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1. Sliding Mode Control Scheme of a Cascaded H-Bridge Multilevel Active Front End Rectifier

   https://doi.org/10.1109/PEDG51384.2021.9494173  

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

   In this study, a sliding mode control (SMC) scheme is proposed for the single-phase cascaded H-bridge (CHB) multilevel active front end (AFE) rectifier with LCL filter. A PI controller is employed to control the DC voltage of the rectifier modules and to obtain the amplitude for the reference grid current. The SMC based current control scheme uses the grid current and filter capacitor voltage feedbacks. The resonance of the LCL filter is damped using the voltage feedback of the capacitor. Therefore, the requirement for additional damping circuitry is removed. Simulation and experimental results are presented to verify the performance of the SMC for the CHB multilevel AFE rectifier. The overall proposed control scheme provides almost unity power factor and fast transient response. It is seen from the results that the current drawn from the grid is in sinusoidal waveform with low THD. 

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2. 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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3. Stability Analysis of Multiple Grid-Connected Inverters Using Different Feedback Currents

   https://doi.org/10.1109/PEDG.2018.8447657  

   Carballo, David; Escala, Edgar; Balda, Juan Carlos (June 2018, 2018 9th IEEE International Symposium on Power Electronics for Distributed Generation Systems (PEDG))

   Distributed generation is gaining greater penetration levels in distribution grids due to government incentives for integrating distributed energy resources (DERs) and DER cost reductions. The frequency response of a grid-connected single inverter changes as other inverters are connected in parallel due to the couplings among grid inductance and/or inverter output filters. The selection of the inverter- or grid-side currents as feedback control signals is then not trivial because each one has tradeoffs. This paper analyses the system stability for multiple parallel- and grid-connected inverters using the inverter- or gridside currents as feedback signals. Modeling of both feedback signals is performed using the current separation technique. Moreover, the stability range for different conditions including active damping is analyzed through the root locus technique. The grid-side current has a wider range of stability, but the inverterside current allows for higher values of the proportional gain near the critical frequency and no extra sensors are needed since measurement of the inverter current is needed for protection in high-power applications. 

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4. 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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5. 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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