Adaptive backstepping droop controller design for multi-terminal high-voltage direct current systems

Wind power is one of the most developed renewable energy resources worldwide. To integrate offshore wind farms to onshore grids, the high-voltage direct current (HVDC) transmission cables interfaced with voltage source converters (VSCs) are considered to be a better solution than conventional approaches. Proper DC voltage indicates successive power transfer. To connect more than one onshore grid, the DC voltage droop control is one of the most popular methods to share the control burden between different terminals. However, the challenges are that small droop gains will cause voltage deviations, while higher droop gain settings will cause large oscillations. This study aims to enhance the performance of the traditional droop controller by considering the DC cable dynamics. Based on the backstepping control concept, DC cables are modelled with a series of capacitors and inductors. The final droop control law is deduced step-by-step from the original remote side. At each step the control error from the previous step is considered. Simulation results show that both the voltage deviations and oscillations can be effectively reduced using the proposed method. Further, power sharing between different terminals can be effectively simplified such that it correlates linearly with the droop gains, thus enabling simple yet accurate system operation and control.

Analysis of voltage source converter-based high-voltage direct current under DC line-to-earth fault

DC line faults on high-voltage direct current (HVDC) systems utilising voltage source converters (VSCs) are a major issue for multi-terminal HVDC systems in which complete isolation of the faulted system is not a viable option. Of these faults, single line-to-earth faults are the most common fault scenario. To better understand the system under such faults, this study analyses the behaviour of HVDC systems based on both conventional two-level converter and multilevel modular converter technology, experiencing a permanent line-to-earth fault. Operation of the proposed system under two different earthing configurations of converter side AC transformer earthed with converter unearthed, and both converter and AC transformer unearthed, was analysed and simulated, with particular attention paid to the converter operation. It was observed that the development of potential earth loops within the system as a result of DC line-to-earth faults leads to substantial overcurrent and results in oscillations depending on the earthing configuration.

Characterization of a high-density, direct-current reflex discharge plasma source operating in Ar and N-2

The characterization of a direct current, low-pressure, and high-density reflex discharge plasma source operating in argon and in nitrogen, over a range of pressures 1.0-10(-2) mbar, discharge currents 20-200 mA, and magnetic fields 0-120 G, and its parametric characterization is presented. Both external parameters, such as the breakdown potential and the discharge voltage-current characteristic, and internal parameters, like the charge carrier's temperature and density, plasma potential, floating potential, and electron energy distribution function, were measured. The electron energy distribution functions are bi-Maxwellian, but some structure is observed in these functions in nitrogen plasmas. There is experimental evidence for the existence of three groups of electrons within this reflex discharge plasma. Due to the enhanced hollow cathode effect by the magnetic trapping of electrons, the density of the cold group of electrons is as high as 10(18) m(-3), and the temperature is as low as a few tenths of an electron volt. The bulk plasma density scales with the dissipated power. Another important feature of this reflex plasma source is its high degree of uniformity, while the discharge bulk region is free of electric field. (C) 2002 American Institute of Physics.

Asymptotic layer-type model of high pressure direct current glow discharge in electronegative gases

Here a self-consistent one-dimensional continuum model is presented for a narrow gap plane-parallel dc glow discharge. The governing equations consist of continuity and momentum equations for positive and negative ions and electrons coupled with Poisson's equation. A singular perturbation method is developed for the analysis of high pressure dc glow discharge. The kinetic processes of the ionization, electron attachment, and ion-ion recombination are included in the model. Explicit results are obtained for the asymptotic limits: delta=(r(D)/L)(2)--> 0, omega=(r(S)/L)(2)--> 0, where r(D) is the Debye radius, r(S) is recombination length, and L is the gap length. The discharge gap divides naturally into four layers with multiple space scales: anode fall region, positive column, transitional region, cathode fall region and diffusion layer adjacent to the cathode surface, its formation is discussed. The effects of the gas pressure, gap spacing and dc voltage on the electrical properties of the layers and its dimension are investigated. (C) 2000 American Institute of Physics. [S0021-8979(00)00813-6].

HIGH VOLTAGE PULSED CURRENT STIMULATION OF THE SCIATIC NERVE IN RATS: ANALYSIS BY THE SFI

Objective: To analyze the efficiency of high voltage pulsed current (HVPC) with early application in three different sites, in the regeneration of the sciatic nerve in rats submitted to crush injury, the sciatic functional index (SFI) was used to assess the functional recovery. Methods: After crushing of the nerve, 57 animals were submitted to cathodal HVPC at frequency of 50Hz and voltage of 100V, 20 minutes per day, 5 days per week. The rats were divided into five groups: control group; ganglion group; ganglion + muscle group; muscle group; and sham group. The SFI was determined weekly for seven weeks, from the preoperative period to the 6th postoperative week. Results: Compared with the control group, the results showed a significantly better performance of group 2 for the first 3 weeks; group 3 showed significantly better performance in the third week; and group 4 showed a significantly negative performance during the 481 and 6th weeks. Conclusion: Early application of HVPC had a positive effect in the treatment of the spinal cord region and the sciatic nerve root ganglion with a dispersive electrode on the contralateral lumbar region or on the gastrocnemius. However, HVPC had a negative effect in the treatment with an active electrode on the gastrocnemius and a dispersive electrode on the contralateral thigh. Level of evidence II, Prospective comparative study.

Droop setting design for multi-terminal HVDC grids considering voltage deviation impacts

In multi-terminal high voltage direct current (HVDC) grids, the widely deployed droop control strategies will cause a non-uniform voltage deviation on the power flow, which is determined by the network topology and droop settings. This voltage deviation results in an inconsistent power flow pattern when the dispatch references are changed, which could be detrimental to the operation and seamless integration of HVDC grids. In this paper, a novel droop setting design method is proposed to address this problem for a more precise power dispatch. The effects of voltage deviations on the power sharing accuracy and transmission loss are analysed. This paper shows that there is a trade-off between minimizing the voltage deviation, ensuring a proper power delivery and reducing the total transmission loss in the droop setting design. The efficacy of the proposed method is confirmed by simulation studies.

Multiterminal DC wind farm collection grid internal fault analysis and protection design

The multiterminal dc wind farm is a promising topology with a voltage-source inverter (VSI) connection at the onshore grid. Voltage-source converters (VSCs) are robust to ac-side fault conditions. However, they are vulnerable to dc faults on the dc side of the converter. This paper analyzes dc faults, their transients, and the resulting protection issues. Overcurrent faults are analyzed in detail and provide an insight into protection system design. The radial wind farm topology with star or string connection is considered. The outcomes may be applicable for VSCs in the multi-VSC dc wind farm collection grid and VSC-based high-voltage direct current (HVDC) offshore transmission systems.

A novel high voltage pulsed power supply based on low voltage switch-capacitor units

This paper presents a high voltage pulsed power system based on low voltage switch-capacitor units connected to a current source for several applications such as plasma systems. A buck-boost converter topology is used to utilize the current source and a series of low voltage switch-capacitor units is connected to the current source in order to provide high voltage with high voltage stress (dv/dt) as demanded by loads. This pulsed power converter is flexible in terms of energy control, in that the stored energy in the current source can be adjusted by changing the current magnitude to significantly improve the efficiency of various systems with different requirements. Output voltage magnitude and stress (dv/dt) can be controlled by a proper selection of components and control algorithm to turn on and off switching devices.

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.

Some of the design considerations in power generation from offshore wind farms

The global efforts to reduce carbon emissions from power generation have favoured renewable energy resources such as wind and solar in recent years. The generation of power from the renewable energy resources has become attractive because of various incentives provided by government policies supporting green power. Among the various available renewable energy resources, the power generation from wind has seen tremendous growth in the last decade. This article discusses various advantages of the upcoming offshore wind technology and associated considerations related to their construction. The conventional configuration of the offshore wind farm is based on the alternative current internal links. With the recent advances of improved commercialised converters, voltage source converters based high voltage direct current link for offshore wind farms is gaining popularity. The planning and construction phases of offshore wind farms, including related environmental issues, are discussed here.

Application of power electronics in improving power quality and supply efficiency of AC traction networks

Major advances in power electronics during recent years have prompted considerable interest within the traction community. The capability of new technologies to reduce the AC railway networks' effect on power quality and improve their supply efficiency is expected to significantly decrease the cost of electric rail supply systems. Of particular interest are Static Frequency Converter (SFC), Rail Power Conditioner (RPC), High Voltage Direct Current (HVDC) and Energy Storage Systems (ESS) solutions. Substantial impacts on future feasibility of railway electrification are anticipated. Aurizon, Australia's largest heavy haul railway operator, has recently commissioned the world's first 50Hz/50Hz SFC installation and is currently investigating SFC, RPC, HVDC and ESS solutions. This paper presents a summary of current and emerging technologies with a particular focus on the potential techno-economic benefits.

Simulation of surface and volume stress for high voltage transmission insulators

The bulk of power transmission from the generating stations to the load centres is carried through overhead lines. The distances involved could span several hundreds of kilometres. To minimize line losses, power transmission over such long distances is carried out at high voltages (several hundreds of kV). A network of outdoor lines operating at different voltages has been found to be the most economical method of power delivery. The disc insulators perform dual task of mechanically supporting and electrically isolating the live phase conductors from the support tower. These insulators have to perform under various environmental conditions; hence the electrical stress distribution along the insulators governs the possible flashover, which is quite detrimental to the system. In view of this the present investigation aims to simulate the surface electric field stress on different types of porcelain/ceramic insulators; both normal and anti-fog type discs which are used for high voltage transmission/distribution systems are considered. The surface charge simulation method is employed for the field computation to simulate potential, electric field, surface and bulk/volume stress.

Distributed MPC for Frequency Regulation in Multi-Terminal HVDC Grids

High Voltage Direct Current (HVDC) lines allow large quantities of power to be
transferred between two points in an electrical power system. A Multi-Terminal HVDC (MTDC) grid consists of a meshed network of HVDC lines, and this allows energy reserves to be shared between a number of AC areas in an efficient manner. Secondary Frequency Control (SFC) algorithms return the frequencies in areas connected by AC or DC lines to their original setpoints after Primary Frequency Controllers have been called following a contingency. Where multiple
TSOs are responsible for different parts of a MTDC grid it may not be possible to implement SFC from a centralised location. Thus, in this paper a simple gain based distributed Model Predictive Control strategy is proposed for Secondary Frequency Control of MTDC grids which allows TSOs to cooperatively perform SFC without the need for centralised coordination.

Analysis of Transmission Loss with Droop Control in a Multi-Terminal HVDC System

High Voltage Direct Current (HVDC) electric power transmission is a promising technology for integrating offshore wind farms and interconnecting power grids in different regions. In order to maintain the DC voltage, droop control has been widely used. Transmission line loss constitutes an import part of the total power loss in a multi-terminal HVDC scheme. In this paper, the relation between droop controller design and transmission loss has been investigated. Different MTDC layout configurations are compared to examine the effect of droop controller design on the transmission loss.

Szenarien zur zukünftigen Stromversorgung, kostenoptimierte Variationen zur Versorgung Europas und seiner Nachbarn mit Strom aus erneuerbaren Energien

Sowohl die Ressourcenproblematik als auch die drohenden Ausmaße der Klimaänderung lassen einen Umstieg auf andere Energiequellen langfristig unausweichlich erscheinen und mittelfristig als dringend geboten. Unabhängig von der Frage, auf welchem Niveau sich der Energiebedarf stabilisieren lässt, bleibt dabei zu klären, welche Möglichkeiten sich aus technischer und wirtschaftlicher Sicht in Zukunft zur Deckung unseres Energiebedarfs anbieten. Eine aussichtsreiche Option besteht in der Nutzung regenerativer Energien in ihrer ganzen Vielfalt. Die Arbeit "Szenarien zur zukünftigen Stromversorgung, kostenoptimierte Variationen zur Versorgung Europas und seiner Nachbarn mit Strom aus erneuerbaren Energien" konzentriert sich mit der Stromversorgung auf einen Teilaspekt der Energieversorgung, der zunehmend an Wichtigkeit gewinnt und als ein Schlüssel zur nachhaltigen Energieversorgung interpretiert werden kann. Die Stromversorgung ist heute weltweit für etwa die Hälfte des anthropogenen CO2-Ausstoßes verantwortlich. In dieser Arbeit wurden anhand verschiedener Szenarien Möglichkeiten einer weitgehend CO2–neutralen Stromversorgung für Europa und seine nähere Umgebung untersucht, wobei das Szenariogebiet etwa 1,1 Mrd. Einwohner und einen Stromverbrauch von knapp 4000 TWh/a umfasst. Dabei wurde untersucht, wie die Stromversorgung aufgebaut sein sollte, damit sie möglichst kostengünstig verwirklicht werden kann. Diese Frage wurde beispielsweise für Szenarien untersucht, in denen ausschließlich heute marktverfügbare Techniken berücksichtigt wurden. Auch der Einfluss der Nutzung einiger neuer Technologien, die bisher noch in Entwicklung sind, auf die optimale Gestaltung der Stromversorgung, wurde anhand einiger Beispiele untersucht. Die Konzeption der zukünftigen Stromversorgung sollte dabei nach Möglichkeit objektiven Kriterien gehorchen, die auch die Vergleichbarkeit verschiedener Versorgungsansätze gewährleisten. Dafür wurde ein Optimierungsansatz gewählt, mit dessen Hilfe sowohl bei der Konfiguration als auch beim rechnerischen Betrieb des Stromversorgungssystems weitgehend auf subjektive Entscheidungsprozesse verzichtet werden kann. Die Optimierung hatte zum Ziel, für die definierte möglichst realitätsnahe Versorgungsaufgabe den idealen Kraftwerks- und Leitungspark zu bestimmen, der eine kostenoptimale Stromversorgung gewährleistet. Als Erzeugungsoptionen werden dabei u.a. die Nutzung Regenerativer Energien durch Wasserkraftwerke, Windenergiekonverter, Fallwindkraftwerke, Biomassekraftwerke sowie solare und geothermische Kraftwerke berücksichtigt. Abhängig von den gewählten Randbedingungen ergaben sich dabei unterschiedliche Szenarien. Das Ziel der Arbeit war, mit Hilfe unterschiedlicher Szenarien eine breite Basis als Entscheidungsgrundlage für zukünftige politische Weichenstellungen zu schaffen. Die Szenarien zeigen Optionen für eine zukünftige Gestaltung der Stromversorgung auf, machen Auswirkungen verschiedener – auch politischer – Rahmenbedingungen deutlich und stellen so die geforderte Entscheidungsgrundlage bereit. Als Grundlage für die Erstellung der Szenarien mussten die verschiedenen Potentiale erneuerbarer Energien in hoher zeitlicher und räumlicher Auflösung ermittelt werden, mit denen es erstmals möglich war, die Fragen einer großräumigen regenerativen Stromversorgung ohne ungesicherte Annahmen anhand einer verlässlichen Datengrundlage anzugehen. Auch die Charakteristika der verschiedensten Energiewandlungs- und Transportsysteme mussten studiert werden und sind wie deren Kosten und die verschiedenen Potentiale in der vorliegenden Arbeit ausführlich diskutiert. Als Ausgangsszenario und Bezugspunkt dient ein konservatives Grundszenario. Hierbei handelt es sich um ein Szenario für eine Stromversorgung unter ausschließlicher Nutzung erneuerbarer Energien, die wiederum ausschließlich auf heute bereits entwickelte Technologien zurückgreift und dabei für alle Komponenten die heutigen Kosten zugrundelegt. Dieses Grundszenario ist dementsprechend auch als eine Art konservative Worst-Case-Abschätzung für unsere Zukunftsoptionen bei der regenerativen Stromversorgung zu verstehen. Als Ergebnis der Optimierung basiert die Stromversorgung beim Grundszenario zum größten Teil auf der Stromproduktion aus Windkraft. Biomasse und schon heute bestehende Wasserkraft übernehmen den überwiegenden Teil der Backup-Aufgaben innerhalb des – mit leistungsstarker HGÜ (Hochspannungs–Gleichstrom–Übertragung) verknüpften – Stromversorgungsgebiets. Die Stromgestehungskosten liegen mit 4,65 €ct / kWh sehr nahe am heute Üblichen. Sie liegen niedriger als die heutigen Preisen an der Strombörse. In allen Szenarien – außer relativ teuren, restriktiv ”dezentralen” unter Ausschluss großräumig länderübergreifenden Stromtransports – spielt der Stromtransport eine wichtige Rolle. Er wird genutzt, um Ausgleichseffekte bei der dargebotsabhängigen Stromproduktion aus erneuerbaren Quellen zu realisieren, gute kostengünstige Potentiale nutzbar zu machen und um die Speicherwasserkraft sowie die dezentral genutzte Biomasse mit ihrer Speicherfähigkeit für großräumige Backup-Aufgaben zu erschließen. Damit erweist sich der Stromtransport als einer der Schlüssel zu einer kostengünstigen Stromversorgung. Dies wiederum kann als Handlungsempfehlung bei politischen Weichenstellungen interpretiert werden, die demnach gezielt auf internationale Kooperation im Bereich der Nutzung erneuerbarer Energien setzen und insbesondere den großräumigen Stromtransport mit einbeziehen sollten. Die Szenarien stellen detaillierte und verlässliche Grundlagen für wichtige politische und technologische Zukunftsentscheidungen zur Verfügung. Sie zeigen, dass bei internationaler Kooperation selbst bei konservativen Annahmen eine rein regenerative Stromversorgung möglich ist, die wirtschaftlich ohne Probleme zu realisieren wäre und verweisen den Handlungsbedarf in den Bereich der Politik. Eine wesentliche Aufgabe der Politik läge darin, die internationale Kooperation zu organisieren und Instrumente für eine Umgestaltung der Stromversorgung zu entwickeln. Dabei kann davon ausgegangen werden, dass nicht nur ein sinnvoller Weg zu einer CO2–neutralen Stromversorgung beschritten würde, sondern sich darüber hinaus ausgezeichnete Entwicklungsperspektiven für die ärmeren Nachbarstaaten der EU und Europas eröffnen.