Gearing up

Gearbox failure remains one of the most serious incidents a turbine operator has to deal with. Replacing one is a costly and challenging task, particularly offshore – nearly half of unscheduled operations and maintenance costs relate to the gearbox. It is no wonder manufacturers, specialist designers and after-market suppliers are constantly innovating to improve reliability and extend lifetime.

The rise of the direct-drive turbine is perhaps the primary example, dispensing with the gearbox altogether. Fewer rotating parts should mean fewer failures overall, but there have been numerous direct-drive problems documented over the past six or seven years. Monitoring over 5GW of turbine capacity for its clients, Romax has the data to prove the point. “Direct-drive turbines are not a silver bullet,” says Dr John Coultate, head of engineering development at Romax Insight. “The biggest reliability challenge is the main bearings, which keep the mainshaft and rotor aligned. We’ve seen plenty of issues with those.”

Monitoring those inaccessible direct-drive main bearings can also be challenging. “It can be hard to get a sensor close enough and it’s tough if the bearing needs replacing,” says Coultate. “You might need to take out the whole drivetrain and even the whole nacelle.”

The growing use of medium-speed gearboxes in larger turbines is also a reaction to previous unreliability. With no high-speed gears and only one or two stages, medium-speed boxes should have lower wear rates as well as being lighter and smaller. Compact hybrid or integrated designs, where the gearbox and generator share the same frame, bearings and shaft enhance these latter benefits. “We’ve seen a lot of high-speed-shaft bearing failures and there’s a definite industry shift to medium-speed,” says Coultate. “When you look at overall levelised cost of energy, medium-speed drivetrains are optimal for large turbines, though it’s too early to tell whether they are much more reliable.”

Romax’s work with the National Renewable Energy Lab shows the way ahead. The Next-Generation Drivetrain Architecture project produced an integrated, medium-speed, mediumvoltage drivetrain that, at 5MW scale, promised a 31% reduction in system cost. As well as innovations like planetary stage journal bearings, the gearbox design also addressed other reliability issues such as unequal loading of planetary gear bearings due to slight differences in tolerance. It uses flexible planetary pins to improve alignment of the different planetary stages, successfully equalising load-sharing under high loads, and thus extending bearing and gear life. “The biggest challenge in today’s medium-speed designs is maintenance as the components tend to be very closely coupled,” says Coultate. “Can you remove the individual part uptower or do you need to crane out the whole gearbox?”

Advance analysis

The latest designs prioritise this need for easier inspection and maintenance. That can mean shaft bearing replacement on site or simply leaving enough room to insert an endoscope to check internal wear. The increasingly common condition monitoring systems (CMS) allow engineers to spot problems in advance. Analysis typically relies on vibration sensors to spot failing bearings. Debris and other oil condition detectors, though rarely fitted, add valuable information too.

Romax doesn’t process vibration data at the turbine itself but instead feeds it to its remote Fleet Monitor cloud application. There it can combine the data with past failure statistics, SCADA data, oil sample results and other information. Analysts use Fleet Monitor to run sophisticated signal-processing algorithms that, when combined with statistical analysis, can single out vibrations caused by true faults while ignoring those caused by normal operation.

Romax believes that its analysis of vibration data can give up to two years’ warning that main or planetary bearings are on the way out. Today’s gearboxes might then permit inexpensive uptower repairs to intermediate and even planetary stages. If a new gearbox is indeed required, this advance alert lets operators avoid downtime. Sourcing a suitable crane for multi-MW machines at short notice is a tough ask, let alone a gearbox.

“Predictive maintenance means running a site proactively rather than reactively,” explains Coultate. “The failure will still happen, but it’s the way you manage that failure. You prevent catastrophic damage, the turbine is only offline for a day or two, you hire the crane once, and you pay the lowest price for components and staff.” With this ability to predict component life, it would also be possible to adopt aerospace’s preventive maintenance approach and replace many more parts when they get close to ‘life-expired’. However, doing this would also be prohibitively expensive.

Coultate describes predictive maintenance as the “sweet spot between reactive and preventive maintenance”. One of the cardinal rules of preventive maintenance is regular oil inspection and replacement. With gear tooth damage a common occurrence, and excessive bearing wear or breakage contributing to well over half of turbine gearbox failures, correct lubrication is vital for longer life. With their higher viscosity index, synthetic oils perform better than mineral ones, as they are able to stay close to the specified grade regardless of operating temperature.

Additive solution

Then there are additives such as those supplied by Afton Chemical and REWITEC that aim to protect against bearing micropitting and white etching. REWITEC says its DuraGear W100 additive can actually repair damaged bearing surfaces. DuraGear features silicon nanoparticles that are carried by gearbox lubricants and bond to damaged bearing surfaces to create a ceramic coating.

“The product reacts chemically where you have high loads and a thin oil film,” says Stefan Bill, REWITEC’s founder and managing director. “That fills up the holes in gear teeth or bearings.” Bill cites numerous tests and field trials that back up this claim. Partners have included academic institutions such the University of Giessen, bearing specialists like SKF and operators including Iberdrola.

Modelling work by Sentient Science found that the lower surface roughness achieved by applying W100 to Winergy 4410.2 gearboxes should increase gear life by a factor of at least 2.6. Planetary bearings would last 3.3 times longer. For owners looking at gearboxes with damaged bearings, simply adding a few litres of fluid is an attractive alternative to repair or replacement.

Torque strain

But Paul Baker, vice-president of sales and engineering at AeroTorque, thinks much of the cracking, pitting, spalling and flaking issues seen in gearboxes aren’t simply due to inadequate design or poor lubrication. Instead, he blames the forces caused by transient reverse torque loadings and the strain rates (rapid rate of change of torque) involved. When a rotor slows suddenly, the gearbox is caught between two large masses – the rotor and the generator – trying to rotate in opposite directions. This results in brief but often enormous reverse loadings on the gear teeth and bearing surfaces.

“Reversing the load by 180° on a roller bearing hits the unloaded side right where the rollers are poorly aligned and lubricated,” Baker says. “There’s a huge concentration of load on just a few rollers, so you get slipping and skidding, radial impact damage, and increased wear.”

Grid faults or gusts of wind cause some reverse torque events but the control system’s stopping protocols influence the severity of most of the events. Mild reverse pitch alone might be benign but slowing the rotor more quickly might require combining rapid pitch reversal with increasing amounts of mechanical braking. ‘Stick/slip’ during the latter’s application can cause rapid oscillations between positive and negative torque.

Fitting AeroTorque’s WindTC torque damper between the gearbox and generator reduces peak reverse torque loads on the gearbox by 50–70%, and also reduces strain rate by over 50%. This is effectively a giant, purely mechanical friction clutch that slips at preset torque levels. With the rotor driving the generator in the normal forward direction, WindTC will only slip when the torque approaches double the normal operating level – which almost never happens. But when torque reverses, it will slip from around 40% of normal torque load, resetting itself when it returns to positive. During the most damaging events where torque oscillates rapidly between positive and negative, the device constantly slips and resets itself. “It will slip momentarily to damp the torsional energy before ‘torsional wind up’ happens,” explains Baker. “It only slips a few degrees, which means it doesn’t heat up and it doesn’t wear out.”

Though all wind turbines use SCADA systems to monitor operational conditions, such as wind speed, temperature and atmospheric pressure, their ten-minute sample intervals miss these fleeting torque reversals. Likewise, CMS sensors rarely, if ever, record torque. Yet, field trials from AeroTorque show that reverse torque events are commonplace. A fault condition like a suspected failed pitch thyristor is one example. “Designers used to think that reverse torque only happened during emergency stops, but that’s absolutely not the case,” Baker says. “Emergency stops are bad, but it’s the frequent events that are the real problem.”

“SCADA data is no use for looking at shorter events,” notes Romax’s Coultate. “We install temporary strain gauges if we want to measure torque and off-axis loads.” More real-time torque data should become available as more blade root strain gauges are installed on large turbines to manage pitch control.

Cost factors

Despite the many after-market solutions available to predict and prevent gearbox failure, the extra installed cost remains the stumbling block. Balancing spend against any theoretical lifetime extension is a tough decision. Baker says, “For any manufacturer, installed cost is more important than long-term reliability. Focusing on reliability might make it too expensive and hurt sales.” Here, smart developers and investors can use historic failure data to predict which equipment to buy to give the lowest O&M costs over the life of their new wind farm project. This way, they can consider many different scenarios; for example, using different gearbox suppliers or replacing failed gearboxes with refurbished rather than new ones. They can then choose the approach with the lowest cost and risk.

Despite it being standard for offshore machines, few operators retrofit CMS to older, smaller onshore turbines because of the installation cost. Developments such as Romax’s ecoCMS offer an affordable alternative, employing a simple electronic architecture that still delivers most of the benefits of a standard CMS. Encouragingly, REWITEC is currently working with aftermarket supplier ZF to use its additives in new gearboxes while AeroTorque is in talks with various manufacturers about fitting its WindTM device to new turbines. But regular inspections and oil changes should keep the wheels turning for as long as possible.