149 research outputs found

    Developing A Medium-Voltage Three-Phase Current Compensator Using Modular Switching Positions

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    The objective of this thesis is to present the context, application, theory, design, construction, and testing of a proposed solution to unbalanced current loading on three-phase four-wire systems. This solution, known as the Medium-Voltage Unbalanced Current Static Compensator or MV-UCSC, is designed to recirculate currents between the three phases of adistribution system. Through this redistribution of the currents negative- and zero-sequence current components are eliminated and a balanced load is seen upstream from the point of installation. The MV-UCSC as it operates in the distribution system is presented followed by its effect on traditional compensation equipment. The construction of the MV-UCSC as well as 13.8 kV simulations are then shown. Development of the switching positions required by the MVUCSC is then given followed by a variation on this switching position with the intent to reduce part count. Finally, the testing the 13.8 kV three-phase four-wire, neutral-point-clamped, elevenlevel, flying-capacitor-based MV-UCSC connected directly to the grid is presented

    The Essential Role and the Continuous Evolution of Modulation Techniques for Voltage-Source Inverters in the Past, Present, and Future Power Electronics

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    The cost reduction of power-electronic devices, the increase in their reliability, efficiency, and power capability, and lower development times, together with more demanding application requirements, has driven the development of several new inverter topologies recently introduced in the industry, particularly medium-voltage converters. New more complex inverter topologies and new application fields come along with additional control challenges, such as voltage imbalances, power-quality issues, higher efficiency needs, and fault-tolerant operation, which necessarily requires the parallel development of modulation schemes. Therefore, recently, there have been significant advances in the field of modulation of dc/ac converters, which conceptually has been dominated during the last several decades almost exclusively by classic pulse-width modulation (PWM) methods. This paper aims to concentrate and discuss the latest developments on this exciting technology, to provide insight on where the state-of-the-art stands today, and analyze the trends and challenges driving its future

    Five-Level Flying Capacitor Converter used as a Static Compensator for Current Unbalances in Three-Phase Distribution Systems

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    This thesis presents and evaluates a solution for unbalanced current loading in three-phase distribution systems. The proposed solution uses the flying capacitor multilevel converter as its main topology for an application known as Unbalanced Current Static Compensator. The fundamental theory, controller design and prototype construction will be presented along with the experimental results. The Unbalanced Current Static Compensator main objective is the balancing of the up-stream currents from the installation point to eliminate the negative- and zero-sequence currents originated by unbalanced single-phase loads. Three separate single-phase flying capacitor converters are controlled independently using a d-q rotating reference frame algorithm to allow easier compensation of reactive power. Simulations of the system were developed in MATLAB/SIMULINK™ in order to validate the design parameters; then, testing of the UCSC prototype was performed to confirm the control algorithm functionality. Finally, experimental result are presented and analyzed

    Study and evaluation of distributed power electronic converters in photovoltaic generation applications

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    This research project has proposed a new modulation technique called “Local Carrier Pulse Width Modulation” (LC-PWM) for MMCs with different cell voltages, taking into account the measured cell voltages to generate switching sequences with more accurate timing. It also adapts the modulator sampling period to improve the transitions from level to level, an important issue to reduce noise at the internal circulating currents. As a result, the new modulation LC-PWM technique reduces the output distortion in a wider range of voltage situations. Furthermore, it effectively eliminates unnecessary AC components of circulating currents, resulting in lower power losses and higher MMC efficiency.Departamento de Tecnología ElectrónicaDoctorado en Ingeniería Industria

    Thermal regulation and balancing in modular multilevel converters

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    Modular multilevel converters (MMCs) are envisaged as the key power electronic converter topology to enable a multi-terminal pan-European high voltage direct current (HVDC) Supergrid for the interconnection of offshore wind farms and exchange of energy between different countries. A key feature of MMCs in the large number of semiconductor devices employed in each converter station, distributed over a stack of series-connected sub-modules (SMs). These semiconductors possess strict thermal limits, which can constrain the operating range on the converter by limiting its capability of providing enhanced functionalities to the AC grid such as short-term power overloads. Furthermore, due to different loading conditions and ageing, significant temperature differences can exist between SMs which can lead to a very different lifetime expectancies for the semiconductor modules. This thesis proposes active thermal control methodologies to act of two distinct converter levels. Firstly, two novel dynamic rating strategies are proposed to define the converter current injection limit as a response to the maximum semiconductor temperature feedback. This enables the exploitation of the semiconductors thermal headroom to provide short-term power overloads, which can be used for the improvement of the frequency support of a power-distressed AC grid. Secondly, a SM-level temperature regulation and balancing algorithm is proposed, aiming at the equalisation of the maximum semiconductor die temperature in all the SMs of an MMC arm. The proposed methods are validated in a detailed and combined electro-thermal simulation model with 3 and 10 SMs per arm developed in MATLAB®/Simulink® using PLECS® Blockset. An experimental platform has been designed and utilised to verify the effectiveness of the dynamic rating strategies and the SM temperature regulation and balancing strategy

    Virtual Synchronous Machine Control with Adaptive Inertia Applicable to an MMC Terminal

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    Renewable energy sources (RES) penetration levels are increasing in the power grid. However, it does not have inertia as a traditional synchronous generator, causing a reduction in the inertia and damping in the power grid, impacting the stability during power changes in the grid, causing large frequency deviations. The virtual synchronous machine (VSM) concept has become an attractive solution to emulate the synchronous machine characteristics and supply the inertia and damping property in the system. It consists of emulating the synchronous machine’s static and dynamic properties by power electronic converters and energy storage systems. Nevertheless, the implementation and design of the VSM is a challenge since it must be flexible in the presence of load fluctuations, preventing the oscillations and frequency overshoot from increasing during system disturbances. Hence, the VSM with adaptive inertia has become a potential solution because it provides the inertia and damping factor to the grid according to the load variations and different RES penetration levels in the system. Therefore, the inertia estimation is necessary to use the special techniques that guarantee the balance between the power and frequency response..

    Multilevel Converter Topologies for Utility Scale Solar Photovoltaic Power Systems

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    Renewable energy technologies have been growing in their installed capacity rapidly over the past few years. This growth in solar, wind and other technologies is fueled by state incentives, renewable energy mandates, increased fossil fuel prices and environmental consciousness. Utility scale systems form a substantial portion of electricity capacity addition in modern times. This sets the stage for research activity to explore new efficient, compact and alternative power electronic topologies to integrate sources like photovoltaics (PV) to the utility grid, some of which are multilevel topologies. Multilevel topologies allow for use of lower voltage semiconductor devices than two-level converters. They also produce lower distortion output voltage waveforms. This dissertation proposes a cascaded multilevel converter with medium frequency AC link which reduces the size of DC bus capacitor and also eliminates power imbalance between the three phases. A control strategy which modulates the output voltage magnitude and phase angle of the inverter cells is proposed. This improves differential power processing amongst cells while keeping the voltage and current ratings of the devices low. A battery energy storage system for the multilevel PV converter has also been proposed. Renewable technologies such as PV and wind suffer from varying degrees of intermittency, depending on the geographical location. With increased installation of these sources, management of intermittency is critical to the stability of the grid. The proposed battery system is rated at 10% of the plant it is designed to support. Energy is stored and extracted by means of a bidirectional DC-DC converter connected to the PV DC bus. Different battery chemistries available for this application are also discussed. In this dissertation, the analyses of common mode voltages and currents in various PV topologies are detailed. The grid integration of PV power employs a combination of pulse width modulation (PWM) DC-DC converters and inverters. Due to their fast switching nature a common mode voltage is generated with respect to the ground, inducing a circulating current through the ground capacitance. Common mode voltages lead to increased voltage stress, electromagnetic interference and malfunctioning of ground fault protection systems. Common mode voltages and currents present in high and low power PV systems are analyzed and mitigation strategies such as common mode filter and transformer shielding are proposed to minimize them

    A clamping circuit based voltage measurement system for high frequency flying capacitor multilevel inverters

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    In an era where high-frequency flying capacitor (FC) multilevel inverters (MLI) are increasingly gaining attention in energy conversion systems that push the boundaries of power density, the need for a compact, fast, and accurate FC voltage monitoring is also increasing. In this paper we designed and developed a new FC measurement system, based on precise sampling of the inverter switching node voltage, through a bidirectional clamping circuit. The deviation of FC voltages from their nominal values are extracted by solving a set of linear equations. With a single sensor per phase and no isolation requirements, as opposed to dozens of sensors in traditional FC monitoring, our approach results in significantly lower cost, complexity, and circuit-size. Detailed device-level simulations in LTspice and system-scale simulations in Matlab, validate the accuracy and speed of the proposed measurement system and the balancing strategy in steady state, abrupt load change and imbalance conditions. Experiments carried out in a 3-phase Gallium-Nitride 5-level inverter prototype, reveal a gain in precision and bandwidth that is more than 30 times that of conventional methods, at a fraction of their cost and footprint. The recorded performance renders the developed sensor an ideal solution for fast MLIs based on wide-bandgap technolog

    A New MMC Topology Which Decreases the Sub Module Voltage Fluctuations at Lower Switching Frequencies and Improves Converter Efficiency

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    Modular Multi-level inverters (MMCs) are becoming more common because of their suitability for applications in smart grids and multi-terminal HVDC transmission networks. The comparative study between the two classic topologies of MMC (AC side cascaded and DC side cascaded topologies) indicates some disadvantages which can affect their performance. The sub module voltage ripple and switching losses are one of the main issues and the reason for the appearance of the circulating current is sub module capacitor voltage ripple. Hence, the sub module capacitor needs to be large enough to constrain the voltage ripple when operating at lower switching frequencies. However, this is prohibitively uneconomical for the high voltage applications. There is always a trade off in MMC design between the switching frequency and sub module voltage ripple
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