## 3 Results
### Technical Analysis
#### 3.2.4 Power Quality Issues  
The power quality issues described in the analyzed papers were very varied, from frequency variations to high-voltage THD, from voltage sags/swells to low-power factors. Considering the number of PQ issues considered, 88 studies focused on one problem, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **Inference Strategy** & **Efficacy** & **Efficiency** \\ \hline
[90] & _Takagi–Sugeno_ & Simulations & Proposed method results are compared to results obtained with a PI controller or without a PQ improvement apparatus \\ \hline
[91] & _Mamdani_ & Simulations and experimental tests & Comparison with a PI controller \\ \hline
[92] & _Takagi–Sugeno_ & Simulations & Comparison with a conventional PI controller \\ \hline
[94] & _Mamdani_ & Simulations & Comparison between different versions of the proposed fuzzy-based on-load tap changer control (OLTC) and a classic OLTC \\ \hline
[95] & _Product inference_ & Simulations & Comparison with a PI controller \\ \hline
[96] & _Explained but not classified_ & Simulations and experimental tests & Simulation results of the proposed method compared with results associated with a PI controller \\ \hline
[89] & _Takagi–Sugeno_ & Simulations and experimental tests & No additional methodology is tested, only the proposed method considering diverse situations \\ \hline
[97] & _Mamdani_ & Simulations & Comparison with a PID controller \\ \hline
[98] & _Takagi–Sugeno_ & Simulations & Proposed adaptive fuzzy compared with conventional fuzzy controller \\ \hline
[99] & _Min inference_ & Simulations & Results comparison without the control system \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Relevant papers considering the inference strategy of the FLCs. Figure 5: Article distribution regarding inference strategy. T-S—Takagi–Sugeno. whereas the other 46 were on multiple issues. Figure 6 illustrates these aspects, as one can see that the first seven categories describe only one disturbance, e.g., voltage sag/swell or harmonics/THD, and the last eight categories describe multiple disturbances, such as frequency variations, harmonics/THD, and reactive power (stability power systems). Assessing the data from the chart in Figure 6, we deduced that diminishing harmonic distortions and decreasing THD was the goal of almost a quarter of the studies that used fuzzy logic control, i.e., 33 papers. Furthermore, the number of studies that focused only on one disturbance represented more than 65% of the total of 134 analyzed papers. The next most studied PQ issues were voltage variations (category 6) and the combination of frequency variations, harmonics/THD, and reactive power (category 8), which influence the stability of power systems. Closely behind, 12 studies focused on the power system's frequency control and the combinations of harmonics with reactive power, frequency variations, voltage variations, harmonics/THD, and reactive power. Table 7 lists 10 papers that are relevant to the categories mentioned in Figure 6. The table provides information about the PQ issues addressed in each paper, a brief summary of the proposed method, and the results of the efficacy and efficiency testing of the fuzzy-based control systems.