# New Tendencies in Wind Energy Operation and Maintenance
## 3 Overview of Wind Farm O&M Research
### Reliability (RELI)  
The reliability based on the characteristics of wind energy is the basic signature quality of the wind turbine, so it is necessary to apply a proposal [27] to meet the same objectives; technical reliabilities include design [34], manufacturing and maintenance. The most reliable and sustainable [35] systems have a lower environmental impact and high energy and exergy efficiency. The enormous proliferation of wind farms around the world has emerged as an alternative to traditional power generation and as a result of economic issues that require control systems to [27] optimize the availability and benefits. [PERSON] et al. [36] classified the main designs, focusing on reliability, to collect and compare data within a selection of the most important studies of the literature. The results obtained by the application of several studies indicate the importance of the technical availability factor [37] in annual energy production. Modern onshore wind turbines reach values of 95-99% availability. However, these values only give information from the time that wind turbines are operational and they do not treat the energy produced compared to its theoretical production. This availability, based on energy, differs from the availability based on time due to the fact that forced inactivity times tend to occur during periods of high production potential. The objective of the optimization model [38] is the combination of system maintenance costs and revenue loss of wind energy, which is mainly limited by the reliability of power generation and maximum load regulation adequacy. The proposed model is important to extend classical scheduling maintenance [39] to include the expected market prices and weather conditions. [PERSON] et al. [21] examined the effect of failure rates depending on the wind speed on energy production. [PERSON] et al. [31] focused on the influence of weather and wind turbines based on the failure rate and downtime, to try to understand the causes and consequences of failure. To assess the ability for system maintenance, system reliability is one of the key performance indicators, for instance, in the work [40], Vensim software was applied for the modeling of dynamic systems and software simulation with the objective of evaluating and controlling reliability. Wind turbine reliability analysis is based on the systemic approach of the wind turbine subsystems, as their total reliability depends directly on the reliability of the subsystems. The model used in [14] was a method for identifying likely causes of system failure called fault tree analysis. Simplification of the energy generator units [41] keeps the manufacturing and maintenance costs down and increases reliability. [PERSON] et al. [29] presented the approaches to achieve reliability at the component level and at the level of electrical components. A model of the optimizing maintenance interval for the wind turbine gearbox considering the profit maximization component per unit time is presented in [42]. [PERSON] and [PERSON] [43] considered a failure mode with a corresponding limit state in relation to estimating the reliability, considering failures, for example, due to a fault in an electrical component or loss of grid. The potential benefits of simplifying multiple power converters in each turbine, implying a saving cost, reduced losses and maintenance and increased [44] reliability of the system, were analyzed, focusing on total energy extraction. Additionally, due to the regulation of the electricity market, the distribution network reliability becomes more and more important, and the test after installation or after repair of failures significantly reduces the failure rate during normal operation. [PERSON] and al. [45] described the evolution of the very low frequency technology testing. An intelligent alarm management system [46] for wind farms improves the wind turbine reliability and would reduce downtime, increase availability and lead to a well-organized maintenance program.