Guest Editorial Special Issue on Fault Tolerant Operation and Stability Enhancement of Power Electronics Dominated Grids

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The climate emergency necessitates faster and wide-scale decarbonization of power grids and daily economic activities. As a result, it has triggered large-scale deployments of inertia-less power electronics renewable power generators with intermittent output powers as replacements for existing fossil fuel-based power plants which can dispatch their output powers and possess massive inertia. Power grid decarbonization in this manner presents significant operational and security challenges and exacerbates the risks of instability due to several factors such as low inertia, lack of spinning reserve to quickly nullify active power mismatch between demand and supply, and insufficient fault current for the correct operation of protection systems. Also, the inability to source or sink large active powers in weak ac grids that may result from decommissioning of a large number of existing generators is among the major concerns. Additionally, the transition toward power electronics dominated power systems that consist of numerous grid-following and grid-forming converters and HVdc systems is another major technical challenge that grid operators may face as many of the fundamentals that will dictate steady-state and transient behaviors in the future grid may differ, including methods for assessing grid stability. Modern power converters are versatile and can address many potential challenges that may emerge, nonetheless, their complex dynamics spread over a range of frequencies and must be understood. The highlighted challenges call for new control, protection, technologies, and solutions to be developed to enable the following: safe and reliable operation of power grids during the decarbonization period, in which coexistence of grid-forming and grid-following converters and conventional power plants must be managed carefully, and after the transition to zero-carbon energy systems; increased integration of renewable power generations at all voltage levels; holistic strategies for ac and dc fault handling in both onshore and offshore ac or dc grids; and ways for improving stability and extending active power sourcing and sinking limits in weak ac grids. This Special Issue aimed to foster and document the latest research that addresses the abovementioned emerging challenges. In response to the call for papers, the total number of manuscripts received was 63, and 42 papers were accepted for publication. The submitted manuscripts came from 19 different countries, covering all ten IEEE regions, i.e., China, Brazil, Finland, India, Hong Kong, France, Denmark, Iran, the U.K., the USA, France, Australia, Germany, South Africa, Spain, Canada, Mexico, Belgium, and Taiwan. From the accepted papers, 25 papers address issues directly relevant to this special topic. Broadly, the papers can be categorized as follows: converter topologies, faults and protection, control and stability, and review.

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Keeping the synchronization of a grid feeding converter (GFC) with a weak grid during deep voltage sags has been introduced as a serious challenge in converter-interfaced renewable energy sources dominated weak grids. To deal with this challenge, a simple yet effective solution based on the virtual inertia concept is proposed in this paper. This method is realized by adding a correction term to the DC-link voltage controller, which adjusts the active and reactive current set points and enables the converter to remain synchronized to the grid during severe faults. Closed-loop dynamics of the system in the presence of the parametric uncertainty of the grid-side impedance has been studied, in both normal and fault conditions with different voltage drops. Along these, system performance has been investigated, and in comparison with previous methods, it is revealed that the direct inertial support gain may possibly cause instability and do not propose a stable synchronization process to the GFC under deep faults. The performance of the proposed method has been verified by real-time laboratory results for different resistive/inductive weak grids with various levels of voltage sags. Real-time verification demonstrates the effectiveness of the proposed control in stabilizing GFCs for inertia emulation and its role in a better synchrony process.