## 2 Literature Overview
Constraints related to the test network are summarized in second group as outlined below:
* The values of \(\mathit{THD_{V}}\) and/or \(\mathit{THD_{I}}\) at all buses must be lower than the specified prescribed values based on the standards and regulations that the authors refer to. * The individual values of voltage and/or current harmonics at all buses must be lower than the specified prescribed values, depending on the standards and regulations referenced by the authors. Based on the gap in the literature, this paper focuses on implementing APFs on a radial low-voltage distribution feeder, considering the variability of production and consumption at each bus. The positioning and sizing of the APF will be based on the load range at the buses over time rather than relying on a predetermined operating point at the installation site. As the authors in the literature overview often overlook nonlinearity throughout the whole network, this paper models a full-scale nonlinear state of the network throughout the entire network. Additionally, this paper focuses on an optimization framework that allows monitoring the network's state with respect to power changes and harmonics spectrum during the optimization problem. The co-simulation process described in this paper integrates a metaheuristic optimization method with a simulation tool designed for modeling and analyzing power networks. Specifically, the black-box optimization problem in this co-simulation setup involves an LV radial distribution network model created using DlgSILENT PowerFactory 2022, SP4, which is a specialized power system modeling and analysis simulation tool. This integration results in a user-friendly and time-efficient approach, yielding a more realistic three-phase and unbalanced network model than the traditional manual coding process for developing such a model. The scientific contributions of the paper can be listed as the following:  
* Developing a co-simulation optimization model to determine the parameters and optimal settings of the parallel APF in an unbalanced low-voltage network. * Optimal allocation of APFs with variability of harmonic spectra of different load types. According to this paper's authors, research in optimizing the sizing and placement of APFs should focus on improving the efficiency of optimization processes, particularly in low-voltage networks. Such optimization could offer significant benefits, including enhanced power quality and reduced installation costs. Additionally, the authors focused their research on optimizing the control of a group of APFs, which enables the generation of current or voltage waveforms that monitor the \(\mathit{THD_{V}}\) across all nodes in the network. These waveforms are distinguished from the waveforms when the APF is monitored locally, i.e., when only the connection point is observed. In this context, in addition to the optimal sizing and placement of the APF, the authors propose an algorithm to determine the optimal operating point of the APF, taking into account the current load of the network. At the same time, a co-simulation framework enables more user-friendly integration of a three-phase network model, making it possible to analyze an unbalanced network state, resulting in a more realistic model and reduced coding time. The position and dimensioning of the APFwill be determined by the load in specific nodes rather than by the operating point set on the node where the APF is installed. The paper, through seven sections, first elaborates on the problem of optimal allocation by defining the objective function and associated constraints. Furthermore, the paper evaluates and analyzes the obtained results in detail through the algorithm and optimization framework. Finally, certain conclusions are drawn as well as future research.