Browsing by Author "Tafti, Hossein Dehghani"
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Item Analysis of the Average Neutral-Point Current Limits of the Neutral-Point-Clamped Converter under Three-Level Modulation(IEEE Computer Society, 2020-10-18) Jiang, Zhuolin; Beniwal, Neha; Ceballos, Salvador; Pou, Josep; Tafti, Hossein Dehghani; Farivar, Glen G.; POWER ELECTRONICS AND SYSTEM EQUIPMENTThe neutral-point-clamped (NPC) converter employed in applications like bipolar dc-bus, back-to-back converters, power compensator, etc., requires a nonzero average neutral-point (NP) current injection under certain operating conditions. In order to better implement the average NP current injection, this paper studies the maximum and minimum average NP current that can be injected with an NPC converter. For this study, a three-level modulation technique is employed in which one of the three phases is selected to switch between the positive dc-rail and the negative dc-rail while the other two phases switch in two consecutive voltage levels. The maximum and minimum average NP current injection limits are analyzed under different loading conditions. The presented study also evaluates the effects of modulation index and operating power factor on the limits.Item Analytical Derivation of Intersubmodule Active Power Disparity Limits in Modular Multilevel Converter-Based Battery Energy Storage Systems(2021-03) Liang, Gaowen; Tafti, Hossein Dehghani; Farivar, Glen G.; Pou, Josep; Townsend, Christopher D.; Konstantinou, Georgios; Ceballos, Salvador; POWER ELECTRONICS AND SYSTEM EQUIPMENTDue to a dramatic increase in grid-connected renewable energy resources, energy storage systems are interesting and important for future power systems, among which the modular multilevel converter (MMC)-based battery energy storage systems (BESSs) are one of the most modular, efficient, and flexible topologies. Uneven active power distribution among submodules (SMs) in the arms of an MMC-based BESS is necessary for certain applications. The main contribution of this article is to present a general analysis of the inter-SM active power disparity problem which incorporates the inherent operational constraints of the MMC converter. An analytical method to derive inter-SM active power disparity limits is introduced. The proposed analysis can help facilitate the design of MMC-based BESS for applications such as recycled batteries and hybrid battery chemistries, which can both require significant inter-SM active power disparity. The analysis formulates a criteria vector and criterion value that describes whether an MMC-based BESS is capable of supplying demanded output powers while subject to inter-SM active power disparity. Simulation and experimental results are obtained on a single-phase system with varying numbers of SMs per arm, which verifies the feasibility and generality of the proposed analytical method.Item Band-Limited Three-Level Modulation for Balancing Capacitor Voltages in Neutral-Point-Clamped Converters(2020-09-01) Beniwal, Neha; Townsend, Christopher David; Farivar, Glen Ghias; Pou, Josep; Ceballos, Salvador; Tafti, Hossein Dehghani; POWER ELECTRONICS AND SYSTEM EQUIPMENTIn this article, a band-limited three-level modulation scheme is proposed for a three-phase neutral-point-clamped (NPC) converter with an aim to balance the average capacitor voltages under all operating conditions. When conventional carrier-based pulsewidth modulation is applied, the amplitude of neutral point (NP)-voltage ripple tends to increase under linear imbalances, while the NP voltage can shift, leading to instability in the presence of nonlinear loads. A previously proposed three-level modulation scheme was able to eliminate all low-frequency capacitor voltage ripple by switching a phase between three levels in each switching period. This approach significantly increased the switching loss. The proposed modulation scheme limits the increase in switching loss by employing two operational bands, one fixed band applied to the capacitor voltages and another variable band applied to the phase currents. If the instantaneous capacitor voltages exceed the voltage band, then an appropriate phase-leg is switched in three levels to guarantee that the capacitor voltages cannot diverge further. If the instantaneous capacitor voltages reside within the voltage band, a variable current band is used to regulate switching in three voltage levels in order to obtain lower switching losses as compared to the conventional three-level modulation scheme. Therefore, the proposed modulation scheme presents a tradeoff between switching losses and capacitor voltage ripple. The experimental results are obtained under different loading conditions for validating the performance of the proposed modulation scheme.Item Comparative Analysis of Flexible Power Point Tracking Algorithms in Photovoltaic Systems(Institute of Electrical and Electronics Engineers Inc., 2020-10-11) Tafti, Hossein Dehghani; Konstantinou, Georgios; Townsend, Christopher D.; Farivar, Glen G.; Ceballos, Salvador; Pou, Josep; Fletcher, John E.; POWER ELECTRONICS AND SYSTEM EQUIPMENTSeveral flexible power point tracking (FPPT) algorithms have been recently proposed in the literature to provide various grid support functionalities in photovoltaic (PV) systems. These algorithms aim to regulate the PV power to a specific value, imposed by grid codes and according to operational conditions. To obtain a fair comparison between several FPPT algorithms, each algorithm must be designed with its optimum parameter values. The main contribution of this paper is to provide a detailed analysis of the effect of various parameters of three FPPT algorithms on their transient or steady-state performance. The results of this analysis are used to obtain an optimum tuning of the parameters of each algorithm to attain an enhanced performance in both transient and steady-state operating conditions. Finally, the performance of these algorithms is compared using simulation and an experimental laboratory systems.Item A Dual-Mode Modulation Technique for Controlling the Average Neutral Point Current in Neutral-Point-Clamped Converters(2021-05) Beniwal, Neha; Farivar, Glen Ghias; Tafti, Hossein Dehghani; Pou, Josep; Ceballos, Salvador; Konstantinou, Georgios; Townsend, Christopher D.; POWER ELECTRONICS AND SYSTEM EQUIPMENTThis article presents a dual-mode modulation technique that aims to control the average current flow into the neutral point (NP) of the NP-clamped (NPC) converter without the need for any additional hardware. The two modes of operation are normal mode and compensating mode. In the normal operation mode, all the three phases switch between two consecutive voltage levels (between the positive or negative dc-rail and the NP) in a switching period. In the COM, at least one of the phases switches between the positive and negative dc-rails in a switching period. An analytical solution is developed to determine the duration of these two operation modes within each fundamental cycle based on the converter's operating condition. An advantage of this solution is that it can be generalized for balancing the capacitor voltages in all applications employing NPC converters. The proposed solution also determines the maximum average NP current injection capacity of the NPC converter under dual-mode modulation technique, which indicates the stable operating range of the converter. The performance of the proposed modulation technique is validated experimentally for various loading conditions.Item Dynamic Reserve Power Point Tracking in Grid-Connected Photovoltaic Power Plants(2023-05-01) Narang, Aditi; Farivar, Glen G.; Tafti, Hossein Dehghani; Ceballos, Salvador; Beniwal, Neha; Pou, Josep; Townsend, Christopher D.; Konstantinou, Georgios; POWER ELECTRONICS AND SYSTEM EQUIPMENTThis article introduces a dynamic power reserve control methodology called reserve power point tracking (RPPT) for grid-connected photovoltaic (PV) plants. The proposed RPPT methodology is employed to ensure availability of the required power reserve to support the grid and accordingly facilitate high penetration of PV generation in the grid. Implementing this control methodology does not require any extra hardware. The proposed methodology regulates the average PV power dynamically by periodically operating on and off the maximum power point (MPP) in order to inject a constant desired power into the grid. Tracking a desired power reference implies that the proposed methodology is a form of flexible power point tracking (FPPT). However, unlike a traditional FPPT, the proposed methodology also provides updated information of the available maximum PV power. Hence, the RPPT fulfills both FPPT and maximum power point tracking (MPPT) functionalities simultaneously. The proposed methodology extracts the MPP information and uses this information to calculate and regulate the amount of PV reserve power. One of the main advantages of the proposed algorithm is its applicability under partial shading conditions. Its effectiveness is demonstrated by experimental results under changing solar irradiance, grid frequency deviation, and partial shading conditions.Item Effect of Capacitor Voltage Ripples on Submodule Active Power Control Limits of Cascaded Multilevel Converters(2022-06-01) Liang, Gaowen; Tafti, Hossein Dehghani; Farivar, Glen G.; Pou, Josep; Townsend, Christopher D.; Konstantinou, Georgios; Ceballos, Salvador; POWER ELECTRONICS AND SYSTEM EQUIPMENTIn the operation of cascaded H-bridge converters and modular multilevel converters with energy storage or renewable power resources, unbalanced active power distribution among the submodules (SMs) is unavoidable. Depending on the operating conditions, there are certain upper and lower limits on the active power that can be processed by a single SM or a subset of SMs. The control system needs to restrict the SM power references to these limits, hence, accurate knowledge of the power limits is important. In existing methods to derive the power limits, the SM capacitor voltages are assumed to have negligible ripples, whereas in practice the ripples can be considerable. This article analyzes the effect of capacitor voltage ripples on the SM active power control limits and highlights the importance of considering the ripple effect. A methodology is proposed to accurately incorporate capacitor voltage ripples in the derivation of SM active power control limits. Simulation and experimental results are provided to evaluate the effectiveness of the proposed methodology.Item Feedforward Modulation for the Neutral-Point-Clamped Converter with Confined Capacitor Voltage Ripples and Reduced Switching Power Losses(2020-04) Beniwal, Neha; Pou, Josep; Ceballos, Salvador; Townsend, Christopher David; Konstantinou, Georgios; Tafti, Hossein Dehghani; Farivar, Glen Ghias; POWER ELECTRONICS AND SYSTEM EQUIPMENTThis article presents a new modulation technique with feedforward compensation for the three-phase three-level neutral-point-clamped converter. With the proposed technique, the capacitor voltage ripples are allowed to vary within certain limits. This enables an optimized design of the converter since the maximum capacitor voltages are predefined. Furthermore, the proposed modulation technique is able to reduce switching power losses compared to modulation techniques that eliminate capacitor voltage ripples completely. The proposed technique is therefore a tradeoff solution between aforementioned techniques and traditional modulation techniques where the capacitor voltage ripples are not limited. In the proposed technique, if the capacitor voltages are within the tolerable specified range, all the phases switch in two consecutive voltage levels in steady state. When the capacitor voltages go beyond the specified limits, one of the three phases is chosen to switch in three levels to inject an appropriate neutral point current for capacitor voltage balance. As the capacitor voltages are allowed to oscillate, the modulation technique is provided with feedforward compensation to avoid producing low-frequency distortion on the output voltages of the converter. Experimental results are presented operating under different loading conditions.Item Grid-Connected Energy Storage Systems: State-of-the-Art and Emerging Technologies(2023-04-01) Farivar, Glen G.; Manalastas, William; Tafti, Hossein Dehghani; Ceballos, Salvador; Sanchez-Ruiz, Alain; Lovell, Emma C.; Konstantinou, Georgios; Townsend, Christopher D.; Srinivasan, Madhavi; Pou, Josep; POWER ELECTRONICS AND SYSTEM EQUIPMENTHigh penetration of renewable energy resources in the power system results in various new challenges for power system operators. One of the promising solutions to sustain the quality and reliability of the power system is the integration of energy storage systems (ESSs). This article investigates the current and emerging trends and technologies for grid-connected ESSs. Different technologies of ESSs categorized as mechanical, electrical, electrochemical, chemical, and thermal are briefly explained. Especially, a detailed review of battery ESSs (BESSs) is provided as they are attracting much attention owing, in part, to the ongoing electrification of transportation. Then, the services that grid-connected ESSs provide to the grid are discussed. Grid connection of the BESSs requires power electronic converters. Therefore, a survey of popular power converter topologies, including transformer-based, transformerless with distributed or common dc-link, and hybrid systems, along with some discussions for implementing advanced grid support functionalities in the BESS control, is presented. Furthermore, the requirements of new standards and grid codes for grid-connected BESSs are reviewed for several countries around the globe. Finally, emerging technologies, including flexible power control of photovoltaic systems, hydrogen, and second-life batteries from electric vehicles, are discussed in this article.Item Minimizing Energy Storage Utilization in a Stand-Alone DC Microgrid Using Photovoltaic Flexible Power Control(2021-09) Yan, Hein Wai; Narang, Aditi; Tafti, Hossein Dehghani; Farivar, Glen G.; Ceballos, Salvador; Pou, Josep; POWER ELECTRONICS AND SYSTEM EQUIPMENTDC microgrids (dcMGs) are gaining popularity for photovoltaic (PV) applications as the demand for PV generation continues to grow exponentially. A hybrid control strategy for a PV and battery energy storage system (BESS) in a stand-alone dcMG is proposed in this paper. In contrast to the conventional control strategies that regulate the dc-link voltage only with the BESS, the proposed control strategy exploits both the PV system and the BESS to regulate the dc-link voltage. The PV acts as the primary dc voltage regulator allowing for the battery to remain standby as a secondary dc voltage regulating resource. As a result, the proposed control strategy minimizes the utilization of the BESS in order to prolong its lifetime while maintaining the state-of-charge (SoC) of the battery within a desired range. To achieve that, the flexible power point tracking (FPPT) concept is applied to the PV system to enhance the dynamic performance of the dcMG by adaptively adjusting the PV output power according to the load profile. The performance of the proposed control strategy is verified with experimental results. Furthermore, the effectiveness of the proposed control strategy on prolonging the lifetime of a lithium-ion battery and a lead-acid battery is investigated via a simulation case study with one-day load and irradiance curve profiles.