HVDC transmission systems: Bipolar back-to-back diode clamped multilevel converter with fastoptimum-predictive control and capacitor balancing strategy

Voltage source multilevel power converter structures are being considered for high power high voltage applications where they have well known advantages. Recently, full back-to-back connected multilevel neutral diode clamped converters (NPC) have been used in high voltage direct current (HVDC) transmission systems. Bipolar back-to-back connection of NPCs have advantages in long distance HVDC transmission systems, but highly increased difficulties to balance the dc capacitor voltage dividers on both sending and receiving end NPCs. This paper proposes a fast optimum-predictive controller to balance the dc capacitor voltages and to control the power flow in a long distance HVDCsystem using bipolar back-to-back connected NPCs. For both converter sides, the control strategy considers active and reactive power to establish ac grid currents on sending and receiving ends, while guaranteeing the balancing of both NPC dc bus capacitor voltages. Furthermore, the fast predictivecontroller minimizes the semiconductor switching frequency to reduce global switching losses. The performance and robustness of the new fast predictive control strategy and the associated dc capacitors voltage balancing are evaluated. (C) 2011 Elsevier B.V. All rights reserved.

Limitações dos sistemas de transmissão AC quando aplicados a parques eólicos offshore

De modo a garantir as metas propostas pela União Europeia de diminuição de emissão de gases poluentes, alguns países membros apostaram na tecnologia eólica offshore. Com a evolução tecnológica, estes países pretendem aumentar as potências instaladas nos parques eólicos offshore, garantindo a sua otimização ao estarem em águas profundas, tendo assim um maior aproveitamento do potencial eólico. Com esta dissertação, realizou-se um estudo para analisar, sob o ponto de vista de modelos, as limitações dos sistemas de transmissão em HVAC (High Voltage Alternating Current) quando estabelecidos em cabos submarinos. Os mecanismos dos sistemas de alimentação em HVAC, na forma tradicional (linhas aérea), são confrontados com condicionalismos. Este tipo de infraestrutura submarina impõe restrições, pelo simples facto dos parâmetros de capacidade e indutância serem bastante significativos, que para uma situação de trânsito de energia, inviabilizam o transporte de energia, devido ao elevado consumo de potência reativa, bem como os seus custos extremamente elevados. Assim, os sistemas de alimentação em HVAC quando aplicados a parques eólicos offshore apresentam limitações. O recurso à tecnologia HVDC (High Voltage Direct Current), poderá ser a solução que se revela mais adequada para os sistemas de transmissão de energia associados aos parques eólicos offshore. Para além dos fatores técnicos é fundamental considerar os custos associados à construção e exploração do parque eólico, sendo fundamental analisar todos os dados relacionados com o projeto.

Proteção de linhas de transmissão de sistemas VSC-HVDC utilizando limitadores de corrente de falta

Tecnologias HVDC que utilizam conversores do tipo fonte de tensão, o VSC-HVDC, ainda não são completamente difundidas e aplicadas no Brasil, em contraste com outros países que começaram a estudar e empregar este tipo de transmissão. Comparado com o HVDC tradicional, o VSC-HVDC é uma tecnologia de transmissão mais eficiente e pode superar deficiências encontradas na transmissão em corrente contínua convencional. O VSC-HVDC pode ser utilizado de maneira mais eficiente nas novas redes de energia, para alimentar ilhas, integração de geração eólica, renovação das linhas em centros urbanos, aplicações multiterminais e conexão com sistemas fracos. Por se tratar de uma tecnologia recente, o VSC-HVDC ainda não é amplamente adotado e uma das principais limitações da utilização destes sistemas é a sua fragilidade diante faltas na linha de corrente contínua. Neste contexto, limitadores de corrente de falta (LCF) podem ser utilizados para minimizar o impacto das faltas. A ação dos limitadores é benéfica ao sistema durante condições de falta, contudo, ainda assim é necessária a atuação do sistema de proteção para extinguir a condição faltosa. Portanto, este trabalho visa propor e avaliar um novo esquema de proteção que opere de maneira seletiva e confiável para sistemas VSC-HVDC na presença de LCF baseados em materiais supercondutores ou LCF indutivos. Para tanto, foram implementadas quatro funções de proteção tradicionais das linhas em CC, a saber: direcional de corrente, diferencial, sobrecorrente com restrição de tensão e ondas viajantes, e ainda, foi proposta uma nova função de proteção, a de condutância, a qual apresentou o menor tempo de identificação de falta, considerando as faltas mais severas. Adicionalmente, foi avaliado o comportamento destas funções quando o sistema apresenta os LCF em série com a linha. Foi demonstrado que é possível extrair os benefícios dos LCF sem deteriorar a qualidade dos resultados das funções de proteção, o que aumenta a segurança e confiabilidade dos sistemas VSC-HVDC, uma vez que os impactos das faltas são minimizados e as mesmas são identificadas em um curto intervalo de tempo.

Protection schemes for meshed VSC-HVDC transmission systems for large-scale offshore wind farms

This chapter discusses network protection of high-voltage direct current (HVDC) transmission systems for large-scale offshore wind farms where the HVDC system utilizes voltage-source converters. The multi-terminal HVDC network topology and protection allocation and configuration are discussed with DC circuit breaker and protection relay configurations studied for different fault conditions. A detailed protection scheme is designed with a solution that does not require relay communication. Advanced understanding of protection system design and operation is necessary for reliable and safe operation of the meshed HVDC system under fault conditions. Meshed-HVDC systems are important as they will be used to interconnect large-scale offshore wind generation projects. Offshore wind generation is growing rapidly and offers a means of securing energy supply and addressing emissions targets whilst minimising community impacts. There are ambitious plans concerning such projects in Europe and in the Asia-Pacific region which will all require a reliable yet economic system to generate, collect, and transmit electrical power from renewable resources. Collective offshore wind farms are efficient and have potential as a significant low-carbon energy source. However, this requires a reliable collection and transmission system. Offshore wind power generation is a relatively new area and lacks systematic analysis of faults and associated operational experience to enhance further development. Appropriate fault protection schemes are required and this chapter highlights the process of developing and assessing such schemes. The chapter illustrates the basic meshed topology, identifies the need for distance evaluation, and appropriate cable models, then details the design and operation of the protection scheme with simulation results used to illustrate operation. © Springer Science+Business Media Singapore 2014.

Chain-link based HVDC voltage source converter using current injection

VSC converters are becoming more prevalent for HVDC applications. Two circuits are commercially available at present, a traditional six-switch, PWM inverter, implemented using series connected IGBTs - ABBs HVDC Light®, and the other a modular multi-level converter (MMC) - Siemens HVDC-PLUS. This paper presents an alternative MMC topology, which utilises a novel current injection technique, and exhibits several desirable characteristics.

A novel approach for Modeling the steady-state VSC-based multiline FACTS controllers and their operational constraints

A new, simple approach for modeling and assessing the operation and response of the multiline voltage-source controller (VSC)-based flexible ac transmission system controllers, namely the generalized interline power-flow controller (GIPFC) and the interline power-flow controller (IPFC), is presented in this paper. The model and the analysis developed are based on the converters` power balance method which makes use of the d-q orthogonal coordinates to thereafter present a direct solution for these controllers through a quadratic equation. The main constraints and limitations that such devices present while controlling the two independent ac systems considered, will also be evaluated. In order to examine and validate the steady-state model initially proposed, a phase-shift VSC-based GIPFC was also built in the Alternate Transients Program program whose results are also included in this paper. Where applicable, a comparative evaluation between the GIPFC and the IPFC is also presented.

Simulation of offshore wind system with two-level converters: HVDC power transmission

A new integrated mathematical model for the simulation of offshore wind energy conversion system performance is presented in this paper. The mathematical model considers an offshore variable-speed turbine in deep water equipped with a permanent magnet synchronous generator using full-power two-level converter, converting the energy of a variable frequency source in injected energy into the electric network with constant frequency, through a high voltage DC transmission submarine cable. The mathematical model for the drive train is a concentrate two mass model which incorporates the dynamic for the structure and tower due to the need to emulate the effects of the moving surface. Controller strategy considered is a proportional integral one. Also, pulse width modulation using space vector modulation supplemented with sliding mode is used for trigger the transistor of the converter. Finally, a case study is presented to access the system performance. © 2014 IEEE.

Simulation of offshore wind system with two levels converters: HVDC power transmission

A new integrated mathematical model for the simulation of offshore wind energy conversion system performance is presented in this paper. The mathematical model considers an offshore variable-speed turbine in deep water equipped with a permanent magnet synchronous generator using full-power two-level converter, converting the energy of a variable frequency source in injected energy into the electric network with constant frequency, through a high voltage DC transmission submarine cable. The mathematical model for the drive train is a concentrate two mass model which incorporates the dynamic for the structure and tower due to the need to emulate the effects of the moving surface. Controller strategy considered is a proportional integral one. Also, pulse width modulation using space vector modulation supplemented with sliding mode is used for trigger the transistor of the converter. Finally, a case study is presented to access the system performance. © 2014 IEEE.

Simulation of rectifier voltage malfunction on OWECS, four-level converter, HVDC light link: Smart grid context tool

This paper presents a model for the simulation of an offshore wind system having a rectifier input voltage malfunction at one phase. The offshore wind system model comprises a variable-speed wind turbine supported on a floating platform, equipped with a permanent magnet synchronous generator using full-power four-level neutral point clamped converter. The link from the offshore floating platform to the onshore electrical grid is done through a light high voltage direct current submarine cable. The drive train is modeled by a three-mass model. Considerations about the smart grid context are offered for the use of the model in such a context. The rectifier voltage malfunction domino effect is presented as a case study to show capabilities of the model. (C) 2015 Elsevier Ltd. All rights reserved.

Simulation of offshore wind system with three-level converters: HVDC power transmission in cloud scope

This paper is on a simulation for offshore wind systems in deep water under cloud scope. The system is equipped with a permanent magnet synchronous generator and a full-power three-level converter, converting the electric energy at variable frequency in one at constant frequency. The control strategies for the three-level are based on proportional integral controllers. The electric energy is injected through a HVDC transmission submarine cable into the grid. The drive train is modeled by a three-mass model taking into account the resistant stiffness torque, structure and tower in the deep water due to the moving surface elevation. Conclusions are taken on the influence of the moving surface on the energy conversion. © IFIP International Federation for Information Processing 2015.

Simulation of a rectifier malfunction on a offshore wind system with HVDC transmission

A new integrated mathematical model for the simulation of an offshore wind system having a rectifier input voltage malfunction at one phase is presented in this paper. The mathematical model considers an offshore variable-speed wind turbine on a floating platform, equipped with a permanent magnet synchronous generator using full-power three-level converter to inject energy into the electric network, through a high voltage direct current transmission submarine cable. The model for the drive train is a discrete three mass, incorporating the dynamic of the moving surface. A case study is presented to access conclusion about the malfunction.

Impacts of increased levels of wind penetration on the Electric Power System of the Åland Islands

The application of VSC-HVDC technology throughout the world has turned out to be an efficient solution regarding a large share of wind power in different power systems. This technology enhances the overall reliability of the grid by utilization of the active and reactive power control schemes which allows to maintain frequency and voltage on busbars of the end-consumers at the required level stated by the network operator. This master’s thesis is focused on the existing and planned wind farms as well as electric power system of the Åland Islands. The goal is to analyze the wind conditions of the islands and appropriately predict a possible production of the existing and planned wind farms with a help of WAsP software program. Further, to investigate the influence of increased wind power it is necessary to develop a simulation model of the electric grid and VSC-HVDC system in PSCAD and examine grid response to different wind power production cases with respect to the grid code requirements and ensure the stability of the power system.

Studie av subsynkron interaktion (SSTI) mellan ett HVDC system med spänningsstyva strömriktare och en närliggande generator

Syftet med examensarbetet är att studera den subsynkrona interaktionen (subsynchronous torsional interaktion, SSTI) mellan ett HVDC system med spänningsstyva strömriktare och en närliggande generator. Studien utförs med hjälp av datorsimuleringar och fokuserar på att studera dämpkaraktäristiken för SSTI. Studien ska visa hur faktorer såsom den överförda effektens storlek och riktning, vald driftmod och kraftsystemets kortslutningseffekt påverkar dämpkaraktäristiken. Rapporten beskriver subsynkron oscillation (SSO) och tillvägagångssättet vid en SSTI undersökning. En kort introduktion till HVDC Light ges. Studien visar att HVDC Light ökar dämpningen för det frekvensområde som är intressant. Resultaten presenteras som diagram med beräknad dämpkaraktäristik.

Metodologia para o cálculo de harmônicas em conversores de potência no sistema HVDC

Pós-graduação em Engenharia Elétrica - FEIS

Análise do suporte de reativos em um sistema elétrico de potência com link HVDC

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)