Synchronization of single-phase inverters is a challenging task due to the difficulty of deriving a rotating voltage frame, in the absence of adequate information from the other two phases. Moreover, modern standards, such as the fault ridethrough (FRT) directives, require inverter-based distributed generators (IBDGs) to respond as fast as possible to grid disturbances; thus, it is necessary to rely on accurate and fast synchronization algorithms. The scope of this paper is to propose a new synchronization technique, which satisfies three important requirements: a) fast dynamic response, b) adequate double frequency rejection, c) low computational complexity. Our synchronization technique is a flexible method relying on variablelength transfer delay, which calculates network frequency by analyzing voltage angle differentials. To enhance this differentiation process and address potential discontinuities, we introduce Heaviside-based functions. Both simulations conducted in MATLAB/Simulink and experimental trials demonstrate that our proposed synchronization method outperforms the most widely adopted existing techniques in terms of dynamic response and computational efficiency. Due to its excellent dynamic performance, the proposed method can offer a stable FRT capability, with fast detection of the voltage dips and seamless resynchronization following fault clearance, all while preventing DC-link overvoltage issues.

This study explores the challenges of frequency instability in isolated island power systems predominantly powered by fluctuating renewable sources like solar panels and wind turbines. These non-interconnected islands with a high reliance on renewables are prone to significant frequency fluctuations due to the variable nature of renewable energy sources and their inherently low inertia. The academic community has identified fast-reacting energy storage systems, such as batteries and pump storage, as potential solutions to offset these frequency variations through rapid frequency response services during power imbalances. However, simulations conducted in this paper, suggest that storage systems might struggle to effectively counter severe frequency excursions. This limitation stems from inherent delays in measurement, computation, and communication within their control systems. To improve the stability of these renewable-heavy, non-interconnected islands, we propose a viable approach: integrating renewables behind the energy storage systems. This method totally decouples renewables from the grid, ensuring that the network frequency and the rotational speed of thermal generators remain unaffected by abrupt changes in renewable power output. Adopting this strategy could facilitate the integration of additional renewable energy sources in non-interconnected islands without compromising the stability of their power systems.

This paper presents a sophisticated centralized secondary controller tailored for inverter-based, islanded Microgrids (MGs) that are prone to unforeseen network partitions triggered by faults activating protective devices. In typical radialconfigured MGs, a line fault can cause protective devices to isolate the faulted line, thereby splitting the MG into two electrically independent sub-microgrids (SMGs), while retaining the existing communication and control framework. Diverging from both centralized and distributed traditional secondary controllers, which often fail to normalize the frequency in one of the split SMGs, the proposed controller exhibits exceptional performance. Through simulation studies on 6-bus and 13-bus islanded MG setups, our controller has demonstrated not only its ability to swiftly restore nominal frequency in both SMGs within a few seconds, but also to ensure fair power distribution among the distributed generators (DGs) serving the SMGs. This rapid frequency stabilization underscores the controller's effectiveness in maintaining stable frequency levels immediately following a fault.

This paper examines the steady-state impacts of cyberattacks, communication packet losses, and communication breakdowns on the consensus secondary control of islanded Microgrids (MGs). Four (4) scenarios are analyzed, encompassing various communication dysfunctions, such as Denial-of-Service (DoS) and False Data Injection (FDI) attacks, communication packet loss, and complete breakdowns of communication links. Our goal is to offer a clear and concise interpretation of how these communication disruptions, if undetected, can influence the steady-state performance of the consensus-based distributed secondary control of islanded MGs. Based on the simulation results, communication dysfunctions can deteriorate the secondary control of islanded MGs, causing considerable degradation of the uniform power sharing of DGs and compromising the restoration of frequency.

Fuse saving protection is usually applied in distribution networks, through auto-reclosing, to prevent fuse burning under transient faults. The modern fault ride-through (FRT) standards, which require distributed generators (DGs) to remain connected during the fault, may lead to the miscoordination of recloser-fuse. This paper proposes a discrete coordinate-descent (DCD) approach that optimizes the FRT of DGs so that the coordination between recloser and fuse is preserved. First, the method performs a short-circuit calculation (SCC) considering the currents contributed by the DGs, according to the FRT standards of each country. Then, if a miscoordination between recloser-fuses is observed, it optimally scales down the DG currents, until the coordination is restored. The proposed method has simple implementation, low communication requirements and offers adequate voltage support during the fault. Simulations were carried out in the IEEE 13-bus and the IEEE 8500-node network to highlight the distinct features of the proposed approach against the state-of-the-art current limiting approaches.

This paper presents a comprehensive three-bus equivalent circuit model of three-phase step voltage regulators. The proposed model can be efficiently integrated in the Z-bus power flow method and can accurately simulate any configuration of step voltage regulators. In contrast to the conventional step voltage regulator models that include the tap variables inside the YBUS matrix of the network, the proposed model simulates them in the form of current sources, outside the YBUS matrix. As a result, the re-factorization of the YBUS matrix is avoided after every tap change reducing significantly the computational burden of the power flow. Furthermore, possible convergence issues caused by the low impedance of step voltage regulators are addressed by introducing fictitious impedances, without, however, affecting the accuracy of the model. The results of the proposed step voltage regulator model are compared against well-known commercial softwares such as Simulink and OpenDSS using the IEEE 4-Bus and an 8-Bus network. According to the simulations, the proposed model outputs almost identical results with Simulink and OpenDSS confirming its high accuracy. Furthermore, the proposed 3-bus equivalent model is compared against a recently published conventional step voltage regulator model in the IEEE 8500-Node test feeder. Simulation results indicate that the proposed step voltage regulator model produces as accurate results as the conventional one, while its computation time is significantly lower. More specifically, in the large IEEE 8500-node network consisting of four SVRs, the proposed model can reduce the computation time of power flow around one minute for every tap variation. Therefore, the proposed step voltage regulator model can constitute an efficient simulation tool in applications where subsequent tap variations are required.

This paper proposes a comprehensive optimization approach based on linear programming (LP) for the installation of multiple hybrid power plants (HPPs) in non-interconnected islands. Contrary to the current state-of-the-art solutions, the proposed approach optimizes simultaneously the size, location, and technology of each HPP in order to minimize the long-term electricity cost of the island. The optimization problem is formulated as a LP problem to ensure convergence and global optimum solution. Moreover, a series of system constraints are included in the optimization problem, e.g., power reserves, transmission constraints, etc., to ensure the secure and reliable operation of the grid; this is compatible with the actual preventive measures imposed by the network operator in real non-interconnected islands. Simulations are executed in a real Greek Island (Rhodes), confirming the applicability of the proposed method as an optimization tool for network planning studies in non-interconnected islands.

The secondary control is applied in islanded Microgrids (MGs), after the primary control, in order to restore the buses voltage and frequency to specified values. The existing power flow methods can accurately calculate the power flow for droop-controlled islanded MGs, but in many cases, they cannot calculate the steady-state solution of the MG after the action of secondary controllers. The main challenge in the steady-state modelling of the secondary layer lies in that the secondary controllers consist of integral parts, which can integrate functions with different integral histories, and therefore, under certain circumstances, can imply inaccurate power sharing between the distributed generation units (DGs). This phenomenon is most pronounced under communication failures, as will be shown in the simulations. In this way, this paper proposes a power flow method for calculating, accurately, the steady-state solution of hierarchically controlled islanded AC MGs, including droop-based primary control and secondary control. The paper includes four main features: a) generalized implementation in several communication strategies e.g., centralized, decentralized, consensus, distributed averaging, b) precise simulation of communication links and integral parts of secondary controllers, c) low computation time, and d) accurate 4-wire network representation. Simulations were executed to validate the proposed method against Simulink and to highlight the importance of an accurate modelling of secondary control in the power flow method for islanded MGs

In this manuscript, a novel Δ-circuit approach is proposed, which enables the fast calculation of fault currents in large islanded AC microgrids (MGs), supplied by inverter-based distributed generators (IBDGs) with virtual impedance current limiters (VICLs). The concept of virtual impedance for limiting the fault current of IBDGs has gained the interest of research community in the recent years, due to the strong advantages it offers. Moreover, Δ-circuit is an efficient approach, which has been widely applied in the past, for the calculation of short?circuit currents of transmission and distribution networks. However, the traditional Δ-circuit, in its current form, is not applicable in islanded MGs, due to the particular characteristics of such networks, e.g., the absence of a slack bus. To overcome this issue, a novel Δ-circuit approach is proposed in this paper, with the following distinct features: a) precise simulation of islanded MGs, b) fast computational performance, c) generic applicability in all types of faults e.g., single-line, 2-line or 3-line faults, d) simple extension to other DG current limiting modes, e.g., latched limit strategy etc. The proposed approach is validated through the time-domain software of Matlab Simulink, in a 9-bus and 13-bus islanded MG. The computational performance of the proposed fault analysis method is further tested in a modified islanded version of the IEEE 8500-node network.

This paper deals with a new smooth line-switching method that facilitates the network reconfiguration of islanded networks. Its distinct features include the ability to handle network asymmetries and to minimize the line current during the switching action. This is attained by developing a three-phase sensitivity-based method to determine the operating set-points of distributed generators (DGs) that minimize the current of the candidate line participating in the switching action. These set-points correspond to the positive-sequence powers as well as the negative- and zero-sequence currents of DGs. Furthermore, the network constraints such as voltage limits and power limits of DGs are always satisfied. Simulations are performed in a balanced 33-bus islanded network as well as in the unbalanced IEEE 8500-node network to evaluate the performance of the proposed method.

Power flow is an integral part of distribution system planning, monitoring, operation, and analysis. This two-part paper proposes a sensitivity-based three-phase weather-dependent power flow approach for accurately simulating distribution networks with local voltage controllers (LVC). This part II, firstly, presents simulation results of the proposed approach in an 8-Bus and 7-Bus network, which are validated using dynamic simulation. Secondly, simulation results for the IEEE 8500-node network are also presented. An extensive comparison is conducted between the proposed sensitivity-based approach and the other existing power flow approaches with respect to result accuracy and convergence speed. Moreover, the influence of weather and magnetic effects on the power flow results and the LVC states is also investigated. Simulation results confirm that the proposed sensitivity-based approach produces more accurate results than the existing approaches since it considers the actual switching sequence of LVCs as well as the weather and magnetic effects on the network. Moreover, the proposed algorithm exhibits accelerated convergence due to the usage of the sensitivity parameters, which makes it an important tool for distribution system analysis.

This manuscript proposes a time-series temperature-dependent power flow method for unbalanced distribution networks consisting of underground cables. A thermal circuit model for unbalanced three-phase multi-core cables is developed to estimate the conductor temperature and resistance of Medium and Low Voltage distribution networks. More specifically, a novel approach is proposed to model and estimate the parameters of the three-phase thermal circuit of 3/4-core cables, using the results of Finite Element Method and Particle Swarm Optimization. The proposed approach is generic and can be accurately applied to any kind of 3- or 4-core cables buried in homogeneous or non-homogeneous soil. Furthermore, it is applicable in cases where one or more adjacent cables exist. Using the proposed approach, the conductor temperature of each phase can be individually and precisely calculated even in networks with highly unbalanced loads. The proposed approach is expected to be an important tool for simulating the steady state of unbalanced distribution networks and estimating the conductor temperatures. The proposed thermal circuit is validated using two 4-core LV and one 3-core MV cables buried in different depths in homogeneous or non-homogeneous soil. Time-series power flow for a whole year is performed in a 25-bus unbalanced LV network consisting of multicore underground cables.