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Distribution network is a part of power systems’ general structure which is considered as the last chain in energy generation and transmission to the consumers. One of the most important issues in distribution lines is occurrence of a fault; regarding the innate structure of distribution networks and their numerous branches, fault location is of high importance. Impedance methods are the most common way used to locate the fault. This method faces problems such as multi-response properties, not being able to recognize the correct fault section and not being able to provide a single correct response. In the present study, possible fault locations are recognized first using the improved impedance method. Then, its natural frequencies are determined. Since there are more than one possible fault locations, it is necessary to identify the fault section in order to determine the fault location accurately. The fault section and its main location are determined through simulating the fault in each section of possible locations by small steps and through accommodating the determined frequency to the existing natural frequency registered in the original fault voltage form. The aforementioned method was applied in Salim’s 11-node network in which effect of fault inception angles, effect of different resistors and fault type were tested. Accuracy and efficiency of the proposed method has been verified.

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There is an increasing demand for low-cost single-phase DC–AC inverters in many applications such as PV systems. PV system may be used without the transformer to improve efficiency and make the whole system lighter, smaller, and easier to install. Using transformerless topology, the system efficiency may be increased by about 2%, and the related cost may be decreased by about 25%. There are large leakage currents in transformerless topologies, especially in photovoltaic systems, where safety issues and electromagnetic interference problems often occur. To overcome such disadvantages, common-ground topologies can be used, which minimize the leakage current of the transformerless inverter ones. The transformerless semi-quasi-Z-source inverter (SqZS) with a common-ground structure offers many advantages over conventional single-phase inverters. Leakage current elimination, high power density, lower components, and low-cost features make it an attractive option as a micro-inverter in photovoltaic applications. The basic topology of SqZSI is especially suitable for a PV module in the low-voltage application as a low-cost micro-inverter with high-voltage SiC switching devices. However, the unity voltage gain is one of the disadvantages of the SqZS inverter, which is referred to as a drawback; In other words, the conventional structure of SqZS is not able to step up the voltage and the maximum amplitude of AC voltage that can be extracted is equal to the input DC voltage; Therefore, in this paper, a modified structure of single-phase inverter (MSqZS) is proposed to achieve voltage boost capability. The voltage boost is achieved in a single-stage conversion just by adding an additional series DC blocking capacitor to the basic inverter. It also maintains the common-ground feature. Nonlinear sinusoidal modulation (NLSPWM) is modified to allow the SqZS basic structure to achieve high voltage gain. However, the proposed inverter is modified in topology and modulation; its complexity is not increased in comparison to the basic SqZSI. The proposed inverter has the least number of components than the similar step-up common-ground topologies. In this paper, the closed-loop control is proposed to improve the performance of the MSqZS under variable input voltage as well as output load and compensation for the undesired and non-ideal effect of the parasitic elements. In addition, the proposed inverter is also capable to generate reactive power. Also, the design considerations for series capacitor is analysed for proper capacitor selection. The simulation and experimental results under various closed-loop and open-loop scenarios comply with the IEEE Std 1547 and verify the appropriate performance of the proposed inverter.

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Nowadays, renewable energy is increasingly used in smart grids and microgrids to reduce the use of fossil fuels and improve network efficiency. Like all power system devices, microgrids are subject to transient and steady-state faults, such as short circuits. These faults impair reliability and consumer dissatisfaction. To accurately, automatically, and economically determine the location of a fault, a robust fault location method is needed to stabilize and repair the damaged part of the network. Given the access to the data of all nodes, the fault in these networks can be located based on the data on the two terminals. Accordingly, this paper proposes a method for determining fault distance and faulty section in the island and grid-connected microgrids. The proposed method uses distributed parameters line model and calculates the location of double-phase faults in the microgrid based on voltage and current data on both sides of each section, taking renewable energies and electric vehicles into account. At first, the measurement devices receive and store the current and voltage data at the beginning and end of each section. If a fault occurs, the fault distance is determined by calculating the difference between voltages and currents on both sides of the fault. According to the sampling rate, many voltage and current samples are obtained during the fault. The proposed method calculates a fault distance for each sample. As a result, many fault distances are obtained. These calculations are done for all sections. In the next step, the distances obtained for each section are plotted on the coordinate axis, and a curve is obtained for each section. Among the curves obtained, one curve has a global minimum, which indicates the faulty section. Other curves are ascending or descending. In addition, the global minimum point indicates the calculated distance of the fault from the beginning of the section. This method is not sensitive to electric vehicle models and distributed generation sources and uses only less than half-cycle data to execute the algorithm. The performance of the method is investigated with the simulation of a 9-bus microgrid in MATLAB/SIMULINK. The effects of changes in line parameters (two scenarios), different fault locations, fault resistance (0, 25, and 50 Ω), fault inception angles (36, 90, 180, and 270 degrees), different DGs operation modes (three scenarios), and measurements error (±3%) are studied. The maximum and minimum errors of this method are obtained to be 0.97% and 0.02%, respectively. The results indicate the high accuracy of the proposed method compared to other fault location methods.