Systematic independent technical inspections and independent real-time, around-the-clock condition monitoring of wind turbines, which are rarely done in South Africa, will improve their operational efficiency, maintenance and life span, advocates Cape Town-based renewable-energy consultant etaWind.
Since the South African wind-energy sector is still in its infancy, there are no binding regulations regarding independent technical inspections, but “internationally . . . a regime based on periodic inspections and condition-based inspections is preferable”, says etaWind MD Felix Bielefeld.
He explains that wind turbines are generally inspected after commissioning, with further inspections carried out before the warranty expires – typically after two years of operation. The aim of these inspections is to identify any damages while the manufacturer is still liable for the repair of the product. Besides damage assessments, which are often requested by insurance companies or operators, other inspections rarely take place.
“Most of the wind projects in South Africa have long-term full-service contracts with their turbine manufacturer, providing owners with a perceived safety net, often resulting in a minimal inspection regime being employed by a project operator,” adds Bielefeld.
However, yearly inspections – auxiliary to the owner’s visual examinations – in conjunction with condition-monitoring findings and maintenance reports, will enable operators to identify potentially fatal flaws and determine the potential need for more in-depth inspections.
The advantage of independent expert reports and condition monitoring is threefold. It can help regulate the maintenance provider or turbine manufacturer, provide highly beneficial and comprehensive life-cycle documentation, as well as enable the owner to make informed decisions and subsequently ensure the long-term success of the project.
etaWind’s inspections cover the entire turbine, including the rotor blades, foundation, mainframe and tower, as well as major components, such as the gearbox and generator. Videoendoscopy and vibration measurements are key to identifying initial damages on the drivetrain and are, therefore, part of the inspection process.
Condition Monitoring Concept
The condition monitoring (CM) of wind turbines refers to vibration measurement and analysis using accelerometers on the drivetrain. CM systems consist of specific software applications combined with hardware – mainly accelerometers and other sensors – to analyse slow-turning components that emit low-energy vibrations and are, thus, difficult to analyse.
Bielefeld says the value-add for the owner and operator is the application of a CM concept, which is the intelligent combination of the CM systems, machine diagnostic capabilities and drivetrain technology. The direct relationship between the independent monitoring centre and the owner supports operations throughout the duration of the full-service contract and the time thereafter. He adds that monitoring centres independent of the wind turbine manufacture always put the interest of the owner first.
State-of-the-art CM systems can measure up to 16 accelerometers time-synchronously to the rotational speed of the turbine and can include information from the wind-turbine control system. The number of sensors used depends on the constitution of the drivetrain, though a proper monitoring system comprises at least eight accelerometers, a sensor to measure the rotational speed of the generator and a sensor to measure the horizontal movement of the nacelle.
A key aspect of CM, he explains, is the comparison of the actual vibration data with the established baseline of the specific turbine. The longer the wind turbine is monitored, the more defined the baseline, which increases diagnostic confidence. After an initial adjustment phase, in which the operational characteristics of the wind turbine are synchronised completely with the CM concept, the system becomes fully functional and then the warning and alarm thresholds are set by the diagnostic engineers.
etaWind and its partners established a life-cycle management database containing information of thousands of analysed drivetrains, allowing for the cross-referencing of data and increasing diagnostic confidence, Bielefeld adds.
Bielefeld asserts that wind turbines should be thought of as small power stations and, as such, technicians should have a firm grasp of the mechanical, electrical and hydraulic systems involved, as well as an understanding of the control systems to thoroughly investigate problems and faults.
“Technical inspections can often be divided into data gathering and data assessment. Consequently, some on-site work might be performed by experienced technicians, whereas data analysis is usually carried out by an engineer,” he says, adding that etaWind only partners with companies that have at least ‘ten years’ experience, apply proven methods and use the latest inspection technology.
The company provides on-the-job training and offers overseas training opportunities for its employees to expose them to a range of turbine models and inspection methods.
It has also partnered with the South African Renewable Energy Technology Centre to assist in boosting the local knowledge base and training wind-energy service technicians.