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1. A Control Scheme Based on Lyapunov Function for Cascaded H-Bridge Multilevel Active Rectifiers

   https://doi.org/10.1109/APEC39645.2020.9124234  

   Jean-Pierre, Garry ; Altin, Necmi ; Shafei, Ahmad El ; Nasiri, Adel ( March 2020 , 2020 IEEE Applied Power Electronics Conference and Exposition (APEC))

   null (Ed.)

   The cascaded H-bridge multilevel active rectifier is a prominent converter configuration. It presents compelling advantages, including high adjustability for a number of applications, such as in solid-state transformers, traction applications, medium and high power motor drives and battery chargers. However, when the H-bridge is operating under an unbalanced load and asymmetrical voltage conditions, it becomes important to design advanced control strategies to maintain the stability of the system. In this study, a Lyapunov-function based control method is proposed for controlling the single-phase cascaded H-bridge active rectifier to achieve global asymptotic stability. A capacitor voltage feedback is added to the conventional Lyapunov-function based stabilizing control method to minimize the resonance of the LCL filter. Additionally, a Proportional-Resonant (PR) control approach is adopted to obtain the reference current signal. This increases the robustness of the current control scheme. A DC voltage balancing control procedure is also employed to prevent the unbalanced DC voltage conditions among the H-bridges. The DC voltage is controlled via a PI controller. The capability of the control approach is verified with simulation and experimental studies. 

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2. A Matrix Autotransformer Switched-Capacitor Converter for Data Center Application

   https://doi.org/10.1109/TPEL.2023.3308888  

   Qiu, Maohang ; Wei, Mengxuan ; Liu, Xiaoyan ; Meng, Haoran ; Cao, Dong ( December 2023 , IEEE Transactions on Power Electronics)

   This article proposes a matrix auto-transformer switched-capacitor dc–dc converter to achieve a high voltage conversion ratio, high efficiency, and high power density for 48-V data-center applications. On the high-voltage side, the proposed converter can fully leverage the benefits of high-performance low voltage stress devices similar to the multilevel modular switched-capacitor converter. Compared with the traditional isolated LLC converter with a matrix transformer, the proposed solution utilized a matrix autotransformer concept with merged primary and secondary side windings, thus leading to reduced transformer winding loss. The resonant inductor could be integrated into the transformer similar to the LLC converter. Because of the matrix autotransformer design, it can achieve a current doubler rectifier on the low voltage side. For less than 8-V low output voltage application, the current doubler rectifier design can fully utilize the best figure-of-merit 25-V device, which is more efficient than the full-bridge rectifier solution using two 25-V devices during the operation. All the devices can achieve zero voltage switching or zero current switching and can be naturally clamped without additional clamping circuits. A 500-W 48-V to 6-V dc–dc converter hardware prototype has been developed with optimized device selection and integrated matrix autotransformer design. Both simulation and experiment results have been provided to validate the features and benefits of the proposed converter. The maximum efficiency of the proposed converter can reach 98.33%. 

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3. Model Predictive Current Control with Inherent Damping for Inverters under Weak Grid Conditions

   Lee, S. and ( January 2019 , Proceedings of the 52nd Frontiers of Power Conference)

   null (Ed.)

   Increased capacity associated with renewable energy sources has created a need for improved methods for controlling power flows from inverter-based generation. This research provides a comparative study of finite-control-set model predictive current control (FCS-MPC-based) with respect to conventional proportional-integral-based (PI-based) synchronous current control for a three-phase voltage source inverter (VSI). The inverter is accompanied by an inductive-capacitive-inductive (LCL) filter to attenuate pulse width modulation (PWM) switching harmonics. However, an LCL filter introduces a resonance near to the control stability boundary, giving rise to substantial complexity from a control perspective. In order to avoid potential instability caused by the resonance, active damping can be included in the PI-based current control. Though properly designed active damping can improve inverter stability, in practice the robustness of standard PI control is not attainable due to variability in the grid inductance at the point of common coupling (PCC). This is due to impedance variations causing large shifts in the LCL resonance frequency. Weak grid conditions (i.e., a low short-circuit ratio) and a correspondingly high line impedance are particularly susceptible to LCL induced resonance instabilities. As an approach to operate with grid impedance variations and weak grid conditions, FCS-MPC has the potential to produce superior performance compared to PI-based current control methods. This comparative study indicates that FCS-MPC has improved resonance damping and fast dynamic capability in a system with renewable energy sources under weak grid conditions. Detailed results from MATLAB/SimPower are presented to validate the suggested FCS-MPC method where it is robust to uncertainty in the grid impedance variations. Overall results indicate an improvement over conventional PI-based current control methods. 

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4. 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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5. A Control Scheme for a DC Extreme Fast Charger with RMS Current Minimization

   https://doi.org/10.1109/PEDG51384.2021.9494232  

   Jean-Pierre, Garry ; Beheshtaein, Siavash ; Altin, Necmi ; Nasiri, Adel ( June 2021 , 2021 IEEE 12th Annual International Symposium on Power Electronics for Distributed Generation Systems)

   In this study, a power converter topology and control schemes for the power converter stages are proposed for a DC extreme fast charger. The proposed system is composed of a cascaded H-bridge (CHB) converter as the active front end (AFE), and an input series output parallel (ISOP), which includes three parallel connected dual active bridge (DAB) cells. A modified Lyapunov Function (LF) based control strategy is applied to obtain high current control response for the AFE. An additional controller to remove the voltage unbalances among the H-bridges is also presented. Additionally, the triple phase-shift (TPS) control method is applied for the ISOP DAB converter. A Lagrange Multiplier (LM) based optimization study is performed to minimize the RMS current of the transformer. The performance of the proposed converter topology and control strategies is validated with MATLAB/Simulink simulations. 

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