## 1 Introduction
In Park and Park [14], a non-MPPT control scheme, called power-weakening control, was developed for excessive power conditioning. In the work by [PERSON] et al. [15], a non-MPPT operation mode of a PV system considering the fault current limiting approach is explored. However, it did not focus on the controller performance under fault conditions. In [PERSON] et al. [16], a modified second-order generalized integrator phase-locked loop (SOGI-PLL) scheme for enhancing the zero voltage ride-through (ZVRT) in a single-phase grid-connected PV system is proposed. Compared to enhanced phase-locked loop (EPLL), the SOGI-PLL approach is faster. However, in SOGI-PLL, overshoot is observed in the process of amplitude detection. In the research by [PERSON] et al. [17], a discrete-time LVRT controller is proposed for a proportional-integral (PI)-controller, in parallel with two modified digital resonant compensators, to mitigate the second-order harmonic voltage oscillation in the DC-link, as well as the positive sequence third-order harmonic current injection into the grid under asymmetrical fault conditions. This study does not discuss the proposed controller's performance under symmetrical fault conditions. In [PERSON] et al. [18], a radial basis function network (RBFN) MPPT controller is proposed, in order to achieve a fast and stable response for the real power control of a hybrid power generation system. This study demonstrated that an RBFN-based MPPT quickly stabilizes at the maximum power point after intense changes. In [PERSON] and [PERSON] [19], dynamic operation and control strategies for a micro-grid are analyzed. A GRNN (general regression neural network) with an improved PSO (particle swarm optimization) algorithm-based MPPT controller is developed to investigate the performance of the PV generation system under transient conditions. A comparison is made with the perturb and observe (P&O) method, which shows that the GRNN controller does not oscillate during tracking to save energy; on the other hand, the P&O method retains oscillating during tracking. It shows a better performance on both transient responses and stability. Unfortunately, these techniques can be costly and difficult to implement. Hence, this literature review infers that the purpose of LVRT enhancement cannot ignore MPPT control techniques. In the work by [PERSON] et al. [20], a current and voltage-limiting strategy to enhance the LVRT capability of inverter-based micro grids (MGs) under symmetrical and asymmetrical faults is proposed. By fixing the current limiting factor (CLF) of the inductor, the current indirectly limits the inverter output voltage of the SPV inverter, and consequently no voltage limit is required in the inner current control loop as proposed in [PERSON] et al. [21]. In the work by [PERSON] [22], a novel unsymmetrical faults analysis method based on the actual three phase models and the boundary conditions to develop completely the network characteristics of micro-grid distribution system (MGDS) is proposed. Two primary matrices (\(\text{B}_{\text{I}}\) and \(\text{Z}_{\text{V-BC}}\)), which are built from the topological characteristics of MGD networks, are used to achieve the power flow solutions. Test results show that the proposed method can achieve high accuracy and efficiency with a lower memory requirement, which could be a useful future smart grid application. A direct building algorithm for micro-grid distribution ground fault (MGDGF) analysis is proposed in [23]. The test results demonstrate that the proposed algorithm is suitable for MGDS, and the ill-conditioned problem that usually occurs in the traditional solution techniques can be avoided here. To overcome the switching frequency limitation problem of the conventional boost converter, quadratic and double cascade boost converters for renewable energy application are proposed in the work by [PERSON] et al. [24], which achieves better efficiency.