# New Tendencies in Wind Energy Operation and Maintenance
## 2 Materials and Method  
To achieve the review of the research carried out on the maintenance of wind farms from 2007 to 2020, it is necessary to identify the different fields of O&M cost that are going to be studied. The definition of each group of variables is included below.  
1. Operation and maintenance of wind farms (O&M): The term \"operation and maintenance\" is studied as a variable in itself, since it appears as a generic keyword in the research conducted.
2. Failure rate and analysis (FAIL): Investigations involving issues related to equipment failures are included in this variable.
3. Reliability (RELI): Reliability, availability and maintainability are attributes of the design of systems that have a significant impact on the maintenance of a developed system.
4. Condition-monitoring system (C_M): This variable is the grouping of research that deals with condition monitoring. This tool searches the processes of monitoring a condition parameter in machinery, in order to identify a significant change that is indicative of a fault developing. It is an important component of predictive maintenance.
5. Maintenance strategies (M_STRA): A maintenance strategy defines the rules for the sequence of planned maintenance work; therefore, the different types of maintenance under investigation are grouped in this variable.
6. Maintenance and life cycle cost (M_COST): Research covering topics related to equipment costs are taken into account in this variable.
7. Maintenance optimization (M_OPT): A variable that meets the keywords \"optimization and maintenance processes\".  
To facilitate the study of these fields of O&M cost (Figure 2), a group of keywords is defined for each field (Table 2). It should be noted that several experts, with more than 20 years of professional experience, helped the authors to identify the most important keywords and their grouping methodology. These experts are responsible for the maintenance of several wind farms in the region of Galicia (Spain), affiliated with private companies.  
_Appl. Sci._**2021**, _11_, 1386  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Group** & **Number of Keywords** & **Keywords** \\ \hline O\&M & 1 & Operation and maintenance \\ \hline \multirow{6}{*}{FAIL} & 1 & Failure rate \\  & 2 & Fault detection \\  & 3 & Failure of gearboxes \\  & 4 & Failure analysis \\  & 5 & Failure \\  & 6 & Failure (mechanical) \\ \hline \multirow{6}{*}{RELI} & 1 & Reliability \\  & 2 & Availability \\  & 3 & Maintainability \\  & 4 & Profitability \\  & 5 & Reliability analysis \\ \hline \multirow{6}{*}{C\_M} & 1 & Condition monitoring \\  & 2 & Condition-monitoring systems \\ \cline{1-1}  & 3 & Algorithms \\ \cline{1-1}  & 4 & Condition-based maintenance \\ \hline \multirow{6}{*}{M\_STRA} & 1 & Maintenance strategies \\  & 2 & Low maintenance \\ \cline{1-1}  & 3 & Turbine maintenance \\ \cline{1-1}  & 4 & Maintenance planning \\ \cline{1-1}  & 5 & Maintenance free \\ \cline{1-1}  & 6 & Preventive maintenance \\ \cline{1-1}  & 7 & Predictive maintenance \\ \cline{1-1}  & 8 & Maintenance activity \\ \cline{1-1}  & 9 & Maintenance action \\ \hline \hline \end{tabular}
\end{table}
Table 2: Keywords grouped in the grouping variables.  
Figure 2: Outline of wind energy O&M cost fields.  
Subsequently, a search is carried out for the different keywords in the Scopus database. The search in Scopus is done by searching for the topic \"wind energy\" and each keyword in the title, abstract or keywords. A statistical study of the grouped variables is then carried out. After this study, it will be possible to obtain conclusions on the most important keywords in this area.