## 3 Results  
### Bibliometric Analysis  
The set of papers considered in this review includes publications from journals and conference proceedings. The results of the paper selection process from the Web of Science were as follows:  
* Initial unfiltered set of papers--460 papers. This set contains the raw list of papers used in the review. * Initial set of papers--278 papers. The initial set is a list of papers that correspond to the subject of power quality improvement using fuzzy control. * Intermediary set of papers--122 papers. These articles were the most relevant of the initial set of papers that were published in the last 15 years (2009-2023). * Final set of papers--135 papers. The final set of papers that was considered included the previous ones plus 13 more papers available before 2009. Going forward, all recommendations and observations we make are regarding this final set of papers. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Analysis Criteria** & **Details** & **Examples** \\ \hline \multirow{3}{*}{Control system} & Control systems may include not only a typical FLC, but also other elements from conventional methods or other AI techniques. & Typical FLC (Type-1 fuzzy) Fuzzy-PI \\  & The number of inputs and outputs of the controller, the number of linguistic variables, the types of fuzzy membership functions, and defuzzification methods. & Hysteresis fuzzy control \\  & The number of inputs and outputs of the controller, the number of linguistic & Triangular, trapezoidal, and gaussian fuzzy numbers \\  & variables, the types of fuzzy membership functions, and defuzzification methods. & Center of gravity \\ \hline \multirow{3}{*}{FLC’s inference strategy} & The inference strategy refers to the way the output is obtained. It is implemented in the FLC to realize the aggregation of the rules and combine them. & Mamdani \\  & The type of disturbance affecting the power quality in power systems [38]. & Frequency variation \\  & The number of equations of the rules and combine them. & Voltage variation, harmonics, \\ \hline \multirow{3}{*}{PQ issue} & Devices used for PQ improvement & SVC \\  & depend on the type of PQ issue and its location in the power system [39]. & UPQC \\  & The control systems were implemented & DVR \\  & The control systems were implemented & STATCOM \\ \hline \multirow{3}{*}{Implementation (efficacy proof)} & The control systems were implemented & \multirow{3}{*}{Limitations} \\  & and tested using simulations, that is, employing appropriate environments like MATLAB and DlgSILENT, or experimental laboratory or infield tests. & Laboratory or on-site hardware \\ \hline \multirow{3}{*}{Assessing efficiency and improvements} & Was the control system assessed for improvement and/or was the fuzzy controller tested considering different fuzzy numbers, inferences, and/or defuzzification methods? & Yes/No \\ \cline{1-1}  & Was the control system assessed for improvement and/or was the fuzzy controller tested considering different fuzzy numbers, inferences, and/or defuzzification methods? & Yes—testing diverse features of FLCs or enriching FLCs with additional elements \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Assessment criteria of the article set—overview of the criteria used to classify and assess the articles in the final set. The first noticeable applications of fuzzy control in PQ were in the 1990s. An assessment of the publications from the 1990s and 2000s showed that, yearly, less than ten articles were published in the research area of interest. Later, the number of articles grew substantially, with a maximum of 43 in 2016. When focusing on the most relevant journal, _IEEE Access_ was the most prolific, with 13 publications, followed by _Electric Power Systems Research, IEEE Industrial Electronics Society_, and _IET Generation, Transmission & Distribution_, with more than 8 articles each. Regarding conference proceedings, relevant papers from the initial set were distributed among many of them, but the majority were found in the proceedings published before 2020.