Taking the grind out of ground rings – a brush with the future of wind power technology

Demand for wind power is growing rapidly across the USA. While it offers undoubted benefits, it also carries common technological issues, including the electrical erosion of ground rings in wind turbine generators. Here, George Finley, Wind Segment Manager with the Electrical Carbon business of Morgan Advanced Materials, explores the challenges and costs, and outlines a pioneering solution.

Although a relative newcomer to the energy market, wind power is growing rapidly as the global demand for clean, efficient and sustainable energy rises. According to the Global Wind Energy Council, the sector grew 25 per cent each year between 2002 and 2007. It’s now estimated that the Global Wind Energy Market will surpass a revenue collection of $170 billion by 2024 (Global Market Insights).

Indeed, the industry is now set for further unprecedented growth and development. Offshore wind turbines particularly are getting more and more powerful by the day. GE has already announced that it will deploy the world’s most powerful offshore wind turbine in 2021, the 12-megawatt Haliade-X. This technological marvel is set to generate 45 per cent more electricity than the largest turbine in 2018’s market.

However, while offshore wind turbines are grabbing all of the headlines, it is onshore wind turbine sizes that produce the most electrical energy. Onshore wind turbines are set to remain at capacities of 2-megawatt and 3-megawatts, meaning volume, and uptime, is the key here. However, onshore wind farm operators face a battle to keep their turbines operational, due to the phenomenon of ground ring grooving.

Ground rings are a feature of wind turbines designed to help divert powerful shaft currents (which may reach 60 amps/1,200 volts or even higher), Their core function is to prevent damage to bearings and other equipment.

Essentially, ground rings work by transferring current through the use of brushes, which are held in contact with the ring usually via a ‘constant force spring’. As long as there is contact, the current will be diverted effectively. Unfortunately however, this contact is hard to ensure, partly because of overall wear and tear resulting from misaligned shafts and components. Brushes also therefore become mis-aligned and may lose contact with the vibrating slip ring.

Even a variation of one or two degrees can make a difference. When this happens, the air gap between the ring and the brush creates a high inductance of the circuit, and arcing electrical currents then eat away at the metal, causing grooves in the slip ring (and also occasionally impacting on bearings). As the grooving develops and metal continues to erode, the gap between the ring and the holder is greater, causing even more instability. This effect is often referred to as ‘footprinting’, ‘ghosting’ or ‘photo imaging’.

Electrically Explosive

Apart from reducing the effectiveness of the ground ring, arcing has a potentially far more damaging consequence. As the metal erodes it becomes like sandpaper, with fine particles ‘dusting’ the brushes themselves. This dust contains conductive material (carbon and copper) which cause a current to flow between phases or phase to ground, leading to destructive flashover and the possibility of an electrical explosion. While this does not typically present a risk to people, it can cause catastrophic damage to the turbine.

The immediate solution is for the ring to be re-machined in-situ or taken away for repair, failing that, for a replacement ring to be installed. Either way, the net result is inevitably downtime and expense. 
A replacement slip ring can cost up to $5,000, while bearings are typically $5,000-$10,000 each. Additionally, when brushes lose contact with the ground ring, expensive electrical equipment and circuit boards can also be damaged, while each day of downtime typically represents an estimated $1,500 of lost revenue.

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

The idea is that if one brush loses contact, the other acts as a fail-safe. In practice however, the brushes are placed too close together and experience a ‘bouncing’ effect which causes both to simultaneously lose contact. Equally, the brush holders themselves are not adjustable and located too far from the ring, potentially causing instability. It’s an improvement, but the underlying problem still remains.

A New Dawn for Wind Power

In search of a definitive answer to this persistent challenge, Morgan’s Electrical Carbon business has developed a pioneering innovation – a fully adjustable drop in replacement ground brush holder system. Instead of two brushes, this new invention deploys four within the same mounting space, facilitating an easy upgrade. Simply unbolt and remove the old holders, and the new ones can be bolted in to replace them.

The brushes themselves are manufactured 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 helps reduce friction chatter (bouncing of the brushes), and therefore reduces dusting and extends the brush life. This proprietary treatment also is not a sticky resin, so dust is less likely to collect, making brushes easier to clean.

The carbon holders are also fully adjustable to increase the distance between brushes and improve stability and contact - typically, they have three times the coverage of the OEM design.

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

As the demand for renewable energy continues to gain traction, it is important that technology develops alongside it. Morgan Advanced Materials’ WTG1 brush holder is a perfect example. Precision machined from aerospace materials, it reduces ground ring damage, enabling the safe and reliable dissipation of dangerous current and voltage spikes. In turn, this prolongs the life of bearings and equipment and safeguards your investment – a sustainable solution for what, after all, is a sustainable energy source.

For more information about Morgan’s Electrical Carbon business and how they can support in the design and installation of brush holders please visit: http://www.morganelectricalmaterials.com/wind