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Living up to the lightning challenge

As highly efficient power plant offering an availability rate close to 100%, wind turbines must be efficiently protected against lightning. Poor surge protection solutions are currently causing interruptions in production that are leading to unacceptable costs for wind plant owners

Wind turbines have always been at moderate risk of being struck by lightning, but as they grow taller and larger the exposure rate has increased significantly. What is not increasing at the same rate is the industry's willingness to deal with the challenges of lightning and how best to protect wind turbines from strike damage. Simply meeting standards for lightning protection of wind turbines laid down by the likes of the International Electrotechnical Committee (IEC) is no guarantee of a safe haven. Damage from lightning is an increasing problem for wind turbine owners.

Thunderstorms and lightning in connection with wind turbines are not fully understood. A lightning hit is not easy to capture and measure, making the electrical parameters difficult to categorise and standardise with an accurate statistical background. Even though the commonly used standards give electrical parameters of lightning events, they generally represent only data from downward lightning strikes, which are the minority of those expected to affect large wind turbines (box). Protection solutions need to start going beyond the minimum requirements of the existing standards.

Researching the effects of a direct lightning strike on what are referred to as "air termination points," such as blade receptors and rods where the lightning strikes the wind turbine system, has become a priority for many leading manufacturers, as has the development of lightning protection equipment that can sustain multiple and repetitive lightning strikes. But it has also become evident that lightning strikes, repetitive or not, produce indirect effects -- overvoltages induced into the electrical system that in cases of inadequate protection lead to serious damage of the wind turbine's electrical and electronic components. To reach an acceptable degree of reliability, in line with conventional power plants, critical electrical and electronic components need to be protected using more advanced, effective protection technologies.

Costly downtime

In most cases it is insufficient to use commodity protection equipment and/or conventional protection concepts for the protection of today's state-of-the-art turbines. Lightning and surge protection is a challenge for wind turbine manufacturers and wind farm operators. The lightning strike density on wind farms is far beyond what would normally be expected in a conventional power plant -- and yet the expectations of operational reliability and low maintenance cost for wind farms are high.

Any interruption in power generation is costly for a wind farm owner, especially on sites where low accessibility or bad weather conditions can result in standstills for weeks or even months. Manufacturers of modern machines are highly focused on designing systems to avoid all possible failures. These days, assessment of lightning risks should form an integral part of any wind farm project business plan.

Statistical data, useful for estimating the lightning strike density and consequently the number of direct strikes per year, is available for most geographical locations around the world. The probability that lightning will cause serious damage must be considered along with the electrical protection design and concept. Robust surge protection design should minimise lightning damage and associated power production losses.

Surge protection

Electrical and electronic components cannot withstand the effect of direct and multiple hits by lightning. For this reason wind turbines are equipped with surge protection devices (SPDs). These modules are generally installed on the power supply connection to the equipment between the live conductor (phase) and the protective earth. In three phase systems, a SPD module is installed between each of the individual phase conductors and the protective earth. When a surge arises on the power terminals after a direct strike on a turbine, the SPD is able to clamp the voltage to a low safe value. During a surge, the associated energy will dissipate in the SPD and not towards the equipment. If everything is designed correctly, the function of the SPD and the operation of the equipment will continue as if no strike had occurred and the wind turbine will carry on producing power.

SPDs used in wind turbines must be able to withstand such multiple strikes without significant wearing or malfunctioning. But the surge protection commonly used today is based on conventional SPD technology, which cannot be expected to reach this level of performance. A typical issue with conventional SPDs is that they start to wear out after one or a few surges due to energy heating. Suddenly, during one of the next surge events, these SPDs reach their "end of life mode," leaving the equipment unprotected during the current and the next surge events.

Conventional SPDs based on metal oxide varistors (MOVs) were originally designed for light industrial applications. They have been used in wind turbines for many years. But little further development seems to have taken place to improve and modify MOVs for the specific challenges posed by their use on wind turbines.

Problem identified

During a fatal surge event, when the SPD suddenly expires, it should ideally turn into a short circuit mode, where the power system's current would start to flow through the SPD from the phase conductor to protective earth. But most manufacturers of conventional surge protection devices are employing fuses to simply disconnect the SPD to avoid unsafe end of life modes. As a result, during lightning storms there are situations when the protectors are disconnected -- actually protecting themselves and as a result leaving the equipment they are supposed to be protecting totally unprotected. The approach is clearly wrong and must be taken into consideration when protecting mission critical systems.

With conventional methods of lightning protection identified as lacking, wind power project developers and wind turbine manufacturers need to consider using more sophisticated lightning protection solutions. If the use of fuses (figure 1) could be avoided and if proper design could lead to SPD lifetime matching that of the wind turbine, failures would be fewer and maintenance requirements reduced.

As well as being far more robust and better designed for the job, protection components must also be designed to stay relatively cool even under harsh lightning surge conditions. Under no circumstances must they heat up to the extent they cause a fire in the wind turbine nacelle. All these considerations point to the need for more innovative protection concepts. This may well demand radical changes in the way SPDs are designed. Expecting an SPD to consist of no more than a small narrow plastic housing containing all necessary surge suppression components is not the right approach.

Cost effective

Properly designed surge protection will always be cost effective, even if using components of a higher price than conventional parts. If the surge protection device can be installed without the need for overcurrent devices such as fuses, the cost of these parts will easily compensate for a higher priced SPD.

Integrating parallel MOVs in a single housing and simply multiplying the individual capacity rates to obtain a higher total suppression capability will not work as intended. MOVs are non linear devices so it will never be possible to achieve effective protection with more MOVs in parallel (figure 2). One of the MOVs will always start conducting before the others. This will result in an immediate fall in surge rate capability since the rest of the parallel MOVs will never support the first MOV as intended. The first working MOV will fast be overloaded, damaged, and will soon fail.

New Methods

The good news for the owners of wind plant and turbine manufacturers is that new technologies are coming on to the market which are able to conduct high surge currents of more than 100 kA many hundreds of times without any significant wearing or erosion. These new SPDs feature design parameters that preclude common failures associated with conventional SPD technology, such as the elimination of internal fuses or thermal disconnects. The incorporation of a robust industrial grade MOV in an aluminium housing offers lifetime protection against surges by resolving the ageing issues which seriously impact the performance of traditional SPDs.

The new protection methods have already been tested for safe operation to high levels of short circuit currents and can sustain multiple and successive lightning strikes and power surges without requiring maintenance. They are classified as Class I SPDs according to the international IEC 61643-1 standard for surge protection devices and in the United States are fully compliant to the new UL 1449 second edition safety standard, which was revised in February. In addition, the protection modules can absorb and efficiently dissipate intense power surges.

Wind turbine customers should not be expected to accept anything less than the best system-protection knowledge and technology available. With wind turbine manufacturers clearly prepared to provide this, sub suppliers of protection components should also be expected to keep up with today's technology evolution.

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