3,049 research outputs found

    Linearized large signal modeling, analysis, and control design of phase-controlled series-parallel resonant converters using state feedback

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    This paper proposes a linearized large signal state-space model for the fixed-frequency phase-controlled series-parallel resonant converter. The proposed model utilizes state feedback of the output filter inductor current to perform linearization. The model combines multiple-frequency and average state-space modeling techniques to generate an aggregate model with dc state variables that are relatively easier to control and slower than the fast resonant tank dynamics. The main objective of the linearized model is to provide a linear representation of the converter behavior under large signal variation which is suitable for faster simulation and large signal estimation/calculation of the converter state variables. The model also provides insight into converter dynamics as well as a simplified reduced order transfer function for PI closed-loop design. Experimental and simulation results from a detailed switched converter model are compared with the proposed state-space model output to verify its accuracy and robustness

    Multi-pole voltage source converter HVDC transmission systems

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    This study connects several modular multilevel converters to form multi-pole voltage source converter highvoltage dc (VSC-HVDC) links which are suited for bulk power evacuation, with increased resiliency to ac and dc network faults. The proposed arrangements resemble symmetrical and asymmetrical HVDC links that can be used for bulk power transfer over long distances with reduced transmission losses, and for the creation of multi-terminal supergrids currently being promoted for transitional dc grids in Europe. The technical feasibility of the proposed systems is assessed using simulations on symmetrical and asymmetrical tri-pole VSC-HVDC links, including the case of permanent pole-to-ground dc faults

    Analysis and design of a modular multilevel converter with trapezoidal modulation for medium and high voltage DC-DC transformers

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    Conventional dual active bridge topologies provide galvanic isolation and soft-switching over a reasonable operating range without dedicated resonant circuits. However, scaling the two-level dual active bridge to higher dc voltage levels is impeded by several challenges among which the high dv/dt stress on the coupling transformer insulation. Gating and thermal characteristics of series switch arrays add to the limitations. To avoid the use of standard bulky modular multilevel bridges, this paper analyzes an alternative modulation technique where staircase approximated trapezoidal voltage waveforms are produced; thus alleviating developed dv/dt stresses. Modular design is realized by the utilization of half-bridge chopper cells. Therefore, the analyzed converter is a modular multi-level converter operated in a new mode with no common-mode dc arm currents as well as reduced capacitor size, hence reduced cell footprint. Suitable switching patterns are developed and various design and operation aspects are studied. Soft switching characteristics will be shown to be comparable to those of the two-level dual active bridge. Experimental results from a scaled test rig validate the presented concept

    Influence of third harmonic injection on MMC-based HVDC transmission systems

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    Whilst third harmonic injection is extensively used in modular multilevel converter (MMC) control, its significant advantages over sinusoidal modulation have not been fully explored. This paper evaluates the influence of third harmonic injection on system power losses, submodule capacitance, circulating current, and fault current and mathematical models are derived. Station conduction losses are reduced by 11%, yielding higher efficiency and lowering cooling system capacity. The submodule capacitance is reduced by 24%, which significantly lowers the capital cost, weight, and volume of the station converter. Additionally, the phase energy variation is reduced by around 18%, which benefits circulating current control. Due to the lower AC currents, the semiconductor current stresses are correspondingly reduced. In addition to the performance improvement in normal operation, the third harmonic injection reduces the DC fault currents by 13.4% and thus the fault current stresses on semiconductors and DC circuit breakers are lowered. Simulation of a point-to-point HVDC system demonstrates the effectiveness of the above analysis

    A PWM current source-based DC transmission system for multiple wind turbine interfacing

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    A pulsewidth modulation (PWM) current source wind energy conversion system based on a parallel configuration for high voltage direct current application is proposed. A comparison between the parallel and series configurations for current source-based systems is investigated, which shows the merits of the proposed system. A new control technique for the PWM current source inverter is proposed. It can effectively control the average dc-link voltage with a feed-forward loop, while independently controlling reactive power according to grid code requirements. The system simulation confirms the performance of the proposed system with no interaction between wind turbine modules and satisfying performance with grid integration. Practical implementation further verifies the proposed inverter control. Finally, a brief comparison between conventional line-commutated converter-based systems and the proposed PWM current source converter-based system is presented

    Energy transfer analysis for capacitor voltage balancing of modular multilevel converters

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    Voltage balancing between sub-module (SM) capacitors is essential for reliable operation of the modular multilevel converter (MMC). To facilitate design and understanding of the balancing controllers, this study presents an energy transfer analysis for MMC, which explains how the energy can be independently transmitted from/to one phase, between the upper and lower arms, and among the SMs, of an MMC. Using this analysis, the variables which can be utilized to achieve capacitor voltage balancing are identified. Validity of this study has been verified by experimental results based on a three-phase MMC prototype

    Optical remote sensing of glacier characteristics::A review with focus on the Himalaya

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    The increased availability of remote sensing platforms with appropriate spatial and temporal resolution, global coverage and low financial costs allows for fast, semi-automated, and cost-effective estimates of changes in glacier parameters over large areas. Remote sensing approaches allow for regular monitoring of the properties of alpine glaciers such as ice extent, terminus position, volume and surface elevation, from which glacier mass balance can be inferred. Such methods are particularly useful in remote areas with limited field-based glaciological measurements. This paper reviews advances in the use of visible and infrared remote sensing combined with field methods for estimating glacier parameters, with emphasis on volume/area changes and glacier mass balance. The focus is on the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) sensor and its applicability for monitoring Himalayan glaciers. The methods reviewed are: volumetric changes inferred from digital elevation models (DEMs), glacier delineation algorithms from multi-spectral analysis, changes in glacier area at decadal time scales, and AAR/ELA methods used to calculate yearly mass balances. The current limitations and on-going challenges in using remote sensing for mapping characteristics of mountain glaciers also discussed, specifically in the context of the Himalaya

    A modular multilevel based high-voltage pulse generator for water disinfection applications

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    The role of irreversible electroporation using pulsed electric field (PEF) is to generate high voltage (HV) pulses with a predefined magnitude and duration. These HV pulses are applied to the treatment chamber until decontamination of the sample is completed. In this paper, a new topology for HV rectangular pulse generation for water disinfection applications is introduced. The proposed topology has four arms comprised of series connected half H-bridge modular multilevel converter cells. The rectangular pulse characteristics can be controlled via a software controller without any physical changes in power topology. The converter is capable of generating both bipolar and monopolar HV pulses with micro-second pulse durations at a high frequency rate with different characteristics. Hence, the proposed topology provides flexibility by software control, along with hardware modularity, scalability, and redundancy. Moreover, a cell's capacitance is relatively small which drastically reduces the converter footprint. The adopted charging and discharging process of the cell capacitors in this topology eliminate the need of any voltage measurements or complex control for cell-capacitors voltage balance. Consequently, continuity of converter operation is assured under cell malfunction. In this paper, analysis and cell-capacitor sizing of the proposed topology are detailed. Converter operation is verified using MATLAB/Simulink simulation and scaled experimentation

    Single-stage ac–dc buck–boost converter for medium-voltage high-power applications

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    This study proposes three topologies based on single-stage three-phase ac-dc buck-boost converters suitable for medium-voltage high-power applications. The first two topologies are based on a dual three-phase buck-boost converter, with a three-winding phase-shifted transformer to achieve sinusoidal input currents, with relatively small ac filters. The limitation of these two topologies is the switching devices are exposed either to a high voltage beyond that tolerable by a single device. The third topology is based on three single-phase buck-boost converters; with their dc output terminals connected in series to generate high voltage. By using this approach, voltage stresses on the switching devices are greatly reduced, and sinusoidal input currents with nearly unity power factor is achieved over the entire operating range when using small ac filters. Analysis, PSCAD/EMTDC simulations and experimentation are used to assess the feasibility of the proposed topologies during normal operation. Major findings of this study are discussed and summarised as a comparison between the three topologies

    Enhanced flat-topped modulation for MMC control in HVDC transmission systems

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    Flat-topped modulation is a member of the family of triplen-series injection techniques that have been extensively utilized in PWM inverter systems to increase the DC-link, and hence semiconductor, utilization. We propose the use of an optimized flat-topped modulation scheme for the modular multilevel converter (MMC) control. The optimized flat-topped waveform minimizes the magnitude of the triplen harmonics, particularly compared to the popular space-vector modulation (SVM) technique, while fully utilizing the DC voltage. This has particular advantages if the converter-side of the interfacing transformer is earthed. Under such conditions, the zero-sequence earthing current is affected by the triplen series injected into the sinusoidal modulating functions. Therefore, it is critical to minimize the injected triplen harmonics. The operating principle of the flat-topped scheme is presented and the Fourier coefficients are compared with the SVM technique. Additionally, the influence of the proposed control scheme on MMC performance is evaluated mathematically. Simulation of a point-to-point HVDC link using average model demonstrates the effectiveness of the proposed MMC operational schemes. The third harmonic of the flat-topped modulation is reduced by 33%, which lowers the potential zero-sequence current flowing to earth. Compared to conventional sinusoidal modulation, the submodule capacitance is reduced by 25%. This significantly lowers submodule cost, volume, and weight. Station conduction losses are expected to reduce by 11%, yielding higher efficiency and lowering cooling system capacity. In addition to the improvement under normal operation, the proposed control scheme also reduces the fault current by 13.4%
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