Browsing by Author "Townsend, Christopher D."
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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 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 Discontinuous Modulation of a Cascaded H-Bridge Low-Capacitance StatCom(2022-03-01) Liu, Qingxiang; Rodriguez, Ezequiel; Farivar, Glen G.; Ceballos, Salvador; Townsend, Christopher D.; Leyva, Ramon; Pou, Josep; POWER ELECTRONICS AND SYSTEM EQUIPMENTThis article presents a discontinuous modulation (DM) strategy for static compensators (StatComs) based on a cascaded H-bridge (CHB) converter with a star configuration. The proposed DM strategy considers the capacitor voltage oscillations at twice the fundamental frequency and the effect of zero-sequence voltage injection on the capacitor voltages. Considering these effects is especially important in CHB-StatComs with large capacitor voltage ripples (low-capacitance StatComs), where the assumption of a constant dc-link voltage, which is the basis of conventional DM strategies, does not apply. This article also describes a coherent set of steady-state waveforms for CHB-StatComs under DM. In addition to the well-known benefit of reducing the switching losses, the proposed DM also reduces the dc-link capacitors size and extends the operating range. The viability of the proposed DM strategy is verified experimentally on a small-scale prototype. In addition, simulation results are obtained using a real-scale system to study feasibility of the DM under unbalanced conditions.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 Flexible Power Point Tracking in Photovoltaic Power Plants(IEEE Computer Society, 2021-10-13) Narang, Aditi; Farivar, Glen G.; Dehghani Tafti, Hossein; Ceballos, Salvador; Pou, Josep; Townsend, Christopher D.; Konstantinou, Georgios; POWER ELECTRONICS AND SYSTEM EQUIPMENTFrequency support from photovoltaic (PV) power plants is required with new grid codes. To provide frequency support, the PV system needs to regulate the PV power in such a way that a predefined amount power reserve is kept in the PV system. Accordingly, a dynamic flexible power point tracking algorithm (FPPT) for grid-connected PV power plants is introduced in this paper. The proposed algorithm regulates the average PV power by continuously sweeping some portion of the PV curve that includes the given power reference point. The proposed strategy is different from traditional static FPPT techniques as it ensures a predefined amount of power reserve in the PV system. This power reserve provides the flexibility of increasing the power injected into the grid that can be withdrawn with a fast dynamic. Compared to the traditional solutions, in the proposed algorithm, the PV injects a constant power into the grid without any fluctuations. Effectiveness of the proposed technique in providing frequency support in a two-stage grid-connected PV plan is demonstrated by simulation results.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 Enhancing the Natural Voltage Balancing Capability of Neutral-Point-Clamped Converters under Carrier-Based Pulsewidth Modulation(Institute of Electrical and Electronics Engineers Inc., 2018-07-02) Beniwal, Neha; Pou, Josep; Townsend, Christopher D.; Ceballos, Salvador; POWER ELECTRONICS AND SYSTEM EQUIPMENTIn this paper, a new modulation technique is presented which aims to enhance the natural voltage balancing capability of neutral-point-clamped converters. The traditional carrier-based pulsewidth modulation technique with zero-sequence injection faces some drawbacks, namely, its inability to balance initial capacitor voltage imbalances quickly and the divergence of the capacitor voltages in the presence of nonlinear loads with even harmonics. This paper aims to find a trade-off between voltage balance performance and switching losses by reducing the difference between the average capacitor voltages without incorporating any external circuitry. A reference neutral-point (NP) current is generated using the average difference between the capacitor voltages. Depending on the reference NP current and predicted NP current, appropriate duty cycles are applied to switch the three phases. These duty cycles are directly linked to the voltage unbalance between the capacitor voltages and thus provides the necessary 'push' to mitigate deviations in natural balancing characteristics. To validate the proposed modulation technique, simulation results are presented for linear loads as well as for nonlinear loads with even harmonics.Item Estimation methods and sensor reduction in modular multilevel converters: A review(Institute of Electrical and Electronics Engineers Inc., 2018-07-02) Konstantinou, Georgios; Wickramasinghe, Harith R.; Townsend, Christopher D.; Ceballos, Salvador; Pou, Josep; POWER ELECTRONICS AND SYSTEM EQUIPMENTThe circuit configuration of the modular multilevel converter (MMC) requires a large number of current and voltage measurements as well as communications within the converter arms and between the arm controllers and the main control circuit. In order to reduce circuit complexity and hardware requirements with their associated cost, while adding an additional level of reliability to the operation of the MMC, a number of methods have been proposed for the estimation / observation of the various internal electrical variables or parameters of the MMC. These include estimations for sub-module capacitor voltages, dc-link voltages, arm currents or total arm energy. Such methods aim to eliminate or reduce the sensors in the circuit, reduce communication requirements or both. This paper provides an overview and classification of the different estimation/observation methods for MMCs currently proposed in the literature.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.