@article{rachi_awal_husain_2023, title={Asymmetrical Fault Ride-Through and Power Oscillation Characterization for Grid-Tied Voltage Source Converters}, volume={59}, ISSN={["1939-9367"]}, DOI={10.1109/TIA.2023.3268280}, abstractNote={As the number of converter-interfaced distributed energy resources connected to the power system continues to increase rapidly, recent grid codes require these grid-tied converters to maintain grid connection during faults to ensure power supply security and reliability. In this work, we analyze the oscillation in the injected real and reactive power that a grid-tied voltage source converter (VSC) introduces as it attempts to contribute to the power quality and improve the voltage at the point of common coupling (PCC) during an asymmetrical fault. We propose a new double sequence current reference generation method that can be utilized to derive a closed-form quantification of the peak value of both the real and reactive power oscillation during asymmetrical fault ride-through (AFRT) analytically. The effect of the resulting bus voltage oscillation and ripple current requirement at twice the grid frequency, corresponding to the real power oscillation, on the input DC bus capacitor and upstream converter is analyzed for facilitating system component sizing. Furthermore, the proposed current reference generation formulation helps control the negative-sequence current injection during fault to assist with fault identification. The theoretical analysis is validated through simulation in PLECS for both asymmetrical and symmetrical fault scenarios. Experimental results for a three-phase, grid-tied VSC operating under both asymmetrical and symmetrical faults are provided to evaluate the performance of the proposed current reference generation method and validate the analysis presented}, number={4}, journal={IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS}, author={Rachi, Md Rifat Kaisar and Awal, M. A. and Husain, Iqbal}, year={2023}, pages={4550–4561} }
 @article{awal_rachi_yu_husain_lukic_2023, title={Double Synchronous Unified Virtual Oscillator Control for Asymmetrical Fault Ride-Through in Grid-Forming Voltage Source Converters}, volume={38}, ISSN={["1941-0107"]}, url={https://doi.org/10.1109/TPEL.2022.3227729}, DOI={10.1109/TPEL.2022.3227729}, abstractNote={In this work, a double synchronous unified virtual oscillator controller is proposed for grid-forming voltage source converters to achieve synchronization to the fundamental frequency positive- and negative-sequence components of unbalanced grid voltage without any phase-locked loop. The proposed controller leverages a positive-sequence virtual oscillator and a negative-sequence virtual oscillator, a double-sequence current reference generator, and a double-sequence vector limiter. Under fault conditions, the controller enables to limit the converter output current below/at the maximum value allowable by the converter hardware while retaining synchronization regardless of the nature of grid faults. Consequently, symmetrical and asymmetrical fault ride-through can be achieved without the need for switching to a backup controller. This article presents the implementation and detailed analysis of the double-synchronous structure, which enables simultaneous synchronization to both sequences during current-unconstrained and -constrained operations. Validation of the proposed controller is provided through laboratory hardware experiments.}, number={6}, journal={IEEE TRANSACTIONS ON POWER ELECTRONICS}, author={Awal, M. A. and Rachi, Md Rifat Kaisar and Yu, Hui and Husain, Iqbal and Lukic, Srdjan}, year={2023}, month={Jun}, pages={6759–6763} }
 @article{awal_rachi_yu_schroeder_dannehl_husain_2023, title={Grid-Forming Nature Retaining Fault Ride-Through Control}, ISSN={["1048-2334"]}, DOI={10.1109/APEC43580.2023.10131145}, abstractNote={A ride-through controller is proposed to enable inverter based resources (IBRs) to retain grid-forming (GFM) nature under current-constrained operation, such as faults or overload conditions. In this context, GFM nature for IBR is denoted by a voltage source behind reactance that preserves synchronism with the grid leveraging power-synchronization. A comparative analysis of existing GFM controllers is presented to demonstrate that the current state-of-the-art, either fails to preserve the GFM nature under fault, or offers only sub-optimal ride-through performance with regard to converter capacity uti-lization and transient stability. The proposed solution maximizes capacity utilization while retaining GFM nature under fault as well as enhances transient stability. Furthermore, detection of fault occurrence and/or clearance is not required and the identical control structure is preserved regardless of normal or fault/overload operation. Hence, the proposed controller avoids recurring fault-mode operation observed for existing GFM controllers at fault clearance especially under weak grid conditions. Experimental results are presented to validate the proposed solution.}, journal={2023 IEEE APPLIED POWER ELECTRONICS CONFERENCE AND EXPOSITION, APEC}, author={Awal, M. A. and Rachi, Md Rifat Kaisar and Yu, Hui and Schroeder, Stefan and Dannehl, Jorg and Husain, Iqbal}, year={2023}, pages={2753–2758} }
 @article{rachi_husain_2022, title={Metal Oxide Varistor Design Optimization and Main Breaker Branch Switch Control of a Progressively Switched Hybrid DC Circuit Breaker}, volume={58}, ISSN={["1939-9367"]}, DOI={10.1109/TIA.2022.3156534}, abstractNote={In this article, metal oxide varistor (MOV) design optimization and switching control in the main circuit breaker (MCB) branch of a progressively switched hybrid dc circuit breaker (DCCB) is presented. A progressively switched hybrid DCCB can achieve faster fault isolation with reduced peak fault current magnitude and transient recovery voltage compared with a regular hybrid DCCB due to its modified switching strategy. Consequently, thermal stress on the semiconductor devices in MCB is significantly reduced. Analytical model of the system dynamics during fault isolation with progressive switching is derived to demonstrate the switching scheme’s effect on the energy-absorbing component, MOV, during turn-off process. Derived analytical model in conjunction with the displacement curve of the fast mechanical switch of the hybrid DCCB is utilized to optimize the components of the main circuit breaker branch to reduce MOV degradation through asymmetric energy dissipation. A model of the circuit breaker is built in PSCAD to validate the performance of the proposed optimization method in a 10-kV/250-A system with four stage progressive switching. Additionally, a low voltage system model at 380 V is developed in PLECS for two stage progressive switching that works as the basis of experimental validation. This includes both lookup table-based MOV model and device thermal model for junction temperature estimation. Experimental results are provided for a 380-V system to demonstrate reduced fault current peak in a progressive switching and near uniform energy absorption in optimally selected MOVs.}, number={3}, journal={IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS}, author={Rachi, Md Rifat Kaisar and Husain, Iqbal}, year={2022}, pages={3064–3075} }
 @article{awal_rachi_yu_schroder_husain_2022, title={Symmetrical Components Extraction for Grid-Forming Voltage Source Converters}, ISSN={["2329-3721"]}, DOI={10.1109/ECCE50734.2022.9947944}, abstractNote={Fast and accurate detection of symmetrical components is critical for ride-through of asymmetrical faults in grid-forming (GFM) inverter based resources (IBRs). Compared to their grid-following (GFL) counterparts, faster and stable detection of symmetrical components is desired for GFM IBRs due to their more stringent fault ride-through requirements. In this work, five different time-domain approaches for symmetrical components extraction are evaluated from transient response time, accuracy, and digital implementation perspective. Fundamental grid frequency adaptive implementations of these methods are considered, where their output accuracy is compared at different sampling rates. Based on simulation and experimental results, guidelines are presented for preferred implementation strategies at high versus low and fixed versus variable sampling rates.}, journal={2022 IEEE ENERGY CONVERSION CONGRESS AND EXPOSITION (ECCE)}, author={Awal, M. A. and Rachi, Rifat Kaisar and Yu, Hui and Schroder, Stefan and Husain, Iqbal}, year={2022} }
 @article{awal_rachi_bipu_yu_husain_2021, title={Adaptive Pre-Synchronization and Discrete-Time Implementation for Unified Virtual Oscillator Control1}, ISSN={["2329-3721"]}, DOI={10.1109/ECCE47101.2021.9595171}, abstractNote={Unified virtual oscillator controller (uVOC) is a nonlinear time-domain controller which offers robust synchronization and enhanced fault ride-through for grid-following (GFL) and grid-forming (GFM) converters without the need for switching to a back-up controller. An adaptive pre-synchronization method is proposed for uVOC to enable smooth start-up and seamless connection to an existing grid/network with non-nominal frequency and/or voltage magnitude at the point of coupling (PoC). Furthermore, we evaluate the efficacy of different discretization methods for discrete-time (DT) implementation of the nonlinear dynamics of uVOC and demonstrate that zero-order-hold (ZOH) discretization fails at sampling frequencies up to tens of kHz. DT implementation of uVOC using second-order Runge-Kutta method is presented, which offers a reasonsable compromise between computational overhead and discretization accuracy. In addition, an inductor (L) or an inductor-capacitor-inductor (LCL) type input filter used in typical voltage source converter (VSC) applications leads to voltage deviation at the converter output terminal depending on the power flow. A terminal voltage compensator (TVC) for such voltage deviation is proposed. The efficacy of the proposed methods are demonstrated through laboratory hardware experiments.}, journal={2021 IEEE ENERGY CONVERSION CONGRESS AND EXPOSITION (ECCE)}, author={Awal, M. A. and Rachi, Md Rifat Kaisar and Bipu, Md Rashed Hassan and Yu, Hui and Husain, Iqbal}, year={2021}, pages={3418–3424} }
 @article{rachi_husain_2021, title={Design and Development of A Hybrid DC Circuit Breaker for 380V DC Distribution System}, ISSN={["1048-2334"]}, DOI={10.1109/APEC42165.2021.9487071}, abstractNote={Design and operation of a hybrid DC circuit breaker for 380V DC distribution system is presented in this work. A hybrid DC circuit breaker offers reduced on-state loss compared to its full solid-state alternative while ensuring arc-free, fast current interruption. Power electronic converters’ current limiting capability during overload in a DC distribution system can be leveraged to accommodate slow response time of mechanical switch compared to its solid-state counterpart to increase system efficiency during normal operation. A hybrid circuit breaker for a 380V/10kW system, along with its control and device thermal models, is implemented in PLECS. Operational and thermal simulation results are provided to validate safe breaker operation. Additionally, a compact hybrid DC circuit breaker design is presented and a prototype has been fabricated. Performance of the constructed hybrid DC circuit breaker has been evaluated experimentally for a scaled down 380V/5kW DC system to demonstrate very low on-state loss and arc-free current interruption.}, journal={2021 THIRTY-SIXTH ANNUAL IEEE APPLIED POWER ELECTRONICS CONFERENCE AND EXPOSITION (APEC 2021)}, author={Rachi, Md Rifat Kaisar and Husain, Iqbal}, year={2021}, pages={1122–1127} }
 @article{rachi_cen_bipu_khan_husain_2021, title={Design and Development of a Multi-Port Converter for Marine Microgrid Applications}, ISSN={["2329-3721"]}, DOI={10.1109/ECCE47101.2021.9595808}, abstractNote={In this work, a multi-port converter (MPC) design is presented that works as the key building block of a marine microgrid. An emulated wave energy converter (WEC) serves as the renewable energy resource of this microgrid system. The proposed MPC is comprised of a WEC interfacing port, energy storage port, and a split-phase AC load/grid port. Stable power exchange among these dissimilar energy resources are facilitated through a 400V internal DC bus. The full MPC design along with its control and thermal model is developed and implemented in PLECS. Commercially available SiC power modules have been used to leverage its high switching frequency capability that to achieve compact system design with reduced passive component sizes. Individual controller for each energy resource interfacing converter is analyzed, developed and implemented initially. A state-machine is developed utilizing these discrete controllers for MPC operation where all three ports are active simultaneously. These discretized controllers along with voltage/current sensor models with limited bandwidth and response delay in the feedback path are implemented in PLECS to emulate practical controller deployment. Simulation results are provided to validate both the power stage and the discrete controller design. Furthermore, iterative thermal analysis is carried out to have a realistic estimation of the device junction temperature during rated condition to evaluate the efficacy of the thermal management system. Finally, a virtual prototype design of the MPC is presented.}, journal={2021 IEEE ENERGY CONVERSION CONGRESS AND EXPOSITION (ECCE)}, author={Rachi, Md Rifat Kaisar and Cen, Siye and Bipu, Md Rashed Hassan and Khan, Mehnaz Akhter and Husain, Iqbal}, year={2021}, pages={1184–1190} }
 @article{rachi_khan_husain_2021, title={Local Measurement-Based Protection Coordination System for a Standalone DC Microgrid}, volume={57}, ISSN={["1939-9367"]}, DOI={10.1109/TIA.2021.3091945}, abstractNote={A reliable protection coordination system between the power electronic converters and solid-state device-based fast protection unit in a standalone dc microgrid supplied by a wave energy converter (WEC) and an energy storage unit is presented. A system model with solid-state circuit breakers is developed in Piecewise Linear Electrical Circuit Simulation (PLECS) to analyze the steady-state operation as well as faulted conditions. The effect of both resistive and constant power loads on a current-limited bus controlling converter behavior during an overload event is analyzed theoretically. Also, the variation in the WEC power output and its effect on bus voltage during overload are investigated. The analysis has led to a novel dual current–voltage feedback-based protection coordination system. The method does not require any communication between the solid-state circuit breakers and converters and eliminates the need for adaptive updating of the overcurrent threshold with varying renewable generation output. Bus capacitor discharge during a short-circuit is analyzed theoretically as well to formulate an instantaneous trip threshold selection utilizing device gate driver functionality. The proposed protection coordination system ensures rapid fault isolation and continued power delivery to the healthy segments. An SiC-based bidirectional, solid-state dc circuit breaker prototype is designed and fabricated, which has been used to implement the proposed protection method with a 380-V dc bus-based system.}, number={5}, journal={IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS}, author={Rachi, Md Rifat Kaisar and Khan, Mehnaz Akhter and Husain, Iqbal}, year={2021}, month={Sep}, pages={5332–5344} }
 @article{mackey_rachi_peng_husain_2020, title={Optimization and Control of a Z-Source, Ultrafast Mechanically Switched, High-Efficiency DC Circuit Breaker}, volume={56}, ISSN={["1939-9367"]}, DOI={10.1109/TIA.2020.2970657}, abstractNote={A novel design of the Z-source circuit breaker topology is presented to minimize on-state losses of the protection device. An ultrafast mechanical switch is proposed to commutate the fault current and improve the controllability of the circuit breaker. Replacing the power thyristor in the Z-source circuit breaker and integrating an advanced control scheme reduces energy losses with a low-resistance mechanical contactor. The proposed design facilitates bidirectional current flow, enhances control capability for distributed energy resources, and improves ride-through capabilities during load transients. Z-source circuit breakers utilize an impedance network to create a forced current zero crossing in the event of a fault, allowing the inline thyristor to isolate the fault from the source through reverse bias. However, full load current flows through the thyristor, resulting in high loss and heat generation. The concept is validated, and a proper control scheme is developed for this circuit breaker through an analytical estimation model of the system dynamics during a fault. Simulation and modeling are performed in power systems computer aided design (PSCAD) and piecewise linear electrical circuit simulation (PLECS). Finally, an experimental laboratory prototype is tested to validate the analytical and simulation models and certify the control logic.}, number={3}, journal={IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS}, author={Mackey, Landon and Rachi, Md Rifat Kaisar and Peng, Chang and Husain, Iqbal}, year={2020}, pages={2871–2879} }
 @inproceedings{mackey_rachi_peng_husain_2017, title={Optimization of a Z-source, ultra-fast mechanically switched, high efficiency DC circuit breaker}, DOI={10.1109/ecce.2017.8096665}, abstractNote={A novel modification of the Z-source circuit breaker topology is presented for low voltage applications. An ultra-fast mechanical switch has been used in place of the solid-state switch (thyristor) in the Z-source circuit breaker to reduce the energy loss utilizing the very low resistance of mechanical contactors. The proposed modification also facilitates bi-directional current flow for distributed energy resource integration and improves ride through capabilities during downstream load transients. Existing Z-source circuit breaker designs utilize an impedance network to create a forced zero current crossing in the event of a fault in commutating thyristor to isolate the fault from source. However, all load current must flow through the thyristor during normal operation resulting in high loss due to on-state voltage drop of the solid-state switch. To validate the concept and develop proper control for this circuit breaker, both simulations and experimental studies have been carried out. The proposed breaker has been modelled in PSCAD for analysis. Additionally, an analytical estimation model of the system dynamics during fault has been developed to validate the simulations. A test circuit rated for 400 V and 20 A has been designed, constructed and tested.}, booktitle={2017 ieee energy conversion congress and exposition (ecce)}, author={Mackey, L. and Rachi, M. R. K. and Peng, C. and Husain, I.}, year={2017}, pages={3764–3770} }
 @inproceedings{mackey_rachi_peng_husain_2017, title={Z-source circuit breaker utilizing ultra-fast mechanical switch for high efficiency DC circuit protection}, DOI={10.1109/icdcm.2017.8001084}, abstractNote={A novel modification to Z-Source DC circuit breakers has been proposed to reduce power consumption from its predecessor significantly. The power thyristor serves as the means of circuit isolation and voltage blocking in the event of a fault in traditional Z-Source DC circuit breakers. However, Z-Source circuit breakers direct full load current through the Thyristor. The resulting voltage difference and current flow yield substantial power consumption, heat generation, and reduced efficiency. Integrating a fast-mechanical switch and associated control, a zero current crossing and circuit isolation is achieved without significant on-state switch losses.}, booktitle={2017 IEEE Second International Conference on DC Microgrids (ICDCM)}, author={Mackey, L. and Rachi, M. R. K. and Peng, C. and Husain, I.}, year={2017}, pages={452–458} }