Dave Clark/Condition monitoring specialist

There are three methods for monitoring wind-turbine operations: wind-conditions monitoring, performance monitoring, and condition monitoring. The terms are thrown around so much that many in the industry are confused. Although the terms describe different tasks, they have the same goal: performance and reliability of the wind assets.

An automotive analogy may be useful. For instance, if your car is not getting optimal fuel mileage, you get a tune-up. If that doesn’t work, you take the car to a mechanic for a more detailed analysis. So diagnosing the mileage problem is analogous to monitoring wind conditions. A tune-up is analogous to examining SCADA data, or performance monitoring. And the mechanic is akin to the analyst who finds meaning in the vibration data. Here’s more detail on each.

Monitoring wind conditions

Suppose a turbine’s power production or output is less than its power curve says it should be. Does the problem concern wind conditions or the turbine? Wind conditions monitoring may provide a clue. A wind-resource assessment of the site is performed prior to construction and at least for a year. If the resource assessment is correct, the turbine should produce predictable power. If it doesn’t make rated power, (rated for the available wind) then you will need to benchmark the conditions versus the rated power, essentially plotting the wind versus the turbine output.

The Triton from Second Wind is one of several sodar-based (wind sensor using sound) remote wind sensors. The unit can measure wind speed and direction at hub heights to give operators data to gauge a wind plant’s efficiency.

This is done with wind-conditions monitoring this way: Examine the site’s meteorological data to gauge or plot against the turbine’s output. Look for data from a met tower or remote sensor, such as a sodar unit. “Our systems measure wind conditions that can be used for wind-resource assessments,” says Naomi Pierce of Second Wind, a company that makes portable wind-sensing systems. “The equipment can be used for wind-resource assessments and monitoring operating farms. In the latter case, the equipment monitors wind and wind conditions to gauge the performance of an operating wind turbine.”

Performance monitoring based on wind conditions measures the wind potential that can be plotted against the rated power of a wind asset or site, to determine the production efficiency. Wind performance should show how much power the site can produce. If the turbine does not produce expected power, there might be a problem with the turbine. This also involves power-purchase agreements and warranty implications.

Performance monitoring

Performance monitoring is a great tool for wringing maximum performance out of a farm or fleet. This monitoring involves taking the sensor data from the wind turbine and mining (examining) it for information as to why it’s underperforming. You look for reasons why the rated power was not produced.

A tremendous amount of data comes off of a single turbine. Its sensors monitor characteristics such as:

• Yaw and pitch position
• Temperature
• Wind speed and direction
• Generator speed

There are more aspects, but mining the SCADA data can determine where performance issues lie. Steve Brost, a CMS Engineer and turbine prognostics and health management analyst, says this about performance monitoring:

“We use a software tool called T2 for identifying deviations between two or more data populations.  We use it to identify deviations in trends by comparing the (multivariate) means of data populations and point out the one with the most consistent deviation.  We configure a threshold in the tool to identify the ‘highest hitters’, those that deviate most. Once the tool has identified a repeat offender, a value for that data population is collected into what we call a Cusum (cumulative sum), so we can focus on locations in the turbine with the highest probability of potential failure. For example, the hottest temperature might indicate damage or confirm damage through other measurements such as vibration. The Cusum characteristic is something of an alarm threshold or a filter we can set.

After taking all Cusum values across our fleet, we examine the highest values and cross reference the findings with vibration data to focus on locations that are alarming the most frequently in vibration and temperature or other SCADA trending data such as highest fault count.”

Suppose a turbine’s temperature points to a potential problem. This element is neither sufficiently specific nor nearly as predictive as vibration-condition monitoring. But a “hot” indicator is similar to a “check engine” light in a car. It signals a problem but not specifically what is causing it.

Statistically, every tenth turbine faced relevant damage each year. Costs for a planned repair are on average less than 30% compared to the replacement of a component. (Source: DEWI). Consequential damage can be prevented.

Condition monitoring

Nothing reduces turbine efficiency more than a failed component that halts its power production. Condition monitoring using vibration in particular, aims at identifying specific component degradations. Typically, this includes sensors placed on components that are costly to repair, fail with regularity, or both. Usually and regardless of model, such components include:

• Main bearings
• Low-speed shaft, gearbox
• Planetary section, gearbox
• High-speed shaft section, gearbox
• Generator, drive end
• Generator, non-drive end

Several operational parameters (which performance monitoring can detect) ultimately increase wear on drive-train components. Performance monitoring detects these abnormalities while condition monitoring detects component wear. Hence, it is crucial to perform both. The monitoring methods work together on performance-related failures.

For example, a rated power output may be affected by a misalignment between the gearbox and generator, or a failing high-speed shaft. Yaw deviation, the difference between wind direction and nacelle direction, can also affect power output by causing undue drive train loading. Condition monitoring detects the wear.

As you would expect, not all failures or shortcomings are performance-monitoring detectable or correctable. For instance, looseness, misalignment, imbalance, and most all early detection on gear and bearing failures are impossible with factory sensors. So check the fuel efficiency on your turbine, give it a tune-up, and avoid a trip to the mechanic. Although methods for monitoring wind turbine performance have different names, all work toward the same thing: peak wind-turbine performance.

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Hugh Smith Regional product line marketing manager Miller Fall Protection / Honeywell Franklin, PA www.millerfallprotection.com

A quick-acting braking system that arrests falls within inches, not feet, has safety managers retiring conventional shock-absorbing lanyards. Instead, they favor compact, lightweight, personal fall limiters (PFLs), also known as self-retracting lifelines. As manufacturers introduce PFLs with 100% tie-off fall protection (dual or twin legs), workers can move safely anywhere on a job site without ever being disconnected and at risk of a fall. Whether on a construction site, building scaffolding, or maintaining a wind farm, it’s good to know someone has your back.

The 6-ft shock-absorbing lanyard has been a dominant tool in personal fall protection for years. These lanyards allow for up to 6 ft of free-fall distance before activating, and another 3.5 ft of deceleration distance before arresting a fall. However, a personal fall limiter requires less than 2 ft to arrest free falls. If you have 15 ft of fall clearance or less, you’d better have a retractable on or you’re going to hit the ground.

Safety managers say the primary driver of their decision to swap out lanyards for PFLs is fall-arrest clearance, which is critical to worker safety. Personal fall limiters have evolved into more compact, lightweight, and affordable devices developed in response to workers’ needs for quick stopping action at low fall clearances, and for greater mobility around barriers. Today’s high-strength, high-impact materials let product engineers build smaller units that can withstand required fall forces. Some models accommodate workers up to 400 lbs including tool weight. Advanced designs incorporate a built-in swivel mechanism, and D-ring connectors that easily adapt two lightweight PFLs for continuous 100% tie-off fall protection. This eliminates a need for double-legged shock-absorbing lanyards. With the reduction in size and weight comes a reduced price, making PFLs more affordable than ever.

The latest advancement is the ANSI Z359.1 compliant line of Miller brand retractable lifelines by Honeywell Safety Products–the TurboLite and TwinTurbo personal fall limiters, and the Turbo T-BAK and Twin Turbo T-BAK tie-back personal fall limiters.

Recently one of the nation’s largest manufacturers made a concerted effort to reduce fall hazards, 150 personal fall limiters were added to their arsenal of fall protection equipment in a move that will eventually outlaw lanyards on their job site. All employees, contractors and sub-contractors will be required to comply with the new policy, which incorporates rigorous, hands-on training and inspection, and encourages those working at height to take personal responsibility for their own safety.

Wondering if it’s time to transition from lanyards to PFLs? Think of it this way: for years cars were not equipped with seat belts, then came lap belts, and after that lap-belts plus shoulder harnesses. Then both were incorporated together with quick-acting pretensioners. Seat belts eventually moved around passengers when the car door closed. We complained about the changes, but got used to them. Now we buckle up without even thinking whenever we get in our cars.

Advances in personal fall protection have taken us from the safety belt, to the 6-ft shock absorbing lanyard, to personal fall limiters. You may think you’ll never be in a car accident, and you may think you’ll never actually fall. But if a person does, it could lead to an incapacitating injury or worse. However, with the right fall arrest equipment, a person can go home safe after work. Isn’t that worth it?

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Like stately giants, utility wind turbines are appearing further afield and offshore. As designers tackle the job of building longer, heavier, higher performing turbine blades, wind-farm operators and owners are faced with a different challenge– keeping aging blades in optimum condition.

Traditionally, less attention has been paid to the repair and upkeep of turbine blades versus other components. Instead, preventive maintenance programs have focused on the internal mechanics of turbines due to the predictability of their maintenance requirements. Typical preventive maintenance plans for internal components fall into 3, 6, and 12-month work schedules. By nature, blade repairs are more difficult to plan. Blade damage can arise in manufacturing, transportation, and tower construction and erection. However, maintenance issues more often occur in the field from leading-edge erosion, weather, and other factors. A lack of predictability and historical data complicates preventive maintenance for blades.

A maintenance technician from Wind Energy Services Company sands the substrate of a blade before applying a surface coat.

Commercial turbines can have tip speeds of over 200 miles per hour. At these speeds, rain drops can take on the impact of small stones, and blowing sand has the erosion power of a plasma cutter. Studies have shown blade roughness and accumulated debris on the blades can reduce wind turbine performance by 5 to 30%. Blades that aren’t working efficiently can also create vibration that contributes to gearbox failures.

While Composites One distributes composites and materials for blades and their repair, those who apply them have an entirely personal and unique perspective to the task–which is worthy of broader exposure. Hence, this article.

To minimize downtime and boost energy-capture efficiency, it’s critical to adopt practices early on, including implementing a preventive maintenance program and identifying problem areas. “Return on investment for wind turbines is a long cycle so any downtime has significant impact on the owner,” says Dave Smith, power generation manager for Composites One. “Repairs must be made quickly and cost effectively.”

Gary Kanaby, director of sales for Texas-based Wind Energy Services Company (WES) agrees. “A planned maintenance program can mean the difference between a small repair and damage that incurs the cost of an outside crew, crane rental, and the loss of energy sales while the turbine is down,” he says. “Leading-edge damage affects air foil and air flow around the blade and can cause up to a 5% energy loss.” Kanaby explains that uncorrected erosion will lead to cracking, splitting of the blade tip, and blade separation. Efficiency can be improved by restoring the leading edge to its original airfoil specs.

Visual inspection is the simplest form of preventive maintenance and can be conducted using a camera with a telescopic lens of at least 400 mm, or high-powered binoculars. Joshua Crayton is blade services manager for Rope Partner, which provides turbine maintenance, repair, and inspection services throughout its locations in California, Texas, Canada, and Germany. He says regular inspections are especially important in windy seasons and following lightning storms. “Operators and owners are inheriting their wind farm assets and the responsibility of maintaining blades that are no longer covered by the OEM warranty,” he says. “Like any business, wind farm owners and operators typically run a lean staff and may not have an experienced maintenance technician in-house. Partnering with a service company can help them design a long-term, post warranty, preventive maintenance plan.”

According to Crayton, a maintenance plan should be initiated before the warranty period expires. “A thorough internal and external blade inspection should be scheduled in the warranty period,” he says. “Once owners and operators take over care of a wind farm, these inspections should take place every two years. Personnel can conduct simple ground inspections while on-site, but there is no substitution for a close, visual examination performed uptower.” Trained personnel using standard rope access systems offer an efficient, cost effective, environmentally friendly approach that enables complete 360° access to the tower, nacelle, and blades.

Crayton says consultation can still offer practical insights for wind-farm owners with the capbility to perform blade inspection and repairs in-house. “There are many different types of blades in the field,” he says. “Each construction type carries its own inherent flaws and issues. Consultants can give wind-farm owners an understanding of what to look for. A defect that may be potentially catastrophic for one type of blade may not be as serious for another.” Water ingress, for example, will not have the same impact on a blade core made from polymer foams as it would a blade built predominantly with a balsa-wood core.

Over the last year Crayton’s crew has begun to see an increase in requests from owners and operators for internal and external blade inspection on a site-wide basis. “When we make repairs we are always trying new products to find more efficient ways to get the turbine up and running faster,” he adds. For small fixes such as minor pitting or cleaning debris off blades, technicians can use a variety of abrasives, cleaning solutions, and fillers.

Structural repairs are more often made in the field. Products, such as the Renuvo line from Gurit, allow for prepreg patching right on the blade.

Jim Sadlo, wind energy market development manager for 3M, says some wind farm owners monitor their blades using high-speed telephoto cameras. “The stop action images offer enough clarity to reveal problem areas such as leading edge erosion and other defects on the blade,” he says. Typically, the next step is a visual blade inspection by a service company who analyzes the scope of work and determines the required materials. Wind farms, especially those in Northern climates, have a short window of opportunity to complete repairs. For example, restoring surface damage can require a number of steps. The crew usually masks off the portion of the blade surrounding the work area. A plastic film attached to masking tape works effectively and is easy to haul uptower, says Sadlo. The defective portion of the blade is cut out and then ground using ceramic grinding abrasives. The area has to be rebuilt with fabric and resin according to OEM criteria for strength, density, and structural soundness. After placing the last layer of fabric, filler helps restore the blade’s aerodynamic shape. Several epoxies and polyurethanes are available in easy-to-handle cartridges that offer short cure times. A repair technician can begin sanding in as little as 30 minutes.

Once the repaired section is sanded and painted, wind protection tape can be applied and the tape’s edges sealed and beveled to create optimal aerodynamic characteristics. Sadlo says the tape can also act as a shock absorber to lessen the impact of flying debris such as bugs or hail. This wind protection tape has a new option that allows for easy installation while the blade is vertical. The center section of the tape’s liner can be removed separately from the rest of the liner, making it easier to align to the blade edge and apply the tape from the middle.

When looking for advice on maintenance or repair products, the following companies can offer both materials and expertise.

Resins that cure quickly with UV light instead of heat also help reduce the time crews have to wait between repair steps. David Cripps, global account manager for Gurit Wind Energy says such resins broaden the window for repairs in colder climates because they can be used at temperatures as low as freezing. Available in a paste or prepreg patch, the resin paste can be used on its own to make small repairs. If a laminate must be restored to its original condition, the resin paste can also be used as a primer or wetting agent to help bond the new prepreg–a relatively dry material–to the blade surface. Once the prepreg patch is applied, additional paste can smooth the surface. “With the prepreg method, the amount of resin in the laminate is highly controlled for an accurate fiber-resin ratio,” he says.

Improved materials, such as coatings, are helping wind farm owners and operators maximize the resistance of their blades to the elements and extend service life. Applied to the blade’s exterior by the OEM, these newer coatings can also be reapplied uptower to facilitate a repair. Martin Schoning, sales manager for Bergolin, says the products are fast drying and environmentally friendly. “The blade’s leading edge takes the brunt of damage from erosion, weather elements, and airborne particulates,” he says. “Re-application of the right coating is a key step to increasing the blade’s resistance to abrasion and erosion.”

The need for products that can be used in the field under less than ideal conditions is a component of preventive maintenance. “Turbine downtime costs a lot of money,” says Alistair Smith, technical sales manager for Mankiewicz. “Paints and top coats offer abrasion resistance and absorb some of the energy from sand, hail or any other element that hits the surface. The products must dry fast and last a long time. Delivery systems also have to make the products easy to handle, transport, and use uptower.”

As the industry grows, regular inspection and maintenance of blades, along with products that can support fast fixes are becoming critical tools for minimizing costs associated with reduced efficiency and downtime. “The blade is really the ‘engine’ of the turbine,” says Smith, “it’s the component that is capturing and producing energy. Planned maintenance can keep blades in peak performance which directly translates to kilowatts sold.”

Marcy Offner
Composites One
Arlington Heights, IL
www.compositesone.com

WPE

Oliver Hirschfelder
Global Wind Energy Director
Capital Safety
www.capitalsafety.com

Alex Nino, LUDECAwind, www.ludecawind.com

The green arrows indicate sensor (accelerometer) locations for standard wind turbines.

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.

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.