195 research outputs found

    Reliability Enhancing Control Algorithms for Two-Stage Grid-Tied Inverters

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    In the photovoltaic (PV) generation system, two types of grid-tied inverter systems are usually deployed: the single-stage grid-tied inverter system and the two-stage grid-tied inverter system. In the single-stage grid-tied inverter system, the input of the inverter is directly connected to the PV arrays, while an additional dc-dc stage is inserted between the PV arrays and the dc-ac inverter in the two-stage design. The additional dc-dc stage could provide a stable dc-link voltage to the inverter, which also enables new design possibilities, including the multi-MPPT operation and solar-plus-storage application. Thus, the two-stage grid-tied inverter has been widely used in the PV generation system.As the core component of the PV generation system, the reliability of the grid-tied inverter determines the overall robustness of the system. The two-stage grid-tied inverter system includes three parts: the dc-dc stage, dc-link capacitor, and dc-ac inverter. Thus, the reliability of the two-stage grid-tied inverter relies on the reliability of each part. The dc-dc stage is used to provide a stable dc-link voltage to the inverter. However, when the inverter stage provides constant power to the grid, the load of the dc-dc stage becomes the constant power load (CPL), which will deteriorate the stability of the dc-dc stage. The dc-link capacitor is used to attenuate the voltage ripple on the dc-link and balance the transient power mismatch between the dc-dc stage and the dc-ac stage. However, during the operation of the inverter system, the degradation of the capacitor will reduce the converter reliability, and even result in system failure. The inverter stage is connected to the grid through the output filter, and the LCL type filter has been commonly used due to its superior performance. The resonance of the LCL filter must be properly damped to enhance the inverter stability. However, the grid-side impedance will lead to the resonant frequency drifting of the LCL filter, which will worsen the stability margin of the inverter. Thus, the control design of the two-stage grid-tied inverter system must consider those reliability challenges. In this work, three control algorithms are proposed to solve the reliability challenges. For the dc-dc stage, an uncertainty and disturbance estimator (UDE) based robust voltage control scheme is proposed. The proposed voltage control scheme can actively estimate and compensate for the disturbance of the dc-dc stage. Both the disturbance rejection performance and the stability margin of the dc-dc stage, especially under the CPL, could be enhanced. For the dc-link capacitor, a high-frequency (HF) signal injection based capacitance estimation scheme is proposed. The proposed estimation scheme can monitor the actual dc-link capacitance in real-time. For the inverter stage, an adaptive extremum seeking control (AESC) based LCL filter resonant frequency estimation scheme is proposed. The AESC-based estimation scheme can estimate the resonant frequency of the LCL filter online. All the proposed reliability enhancing control algorithms could enhance the reliability of the two-stage grid-tied inverter system. Detailed theoretical analysis, simulation studies, and comprehensive experimental studies have been performed to validate the effectiveness

    Reliability Enhancing Control Algorithms for Two-Stage Grid-Tied Inverters

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    In the photovoltaic (PV) generation system, two types of grid-tied inverter systems are usually deployed: the single-stage grid-tied inverter system and the two-stage grid-tied inverter system. In the single-stage grid-tied inverter system, the input of the inverter is directly connected to the PV arrays, while an additional dc-dc stage is inserted between the PV arrays and the dc-ac inverter in the two-stage design. The additional dc-dc stage could provide a stable dc-link voltage to the inverter, which also enables new design possibilities, including the multi-MPPT operation and solar-plus-storage application. Thus, the two-stage grid-tied inverter has been widely used in the PV generation system.As the core component of the PV generation system, the reliability of the grid-tied inverter determines the overall robustness of the system. The two-stage grid-tied inverter system includes three parts: the dc-dc stage, dc-link capacitor, and dc-ac inverter. Thus, the reliability of the two-stage grid-tied inverter relies on the reliability of each part. The dc-dc stage is used to provide a stable dc-link voltage to the inverter. However, when the inverter stage provides constant power to the grid, the load of the dc-dc stage becomes the constant power load (CPL), which will deteriorate the stability of the dc-dc stage. The dc-link capacitor is used to attenuate the voltage ripple on the dc-link and balance the transient power mismatch between the dc-dc stage and the dc-ac stage. However, during the operation of the inverter system, the degradation of the capacitor will reduce the converter reliability, and even result in system failure. The inverter stage is connected to the grid through the output filter, and the LCL type filter has been commonly used due to its superior performance. The resonance of the LCL filter must be properly damped to enhance the inverter stability. However, the grid-side impedance will lead to the resonant frequency drifting of the LCL filter, which will worsen the stability margin of the inverter. Thus, the control design of the two-stage grid-tied inverter system must consider those reliability challenges. In this work, three control algorithms are proposed to solve the reliability challenges. For the dc-dc stage, an uncertainty and disturbance estimator (UDE) based robust voltage control scheme is proposed. The proposed voltage control scheme can actively estimate and compensate for the disturbance of the dc-dc stage. Both the disturbance rejection performance and the stability margin of the dc-dc stage, especially under the CPL, could be enhanced. For the dc-link capacitor, a high-frequency (HF) signal injection based capacitance estimation scheme is proposed. The proposed estimation scheme can monitor the actual dc-link capacitance in real-time. For the inverter stage, an adaptive extremum seeking control (AESC) based LCL filter resonant frequency estimation scheme is proposed. The AESC-based estimation scheme can estimate the resonant frequency of the LCL filter online. All the proposed reliability enhancing control algorithms could enhance the reliability of the two-stage grid-tied inverter system. Detailed theoretical analysis, simulation studies, and comprehensive experimental studies have been performed to validate the effectiveness

    Modular, Scalable Battery Systems with Integrated Cell Balancing and DC Bus Power Processing

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    Traditional electric vehicle and stationary battery systems use series-connected battery packs that employ centralized battery management and power processing architecture. Though, these systems meet the basic safety and power requirements with a simple hard- ware structure, the approach results in a battery pack that is energy and power limited by weak cells throughout life and most importantly at end-of-life. The applications of battery systems can benefit significantly from modular, scalable battery systems capable of advanced cell balancing, efficient power processing, and cost gains via reuse beyond first-use application. The design of modular battery systems has unique requirements for the power electronics designer, including architecture, design, modeling and control of power processing converters, and battery balancing methods. This dissertation considers the requirements imposed by electric vehicle and stationary applications and presents design and control of modular battery systems to overcome challenges associated with conventional systems. The modular battery system uses cell or substring-level power converters to combine battery balancing and power processing functionality and opens the door to new opportunities for advanced cell balancing methods. This approach enables balancing control to act on cell-level information, reroute power around weaker cells in a string of cells to optimally deploy the stored energy, and achieve performance gains throughout the life of the battery pack. With this approach, the integrated balancing power converters can achieve system cost and efficiency gains by replacing or eliminating some of the conventional components inside battery systems such as passive balancing circuits and high-voltage, high-power converters. In addition, when coupled with life prognostic based cell balancing control, the modular system can extend the lifetime of a battery pack by up to 40%. The modular architecture design and control concepts developed in this dissertation can be applied to designs of large battery packs and improve battery pack performance, lifetime, size, and cost

    A Review of Current Research Trends in Power-Electronic Innovations in Cyber-Physical Systems.

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    In this paper, a broad overview of the current research trends in power-electronic innovations in cyber-physical systems (CPSs) is presented. The recent advances in semiconductor device technologies, control architectures, and communication methodologies have enabled researchers to develop integrated smart CPSs that can cater to the emerging requirements of smart grids, renewable energy, electric vehicles, trains, ships, internet of things (IoTs), etc. The topics presented in this paper include novel power-distribution architectures, protection techniques considering large renewable integration in smart grids, wireless charging in electric vehicles, simultaneous power and information transmission, multi-hop network-based coordination, power technologies for renewable energy and smart transformer, CPS reliability, transactive smart railway grid, and real-time simulation of shipboard power systems. It is anticipated that the research trends presented in this paper will provide a timely and useful overview to the power-electronics researchers with broad applications in CPSs.post-print2.019 K

    A Control Strategy for Self-Sustained and Flexible DC Nanogrids

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    Microgrids are becoming a potential solution for combining distributed generation units, such as photovoltaic panels, wind turbines and energy storage systems. As a simple and small version of a microgrid, a nanogrid is a power distribution system that is suitable for a single node, such as a small building or a private house. The nanogrid can be flexibly connected to or disconnected from other power entities through a gateway. In most cases, the nanogrid is connected to the utility grid to avoid the power outage and to increase the operational efficiency. However, the current standalone nanogrid model is not suitable because an imbalance between the generated and consumed electrical power might occur. The main objective of this research work is to develop a self-sustained and flexible control strategy for autonomous direct current (DC) nanogrids in remote and rural areas without the need for a communication system. The proposed control strategy for the nanogrids is based upon a hierarchical control, in which the primary control manages the power balance inside the nanogrids and the secondary control is responsible for removing deviation of the DC bus voltage caused by droop operation. The state of charge (SoC) of the battery and the external DC bus voltage are taken into account in the proposed control strategy in order to avoid the overcharge/deep discharge of the battery as well as the collapse of the external DC bus. The control algorithm also ensures a flexible exchange of power inside a nanogrid as well as among multiple nanogrids without any extra digital communication link. Bidirectional power flow among multiple nanogrids is implemented through a dedicated interconnected bidirectional Dual Active Bridge (DAB) DC/DC converter installed inside each nanogrid to ensure a galvanic isolation among multiple, interconnected nanogrids. The proposed control strategy is validated through both simulations and experiments. Simulation and experimental results are used to validate the operation of the proposed control algorithm and prove the resemblance between theory and experiments. However, in order to implement the proposed control strategy, a model of the DC nanogrid has to be developed. For that reason, modeling of every single converter in the system should be conducted. The second important contribution of this research is modeling and control for converters independently, including a bidirectional buck converter and a dual active bridge converter. A small-signal model based on the state-space averaging technique for the bidirectional buck converter is developed, in which only the mean value (i.e. “zeroth” harmonic) of the state variables is taken into account. On the other hand, the generalized state-space averaging-based modeling method is used to obtain the state-space representation of the DAB converter, in which the direct current (DC) component and the fundamental harmonics in the Fourier series expansion of state variables are considered. Transfer functions from control-to-output are determined, which will be used to define two controllers for the current and voltage loops in a cascaded control structure. Simulations and experiments will be used to validate the operation of the proposed method. As aforementioned, modeling and control for each converter in the DC nanogrid is performed separately. Nevertheless, when these converters are connected to form a complete DC nanogrid, they will affect each other and the stability of the entire system is influenced as well. To overcome this problem, a model of the entire system has to be developed and the system stability has to be analyzed. For this purpose, the small-signal transfer function of a DC nanogrid is synthesized from the small-signal transfer functions of every single converter of the system. Using this transfer function, the system stability is analyzed and the secondary controller is designed. Simulation and experimental results are used to verify a stable operation of the DC nanogrid system

    Advanced Control of a Multi-Port Autonomous Reconfigurable Solar Power Plant

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    The multi-port autonomous reconfigurable solar power plant (MARS), which is an integration of photovoltaic (PV) and energy storage system (ESS) to the transmission ac grid and a high-voltage direct current (HVdc) link, is designed to provide frequency response and reject disturbances in the grid with continued operation and reduced transient instability. The complex architecture of the MARS and the intermittent nature of PV underlies the need for developing simple, efficient, and easily generalizable control methods for MARS and MARS-type systems that integrate multiple power sources to the submodules (SMs) in each arm. The presence of different sources such as PV and ESS in each arm of the MARS causes uneven distribution of active power among different SMs present in MARS, thereby leading to unbalanced modules’ capacitor voltages that may impact system stability under various operating conditions. Moreover, in the case of partial shadings, shaded PV SMs will suffer from decreased injected PV power, causing power mismatch between different SMs in the MARS system. An energy balancing control (EBC) method is introduced to balance the capacitor voltages of different types of SMs. Moreover, the system operation region is explored through data-driven method and a machine learning-based EBC criteria are proposed to improve the system efficiency and reduce the switching frequency. The proposed EBC criteria can disable/enable the EBC depending on the MARS input power dispatch commands with high accuracy according to the operation region. To simplify the design process and improved the system performance, the thesis further proposed a neural network-based power mismatch elimination (NNPME) strategy. The NNPME strategy employs ESS to its maximum capacity and the dc and ac circulating currents to transfer power between the SMs, arms, and legs of the MARS and stabilize the system under partial shedding conditions. The aforementioned controls are data-driven methods that require a large amount of simulation data. A model predictive control (MPC) is proposed for more accurate and efficient control of MARS. It can optimally allocate uneven power of ESS and PV in one arm and counteract capacitor voltage deviations. The system dynamic response is largely improved with the implementation of MPC. The proposed advanced controls facilitate the efficient control and energy management of a system with multiple input power sources like MARS to fully utilize its potential with an extended operating region while maintaining high efficiency.Ph.D

    DC Microgrid Protection: A Comprehensive Review

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    A Survey on Multi-Active Bridge DC-DC Converters: Power Flow Decoupling Techniques, Applications, and Challenges

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    Multi-port DC-DC converters are a promising solution for a wide range of applications involving multiple DC sources, storage elements, and loads. Multi-active bridge (MAB) converters have attracted the interest of researchers over the past two decades due to their potential advantages such as high power density, high transfer ratio, and galvanic isolation, for example, compared to other solutions. However, the coupled power flow nature of MAB converters makes their control implementation difficult, and due to the multi-input, multi-output (MIMO) structure of their control systems, a decoupling control strategy must be designed. Various control and topology-level strategies are proposed to mitigate the coupling effect. This paper discusses the operating principles, applications, methods for analyzing power flow, advanced modulation techniques, and small signal modelling of the MAB converter. Having explained the origin of cross-coupling, the existing power flow decoupling methods are reviewed, categorized, and compared in terms of effectiveness and implementation complexity

    High Power, Medium Frequency, and Medium Voltage Transformer Design and Implementation

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    Many industrial applications that require high-power and high-voltage DC-DC conversion are emerging. Space-borne and off-shore wind farms, fleet fast electric vehicle charging stations, large data centers, and smart distribution systems are among the applications. Solid State Transformer (SST) is a promising concept for addressing these emerging applications. It replaces the traditional Low Frequency Transformer (LFT) while offering many advanced features such as VAR compensation, voltage regulation, fault isolation, and DC connectivity. Many technical challenges related to high voltage stress, efficiency, reliability, protection, and insulation must be addressed before the technology is ready for commercial deployment. Among the major challenges in the construction of SSTs are the strategies for connecting to Medium Voltage (MV) level. This issue has primarily been addressed by synthesizing multicellular SST concepts based on modules rated for a fraction of the total MV side voltage and connecting these modules in series at the input side. Silicon Carbide (SiC) semiconductor development enables the fabrication of power semiconductor devices with high blocking voltage capabilities while achieving superior switching and conduction performances. When compared to modular lower voltage converters, these higher voltage semiconductors enable the construction of single-cell SSTs by avoiding the series connection of several modules, resulting in simple, reliable, lighter mass, more power dense, higher efficiency, and cost effective converter structures. This dissertation proposes a solution to this major issue. The proposed work focuses on the development of a dual active bridge with high power, medium voltage, and medium frequency control. This architecture addresses the shortcomings of existing modular systems by providing a more power dense, cost-effective, and efficient solution. For the first time, this topology is investigated on a 700kW system connected to a 13kVdc input to generate 7.2kVdc at the output. The use of 10kV SiC modules and gate drivers in an active neutral point clamped to two level dual active bridge converter is investigated. A special emphasis will be placed on a comprehensive transformer design that employs a multi-physics approach that addresses all magnetic, electrical, insulation, and thermal aspects. The transformer is designed and tested to ensure the system’s viability
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