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Stocking up to avoid shutting down

The drive to reduce the cost of energy over the lifetime of a wind farm has pushed efficient operations-and-maintenance (O&M) practice up the list of priorities for developers, especially as these costs can equal 10-15% of total generating costs.

The condition and functionality of the main components of a project's wind turbines and the major electrical equipment at its substation are critical to maximise power output and minimise downtime over the lifetime of a project. Understanding how components deteriorate and optimising stock levels accordingly can ensure a wind farm's continued power output without excessive investment in costly spare parts.

Main components

The major components of a wind turbine are the blades, the main bearing, the gearbox (if there is one) and the generator. "The most important in terms of maintenance is the gearbox, followed by the generator, then the blades," says Eduardo Garcia Perez, head of central maintenance for Europe at EDP Renewables, which develops, owns and operates wind farms in several countries across Europe with an installed capacity of almost 4GW.

According to Garcia Perez, around 1-5% of the main wind turbine components fail each year. The exact failure rate of each component varies, depending on the turbine model, its age, the wind and weather conditions where it operates and quality of the maintenance programme of the wind farm operator.

The blades of the rotor capture the kinetic energy of the wind, which is converted to mechanical energy in the form of low-speed torque in the main shaft. They are the "weak point" of a wind turbine, says Garcia Perez, due to being exposed to tough environmental conditions over a long time period.

During the first five years, usually the warranty period, blades typically have relatively few problems provided they are sufficiently maintained. However, beyond that they need a high level of attention through visual inspections. Blades have a composite structure and are exposed to the elements over a period of 20 years. "They are subject to ultra-violet radiation and temperature changes," Garcia Perez explains. "All of this cannot be tested at a test site over a threeto four-month period."

EDP carries out regular inspections of the blades in its turbine fleet while they are operational, from the ground with telescopes and using a crane platform to check for defects. It is after between eight and nine years of operation that the condition and durability of the wind turbine blades could present problems, says Garcia Perez. However, because most of the turbine blades in operation on megawatt-clas turbines are not yet that old, there is no experience in understanding their degradation beyond that point.

Because the maintenance work is done onsite, blade maintenance is less costly and so less critical than similar work for either the gearbox or generator.

The gearbox is necessary to increase the rotational speed at high torque of the main shaft to the higher speed required to rotate the generator. Gearboxes usually degrade through wear, pitting and cracking on teeth and the bearing.

"For our gearboxes we obtain maintenance information from oil analysis, Scada (wind farm supervisory control and data acquisition) data, endoscopies and even noise," says Garcia Perez. "With the information, we try to establish both a trend analysis and a comparison of each machine with the rest of the fleet. A gearbox takes six to eight weeks to repair and we always have them in stock."

Gearboxes last about eight to ten years but sometimes, depending on the model, they can fail after only five years.

The generator takes the mechanical power from the high-speed low-torque shaft and converts it into the electricity that will then go via a substation to the grid. Generators usually fail by a degrading of the isolation material in the rotor or stator, due to exposure to electromagnetic fields.

"Generators are inspected externally, (we) retouch electric connections and frame bolts and grease the bearings," says Garcia Perez. "Sometimes we apply vibration analysis for bearings and measure conductivity and isolation. It is also important to check the alignment with the gearbox. Sometimes, generators fail mechanically due to damage to the wire connections."

Estimating component needs can be difficult, as some generator models can last between five and eight years while others run for more than 15 years without any problem, he adds.

Another key component is the main bearing, which supports the main shaft. This transmits the torque from the rotor to the gearbox and is subject to radial and axial loads from the blades. Regular greasing to keep main bearings lubricated ensures they normally last more than ten years until they need to be replaced.

Emerging market

Until recently, wind-turbine manufacturers dominated the post-warranty component maintenance market. However, a service market is emerging with energy service companies (ESCOs) specialising in maintenance and repairs competing with manufacturers for post-warranty service contracts. Developers such as EDP Renewables are also starting to manage their own post-warranty maintenance.

A significant proportion of EDP's European capacity of almost 4GW covers a variety of turbine models and sizes, according to Garcia Perez. Two years ago, the company started managing its own stock levels. The local warehouse at each EDP wind farm is stocked with adequate levels of small parts, such as sensors, bolts hydraulic materials, oils, greases and filters. These stock levels are initially set using recommendations from the turbine manufacturer and will subsequently be adjusted by the local wind-farm manager and entered into a computerised system.

Major components will almost always be the original manufacturers' own parts stored in the warehouse and the system for calculating stock levels - still a work in progress for EDP - is more complex (see below).

State of flux

Garcia Perez believes maintenance management is in flux. Once, manufacturers dominated, charging premium rates for post-warranty maintenance as they monopolised expertise. Now a market is developing where manufacturers, ESCOs and developers compete. In the future, Garcia Perez sees a possibility of a post-competitive system: performance data-sharing on stock across the industry.

TAKING STOCK CENTRAL DATABASE MANAGES SPARES FOR EDP RENEWABLES' EUROPEAN PROJECTS

While EDP Renewables keeps spare small parts at each wind farm, major components are stored and managed in a central warehouse. Eduardo Garcia Perez, the company's head of central maintenance for Europe, explains that the procedure for calculating stock levels begins while the parts are still under warranty. Over this period of roughly five years EDP will keep a record of all failures or defects for each main component. Six months before the warranty expires Garcia Perez's team begins to create a database with all the recorded failures and the point in the five-year period when the failures or defects occurred.

Once this is done, he says, and accounting for the number of turbines in the fleet, the number of failures for the subsequent three years is predicted and stock levels are adjusted accordingly.

The whole process is then repeated year-on-year and, as more data accumulates, a more accurate picture of component failure and stock requirements can be drawn. The database records all defects and failures of the major components of each turbine and accounts for any adjustments to the number of turbines in the fleet, changing the stock levels if necessary.

"To have a big size of fleet always helps," he adds. "For one thing, it means it is possible to share major components such as gearboxes, generators and main bearings, so the ratio of stock over covered fleet decreases, meaning lower investment costs for spare parts. A bigger fleet also makes it possible to obtain better commercial conditions in terms of price and delivery time because of the higher volume.

"In our central warehouse in Spain you will see for our post-warranty European fleet of wind turbines one generator and one gearbox for each model - unless it is a model with known problems where we will have maybe five gearboxes and five generators," says Garcia Perez. "The total is probably 30 gearboxes, 30 generators, no blades, ten main bearings and 15 transformers."

However, EDP is yet to carry out detailed financial calculations on the cost of the process. "We are still at the stage of managing our own repairs and calculating the correct level of spare parts," says Garcia Perez. "We have not yet done a financial analysis, that will be the next stage."

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