04-10-2012, 03:31 PM
Improving Wind Turbine Gearbox Reliability
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Background
The wind energy industry has experienced high gearbox failure rates from its inception [1]. Early wind turbine designs were fraught with fundamental gearbox design errors compounded by consistent under-estimation of the operating loads. The industry has learned from these problems over the past two decades with wind turbine manufacturers, gear designers, bearing manufacturers, consultants, and lubrication engineers all working together to improve load prediction, design, fabrication, and operation. This collaboration has resulted in internationally recognized gearbox wind turbine design standards [2]. Despite reasonable adherence to these accepted design practices, wind turbine gearboxes have yet to achieve their design life goals of twenty years, with most systems requiring significant repair or overhaul well before the intended life is reached [3,4,5]. Since gearboxes are one of the most expensive components of the wind turbine system, the higherthan-expected failure rates are adding to the cost of wind energy. In addition, the future uncertainty of gearbox life expectancy is contributing to wind turbine price escalation. Turbine manufacturers add large contingencies to the sales price to cover the warranty risk due to the possibility of premature gearbox failures. In addition, owners and operators build contingency funds into the project financing and income expectations for problems that may show up after the warranty expires. To help bring the cost of wind energy back to a decreasing trajectory, a significant increase in long-term gearbox reliability needs to be demonstrated.
Observations on the Basic Problems
While it is premature to draw firm conclusions about the nature of these failures, some reasonable observations have been made to help narrow the course and scope of this project.
1. Most of the problems with the current fleet of wind turbine gearboxes are generic in nature, meaning that the problems are not specific to a single gear manufacturer or turbine model. Over the years, most wind turbine gearbox designs have converged to a similar architecture with only a few exceptions. Therefore, there is an opportunity to collaborate among the many stakeholders in the wind turbine gearbox supply chain to find root causes of failures and investigate solutions that may advance the collective understanding of the industry.
2. The preponderance of gearbox failures suggests that poor adherence to accepted gear industry practices, or otherwise poor workmanship, is NOT the primary source of failures. Of course, some failures have been directly attributed to quality issues, and further improvements in this area are not precluded from consideration, but we assume that manufacturers are capable of identifying and correcting quality control problems on their own if they choose to do so. Therefore, the target of this project will be the greater problem of identifying and correcting deficiencies in the design process that may be diminishing the life of the fleet.
3. Most gearbox failures do not begin as gear failures or gear-tooth design deficiencies. The observed failures appear to initiate at several specific bearing locations under certain applications, which may later advance into the gear teeth as bearing debris and excess clearances cause surface wear and misalignments. Anecdotally, field-failure assessments indicate that up to 10% of gearbox failures may be manufacturing anomalies and quality issues that are gear related, but this is not the primary source of the problem.
4. The majority of wind turbine gearbox failures appear to initiate in the bearings. These failures are occurring in spite of the fact that most gearboxes have been designed and developed using the best bearing-design practices available. Therefore, the initial focus of this project will be on discovering weaknesses in wind turbine gearbox bearing applications and deficiencies in the design process.
Gearbox Reliability Collaborative
Many of the gearbox problems described above may be the direct result of institutional barriers that hinder communication and feedback during the design, operation, and maintenance of turbines. In isolation, it is very difficult for single entities in the supply chain to find proper solutions. Hence, a collaborative is needed to bring together the various portions of the design process, and to share information needed to address the problems. This promises to be one of the more challenging parts of this project, as information sharing introduces perceived risk to the protection of intellectual property, which is guarded dearly by most companies. A goal of this project is to establish this cooperative framework while protecting the intellectual property rights of all parties. These concerns will be addressed through legal agreements with NREL, and will be further mitigated since the project does not focus on any manufacturer’s specific design. The collaborative is operated by NREL staff and expert consultants hired by NREL to guarantee privacy of commercially sensitive information and data. In addition, a goal of the collaborative is to engage key representatives of the supply chain, including turbine owners, operators, gearbox manufacturers, bearing manufacturers, lubrication companies, and wind turbine manufacturers. Each party holds information and experience that is needed to guide the project, supply the components, and interpret results of the test. The collaborative partners will benefit by having input throughout the testing setup and execution, and will have access to data within the agreements established by the cooperative. Results will be released by the GRC as agreed upon by its members.
Dynamometer Testing
The National Renewable Energy Laboratory operates a 2.5-MW dynamometer test facility funded by the U.S. Department of Energy at its National Wind Technology Center in Golden, CO that is dedicated to the testing of wind turbine drive trains [12]. Since 1999, this facility has been in continuous operation providing testing services to prototype and production wind turbine drive trains up to 2 MW in size. NREL plans to use this facility and its support staff to conduct full-scale tests on the 750-kW drivetrain selected. A schematic of the facility is shown in Figure 4.
One of the benefits of using a full scale drive-train test facility is that the time to evaluate new configurations can be reduced by an order of magnitude or more (compared to field testing) since loading conditions can be repeated and accelerated as needed. Instrumentation is easier to install and maintain, and the results can often be observed first-hand from a safe vantage point.
One limitation is that the prescribed loading in the test facility is currently capable of applying only low-speed shaft torque with a very simple single-point transverse load (up to 100 kips) that might represent shaft bending load due to gravity, but not in a dynamic situation. Plans are underway to upgrade the facility to enable more complex dynamic-load combinations, including low-speed shaft bending in two directions, shear loading, as well as a reversing axial thrust component. This additional loading capability will enable better simulation of the actual field conditions in real-time operation.