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1. Fast Detection of Open Circuit Device Faults and Fault Tolerant Operation of Stacked Multilevel Converters

   https://doi.org/10.1109/APEC.2019.8721923  

   Hekmati, Parham; Brown, Ian P.; Shen, Z. John (March 2019, 2019 IEEE Applied Power Electronics Conference and Exposition (APEC))

   This paper proposes a simple and fast technique for power device open circuit (OC) fault detection in stacked multicell converters (SMCs). A mitigation technique allowing for fault-tolerant operation using a simple front-end routing circuit is also proposed for SMCs. The fault detection concept only needs to sense the voltage and direction of current at the output terminal of the SMC to detect and localize an OC switch fault to a particular rail of the SMC. The proposed technique compares the measured and expected voltage levels considering the commanded switch states and the direction of the terminal current flow. Once an OC fault is detected and localized, the front-end routing circuit will be activated to reconfigure the SMC converter to a simple flying capacitor multilevel converter (FCMC) to maintain the output power flow with a reduced number of voltage levels. A window detector circuit is proposed to track the output voltage level and current direction with high bandwidth. Simulations were performed to validate the fault detection method and router performance. The functionality of windows detector is investigated with a hardware prototype 7 level 300 V SMC. 

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2. Prognostic Health Monitoring of DC Microgrid with Fault Detection and Localization using Machine Learning Techniques

   https://doi.org/10.1109/ECCE53617.2023.10362739  

   Bohara, Bharat; Krishnamoorthy, Harish S. (October 2023, IEEE)

   DC microgrids incorporate several converters for distributed energy resources connected to different passive and active loads. The complex interactions between the converters and components and their potential failures can significantly affect the grids' resilience and health; hence, they must be continually assessed and monitored. This paper presents a machine learning-assisted prognostic health monitoring (PHM) and diagnosis approach, enabling progressive interactions between the converters at multiple nodes to dynamically examine the grid's (or micro-grid's) health in real time. By measuring the resulting impedance at the power converters' terminals at various grid nodes, a neural network-based classifier helps detect the grid's health condition and identify the potential fault-prone zones, along with the type and location of the fault type in the grid topology. For a faulty grid, a Naive Bayes and a support vector machine (SVM)-based classifiers are used to locate and identify the faulty type, respectively. A separate neural network-based regression model predicts the source power delivered and the loads at different terminals in a healthy grid network. The proposed concepts are supported by detailed analysis and simulation results in a simple four-terminal DC microgrid topology and a standard IEEE 5 Bus system. 

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3. A Fault-Tolerant Hybrid Cascaded H-Bridge Topology

   https://doi.org/10.1109/ECCE.2019.8912836  

   Mhiesan, Haider; Mantooth, Alan; Siwakoti, Yam P. (September 2019, 2019 IEEE Energy Conversion Congress and Exposition (ECCE))

   null (Ed.)

   This paper presents the fault-tolerant operation for a cascaded H-bridge (CHB) inverter. The added features ensure reliable and robust operation in the event of a fault. The proposed strategy uses an additional cross-coupled CHB (X-CHB) unit in companion with the existing CHB to support the output voltage and ensure continuity of operation in case of an open/short circuit fault. The operation of the proposed X-CHB inverter is described in detail. Simulation and experimental verification of the proposed concept is demonstrated using a seven-level CHB. Both simulation and experimental results validate the fault-tolerant operation of the CHB for a battery energy storage system (BESS) in case of switch faults such as open/short-circuit switch faults or dc-source or battery failure. 

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4. Protection in Inverter-Dominated Grids: Fault Behavior of Grid-Following vs. Grid-Forming Inverters and Mixed Architectures—A Review

   https://doi.org/10.3390/en19030684  

   Nurunnabi, Md; Li, Shuhui (February 2026, Energies)

   The rapid rise of inverter-based resources (IBRs) such as solar, wind, and battery energy storage is transforming power grids and creating new challenges for protection. Unlike synchronous generators, many IBRs are interfaced through grid-following (GFL) inverters that operate as controlled current sources and rely on an external voltage reference, resulting in fault responses that are current-limited and controller-shaped. These characteristics reduce fault current magnitude and can undermine conventional protection schemes. In contrast, emerging grid-forming (GFM) inverters behave as voltage sources that establish local voltage and frequency, offering improved disturbance support but still transitioning to current-limited operation under severe faults. This review summarizes GFL versus GFM operating principles and deployments, compares their behavior under balanced and unbalanced faults, and evaluates protection impacts using a protection-relevant taxonomy supported by illustrative electromagnetic transient (EMT) case studies. Key challenges, including underreach/overreach of impedance-based elements, reduced overcurrent sensitivity, and directional misoperation, are identified. Mitigation options are discussed, spanning adaptive/supervised relaying, communication-assisted and differential protection, and inverter-side fault current shaping and GFM integration. The implications of IEEE 1547-2018 and IEEE 2800-2022 are reviewed to clarify ride-through and support requirements that constrain protection design in high-IBR systems. 

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5. Control Methodologies to Mitigate and Regulate Second-Order Ripples in DC–AC Conversions and Microgrids: A Brief Review

   https://doi.org/10.3390/en16020817  

   Chaturvedi, Shivam; Wang, Mengqi; Fan, Yaoyu; Fulwani, Deepak; Hollweg, Guilherme Vieira; Khan, Shahid Aziz; Su, Wencong (January 2023, Energies)

   Second-order ripples occur in the voltage and current during any DC–AC power conversion. These conversions occur in the voltage source inverters (VSIs), current source inverters (CSIs), and various single-stage inverters (SSIs) topologies. The second-order ripples lead to oscillating source node currents and DC bus voltages when there is an interconnection between the AC and DC microgrids or when an AC load is connected to the DC bus of the microgrid. Second-order ripples have various detrimental effects on the sources and the battery storage. In the storage battery, they lead to the depletion of electrodes. They also lead to stress in the converter or inverter components. This may lead to the failure of a component and hence affect the reliability of the system. Furthermore, the second-order ripple currents (SRCs) lead to ripple torque in wind turbines and lead to mechanical stress. SRCs cause a rise in the temperature of photovoltaic panels. An increase in the temperature of PV panels leads to a reduction in the power generated. Furthermore, the second-order voltage and current oscillations lead to a varying maximum power point in PV panels. Hence, the maximum power may not be extracted from it. To mitigate SRCs, oversizing of the components is needed. To improve the lifespan of the sources, storage, and converter components, the SRCs must be mitigated or kept within the desired limits. In the literature, different methodologies have been proposed to mitigate and regulate these second-order ripple components. This manuscript presents a comprehensive review of different effects of second-order ripples on different sources and the methodologies adopted to mitigate the ripples. Different active power decoupling methodologies, virtual impedance-based methodologies, pulse width modulation-based signal injection methodologies, and control methods adopted in distributed power generation methods for DC microgrids have been presented. The application of ripple control methods spans from single converters such as SSIs and VSIs to a network of interconnected converters. Furthermore, different challenges in the field of virtual impedance control and ripple mitigation in distributed power generation environments are discussed. This paper brings a review regarding control methodologies to mitigate and regulate second-order ripples in DC–AC conversions and microgrids. 

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