Bearing failure in wind turbine and lubrication and wear.

Abstract: Premature bearing failures of wind turbine generators are occurring frequently. The reasons are mani2 fold, but based on application studies and bearing investigations two main root causes have been identified: electrical current passage, electrical erosion respectively, due to frequency converter supply of doubly - fed induction generators and lubrication and wear related problems. Strives to give an overview on typical root causes for premature bearing failures in wind turbine generator applications and introduces extra - large (XL) hybrid deep groove ball bearings as a new SKF solution to significantly reduce generator bearing failures and to increase the overall reliability and operating availability of wind turbines.

Key words: wind turbine generators; bearings; solution

0 Introduction

Power electronic technology has been applied more and more in variable speed control. This technology has replaced many traditional speed control methods, such as mechanical gearbox drive and hydraulic drive. The application of frequency converter enables the motor to adjust speed freely from zero to rated speed and keep it running at a suitable working point. Therefore, the efficiency and dynamic response capacity of the motor are improved and the energy consumption is reduced. The same technology is also applied to wind turbines. In the past 5 or 6 years, the "doubly fed generator" in large-scale wind power generation equipment has become a typical application and has become more and more popular. This technology can enable the wind turbine to operate in a relatively wide speed range. The rotor winding of the doubly fed generator is powered by the frequency converter. The motor can optimize the power generation energy and maintain a stable line frequency under the condition of changing the rotor speed, and regulate the active power and reactive power. However, due to the introduction of the frequency converter system, the bearing failure caused by overcurrent occurs frequently and becomes the main cause of the early failure of the generator. At the same time, it is confirmed that another main reason for bearing failure is lubrication and corresponding wear.

This paper expounds the fundamental cause of early bearing failure in the application of wind turbine, and introduces the super large hybrid ceramic ball bearing technology. This technology proposed by SKF can significantly reduce the bearing failure in the generator and increase the overall reliability and operation stability of the equipment.

1. Bearing failure in wind turbine

Among the parts of various mechanical equipment, the bearing is the most important part. The failure of the bearing in the motor may also be caused by many other reasons, such as exceeding the expected load, improper lubrication, improper handling during transportation and shutdown, electric corrosion, damage caused by installation and disassembly, internal pollution of the bearing, seal failure, improper tolerance and matching of the shaft and bearing chamber, etc. Each factor will cause different damage, and at the same time, will leave their own unique marks in the bearing. Through the study of application examples and bearings, it is found that the bearing failure in wind turbines is directly or indirectly related to overcurrent, lubrication and wear.

1.1 Electric corrosion

In the doubly fed generator supplied by frequency converter, the problem of current flowing through the bearing is very common. This phenomenon is the so-called electric corrosion. Electrical corrosion of bearings usually occurs when current flows from one raceway to another through the rolling elements. The damage degree of electric erosion to the bearing depends on the discharge energy and duration, but the damage effect is basically similar, including: small electric erosion pits on the rolling elements and raceways, rapid degradation of lubrication, washboard pattern in the second stage of failure and corresponding bearing failure.

The heat generated during the discharge will melt the bearing material surface and produce pits. Meanwhile, the molten metal chips will transfer and leave the original position. The metal on the surface of the etched pit will be re hardened and become more brittle than the previous bearing material. Below the re hardening layer is an annealed layer, which is softer than the surrounding material. The damaged bearing surface looks dark and is characterized by many small melting pits. These tiny pits are all over the surface of the rolling element and the raceway. The size of the pits is very small. Whether on the inner ring, the outer ring or the rolling element, its diameter is usually only 5-8 μ m. Therefore, the shape of these pits can only be seen under a very high power microscope.

In addition, the current discharge also leads to the degeneration and rapid degradation of the internal lubrication of the bearing. The high temperature makes the additives in the lubricating machine react with the base oil, resulting in the combustion and carbonization of the base oil, and the additives will be quickly exhausted. Therefore, the lubricating machine will become harder or even blacker (Fig. 1) and cannot meet the requirements of lubrication. The rapid failure of lubricating machine is also a typical failure mode caused by overcurrent.

Fig. 1 segment of ball bearing cage (from lubrication to blackening due to overcurrent)

1.2 Lubrication

Sufficient lubrication is an important condition for stable operation of rolling bearing to reach its service life. The role of the lubricant is to form a protective oil film, separate the rolling contact surface, and prevent direct contact between metals. The lubricant shall also protect the corresponding parts from corrosion. When lubricating grease is used as a lubricant (commonly used in wind turbines), its function needs to be expanded, such as protecting the bearing from solid particles, dust and water pollution.

Grease has some important properties, including viscosity, oil film forming ability and density. The most important factors affecting the oil film thickness are the bearing size, speed, temperature, load and base oil viscosity. In order to obtain some special properties of lubricating grease, some additives are often required, for example, anti-rust additives can prevent metal rusting and oxidation of lubricating grease.

Wind turbines are installed in all parts of the world, experiencing different climatic environments and adapting to different environmental changes. For example, the ambient temperature is related to the region, latitude and season where the wind turbine is installed. Similarly, the humidity of wind farms installed in the North Sea and near the desert will be very different. The lubricant in the wind turbine should adapt to these changes. Sometimes the parameters under specific working environment (such as temperature) are very difficult to predict accurately, resulting in poor lubrication performance due to improper selection of lubricating grease. Insufficient lubrication will lead to fatigue and wear of the metal surface, thus reducing the life of the bearing. If the lubricating film between the rolling element and the raceway is too thin, the metal surface will not be fully isolated, and direct metal to metal contact will occur. Such a situation may be caused by pollution, insufficient lubrication, etc.

It is very important to supplement the lubrication of the bearing regularly to ensure its good operation and reach the expected service life. Generally, in the installed wind turbines, the re-lubrication of the generator is carried out manually in routine maintenance. In this process, the workers will climb to the top of the tower for work every 6 months. Sometimes, during routine maintenance, the re lubrication of the generator will be forgotten; Sometimes such time interval is too long for the actual working condition of the generator bearing.

1.3 Wear

Generally, there is little wear inside the bearing. When the external particles enter the bearing or the lubrication is insufficient, it will lead to wear. Vibration of the bearing may also cause wear during shutdown.

The application of wind turbine should not only adapt to the harsh environment during operation, but also adapt to the shutdown condition during maintenance. Due to the model, the bearings in the generator are usually open deep groove ball bearings. The bearing is protected by an external labyrinth seal. Even so, there is still a risk of contaminants entering the bearing chamber. The most common sources of pollution include: pollution caused by insufficient lubrication (such as dry high-quality particles), damaged seals, environmental pollution particles, and pollution caused by improper installation and replacement of bearing chambers. These pollution particles will cause fatigue and spallation on the raceway of the bearing after rolling of the rolling body (Fig. 2).

Fig. 2 traces on the raceway surface caused by rolling over contaminated particles

During transportation, if the rotor shaft of the generator is poorly fixed and the vibration occurs in the clearance of the bearing, these may damage the bearing. Similarly, if the generator is in the shutdown state, the vibration over a period of time will also cause bearing damage. It is very common that the wind turbine designed before still does not stop rotating when it is stopped. Therefore, in the updated design, the blades still rotate slowly during shutdown. However, when the generator is stationary, there is no lubricant between the parts in contact with each other in the bearing to form an oil film, and thus direct metal contact between the rolling elements and the raceway is generated. If the external vibration is introduced again, the vibration will cause the rolling element to have a very small displacement with respect to the raceway. Under such displacement, the small cold welding points and wear between metals will damage the raceway and cause fretting corrosion. Such damage usually occurs at the place where the rollers are equally spaced, showing changes in color or gloss. This damage is the so-called "pseudo Brinell indentation" (Fig. 3).

Fig. 3 pseudo Brinell indentation on cylindrical roller bearing caused by shutdown vibration

2. Mixed ceramic bearing

The hybrid ceramic bearing (Fig. 4) is composed of a bearing ring made of bearing steel and rolling elements made of bearing grade silicon nitride material. Silicon nitride is a kind of ceramic material, which has many beneficial characteristics: high hardness, electrical insulation, low density and stability; It also has excellent electrical insulation characteristics. Hybrid ceramic bearings have higher speed capability and longer service life under most working conditions compared with ordinary all steel bearings.

Fig. 4 hybrid ceramic bearing composed of bearing steel ferrules and bearing grade silicon nitride (ceramic) rolling elements

A very important characteristic of silicon nitride material is that it has very good electrical insulation performance. This performance prevents the bearing from being damaged by overcurrent, and increases the service life of the bearing in the variable-speed drive motor supplied by the frequency converter. The density of silicon nitride material is 40% of that of steel material, which makes the rolling body of the bearing lighter and has less inertia, which means that the force on the cage is reduced during rapid start and stop, and the bearing friction is reduced when compared with the all steel bearing during high-speed operation. Lower friction means lower operating temperature, longer re-lubrication time interval and lubricant life. Therefore, the hybrid ceramic bearing has better performance at high speed. SKF hybrid ceramic bearing is not very sensitive to insufficient lubrication, which makes it have longer service life than all steel bearings under more severe dynamic environment or under low operating viscosity lubrication conditions.

Compared with steel, silicon nitride material has higher hardness and greater elastic modulus, which increases the robustness of the bearing and extends the service life of the hybrid ceramic bearing in the polluted environment. When operating in poor lubrication and harsh environment, the performance of hybrid ceramic bearing on wear resistance is far better than that of all steel bearing. On the other hand, the ball made of silicon nitride material with the same size has a smaller thermal expansion than the ball made of steel, which reduces the sensitivity of the bearing due to the thermal distribution and makes the preload control more accurate. In order to ensure the best quality, SKF uses a set of complex technical requirements for silicon nitride materials for bearing rolling elements to control the quality. This technical requirement includes material strength, macro and micro structure of material, hardness, rigidity, fatigue performance of rolling contact and appearance of finished surface of rolling element. Each component of the hybrid ceramic bearing is selected according to the high requirements of the silicon nitride material of the rolling element.

The above characteristics of the mixed ceramic material can solve the problems in the wind turbine as described in the first part. SKF's super large hybrid ceramic deep groove ball bearing is a solution that can greatly reduce the early failure of bearings in wind turbines.

3. Super large mixed ceramic deep groove ball bearing

From a technical point of view, hybrid ceramic bearings are the most reliable solution for wind turbine applications. Because it not only has a very good electrical insulation material, it can completely eliminate the high-frequency current flowing through the bearing, but also has excellent tribological performance under severe operating conditions.

SKF has developed a series of super large hybrid ceramic deep groove ball bearings (see Table 1). This series of products optimizes the product performance for the application of wind turbines and covers the main models in the mainstream wind farms.

In the traditional design, two deep groove ball bearings of the same model are arranged at the driving section and the non-driving end of the wind turbine. In order to avoid over-current and solve the problem of electric corrosion, it is suggested to replace all steel deep groove ball bearings with super large hybrid ceramic deep groove ball bearings in the bearings of doubly fed generators. This can solve the wear caused by poor lubrication and pollution, and improve the operation reliability.

Table 1 data of super large mixed ceramic deep groove ball bearing

Note: supplementary suffix: hc52 ceramic rolling element; C3, c3p2 radial clearance; S02 heat treatment stability; Va9702 wind turbine application special design

It is very convenient to replace the existing deep groove ball bearing with SKF super large mixed ceramic deep groove ball bearing. According to the requirements of iso15:1998, this bearing has the same external dimension as the ordinary bearing. It does not need to redesign the external parts of the bearing, nor does it need special tools for installation. As long as it is handled in the same way as the ordinary bearing.

4. Conclusion

Due to the special design of doubly fed wind turbine and the introduction of frequency converter, high-frequency stray bearing overcurrent is very easy to occur. Therefore, electric corrosion and early failure of bearings often occur, which requires electrical insulation to be applied to the bearings at both ends. In addition to electric corrosion, insufficient lubrication and wear have also become an important reason for the failure of wind turbines. By using SKF insulating mixed ceramic bearing, current can be prevented from passing through the rolling contact part of the generator bearing. Due to the use of superior materials and quality control, SKF hybrid ceramic bearings have better technical reliability and more economical long-term cost efficiency against early bearing failure due to stray current and poor lubrication. The operational reliability and availability of wind turbines have been improved, and the cost per kilowatt has been reduced. The hybrid ceramic deep groove ball bearing has standard external dimensions, and can be replaced with the same size all steel deep groove ball bearing without special tools and extra work.

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