A wind turbine is only as good as the sum of its parts. However, choosing components that operate with minimal maintenance or repair needs is a challenge at wind farms. Almost without exception, turbines operate under extreme conditions that put these powerful machines under considerable stress and wear.
Over time, operations and maintenance costs can easily add up to 25% or more of the total levelized cost per kilowatt-hour produced over the lifetime of a turbine. One way to prolong component health is to choose quality maintenance products to compliment a rigorous O&M plan.
To this end, engineers and manufacturers are continually improving turbine components for greater durability and reliability. To get an idea of what’s new, here’s an update on recent developments for a few important turbine components.
A slip ring is used to transfer electric current from a stationary to a rotating unit. Wind turbines require reliable transmission of power and data signals from the nacelle to the control system for the rotary blades, and this is where a slip ring functions. The electrical connections from the top box to the hub pass through a slip ring, which allows the cabling to rotate. In some turbines, the center of the slip ring rotates while the outer portion is stationary.
In other units, the outer portion rotates while the center is held stationary.
Regardless of the design, it is important to consider the quality of the materials used, the annual maintenance required, and the overall life expectancy when selecting slip rings for wind turbines. Although slip rings once had a reputation for power losses and limited capacity, that is no longer the case. A wide selection of circuitry is available depending on the power requirements, with many combinations of amperage and voltage (ac or dc). Advanced designs can transfer higher wattage with decreasing power loss.
For example, supplier United Equipment Accessories’ (UEA) slip rings have handled over 55 kW for pitch control motor use with circuits rated over 100 amps and 690 VAC. The company points out that wattage transfer capacity and power losses may be affected by various factors and recommends custom-designed slip rings in many turbine applications to ensure proper capacity and function. UEA also says material considerations are important for reducing O&M. For example, the use of solid silver rings rather than wearable plating better maintains communication circuit efficiency and reliability.
Brushes are a small but important part of a wind turbine. A brush is an electrical conductor that works with slip rings and brush holders, protecting components from static electricity and lightning strikes. Worn or low-quality brushes can wear on a slip ring, causing it to degrade prematurely.
Although copper brushes are widely used in the wind industry, turbine manufacturers and operators are reconsidering the use of silver brushes. Copper has typically been the metal of choice because it is fairly reliable and cost-effective.
However, according to manufacturer BGB Technology, silver brushes are proving more effective for longevity and lifetime performance. In fact, silver brushes are typically used in offshore wind turbines because of their wider operating range, reliable conductivity, and durability in harsh conditions. Wind operators who select silver brushes for offshore or onshore use may pay more now, but benefit from fewer repairs and brush changes long term.
Wind turbines are built with emergency pitch-control systems to protect the asset from damage during excessive wind speeds or a grid power loss. The pitch system is vital to safe operation, shifting a turbine’s blades out of the wind and slowing down the rotor to stop the turbine from spinning out of control.
According to industry data, however, pitch-control system failures account for nearly a quarter of all downtime in wind turbines. Unplanned maintenance costs relating to battery problems in pitch-control systems include voltage faults, degraded performance in cold and hot weather, and unexpected tower climbs to replace failed battery systems.
An alternative to battery-based systems is ultracapacitor-based energy storage for the pitch system. Ultracapacitors are high-powered devices that store charge electrostatically. In contrast, lead-acid batteries operate electrochemically with inherent disadvantages due to the nature of their chemical process.
According to Maxwell Technologies, a global developer of power storage and delivery solutions, ultracapacitors offer much greater efficiency and reliability in emergency pitch controls and require no scheduled maintenance for 10 years or longer. This contributes to greater wind-turbine uptime and reduced O&M costs.
Strong wind loads mean great demands are placed on a turbine’s gearbox and main-shaft bearings. Bearings that can provide the highest possible performance potential in a compact design are ideal for reducing the overall component size, weight, and manufacturing costs in wind turbines. Recently, tapered roller bearings have demonstrated desirable performance in a number of new wind-turbine applications when compared with conventional, spherical roller.
Spherical bearings are composed of raceways that are rounded (spherical) on the inside axially. In tapered bearings, which are smaller in size, the rings and the rollers are tapered in the shape of truncated cones to simultaneously support axial and radial loads.
The Timken Company, a global manufacturer of bearings and related components, has found that tapered bearings offer an increased power density, thereby reducing the overall cost of energy. What’s more is they can bear both thrust and radial loads, providing high performance in harsh conditions and unpredictable changes in wind speed and direction.
However, Timken says that it is imperative that the specific demands of the application are first considered. This means there is no one correct bearing for all applications, onshore or offshore. For example, in extreme conditions — and particularly in offshore turbines — a bearing’s geometry, clearances, and load capacity are typically custom-engineered to meet the specific operational conditions.
Now that the U.S. offshore wind sector has one working wind farm and more projects in the pipeline, it is time to start thinking about subsea cables that transmit electricity to a mainland grid. Industry stats show that cable damage accounts for as much as 80% of insurance claims at offshore wind farms.
Unlike onshore wind-farm installations, offshore cabling routes are also more difficult to access, let alone install or repair cable effectively. PMI Industries, which specializes in manufacturing and testing underwater cable hardware, says cable faults typically occur when subsea equipment is deployed from a vessel or retrieved from the water and fails due to the extreme tension placed on the attached subsea cables.
The stress and tension build at the equipment’s attachment point unless a proper underwater cable termination or grip is in place. PMI suggests using full-strength, underwater helical cable terminations during deployment and retrieval of subsea equipment to prevent cable faults. With a helical termination, stresses that would occur at a localized point on the cable are dispersed over the length of the cable.
One benefit of the company’s helical wire terminal system is that it installs anywhere along the length of a cable without access to the cable end. In addition, it needs zero tools or cable prep.
One other idea to reduce cable faults is to use an integrated protection system for offshore wind-farm cables in wind turbines and substation platforms. Trelleborg’s Njordguard cable protection uses API 17L-certified Uraduct material, which is highly abrasion-resistant and can travel over the seabed floor without drag or snagging risks. Trellebrog says the extendable cable system fits monopile and J-tube applications, requires minimal assembly, and can be manufactured to meet any diameter cable.
Most notably for offshore wind, the system improves safety because it installs, and can be removed and reused, without the use of divers or underwater remote-operating vehicles.