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Across the USA, demand for wind power is rapidly growing. Although it provides definite benefits, it also experiences common technological problems, which includes the electrical erosion of ground rings in wind turbine generators. Wind Segment Manager with the Electrical Carbon business of Morgan Advanced Materials, George Finley, looks at the challenges and their costs, and outlines a pioneering solution.

Whilst it is relatively new to the energy market, wind power is quickly increasing as the global demand for sustainable, clean, and efficient energy rises. The sector grew 25 percent per year between 2002 and 2007 according to the Global Wind Energy Council. It is now thought that the Global Wind Energy Market will exceed a revenue collection of $170 billion by 2024 (Global Market Insights).

The Global Wind Energy Market industry is set for even more unprecedented growth and development. Offshore wind turbines especially, are becoming more and more powerful every day. GE has already announced that in 2021 it will deploy the 12-megawatt Haliade-X, the world’s most powerful offshore wind turbine. This technological marvel is set to produce 45 percent more electricity than the biggest turbine in 2018’s market.

Nonetheless, whilst offshore wind turbines are stealing all of the headlines, it is onshore wind turbine sizes that generate the most electrical energy. Onshore wind turbines are set to stay at capacities of 2-megawatt and 3-megawatts, meaning volume, and uptime, is paramount. Yet, due to the phenomenon of ground ring grooving, onshore wind farm operators face a battle to keep their turbines in operation.

Designed to help divert powerful shaft currents (which may reach 60 amps/1,200 volts or even higher), ground rings are a feature of wind turbines which have the core function of stopping damage to bearings and other equipment.

Ground rings function by transferring current through the utilization of brushes, which are held in contact with the ring, normally via a ‘constant force spring.’ The current will be effectively diverted as long as contact exists. However, this contact can be hard to verify, partially due to overall wear and tear as a result of misaligned shafts and components. Therefore, brushes also become misaligned and could lose contact with the vibrating slip ring.

A variation of even one or two degrees can make a difference. When this occurs, the air gap between the ring and the brush causes a high inductance of the circuit, and arcing electrical currents subsequently eat away at the metal, causing grooves in the slip ring (and also impacting on bearings occasionally).

As the grooving develops and the metal carries on eroding, the gap between the ring and the holder grows, which results in even more instability. This effect is commonly called ‘footprinting’, ‘ghosting’ or ‘photo imaging.’

Aside from decreasing the ground rings effectiveness, arcing has a consequence which is potentially far more damaging. As the metal gradually erodes it becomes sandpaper-like, with fine particles ‘dusting’ the brushes themselves. This dust carries conductive material (carbon and copper) which will cause a current to flow between phases or phase to ground, resulting in destructive flashover and the risk of an electrical explosion.

Although this does not normally put people at risk, it can result in damage which can be catastrophic to the turbine. The immediate solution is for the ring to be taken away for repair, re-machined in-situ or for a replacement ring to be installed. Inevitably, the net result is downtime and expense.

Bearings are around $5,000-$10,000 each and a replacement slip ring can cost up to $5,000. Plus, expensive electrical equipment and circuit boards can also be damaged when brushes lose contact with the ground ring. Each day of downtime usually represents an estimated $1,500 of lost revenue.

This is an important problem, and manufacturers have set about achieving a solution. The answer has been the introduction of a brush holder which incorporates two brushes, covering some 40° (1/9th) of the ring.

The proposal is that if one of the brushes loses contact, the other acts as a failsafe. However, in practice, the brushes are positioned too near to each other and encounter a ‘bouncing’ effect which causes both to lose contact simultaneously. Equally, the brush holders themselves are located too far from the ring and are not adjustable, potentially causing instability. It is an improvement, but the underlying problem still persists.

Morgan’s Electrical Carbon business has developed a pioneering innovation – a fully adjustable drop in replacement ground brush holder system, whilst searching for a definitive answer to this persistent challenge. Instead of dual brushes, this new invention stations four inside the same mounting space, which facilitates an easy upgrade. Simply unbolt and take away the old holders, and the new ones can be bolted in to take their place.

The brushes are made from an aerospace grade of copper graphite material. The material is specially treated to withstand low and high humidity environments and forms a low friction film, or patina, on the slip ring.

This low friction film then aids in decreasing friction chatter (bouncing of the brushes), and so reduces dusting and extends the brush life. This proprietary treatment makes brushes easier to clean as it is not a sticky resin, so it is less likely to collect dust. To improve stability and contact and increase the distance between brushes, the carbon holders are also fully adjustable – usually, they have three times the coverage of the OEM design.

Other features include quick disconnect brush terminals for easy maintenance/replacement and a full scope of options to meet all of the needs of the machine. The unit cost of a Morgan brush holder is remarkably lower than for a replacement ring or bearing, even more so when other factors like downtime are taken into account.

Morgan Advanced Materials’ WTG1 brush holder is a perfect example of how, as the demand for renewable energy keeps gaining traction, it is crucial that technology develops alongside it.

Precision machined from aerospace materials, it minimizes ground ring damage, allowing the safe and reliable dissipation of dangerous current and voltage spikes. This prolongs the life of equipment and bearings and so safeguards your investment – a sustainable solution for what, after all, is a sustainable energy source.

This information has been sourced, reviewed and adapted from materials provided by Morgan Advanced Materials.

For more information on this source please visit Morgan Advanced Materials.

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