## 4 Discussion
Indeed, in [146], the fuzzy-based approach reduces the reactive power ripples by 40.5% in comparison with the conventional method (direct power control). For electricity consumers, reactive power control is significant as they must maintain the power factor at the connection point within a certain limit. Thus [132] presents an FLC that controls an SVC, and shows using MATLAB simulations the difference between the Mamdani and Takagi-Sugeno inferences strategies. In [147], the author describes an active power factor correction scheme that contains a boost converter controlled by an FLC with parameters optimized by a pattern search algorithm. According to simulations, the proposed method is effective and more efficient (PF = 0.99987, THD = 1.65%) than a PI controller (PF = 0.99945, THD = 2.35%). Unbalanced three-phase voltages and currents is an issue that appears in three-phase power systems. In the literature, this problem is treated together with other PQ issues that often occur in single-phase power systems as well. For example, the authors of [99] demonstrated, through simulations, the usage of a three-phase shunt APF controlled by a self-tuning control system containing an FLC in decreasing the THD and balancing the three-phase systems of currents and voltages. The authors of [148] present an adaptive hysteresis and FLC that controls a three-phase APF. This PQ improvement system proved to give superior results in simulations and experimental laboratory tests than a PI controller. Considering a holistic approach to PQ improvement, that is, a reduction in all PQ disturbances, the selection of a proper control apparatus is essential. The literature demonstrates that using UPQCs controlled by hybrid FCSs is an excellent choice. The authors of [87] present the use of three type-1 fuzzy controllers to obtain switching signals for PWM logic control that command the UPQC. Simulations showed the efficacy of the proposed method in reducing THD (current from 26.58% to 5.76, and voltage from 46.93% to 3.67%), voltage sags/swells, and unbalance. A multi-feeder UPQC controlled by two FCSs is described in [67], in which a type-1 fuzzy and a fuzzy-PI controller are designed and compared through simulations. The hybrid FCS performed better in decreasing THD, unbalance, and voltage variations. The authors of [113] propose a hybrid FCS that uses two FLCs (Mamdani and Takagi-Sugeno) and the feedback integral phase-locked-loop (PLL) strategy to control a UPQC. They showed the results of the experimental assessment which proved that their method is superior to a Takagi-Sugeno-modified PLL technique in improving harmonic distortions, voltage sags, and unbalance. Indeed, with the proposed method, the THD was 2.13%, and the Takagi-Sugeno method gave a THD of 3.56%. The tendency in recent years has been to study and propose fuzzy control inverters that connect distributed generation units but also reduce PQ issues. For example [100] describes a fuzzy space vector pulse-width modulation technique to control the inverter and improve PQ in a microgrid, without the need to depend on the utility grid. The proposed method efficacy was tested through simulations and plotting voltage, frequency, harmonics, and reactive power characteristics. The authors concluded that their method is superior to the conventional ST-PWM control, as the THD was 1.18% in comparison to 1.53%, and there were less ripples in reactive power. An FLC-based Improved Second-order Generalized Integrator (I-SOGI) scheme that controlled the assembly of a Z-source inverter that compensated a DVR solved PQ issues like balanced and unbalanced voltage sags, swells, and harmonics [131]. Additionally, the authors used the FLC (2 inputs-3 outputs) to tune the PID controller's \(k\) parameters. The simulation showed that the proposed method gave better results than other approaches from the literature. The proposed method's THD was 2%, and the results for the other methods were as follows: ANN--7.5%; RFA--5.5%; and ASO--4%.