IEC 61400-24 lightning protection of wind turbines

                           IEC 61400-24 lightning protection of wind turbines

 

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.

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.

 

 

I. 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).

 

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.

 

II.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.

III. 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.

 

IV.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.

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 . 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 , unless a high voltage insulation coordination study is made to show that high voltage arresters are not needed.