Alex Nino, LUDECAwind, www.ludecawind.com
The wind energy industry is growing significantly in generating capacity and volume. As demand increases, so does output power of wind turbines. OEM’s across the globe are having extreme difficulty testing newer and larger wind turbines because conditions vary from country to country, thus producing a negative impact on system availability and reliability. In 2003, as OEMs were inroducing relatively large 1+MW turbines, their gearboxes began failing at a staggering rate. In Europe, system insurers introduced a revision in their policies and cancelled all old contracts. A revised clause stipulated that “all roller bearings in the drive train must be replaced after 5 years or 40,000 operating hours, unless a suitable Condition Monitoring Systems (CMS) is installed.” Insurers found value and protection in the function and fault diagnostics of condition monitoring systems.
Function diagnosis refers to the measurement and data collection of functional and operating parameters, and overall vibration values. This information is required for the proper analysis and long-term operation of rotating machinery. Fault diagnosis is the determination (cause) of damage conditions of machinery and its components.
According to DIN ISO, condition-based maintenance for wind-power plants aims to maintain, visually inspect, measure, and analyze the condition of the turbines and perform required repairs. However, how can we measure and evaluate vibration components of wind turbines when they have been excluded from many of these international standards? For example, ISO 10816-3 explicitly excludes wind-power plants.
Condition-monitoring systems have made it clear that wind turbines are complex machines in which overall vibration values must be systematically determined and evaluation references made available. Points to consider when writing evaluation guidelines for wind-power plants include:
• Function and structural design of wind turbines and their components
• Interaction between the individual drivetrain components (modules) being tested
• Information and experience regarding the possible faults and damages occurring in the individual testing modules during operation and their economic impact
• Knowledge of operation-related and machine-related vibration influences, the diagnosis procedures that must be adhered to and their respective limits
The recent VDI 3834, established and released in 2009, takes into consideration the special requirements for evaluating wind turbine components. The guideline is set for turbines ranging from 100 kW to 3 MW.
Measuring methods
Varying characteristics of wind turbine operation and wind conditions require collecting vibration data using piezoelectric accelerometers which can measure frequencies from 0.1 Hz to 6 kHz, as defined by VDI 3834. Other important criteria for proper data collection is a minimum load of 20%. Because of natural fluctuations in wind loads, the VDI 3834 specifies measurement periods from 1 to 10 minutes. Such long evaluation periods provide a stable and meaningful root mean square (rms) vibration data for slow rotating compenents.
RIGHT: The chart lists permissible evaluation velocities in mm/s. For example, a gearbox (VDI abbreviation GBX) is running well when vibration velocity is less than 3.5 mm/s. Watch it more closely between 3.5 and 5.6 mm/s (the yellow band) and expect a crane callout above 5.6 mm/s. LEFT: The chart lists permissible evaluation accelerations in mm/s2. For instance, a generator is running well when vibration accelerations are less than 10 mm/s2. Watch it more closely between 10 and 16 mm/s2 (the yellow band). Expect trouble above 16 mm/s2.
Characteristic valuesThe VDI guideline separates drivetrain components into their main groups and assigns overall vibration values to the most important ones. These component-specific vibrations can be classified and identified. The VDI 3834 is based on statistical analysis of vibration measurements from more than 450 wind turbines, and defines threshold values in terms of vibration velocity in mm/s and vibration acceleration in m/s2 for the generator, gear, main bearing, nacelle, and tower. VDI 3834 also recommends warning and alarm thresholds. Threshold values are defined as component-specific frequency bands.
Detecting generator faults early calls for monitoring vibration magnitudes. The graphical trend displays the increase in vibration amplitude over time and response. As can be seen, the turbine was shutdown for repair. Turbine start up began after more than month out of production.
Assessing and reducing vibrations
Level 1 monitoring differentiates between remote monitoring of these overall vibration values and the remote monitoring of characteristic diagnosis values. The practice is not new to general industry. Vibration values from ISO 10816-3 are used to monitor the general vibration conditions while other guidelines use frequency-based or order-based characteristic trending values. Based on overall vibration values, it is now possible to assess and compare the vibration levels of wind turbines. The early detection and reduction of elevated vibration levels will extend wind turbine operational life.
Corrective measures
The required measures can be identified by means of condition diagnosis. Diagnosis specialists use amplitude spectra, envelope spectra, time signals, or spectra, or both to detect unusual vibration signals, identify dominant excitations, and evaluate frequency-specific trends using the water fall display function.
The polar plot shows the balancing result (amplitude and phase) of the wind turbine (initial unbalance, trim corrections, and final result). The chart graph displays the DIN ISO 1940-1 standard for rotor unbalance. In the wind power industry, the selected tolerance grade is G16 (as pointed about by the red line). For example, a wind turbine running 12 rpm the permissible residual unbalance to abide by is 16,000 g-mm/kg
Here are a few examples on how to increase the uptime and availability of wind turbines using results from vibration analysis:
Detecting vibration results from a generator faults: The accompanying illustration Detecting generator faults shows the trend of a wind-turbine generator in which an increase in vibration amplitude indicates an ongoing machine fault several weeks in advance. After replacing the generator, overall vibration values returned to normal. It should be noted that such vibration changes only arise if the affected drive train component is dominant in the frequency band. Overall vibration values do not rise when the damage is not dominant in the amplitude spectra.
Identifying deviations in the drivetrain alignment: Telemonitoring of a wind turbine in How alignment reduces vibration shows an increase in overall vibration values. Frequency analysis showed additional vibrations from poor misalignment between the generator and high-speed side of the gearbox. The machine was then aligned using accurate laser-alignment equipment with defined alignment targets. Overall vibration values dropped significantly.
Reducing blade imbalance: Unbalanced rotor blades can lead to rotational excitations and a load increase on bearings and other drivetrain components. Although frequencies are relatively low in wind turbines, resultant amplitudes can range up to 100 mm/s. Measurements must be taken with linear vibration sensors and longer measurement periods, as prescribed by VDI 3834. Applying the recommended G16 balancing grade for rotor blades, Permissible residual unbalance shows the resultant dynamic field balancing. In this example, other vibrations caused by imbalance, were reduced to the point where the difference was noticeable across the entire nacelle.
he graphical trend plot displays the vibration velocity before and after an alignment of the high speed side of the wind turbine.
These examples illustrate the targeted use of measuring and testing techniques that make it possible to reduce vibrations in working wind turbines. VDI 3834 lets manufacturers and operators assess the vibration condition of wind turbines and reduce them by implementing specific corrective measures in order to reach new threshold values.
Filed Under: O&M
Any Vibration Standard specifically aimed at DIRECT-DRIVE wind turbines, i.e. with NO Gearbox but low-speed multi-pole synchronous generators (either wound rotor or permanent magnet excitation), directly coupled by a flange, shaft [or embedded in] to the rotor hub?
VDE 3834 specifically states in its Title that it is devoted to Gearbox-type wind turbines. How can vibrations on a direct-drive Generator component be assessed and evaluated? Are there any acceptable vibration levels stipulated or recommended for the direct-drive case?
Thanks for your clarification and comments.