Bearings must withstand some tough conditions, one of the most challenging of which is inside a wind turbine generator. As well as punishing environmental conditions, it’s difficult and costly to change these bearings. They must withstand whatever is thrown at them.
In addition, there is another challenge which lies in the nature of electrical machines, including generators. When electrical current passes through rolling bearings, there is a risk of electrical erosion which can threaten machine reliability. This erosion can damage and degrade bearings especially frequent converter driven machines.

Outdoor conditions

The first sign of the problem is usually a temperature rise. Condition monitoring systems will pick this up, though it will manifest itself in other ways too such as through the appearance of blackened lubricant (which has been burnt due to the raised temperature).
At the same time, bearing vibration will start to occur. This will be detected by increased machine noise, or picked up by vibration monitoring sensors, if they are installed. It happens because the high frequency current flowing through the bearing is causing micro-pitting on the surface, leading to the vibration.
Generators used in wind turbines are even more prone to damage: they must survive punishing outdoor conditions (including fluctuating temperatures, changing load conditions, and salty sea air); and their remoteness means they must be effectively maintenance-free.
The only way to get 100% confirmation that bearing damage has been caused by electrical currents is to dismount the bearing and send it to its manufacturer for analysis where it may need to be cut and analysed. If electrical erosion is confirmed as the underlying reason, this at least means that the problem has been diagnosed and can be dealt with.
The first step towards a reliable solution is to ensure that the electrical system is properly set up. Including power supply, switching elements, cables, brushes and grounding.
It is also vital to use the right components to achieve a finely tuned system. These will not completely solve the problem, but will help to extend bearing life in the face of factors such as current leakage.

Insulated success

An added layer of security, helping to protect the bearing and lengthen the life in such situations, is to use an insulated bearing. A special coating prevents the passing of electric current, even under conditions of high humidity. As humidity reduces the function of standard insulation layer, the new protective sealant on SKF’s Insocoat is particularly useful in countries with challenging climate conditions as found in China and India.
These bearings are widely used in wind turbine generators. They have an aluminium oxide coating that is applied using a special plasma spray process. Ensuring an even thickness of coating helps to offer higher protection as current will always find the ‘weakest point’. The single layer can be applied either to the exterior surface of the inner or the outer ring.
For generators and larger bearings in general coating on the inner ring is preferable, and is available for most sizes of bearing. However, it is wise to check in advance that it is available for specific applications. Coating the inner ring will lower the capacitance of the bearing system, and therefore raise the impedance, and so improve protection against current effects.

Extend bearing life

Insocoat bearings work over the same temperature range and have the same size as standard bearings, so can be substituted for them directly so there are no problems with mounting the bearing. Swapping out standard bearings and replacing them with Insocoat equivalents is a straightforward process, requiring no special tools or conditions. 
To date, SKF has offered these bearings with a 100micron layer. However, a new generation is being rolled out with a 300micron layer which will provide a greater level of protection.
Unfortunately, electrical currents will always win out in the end, so any solution based on insulating layers can only alleviate – not eliminate – problems involving stray currents. However, the added protection given by SKF-Insocoat bearings helps to extend bearing lifetime and prevent machine downtime, boosting the economy and efficiency of wind turbine energy generation.
Samantha Joubert
Tel: (011) 821 3602