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1. Modeling and Stability Analysis of Grid-Connected Inverters with Different LCL Filter Parameters

   https://doi.org/10.1109/eGRID.2018.8598670  

   Carballo, David; Escala, Edgar; Balda, Juan Carlos (November 2018, 2018 IEEE Electronic Power Grid (eGrid))

   Microgrid are gaining popularity due to several advantages like potential for fuel savings and resiliency in case of grid catastrophic failures. In a microgrid, many energy sources like wind and solar farms are connected to the grid through inverters with different power ratings and LCL filter parameters. The inverters incorporated in these systems might have a different frequency response and stability ranges than those inverters with identical LCL filter values. This paper establishes the model and analyzes the stability of a system with multiple paralleled- and grid-connected inverters with different LCL filter paramenters using the grid-side currents as feedback signals. The analysis results showed that a method similar to the interactive and common current analysis technique used on inverters with identical LCL filters can be implemented on a system with different LCL filers to calculate the maximum values of the inverters’ current controller gains without having to derive the complicated equations of the MIMO system. 

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2. Impact of Solar Inverter Dynamics during Grid Restoration Period on Protection Schemes Based on Negative-Sequence Components

   https://doi.org/10.3390/en15124360  

   Ekic, Almir; Wu, Di; Jiang, John N. (June 2022, Energies)

   The growing penetration of renewable resources such as wind and solar into the electric power grid through power electronic inverters is challenging grid protection. Due to the advanced inverter control algorithms, the inverter-based resources present fault responses different from conventional generators, which can fundamentally affect the way that the power grid is protected. This paper studied solar inverter dynamics focused on negative-sequence quantities during the restoration period following a grid disturbance by using a real-time digital simulator. It was found that solar inverters can act as negative-sequence sources to inject negative-sequence currents into the grid during the restoration period. The negative-sequence current can be affected by different operating conditions such as the number of inverters in service, grid strength, and grid fault types. Such negative-sequence responses can adversely impact the performance of protection schemes based on negative-sequence components and potentially cause relay maloperations during the grid restoration period, thus making system protection less secure and reliable. 

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3. Performance Evolution of Combined Grid-Forming and Grid-Following Inverters with Different Filtering Mechanisms

   https://doi.org/10.23919/ICPE2023-ECCEAsia54778.2023.10213884  

   Nurunnabi, Md; Li, Shuhui; Mondal, Hahnemann; Hong, Yang-Ki; Choi, Minyeong; Won, Hoyun (May 2023, 2023 11th International Conference on Power Electronics and ECCE Asia (ICPE 2023 - ECCE Asia))

   This paper discusses the challenges faced by electric power systems due to the increasing use of inverter-based renewable energy resources (IBRs) operating in grid-following mode (GFL) and the limited support they provide for the grid’s reliability and stability. With increased IBRs connected to the grid, electric utilities are increasingly requiring IBRs to behave like traditional grid-forming (GFM) synchronous generators to provide support for inertia, frequency, voltage, black start capability, and more. The paper focuses on developing GFM inverter technologies with L, LC, and LCL filters and investigates the performance of combined GFM and GFL inverters with different filtering mechanisms when supplying different types of loads. It also emphasizes achieving voltage controllability at the point of common coupling of the GFM with the rest of an AC system. EMT simulation is utilized to investigate the interaction of combined GFM and GFL inverters with different filtering mechanisms. The research results will assist electric utilities in ensuring the reliability and stability of electric power systems in the future. 

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4. Machine Learning-Based Protection and Fault Identification of 100% Inverter-Based Microgrids

   https://doi.org/10.1109/ISIE54533.2024.10595742  

   Beikbabaei, Milad; Lindemann, Michael; Kapourchali, Mohammad Heidari; Mehrizi-Sani, Ali (June 2024, IEEE)

   100% inverter-based renewable units are becoming more prevalent, introducing new challenges in the protection of microgrids that incorporate these resources. This is particularly due to low fault currents and bidirectional flows. Previous work has studied the protection of microgrids with high penetration of inverter-interfaced distributed generators; however, very few have studied the protection of a 100% inverter-based microgrid. This work proposes machine learning (ML)–based protection solutions using local electrical measurements that consider implementation challenges and effectively combine short-circuit fault detection and type identification. A decision tree method is used to analyze a wide range of fault scenarios. PSCAD/EMTDC simulation environment is used to create a dataset for training and testing the proposed method. The effectiveness of the proposed methods is examined under seven distinct fault types, each featuring varying fault resistance, in a 100% inverter-based microgrid consisting of four inverters. 

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5. Load Sharing Scheme Incorporating Power Security Margins for Parallel Operation of Voltage Source Inverters

   https://doi.org/10.3390/en14185825  

   Eyisi, Chiebuka; Li, Qifeng (September 2021, Energies)

   null (Ed.)

   The interconnection of distributed energy resources (DERs) in microgrids (MGs) operating in both islanded and grid-connected modes require coordinated control strategies. DERs are interfaced with voltage source inverters (VSIs) enabling interconnection. This paper proposes a load demand sharing scheme for the parallel operation of VSIs in an islanded voltage source inverter-based microgrid (VSI-MG). The ride-through capability of a heavily loaded VSI-MG, where some of the VSIs are fully loaded due to the occurrence of an event is investigated. In developing analytical equations to model the VSI, the concept of virtual synchronous machines (VSM) is applied to enable the VSI mimic the inertia effect of synchronous machines. A power frame transformation (PFT) that takes the line ratios of the MG network into account is also incorporated to yield satisfactory transient responses of both network frequency and bus voltages in the MG network. A Jacobian-based method is then developed to take into account the operational capacity of each VSI in the VSI-MG. The resulting amendable droop control constrains the VSIs within their power capabilities when an event occurs. Simulation results presented within demonstrate the effectiveness of the proposed procedure which has great potential to facilitate efforts in maintaining system reliability and resiliency. 

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