Operation of meshed high voltage direct current (HVDC) overlay grids: from operational planning to real time operation

Energy turnaround from conventional to renewable energy generation needs bulk power long distance transmission. This new transmission objective can be meet with an HVDC overlay grid spanning the existing AC transmission system. This thesis proposes an operation management strategy for future HVDC overlay grids subdivided in tertiary, secondary and primary control instances. The tertiary control ensures coordination among HVDC converters and with the AC system. It determines converter reference values on a regular basis. It is proposed for the case of having multiple as well as a single system operator responsible for the overlay HVDC grid. The secondary control instance locally adapts tertiary control’s converter references to the actual grid requirements (e.g. after disturbances). The primary control ensures DC energy balance. Therefore, a continuous p-v-characteristic is proposed as well as two appropriate parameterization methods. One emulates piecewise linear p-v-characteristics and the other performs an automatic parameterization according to available balancing power provision capabilities on related AC point of common coupling. All control methods are validated by numerical case studies.Weltweit aber besonders in Europa steigt der Bedarf große Leistungen über weite Strecken zu transportieren. Dies ist hauptsächlich in der Energiewende und dem damit zusammenhängenden stark ansteigenden Anteil Erneuerbarer Energien und deren Erzeugungszentren begründet. Ein bedeutender Teil der Erneuerbaren Energien wird zukünftig weitab der Lastzentren produziert. Zur Lösung dieser daraus resultierenden neuen Transportaufgabe ist die Hochspannungsgleichstromübertragung (HGÜ) besonders geeignet. Eine redundante und damit auch wirtschaftliche Ausführung stellt das vermaschte HGÜ-Netz dar, das in der Energieversorgungsnetzhiearchie eine neue Netzebene dargestellt und somit als Overlay-HGÜ-Netz bezeichnet wird. Diese Arbeit widmet sich der Fragestellung der Betriebsführung eines Overlaynetzes. Dazu wird eine dreistufige Betriebsführung vorgeschlagen. In Anlehnung an die im europäischen AC-Verbundnetz bestehende Dreiteiligkeit wird eine Untergliederung in folgende Regelungsinstanzen vorgenommen: Tertiär-, Sekundär und Primärregelung. Die Tertiärregelung übernimmt die Koordinierungsaufgabe der Umrichter untereinander und mit dem unterlagerten AC-Netz im Rahmen einer Betriebsplanung. Es ist ein betriebstypisches Aktualisierungsintervall von 15 Minuten vorgesehen, indem die Umrichtersollwerte vorgegeben werden. Deren Bestimmung erfolgt durch ein auf dieses nichtlineare Problem zugeschnittenen AC/DC Optimal Power Flow. Dieses Verfahren fußt auf der Verfügbarkeit aller AC- und DC-Netzinformationen im Gebiet des Overlaynetzes. Im Falle einer föderalen Organisation eines HGÜ-Overlaynetzes in Europa müssen die Zielsetzungen mehrere Übertragungsnetzbetreiber (ÜNB) bei der Bestimmung eines Umrichtersollwertfahrplans berücksichtig werden. Für diesen Fall wird hier eine Methode vorgeschlagen, die mittels eines Aushandlungsprozesses die ÜNB spezifischen Kostenfunktionen für den Einsatz von HGÜ-Umrichtern in der entsprechenden Regelzone zu einer für das gesamte Overlaynetz gültigen Zielfunktion konsolidiert. Dabei werden Grenzwerte der einzelnen beteiligten ÜNB ebenso berücksichtigt wie lokale Zielfunktionen. Die Sekundärregelung passt die von der Tertiärregelung vorgegebenen Umrichtersollwerte innerhalb des 15-min-Betriebsintervalls vor allem im Fall von Störungen an. Dafür wird ein Verfahren vorgeschlagen, das sich der Informationen eines Weitbereichsüberwachungssystems bedient, um signifikante Abweichung der geplanten Leistungsflüsse zu erfassen. Die Umrichterwirkleistungssollwerte werden entsprechend angepasst. Eine Aufteilung von unplanmäßigen Leistungsflüssen zwischen AC und DC-Netz sorgt für eine Entlastung des AC-Netzes und beugt Betriebsmittelüberlastungen und dadurch verursachten Instabilitätsphänomenen vor. Die Primärregelung gewährleistet das Gleichgewicht zwischen ein- und ausgespeister Wirkleistung in das / aus dem HGÜ-Overlaynetz. Ist die diesbezügliche Leistungsbilanz ausgewogen, ist das Energiegleichgewicht, die sogenanntes Energiestabilität, gewahrt. Die DC-Zeitkonstanten sind klein. Nur eine dezentral (am Umrichterstandort) angeordnete Regelung kann zeitlich angemessen reagieren. Diese nutzt eine p-u-Regelcharakteristik, die die Umrichtersollleistung entsprechend der Abweichung von der DC-Sollspannung anpasst. Dafür werden eine kontinuierliche p-u-Charakteristik sowie Verfahren zu deren Parametrierung vorgeschlagen. Für die Bereitstellung von DC-Regelleistung besonders geeignete AC-Knoten können so angemessen für das HGÜ-Overlaynetz genutzt werden. Die Funktionalität des hier vorgeschlagenen dreiteiligen Bertriebsführungsverfahrens für vermaschte HGÜ-Netze wird anhand von numerischen Fallstudien auf Basis einer typischen Netztsituation in Zentraleuropa validiert.There is an increasing demand for long distance bulk power transmission worldwide and particularly in Europe. Energy turnaround from conventional to renewable energy generation is one of the main drivers. This implies that a significant percentage of electricity production is generated remotely from load centers, by huge wind farms, for example. This new transmission objective can be met with high voltage direct current (HVDC) transmission. An HVDC grid is favored for redundancy as well as economic reasons. As this HVDC grid will be a new network layer above the existing AC transmission layer, it is referred to as an “overlay” HVDC grid. This thesis proposes a three stage operation management strategy for future HVDC overlay grids. The architecture is comprised of tertiary, secondary and primary control instances which reflect the hierarchy of AC system operation. All control methods have been validated by numerical case studies on a reference grid which is a representative of a typical interconnected network situation in central Europe. The proposed tertiary control ensures coordination among all HVDC converters and with the underlaying AC system. It serves as an example of converter reference value determination in a 15 minutes time interval. Therefore a mixed AC/DC optimal power flow method is proposed which is capable of solving this nonlinear optimization problem based on a complete set of topological and other state information of the entire grid. In the event of having different transmission system operators (TSO) operating only a subset of converters of the HVDC overlay grid, the optimization problem becomes increasingly complex since each TSO might have its own optimization objectives. This problem is addressed by another multiple objective function approach. The proposed method superimposes particular cost functions of related TSO which yields system wide cost functions as a basis for AC/DC power flow optimization. The Secondary control instance adapts the tertiary control’s converter reference values within the 15 minute interval to the actual grid requirements, particularly in the event of grid disturbances. An algorithm is proposed that identifies significant deviations from the actual power flow schedule by a wide are monitoring system. Converter power references are adapted in order to optimally share the deviations between the AC system and the HVDC overlay grid. Since data availability is key for the robust operation of this method, backup mechanisms for data acquisition is also proposed. The Primary control ensures DC energy balance, which is referred to as the energy stability of HVDC grids. Converter reference values for active power need to be adjusted in the event of a mismatch between active power fed to and drawn from the HVDC grid. As the time constants within a DC grid are very small, this is a fast, local control based on p v characteristics; the converter’s power reference is adjusted in accordance with deviation of the DC node voltage from its reference. Furthermore, a continuous p v characteristic is proposed as well as two appropriate parameterization methods. One emulates already existing piecewise linear p v characteristics for DC node voltage control and the other performs an automatic parameterization according to available balancing power provision capabilities on related AC point of common couplings. The latter significantly reduces the additional loading of the AC transmission grid with DC balancing power flows as the AC nodes, which are the most technically feasible, are utilized to provide the most DC balancing power

Integration der Betriebsführung eines HGÜ-Overlay-Netzes in die Leitungs-Frequenz-Regelung eines Drehstrom-Verbundnetzes

Der Anteil Erneuerbarer Energien in Deutschland und in Europa allgemein nimmt signifikant zu, um so den Ausstoß von Kohlenstoffdioxid zu senken. Die Errichtung eines Großteils derartiger Erzeugungsanlagen kann aufgrund von klimatischen Gegebenheiten nur an wenigen Orten erfolgen, die Lasten bleiben weiterhin dezentral verteilt und konventionelle Kraftwerke werden sukzessive abgeschaltet. Daher wird ein durchgängig automatisiertes und leistungsfähiges Elektroenergieübertragungsnetz zum Langstreckentransport großer Energiemengen unverzichtbar. Eine mögliche Lösung dieser Herausforderung ist ein europaweit vermaschtes Overlay-Netz. Dieses kann auf der HGÜ-Technologie basieren und verbindet Regionen großer Einspeisung aus regenerativen Quellen mit Lastzentren. Aufgrund seiner technologischen Eigenschaften gegenüber einem AC-Netz muss ein HGÜ-Overlay-Netz Systemdienstleistungen wie die Leistungs-Frequenz-Regelung aktiv unterstützen, damit die Systemstabilität und Versorgungssicherheit ebenso sichergestellt werden können, wie mit dem heutigen auf allen Netzebenen realisierten AC-Netz. In dieser Arbeit wird ein neuartiges Verfahren für die Beteiligung eines HGÜ-Overlay-Netzes am Regelleistungstransport entwickelt. Gleichzeitig realisiert diese Betriebsführung auch die anteilige Übernahme anderweitig hervorgerufener Leistungsflüsse z.B. durch Energiehandelsgeschäfte. Numerische Fallstudien zeigen einige Vorteile dieses Verfahrens bei unterschiedlichen Störungen des AC-DC-Übertragungsnetzes auf. Es wird gezeigt, dass ein HGÜ-Overlay-Netz mit der entwickelten Betriebsführung das AC-Netz um einen einstellbaren Anteil der Leistungsflüsse entlastet und die Leitungs-Frequenz-Regelung z.B. im Hinblick auf Schnelligkeit und Frequenzabweichungen verbessert

Non-telecommunication based DC line fault detection methodology for meshed HVDC grids

One of the most challenging aspects regarding realisation of meshed high-voltage direct current (HVDC) systems is the handling of DC line faults. Due to the specific DC fault current phenomena, AC fault detection and clearance methods are not applicable in meshed HVDC grids. New fault clearing algorithms and equipment need to be developed. Therefore it is necessary to be aware of the DC fault current characteristics. Starting from a travelling wave approach simplifications for fault currents in low - and high impedance earthed DC grids are derived. On this basis, a non-telecommunication based DC fault detection algorithm is designed. Due to its decentralised implementation, it is very fast acting, reliable and economic in comparison with centralised approaches which are characterised by a parallel telecommunication system. Furthermore it is able to detect pole to pole as well as pole to earth faults in low- and high-impedance earthed DC grid. Finally this fault detection algorithm is validated by numerical case studies

Mapping human pressures on biodiversity across the planet uncovers anthropogenic threat complexes

Abstract Climate change and other anthropogenic drivers of biodiversity change are unequally distributed across the world. Overlap in the distributions of different drivers have important implications for biodiversity change attribution and the potential for interactive effects. However, the spatial relationships among different drivers and whether they differ between the terrestrial and marine realm has yet to be examined. We compiled global gridded datasets on climate change, land-use, resource exploitation, pollution, alien species potential and human population density. We used multivariate statistics to examine the spatial relationships among the drivers and to characterize the typical combinations of drivers experienced by different regions of the world. We found stronger positive correlations among drivers in the terrestrial than in the marine realm, leading to areas with high intensities of multiple drivers on land. Climate change tended to be negatively correlated with other drivers in the terrestrial realm (e.g. in the tundra and boreal forest with high climate change but low human use and pollution), whereas the opposite was true in the marine realm (e.g. in the Indo-Pacific with high climate change and high fishing). We show that different regions of the world can be defined by Anthropogenic Threat Complexes (ATCs), distinguished by different sets of drivers with varying intensities. We identify 11 ATCs that can be used to test hypotheses about patterns of biodiversity and ecosystem change, especially about the joint effects of multiple drivers. Our global analysis highlights the broad conservation priorities needed to mitigate the impacts of anthropogenic change, with different priorities emerging on land and in the ocean, and in different parts of the world.Peer reviewe

A continuous DC voltage control function for meshed HVDC grids, and the impact of the underlying future AC grid due to renewable in-feed

This paper discusses the development of a continuous high voltage direct current (HVDC) voltage control function for future meshed HVDC grids. Starting from a state-of-the-art discontinuous voltage droop control, which is used in point-to-point links, a continuously differentiable control function is developed. Using a mathematical description, the control function is transformed to meet the HVDC voltage control strategy for meshed HVDC grids. The control function's stability is discussed and its robustness is shown on an example. The example discusses and compares the current and future characteristics of the underlying AC transmission grid and the impact renewable in-feed has to the control function

High Working Memory Capacity at the Cost of Precision?

Working memory capacity (WMC) varies tremendously among individuals. Here, we investigate the possibility that subjects with high WMC use this limited resource more efficiently by reducing the precision with which they store information in demanding tasks. Task difficulty was increased by (a) presenting more objects to be memorized, (b) informing subjects only after the encoding phase about the relevant objects, and (c) delivering distracting features at retrieval. Precision was assessed by means of a continuous delayed-estimation task, in which object features had to be estimated from memory. High WMC subjects did not show a stronger drop in precision in difficult tasks. Instead, a positive correlation between precision and general WMC emerged. These findings suggest that high WMC subjects do not necessarily trade in quantity for quality when forming working memory (WM) representations under increasing demand. Instead, they seem to be able to devote more cognitive resources to support WM storage

Meshed HVDC transmission in the content of sustainable power transmission for the future and its environmental impact

When talking about future power systems, power transmissions cannot be omitted. While it makes sense to generate RE where the potential is highest, it often means that volatile RE has to be transmitted over long distances. HVDC is favored for modern transmission to reinforce AC transmission. Around the world, HVDC is discussed and implemented in grid expansion plans. These plans range from point-to-point links towards meshed grids. This paper explores these grids and their ability to transmit RE in bulk and over long distances. It is shown to prevent massive AC grid expansion, and therefore positively impact the environment

Sit-to-stand biomechanics : a systematic review

Objective: This review focuses on studies that enhance our knowledge of the biomechanics of the sit-to-stand (STS) quantitatively. The questions asked and the methods of study have led to the partitioning of the STS into components, or phases. Our goal is to summarize the methodological approaches and define the phases of a STS transfer in order to provide a standard practice for STS. Data Sources: Electronic database searches were conducted in PubMed and Google Scholar from inception to February 2021. Study Selection: Articles with a combination of the keywords “sit-to-stand”, “biomechanics”, “stability” and “postural control” were searched. Only the articles that provided quantitative biomechanical insights of the STS movement were included. Data Extraction: We categorized the studies by the questions posed, the measurement techniques used, and the definition of phases. Data Synthesis: Seventy-two papers published between 1976 and 2021 that provided quantitative biomechanical insights of the STS movement itself were included in this review. Among the studies that qualified for this review, research interest in the STS may be parsed into the following categories: a) how the movement is performed, b) use of the STS to understand postural stability, and c) use of the STS to understand clinical differences in performance. Conclusion: Definitions of events in the STS that define phases are inconsistent. The major reason identified is due to differences in the purpose of the studies or equipment used. Four studies dividing STS into 2 phases, eight studies divided STS into 3 phases, and 2 studies divided STS into 4 phases and 2 studies divided STS into 5 phases. At a minimum, the STS is typically deconstructed into two phases, with seat-off being the demarcation of transition between phases. This is based on the characteristics of muscle power or on select kinematic events. However, 10 different ways in which the event of ‘seat-off’ has been defined shows the impact of differences in measurement technique. These differences between studies result in contradictory definitions of the same event. These findings reveal a need for standardization of event definition and recommendations on measurement techniques.Kinesiology and Health Educatio