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Increasing power demand, aging distribution systems and concerns towards greenhouse gas emissions have resulted in the increased occurrence of distributed generation (DG) within distribution networks. The conventional protection methods designed for passive radial distribution networks may become redundant with large bidirectional power flow and dynamic network topology. Additionally, the anti-islanding protection currently employed limits the benefits of wide-scale DG installation and autonomous operation. The microgrid concept can solve these problems, but several challenges must be overcome before practical implementation. Besides bi-directional power flow, the vast variance between the fault current in grid-connected and autonomous mode and the arbitrary output impedance of the inverter-interfaced DG units in fault conditions and current limiting mode pose a challenge to the protection schemes that use traditional overcurrent protection devices. Many researchers have proposed various techniques, but a robust protection scheme capable of protecting microgrids against different faults for both modes of operation under dynamic network topologies and being financially viable is still to be developed. Hence, the main objective of this paper is to critically review various AC microgrid protection methods proposed in the literature, focusing on analysing the recent protection approaches using modern intelligent techniques. Open research problems and future research trends in AC microgrid protection are also presented in this research. ; No Full Text

Three-phase three-leg inverters represent the majority of inverters that are integrated into low voltage power grids. They can develop positive- and negative-sequence components during unbalanced conditions including asymmetrical faults, but not zero-sequence components. This indicates that a sequence-based control strategy is required for comprehensive control of inverter currents and voltages. Positive- and negative-sequence components must be controlled separately with their own dedicated controllers in such a scheme. Given this critical need, a sequence-based control technique for grid-forming and grid-feeding inverter-interfaced distributed generators is proposed. As a necessary part of the fault ride-through capability of inverter-interfaced distributed generators, a current limiter is developed, which can limit inverter currents at a pre-defined threshold for both balanced and unbalanced faults. Whilst the current regulators work in the synchronous reference frame with simple PI controllers, the limiter works in the natural reference frame and thus treats each phase current separately, which results in current limiting at the threshold for all phases. The advantage of the proposed limiter is that it acts instantaneously such that the inverter current can be limited from the start of the disturbance. The simulation results in MATLAB/SIMULINK reveal promising performance of the proposed control strategy with the proposed current limiting.

Recently, microgrids (MGs) have attracted considerable attention as a high-quality and reliable source of electricity. In this paper, energy management in MGs is addressed in the light of economic efficiency, environmental restrictions, and reliability improvement via: 1) optimizing the type and capacity of distributed generation (DG) sources, as well as the capacity of storage devices (SD); and 2) developing an operational strategy (OS) for energy management in MGs. A master-slave objective function based on net present value (as an economic indicator) is proposed. Such objective function is solved using a hybrid optimization method. This method includes two steps. In the first step, 2-D slave object functions (SOFs), operating costs, and consumer outage cost (as a reliability index) are minimized by quadratic programming and particle swarm optimization (PSO) algorithms, respectively. Then Pareto curve is drawn for SOF and fuzzy logic is employed to select the best SOF solution, OS, from Pareto curve. In the second step, using the best OS from step one for any iteration, PSO algorithms employed to solve master objective function, and to determine the optimum capacity and type of DGs and SDs. The results show that the proposed framework can be considered as an efficient tool in planning and energy management of MGs.

The stable operation of conventional power systems greatly depends on coherent impedances of the bulk power networks’ elements. However, penetration of inverter interfaced distributed generation (IIDG) units put the stability of modern power systems into a risk due the vague and arbitrary output impedance of IIDG units. Besides, the impedance specification of IIDGs can only be established by means of a virtual impedance loop, which needs extra control efforts also imposes voltage drops. Especially, the virtual impedance depends on the output current and cannot be thus freely adjusted. To this end, an optimal voltage regulator (OVR) is proposed for controlling IIDG units to achieve a free/wide range of impedance shaping. The OVR facilitates the optimal impedance shaping based on the control requirement and grid's impedance characteristics, which makes the IIDG units consistent with the power network thus contributing to stabilizing modern power systems. The OVR's control system is based on the state feedback control and the impedance shaping is achieved through an appropriate feedback gain adjustment process. Simulation results prove the effectiveness of the method to achieve the desired impedance shaping.

Fault ride-through (FRT) is essential for inverter-interfaced distributed generation (IIDG) units to protect semiconductor switches from being imposed to overcurrent conditions while the transients are securely passed. To this end, a current limiting strategy is adopted for IIDG units, mostly embedded in control loops, to limit the current within the withstand-able band and to make the IIDG units stay connected to the (micro) grid during the transient. However, the FRT/current limiting of grid-forming inverters affects the transient stability of the autonomous droop-based microgrids and may make them unstable, which yet has not been well-explored in the literature. This issue is considered in this work through a scrupulous observation of the second-order nonlinear differential equation describing the frequency-phase angle dynamics and investigating the problem through the Lyapunov theory. The Chetaev’s instability theorem, which is developed based on the Lyapunov direct method, is used to explore the instability conditions due to the FRT. It is revealed that the phase angle variation, as a consequence of the current limiting, and the arbitrary/resistive transient impedance of grid-forming inverters make the system unstable. Numerical and time-domain results through MATLAB/Simulink platform prove the validity of the models.

Abstract The islanding capability of Microgrids (MGs), when a fault happens in the grid, is seen as one major driver in enhancing the reliable of MGs. However, the use of power electronic interfaces based-MGs faces some difficulties in maintaining stability in an islanding mode due to the low capacity of installed DGs as well as the low speed response of MGs to any load changes. In fact, the lack of availability of spinning reserves makes fast response to load changes, as well as balancing between power generation and load demand difficult. This leads to deviation in voltage and frequency from their (pre-determined) permitted values. In this paper, a new MG’s topology along with a novel control strategy is proposed for stabilizing MGs in different operation modes. Battery storage is used to address the slow response problem of micro sources (MSs) to load changes, and to balance load demands and power generation in the islanding mode of MGs. A droop control idea is adopted in the lowest level of a hierarchy controller to enhance cooperation between power electronic inverters, and to improve load dispatch and voltage regulation. The voltage variation of the MG’s main bus is considered as a load change criterion, instead of the output power of converters, to increase the response speed of the control system. The effectiveness of the proposed control strategy is assessed using MATLA/SIMULINK. It is shown that the proposed control strategy improves the operation of MGs in grid connected and islanding modes where soft transient between these two modes are achieved.

Abstract Recently, microgrids have attracted considerable attention as a high-quality and reliable source of electricity. In this work energy management in microgrids is addressed in light of economic and environmental restrictions through (a) development of an operational strategy for energy management in microgrids and (b) determination of type and capacity of distributed generation (DG) sources as well as the capacity of storage devices (SD) based on optimization. Net present value is used as an economic indicator for justification of investment in microgrids. The proposed NPV-based objective function accounts for the expenses including the initial investment costs, operational strategy costs, purchase of electricity from the utility, maintenance and operational costs, as well as revenues including those associated with reduction in non-delivered energy, the credit for reduction in levels of environmental pollution, and sales of electricity back to the utility. The optimal solution maximizing the objective function is obtained using a hybrid optimization method which combines the quadratic programming (QP) and the particle swarm optimization (PSO) algorithms to determine the optimum capacity of the sources as well as the appropriate operational strategy for the microgrid. The fuzzy set theory is employed to account for the uncertainties associated with electrical power price. Application of the proposed method under different operational scenarios serves to demonstrate the efficiency of the proposed scheme.