Strategic simulation and testing help Moventas slash wind turbine gearbox weight

Wind farms are booming, with a substantial 16 percent annual increase in megawatt production in 2014. Currently, there are 83 countries worldwide producing wind energy, with the European Union, China and the United States having the most installed wind power capacity.

In Europe, the North Sea is practically brimming with wind farms. Countries like Denmark produce 39 percent of their electricity using wind power. On the other hand, China installed more than half of the world’s wind turbine capacity last year. Any way you look at it, wind energy is big business. Meeting the market demand for more wind turbines has seriously pressured the industry and suppliers such as the Finnish company Moventas, a leader in wind turbine gearboxes.

“There are two main competitive trends in the market: making wind energy more economical and making the turbines as productive as possible,” explains Jari Toikkanen, head of research and development at Moventas. Toikkanen, whose role includes managing conceptual design and analysis, adds, “This, of course, affects the wind turbine suppliers as well.”

Since 2007, Moventas has counted on state-of-the-art Simcenter™ solutions, including Simcenter Testlab™ software and Simcenter SCADAS™ hardware for data acquisition, vibration analysis and design troubleshooting. Faced with a fast-changing industry, new regulations and tough customer requirements, Toikkanen and his team rely on their simulation and testing solutions to expand and strengthen the analysis and measurement scope and fulfill very specific Moventas process requirements during the verification phase.

“By implementing a strict, step-by-step simulation and testing measurement procedure, we offer our customers an extremely reliable design process as well as gearboxes with superb product properties delivered within a very competitive time-to-market timeframe,” explains Toikkanen. “In addition, Moventas gearboxes feature a lightweight design and minimized torsional and structural vibrations. This is a significant benefit since the newer, high-tower IEC III-class turbines need to be tonality-free.”

Blade spans the height of a 30-story building

The arrival of towering 220-meter turbines with massive 160-meter blade spans has changed the game in the past five years. “We used to make gearbox turbines that weighed 30 tons nine years ago,” states Toikkanen. “Today, we are targeting below 20 tons. This being said, we are not even near the same material spend level as five years ago. We are looking at a significant reduction also in component pricing. This is valid throughout most of the supply chain. It is why the price of wind energy is dropping and why we are seeing more volume and growth in the market today.”

From an engineering point of view, designing, developing and testing an efficient and dependable megaturbine is a big challenge, to say the least. Consider this fact: each turbine blade is just slightly shorter than the total wingspan of an Airbus A380. Engineers must not only deal with the dynamics of these huge blades, they also need to handle heavy components at heights ranging from 100 to 220 meters in offshore and adverse conditions.

“As a market, we certainly need advanced simulation and testing analysis tools to ensure a highly reliable design process and high-quality end product,” says Toikkanen. “And, as a supplier, it is in our best interest to have watertight sets of validation data backing our products. Simcenter Testlab software and Simcenter SCADAS hardware are critical tools to our success here.”

Bigger blades means different dynamics

These offshore giants with 30-story blade spans significantly impact the gearbox dynamics. Besides the typical engineering issues, new challenges are arising, such as how to handle higher input torque with less development time and fewer onsite testing capabilities.

“Rotor dynamics are different today with these mega-sized blades,” says Toikkanen. “It means that the turbine can gather wind from a lower speed than previously possible. This makes the turbine more flexible, more efficient and increases the amount of energy it can produce in a lifetime.”

However, longer blades provide more engineering challenges for the team at Moventas. Engineers must determine how to handle the higher input torque on the turbines and gearboxes when the gearboxes are smaller and lighter and use less materials for cost reasons. They must also do this with less vibration testing, and still meet new, stricter regulations such as the International Electrotechnical Commission (IEC) 61400 standard that requires a mandatory torsional dynamics analysis.

“In the short term, our goal is to help our customers make the best possible turbine,” Toikkanen says. “We have looked at a variety of solutions from conceptual design and changing the gear placement. We try to find an optimal design for smaller gearboxes as well as material characteristics, like surface roughness and coatings. In a second phase, we will investigate new technology and material innovation.”

The all-important cost of wind energy

One of the renewable energy industry’s prime metrics is levelized cost of energy (LCOE). It is driving the wind energy segment to optimize wind turbine efficiency and online availability from an economical perspective. As part of the supply chain, Moventas needs to focus design on minimized material usage in a size that is as compact as possible. However, these lightweight gearbox packages go into even bigger and more efficient wind turbines. For the drivetrain, this means a high torque density. Less material tends to lead to a less rigid construction as well as larger secondary loads, like deflections and lower natural modes.

“With all this in play in the big picture of LCOE, it is so important to really understand the drivetrain dynamic behavior,” says Toikkanen. “At Moventas, we set a strategic target to improve gear and drivetrain torque density over the years. As a result, today, Moventas has updated the prototype design process, product portfolio and verification tools to a state-of-the-art level.”

How to test a 220-meter offshore giant

One can imagine that testing and certifying a wind turbine is quite different than testing and certifying a washing machine or a car. The new giant turbines easily produce structural resonances and sound pollution, situations that are more critical than previous generations of turbines. Poor torsional dynamics can be one of the culprits causing the noise pollution, which is why it is so important to minimize excitations during the design phase.

Typically, Moventas engineers perform extensive modal impact testing so that component resonances do not match the excitation frequencies of the surrounding structure or gear mesh frequencies, which would potentially excite damaging vibrations in the framework, rotor blades, drive shafts or the tower itself. Generally speaking, the goal is to avoid the modal frequency range of 80 to 250 Hz for the torque arm and 500 to 1600 Hz for the rest of the housing structure. When resonances are identified within or near these ranges, engineers shift the modal frequencies by modifying the geometry of the gearbox components and torque arm – typically optimizing stiffness properties by changing part thicknesses and shapes.

Toikkanen notes that the process is complicated by the variable gearing frequencies that excite gearbox and torque arm vibration modes at different rotor blade speeds – from an input rotation of a few revolutions per minute for a light breeze to a maximum of ten times that for galeforce winds.

Improving torsional dynamics

Because of the new regulations, torsional dynamics is a primary concern, especially with the smaller, more compact gearbox design. Manufacturers like Moventas need to know exactly how the gearbox will operate and what the internal dynamics will be under different operating conditions.

“You need to know exactly how it behaves,” Toikkanen explains. “This is key aspect of a reliable machine. The wind turbine driveline consists of many components. It is known that the dynamics factors inside the turbine components can differ even 15 percent from simplified rotor loads.”

Why the difference? Toikkanen is quick to point out that there are hidden loads inside the whole turbine structure created by traditional design methods. The trick is to understand these loads using simulation software, and then back the simulated evidence with a solid testing procedure that includes proving certain torsional dynamics and structural deflections. Local behavior of tooth- or bearing-surface contact is heavily dependent on load sharing – for example, between multiple planet wheels and the dynamic loading component – which cumulatively influence the final loads.

“If you don’t instigate a process like this, there is a risk of over-compensating or neglecting the most important internal loading cases on the drivetrain component,” says Toikkanen. “Our Moventas virtual design environment, MoVE, is created to support detailed drivetrain analysis starting from the conceptual phase of the gear unit. If your simulation and testing process isn’t dependable, you will be forced to use unrealistically high safety margins for the loading factors. This makes the design heavy and, honestly, non-competitive.”

By honing its design process to perfection, Moventas took 10 tons of weight off its gearbox design in the past decade. Today, the team tests torsional measurements early in the design process, finding the frequencies on molds and other components, in order to validate that the simulation was correct and the designs can continue to be used dependably further down the chain.

“Simcenter Testlab software and Simcenter SCADAS hardware play an important role in the early design stages to confirm the simulation results, but we also use our testing solution extensively for qualification requirements, like the IEC 61400,” says Toikkanen. “Moventas and other manufacturers are required to follow strict rules and measurement procedures to classify their products. So a solid testing solution is not only good to have, it is a requirement.”

Biggest challenge: prototype testing

One of the biggest challenges is the fullscale prototype testing where the Moventas team determines the gearbox reliability and the actual torsional frequencies. To begin, they run setups on test rigs in factory conditions, which is different than in the actual environment. The test setup follows strict guidelines and usually requires hundreds of channel signals. “You need to have proper measurements early in the process to verify and validate your simulations,” says Toikkanen. “You build this up over years of experience and practice. What you don’t want to see is failure in the prototype. This can mean a four-month delay right away. Nobody wants to troubleshoot anymore. If you find the mistake when you are testing the final prototype, it is a losing game at that point.”

When the team members from Moventas go onsite, which they do more and more as their business expands into lifecycle management and service requirements, it is a different story. Having a dependable, fast and efficient measurement solution is essential when you are climbing up 120-meter-high towers to test.

“Simcenter SCADAS hardware is very interesting for the wind market, especially with the bigger turbines,” Toikkanen says. “We use our eight-channel Simcenter SCADAS Mobile hardware quite frequently. It is very beneficial to have the equipment as small as possible, with easy user interfaces and pre-setups of the measurement that they can just plug in and go. Time is money up there when you are testing a turbine. You have to climb up and carry the equipment to start, but there is also a cost factor because you stop the turbine for four or five hours to test it. This can easily cost more than 500 euros of lost energy revenue per turbine. You don’t want to run the risk of having to redo tests when you are testing a wind farm.”