Lightning Protection for Wind Turbines

Transcription

1 Lightning Protection for Wind Turbines Electrical Engineering & Computer Science Article June 2017 Authors Alex Z. Kattamis, Ph.D., P.E., CFEI and Matthew Pooley, Ph.D. Introduction Over the past few decades, the number of wind farms has grown dramatically, beginning with the first large (utility-scale) wind farm installed in California in By 2008, the U.S. installed capacity reached 25.4 Gigawatts and according to the U.S. Department of Energy, the installed capacity of wind power in the U.S. reached 74 GW in 2015, accounting for 4.7% of generation in outpacing solar at 0.6%.[ 1 ] A similar trend is taking place worldwide, reaching 456 GW in 2016.[ 2 ]. Lightning plays a lead role in the failures and outages at wind farms. In 2012, lightning accounted for 23.4% of wind turbine failures.[ 3 ] Therefore, it is critical that the design and installation of a lightning protection system (LPS) for the wind turbine be done correctly The LPS for the protection of residences and other structures generally consists of four main components: a network of strike termination devices (lightning rods); a network of conductors to move lightning energy from the strike termination devices toward earth; a network for ground terminations (ground rods); and equipotential bonding and surge protection devices. Relevant standards such as IEC , Wind turbine generator systems Part 24: Lightning protection, should be applied in order to develop a robust LPS for wind turbines. Damage Mechanisms In the case of wind turbines, there are a few issues specific to their intrinsic properties and geometries. Certain components, such as the tower and earth grounding system, are unremarkable as the challenges of protecting these portions do not require particularly inventive engineering. Herein we will focus on the challenging aspects, including blades, nacelle enclosures, electronics within the nacelle, and bearings. Figure 1 shows the various components of a wind turbine that are to be discussed Figure 1. Diagram of the main components of a wind turbine

2 Damage to each of the components in question have different sensitivities of various modes of failure and can lead to different types of outages: 1. Blades: lightning strikes can cause structural damage resulting in catastrophic failure and long-term outage. 2. Nacelle enclosure: electrical arcing and localized currents can cause physical damage. 3. Electronics within the nacelle: electronic components that form part of the lightning path to Earth can experience damaging currents; in addition, these systems can be damaged by induced electromagnetic fields cause by time-varying lightning currents. 4. Bearings: lightning current and arcing can cause surface damage to bearing components, leading to long-term degradation and reduction in lifetime. Blades Generally, wind turbine blades are fabricated from composite, non-conducting materials. This means that a blade taking a direct lightning strike cannot easily absorb and transfer the electrical energy, resulting in often-catastrophic mechanical damage. See Figure 2. Figure 2. Example of a wind turbine blade destroyed via a lightning strike Furthermore, many modern blades have hollow structures to reduce total blade weight. It is typically preferable for a lightning strike to travel over the surface structure of a blade rather than through the internal structure. This is because lightning arcs through the interior structure of the blade, which can damage structural supports or generate gas that may be trapped, are more likely to lead to catastrophic failure of the blade.

3 Manufactures have developed methods to mitigate the problems introduced by non-conductive materials and hollow blade designs. One approach is the incorporation of internal down conductors, conductive surfaces and/or meshes, and lightning attachment points as shown in the Figure 3. Figure 3. Internal conductor layout including lightning attachment point and down conductors The use of a conducting outer surface may seem like an easy way to minimize damage to blades during lightning strikes. The option of using a metallic coating on the exterior surface of the blade, however, is often not feasible because the coating must be thick enough to carry the lightning current without incurring damage, and this adds substantial weight to the blades. Furthermore, problems of erosion and corrosion have occurred to such coatings when impacted by lightning. [4] To overcome this engineering challenge, a multi-component system of surface-conductors, lightning receptors, and internal conducting structure may be used. Typically, a conducting mesh may be incorporated into the tip region of the blade that, due to the geometry, is often the location at which lightning strikes. The conducting mesh is connected via lightning receptors to an internal down conductor which enables the lightning current to travel to the pitch and yaw bearings and then into the blade hub. This solution requires conducting material to cover only the top portion of the blade, therefore drastically reducing the total weight of the mesh material. Nacelle and Enclosed Electronic Systems The next critical part of the lightning protection, as it relates to wind turbines, is the protection of the nacelle and the components within. The nacelle is a housing that encloses the generator, gear box and associated electronic systems as shown in Figure 4. Figure 4. Example nacelle and internal components

4 The nacelle must provide protection from a direct strike as well as protection against any induced voltage transients caused by a strike to the blades. Electronic systems within the nacelle enclosure include: control systems, power regulators and distribution electronics, SCADA systems, and communication systems. These semiconductor-based electronics are generally sensitive to surges in voltage or pulses of current. The turbine will shut down if critical electronic systems are damaged. Such electronic systems are not only susceptible to the direct currents of a strike but also to induced currents. Typical approaches to protecting these electronic systems include: the nacelle acting as a Faraday cage; SPDs; the use of optical links for communication; and, equipotential bonding. The nacelle housing, if conductive, can be used as a Faraday cage protecting the electronic systems from both direct strike and induced voltages. For the case of non-conducting nacelle materials, such as GRP, metal components such as meshes can be implemented to create a Faraday cage. Regardless of material or cage configuration, it is critical that the conducting portions be properly bonding to the nacelle bed-plate and to the tower so that the resistance of the electrical pathway to Earth is minimized. For systems that do not require physical connections outside of the Faraday cage, optical interconnects may be used as a bridge between protected electronics within the Faraday cage and the outside environment. Optical connections may be appropriate for communication systems; however, it is not practical to use optical connections for all pathways that traverse the Faraday cage boundary. For example, cables that transfer power from the turbine generator require a physical conductive connection. SPDs can be installed for wires and cables that cross the boundary of the Faraday cage. These can help minimize the magnitude of over-voltages or current spikes to which electronic systems may be exposed. SPDs are unable to fully isolate electronic systems within the Faraday cage from lightning strike effects, and damage to electronic systems within the Nacelle is a leading cause of turbine failure. Consequently, when designing these electronics systems, the practicalities of repairing these systems should be considered. For example, electronics systems susceptible to damage should be accessible and designed in a modularized way to enable convenient replacement of effected components. When lightning current flows towards Earth, any bottlenecks due to local regions of the path that have higher impedance can cause the current to arc between components as the current impulse travels down the turbine structure. Equipotential bonding is the practice of electrically connecting regions to minimize such arcing. In particular, equipotential bonding of components within the nacelle, and between different portions of non-conducting nacelles, may be used to reduce arcing between these regions during a lightning strike. Bearings Bearings are used to enable rotation of the blade pitch/yaw, as well as rotation of the blade-hub. The bearings provide the physical connection between the blades and the blade-hub, and between the bladehub and the nacelle. Consequently, the bearings are often included in the most direct electrical pathway from a lightning strike location to the Earth. Generally, 100% of the lightning current will pass through the pitch and yaw bearing of a blade that receives a strike, and 80% of lightning current transfers into the nacelle via the low-speed bearing.[ 5 ] Fortunately, bearings provide a relatively robust electrical pathway and do not often undergo catastrophic failure due to lightning current; however, bearings may be damaged during lightning strikes. For example, arcing to and from bearings through lubricating fluid or air-gaps can cause pitting on the bearing surface, and surface coatings may be damaged by the heat caused by high current flow during a lightning strike. These damage mechanisms may reduce the operational lifetime of a bearing.

5 Though bonding straps and slip rings can, in principle, be used to provide alternate parallel electrical pathways by which current may be diverted around the bearings, such solutions are in practice unreliable and introduce additional engineering complexity. For this reason, it is typical for the bearings to form part of the pathway to Earth for a lightning strike. In fact, a direct pathway taken through the bearings is often the preferred path to Earth because the bearings are generally less susceptible to lightning-caused damage than other components. Conclusion With the growing use of wind turbines for power generation, the role of LPS to limit outages and system downtime will become increasingly critical. In addition to designing wind turbines that can handle minimal damage from lightning strikes, it is also important to consider designs that minimize the time, complexity, and cost of repairs. Reduced probability of damage, combined with cheaper and faster repair mechanisms, is likely to significantly reduce the economic impact of unavoidable lightning strikes. Exponent s team of consultants can assist in mitigating the risk of lightning strike damage to wind turbines. Prior to a lightning event, Exponent personnel can advise regarding structural protection mechanisms, such as grounding electrodes and equipotential bonding, as well as electrical protection for internal components, such as electrical isolation and modifications to electrical systems to reduce complexity of future repairs. Following a lightning event, Exponent can assess damage and investigate failure mechanisms to suggest modifications that may increase robustness to future strikes Gcube-Insurnace. GCube top 5 US wind energy insurance claims report, 2012 available at 4 M.I. Lorentzou, N.D. Hatziargyriou, I. Cotton Key issues in lightning protection of wind turbines WSEAS Conference On Circuits, Greece (2004) 5 Ibid