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

Protecting valuable resources using optimal control theory and feedback strategies for plant disease management

Mathematical models of tree diseases often have little to say about how to manage established epidemics. Models often show that it is too late for successful disease eradication, but few study what management could still be beneficial. This study focusses on finding effective control strategies for managing sudden oak death, a tree disease caused by Phytophthora ramorum. Sudden oak death is a devastating disease spreading through forests in California and southwestern Oregon. The disease is well established and eradication is no longer possible. The ongoing spread of sudden oak death is threatening high value tree resources, including national parks, and culturally and ecologically important species like tanoak. In this thesis we show how the allocation of limited resources for controlling sudden oak death can be optimised to protect these valuable trees. We use simple, approximate models of sudden oak death dynamics, to which we apply the mathematical framework of optimal control theory. Applying the optimised controls from the approximate model to a complex, spatial simulation model, we demonstrate that the framework finds effective strategies for protecting tanoak, whilst also conserving biodiversity. When applied to the problem of protecting Redwood National Park, which is under threat from a nearby outbreak of sudden oak death, the framework finds spatial strategies that balance protective barriers with control at the epidemic wavefront. Because of the number of variables in the system, computational and numerical limitations restrict the control optimisation to relatively simple approximate models. We show how a lack of accuracy in the approximate model can be accounted for by using model predictive control, from control systems engineering: an approach coupling feedback with optimal control theory. Continued surveillance of the complex system, and re-optimisation of the control strategy, ensures that the result remains close to optimal, and leads to highly effective disease management. In this thesis we show how the machinery of optimal control theory can inform plant disease management, protecting valuable resources from sudden oak death. Incorporating feedback into the application of the resulting strategies ensures control remains effective over long timescales, and is robust to uncertainties and stochasticity in the system. Local management of sudden oak death is still possible, and our results show how this can be achieved

Heterogeneous and hybrid control with application in automotive systems

Control systems for automotive systems have acquired a new level of complexity. To fulfill the requirements of the controller specifications new technologies are needed. In many cases high performance and robust control cannot be provided by a simple conventional controller anymore. In this case hybrid combinations of local controllers, gain scheduled controllers and global stabilisation concepts are necessary. A considerable number of state-of-the-art automotive controllers (anti-lock brake system (ABS), electronic stabilising program (ESP)) already incorporate heterogeneous and hybrid control concepts as ad-hoc solutions. In this work a heterogeneous/hybrid control system is developed for a test vehicle in order to solve a clearly specified and relevant automotive control problem. The control system will be evaluated against a state-of-the-art conventional controller to clearly show the benefits and advantages arising from the novel approach. A multiple model-based observer/estimator for the estimation of parameters is developed to reset the parameter estimate in a conventional Lyapunov based nonlinear adaptive controller. The advantage of combining both approaches is that the performance of the controller with respect to disturbances can be improved considerably because a reduced controller gain will increase the robustness of the approach with respect to noise and unmodelled dynamics. Several alternative resetting criteria are developed based on a control Lyapunov function, such that resetting guarantees a decrease in the Lyapunov function. Since ABS systems have to operate on different possibly fast changing road surfaces the application of hybrid methodologies is apparent. Four different model based wheel slip controllers will be presented: two nonlinear approaches combined with parameter resetting, a simple linear controller that has been designed using the technique of simultaneously stabilising a set of linear plants as well as a sub-optimal linear quadratic (LQ)-controller. All wheel slip controllers operate as low level controllers in a modular structure that has been developed for the ABS problem. The controllers will be applied to a real Mercedes E-class passenger car. The vehicle is equipped with a brake-by-wire system and electromechanical brake actuators. Extensive real life tests show the benefits of the hybrid approaches in a fast changing environment

Safe and accurate MAV Control, navigation and manipulation

This work focuses on the problem of precise, aggressive and safe Micro Aerial Vehicle (MAV) navigation as well as deployment in applications which require physical interaction with the environment. To address these issues, we propose three different MAV model based control algorithms that rely on the concept of receding horizon control. As a starting point, we present a computationally cheap algorithm which utilizes an approximate linear model of the system around hover and is thus maximally accurate for slow reference maneuvers. Aiming at overcoming the limitations of the linear model parameterisation, we present an extension to the first controller which relies on the true nonlinear dynamics of the system. This approach, even though computationally more intense, ensures that the control model is always valid and allows tracking of full state aggressive trajectories. The last controller addresses the topic of aerial manipulation in which the versatility of aerial vehicles is combined with the manipulation capabilities of robotic arms. The proposed method relies on the formulation of a hybrid nonlinear MAV-arm model which also takes into account the effects of contact with the environment. Finally, in order to enable safe operation despite the potential loss of an actuator, we propose a supervisory algorithm which estimates the health status of each motor. We further showcase how this can be used in conjunction with the nonlinear controllers described above for fault tolerant MAV flight. While all the developed algorithms are formulated and tested using our specific MAV platforms (consisting of underactuated hexacopters for the free flight experiments, hexacopter-delta arm system for the manipulation experiments), we further discuss how these can be applied to other underactuated/overactuated MAVs and robotic arm platforms. The same applies to the fault tolerant control where we discuss different stabilisation techniques depending on the capabilities of the available hardware. Even though the primary focus of this work is on feedback control, we thoroughly describe the custom hardware platforms used for the experimental evaluation, the state estimation algorithms which provide the basis for control as well as the parameter identification required for the formulation of the various control models. We showcase all the developed algorithms in experimental scenarios designed to highlight the corresponding strengths and weaknesses as well as show that the proposed methods can run in realtime on commercially available hardware.Open Acces

Dynamic Interactions of a Double-stage Photovoltaic Power Converter: Modelling and Control

Photovoltaic (PV) systems are a promising renewable source to achieve green energy targets and be part of the electricity generation. Lots of efforts have been devoted to increase the penetration level of PV systems and its share in the generated electricity. Power quality is one of the challenges that impact the penetration level of PV systems. It is important to ensure high power quality from PV systems to allow more installations to the grid. So, PV power quality issues have to be addressed properly. It was reported that the poor power quality of the PV systems might be caused by many reasons such as the large amount of PV power fluctuation, the low level of current from the PV system, and large populations of PV inverters. In addition to the aforementioned reasons, recently it was suggested that perturb and observe (P&O) controller is another source of harmonics which result in a deprived PV power quality. This newly reported problem is based on experimental observations without full understanding of the generation mechanism of these harmonics in the PV system, the relation between the P&O controller design and the generated harmonics, and the effect of these harmonics on the rest of the system. Thus, in-depth analysis of the harmonics in PV systems due to P&O controller and a solution to eliminate these harmonics are demanded. Therefore, in this research an investigation is carried out to explore P&O related harmonics in a double-stage grid-connected PV system. First, regarding the P&O related harmonics full explanation of how harmonics are generated due to the perturbing nature of the P&O controller is provided, a modelling approach is suggested to identify the frequency and the amplitude of the variations in the DC bus due to the P&O controller, the effect of different factors (e.g. weather conditions, system parameters, system operating point, and P&O architecture) on the induced harmonics are investigated. Secondly, regarding the effect of the P&O related harmonics on the rest of the system an intense simulation analysis is provided to explore the possible effect of the P&O related harmonics on increasing the interaction between the system power stages. This can help to set system design recommendations and guidelines such as sizing the dc-link capacitance and designing the system controllers. Finally, a novel mitigation solution is proposed to supress the P&O related harmonics. That can help to reduce the dynamic interaction between system power stages and improve the power quality of the PV system

Observer based active fault tolerant control of descriptor systems

The active fault tolerant control (AFTC) uses the information provided by fault detection and fault diagnosis (FDD) or fault estimation (FE) systems offering an opportunity to improve the safety, reliability and survivability for complex modern systems. However, in the majority of the literature the roles of FDD/FE and reconfigurable control are described as separate design issues often using a standard state space (i.e. non-descriptor) system model approach. These separate FDD/FE and reconfigurable control designs may not achieve desired stability and robustness performance when combined within a closed-loop system.This work describes a new approach to the integration of FE and fault compensation as a form of AFTC within the context of a descriptor system rather than standard state space system. The proposed descriptor system approach has an integrated controller and observer design strategy offering better design flexibility compared with the equivalent approach using a standard state space system. An extended state observer (ESO) is developed to achieve state and fault estimation based on a joint linear matrix inequality (LMI) approach to pole-placement and H∞ optimization to minimize the effects of bounded exogenous disturbance and modelling uncertainty. A novel proportional derivative (PD)-ESO is introduced to achieve enhanced estimation performance, making use of the additional derivative gain. The proposed approaches are evaluated using a common numerical example adapted from the recent literature and the simulation results demonstrate clearly the feasibility and power of the integrated estimation and control AFTC strategy. The proposed AFTC design strategy is extended to an LPV descriptor system framework as a way of dealing with the robustness and stability of the system with bounded parameter variations arising from the non-linear system, where a numerical example demonstrates the feasibility of the use of the PD-ESO for FE and compensation integrated within the AFTC system.A non-linear offshore wind turbine benchmark system is studied as an application of the proposed design strategy. The proposed AFTC scheme uses the existing industry standard wind turbine generator angular speed reference control system as a “baseline” control within the AFTC scheme. The simulation results demonstrate the added value of the new AFTC system in terms of good fault tolerance properties, compared with the existing baseline system

Battery SMART charge controller/combined co-gen grid connected inverter design and simulation

Electricity generation and distribution is undergoing significant change under the influences of energy security, climate change, technological development, and economics. Technologies that have introduced two-way power flow onto a distribution grid that was designed for one-way power flow are creating challenges and opportunities for innovation in the electricity distribution sector. These technologies include solar photovoltaics (PV), wind turbines, and battery energy storage systems (BESS). As the newest technology, BESS present opportunities to both the electricity distribution network service provider (DNSP) and the consumer. This dissertation focused primarily on the consumer side of the switchboard, modelling and analysing the economics and some of the technical issues for an economic-mediated battery controller as part of a grid-tied residential hybrid renewable energy system (HRES) that consists of a BESS, 1 kW wind turbine, and 10 kW PV array. The geographical context of this project is Nambour, Queensland; PV and wind power calculations were based on Nambour’s meteorological history. Residential energy consumption was modelled as a ‘typical’ Nambour residential customer. The technological context was such that costs and choices applied at mid-2016. The tariff context used was the recently introduced TOU tariff 12, which played a significant role in the timing and logic development of the battery charge controller algorithm. From a technical standpoint, the charge controller algorithm was a major achievement of the present work. In developing the algorithm, it was found that the use of data from individual system components could be used to formulate the optimum mix of power sourced from or sunk to both the grid and the BESS. The output of this formulation was then demonstrated as a data input used for the control of the switching patterns of the BESS power electronics, a two-quadrant DC-DC converter (chopper). The other major achievement of the current work was the finding that although BESS economics continue to improve, they generally still need to achieve further cost reductions in order to realise economic feasibility for the modelled context. It was also found that economic feasibility is more likely to be reached more quickly under conditions of high energy consumption, high inflation, high peak TOU tariff, and low discount rate