Written by George Finley, Wind Segment Manager
Electrical Carbon Business, Morgan Advanced Materials
While the promise of offshore wind is grabbing many headlines of late, it is onshore wind that is currently producing the majority of the renewable electrical energy in the United States. For the most part, onshore wind turbines are set at capacities of two to three-megawatts, meaning component reliability is key to production.
Given the harsh conditions in which turbines operate, this is typically easier said than done.
It is, therefore, important to ensure all turbine components are subject to regular operations and maintenance checks — even small devices, such as ground rings and brushes.
Understanding ground rings
One operational challenge onshore wind operators face is the phenomenon of ground-ring grooving. Wind-turbine ground rings 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.
Ground rings work by transferring current through the use of brushes, which are typically held in contact with the ring with a constant force spring. This means that as long as there is contact, the current will be diverted effectively. The brush is an electrical conductor that works with slip rings and brush holders, protecting vital components from parasitic currents and static electricity. Worn or low-quality brushes can wear on a slip ring, causing it to degrade prematurely or force early component replacement.
In addition, wear and tear may result in misaligned shafts and components — which also cause wear and tear on components. For example, brushes may become misaligned and lose contact with the vibrating slip ring. Unfortunately, it takes little movement for this to occur and even a variation of one or two degrees can impact proper function. If this happens, the air gap between the ring and brush causes a high inductance of the circuit and arcing electrical currents slowly eat away at the metal. This leads to grooves in the slip ring and, occasionally, will affect the bearings, too.
As the metal erodes and grooves deepen, it creates a growing gap between the ring and holder, causing greater instability between the slip ring and brush. This effect has several names and is referred to as footprinting, ghosting, or photo imaging.
An arcing risk
Arcing has the potential to cause far more damage than reduced ground ring effectiveness. When metal erodes, the result is a sandpaper-like surface with fine dust particles layering the brushes. This dust contains conductive material (carbon and copper), which may cause a current to flow between phases or phase to ground. This may lead to a destructive flashover or, even worse, an electrical explosion.
The result: typically this means catastrophic damage to the wind turbine.
Damage to the turbine components must then be assessed and repaired. For example, the slip ring may require re-machining in-situ or offsite repair. Failing that, replacement is necessary at the cost of up to $5,000 per ring. If new bearings are needed, expect a cost of between $5,000 and $10,000 each. What’s more: when brushes lose contact with the ground ring, costly damage to electrical equipment and circuit boards is ß. And a single day of downtime represents an estimated $1,500 of lost revenue.
To reduce the likelihood of such a destructive event from occurring, wind operators typically use a brush holder that accommodates two brushes — covering some 40° (1/9th) of the ring. The idea is that if one brush loses contact, the other works as a failsafe. However, in practice, this creates a ‘bouncing’ effect between the two side-by-side brushes, called friction chatter, and causes both to lose contact simultaneously. In addition, brush holders come as fixed and unmodified devices and are generally placed a fair distance from the ring, which does little to fix the problem.
An adjustable idea
A new idea to reduce slip ring and brush wear uses a fully adjustable drop-in replacement ground brush holder system. Instead of two brushes, this new invention deploys four within the same mounting space. This means upgrading is easy and simply involves unbolting and removing the old brush holders, and bolting in the new one.
These brushes are durable and manufactured from aerospace-grade 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. The low-friction film reduces friction chatter (the bouncing of the brushes) and dusting, and extends the brush life.
Additionally, the proprietary treatment is not a sticky resin so, if dust occurs, it is unlikely to collect or stick, making brushes easier to clean.
The carbon holders are also fully adjustable to increase the distance between brushes and improve stability and contact. This gap provides up to three times the coverage of the typical OEM design in a turbine. Other features include quick disconnect brush terminals for simple maintenance or replacement. The unit cost of this new brush holder is also significantly lower than for a replacement ring or bearing, and particularly when turbine downtime is considered for component repairs.
Reliability is critical in wind applications that must withstand harsh conditions. A word to the wise: consider wind-turbine components carefully, even the small ones, and mitigate the risk of wear whenever possible.