Lightning and surge protection for wind turbine system

With the growing awareness of the global warming and the limits to our fossil based fuels, the need to find better renewable source of energy is becoming apparent. The use of wind energy is a rapidly growing industry. Such installation are generally located on open and elevated terrain and as such present attractive capture points for lightning discharges. If reliable supply is to be maintained it is important that sources of over-voltage damage are mitigated. LSP provides an extensive range of surge protection devices suited to both direct and partial lightning currents.

LSP has a full suite of surge protection products available for wind turbine applications. The offering from LSP to various DIN rail mounted protection products and surge and lightning monitoring. As we enter a time in history when the push towards green energy and technology is continually causing more wind farms to be built, and current wind farms to be expanded, both turbine manufacturers and wind farm owners/operators are increasingly aware of the costs associated with lightning strikes. The monetary damage that operators sustain when there is an instance of a lightning strike comes in two forms, the costs associated with replacement of machinery due to physical damage and the costs associated with the system being offline and not producing power. Turbine electrical systems face the continual challenges of the landscape that surrounds them, with wind turbines generally being the tallest structures in an installation. Due to the harsh weather that they will be exposed to, combined with the expectations of a turbine being struck by lightning several times throughout its lifespan, costs of equipment replacement and repair must be factored into the business plan of any wind farm operator. The direct and indirect lightning strike damage is created by intense electromagnetic fields that create transient overvoltages. These overvoltages are then passed through the electrical system directly to sensitive equipment within the turbine itself. The surge propagates through the system producing both immediate and latent damage to circuitry and computerized equipment. Components such as generators, transformers, and power converters as well as control electronics, communication and SCADA systems are potentially damaged by lighting created surges. Direct and immediate damage may be obvious, but latent damage that occurs as a result of multiple strikes or repeated exposure to surges can occur to key power components within an effected wind turbine, many times this damage is not covered by manufacturer’s warranties, and thus the costs for repair and replacement fall on operators.

Offline costs are another major factor that must be figured into any business plan associated with a wind farm. These costs come when a turbine is disabled and must be worked on by a service team, or have components replaced which involves both purchase, transport and installation costs. The revenues that can be lost due to a single lightning strike can be significant, and the latent damage that is produced over time adds to that total. LSP’s wind turbine protection product significantly reduces the associated costs by being able to withstand multiple lightning surges without failure, even after multiple instances of strike.

The case for surge protection systems for wind trubines

The continual change in climate conditions combined with the increasing dependence upon fossil fuels has provided a great in interest in sustainable, renewable energy resources worldwide. One of the most promising technologies in green energy is wind power, which except for high startup costs would be the choice of many nations worldwide. For example, in Portugal, the wind power production goal from 2006 to 2010 was to increase to 25% the total energy production of wind power, a goal which was achieved and even surpassed in later years. While aggressive government programs pushing wind and solar energy production have expanded wind industry substantially, with this increase in the number of wind turbines comes an increase in the likelihood of turbines being struck by lightning. Direct strikes to wind turbines have become recognized as a serious problem, and there are unique issues that make lightning protection more challenging in wind energy than in other industries.

The construction of wind turbines is unique, and these tall mostly-metal structures are very susceptible to damage from lightning strikes. They are also difficult to protect using conventional surge protection technologies which mainly sacrifice themselves after a single surge. Wind turbines can rise more than 150 meters in height, and are located typically on high ground in remote areas that are exposed to the elements, including lightning strikes. The most exposed components of a wind turbine are the blades and nacelle, and these are generally made of composite materials which are unable to sustain a direct lightning strike. A typical direct strike generally happens to the blades, creating a situation where the surge travels all through the turbine components within the windmill and potentially to all electrically-connected areas of the farm. The areas typically used for wind farms present poor earthing conditions, and the modern wind farm has processing electronics which are incredibly sensitive. All of these issues make the protection of wind turbines from lightning-related damage most challenging.

Within the wind turbine structure itself, the electronics and bearings are very susceptible to lightning damage. Maintenance costs associated with wind turbines is high due to the difficulties in replacing these components. Bringing technologies which can improve statistical averages for necessary component replacement is a source of great discussion within most board rooms and governmental agencies involved with wind production. The robust nature of surge protection product line is unique among surge protection technologies because it continues to protect the equipment even when activated, and there is no need for replacement or resetting after a lightning surge. This allows wind power generators to remain online for longer periods. Any improvements to the statistical averages of offline statuses and times that turbines are down for maintenance will ultimately bring further costs to the consumer.

Preventing damage to low-voltage and control circuits is crucial, as studies have shown that more than 50% of wind turbine failures are caused by breakdowns of these types of components. Documented breakdowns of equipment attributed to direct and induced lightning strikes and backflow surges which propagate just after a lightning strike, are common. Lightning arrestors installed to the power grid side of systems are grounded together with the low voltage side in order to decrease grounding resistance, increasing the ability of the entire chain to withstand a strike to a single wind turbine.

Lightning and surge protection for wind turbines

This article describes the implementation of lightning and surge protection measures for electrical and electronic devices and systems in a wind turbine.

Wind turbines are highly vulnerable to the effects of direct lightning strikes due to their vast exposed surface and height. Since the risk of lightning striking a wind turbine increases quadratically with its height, it can be estimated that a multi-megawatt wind turbine is hit by a direct lightning strike roughly every twelve months.

The feed-in compensation must amortise the high investment costs within a few years, meaning that downtime as a result of lightning and surge damage and associated re-pair costs must be avoided. This is why comprehensive lightning and surge protection measures are essential.

When planning a lightning protection system for wind turbines, not only cloud-to-earth flashes, but also earth-to-cloud flashes, so-called upward leaders, must be considered for objects with a height of more than 60 m in exposed locations. The high electrical charge of these upward leaders must be particularly taken into account for the protection of the rotor blades and selecting suitable lightning current arresters.

Standardisation-Lightning and surge protection for wind turbine system
The protection concept should be based on the international standards IEC 61400-24, IEC 62305 standard series and the guidelines of the Germanischer Lloyd classification society.

Protection measures
IEC 61400-24 recommends the selection of all sub-components of the lightning protection system of a wind turbine according to lightning protection level (LPL) I, unless a risk analysis demonstrates that a lower LPL is sufficient. A risk analysis may also reveal that different sub-components have different LPLs. IEC 61400-24 recommends that the lightning protection system be based on a comprehensive lightning protection concept.

The Lightning and surge protection for wind turbine system consists of an external lightning protection system (LPS) and surge protection measures (SPMs) to protect electrical and electronic equipment. To plan protection measures, it is advisable to subdivide the wind turbine into lightning protection zones (LPZs).

The Lightning and surge protection for wind turbine system protects two sub-systems which can only be found in wind turbines, namely the rotor blades and the mechanical power train.

IEC 61400-24 describes in detail how to protect these special parts of a wind turbine and how to prove the effectiveness of the lightning protection measures.

According to this standard, it is advisable to carry out high-voltage tests to verify the lightning current withstand capability of the relevant systems with the first stroke and the long stroke, if possible, in a common discharge.

The complex problems with regard to the protection of the rotor blades and rotably mounted parts/bearings must be examined in detail and depend on the component manufacturer and type. The IEC 61400-24 standard provides important information in this respect.

Lightning protection zone concept
The lightning protection zone concept is a structuring measure to create a defined EMC environment in an object. The defined EMC environment is specified by the immunity of the electrical equipment used. The lightning protection zone concept allows for the reduction conducted and radiated interference at the boundaries to defined values. For this reason, the object to be protected is subdivided into protection zones.

The rolling sphere method may be used to determine LPZ 0A, namely the parts of a wind turbine which may be subjected to direct lightning strikes, and LPZ 0B, namely the parts of a wind turbine which are protected from direct lightning strikes by external air-termination systems or air-termination systems integrated in parts of a wind turbine (in the rotor blade, for example).

According to IEC 61400-24, the rolling sphere method must not be used for rotor blades themselves. For this reason, the design of the air-termination system should be tested according to chapter 8.2.3 of the IEC 61400-24 standard.

Fig. 1 shows a typical application of the rolling sphere method, while Fig. 2 illustrates the possible division of a wind turbine into different lightning protection zones. The division into lightning protection zones depends on the design of the wind turbine. Therefore, the structure of the wind turbine should be observed.

It is, however, decisive that the lightning parameters injected from outside of the wind turbine into LPZ 0A are reduced by suitable shielding measures and surge protective devices at all zone boundaries so that the electrical and electronic devices and systems inside the wind turbine can be operated safely.

Shielding measures
The casing should be designed as an encapsulated metal shield. This means that a volume with an electromagnetic field which is considerably lower than the field outside of the wind turbine is achieved in the casing.

In accordance with IEC 61400-24, a tubular steel tower, used predominantly for large wind turbines, can be considered an almost perfect Faraday cage, best suitable for electromagnetic shielding. The switchgear and control cabinets in the casing or “nacelle” and, if any, in the operation building, should also be made of metal. The connecting cables should feature an external shield capable of carrying lightning currents.

Shielded cables are only resistant to EMC interference if the shields are connected to the equipotential bonding on both ends. The shields must be contacted by means of fully (360°) contacting terminals without installing EMC-incompatible long connecting cables on the wind turbine.

Magnetic shielding and cable routeing should be performed according to section 4 of IEC 62305-4. For this reason, the general guidelines for an EMC-compatible installation practice according to IEC/TR 61000-5-2 should be used.

Shielding measures include, for example:

Installation of a metal braid on GRP-coated nacelles.
Metal tower.
Metal switchgear cabinets.
Metal control cabinets.
Lightning current carrying shielded connecting cables (metal cable duct, shielded pipe or the like).
Cable shielding.

External lightning protection measures
The function of the external LPS is to intercept direct lightning strikes including lightning strikes into the tower of the wind turbine and to discharge the lightning current from the point of strike to the ground. It is also used to distribute the lightning current in the ground without thermal or mechanical damage or dangerous sparking which may cause fire or explosion and endanger people.

The potential points of strike for a wind turbine (except the rotor blades) can be determined by means of the rolling sphere method shown in Fig. 1. For wind turbines, it is advisable to use class LPS I. Therefore, a rolling sphere with a radius r = 20 m is rolled over the wind turbine to determine the points of strike. Air-termination systems are required where the sphere contacts the wind turbine.

The nacelle/casing construction should be integrated in the lightning protection system to ensure that lightning strikes in the nacelle hit either natural metal parts capable of withstanding this load or an air-termination system designed for this purpose. Nacelles with GRP coating should be fitted with an air-termination system and down conductors forming a cage around the nacelle.

The air-termination system including the bare conductors in this cage should be capable of withstanding lightning strikes according to the lightning protection level selected. Further conductors in the Faraday cage should be designed in such a way that they withstand the share of lightning current to which they may be subjected. In compliance with IEC 61400-24, air-termination systems for protecting measurement equipment mounted outside of the nacelle should be designed in compliance with the general requirements of IEC 62305-3 and down conductors should be connected to the cage described above.

“Natural components” made of conductive materials which are installed permanently in/on a wind turbine and remain unchanged (e.g. lightning protection system of the rotor blades, bearings, mainframes, hybrid tower, etc.) may be integrated in the LPS. If wind turbines are of a metal construction, it can be assumed that they fulfil the requirements for an external lightning protection system of class of LPS I according to IEC 62305.

This requires that the lightning strike be intercepted safely by the LPS of the rotor blades so that it can be discharged to the earth-termination system via natural components such as bearings, mainframes, the tower and/or bypass systems (e.g. open spark gaps, carbon brushes).

Air-termination system/down conductor
As shown in Fig. 1, the rotor blades; nacelle including superstructures; the rotor hub and the tower of the wind turbine may be hit by lightning.
If they can intercept the maximum lightning impulse current of 200 kA safely and can discharge it to the earth-termination system, they can be used as “natural components” of the air-termination system of the wind turbine’s external lightning protection system.

Metallic receptors, which represent defined points of strike for lightning strikes, are frequently installed along the GRP blade to protect the rotor blades against damage due to lightning. A down conductor is routed from the receptor to the blade root. In case of a lightning strike, it can be assumed that the lightning strike hits the blade tip (receptor) and is then dis- charged via the down conductor inside the blade to the earth-termination system via the nacelle and the tower.

Earth-termination system
The earth-termination system of a wind turbine must perform several functions such as personal protection, EMC protection and lightning protection.

An effective earth-termination system (see Fig. 3) is essential to distribute lightning currents and to prevent the wind turbine from being destroyed. Moreover, the earth-termination system must protect humans and animals against electric shock. In case of a lightning strike, the earth-termination system must discharge high lightning currents to the ground and distribute them in the ground without dangerous thermal and/or electrodynamic effects.

In general, it is important to establish an earth-termination system for a wind turbine which is used to protect the wind turbine against lightning strikes and to earth the power supply system.

Note: Electrical high-voltage regulations such as Cenelec HO 637 S1 or applicable national standards specify how to design an earth-termination system to prevent high touch and step voltages caused by short-circuits in high or medium-voltage systems. With regard to the protection of persons, the IEC 61400-24 standard refers to IEC//TS 60479-1 and IEC 60479-4.

Arrangement of earth electrodes

IEC 62305-3 describes two basic types of earth electrode arrangements for wind turbines:

Type A: According to Annex I of IEC 61400-24, this arrangement must not be used for wind turbines, but it can be used for annexes (for example, buildings containing measurement equipment or office sheds in connection to a wind farm). Type A earth electrode arrangements consist of horizontal or vertical earth electrodes connected by at least two down conductors on the building.

Type B: According to Annex I of IEC 61400-24, this arrangement must be used for wind turbines. It either consists of an external ring earth electrode installed in the ground or a foundation earth electrode. Ring earth electrodes and metal parts in the foundation must be connected to the tower construction.

The reinforcement of the tower foundation should be integrated in the earthing concept of a wind turbine. The earth-termination system of the tower base and the operation building should be connected by means of a meshed network of earth electrodes to gain an earth-termination system ranging over as large an area as possible. To prevent excessive step voltages as a result of a lightning strike, potential controlling and corrosion-resistant ring earth electrodes (made of stainless steel) must be installed around the tower base to ensure protection of persons (see Fig. 3).

Foundation earth electrodes

Foundation earth electrodes make technical and economic sense and are, for instance, required in the German Technical Connection Conditions (TAB) of power supply companies. Foundation earth electrodes are part of the electrical installation and fulfil essential safety functions. For this reason, they must be installed by electrically skilled persons or under supervision of an electrically skilled person.

Metals used for earth electrodes must comply with the materials listed in Table 7 of IEC 62305-3. The corrosion behaviour of metal in the ground must always be observed. Foundation earth electrodes must be made of galvanised or non-galvanised steel (round or strip steel). Round steel must have a minimum diameter of 10 mm. Strip steel must have minimum dimensions of 30 x 3,5 mm. Note that this material must be covered with at least 5 cm concrete (corrosion protection). The foundation earth electrode must be connected with the main equipotential bonding bar in the wind turbine. Corrosion-resistant connections must be established via fixed earthing points of terminal lugs made of stainless steel. Moreover, a ring earth electrode made of stainless steel must be installed in the ground.

Protection at the transition from LPZ 0A to LPZ 1

To ensure safe operation of electrical and electronic devices, the boundaries of the LPZs must be shielded against radiated interference and protected against conducted interference (see Figs. 2 and 4). Surge protective devices capable of discharging high lightning currents without destruction must be installed at the transition from LPZ 0A to LPZ 1 (also referred to as “lightning equipotential bonding”). These surge protective devices are referred to as class I lightning current arresters and are tested by means of impulse currents of 10/350 μs waveform. At the transition from LPZ 0B to LPZ 1 and LPZ 1 and higher only low-energy impulse currents caused by voltages induced outside the system or surges generated in the system must be coped with. These surge protective devices are referred to as class II surge arresters and are tested by means of impulse currents of 8/20 μs waveform.

According to the lightning protection zone concept, all incoming cables and lines must be integrated in the lightning equipotential bonding without exception by means of class I lightning current arresters at the boundary from LPZ 0A to LPZ 1 or from LPZ 0A to LPZ 2.

Another local equipotential bonding, in which all cables and lines entering this boundary must be integrated, must be installed for every further zone boundary within the volume to be protected.

Type 2 surge arresters must be installed at the transition from LPZ 0B to LPZ 1 and from LPZ 1 to LPZ 2, whereas class III surge arresters must be installed at the transition from LPZ 2 to LPZ 3. The function of class II and class III surge arresters is to reduce the residual interference of the upstream protection stages and to limit the surges induced or generated within the wind turbine.

Selecting SPDs based on voltage protection level (Up) and equipment immunity

To describe the Up in an LPZ, the immunity levels of the equipment within an LPZ must be defined, e.g. for power lines and connections of equipment according to IEC 61000-4-5 and IEC 60664-1; for telecommunication lines and connections of equipment according to IEC 61000-4-5, ITU-T K.20 and ITU-T K.21, and for other lines and connections of equipment according to manufacturer’s instructions.

Manufacturers of electrical and electronic components should be able to provide the required information on the immunity level according to the EMC standards. Otherwise, the wind turbine manufacturer should perform tests to determine the immunity level. The defined immunity level of components in an LPZ directly defines the required voltage protection level for the LPZ boundaries. The immunity of a system must be proven, where applicable, with all SPDs installed and the equipment to be protected.

Power supply protection

The transformer of a wind turbine may be installed at different locations (in a separate distribution station, in the tower base, in the tower, in the nacelle). In case of large wind turbines, for example, the unshielded 20 kV cable in the tower base is routed to the medium-voltage switchgear installations consisting of vacuum circuit breaker, mechanically locked selector switch disconnector, outgoing earthing switch and protective relay.

The MV cables are routed from the MV switchgear installation in the tower of the wind turbine to the transformer situated in the nacelle. The transformer feeds the control cabinet in the tower base, the switchgear cabinet in the nacelle and the pitch system in the hub by means of a TN-C system (L1; L2; L3; PEN conductor; 3PhY; 3 W+G). The switchgear cabinet in the nacelle supplies the electrical equipment with an AC voltage of 230/400 V.

According to IEC 60364-4-44, all electrical equipment installed in a wind turbine must have a specific rated impulse withstand voltage according to the nominal voltage of the wind turbine. This means that the surge arresters to be installed must have at least the specified voltage protection level depending on the nominal voltage of the system. Surge arresters used to protect 400/690 V power supply systems must have a minimum voltage protection level Up ≤2,5 kV, whereas surge arrester used to protect 230/400 V power supply systems must have a voltage protection level Up ≤1,5 kV to ensure protection of sensitive electrical/electronic equipment. To fulfil this requirement, surge protective devices for 400/690 V power supply systems which are capable of conducting lightning currents of 10/350 μs waveform without destruction and ensure a voltage protection level Up ≤2,5 kV must be installed.

230/400 V power supply systems

The voltage supply of the control cabinet in the tower base, the switchgear cabinet in the nacelle and the pitch system in the hub by means of a 230/400 V TN-C system (3PhY, 3W+G) should be protected by class II surge arresters such as SLP40-275/3S.

Protection of the aircraft warning light

The aircraft warning light on the sensor mast in LPZ 0B should be protected by means of a class II surge arrester at the relevant zone transitions (LPZ 0B → 1, LPZ 1 → 2) (Table 1).

400/690V power supply systems Co-ordinated single-pole lightning current arresters with a high follow current limitation for 400/690 V power supply systems such as SLP40-750/3S, must be in- stalled to protect the 400/690 V transformer, inverters, mains filters and measurement equipment.

Protection of the generator lines

Considering high voltage tolerances, class II surge arresters for nominal voltages up to 1000 V must be installed to protect the rotor winding of the generator and the supply line of the inverter. An additional spark-gap-based arrester with a rated power frequency withstand voltage UN/AC = 2,2 kV (50 Hz) is used for potential isolation and to prevent the varistor-based arresters from operating prematurely due to voltage fluctuations which may occur during the operation of the inverter. A modular three-pole class II surge arrester with an increased rated voltage of the varistor for 690 V systems is installed on each side of the stator of the generator.

Modular three-pole class II surge arresters of type SLP40-750/3S are designed specifically for wind turbines. They have a rated voltage of the varistor Umov of 750 V AC, considering voltage fluctuations which may occur during operation.

Surge arresters for IT systems

Surge arresters for protecting electronic equipment in telecommunication and signalling networks against the indirect and direct effects of lightning strikes and other transient surges are described in IEC 61643-21 and are installed at the zone boundaries in conformity with the lightning protection zone concept.

Multi-stage arresters must be designed without blind spots. It must be ensured that the different protection stages are co-ordinated with one another, otherwise not all protection stages will be activated, causing faults in the surge protective device.

In the majority of cases, glass fibre cables are used for routing IT lines into a wind turbine and for connecting the control cabinets from the tower base to the nacelle. The cabling between the actuators and sensors and the control cabinets is implemented by shielded copper cables. Since interference by an electromagnetic environment is excluded, the glass fibre cables do not have to be protected by surge arresters unless the glass fibre cable has a metallic sheath which must be integrated directly into the equipotential bonding or by means of surge protective devices.

In general, the following shielded signal lines connecting the actuators and sensors with the control cabinets must be protected by surge protective devices:

Signal lines of the weather station on the sensor mast.
Signal lines routed between the nacelle and the pitch system in the hub.
Signal lines for the pitch system.

Signal lines of the weather station

The signal lines (4 – 20 mA interfaces) between the sensors of the weather station and the switchgear cabinet are routed from LPZ 0B to LPZ 2 and can be protected by means of FLD2-24. These space-saving combined arresters protect two or four single lines with common reference potential as well as unbalanced interfaces and are available with direct or indirect shield earthing. Two flexible spring terminals for permanent low-impedance shield contact with the protected and unprotected side of the arrester are used for shield earthing.

Laboratory tests according to IEC 61400-24

IEC 61400-24 describes two basic methods to perform system level immunity tests for wind turbines:

During impulse current tests under operating conditions, impulse currents or partial lightning currents are injected in the individual lines of a control system while supply voltage is present. In doing so, the equipment to be protected including all SPDs is subjected to an impulse current test.
The second test method simulates the electromagnetic effects of the lightning electromagnetic impulses (LEMPs). The full lightning current is injected into the structure which discharges the lightning current and the behaviour of the electrical system is analysed by means of simulating the cabling under operating conditions as realistically as possible. The lightning current steepness is a decisive test parameter.