| EPE 2015 - DS2h: Power Electronics in Transmission and Distribution Systems; HVDC & FACT's | ||
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 | A modular and scalable HVDC Current Flow Controller
By Viktor HOFMANN | |
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Abstract: Due to technical and economic benefits there is an increasing interest in establishing an overlaying HVDC grid. Therefore, a current flow control is essential. In this paper a modular and scalable HVDC current flow controller is presented, which allows a direct exchange of power between two or more cables. In the first section the functional principle of the HVDC Balancing MMC is presented and a fundamental analysis of the steady state operation is done. In the second section an average loss calculation is described. In both sections the general results are discussed for a chosen point of operation.
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| | A Modular Multilevel Flying Capacitor Converter-Based STATCOM for Reactive Power Control in Distribution Systems
By CHIGOZIE JOHN NWOBU | |
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Abstract: This paper presents a simulation study for a built prototype of modular multi-level flying capacitor cascade converter as a STATCOM. The converter modulation scheme applied is based on Phase Shifted PWM and the two scenarios which require compensation are investigated to verify this topology. The two scenarios are PCC voltage regulation and unity power factor correction through reactive power compensation. Simulation results verify the performance of the chosen topology
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| | AC line voltage controller for grid integration of renewable energy sources
By Alain GERMANIER | |
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Abstract: Distributed Renewable Energy generation can affect the power quality of local low voltage distribution grids. Large hydro, wind or solar power plants combined with too weak power distributed infrastructures will generate line voltage increase at the feeding point. A solution for automatic line voltage correction, based on a static AC/AC static converter and a transformer is presented here. Emphasis in the design was put on flexibility, high efficiency, dynamic performance and costs. In case of asymmetric loads, each phase can be controlled individually. In comparison to fully inductive voltage stabilizer, the system has very low or even positive impact on grid impedance and harmonic distortion. The system can be installed anywhere along the line. Thanks to an embedded communication and control interface it can be integrated into smart grid applications. The stabilization is possible by increasing or decreasing the line voltage. Decoupling power electronics and transformer allows modular system sizing in voltage and power ranges. A three-phase single stage power electronics system with its own DSP control system is fully implemented and tested. Results in term of efficiency and dynamic performances are presented in this paper.
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| | Active Filtering Based Current Injection Method for Multi Modal SSR damping in an AC/DC System
By TIBIN JOSEPH | |
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Abstract: The rapid expansion in power transmission for the integration of large-scale renewables complemented byinfrastructure reinforcements in the form of series compensation poses the threat of subsynchronous resonance (SSR). However, existing and future high-voltage direct current (HVDC) links can be effectively used to mitigate this undesirable phenomenon. In this line, this paper presents an auxiliary control loop embedded in a voltage source converter (VSC) based HVDC system to damp torsional interactions -a form of SSR involving turbogenerators and series-compensated transmission lines. The proposed damping scheme employs modal filters to identify SSR upon occurrence and then injects currents at subsynchronous frequency to damp it. The SSR damper has been tested in the well-known IEEE First Benchmark Model, which has been upgraded with a VSC-HVDC link to form an integrated AC/DC system. Simulations have been performed in PSCAD, with eigenvalue analysis carried out in MATLAB for the small-signal stability assessment of the AC/DC system. Results show that the proposed scheme effectively damps SSR irrespectively of the torsional mode being excited.
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| | Analysis and semiconductor based comparison of energy diverting converter topologies for HVDC transmission systems
By Andre BIRKEL | |
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Abstract: Energy diverting converter topologies are important power electronic components for HVDC connected offshore wind parks to meet grid code requirements. In this paper, three different types of energy diverting converter topologies are investigated. In the first section, the topologies are introduced and compared under general aspects. Afterwards, an approach for a sinusoidal voltage modulation of a MMC based braker is presented. Within its operation range it allows the use of half bridge submodules and also the use of full bridge submodules. Finally the topologies are compared to each other under aspects of braking performance, including a detailed loss calculation for the MMC braker topology.
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| | Analysis of Deviations on the Optimal Power Flow Operation of MTDC Networks: A Comparison between Droop Control and the DVC Strategy
By Rodrigo TEIXEIRA PINTO | |
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Abstract: There is a growing interest in the development of high-voltage multi-terminal direct current (MTDC)networks for integration of renewable energy sources, reinforcement of existing ac networks and feedingremote locations. Although many studies have investigated power flow control strategies for MTDCnetworks, few studies have looked into the possible sources of load flow errors. The aim of this paper isto analyze the effect of errors in the OPF operation of these future HVDC grids for three different sources:voltage measurement errors;variation of cable resistance with temperature and error in the prediction ofthe generated power from renewable sources. To perform the analysis a direct current OPF tool is used tocompare two different control strategies, namely droop control and the distributed voltage control (DVC)strategy. The OPF object function is the minimization of losses in the MTDC network, which is basedon the recently built 5-terminal HVDC grid in Zhoushan, China.
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| | Conception of a Modular Multilevel Converter in a Multi Terminal DC/AC transmission network
By Daniel SIEMASZKO | |
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Abstract: In the frame of a collaboration project within Swiss Competence Centre for Energy Research in the Future Swiss Electrical Infrastructure section, a Multi Terminal DC pilot has been implemented for studying the interaction between an HVDC and a high voltage AC grid. For the interaction with the medium voltage AC grid, a Modular Multilevel Converter is being designed for its features in low harmonic content, scalability, and flexibility. The energy buffering of the converter will allow the converter to sustain either AC or DC grids. The operation of the converter is described in details and simulated.
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| | Current-fed GaN Front-end converter for ISOP-IPOS converter-based high power density DC distribution system
By Yusuke HAYASHI | |
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Abstract: A current-fed ac-dc converter using Gallium Nitride (GaN) power devices has been proposed to realize high power density ISOP (Input Series and Output Parallel)-IPOS (Input Parallel and Output Series) converter-based dc distribution system. The current-fed converter becomes the strong candidate in the ISOP-IPOS converter-based dc distribution system because this system expands the possibility of the ac-dc converter design without taking the converter I/O voltages and its output power into account. An 1 kW prototype with the efficiency of 95.5\% has been developed by using GaN-HEMTs to show the feasibility. Design consideration for the GaN converter has been also conducted quantitatively taking the low on-resistance, the high-speed switching behavior and its own device structure into account. The ISOP-IPOS converter-based dc distribution system takes full advantage of the current-fed converter and the converter using GaN devices contributes to realizing higher power density dc distribution system.
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| | Decentral control of a multi-terminal HVDC system with automatic exchange of instantaneous and primary reserve power across AC grids
By Florian FEIN | |
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Abstract: A control strategy for HVDC converters in future wide-area multi-terminal HVDC grids is developed. The control strategy is inspired by the dynamic behaviour of common motor-generator-sets used for electromechanical AC-DC conversion in the medium power range. With the new control strategy HVDC converters are on the one hand suitable for distributed control of DC voltage and they also contribute to the control and the stability of AC voltage and AC grid system frequency on the other hand. This ensures that a DC grid behaves like a network of conventional AC line interconnectors, that automatically transmits instantaneous- and primary reserve power if there is a disturbance in any of the connected AC nodes. The electric behaviour of one HVDC converter with the new control strategy is studied in simulation.
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| | Direct and indirect adaptive control for synchronous generator semiconductor's excitation system
By JOZEF RITONJA | |
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Abstract: The paper discusses the stability problem of a synchronous generator connected to an infinite bus. From the comprehensive and systematic eigenvalue analysis of the simplified linearized mathematical model of the synchronous generator it is evident that the variations in the dynamics of the synchronous generators are considerable. Therefor the conventional power system stabilizers with fixed parameters do not ensure damping of the contemporary synchronous generators operating in modern power systems at peak performances. It was found out that the adaptive control represents the most logic choice for high performance power system stabilizers. In the paper a detailed survey of the available adaptive control concepts is presented. Featured is the possibility of application of (-) direct adaptive control, where a signal from controller is calculated directly without intermediate identification of the controlled plant, as well as (-) indirect adaptive control with separate identification of the controlled plant and the controller with variable parameters calculated from identified model.
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| | Extension of Power Transmission Capacity in MMC-based HVDC Systems through Dynamic Temperature-Dependent Current Limits
By Jorge GONCALVES | |
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Abstract: This paper proposes the extension of the current control loops used in Modular Multilevel Convertersto include semiconductors junction temperature as an operational constraint. A new controller is addedto the inner current loop to modulate the current limits as a function of the temperature, providing anextension of the power transmission capacity without violating the thermal limits of thesemiconductors. A numerical method to investigate the operation of converters with temperature-dependent current limits is proposed and the stability of the controller is analyzed and validatedthrough simulations under steady and transient operations.
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| | High power, High-Boost, Resonant-Type DC-DC Converters as Power Interfaces for Interconnecting Wind Generators to Main Grids from MV DC Collectors: Design and P
By G.-ADOLFO ANAYA-RUIZ | |
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Abstract: This paper presents two resonant boost DC-DC converters. Both soft-switched configurations can beused to applications in DC grids and have high voltage-gain. These converters are designed to operateas a converter station at the wind generation-side of a HVDC-system linking wind power generationinto the transmission network. At initial sections the proposed converters are thoroughly explained. Atfinal sections the steady-state response of voltage and load variations are evaluated for bothconverters.
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| | Impact of different control algorithms on Modular Multilevel Converters electrical waveforms and losses
By Francois GRUSON | |
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Abstract: Modular Multilevel Converters (MMC) are becoming increasingly popular with the development of HVDC connection and, in the future, Multi Terminal DC grid. A lot of publications have been published about this topology these last years since it was first proposed. Many of them deal with converter control methods, other address the method of estimating losses. Usually, the proposed losses estimation techniques are associated to simple control methods For VSC (Voltage Sources Converters) topology, the losses minimization is based on the limitation of the RMS currents values. This hypothesis is usually extended to the control of MMC, by limiting the differential currents to their DC component, without really being checked.This paper investigates the impact of two control algorithms variants on electrical quantities (currents, capacitor voltages ripple, losses). From the published results, it is shown that in some cases the usual choice is not the best one.
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| | Individual capacitor voltage balancing in H-bridge cascaded multilevel STATCOM at zero current operating mode
By Ehsan BEHROUZIAN | |
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Abstract: Individual capacitor voltage balancing is one of the challenges in the field of multilevel converters, especiallywhen the phase legs of the converter are connected in star configuration. This issue can be even more problematic when the converter is operating close to zero average current operating mode. This paper first shows the issue related to the capacitor voltage balancing at zero average-current mode and second proposes a novel algorithm to overcome this problem. The method proposes a modulation technique for individual balancing in H-bridge cascaded multilevel converters operated at zero average-current mode. The proposed algorithm modifies the conventional sorting algorithm based on current ripple. The individual DC-link voltage control method is applied to a 19-level star-connected cascaded converter in PSCAD and simulation results verify the ability of the proposed method in maintaining the balancing of the capacitors voltages at zero current injection mode. Several practical limitation such as forward voltage drop over semiconductor elements, noise in measured data, delay in the digital controller, switching deadtime, and grid voltage harmonics are also considered in the model.
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| | Instantaneous Active and Reactive Power in Six-Phase Systems
By SAMIR AHMAD MUSSA | |
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Abstract: This paper proposes a method for calculation of the instantaneous active and reactive power in six-phase system, derived from the Akagi three-phase instantaneous power theory, the proposed method is physical analyzed and the results are validated by simulation.
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| | Limitations of the H-Bridge Multilevel STATCOMs in Compensation of Current Imbalance
By DURO BASIC | |
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Abstract: In this paper we investigate limitations of H-bridge multilevel STATCOMs in the compensation ofcurrent imbalance in industrial applications. In either star or delta topology of the STATCOMconverters, the zero sequence injection is used to balance power flows (i.e. to balance dc bus capacitorvoltages) in the arm converters. In imbalanced situations the compensation capability of the currentimbalance is limited by the level of the required zero sequence injection.
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| | Local and primary controls of a Multi-terminal HVDC grid in an experimental setup.
By Miguel JIMENEZ CARRIZOSA | |
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Abstract: This paper focuses on the experimental implementation of local and primary controls for MT-HVDC in a real test bench (Pnom=6 kW). A local control with two different time scales, one for the current and the other for the voltage, and primary control (droop control), which is responsible to vary the reference voltage when a disturbance appears in the network, are addressed in this paper.
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| | Modular DC/DC Structure with Multiple Power Flow Paths for Smart Transformer Applications
By Giampaolo BUTICCHI | |
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Abstract: Smart Transformers (ST), solid-state transformers with advanced control functionalities, are expected to have an important market by 2020. However, still important technological barriers, in terms of reliability and efficiency, exist and only new semiconductor devices cannot solve them. The use of advanced power converter topologies and architectures can play a role too.In this paper, a particular structure for the isolated DC/DC converter, core of a typical three-stage ST, is investigated. By employing the concept of Multiple Active Bridge, a modular and redundant architecture, a power converter characterized by multiple power routing paths is investigated.
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| | Operation of a Five-level Current Source Converter based PV Power Plant as a STATCOM
By Vishal VEKHANDE | |
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Abstract: In this paper, two control strategies based on space vector modulation (SVM) technique are developedfor a five-level current source converter (CSC) based STATCOM. In case of control strategy-1, DC-linkcurrents are kept constant, and the reactive power is varied by changing the modulation index. On theother hand, in case of control strategy-2, DC-link currents are varied in accordance with reactive powerrequirement, keeping the modulation index constant. Therefore, in case of control strategy-2, lossesreduce when the converter feeds smaller amount of the reactive power. Whereas, the control strategy-1exhibits faster dynamic response to the changing reactive power commands compared to that in controlstrategy-2.
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| | Operation Range of HVDC-MMC with Circulating Current Suppression and Energy Balancing Control
By Andrew THE | |
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Abstract: Many control strategies have been proposed to optimize the operation of the Modular MultilevelConverter (MMC) in HVDC applications, including the suppression of the circulating current and thebalancing of arm energies. Applying these control strategies, we present an analysis of the operationrange of the MMC, visualized as P-Q-charts. The analysis is performed with three different parameterswhich limit the operation range of the MMC: submodule voltage ripple, minimum value of the voltagereference and converter losses. The first two parameters define the voltage limit, and the third one thecurrent limit of the converter. They are therefore important parameters for the converter design.
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| | Power Module Voltage Balancing Method for a ±350 kV/ 1000 MW Modular Multilevel Converter
By Zixin LI | |
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Abstract: In this paper, a power module capacitor voltage balancing method is presented for a ±350 kV/ 1000 MW modular multilevel converter (MMC) oriented for a back-to-back grid interconnecting project. In this method, a Balancing Adjusting Number (BAN) is proposed to adjust the voltage balancing effect and the switching frequency of the power modules. An accurate switching frequency calculation method is also presented. Verification results show the effectiveness of the proposed voltage balancing method and the correctness of the switching frequency calculation method.
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| | Scaling method for a multi-terminal DC experimental test rig
By Marc CHEAH-MANE | |
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Abstract: This paper demonstrates a scaling method for an experimental test rig to represent the steady state operation of different HVDC transmission systems. The method is based on a parameter conversion between the experimental rig and the HVDC system under study. A virtual resistance is added to the DC cable resistance of the experimental test rig in order to achieve the same steady state operation as the real HVDC system. A droop control correction in the VSCs was used to implement the virtual resistance. A Hardware-in-the-loop set-up formed by laboratory-scale power electronic devices and a Real Time Simulator was used to test this scaling method. Three case studies modelled in PSCAD/EMTDC were used to verify that the results show good agreement.
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| | Side-by-side connection of LCC-HVDC links to form a DC grid
By Yingmei LIU | |
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Abstract: HVDC is suitable for bulk-power and long-distance transmission. Most of the existing HVDC systems are point-to-point LCC-HVDCs. By interconnecting existing LCC links can achieve more economical benefits and higher flexibility of power transfer. However, due to different voltage levels and control modes of these links, it is impossible to interconnect them together directly through dc cables. The side-by-side connection of LCC links using DC transformers at a geographic crossing point or close proximity between the LCC links is proposed. Such connection is to achieve the benefits of dc grid operation without incurring vast amount of costs by constructing new DC transmission lines but making full use of existing LCC lines. Two LCC links tied by a DC transformer are tested as a case study. The DC transformer based on full-bridge sub-module is designed for coping with power reversal, temporary DC and AC faults and reduced voltage operation. And its control system is designed to match the operation of LCC links.
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| | System fault test of SiC device applied 6.6kV transformerless D-STATCOM
By Yushi KOYAMA | |
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Abstract: This paper reports the investigation results for transformerless Distribution-Static SynchronousCompensator (D-STATCOM) with Modular Multilevel Converter (MMC) topology. The DSTATCOMhas cascaded Silicon-Insulated Gate Bipolar Transistor (Si-IGBT) inverter cells andSilicon Carbide-Junction Field Effect Transistor (SiC-JFET) inverter cells. SiC-JFET inverter cells areoperated with Pulse Width Modulation (PWM) since the low switching losses. The Si-IGBT invertercells have higher DC voltage than the SiC-JFET inverter cells, and they are operated with one-pulseoperation that output one positive and one negative pulses during a cycle. MMC topology and highvoltage Si-IGBT inverter cells realized connecting the D-STATCOM to 6.6 kV distribution systemwithout transformer.A prototype model of the D-STATCOM rated at 6.6 kV, 100 kVA was built and its field test wasexecuted. The experimental results prove the stable rated operation and Fault-Ride-Throughperformance.
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