Keeping our wind energy secure

P&E talks to four industry experts about the challenges of wind safety for onshore and offshore wind farms. With EWT’s Eric Bakker, Ricardo Moro of Global Energy Services, Trevor Howes of Orga Aviation BV and Electricon’s Kim Bertelsen.

“Special focus must be on systems that have an influence on turbine control”
-Kim Bertelsen

P&E. How can wind farms be protected against lightening and direct strikes?
Kim Bertelsen. Each individual wind turbine in a wind farm has to have an integrated lightning protection system, which can efficiently intercept with the direct lightning strike. This lightning protection system should conform to the IEC61400-24 standard including air termination systems, down conduction and earthing systems. All systems and subsystems in the protection chain must be tested and verified to prove sufficient protection.

Damages to structural parts, such as blades and main bearing as well as power and control equipment, must be limited. The entire route of the lightning current should be determined by the engineers - and not being just coincidental. A lightning protection system should not only protect against the direct effect of lightning, but also the indirect effect protection should be thought into the design in the early stage.

Special focus must be on systems that have an influence on turbine control and are installed in zones where the lightning impact is high. This is especially in wind sensors, blade sensors and pitch-systems, and must have increased protection installed. The lifetime of all protection measures must be considered since replacement and repair will always be costly. Regular maintenance is necessary, but components with a long lifetime should be preferred.

Trevor Howes. As our obstacle light systems are one of the very few products that are mounted outside and on top of the nacelle these are amongst the first products to be exposed to the effects of lightning discharges. After conducting extensive tests on simulation apparatus, and confirmed through the experience of having thousands of obstacle light systems operating worldwide, we have designed our products to ensure that the direct and residual effects of lightning discharges are safely managed outside the obstacle lights rather than passing to the electrical ground through the obstacle light system, which will often cause critical failure. This does require that the obstacle light systems are properly bonded to a primary ground path during installation - without that it's impossible to be sure that the obstacle light system is not going to be damaged which, if that happens, leaves the wind turbine without the required air safety marking warnings. 

Eric Bakker. Lightning still is one of the most common reasons for catastrophic failure of a wind turbine. Since it is impossible to avoid lightning strike the turbine, well-designed protection is a necessity for wind turbines. The lightning protection systems have improved significantly and EWT offers both an active and a passive system for its customers. The passive system consists of several lightning receptors and a metal tip on each turbine blade, which will guide over-voltage and currents adequately to the ground in case of a lightning strike, minimizing damage to humans and equipment. In addition to this the control system of the wind turbine and vulnerable electronic parts are well protected in order to avoid damage but also to make sure that essential signals will not be disturbed.

The active system predicts and detects lightning in the vicinity of the wind park, a technique currently used at airports and golf courses. The wind park will be switched off when lightning has been detected within a set range. In this way, further damage to the turbine and its environment will be prevented in those rare cases that a lightning strike couldn't be defused by the passive system. This system is primarily used at sites where wind turbines are co-located with very sensitive installations like refineries.

Ricardo Moro. Wind turbines have three lightning protection systems, one in each blade. It consists of a metal piece in the blade tip, which is connected to a cable that extends from that metal piece to the ground in order to conduct the electrical discharge as safely as possible. The substation is protected by a lightning rod more or less as any conventional building. Apart from this, all electrical components, mainly the transformers, are designed and manufactured with an isolation level, which is part of the product specification.

What are the differing challenges between onshore and offshore wind farm safety?
RM. Main challenges in offshore wind farm safety are those related to accessibility - there is no 100 percent safe way to access a wind turbine. The usual way is to carry technicians on a boat from the coast to the turbine. Depending on the distance and the sea conditions this trip can lead to tiredness and sickness. When the boat is close to the turbine, there is a risk in the manoeuvre of transferring those from the boat to the turbine. This operation can only be done when currents and wave levels allow for it - less than two meters waves.

Many efforts are taking place in order to develop safer access methods but an optimum solution has not been found yet. When the sea is rough and boat access is impossible, a helicopter can be used to transfer the technicians from the coast to the turbine. In this case, the helicopter unloads the tech upon a platform situated on top of the nacelle. There is an obvious risk factor in this operation. This leads to higher demands with respect to training -first aid, personal survival techniques, fire prevention, fire fighting and helicopter rescue - and PPE - survival suit, locator beacons.

KB. The main difference is that offshore wind farm has limited and costly accessibility. In case of a minor failure to an electrical component - possibly destroyed by lightning or over voltages - the consequential repair costs and downtime losses will be way out of proportions compared to the cost of an efficient and innovative protection solution.

The lightning strike density - numbers of lightning flashes - is dependant on local weather conditions. There are areas in the world where the turbine will experience several hundreds of direct strikes during its lifetime. It is not a matter of 'if' but 'how many times' the wind turbine is affected by lightning. The safety of personnel staying in the offshore wind turbine during a thunderstorm must be considered. For onshore sites the normal procedure is to leave the wind turbine, but when working offshore such fast evacuation is not an option. A safe location inside the turbine must be defined, where service personnel can stay until the thunderstorm warning is over.

TH. For us the issue of safety relates to the ongoing reliable operation of the obstacle light systems. These systems are used to provide the necessary visual marking of the wind farm to any air traffic that may be operating in the vicinity, often low flying emergency services. Without these the pilots are deprived of their visual cue and at nighttime are essentially flying blind. As Orga has been supplying obstacle-marking systems to the offshore oil and gas industries worldwide for more than 30 years we have brought experience, knowledge and expertise into our product designs. The probability that access to the obstacle light system is restricted by weather conditions and access costs means that we need to design obstacle light systems with high availability times and work closely with our customers to ensure correct installation so that the air safety marking systems withstand the operating conditions encountered in offshore environments.

EB. In offshore parks, the safety situation is much more challenging due to waves, rougher weather conditions and limited accessibility of the turbines. Even though the industry has made tremendous progress on safety, the track record is still significantly poorer than the incident levels acceptable in the oil and gas industry. The offshore wind industry should therefore tap into the expertise of the sub contractors, many from the oil and gas industry, to improve the safety record of the industry.

However, significant improvements can also be made on safety in onshore wind. Keeping the pressure on is key for the reputation of the industry. Within EWT, for example, project managers are incentivized to deliver projects without safety incidents, and strict procedures are in place to help prevent those.

How can technologies ensure turbine reliability and keep costs low?
EB. Turbine technology is key to long-term reliability and wind park economics. Gearboxes are especially critical in this respect. Research (DEWI) shows that 30 percent of the downtime of a wind turbine is due to issues with gearboxes. Also in terms of costs over the lifetime of the turbine, an additional 10 percent of the original capex needs to be budgeted to overhaul gearboxes. Applying direct drive technology, which eliminates the gearbox, will therefore lead to a significant reduction of downtime and operational costs. Other advantages of this technology are higher yields and improved manufacturability, which all contribute to best-in-class cost per kWh levels. As one of the few wind turbine manufacturers in the world, EWT has an extensive track record with this direct drive technology and is applying it in all its turbine models (750kW, 900kW and most recently its 2MW).

RM. It is part of the OEM activity to improve designs for higher reliability, by the right selection of materials and the best product design. Computer-based design and simulation is the basic tool in this stage. Test procedures and test rigs are developed to ensure the quality of the components, such as blades, gearboxes and generators. With respect to testing the whole turbine, prototypes are installed in the field and closely supervised in order to detect any significant problem. Ideally, prototypes are tested for a significant period of time prior to start up of serial production.

There are two types of maintenance technologies. Predictive maintenance is one - there is a range of monitoring techniques that show whether a turbine is working in proper conditions, mainly vibration and temperature monitoring, but also noise or extensometry techniques. Each turbine has its own signature and whenever any of the parameters go out of range, it indicates that a component needs to be replaced or adjusted.

Preventive maintenance is another - the turbine manuals describe the kind of routine controls and maintenance actions that are required. By properly following such indications, the maintenance company can minimize downtime due to unexpected failures. With the proper statistical tools, it is possible to optimize the frequency and scope of those maintenance actions.

KB. By using modern technology in air termination and surge protection - and especially by using new methods in test and verification - it can be ensured that the turbine remains operational during and after a direct lightning strike to the wind turbine structure. Surges and over-voltages, due to lightning strikes or internal switching, are the most common source of isolation breakdown in electrical machines and systems. Such influences can be limited by using modern technology.

There is an increasing demand to use standard 'off the shelf" industrial equipment, but such equipment is normally not designed to withstand the electromagnetic environments in a wind turbine application. Special requirements and environmental zoning must be defined to secure adequate equipment immunity against radiated and conducted disturbances.

TH. As turbine heights increase the potential risk of such structures to air traffic also rises.  Already today's larger turbines are already penetrating the 150m low flying air space where the taller broadcast towers were previously the only remote structures encountered by pilots. As a result of the increased hazard presented by wind farms deploying large numbers of these turbines, national and international civil aviation regulations may mandate the use of high intensity obstacle light systems. Recent developments in technology mean that we are now developing high intensity obstacle light systems with long life solid state LED light sources to provide reliable and low maintenance systems with high availability, rather than incorporating the xenon strobe light sources that have been the only choice until now. These type of developments show how a specialist company, such as Orga Aviation, can support the wind industry by developing products with high product reliability for increased safety, and with lower maintenance requirements whilst meeting the more demanding regulatory requirements resulting from the use of larger turbines.

How is wind energy expanding and what is needed to cope with the increasing demand?
TH. As larger numbers of turbines are installed, and as these turbines are increasing in height, the visual impact of the turbines, and specifically of the safety marking obstacle light systems, often becomes an issue in the community impact assessment. The regulations governing the type and intensity of obstacle lights required are based on those needed under worst-case visual flying conditions, and differ from country to country. To help solve some of the apparent conflict between adequate safety marking and visual impact, the time is already here when an industry lead interaction with the civil aviation regulatory community is overdue. Consideration of how to incorporate the possibilities today's technology brings to enable us to operate the obstacle light systems at settings that are applicable to the ambient flying conditions, and even with consideration of the detection of air traffic in the vicinity of the wind farm, would help to overcome many perceived objections.

RM. Europe tends to reduce growth. There will be an increasing number of re-powering projects in wind farms that were constructed 10-15 years ago, which consist of replacing old turbines with new ones that provide much higher output. US, China and India are the new high growth markets. The development of the US market is conditioned to the successful implementation of Obama's plan for renewables.

Favourable legislation is needed. In order to develop wind energy generation, laws and regulations must be passed that incentivise investment in this kind of plants and permit the construction of wind farms in certain areas. Credit availability is also important. The current lack of credit available is reducing the number of projects under development. Not only small investors but also main utilities are slowing down their wind new projects. In some regions a more robust grid is necessary in order to evacuate the additional energy coming from new wind farms. This kind of investment normally requires a long administrative process, which delays the development of new wind energy plants. Finally, wind predictability is key in order to facilitate integration into the overall electrical system. Contrary to other generation systems, it is difficult to make short-term predictions about the energy production that a wind farm will generate. Predictability is a requisite by the electrical authority of each country in order to properly plan the energy generation mix at any time.

EB. The number of installed wind turbines is rising rapidly, driven by societal pressure to cut down greenhouse gas emissions and reduce energy dependency. The challenge will be to integrate those large numbers of turbines in the existing infrastructure, since many of the transmission networks are congested. One way to cope with this is to connect turbines to the weaker distribution networks. To do so, turbine power characteristics need to support those networks. The full capacity synchronous generator from the EWT DirectWind turbine has those characteristics and can therefore be integrated relatively well in weak grids. Obviously, national governments need to come up with a well-coordinated plan to upgrade the national grids as well, but that will take several years.

KB. Wind turbines are constantly increasing in size and complexity. The wind turbines are counted on as power plants in the overall power production planning. If a sudden small interruption occurs it cannot be tolerated that the turbine is disconnected - leaving the grid on its own. The modern wind turbine has to stay connected also during a thunderstorm - and this can no longer be claimed as force majeur. It is decided that these machines should be running out there and this demands the same approach to lightning protection, as well as known from conventional onshore power plants. 

Trevor Howes is General Business Manager at Orga Aviation BV, responsible for Orga Aviation's activities as a leading provider of obstacle marking systems worldwide. He has 30 years experience of advanced technology manufacturing companies, with previous R&D, manufacturing and commercial roles in lithium battery and photovoltaic industries before joining Orga 15 years ago.

Eric Bakker has been CEO of EWT since April 2009, joining the company from BP Alternative Energy where he was President of Wind Power Europe, Middle East and Africa. At BP he has been responsible for the development of over 700 MW of wind energy and two M&A transactions leading to one of the largest wind development portfolios in the US.

Ricardo Moro is Chief Executive Officer at Global Energy Services, an independent services provider to the energy sector and world leader in the wind market with 4400 employees active in Europe, North America and North Africa. He is an industrial engineer and has over 20 years' experience in the renewable energy sector, both in manufacturing and in service activities.

Kim Bertelsen is the owner of Electricon and a lightning protection expert. He has worked with lightning protection and system reliability for more than 16 years, the last 10 years in the wind turbine industry. Kim is participating in the Danish National Committee of the IEC and has a seat in the International IEC PT 24 group responsible for wind turbine lightning protection.