Modular multilevel converter losses model for HVdc applications

Multi-terminal high voltage dc (HVdc) grids can eventually became a feasible solution to transport energy to remote and/ or distant areas and its exploitation depend, among other things, on the performance of the converter terminals. Therefore, to optimize the power transmission strategy along such a grid, it is necessary to recognize the efficiency of all the converters in all points of operation, namely with the different load conditions. In this vision, the aim of this work is to provide the methodology to model the modular multilevel converter (MMC) efficiency by means of a mathematical expression that can describe, over a broad range of active and reactive power flow combinations, the power losses generated by the semiconductors. According to the presented methodology, a polynomial-based model with a reduced number of coefficients is deducted, in such a way that can be directly used for optimal power flow (OPF) studies. The accuracy of the proposed model is characterized by an absolute relative error, at the worst scenario, approximately equal to 3%.Postprint (author's final draft

Modular Multilevel Converter with Sensorless Diode-Clamped Balancing through Level-Adjusted Phase-Shifted Modulation

Cascaded H-bridge and modular multilevel converters (MMC) are on the rise with emerging applications in renewable energy generation, energy storage, and electric motor drives. However, their well-known advantages come at the price of complicated balancing, high-bandwidth isolated monitoring, and numerous sensors that can prevent MMCs from expanding into highly cost driven markets. Therefore, an obvious trend in research is developing control and topologies that depend less on measurements and benefit from simpler control. Diode-clamped topologies are considered among the more applicable solutions. The main problem with a diode-clamped topology is that it can only balance the module voltages of a string in one direction; therefore, it cannot provide a completely balanced operation. This paper proposes an effective balancing technique for the diode-clamped topology. The proposed solution exploits the dc component of the arm current by introducing a symmetrically level-adjusted phase-shifted modulation scheme, and ensures the balancing current flow is always in the correct direction. The main advantages of this method are sensorless operation, no added computation and control effort, and low overall cost. Analysis and detailed simulations provide insight into the operation of the system as well as the new balancing technique and the experimental results confirm the provided discussions

Controlled transition full-bridge hybrid multilevel converter with chain-links of full-bridge cells

This paper proposes a controlled transition full-bridge (CTFB) hybrid multilevel converter (HMC) for medium and high voltage applications. It employs a full-bridge cell chain-link (FB-CL) between the two legs in each phase to generate multilevel bipolar output voltage. The CTFB-HMC has twice dc voltage utilization or power density of conventional converters due to the bipolar capability of its full-bridge configuration. Hence, for the same power rating and same voltage level number, its total cells per phase are quarter that in modular multilevel converter (MMC), which reduces the hardware installation volume. Also, in the proposed converter, the total device number in the conduction paths is the same as in the half-bridge MMC, leading to low conduction losses. The FB-CL current of the CTFB converter has no dc component, which offers the potential to enhance the transient response. Comparative studies between the CTFB and other multilevel topologies are carried out to clarify its main features. The modulation strategies and parameter sizing of the proposed converter are investigated using a generic case. Simulation and experimental results are used to verify the effectiveness of the proposed approach

DC current flow controllers for meshed HVDC grids

Premi Extraordinari de Doctorat, promoció 2018-2019. Àmbit d’Enginyeria IndustrialMeshed High Voltage Direct Current (HVDC) grids are seen as solution to transmit and exchange high amounts of power across long distances or using submarine cables with high levels of flexibility and redundancy. Also, they can be especially suitable to integrate offshore energy resources such as offshore wind power plants. This thesis focuses on the DC Current Flow Controllers (CFC) for meshed HVDC grids. The CFCs are being thought as power electronics based devices that may be installed in future meshed HVDC grids to aid in the current flow regulation. The concept is similar to Flexible Alternating Current Transmission Systems (FACTS) but applied to HVDC grids. First, an overview of the different CFC concepts proposed in the literature is presented. Then, the modelling and control of a DC/DC CFC converter is developed and the benefits of installing it in a meshed HVDC grid are analysed. The functionality of the previous CFC is also integrated into a DC Circuit Breaker, in order to have a single device with both capability to interrupt DC faults and provide DC current regulation. Afterwards, an interline DC/DC CFC topology is proposed, which has the advantage of a simplified converter structure. It is validated using dynamic simulations and a prototype is built and tested in a meshed DC grid experimental platform. A single CFC may not be enough to regulate the current flows in complex meshed HVDC grids, thus, this work also considers the concept of Distributed CFCs (DCFC) in a meshed HVDC grid, which are being operated selectively, allowing more flexibility when regulating the current flows. Also, multiple lines can be connected to a certain HVDC node. Therefore, the proposed CFC is extended to be connected to any number of HVDC lines and so, be able to control the current circulating through any of them. The obtained multi-port CFC is validated through simulations. Other devices can help to the current regulation in meshed HVDC grids, for example already installed DC/DC converters that adapt the different voltages of the HVDC systems. A transformerless DC/DC topology is analysed in this work and the design of its AC filter addressed. Finally, taking into account that some HVDC links based on Line Commutated Converters (LCC-HVDC) are installed near to potential offshore wind power resources, this work studies the operation and control of a Current Source Converter (CSC) based tapping station connected in series with the HVDC link to integrate offshore wind power.Les xarxes d'alta tensió mallades en contínua, meshed High voltatge Direct Current (HVDC) grids, es presenten com una solució per transportar grans quantitats d'energia a través de llargues distàncies o mitjançant cables submarins amb alts nivells de flexibilitat i redundància. També, són especialment adequades per la captació d'energia de parcs eòlics marins. Aquesta tesi se centra en els controladors del flux de corrent, Current Flow Controllers (CFC), per a xarxes HVDC mallades. Els CFC es plantegen com dispositius d'electrònica de potència que es podrien instal·lar en les futures xarxes HVDC mallades per tal d'ajudar en la regulació dels fluxos de corrent de les línies. Aquest concepte és similar als dispositius FACTS (Flexible AC Transmission Systems), però aplicat a xarxes HVDC. Primer, es realitza un recull de les diferents propostes de CFCs a la literatura. Després, es modelitza i es dissenya el control d'un convertidor DC/DC CFC i s'analitzen els beneficis d'instal·lar-lo en una xarxa HVDC mallada. La funcionalitat de l'anterior CFC s'inclou en els interruptors de contínua, DC Circuit Breakers (DCCB), per tal de tenir un dispositiu amb capacitat d'interropre faltes DC i també controlar corrents. A continuació, es proposa una topologia de CFC simplicada, que es valida per mitjà de simulacions i se'n construeix un prototip que es prova experimentalment al laboratori. Un únic CFC pot no ser suficient per a controlar els fluxos de corrent en xarxes HVDC mallades d'una certa complexitat. És per això, que també s'introdueix el concepte de CFCs distribuïts en diferents nodes de la xarxa i que s'operen de forma selectiva. Vàries línies HVDC poden estar connectades a un node, per aquest motiu, la topologia de CFC anteriorment presentada s'actualitza per tal de poder ser connectada a un nombre qualsevol de línies. La topologia multi-port obtinguda es valida per mitjà de simulacions. Altres dispositius que poden ajudar a controlar els fluxos de corrent són els propis convertidors DC/DC que s'encarreguen d'adaptar la tensió dels sistemes HVDC. S'analitza també un convertidor DC/DC sense transformador AC i es realitza el disseny del seu filtre AC. Finalment, algunes de les línies HVDC basades en tecnologia Line Commutated Converter (LCC) es troben a prop de zones amb energia eòlica potencial. Per aquest motiu, s'estudia l'operació i control d'un convertidor Current Source Converter (CSC) que actua com una estació de tapping per tal d'injectar l'energia d'un parc eòlic marí a la línia LCC-HVDC.Award-winningPostprint (published version

Novel Power Electronic Device Structures for Power Conditioning Applications

The work presented in this thesis contains an investigation into the methods by which the semiconductor device performance can be improved with an aim to reduce the overall losses in the power conversion system. The types of devices discussed and evaluated in this thesis include Silicon MOSFETs, IGBT, CIGBT and GaN HEMT devices. The performance improvement methods suggested in literature usually involve a trade-off of device characteristics with one another. Therefore an investigation into new device technologies and structures is deemed necessary such that the performance trade-off can be avoided or be improved

DC current flow controllers for meshed HVDC grids

Meshed High Voltage Direct Current (HVDC) grids are seen as solution to transmit and exchange high amounts of power across long distances or using submarine cables with high levels of flexibility and redundancy. Also, they can be especially suitable to integrate offshore energy resources such as offshore wind power plants. This thesis focuses on the DC Current Flow Controllers (CFC) for meshed HVDC grids. The CFCs are being thought as power electronics based devices that may be installed in future meshed HVDC grids to aid in the current flow regulation. The concept is similar to Flexible Alternating Current Transmission Systems (FACTS) but applied to HVDC grids. First, an overview of the different CFC concepts proposed in the literature is presented. Then, the modelling and control of a DC/DC CFC converter is developed and the benefits of installing it in a meshed HVDC grid are analysed. The functionality of the previous CFC is also integrated into a DC Circuit Breaker, in order to have a single device with both capability to interrupt DC faults and provide DC current regulation. Afterwards, an interline DC/DC CFC topology is proposed, which has the advantage of a simplified converter structure. It is validated using dynamic simulations and a prototype is built and tested in a meshed DC grid experimental platform. A single CFC may not be enough to regulate the current flows in complex meshed HVDC grids, thus, this work also considers the concept of Distributed CFCs (DCFC) in a meshed HVDC grid, which are being operated selectively, allowing more flexibility when regulating the current flows. Also, multiple lines can be connected to a certain HVDC node. Therefore, the proposed CFC is extended to be connected to any number of HVDC lines and so, be able to control the current circulating through any of them. The obtained multi-port CFC is validated through simulations. Other devices can help to the current regulation in meshed HVDC grids, for example already installed DC/DC converters that adapt the different voltages of the HVDC systems. A transformerless DC/DC topology is analysed in this work and the design of its AC filter addressed. Finally, taking into account that some HVDC links based on Line Commutated Converters (LCC-HVDC) are installed near to potential offshore wind power resources, this work studies the operation and control of a Current Source Converter (CSC) based tapping station connected in series with the HVDC link to integrate offshore wind power.Les xarxes d'alta tensió mallades en contínua, meshed High voltatge Direct Current (HVDC) grids, es presenten com una solució per transportar grans quantitats d'energia a través de llargues distàncies o mitjançant cables submarins amb alts nivells de flexibilitat i redundància. També, són especialment adequades per la captació d'energia de parcs eòlics marins. Aquesta tesi se centra en els controladors del flux de corrent, Current Flow Controllers (CFC), per a xarxes HVDC mallades. Els CFC es plantegen com dispositius d'electrònica de potència que es podrien instal·lar en les futures xarxes HVDC mallades per tal d'ajudar en la regulació dels fluxos de corrent de les línies. Aquest concepte és similar als dispositius FACTS (Flexible AC Transmission Systems), però aplicat a xarxes HVDC. Primer, es realitza un recull de les diferents propostes de CFCs a la literatura. Després, es modelitza i es dissenya el control d'un convertidor DC/DC CFC i s'analitzen els beneficis d'instal·lar-lo en una xarxa HVDC mallada. La funcionalitat de l'anterior CFC s'inclou en els interruptors de contínua, DC Circuit Breakers (DCCB), per tal de tenir un dispositiu amb capacitat d'interropre faltes DC i també controlar corrents. A continuació, es proposa una topologia de CFC simplicada, que es valida per mitjà de simulacions i se'n construeix un prototip que es prova experimentalment al laboratori. Un únic CFC pot no ser suficient per a controlar els fluxos de corrent en xarxes HVDC mallades d'una certa complexitat. És per això, que també s'introdueix el concepte de CFCs distribuïts en diferents nodes de la xarxa i que s'operen de forma selectiva. Vàries línies HVDC poden estar connectades a un node, per aquest motiu, la topologia de CFC anteriorment presentada s'actualitza per tal de poder ser connectada a un nombre qualsevol de línies. La topologia multi-port obtinguda es valida per mitjà de simulacions. Altres dispositius que poden ajudar a controlar els fluxos de corrent són els propis convertidors DC/DC que s'encarreguen d'adaptar la tensió dels sistemes HVDC. S'analitza també un convertidor DC/DC sense transformador AC i es realitza el disseny del seu filtre AC. Finalment, algunes de les línies HVDC basades en tecnologia Line Commutated Converter (LCC) es troben a prop de zones amb energia eòlica potencial. Per aquest motiu, s'estudia l'operació i control d'un convertidor Current Source Converter (CSC) que actua com una estació de tapping per tal d'injectar l'energia d'un parc eòlic marí a la línia LCC-HVDC