@article{alharbi_isik_alfaris_alkuhayli_bhattacharya_2022, title={A Fault Clearance and Restoration Approach for MMC-Based MTDC Grid}, volume={11}, ISSN={["2079-9292"]}, DOI={10.3390/electronics11142127}, abstractNote={With the growth in continuous energy demand, high-voltage Multi-Terminal DC (MTDC) systems are technically and economically feasible to transmit bulk power and integrate additional energy sources. However, the high vulnerability of the MTDC systems to DC faults, especially pole-to-pole (P2P) faults, is technically challenging. The development of DC fault ride-through techniques such as DC circuit breakers is still challenging due to their high cost and complex operation. This paper presents the DC fault clearance and isolation method for an MMC-based MTDC grid without adopting the high-cost DC circuit breakers. Besides, a restoration sequence is proposed to re-energize the DC grid upon clearing the fault. An MMC-based four-terminal DC grid is implemented in a Control-Hardware-in-Loop (CHIL) environment based on Xilinx Virtex-7 FPGAs and Real-Time Digital Simulator (RTDS). The RTDS results show that the MTDC system satisfactorily rides through DC faults and can safely recover after DC faults.}, number={14}, journal={ELECTRONICS}, author={Alharbi, Mohammed and Isik, Semih and Alfaris, Faris E. and Alkuhayli, Abdulaziz and Bhattacharya, Subhashish}, year={2022}, month={Jul} }
 @article{isik_burugula_alharbi_azidehak_bhattacharya_2022, title={Implementation of a Modular Distributed Fault-Tolerant Controller for MMC Applications}, volume={15}, ISSN={["1996-1073"]}, DOI={10.3390/en15228427}, abstractNote={Centralized control algorithm limits the hardware flexibility of a modular multilevel converter (MMC). Therefore, distributed control structure has recently started to be seen in the industry application. Even though distributed controller reduces a single point of failure risk compared to the centralized controller, the failure risk of the entire control systems increases due to the number of local controllers. However, the distributed controller can be programmed in such a way as to replace the faulty local controller and sustain the MMC operation. In this paper, the distributed modular fault-tolerant controller is implemented in a laboratory-scale MMC prototype. The controller is built to control four SMs per phase for the proof-of-concept. Therefore, the MMC prototype is also built by two SMs per arm. The controller capability is validated with experimental and the Opal-RT result-time simulator results in a control-hardware-in-loop (CHIL) environment.}, number={22}, journal={ENERGIES}, author={Isik, Semih and Burugula, Vasishta and Alharbi, Mohammed and Azidehak, Ali and Bhattacharya, Subhashish}, year={2022}, month={Nov} }
 @article{isik_alharbi_bhattacharya_2021, title={An Optimized Circulating Current Control Method Based on PR and PI Controller for MMC Applications}, volume={57}, ISSN={["1939-9367"]}, url={https://doi.org/10.1109/TIA.2021.3092298}, DOI={10.1109/TIA.2021.3092298}, abstractNote={Modular multilevel converter (MMC) is an excellent topology for medium- and high-voltage applications due to its advantages over other multilevel converters. However, the control algorithm design needs meticulous attention as each submodule (SM) capacitor voltage is balanced around their reference. Any voltage inequality between the SM voltage causes the second harmonic-dominated circulating current inside the MMC. The circulating current increases the rms value of the arm currents, component ratings, and the ripple in the capacitor voltages unless it is appropriately controlled. This article proposes an optimized closed-loop circulating current control method based on PR and PI controllers in abc reference frame to prevent high circulating current inside an MMC. The proposed method suppresses the magnitude of circulating current while reducing the ripple in capacitor voltages. The ripple in the dc link voltage is also reduced without any supplementary controller under balanced and unbalanced ac grid conditions. Thus, the proposed circulating current control method reduces the conduction losses and component ratings, while the converter's efficiency and reliability increase. The method's verification is tested on a point-to-point connected MMC-based high voltage direct current system in a real-time simulator with Xilinx FPGA-based MMC emulator and arm controller.}, number={5}, journal={IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Isik, Semih and Alharbi, Mohammed and Bhattacharya, Subhashish}, year={2021}, month={Sep}, pages={5074–5085} }
 @article{isik_alharbi_bhattacharya_2021, title={Comprehensive Analysis of the Control Structures for MMC Applications}, ISSN={["2329-3721"]}, DOI={10.1109/ECCE47101.2021.9595092}, abstractNote={Early MMC applications are controlled with a centralized controller. However, recent MMC controllers are primarily based on a distributed control structure as modularity and scalability features of an MMC can be efficiently utilized with local controllers of the distributed control algorithm. However, the distributed control structure also has a centralized unit for coordinating its local controllers, and the chance of a single point of failure still exists. Therefore, in most recent MMC applications, decentralized control structures are adopted, eliminating the central control unit. Eliminating the central control unit eliminates a single point of failure risk, and the reliability of an MMC theoretically increases. However, this might not always be true for practical MMC applications, as the operation of an MMC relies overall structure of an MMC. Therefore, each arm of an MMC should operate appropriately for the safe and reliable operation of an MMC. This paper evaluates and compares the control structures for MMC applications regarding reliability and area of use.}, journal={2021 IEEE ENERGY CONVERSION CONGRESS AND EXPOSITION (ECCE)}, author={Isik, Semih and Alharbi, Mohammed and Bhattacharya, Subhashish}, year={2021}, pages={2459–2466} }
 @article{isik_burugula_alharbi_bhattacharya_2021, title={Modular Power Flow Enhancer for Transmission Networks under Unbalanced Power Grid Conditions}, ISSN={["1553-572X"]}, DOI={10.1109/IECON48115.2021.9589894}, abstractNote={The three-phase H-bridge or NPC converters are commonly adopted converter topologies for FACTS devices. Both the converters are classified as multilevel converters capable of producing a three-level AC voltage between the phase and the neutral terminals. Either topology is a good solution in the low voltage environment where it is possible to select a switch capable of blocking the required DC bus voltage. If the converter is designed for a high-voltage application, the design stage of these converters may be challenging due to making composite switches for voltage blocking requirements. Besides, there is a need for a large filter for interfacing these converters with the grid to meet the THD requirements as the operating frequency is the line frequency. Therefore, this paper adopts an MMC as an SSSC to enhance power flow in the transmission network and relieve the transmission line against abnormal situations. Besides, PR-based controllers are presented in αβ0 stationary reference frame to provide reliable operation under unbalanced grid conditions. The effectiveness of the MMC-based SSSC and its controller is modeled in FPGAs and integrated with the RTDS through fiber optic cables.}, journal={IECON 2021 - 47TH ANNUAL CONFERENCE OF THE IEEE INDUSTRIAL ELECTRONICS SOCIETY}, author={Isik, Semih and Burugula, Vasishta and Alharbi, Mohammed and Bhattacharya, Subhashish}, year={2021} }
 @article{alharbi_bhattacharya_2019, title={Scale-Up Methodology of a Modular Multilevel Converter for HVdc Applications}, volume={55}, ISSN={["1939-9367"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85071334447&partnerID=MN8TOARS}, DOI={10.1109/TIA.2019.2925055}, abstractNote={Modular multilevel converters (MMCs) are a realistic alternative to the conventional voltage source converters for medium-voltage (MV) and high-voltage direct current (HVdc) applications. The number of submodules (SMs) per arm of the MMC can be as high as 512 to achieve desired high dc voltage levels required for HVdc with a very low total harmonic distortion (THD) (e.g., <0.1%) of the MMC ac-side interface voltage. Although the low THD of the MMC output voltage with a high number of SMs is desirable, the MMC control implementation and complexity is also important to be considered for the high number of SMs. The MMC control complexity significantly increases as the number of SMs increases. Redesigning the number of SMs in MMCs also becomes quite difficult and may require significant control upgrade, which in turn also needs additional tests and validations. This paper presents an MMC scale-up control methodology applicable for MV and HVdc applications. The number of SMs can be conveniently increased or reduced without any significant control modifications. The proposed control method and capacitor voltage balancing algorithm are implemented in the real-time digital simulator and MMC support units based on field-programmable gate array boards. The performance of the proposed MMC control method is investigated for a point-to-point MMC-based HVdc system under various operating conditions.}, number={5}, journal={IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Alharbi, Mohammed and Bhattacharya, Subhashish}, year={2019}, pages={4974–4983} }
 @inproceedings{alharbi_mobarrez_bhattacharya_2017, title={Control and performance analysis methodology for scale-up of MMC submodules for back-to-back HVDC applications}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85020008754&partnerID=MN8TOARS}, DOI={10.1109/apec.2017.7930731}, abstractNote={The modular multilevel converter (MMC) is a promising topology for both Medium Voltage (MV) and High Voltage Direct Current (HVDC) applications. The MMC employs a large number of “submodule” (SM) cascaded in series per phase arm to achieve the MV- or HVDC voltage. The SM numbers per phase arm can be as high as 256 or 512 SMs for +/− 300kV DC bus voltage for typical 300–600MW Back-to-Back (B2B) and also point to point HVDC applications. The MMC AC side interface voltage Total Harmonics Distortion (THD) requirements are normally < 3% which can be achieved by 48-pulse stepped AC waveform. This paper addresses the first step towards scale-up control and performance analysis such that the behavior of a large number of SMs can be predicted by a cumulative set of a smaller number of SMs. The SM capacitor voltage control is analyzed for scale-up of MMC. Simulation results of B2B HVDC application based MMC system with minimum values of MMC components (SMs capacitors and arms inductors) based on a parametric search are performed using Real Time Digital Simulator (RTDS) system.}, booktitle={Conference Proceedings - IEEE Applied Power Electronics Conference and Exposition - APEC}, author={Alharbi, M. and Mobarrez, M. and Bhattacharya, Subhashish}, year={2017}, pages={440–447} }