4.2 Main Issues with Onshore Wind Turbines Components

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1 Graph 2 Causes of Wind Turbine Failure Survey on 15,500 German Machines, (Windstats Newsletter, 2004) 5% 5% 4% 2% 2% 11% 42% Component failure Control system failure Unknown cause High wind Grid failure 8% Lightning Loosening of parts 21% Icing 4.2 Main Issues with Onshore Wind Turbines Components The components of a wind turbine can be affected by a variety of issues. The objective of this section is to give an overview of the main stress and failure factors, with a particular focus on the most relevant components (gearbox, blades and generator). The content is not intended to be a full list of O&M issues and a more detailed description can be found in The Wind Energy Operations and Maintenance 2011 published by Wind Energy Update Drive Train and Bearings The drive train (figure 3) is a fundamental part of the generation process because it transmits to the generator the rotational energy received by the rotor. During operations, drive trains (also indicated as power trains) are subject to a wide range of dynamic loads. The high number of load cycles and operating conditions increase the likelihood of failure events caused by stress and wearing associated with loads and vibrations. 29

2 Connecting shafts, bearings and other machines are all sub-parts of the drive train (the gearbox and generator can also be considered part of it but they will be discussed in more detail later in this section). Because of shaft rotation, a fatigue crack can occur in any point and it can evolve into a proper fracture. Figure 3 Diagram of a Typical Drive Train with Main Connected Components, ( windenergy) Electronic Controls The electronic controls have two functions. Firstly they supervise the functioning of the wind generator by measuring and storing operational data that will be used for statistical purpose when necessary. They are also responsible for most decision-making processes within the safety management of the turbine. A bespoke micro-computer with a large storage capacity is installed in the control cabinet of the nacelle and represents the core part of this component. The electronic controls can be affected by frequently occurring failures but they typically entail minimal downtime because of the short time needed to fix them. Among the causes of failures are: 30

3 4.2.3 Gearbox The blades of a wind turbine are mounted to a low-speed shaft that is connected (through the drive train) to the gearbox (figure 4). This is one of the heaviest and most expensive components and increases the angular speed of the blades from typical values of rpm to the required speed for most generators, 1,000-1,800 rpm. The high intensity of static and dynamic loads and the vibrations placed on the many rotating parts of a gearbox during the generation process determine wear and tear and eventually failures. Gearboxes typically have a relatively low failure frequency, but when it does happen, they are most likely to result in major failures and in turn entail one of the longest downtimes with relevant consequences for productivity. Indeed the complexity of multiple moving parts means a higher tendency for major failures and more demanding maintenance requirements. Among the factors causing deterioration of a gearbox are: Among the type of issues that can evolve in a failure of the gearbox there are: surfaces, 31

4 Figure 4 Example of Gearbox, (www. windpowerengineering. com) Generator Wind turbines typically have an AC generator that converts the mechanical rotational energy into electrical energy. This component is different from other industrial generators because they work with a significantly fluctuating power source. As a matter of fact, the variability of wind conditions can determine continuous change in the speed of the rotor blades, which in turn causes variation of mechanical energy transmitted to the generator. Some of the causes of generator failure are: components, electrical insulation or inadequate design, cooling system), weather-related conditions. 32

5 4.2.5 Hydraulic System The hydraulic system operates various other parts of a wind turbine, like the mechanical brake system, the pitching system and the yaw control system. It is composed of several sub-components including pumps, oil tanks, filters and pressure valves. The system provides the right pressure levels needed when a machine starts or stops working. The hydraulic system can be affected by mechanical failures caused by (among others): filtered according to ISO cleanliness standards), with consequent variation of electrical conductivity, industrial sources, this issue is caused by improper installation in 60% of the cases, poor system design (20%), quality of the components (15%) or overuse of the system (only 5% of the time). Problems to the hydraulic system can cause other issues such as: Mechanical Brake System The mechanical brakes are mainly composed of brake discs, brake pads and callipers and they have two functions. Firstly they are a back-up system to guarantee that the rotational speed of the drive train does not escalate to unsustainable levels if the pitch system fails to operate properly. Secondly, they stop the rotor blades when the machine is not operating. In terms of causes of failure, excessive wear of brake linings could entail brake issues and, in extreme situations, even fire. 33

6 4.2.7 Rotor Blades The blades are possibly the most representative component of a wind turbine. Their length has grown year by year to ensure that they sweep a larger area. Whilst this development has dramatically contributed to making wind energy a more competitive generation technology, the increasing dimension of the blades entails proportionally rising loads and challenging design and transportation-related issues. Rotor blades are made of composite materials because they achieve a better strength and stiffness to weight ratio [1], as well as for their higher resistance to corrosion and insulation properties. Examples of materials used in their production are carbon-fibre reinforcing, wood-epoxy laminates or more commonly fibreglass reinforced plastic (GRP). A rotor blade consists of two main parts: Blade reliability is crucial. Being directly responsible for harvesting the mechanical energy contained by the wind, they are completely exposed to its stochastic effect and turbulences. These challenging conditions determine a highly dynamic loading regime, which already becomes a headache at design stage, when blades have to achieve an extraordinarily delicate balance between structural resistance, aerodynamic properties and noise-related performance. Among the various causes of damage to rotor blades, the most common are: defects, lightning strikes or ice build-up where relevant), and Among the main damaging phenomena are (figures 5 and 6): through the deterioration of the bonding agents used at the interface between the various structural elements of a blade, 34

7 air traps between the piles of a blade or poor infusion of resin in a given area that causes poor or no bonding, environment (high rotational speed) and a variety of erosive agents like sand, rain or hail, lightning rods but a frequent cause of accidents given the increasing tip height of modern generators, with the extreme dynamic loads, Figure 5 Example of Rotor Blade damaged by lightning, ( squarespace.com) 35