The inevitable transition of the power system toward a sustainable and renewable-energy centered power system is accompanied by huge versatility and significant challenges. A corresponding shift in operation strategies, embracing more intelligence and digitization, e.g., a Cyber-Physical System (CPS), is needed to achieve an optimal, reliable and secure operation across all system levels (components, units, plants, grids) and by the use of big data. Digital twins (DTs) are a promising approach to realize CPS. In this paper, their applications in power systems are reviewed comprehensively. The review reveals that there exists a gap between available DT definitions and the requirements for DTs utilized in future power systems. Therefore, by adapting the current definitions to these requirements, a generic definition of a “Digital Twin System (DTS)” is introduced which finally allows proposing a multi-level and arbitrarily extendable “System of Digital Twin Systems (SDTSs)” idea. The SDTSs can be realized with an open-source framework that serves as a central data and communication interface between different DTSs which can interact by “Reporting Modules” and are regulated by “Control Modules” (CMs). Exemplary application scenarios involving multiple system levels are discussed to illustrate the capabilities of the proposed SDTS concept.

A novel three-phase grid-connected inverter topology with a split dc link and LC filter is proposed. It allows for a full parallel connection of multiple inverters simultaneously on both the ac and dc sides, offering high modularity, redundancy, expandability, and overall system reliability. A generic dynamical system model is derived, considering the coupling effects between parallelized inverters, physical constraints, and varying grid impedance. A decentralized proportional- integral state feedback control (PI-SFC) with an extended Luenberger state observer is developed and compared with a conventional PI controller regulating inverter-side currents. A stability analysis shows that both closed-loop control systems remain stable even if an arbitrarily large number of inverters are connected in parallel. Simulations and experiments confirm the functionality and robustness of the closed-loop system under varying grid impedances and during grid faults. For the experimental results, the controllers were implemented on commercially available hardware of the proposed topology. In particular, the PI-SFC allows for better exploitation of the full power of the inverters due to its enhanced controller performance and damping ability. Besides, the exceptional match between simulation and experimental results proves the accuracy of the proposed system model as well.

A novel three-phase grid inverter topology with a split dc link and LC filter is proposed. It allows for a full parallel connection of multiple inverters simultaneously on both the ac and dc sides, offering exceptional modularity, redundancy, expandability, and overall system reliability. A holistic dynamical system model is derived, considering the coupling effects between parallelized inverters, physical constraints, and varying grid impedance. A decentralized proportional-integral state feedback control (PI-SFC) with an extended Luenberger state observer is developed and compared with a conventional PI controller to control the inverterside currents. A stability analysis shows that both closed-loop control systems remain stable even if an arbitrarily large number of grid inverters are connected in parallel. Simulations and experiments confirm the functionality and robustness of the closed-loop system under varying grid impedances and during grid faults. For the experimental results, the controllers were implemented on commercially available hardware of the proposed topology. In particular, the PI-SFC allows for better exploitation of the full power of the inverters due to its better controller performance and damping ability. Finally, the exceptional match between simulation and measurement results proves the accuracy of the proposed system model as well.

In this work, a novel three-phase grid inverter topology with split DC-link and LC-filter is discussed. A proportionalintegral (PI) state-feedback current and voltage controller is designed based on linear quadratic regulator (LQR) theory. The midpoint current ripple is suppressed by introducing the filter capacitor voltage as control target. Moreover, a 3-dimensional Space Vector Modulation (3D-SVM) for this topology is implemented, which allows for larger voltage amplitudes compared to conventional Pulse Width Modulation (PWM). Realistic simulation results show that the combination of PI state-feedback controller and 3D-SVM achieves faster and more accurate closedloop dynamics with less oscillations and better decoupling under varying and unbalanced grid conditions.

As a crucial characteristic of the grid, its varying impedance affects not only the performance of grid-connected inverters but, also and even more severe, system stability. Emulating varying grid impedances to validate inverter control algorithms and system stability offers greater flexibility than physical setups. Conventional power-hardware-in-the-loop (PHIL) systems incorporating grid impedance emulation are confronted inevitably by stability issues and time delays, which require complex modifications. In this paper, a novel PHIL system for grid (impedance) emulation is proposed, where a communication between simulator and controller is established by derivative-free system dynamics. Moreover, an effective method is proposed that eliminates the delay effects and, therefore, also obviates stability issues (almost) completely. High fidelity simulations validate the feasibility and effectiveness of the proposed PHIL system and highlight the achievable (high) accuracy of the emulation in both transient and steady states conditions.