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An Intentional Controlled Islanding (ICI) algorithm that directly determines an islanding solution with minimal power-flowdisruption for any given number of islands, while ensuring that each island contains only coherent generators was previouslyproposed and tested using IEEE test systems. The proposed algorithm is based on an exact Mixed Integer LinearProgramming Formulation (MILP). However, since voltage instability has been found to be one of the main causes of severalrecent blackouts in the world, in this paper, the aforementioned algorithm is extended to consider voltage stability constraints.The extended ICI algorithm guarantees sufficient generation capacity (active and reactive power) to match the loadconsumption within each island. At the same time, it reduces the possibility of voltage collapse by assigning the weakest buses to the islands in such a way to be as close as possible to the sources or sinks of reactive power. The new constraints areexpected to further improve the transient stability of the formed islands.

Renewable sources are expected to be a key element of future power systems. With many challenges ahead, thinking out of the box is necessary in order to equip the power system with additional advanced functionalities and capabilities to combat the anticipated challenges. The associated equipment of distributed generation can be seen as a grid asset upgrade because of the increased number of elements connected to the grid, such as grid side converters (GSC) of photovoltaics and battery storage systems. It therefore makes sense to think of ways to diversify the role of distributed renewables in a way that benefits the grid, consumers, and prosumers in terms of power quality. The work proposes an advanced, multi-function GSC control technique for improving the distribution grid power quality by enabling the injection of asymmetric, DC and harmonic currents in case it is needed. Injecting for example such currents locally through the GSC of a residential photovoltaic system can compensate undesired characteristics of the prosumer loads in order to allow the flow of symmetric and high quality currents between the prosumer and the distribution grid. Experiments and simulation results are presented to validate the performance of proposed control technique.

The integration of modern renewable energy sources is enabled by the grid side converter (GSC) based on power electronic technology. The operation of the GSC is properly regulated by the GSC controller according to sensor measurements (i.e., of the grid voltage, of the line currents, and of the voltage at the dc-link). However, in case of current sensor faults, the operation of the GSC and of the entire renewable system can be critically affected and catastrophic failures may occur if the sensor fault is not accommodated on time. This paper proposes a model-based Fault Detection, Isolation, and Accommodation (FDIA) scheme, which enables the GSC to overcome faults that may occur on the associated current sensors. The sensor fault detection and isolation scheme has been designed based on analytical redundancy relations, while the accommodation of the faults is based on an adaptive estimation scheme. The proposed FDIA scheme has been applied on a modern GSC and the effectiveness of the scheme has been tested under several multiple current sensor faults and under several grid conditions. Furthermore, the FDIA scheme enables the GSC to operate properly (without causing failures or damages to its components) under current sensor faults and thus, the reliability of the GSC is enhanced.

Synchronization is a crucial aspect in grid-tied systems, including single-phase photovoltaic inverters, and it can affect the overall performance of the system. Among prior-art synchronization schemes, the Multi Harmonic Decoupling Cell Phase-Locked Loop (MHDC-PLL) presents a fast response under grid disturbances and high accuracy under harmonic distortions. However, major drawbacks of the MHDC-PLL include increased complexity and inaccurate response under non-nominal frequencies, which may occur in practical applications. Thus, this paper proposes strategies to address these issues. At first, a novel re-formulation of an equivalent decoupling cell is proposed for reducing the implementation complexity, and then a frequency adaptive quadrature signal generator for the MHDC-PLL is proposed to enable an accurate response even under non-nominal frequencies. Simulation and experimental results are provided, which show that the proposed synchronization (i.e., the frequency adaptive MHDC-PLL) can achieve a fast and accurate response under any grid disturbance and/or severe harmonic conditions., This work was supported by the Research Promotion Foundation of Cyprus (RPF, Cyprus, Project KOINA/SOLAR-ERA.NET/0114/02), by Energinet.dk (ForskEL, Denmark) and the SOLAR-ERA.NET (European Union's Seventh Framework Programme). © 2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. L. Hadjidemetriou, Y. Yang, E. Kyriakides and F. Blaabjerg, "A synchronization scheme for single-phase grid-tied inverters under harmonic distortion and grid disturbances," IEEE Trans. Power Electronics, vol. 32, no. 4, pp. 2784-2793, April 2017. DOI: 10.1109/TPEL.2016.2581019. https://www.ieee.org/publications_standards/publications/rights/rights_policies.html

Grid-scale Battery Energy Storage Systems(BESSs) are gaining increasing consideration over the last fewyears since their connection has been shown to improve thedynamic behavior of the power grid. However, existingIntentional Controlled Islanding (ICI) approaches do notconsider the changes in structure and operation that modernpower systems are experiencing due to the increasing integrationof grid-scale BESSs. In this paper a previously proposed ICIalgorithm (for minimal power-flow disruption) is modified toconsider the impact of BESSs on the power system. Morespecifically, the proposed modified ICI algorithm aims todetermine the optimal islanding solution that minimizes thepower imbalance between the generation (i.e., synchronousgenerators and the BESSs) and the consumption (i.e., loads andBESSs) within each island. This is achieved through determiningthe island that each BESS should be, and the optimal activepower set-points for the BESSs that minimize the powerimbalance in the formed islands.

Microgrids composed of solemnly AC-coupled distributed energy resources can be found in many real-life applications while their control has not been researched nearly enough to address some fundamental challenges, the most important of which is overall system reliability and fault tolerance. This paper proposes a droop-based coordinated control scheme for microgrids with AC-coupled units, a method that enables distributed energy resources units to hot swap between current source and voltage source grid-supporting control modes for satisfying load demand and ensuring energy storage systems will constantly be able to form the grid during islanded operation. The proposed control scheme has been realised in MATLAB/Simulink simulation model of a small-scale microgrid of AC-coupled units that corresponds to a real testbed in Northern Greece. Preliminary simulation results, in islanded mode, demonstrate the effectiveness of the proposed control scheme regarding power-sharing accuracy among the resources and state-of-charge balancing among storage units. 1 6

The forward/backward sweep is the most commonly used power flow method in radial Low Voltage Distribution Grids (LVDGs). In most cases, Kron's reduction is used to merge the neutral conductor with the phase conductors. However, important information can be lost by ignoring the effect of the neutral conductor through the Kron's reduction method, since in LVDGs the majority of loads are single-phase connected and are supplied through a phase conductor and the neutral. In this paper, a modification to the forward/backward sweep method is proposed to account for the neutral voltage as well. Test results indicate the significance of knowing the exact configuration of the neutral conductor in an LVDG before conducting power flow studies as it can affect greatly the accuracy of the results. Index Terms-four wire system, low voltage distribution grid, neutral conductor, power flow, radial grid.

Power electronics converters can enable grid support functionalities under abnormal grid conditions. In this paper, a voltage-frequency support strategy based on the reactance to resistance (X/R) ratio of the grid impedance is proposed for a flywheel energy storage system. Unlike conventional support schemes where voltage and frequency support is decoupled, the proposed strategy considers the coupling between voltage and frequency due to the resistive characteristics of the grid impedance in low voltage distribution grids. In addition, the proposed strategy ensures a fair compensation between voltage and frequency support by utilizing two gains that are defined according to the event type. Simulations and experimental tests are carried out using a laboratory setup to validate the proposed strategy. The capabilities of storage systems to provide such tailor-made ancillary services for supporting the distribution grid are demonstrated.

Low-voltage distribution grids face several challenges due to the high penetration of renewable energy sources, the increasing demand (electrification of thermal and transportation sector), and the limited observability (absence of smart metering infrastructure). These challenges impose problems regarding the integrity, stability, power quality and efficiency of distribution grids. Therefore, intelligent solutions are required in order to overcome these problems. This paper proposes a centralized control scheme for phase balancing of low-voltage distribution grids at the substation level. The proposed solution utilizes the advanced capabilities of the grid tied inverters of photovoltaics and storage systems to increase the utilization of distribution lines and to compensate the asymmetric loading conditions of the substation. The solution is based on only one smart meter installed at the distribution substation and utilizes available information and communication technology infrastructure to integrate the solution. The solution achieves significant benefits for the distribution grids, especially regarding their power quality and the effective utilization of their capacity.

This work proposes a sensor-less controller for grid tied photovoltaic (PV) inverters to enable phase balancing functionalities for compensating asymmetric loading conditions imposed by building loads in low voltage distribution grids. For enabling such an advanced functional operation by PV inverter, the first step is to enable the inverter to estimate the equivalent grid impedance. Then, the grid impedance is utilized to approximate the nearby load asymmetries without using any additional current sensors. Finally, advanced control schemes have been developed for PV inverters in order to enable the new phase balancing operation mode, where the inverter can compensate the asymmetric loading conditions of a distribution feeder. The effectiveness of the proposed method has been experimentally validated in a prototype where the grid tied PV inverter is able to compensate nearby load asymmetries and maintain a purely symmetrical interaction with the grid. Further, a simulation-based investigation in a realistic distribution feeder has been performed in order to highlight the benefits of the proposed approach regarding the power quality, the energy losses and the effective utilization of distribution grid capacity.