-24-Lightning-protection
General
The lightning environment for wind turbines, including the parameters of lightning current necessary for the design, analysis, and testing of lightning protection systems, is outlined in IEC 62305-1.
An informative discussion of the lightning phenomenon in relation to wind turbines is included in Annex A.
Lightning current parameters and lightning protection levels (LPL)
In IEC 62305-1, four lightning protection levels (I to IV) are introduced For each LPL, a set of maximum and minimum lightning current parameters is fixed
The lightning current parameters for LPL I will not exceed maximum values with a 99% probability For LPL II, these maximum values are reduced to 75%, and for LPL III and IV, they are further reduced to 50% The reductions are linear for I, Q, and di/dt, while they are quadratic for W/R, with time parameters remaining unchanged.
Table 1 – Maximum values of lightning parameters according to LPL
First short positive stroke LPL
Current parameters Symbol Unit I II III IV
First short negative stroke a LPL
Average steepness di/dt kA/ μ s 100 75 50
Current parameters Symbol Unit I II III IV
Average steepness di/dt kA/ μ s 200 150 100
Current parameters Symbol Unit I II III IV
Current parameters Symbol Unit I II III IV
Flash charge Q flash C 300 225 150 a The use of this wave shape concerns only calculations and not testing
Table 1 outlines the maximum values of lightning current parameters for various lightning protection levels, which are essential for designing lightning protection components These components include the cross-section of conductors, the thickness of metal sheets, the current loading capability of surge protective devices (SPDs), and the necessary separation to prevent dangerous sparking Additionally, these parameters help define testing conditions that simulate the effects of lightning on these components, as referenced in Annex D and IEC 62305-1.
Wind turbines located in regions prone to high instances of upward lightning, especially during winter months, may necessitate enhanced durability for their air termination systems.
Flash charge levels exceeding lightning protection level I, specifically with a Q flash of 300 C, significantly impact the wear and melting of materials This parameter is crucial as it directly influences the maintenance requirements for air termination systems.
The minimum lightning current amplitude values for various Lightning Protection Levels (LPLs) are essential for calculating the rolling sphere radius, which helps establish the lightning protection zone LPZ 0 B, an area not susceptible to lightning strikes Table 2 provides the minimum lightning current parameters and their corresponding rolling sphere radii, which are crucial for the strategic placement of the air termination system and defining the LPZ 0 B.
Table 2 – Minimum values of lightning parameters and related rolling sphere radius corresponding to LPL (Table 6 in IEC 62305-1)
Symbol Unit I II III IV
General
Wind turbines, being tall structures, are highly susceptible to lightning strikes, necessitating effective lightning protection measures This protection is crucial not only to prevent economic losses from damage and revenue loss but also to safeguard the lives of service personnel and minimize maintenance needs.
When designing a lightning protection system, it is crucial to assess the risk of lightning strikes to the structure, particularly for wind turbines Unprotected turbines can suffer damage to their blades, mechanical components, and electrical systems due to lightning Additionally, individuals near wind turbines face risks from step and touch voltages, as well as potential explosions and fires resulting from lightning strikes.
The goal of any lightning protection system is to reduce the hazards to a tolerable level R T
The acceptable risk level for human safety determines the need for additional protection measures When risks fall below this threshold, the decision to implement further safety measures often hinges on an economic analysis, weighing the costs of lightning protection systems against the potential damages they can avert.
It is the responsibility of the authority having jurisdiction to identify the value of tolerable risk
A representative value of tolerable risk R T , where lightning flashes involve loss of human life or permanent injuries is 10 –5 year –1
NOTE Values for tolerable risk are given in IEC 62305-2, Table 7
The likelihood of lightning strikes on structures depends on their height, surrounding terrain, and regional lightning frequency A comprehensive risk assessment can be conducted following IEC 62305-2, but this article provides simplified guidance for evaluating lightning exposure for individual wind turbines, as well as for groups and entire wind farms.
Information about local lightning conditions should be collected whenever possible (for example at high latitudes where winter lightning may pose a special threat)
It's important to note that the accuracy of a risk assessment is directly tied to the quality of the input data, and since the assessment is based on statistical averages of lightning occurrences, users should not anticipate precise short-term predictions for individual wind turbines or wind farms However, such assessments can effectively evaluate the risk reduction achieved through lightning protection measures and facilitate comparisons of risks across various wind turbine projects For additional information, please refer to Annex B.
Assessing the frequency of lightning affecting a wind turbine
The initial step in lightning risk analysis involves estimating the frequency of lightning strikes to wind turbines, as outlined in IEC 62305-2 To accurately assess this frequency, it is essential to gather data on the local ground flash density (Ng), which can typically be obtained from national organizations like weather bureaus In cases where ground flash density data is unavailable, it can be approximated using the relationship: dg = 0.1 T.
N g [km –2 ãyear –1 ] is the annual average ground flash density;
T d [year –1 ] is the number of thunder storm days per year obtained from isokeraunic maps (typically available from the national weather bureau)
The average annual number of dangerous events that may endanger a wind turbine may be separated into:
N D [year –1 ] due to lightning flashes to the wind turbine;
N M [year –1 ] due to lightning flashes near the wind turbine (within 250 m);
N L [year –1 ] due to lightning flashes to the service lines connecting the wind turbine, i.e the power cable and the communication cable connecting the wind turbine;
N I [year –1 ] due to lightning flashes near the service lines connecting the wind turbine, i.e the power cable and the communication cable connecting the wind turbine;
N D,b [year –1 ] due to lightning flashes to a wind turbine or another structure at the
(other) ”b” end of the service lines connecting the wind turbine in question
The average annual frequency of lightning flashes attaching to the wind turbine can be assessed as: d 6 d g
A d [m 2 ] is the collection area of lightning flashes to the structure;
Appropriate values are C d = 1 for wind turbines on flat land and C d = 2 for wind turbines on a hill or a mountain ridge
Wind turbines located in areas prone to lightning, especially winter lightning, may require a higher environmental factor C d to account for the increased risk of upward lightning events.
NOTE 2 Wind turbines placed off shore may have to be assigned an environmental factor C d of 3 to 5 to get a realistic estimate of the frequency of lightning attachment
The collection area of a structure refers to the ground surface that experiences the same annual frequency of lightning strikes as the structure itself For isolated structures, this equivalent collection area is defined by a border line created from the intersection of the ground surface and a straight line with a 1:3 slope, which originates from the upper parts of the structure and rotates around it.
All wind turbines should be represented as a tall mast, with a height equal to the hub height plus one rotor radius This guideline applies to wind turbines featuring any blade type, including those made entirely from non-conductive materials like glass-fibre reinforced plastic.
Figure 1 shows the collection area produced by a wind turbine placed on flat ground Clearly this is a circle with a radius of three times the turbine height
Figure 1 – Collection area of the wind turbine
The following equation can therefore be used when estimating the annual number of lightning flashes to a wind turbine placed on flat ground
H [m] is the height of the wind turbine
In complex terrains, it's essential to take into account the effective height of wind turbines, which includes their installation height, especially when positioned on exposed hills or ridges.
Figure 2) IEC 62305-2 provides guidance for structures in complex terrain or in proximity to other structures
Figure 2 – Effective height, H , of wind turbine exposed on a hill
Furthermore, wind turbines may be endangered by lightning flashes near the wind turbine: d 6 d m g
A m [m 2 ] is the collection area for lightning flashes near the structure which is the area within a distance of 250 m
Implementing effective lightning protection for a wind turbine and its connecting service lines ensures comprehensive safeguarding against damage caused by nearby lightning strikes This protection not only covers the wind turbine itself but also extends to the service lines associated with it.
Large wind turbines typically connect to a high-voltage power cable collection system and an external control center through a communication line These service lines are vulnerable to damage from lightning strikes occurring directly to the lines or in their vicinity.
Figure 3) In case the communication line is an optical fibre connection (which is recommended), the risk of lightning damaging the communication line may be neglected
The number of lightning flashes to a service line connecting a wind turbine can be assessed according to IEC 62305-2, Annex A as:
And the number of lightning flashes near a service line (i.e close enough to affect the line) can be assessed as: i 6 g t e
C d is a location factor: say 1 in flat areas and 2 in hilly terrain;
C e is an environmental factor, which is 1 for rural areas;
A I [m 2 ] is the collection area of lightning flashes to the service line – see Table 3;
A i [m 2 ] is the collection area of lightning flashes near the service line – see Table 3
In the context of wind turbine installations, the transformer factor (C t) is determined by the presence of a transformer between the lightning attachment point and the wind turbine Specifically, C t equals 1 when no transformer is present, while it equals 0.2 when a transformer is involved Given that large wind turbines typically include a high-voltage transformer, it is generally accepted that C t = 0.2 applies to the medium-voltage cables linking the wind turbine to the grid, as outlined in IEC 62305-2, Annex A.
NOTE 3 C d =0 for submarine service lines (submarine high-voltage cables and communication cables)
Table 3 – Collection areas A I and A i of service line depending on whether aerial or buried (corresponds to Table A.3 in IEC 62305-2)
L c [m] is the length of the service line from the wind turbine to the next structure on the line A maximum value L c = 1 000 m should be assumed
H a [m] is the height of the wind turbine connected at the “a” end of the service line
H b [m] is the height of the wind turbine (or other structure) connected at the ”b” end of the service line
H c [m] is the height of the service line conductors above ground ρ [ Ω m] is the resistivity of the soil where the service line is buried A maximum value ρ = 500 Ω m should be assumed
A i is the collection area of lightning flashes near the service line
A I is the collection area of lightning flashes to the service line
Lightning strikes within the narrow area A I can directly impact the cable along its route, while strikes in the broader area A i may induce transients that lead to pin-hole punctures in the cable insulation.
Figure 3 – Collection area of wind turbine of height H a and another structure of height
H b connected by underground cable of length L c
In wind farms, the collection areas of adjacent wind turbines frequently overlap, necessitating a division of these areas between the turbines This division should occur at the points where the 1:3 gradient lines from the tops of the wind turbines intersect.
Assessing the risk of damage
Basic equation
Lightning poses a significant risk to wind turbine installations, leading to potential financial losses This risk can be analyzed as the cumulative effect of various risk components, each represented by a specific equation.
N x [year –1 ] is the number of dangerous events per annum;
P x is the probability of damage to the structure (a function of various protection measures);
This basic equation is to be used for assessing the risk of damage based on the probability of damage of various types and the consequent loss (see Annex B)
In case the risk is found to be too high, protection measures have to be applied as necessary to reduce the risk to less than the tolerable risk R T
NOTE The tolerable risk R T may be stipulated by authorities.
Assessment of risk components due to flashes to the wind turbine (S1)
For evaluation of risk components related to lightning flashes to the wind turbine, the following relationship apply:
– component related to injury to living beings (D1)
– component related to physical damage (D2)
– component related to failure of internal systems (D3)
Parameters to assess these risk components are given in Table 4.
Assessment of the risk component due to flashes near the wind
For evaluation of the risk component related to lightning flashes near the wind turbine, the following relationship applies:
– component related to injury to failure of internal systems (D3)
Parameters to assess these risk components are given in Table 4.
Assessment of risk components due to flashes to a service line
For evaluation of risk components related to lightning flashes to an incoming service line connected to the wind turbine, the following relationship apply:
– component related to injury to living beings (D1)
– component related to physical damage (D2)
– component related to failure of internal systems (D3)
Parameters to assess these risk components are given in Table 4.
Assessment of risk component due to flashes near a service line
For evaluation of the risk component related to lightning flashes near a service line connected to the wind turbine, the following relationship apply:
– component related to failure of internal systems (D3)
For the purpose of this assessment, if (N I – N L ) < 0, then assume (N I – N L ) = 0
Parameters to assess these risk components are given in Table 4
Table 4 – Parameters relevant to the assessment of risk components for wind turbine
(corresponds to Table 8 in IEC 62305-2)
Average annual number of dangerous events due to flashes
N D [year –1 ] to the wind turbine
N M [year –1 ] near the wind turbine
N L [year –1 ] to a service line entering the wind turbine
N I [year –1 ] near a service line entering the wind turbine
N D,b [year –1 ] to a structure at the ”b” end of a service line (see Figure 3)
Probability that a flash to the wind turbine will cause
Probability that a flash near the wind turbine will cause
Probability that a flash to a service line will cause
Probability that a flash near a service line will cause
NOTE Values of loss L t ; L f ; L o ; factors r p , r a , r u , r f reducing the loss and factor h z increasing the loss are given in
General
Unless otherwise shown by risk analysis, all subcomponents should be protected according to LPL-I
Lifetime compliance with a specific Lightning Protection Level (LPL) necessitates site-specific maintenance and inspections The service and maintenance manuals should detail the maintenance and inspection requirements for the lightning protection system, including the earthing system For comprehensive guidelines, refer to the procedures outlined in Clause 12.
A comprehensive risk assessment may indicate that a protection level below LPL-I is the most cost-effective option for certain wind turbines or wind farms Additionally, it can be beneficial to apply varying protection levels; for instance, wind turbine blades could be safeguarded at a higher LPL, while other components that are more affordable to repair or replace might be secured at a lower LPL.
Blades
General
Wind turbine blades are highly susceptible to lightning due to their exposure, experiencing significant effects from electric fields during lightning strikes, as well as the associated lightning currents and magnetic fields For a detailed explanation of the lightning attachment process and the subsequent conduction of current and charge, please refer to Annex A.
Wind turbine blades are placed in lightning protection zone 0 A according to IEC 62305-1 and shall be protected accordingly
A general description of the different issues concerning lightning protection of blades is included in Annex C.
Requirements
Lightning protection for the blade must be adequate to withstand LPL I lightning strikes, unless a risk analysis indicates that LPL-II or LPL-III protection is adequate, ensuring that no structural damage occurs that could affect the blade's performance.
Lightning damage shall be limited to that which can be tolerated until the next scheduled maintenance and inspection.
Verification
To ensure the effectiveness of the air-termination and down-conductor systems in intercepting lightning strikes and conducting lightning currents, verification can be achieved through several methods: conducting high-voltage and high-current tests as outlined in section 8.2.5; demonstrating similarity to a previously verified blade design or one with documented successful lightning protection; or utilizing analysis tools that have been validated through comparison with test results or proven blade protection designs.
To achieve verification by similarity for blades, they must have identical material composition, lightning protection systems, and structural dimensions Any substantial modifications that could influence lightning susceptibility require verification, but assessments matching those of an already verified blade design do not need to be repeated.
The blade manufacturer shall produce documentation that describes which of the above methods are used and the results of the verification.
Protection design considerations
The following subclauses describe the issues which are important for design and incorporation of the lightning protection systems associated with the blade
Lightning air-termination systems are strategically installed on the surface areas of blades to prevent lightning flash attachments or punctures These systems can be integrated into the blade's structure, added as separate components, or consist of a combination of both.
The air-termination system positioning tools outlined in IEC 62305-3, such as the rolling sphere and protective angle, are not applicable to wind turbine blades Consequently, it is essential to verify the air-termination system design in accordance with section 8.2.3.
The manufacturer must ensure that the air termination system is securely mounted and designed to endure expected wear from wind, moisture, and particles Verification should include the presence of lightning protection components in the final blade design, prior to conducting fatigue and other mechanical tests.
To minimize the risk of internal discharges, such as streamers and leaders, all components of the air-termination system, including the mounting of air terminations and connections to the down conductor, must be carefully designed.
The manufacturer must design the air-termination system for easy repair or replacement of parts affected by lightning or environmental factors Over time, air terminations can experience erosion at lightning arc roots, influenced by the charge entering these points and the materials used Blades exposed to frequent lightning strikes may need replacement, emphasizing the importance of selecting durable materials and effective designs to maximize the system's lifespan Additionally, the manufacturer should offer guidance on the inspection and maintenance of the air-termination system.
To ensure the air-termination system's effectiveness and reparability throughout the lifespan of the blade, it is essential that any coatings applied do not compromise its functionality Recommended testing methods for assessing the performance of air terminations are outlined in Annex D.
The manufacturer shall define a procedure for regular inspection of the air-termination systems so that the estimated design life-time and service/replacement intervals can be established and verified
Verification of the air-termination system efficiency shall be done as described in 8.2.3
8.2.4.2 The down conductor system and its connection components
The down conductor system, along with its connection components, is essential for safely directing lightning currents from the air-termination system to the termination point at the root end of the blade.
Connections to the down conductor system must be secure and durable, ensuring the entire lightning protection system can endure the electrical, thermal, and electrodynamic forces of lightning strikes To confirm the system's capability to withstand mechanical stresses in the blades, it is recommended to install the lightning protection system in a blade that undergoes testing as outlined in IEC/TS 61400-23.
The cross-sectional area of the down conductor and the natural conductive components of the blade must effectively carry lightning currents based on the selected Lightning Protection Level (LPL) Metal conductors should generally be chosen in accordance with IEC 62305-3 standards.
Connection component testing must adhere to EN 50164-1 standards without applying conditioning or aging The current test levels should be determined based on the initial short stroke of the chosen Lightning Protection Level (LPL) In cases where non-rigid connections like rotating links, bearings, or spark gaps are utilized, testing should also include long stroke current Additionally, if multiple paths for lightning current are present, the test current amplitudes for each path can be adjusted according to the current distribution among them.
To reduce the risk of internal discharges within the down conductor system and its connection components, all internal parts must be meticulously designed This design strategy aims to prevent electrical discharges from originating in areas other than the external air termination system, thereby minimizing the potential for these discharges to penetrate the blade skin.
Externally mounted down conductors are defined as air-termination systems, hence the requirements in 8.2.4.1 apply
The manufacturer must establish a procedure for the routine inspection of down conductor system components and their connections, ensuring that any parts potentially affected by service environments are assessed This process will allow for the verification of their condition, estimated design lifespan, and service intervals.
Recommended tests for determining the capability of down conductors and connection components are described in Annex D
Verification of the down conductor system and its connection components shall be done as described in 8.2.3
If the blade contains additional conductive components such as conductive structural components, weights, tip brake cables, electrical cables for sensors, and warning lights, these elements must be integrated into the lightning protection system They should be engineered to effectively carry their portion of the lightning current and designed to avoid flashover between conductive parts.
The documentation must include testing aligned with the LPL and analysis to determine if additional conductive components, such as CFC structures, need to be bonded to the lightning protection system.
Bonding methods for these additional conductive components shall be proven by high-current testing as described in 8.2.5.2
In case conductors form parallel current paths within the blade, such conductors shall be interconnected according to IEC 62305-1, and attention must be paid to the effects of electrodynamic forces
8.2.4.4 Electrical field stress impact on composite material design
Wind turbine blades are frequently subjected to high electric fields throughout their operational lifespan due to their elevation and exposure These electric fields, which can be both static and transient, are generated by thunderclouds and can significantly impact the blade structure.
Test methods
The following test methods apply to entire blade designs or sub-sections such as blade tips or laminate coupons
Interception effectiveness of the air termination systems on the blade can be evaluated using the initial leader attachment test described in Annex D, Subclause D.2.1
Development of specific design details surrounding tip receptors, side receptors or similar can gain from the initial leader attachment test described in Annex D, Subclause D.2.1
Enhancing the blade laminate's capacity to prevent internal discharges and protect the blade skin from punctures can be accomplished by increasing the electrical breakdown field strength of the materials The evaluation of the breakdown field strength for insulating composites and coating layers is guided by standards such as IEC 60060-1, IEC 60243-1 (a.c.), IEC 60243-3 (impulse voltage), and IEC 60464-2 (coating).
Electrical activity on insulating surfaces, such as streamers and surface flashovers, can lead to deterioration manifested as tracking and electrical erosion When combined with moisture, this activity can alter the insulating surface's properties, making it more conductive and increasing the risk of direct lightning attachment The tracking resistance of different blade and coating materials can be assessed and compared according to IEC 60587 standards.
Air termination systems are significantly influenced by the lightning flash charge, which is determined by the time integral of the lightning current This phenomenon can be assessed through the high-current physical damage test outlined in Annex D, Clause D.3.
Connection components and parts of the down conductor can be tested by the high-current physical damage test in Annex D, Clause D.3, or the EN 50164-1 without the conditioning/aging applied
The current test waveforms and levels should include the first short stroke and if relevant also the long stroke (continuing current) defined for the selected LPL.
Nacelle and other structural components
General
To ensure effective lightning protection for wind turbines, the nacelle and structural components should utilize the metal structures for air termination, equipotentialisation, shielding, and conduction of lightning currents to the earthing system Additional components, such as air termination systems for meteorological instruments and aircraft warning lights, as well as down conductors and bonding connections, must be designed in accordance with IEC 62305-3 It is also essential to divide the wind turbine into designated lightning protection zones (LPZ) for optimal safety.
Hub
The hub of large wind turbines is a hollow cast iron sphere, measuring 2 to 3 meters in diameter, which provides inherent immunity to lightning due to its material thickness Typically, electrical and mechanical control systems are housed within the hub, with circuits extending to the blades and nacelle To enhance lightning protection, the hub should function as a Faraday cage, utilizing magnetic shields in openings towards the blades and nacelle, often secured by blade flange plates and the main shaft flange When these openings are effectively shielded, the hub's contents do not require additional lightning protection, as the focus shifts to equipotential bonding and transient protection for external systems like blade actuators and electrical controls linked to external circuits.
Spinner
The hub of a wind turbine is typically covered by a glass fibre component known as the spinner, which rotates with the hub Due to the rolling sphere model's indication of potential lightning strikes on the spinner's front end, implementing lightning protection is essential Some wind turbine designs feature external electrical and mechanical control systems, which must be shielded from lightning with air termination systems If no such systems are located beneath the spinner, it may be acceptable to assume the risk of lightning penetrating the spinner without protection However, in most cases, effective lightning protection can be achieved by utilizing the spinner's metal support structure as an air termination system connected to the hub.
Nacelle
The nacelle structure must incorporate effective lightning protection to ensure that lightning strikes either connect to durable metal components or a dedicated lightning air-termination system Nacelles featuring GFRP covers should include a lightning air-termination system and down conductors that create a protective cage around the nacelle, capable of withstanding lightning strikes per the specified protection level Additionally, conductors within this Faraday cage must be designed to handle the lightning currents they may encounter For external instruments on the nacelle, lightning air-termination systems should comply with IEC 62305-3 standards, with down conductors linked to the protective cage.
To enhance shielding against external electric and magnetic fields in nacelles, a metal mesh can be integrated with a GFRP cover, effectively mitigating interference from currents within the mesh Alternatively, all internal circuits can be housed within closed metal conduits or cable trays It is essential to establish an equipotential bonding system that incorporates the major metal structures of the nacelle, as mandated by electrical codes This system will create an efficient equipotential plane for all earthing and bonding connections, ensuring optimal safety and performance.
To effectively manage lightning currents, it is crucial to direct the lightning strikes from the blades directly to the cage, thereby preventing the current from passing through the blade pitch bearings and drive train bearings Various brush systems are employed to divert lightning currents away from these bearings; however, their efficiency can be limited This inefficiency arises from the challenges in designing brush and earth lead systems with sufficiently low impedance to significantly mitigate the current that flows through the main shaft and bearing systems to the nacelle bed plate.
A nacelle cover equipped with a magnetic shield cannot safeguard against the impact of magnetic fields generated by lightning currents within the nacelle, particularly in areas like the main shaft.
Tower
A tubular steel tower, commonly used for large wind turbines, meets the dimensions for down conductors specified in IEC 62305-3 and serves as an effective electromagnetic shield, resembling a Faraday cage This design allows the interior of the tower to be classified as lightning protection zones LPZ1 or LPZ2 To maintain its electromagnetic integrity, it is essential to ensure direct electrical contact along the flanges of tower sections Additionally, the tower and its major metal components should be integrated into the protection earth conductor (PE) and equipotential bonding systems to maximize the benefits of the Faraday cage effect For detailed bonding of metal structures and systems inside the tower, such as ladders, wires, and rails, refer to section 9.3.5.
The nacelle interface is typically sealed with metal platforms and hatches, which also function as an electromagnetic shield for the tower, as discussed in section 8.4.2 regarding lightning protection for the yaw bearing.
Clause 9 discusses the tower interface with the earthing system, emphasizing that if the tower is designed as a Faraday cage, it does not require specific lightning protection for its contents Consequently, the focus for lightning protection shifts to equipotential bonding and transient protection for electrical and control circuits that extend into other lightning protection zones, including the nacelle and the exterior of the tower.
Lattice towers naturally cannot be considered a very effective Faraday cage, although there will be some magnetic field attenuation and lightning current reduction inside the lattice tower
The interior of a lattice tower is classified as LPZ0 B, where lightning conduction should occur through the structural elements of the tower These elements must meet the dimensions specified for down conductors in IEC 62305-3, considering current sharing among parallel paths Additionally, the shields of cables within the lattice towers may require bonding to the tower at specific intervals to prevent insulation puncture, a requirement that should be evaluated through calculations as outlined in IEC 62305-2, Annex D.
In steel reinforced concrete towers, effective lightning down conduction is achieved by utilizing 2 to 4 parallel vertical connections with adequate cross-sections, which are interconnected horizontally at the top, bottom, and every 20 meters in between This strategic bonding of steel reinforcement significantly enhances magnetic field attenuation and reduces lightning current within the tower.
Testing methods
Preliminary testing methods are included in Annex D.
Mechanical drive train and yaw system
General
The wind turbine will in general have a number of bearings for blade pitching, main shaft rotation, gearbox, generator, and yawing systems
Hydraulic or electrical actuator systems are used for control and operation of main components
Bearings and actuator systems have moving parts that directly or indirectly bridge different parts of the wind turbine where lightning current may flow
To ensure safety, all bearings and actuator systems within a lightning current path must be adequately protected to minimize the current flow through these components to an acceptable level.
Bearings
Monitoring bearings in wind turbines is challenging, making post-lightning inspection inadequate Therefore, it is essential to implement well-documented and proven systems for bearing protection.
Protection can be a part of the bearing structure itself or it can be an external system installed across the bearing to bypass the current
To ensure the longevity of bearings, they must be capable of functioning throughout their entire design lifetime, even when exposed to the anticipated number of lightning current penetrations If a bearing cannot maintain operation for its full design life under these conditions, appropriate protective measures must be implemented.
Hydraulic systems
To ensure the integrity of hydraulic systems in the event of a lightning strike, it is crucial to prevent lightning current penetration from impacting these systems Additionally, it is important to address the potential risks of fluid leaks from damaged fittings and the subsequent ignition of hydraulic oil.
Protection measures such as sliding contacts or bonding straps can be used to make the current bypass actuator cylinders
To protect hydraulic tubes from lightning currents, it is essential to implement measures that prevent current penetration When hydraulic tubes are equipped with mechanical armor, this armor must be securely bonded to the machinery's steel structure at both ends Additionally, it is crucial to ensure that the armor has an adequate cross-sectional area to effectively conduct any lightning current it may encounter.
Similar considerations may apply to water cooling systems.
Spark gaps and sliding contacts
To effectively bypass bearings and actuator systems, it's essential to utilize spark gaps or sliding contacts These bypass systems, along with their connecting leads, must exhibit lower impedance than the direct natural current path through the component to ensure optimal performance.
Spark gaps and sliding contacts shall be able to conduct the lightning current it may be exposed to at the place of use in the wind turbine
Both spark gaps and sliding contacts shall be designed to maintain the required performance regardless of environmental effects such as rain, ice, pollution with salt, dust, etc
Spark gaps and sliding contacts are classified as wear parts, necessitating careful calculation and documentation of their service lifetime Regular inspections of these components must be conducted in alignment with the service and maintenance manuals to ensure optimal performance.
Testing
All systems for protection of bearings and actuator systems shall have a documented functionality It is recommended to perform tests with impulse current representing the natural lightning current
It is recommended to perform impulse current tests on full-scale test objects where the important parts of the system are represented in a test mock-up
It shall be demonstrated that the protection system can withstand the damaging effect of the first lightning stroke combined with the long stroke current
When utilizing sliding contacts in a system, it is essential to conduct mechanical tests to ensure system stability, particularly regarding contact wear, both with and without the impact of lightning current erosion The wear must be minimal to ensure uninterrupted operation between scheduled service intervals.
Tests can be done on scaled models, but calculations shall demonstrate the scaling factors and effects
Informative testing methods are included in Annex D, Sublause D.3.4.
Electrical low-voltage systems and electronic systems and installations
General
This clause deals with the protection of the electrical and control systems of a wind turbine against the effects of
• lightning flashes attaching to the wind turbine;
• leader currents developing from the wind turbine;
• indirect lightning flashes (i.e effect through LEMP of lightning flashes not affecting the wind turbine directly)
NOTE 1 Transient overvoltages and surges caused by switching operations in electrical systems (switching electromagnetic impulse, SEMP) must be considered as well However, it is outside the scope of this standard For general information, the reader is referred to IEC 62305-2 Annex F for discussion of switching overvoltages Subclause 8.5.6.9 and Clause F.7 of this standard give some information on the selection of SPDs with regard to overvoltages created within wind turbines
All types of lightning flashes generate lightning electromagnetic impulses (LEMP)
NOTE 2 The general requirements for electrical equipment on machines described in IEC 60204-1 should be observed.
LEMP protection measures (LPMS)
Electrical and control systems in wind turbines are vulnerable to damage from lightning electromagnetic pulses (LEMP), necessitating the implementation of lightning protection measures (LPMS) to prevent system failures To effectively safeguard these systems, a systematic approach based on the lightning protection zones (LPZ) concept, as outlined in IEC 62305-4, is essential LPMS is integral to the LPZ framework for the entire wind turbine, detailed in section 8.5.3, with practical examples provided in Annex E.
The wind turbine manufacturer shall provide a LEMP protection measures system (LPMS) according to IEC 62305-4 for the complete electrical system
NOTE It can be assumed that effective LEMP protection measures also provide effective protection against the effects of indirect lightning flashes
Basic protection measures in an LPMS according to IEC 62305-4 include:
• magnetic and electrical shielding of cables and line routing (system installation) – see Subclause 8.5.5;
• coordinated SPD protection – see Subclause 8.5.6;
• isolation, circuit design, balanced circuits, series impedances, etc
For the LPMS, the following basic information shall be documented (see also Clause 11):
• definition of lightning protection level (LPL) according to IEC 62305-1;
• drawings of the wind turbine defining LPZ and their boundaries;
• circuit diagrams showing SPDs, cable shields and cable shield bonding points
Figures E.5 and E.6 provide basic examples of such documentation.
Lightning protection zones (LPZ)
Wind turbines are categorized into lightning protection zones (LPZ) to systematically safeguard their components from lightning strikes These zones are determined by the potential for direct lightning attachment and the expected intensity of lightning currents and associated electromagnetic fields Effective lightning protection strategies are implemented to ensure that critical systems, such as machinery and electrical controls, can endure the lightning effects specific to their zone For example, overvoltage protection is essential for cables transitioning from a lower LPZ to a higher LPZ, where more sensitive components are located, while internal connections within the same zone may not require such protection This methodology is elaborated in IEC 62305-4, Clause 4, focusing on the design and installation of lightning electromagnetic impulse (LEMP) protection measures.
Further guidance on how to fulfil these requirements is given in Annex E.
Equipotential bonding within the wind turbine
Equipotential bonding, as outlined in IEC 62305-4, is essential in wind turbines to prevent dangerous sparking between conductive components This bonding protects against touch and step voltages during lightning strikes and significantly reduces the risk of damage to electrical and control systems By utilizing low impedance bonding connections, equipotential bonds help maintain safe potential differences among equipment within the wind turbines.
To enhance effectiveness, bonding connections should utilize the wind turbine's large metal structures, including the tower, nacelle bed plate, nacelle frame, and hub These bonding conductors can also mitigate magnetic field levels generated by lightning Strategically placing bonding connections between metal platforms and the tower wall at various points around the platform-tower interface will provide effective electromagnetic shielding for the interior of the tower.
Effective bonding and shielding can significantly reduce damage to wind turbine control systems Additional insights on the necessary bonding for wind turbines can be found in Annex G.
Shielding and line routing
Shielding is the means by which electromagnetic field levels are attenuated The reduction of electromagnetic fields can substantially reduce levels of voltages induced into circuits
To minimize the magnetic field generated within a Lightning Protection Zone (LPZ) by nearby lightning strikes, effective spatial shielding of the LPZ is essential Additionally, to reduce surges induced into the control system through connecting cables, implementing spatial shielding, utilizing shielded cables bonded at both ends, or combining these methods can be highly effective.
Magnetic shielding and line routing according to IEC 62305-4, Clause 4 should be used, and the general guidelines on EMC-correct installation practices described in IEC/TR 61000-5-2 should be followed
When lightning strikes a wind turbine, it generates significant magnetic fields that can induce surge voltages and currents in nearby wiring or the turbine structure itself The intensity of these surges is influenced by the rate of change of the magnetic field and the size of the loop formed by the wiring Therefore, designers must carefully assess the potential magnitude of induced voltages to ensure that these surges remain within the safe limits of the cabling and connected equipment.
The use of shielding and line routing should be documented by analysis and/or testing
Some further considerations about the shielding required in a wind turbine are discussed in Annex G.
Coordinated SPD protection
Coordinated SPD protection consists of a set of SPDs properly selected, coordinated and installed to reduce failures of electrical and electronic systems
NOTE Coordination of SPD protection must include the connecting circuits to provide insulation coordination of complete systems
Coordinated surge protective device (SPD) protection effectively mitigates the impact of lightning surges and switching surges generated internally To safeguard electrical and control systems, a systematic approach involving coordinated SPDs is essential for both low-voltage power systems and control systems For detailed recommendations on SPD protection in wind turbines, refer to Annex F.
According to IEC 62305-4, in an LPMS, SPDs shall be located at the line entrance into each LPZ:
• as close as possible to the boundary of LPZ 1, SPDs tested with I imp (Class I test), as classified in IEC 61643-1, shall be installed;
To ensure optimal protection for equipment, Surge Protective Devices (SPDs) tested with I n (Class II test) as per IEC 61643-1 must be installed as close as possible to the boundary of LPZ 2 or higher, and if necessary, as near as possible to the equipment requiring protection.
NOTE If the length of the circuit between the SPD and the equipment is too long (i.e in general when longer than
10 m), propagation of surges can lead to an oscillation phenomenon – see IEC 62305-4, Subclause D.2.3 and D.2.4
Surge Protective Devices (SPDs) designed to endure partial lightning currents with a typical waveform of 10/350 μs must undergo a specific impulse test current, referred to as I imp For power lines, the appropriate test current I imp is outlined in the Class I test procedure of IEC 61643-1.
Surge Protective Devices (SPDs) designed to endure induced surge currents with an 8/20 μs waveform must undergo a specific impulse test current, referred to as I n For power lines, the appropriate test current I n is outlined in the Class II test procedure of IEC 61643-1.
• IEC 61643-21 for telecommunication and signalling systems
SPDs shall comply with the installation rules given in
• IEC 60364-4-44, IEC 60364-5-53 and IEC 61643-12 for the protection of power systems;
• IEC 61643-22 for the protection of the control and communication systems
The installation sites of Surge Protective Devices (SPDs) must be clearly documented through drawings and wiring diagrams in accordance with the Lightning Protection Management System (LPMS) It is essential to meet the energy coordination requirements outlined in IEC 62305-4 and IEC 61643-12 for SPDs located at various Lightning Protection Zone (LPZ) boundaries, as well as for any surge protection components integrated within equipment.
IEC 62305-4 emphasizes the importance of coordinating Surge Protective Devices (SPDs) within electrical and control systems It mandates that documentation must clearly outline the methods used to achieve effective coordination among SPDs.
Further guidelines for the bonding (earthing) and cabling of electrical and control systems and installations are given in 8.5.4 and 8.5.5 and exemplified in Annex G
SPDs shall withstand the environmental stresses characterising the installation place such as:
At the installation site of wind turbines, specific performance and installation requirements for Surge Protective Devices (SPDs) may be necessary based on environmental conditions Manufacturers should consider these conditions, particularly for critical installation points such as the nacelle and hub.
Maintenance and replacement of SPDs shall be done according to a maintenance plan
SPDs shall be installed in such a way that they can be inspected
NOTE The SPD manufacturer can provide information on SPD service life time
SPD protection of critical parts of the electrical and control systems of wind turbines may require monitoring
8.5.6.8 Selection of SPDs with regard to protection level ( U p ) and system immunity
In order to identify the required protection level U p in an LPZ, it is necessary to establish the immunity levels of the equipment in the LPZ, e.g of
• power lines and equipment terminals according to IEC 61000-4-5 and IEC 60664-1;
• telecom lines and equipment terminals according to IEC 61000-4-5, ITU-T K.20 and ITU-T K.21;
• other lines and equipment terminals according to information obtained from the manufacturer
Electrical and electronic component manufacturers must provide immunity level information in accordance with EMC standards If they cannot, wind turbine manufacturers are responsible for conducting tests to determine the necessary immunity levels.
The established immunity level of components in an LPZ directly defines the necessary protection level to be achieved at the LPZ boundaries
System immunity shall be verified including all SPDs installed and equipment to be protected, if applicable Possible testing methods are described in Annex H
8.5.6.9 Overvoltages created within wind turbines
Wind turbines may require specific surge protective devices (SPDs) to address significant voltage fluctuations and temporary overvoltages in their electrical systems It is essential to analyze and test the relevant components, voltage levels, current levels, and duration to select appropriate SPDs For further guidance, refer to the examples provided in Annex F.
Evidence shall be provided that the selected SPDs can withstand these specific stress levels
8.5.6.10 Selection of SPDs with regard to discharge current I n and impulse current
It is advisable to analyze the lightning current distribution in wind turbines following the guidelines of IEC 62305-4 This analysis enables the selection of Surge Protective Devices (SPDs) based on the discharge current (I n) and impulse current (I imp).
SPDs for circuits that are highly exposed may need a higher rating than those specified in IEC 60364-5-53, or these circuits could be shielded It is essential to identify circuits that face high or repeated stresses through careful analysis If relevant, the wind turbine manufacturer must document these exposed circuits within the electrical and control systems in the wiring diagrams Additional details can be found in Annex F.
8.5.6.11 Selection of SPDs with regard to short-circuit current, the follow current interrupt rating and duty cycle (service lifetime) of the SPDs
The short-circuit withstand current rating of the surge protective device (SPD) combined with the overcurrent protective device (OCPD), such as a fuse, must meet or exceed the maximum short-circuit current anticipated at the installation site Furthermore, when the SPD's follow current interrupting rating is specified, it is essential to verify through calculations or testing that the installed OCPD in the power circuit remains inactive.
NOTE The SPD manufacturer can provide information on SPD service life time
8.5.6.12 Behaviour of SPDs in case of multiple lightning flashes
Wind turbines are frequently struck by lightning, making it essential for surge protection devices (SPDs) to be installed effectively These SPDs must be capable of withstanding multiple lightning flashes to ensure the safety and functionality of the wind turbine installations.
Testing methods for system immunity tests
Preliminary testing methods are included in Annex H.
Electrical high-voltage (HV) power systems
Large wind turbines are linked to an underground high-voltage (HV) cable system through a HV transformer, enabling direct connection to the grid or routing power to a transformer station that increases the voltage to sub-transmission levels, such as 132 kV.
The wind turbine HV transformer may be placed in the back of the nacelle, in the bottom of the tower or next to the wind turbine tower
High-voltage surge protection devices, commonly known as surge arresters, are essential in wind turbine applications to safeguard transformers and high-voltage systems from earth potential rise caused by lightning currents in the turbine's earthing system They also protect against transients from the external high-voltage cable system The necessity for surge arresters on the high-voltage side of transformers should be assessed according to the guidelines outlined in IEC 62305-2, specifically Clause 7 and Annex B.
To evaluate the transient levels from the HV cable system outside wind turbines, specialized transient electrical network simulations are essential, following the guidelines of the IEC 60071 series If these studies are not conducted, it is recommended to install HV surge arresters as a precautionary measure.
HV surge arresters should be metal-oxide surge arresters without gaps in accordance with IEC 60099-4 and should be selected and applied in accordance with IEC 60099-5
Figure 4a – Squirel cage induction generator (SCIG)
Gear WRIG HV trafo Grid
Figure 4b – Wound rotor induction generator (WRIG)
Figure 4 – Examples of placement of HV arresters in two typical main electrical circuits of wind turbines
For optimal protection of high-voltage transformers, surge arresters should ideally be installed at the transformer terminals, as illustrated in Figure 4 However, it is also practical to position surge arresters at the switchgear Typically, a distance of 10 meters is recommended between the surge arresters and the transformer to ensure effective operation.
When considering the installation of arresters, a distance of 40 meters between the arrester and the protected component is feasible, depending on the insulation level of the component If this distance exceeds 40 meters, further analysis is required to determine whether arresters positioned at the base of the tower can adequately protect a transformer located in the nacelle Additionally, if the transformer is situated outside the tower, it is crucial to ensure that its earthing system is connected to the wind turbine's earthing system, ideally forming a single, unified earthing system for optimal safety.
Installing surge protective devices (SPDs) on the low-voltage (LV) side of high-voltage (HV) transformers is a prudent precaution, especially when significant transients may pass through from the HV side It is essential to select SPDs designed for transformer applications, which possess high energy absorption capabilities The transient levels transferred to the LV side are influenced by the transformer’s design and the earthing connection of the LV winding, as detailed in IEC 60071-2, Annex E Therefore, it is advisable to either install SPDs on the LV side or obtain a detailed transformer model from the manufacturer for transient analysis to determine the necessity of SPDs.
NOTE The general requirements for high-voltage systems on machinery in IEC 60204-11 should be observed
9 Earthing of wind turbines and wind farms
General
Basic requirements
The earthing system of a wind turbine must be engineered to ensure adequate protection against damage from lightning strikes, aligning with the Lightning Protection Level (LPL) for which the wind turbine's protection system is specifically designed.
The earthing system must be designed to fulfill four essential requirements: ensuring personal safety by managing step and touch voltages during earth faults, protecting equipment from damage, withstanding thermal and electrodynamic forces during faults, and maintaining long-term mechanical strength and corrosion resistance.
Earth electrode arrangements
Two basic types of earth electrode arrangements that are described in IEC 62305-3 apply to wind turbines:
Type A earthing arrangements, while not suitable for wind turbines, can be utilized for smaller structures such as measurement equipment buildings or office sheds associated with a wind turbine farm These arrangements consist of horizontal or vertical electrodes linked to at least two down conductors on the buildings.
NOTE For further information on type A arrangements, see IEC 62305-3, Subclause 5.4.2.1
The type B arrangement is ideal for wind turbines, consisting of either an external ring earth electrode that maintains contact with the soil for at least 80% of its length or a foundation earth electrode It is essential to connect the ring electrodes and metal components within the foundation to the tower structure for optimal performance.
Earthing system impedance
The earthing system's conventional impedance does not impact the effectiveness of the air termination and down conducting systems It is essential to design the earthing system with minimal impulse impedance to decrease total voltage drop, mitigate earth potential rise, limit partial lightning current flowing into service lines connected to the wind turbine, and reduce the risk of sparks affecting nearby service lines.
The embedded depth and the type of the earth electrodes shall minimise the effects of corrosion, soil drying and freezing and thereby stabilise the conventional earthing resistance
It is recommended that the first metre of a vertical earth electrode should not be regarded as being effective under frost conditions
The components of an earthing system must endure both lightning currents and power system fault currents, in accordance with IEC 62305-3 standards It is essential that the earthing system is designed to effectively dissipate lightning currents into the ground without causing thermal or electrodynamic damage, and that the conductors are kept as short as feasible.
Additional information is included in Annex I, Subclause I.2.2.
Equipotential bonding
General
Equipotentialisation is achieved by interconnecting the LPS with
• external conductive parts and service lines connected to the structure
When lightning equipotential bonding is established to internal systems, part of the lightning current may flow into such systems and this effect shall be taken into account
Achieving lightning equipotential bonding for service lines, including telecommunication and power lines, is crucial and requires consultation with telecommunication network operators, electric power system operators, and relevant authorities, as differing requirements may exist.
Lightning equipotential bonding for metal installations
Lightning equipotential bonding connections shall be made as direct and as straight as possible
Table 5 outlines the minimum cross-sectional values for bonding conductors that link various bonding bars and points, as well as for the conductors that connect these bars and points to the earth termination system.
The minimum values of the cross section of the bonding conductors connecting internal metal installations to the bonding bars/points are listed in Table 6
Table 5 – Minimum dimensions of conductors connecting different bonding bars/points or connecting bonding bars/points to the earth termination system (Table 8 in
Class of LPS Material Cross section mm2 Copper 14 Aluminium 22
Table 6 – Minimum dimensions of conductors connecting internal metal installations to the bonding bar/point (Table 9 in IEC 62305-3)
Class of LPS Material Cross section mm2 Copper 5 Aluminium 8
Electrically insulated LPS
It is not recommended to use an insulated external LPS for wind turbines.
Structural components
General
All structural conducting components of wind turbines can conduct a portion of lightning current, necessitating equipotential bonding of these components.
Metal tubular type tower
The tower shall be considered as the primary protection earth conductor (PE) and equipotential bonding connection
Given the height of the towers, it is essential to anticipate direct lightning strikes and incorporate this consideration into the tower's design All electrical components and significant metal parts capable of conducting lightning must be bonded to the tower The structure should function as a down conductor, ensuring that lightning currents can flow along it freely and without obstruction.
Metal reinforced concrete towers
The tower serves as the main protection earth conductor (PE) and equipotential bonding connection Given its height, the design must account for the likelihood of direct lightning strikes to the tower structure.
External lightning protection systems can be considered for use with concrete towers, but should always be bonded to the steel reinforcement of the tower
Equipotential bonding outlets must be strategically installed at termination points for connecting equipment within the reinforced concrete tower, which should be designed in accordance with section 9.3.6.
Lattice tower
A lattice tower serves to shield its interior from direct lightning strikes and reduces the lightning electromagnetic field, categorizing the space within as LPZ 0 B To ensure effective lightning down conduction, the structural elements of the lattice tower must comply with the dimensions specified for down conductors in IEC 62305-3, while also considering current sharing among parallel paths.
To enhance cable protection within lattice towers, it's effective to position cables in the inner corners of the tower leg metal profiles Additionally, utilizing shielding cable conduits or trays inside the tower can further safeguard the cables.
Systems inside the tower
The interior of the tower will be designated as one or more lightning protection zones (LPZ), where the necessary protection levels for internal equipment will be assessed as outlined in section 8.5.
Ladder systems shall be bonded to the tower at each end, for every 20 m and at every platform
Rails, hoist guides, hydraulic piping, personal protection wires, and other components that traverse a tower must be bonded at both ends Furthermore, for lattice towers, it is advisable to implement bonding every 20 meters, whenever feasible.
The HV transformer earthing system should be bonded to the wind turbine earthing system It is not recommended to use separate earthing systems for power systems and lightning protection.
Concrete foundation
The metal reinforcement in wind turbine foundations is integral to the lightning protection system (LPS) because it is consistently connected to the tower, creating a pathway for lightning or fault currents to reach the ground Therefore, it is essential to treat the metal reinforcement as a crucial component of the LPS.
Ensuring electrical continuity of steelwork in reinforced concrete structures is essential Steel reinforcement is deemed electrically continuous when the primary vertical and horizontal bars are interconnected Connections must be made through welding, clamping, or overlapping by at least 20 times their diameter, securely bound with conductive thread It is crucial to pay special attention to interconnections to avoid localized arcing that can damage the concrete from poor contacts.
The designer must define the connections between reinforcement elements, while the installer is responsible for conducting quality assurance on these connections It is essential to maintain short and straight connections for lightning protection earthing at all times.
When utilizing metal reinforcement for the protective earthing of power systems, it is essential that the thickness of the metal rods and their connections adhere to the specifications outlined in the electrical code for earthing systems.
Outlets for additional bonding, measurement or expansion of the earthing system shall be made at appropriate locations on the foundation.
Rocky area foundation
In rocky areas, the lowest resistivity is normally in the surface of the rock
The B type earth termination system shall be used See Subclause I.1.1 for further information on design details
It is recommended to use at least two concentric ring electrodes for step and touch voltage protection, which may be combined with vertical electrodes drilled into the rock
Rock anchor bolts shall be interconnected to each other and to the ring earthing system If metal reinforced concrete is used, please refer to 9.3.6
In rocky terrains, achieving low earthing resistance may require extensive earthing systems Consequently, the focus should be on controlling surface potential differences to minimize touch and step voltages where people and livestock congregate This can be accomplished by installing one or more ring electrodes around wind turbines and other facilities, alongside implementing surge protection for all service lines connecting the turbines to the power collection and communication systems.
Metal mono-pile foundation
A metal mono-pile foundation is by nature a large earth electrode It shall be used as the primary earth electrode
A ring electrode system for controlling the surface potential gradients close to the foundation may be necessary depending on soil resistivity.
Offshore foundation
Seawater has a significantly lower resistivity compared to most soils, which means that offshore foundations like mono-piles or metal reinforced concrete foundations typically meet earthing system requirements without needing extra measures, such as ring electrodes Additionally, interconnecting offshore foundations, apart from the connection of collection system cable shields, is generally unnecessary.
External earthing systems of copper cannot be used off shore due to corrosion issues.