This paper compares two control schemes in the application control layer of a non-PLL grid-forming (GFM) inverter to gain the insights and understanding of how the two schemes affect the dynamic responses of the GFM inverters and the transition operation of microgrids. Both schemes can achieve smooth microgrid transition operation and power tracking, however, the first scheme expects to exhibit more transients due to the feedforward of the inverse dynamic model of the output filter. Simulation results show that both control schemes achieve successful transition operation with smooth transients, and the second control scheme exhibits slightly better transients in the system frequency during islanding operation and the first one has slightly superior transients in the system frequency during reconnection operation. The power tracking performance in the grid-connected mode is evaluated, which shows that the first scheme has better active power tracking and the second one has better reactive power tracking. The analysis and results are useful to develop reliable control schemes for non-PLL GFM inverters because more and more inverters will work as non-PLL grid-forming sources in the future grids due to better stability and reliability.

Grid-Forming Power Inverters: Control and Applications is the first book dedicated to addressing the operation principles, grid codes, modelling and control of grid-forming power inverters. The book initially discusses the need for this technology due to the substantial annual integration of inverter-based renewable energy resources. The key differences between the traditional grid-following and the emerging grid-forming inverters technologies are explained. Then, the book explores in detail various topics related to grid-forming power inverters, including requirements and grid standards, modelling, control, damping power system oscillations, dynamic stability under large fault events, virtual oscillator-controlled grid-forming inverters, grid-forming inverters interfacing battery energy storage, and islanded operation of grid-forming inverters. Features: Explains the key differences between grid-following and grid-forming inverters Explores the requirements and grid standards for grid-forming inverters Provides detailed modeming of virtual synchronous generators Explains various control strategies for grid-forming inverters Investigates damping of power system oscillations using grid-forming converters Elaborates on the dynamic stability of grid-forming inverters under large fault events Focuses on practical applications

The character of modern power systems is changing rapidly and inverters are taking over a considerable part of the energy generation. A future purely inverter-based grid could be a viable solution, if its technical feasibility can be first validated. The focus of this work lies on inverter dominated microgrids, which are also mentioned as 'hybrid' in several instances throughout the thesis. Hybrid, as far as the energy input of each generator is concerned. Conventional fossil fuel based generators are connected in parallel to renewable energy sources as well as battery systems. The main contributions of this work comprise of: The analysis of detailed models and control structures of grid inverters, synchronous generators and battery packs and the utilization of these models to formulate control strategies for distributed generators. The developed strategies accomplish objectives in a wide time scale, from maintaining stability during faults and synchronization transients as well as optimizing load flow through communication-free distributed control. Die Struktur der modernen Energieversorgung hat sich in den letzten Jahrzehnten massiv geändert. Dezentrale Generatoren, die auf Wechselrichtern basieren, übernehmen einen großen Teil der Energieerzeugung. Ein ausschließlich wechselrichterbasiertes Netz wäre ein realistischer Ansatz, wenn seine technische Machbarkeit verifiziert werden könnte. Die wichtigste Beiträge dieser Arbeit sind: Die Analyse von Modellen und Regelstrukturen von Netzwechselrichtern, Synchrongeneratoren und Batterieanlagen. Die entwickelten Modelle werden verwendet, um Regelstrategien für dezentrale Generatoren in Mittelspannungsinselnetzen zu formulieren. Die erste Strategie ist eine Synchronisationsmethode für netzbildende Wechselrichter. Zweitens wird die Leistungsaufteilung in Mittelspannungsinselnetzen mittels Droop Regelung analysiert. Weiterhin erfolgt die Untersuchung der transienten Lastaufteilung zwischen netzbildenden Einheiten mit unterschiedlichen Zeitkonstanten. Beim Betrieb mehrerer paralleler Wechselrichter wird der Einfluss der Netzimpedanz auf die transiente Lastaufteilung analysiert. Die dritte entworfene Regelstrategie umfasst die Integration der Sekundärregelung in die Primärregelung. Der Ladezustand von Batterien wird mit der Lastaufteilung gekoppelt, um die Autonomie des Netzes zu stärken. Abschließend wird eine Kurzschlussstrategie für netzbildende und netzspeisende Wechselrichter entwickelt. Ziel der Strategie ist die Maximierung des Kurzschlussstromes. Als zusätzliche Randbedingung soll keine Kommunikation zwischen Generatoren stattfinden.

This paper investigates the stability of low-inertia microgrid systems with two control strategies that have different percentages of grid-forming (GFM) inverters. The first control strategy has approximately 50% GFM inverters, and all the battery inverters are working in GFM control mode. Originally, the second control strategy has approximately 10% GFM inverters, with only two battery inverters working in GFM control mode and the rest working in grid-following (GFL) PQ control mode based on current control, which cannot stabilize the microgrid system. Then, the second control strategy is modified to change the GFM inverters from droop control to isochronous control and the GFL battery inverters from traditional current control to voltage control for power control. Both control strategies can maintain system stability; however, the first control strategy can better handle contingency events. The study indicates that 1) a microgrid system with a higher percentage of GFM inverters has better stability; and 2) a microgrid with a lower percentage of GFM inverters can have poor stability, but improved control strategies in inverters can improve system stability. This study improves the understanding of how different percentages of GFM inverters and inverter control strategies affect the system stability of low-inertia microgrids.

This paper develops an improved control strategy of grid-forming (GFM) inverters with fault ride-through (FRT) capabilities to guarantee the stable operation of microgrids under fault conditions, especially islanded microgrids and asymmetrical faults. The proposed control strategy includes the dual control of positive-sequence and negative-sequence control as well as the adaptive virtual impedance (VI) control. Unlike existing works, the proposed strategy applies the VI control for only the d component of the positive-sequence control and leaves the q component of the positive-sequence control and the dq component of the negative-sequence control as zero, thus achieving improved stability and balanced three-phase voltages under asymmetrical faults. The adaptive feature of the VI control guarantees the stability of the GFM inverter under severe faults, which could cause the saturation of the inner current loop, and the instability if the VI is not adaptive. Simulation results of various unbalanced faults with high and low fault impedances show that the proposed control strategy improves the stability of the GFM inverter and achieves stable and balanced output voltages in islanded microgrids. And the algorithm also improves the stability of GFM inverters under balanced faults with high and low fault impedances.