Multi-Resilience, Resilience in ICT-based Multimodal Energy Distribution Systems

Energy distribution systems are increasingly based on the interconnection between different infrastructures (electricity, gas, heat, ICT), transforming them into smart multimodal energy distribution systems (SMEDSs). To realize the energy system transformation towards a highly efficient and renewable energy based system, the coupling of energy sectors (electricity, heat, and mobility) is one of the major facilitators. The distribution system will be faced with significant growth in the number of distributed generators, storages and loads (e.g., heat pumps, electrical vehicles, power to gas units) over the following years and decades. Monitoring and control of a large number of these distributed energy units will increasingly rely on Information and Communications Technology (ICT) in accordance to the trend of digitalization. This development inherently fosters the interdependence between power, gas, heat, and ICT infrastructures in SMEDSs. Challenges to plan and operate a resilient energy infrastructure are inherently present due to naturally limited quality of components. The rising complexity in smart infrastructures as well as their mutual interconnections foster additional challenges to arise both on intra- and inter-system level. The power system is continuously designed to operate more at the edge of acceptable loading and voltage limits, relying on ICT infrastructure for supervision and control under significantly more volatile power flow situations. ICT-sided attacks on smart infrastructures are a reality. Real examples show that the resilience of the energy infrastructure is becoming a major challenge in the future. Ongoing research is mainly in the area of natural disasters or cyber-attacks impacting single infrastructures. An open research question is how resilience in multiple interconnected infrastructures and their mutual interdependencies can be modeled and how overall resilience can be enhanced. This open area is of outmost importance for critical infrastructures and will be addressed in this project proposal.The goal of Multi-Resilience is to evaluate and increase the mutual resilience of SMEDSs coupled through interconnectors. On the one hand, new methods will be developed to model and evaluate resilience of SMEDSs. On the other hand novel resilience-enhancing concepts for the operation of an interconnected infrastructure are investigated to protect against and mitigate intra- and inter-system challenges. For this, the project can rely on work that has already been started for each of the infrastructures independently and expand on this to incorporate the mutual influences of the interconnected systems.

Principal Investigators:
Prof. Dr. Martin Braun
Prof. Dr. Hermann de Meer

Reliable Operation of Inverter-Dominated ICT-Reliant Energy Systems — from Centralized Structures to Agent-Based Decentralized Control

Das Ziel dieses Vorhabens ist es, eine integrierte, mehrschichtige Architektur für die Netzregelung von umrichterdominierten Energiesystemen zu entwickeln, die übergeordnete Regelungs- und Optimierungsziele für verschiedene Ebenen des Energienetzes aufgreift, mittels eines agentenbasierten Kommunikations- und Steuerungssystems verarbeitet und an unterlagerte, umrichterbasierte Erzeuger und Verbraucher sowie Netzregelstationen verteilt, die jeweils einzelne Regelaufgaben übernehmen. Gleichzeitig erfassen bzw. identifizieren die verteilten Einheiten (Umrichter) Informationen über den Systemzustand des Netzes und reichen sie an das Agentensystem weiter, das diese Informationen aggregiert und daraus ein aktuelles und konsistentes Bild vom Zustand der verschiedenen Netzebenen und -abschnitte zusammenstellt. Die Regelung und Optimierung des Netzbetriebs, darunter Lastfluss- sowie Spannungs- und Frequenzregelung, soll dabei dezentral vom Agentensystem durchgeführt werden. Auf diesem Weg werden inhärente Redundanzen des verteilten Systems automatisch genutzt, so dass eine höhere Robustheit/Resilienz gegenüber Kommunikationsfehlern oder Ausfällen von Systemkomponenten erwartet werden kann. Forschungsfragen, die auf Basis dieser mehrschichtigen Systemarchitektur mit ihrer hybriden Netzregelungsstruktur adressiert werden, betreffen die Qualität der verteilten Optimierung, die Auflösung von Reglerkonflikten, das Echtzeitverhalten und die Dynamik der resultierenden Netzregelung (die zum großen Teil auf Umrichtern basiert), die Identifikation von Netzparametern und -zuständen, das überlagerte System-Management zur Sicherstellung der Netzstabilität unter Berücksichtigung des Unbundling, sowie die hierarchische Struktur der Regelung unter Beachtung der Kommunikation zwischen den Ebenen.

Principal Investigators:
Prof. Dr. Lutz Hofmann
Prof. Dr. Sebastian Lehnhoff
Prof. Dr. Axel Mertens

Multimodal Distribution Grid Control for the Decentralized Provision of Ancillary Services

Today, transmission and distribution grid operators ensure quality, security and reliability of the electrical power supply by continuous monitoring and ongoing interventions in the power system management in order to keep grid frequency, voltage and load of power grid equipment within the permissible limits. Voltage and frequency stability are so-called ancillary services which are provided mainly by large power plants. Due to changes in the energy mix towards more renewable energy sources (RES) and therefore the increasing amount of electrical energy production within the distribution grid, ancillary services have to be provided in future more and more by these decentralized energy suppliers as well as by intelligent storage systems and energy consumers.The project presented in this proposal investigates a novel approach for a stable decentralized control of multimodal energy grids. Thereby, the focus is on distribution grids consisting of several coupled energy domains (e.g. electrical, gas, heat). Control techniques shall be developed for these grids to provide ancillary services besides their primary function of supplying loads. Herein, a decentralized cellular approach will be pursued. The term energy cell describes an autonomous area consisting of grids with power generation and consumers of different energy domains. Furthermore, individual cells are coupled and are connected to a large superordinate network, i.e. this project does not investigate islanded energy grids. So-called cell coordinators act as game-theoretical controllers, which negotiate exchange of power with adjacent cells and provide parameters to subordinate controllers of the cascaded controller structure within their respective cell. The game-theoretical control of different multimodal energy cells and their interaction with each other is completely unexplored.The long-time experience of the IEH in the field of electrical power grids and the competences of the IRS required for system-theoretical analysis complement one another optimally in order to perform quantitative and qualitative investigations on such a cellular energy system structure and their control scheme.

Consistent Modeling, Design and Analysis of Multi-layered Hybrid Power Systems with Distributed Control

The objective of this project is to develop methods for the design and analysis of distributed control strategies for hybrid electrical energy systems from the perspective of the overall system. To do so we will build on a combination of methods from hierarchical control, complex network science and nonlinear dynamics. Our ultimate aim is to be able to address questions on structure and control that span multiple layers and time scales of hybrid power systems, and be able to do so with some generality. The ability to answer such questions will become crucial in the future power grid due to a combination of actors that arise from moving from central conventional generation to decentral, intermittent energy sources. On the one hand, balancing intermittent renewable infeed and load on a local level is overly expensive. Thus, extended transmission systems will be needed for an economic energy transition. Such additional transmission capacity will partly take the form of high voltage DC lines, leading to hybrid power systems. On the other hand, the actors (e.g, demand side management control, storage, electric vehicles) that control the system and provide auxiliary services will move to lower grid levels. This will enable and require decentralized, locally controlled subnetworks, so called microgrids. The ability to partition the system at microgrid boundaries will potentially improve the resilience against failures in the transmission level as well as in neighboring networks by preventing cascades. At the same time, this changes the control problem dramatically, and raises the question for the need of dedicated communication infrastructure. The tension between the need for increased global transmission and local, decentralized agents to control the system introduces a deep hierarchy into the design of the control of the electrical power system. The questions that arise in this context are novel in nature. E.g., what is the optimal size of the organizational units, i.e., the microgrids? Is this size static or dynamic itself? The conceptual and methodological nature of our work, combining hierarchical control concepts and sampling based methods for analyzing overall systems, will allow for the application of results to different geographical contexts, with differently developed infrastructure. We will be able to not only answer questions regarding the transformation from bulk fossil-fueled power generation towards increasing renewable infeed in industrialized economies, but also questions pertinent to multi-layer hybrid systems that occur elsewhere. For example, abottom-up electrification approach in rural areas of the global south starting from stand-alone household-based electrical energy systems towards organically growing distribution networks for energy trading, eventually coupled to a transmission network, can also be studied, using the methodological framework to be developed within this project.

Principal Investigators:
Prof. Dr. Jürgen Kurths
Prof. Dr. Jörg Raisch

Multi-Objective Black-Start in ICT-reliant Renewable Energy Systems

This project aims at developing a rapid recovery method for ICT-reliant energy systems after a wide-area power loss. A so-called black-start requires delicate coordination between the ICT network, on the one hand, and the power grid, on the other hand. This is challenging, particularly in future multi-modal energy systems, which require flexible control of energy (power) generation and demand thus yielding specific requirements for the communication system — which is a power consumer itself and dependent on guaranteed power quality. In case of a black-out, the communication network and the power grid have to be brought back to an operational state in parallel — with both systems delicately interacting with one another. The resulting multi-criteria optimization problem that is investigated within this project is oriented at sequentially resupplying generators and consumers — adhering to power system stability constraints. The recovery sequence itself influences the properties of the communication system as well as the informational states (measurability and controllability) of the units such that maximizing the controllability of units during a black-start process becomes an important (potentially conflicting) goal. Within this project an autonomous multi-agent-based approach for solving this highly dynamic and interdependent ICT state-sensitive black-start optimization problem is researched.

Principal Investigators:
Prof. Dr. Sebastian Lehnhoff
Prof. Dr. Hermann de Meer

Formal Stability Assessment of Hybrid Distribution Grids Based on the Correct Modeling of the Effect of Synchronization of the Power Electronics Interfaces

Hybrid grids, which incorporate both AC and DC technologies, use power electronics converters to interface distributed energy resources, energy storage systems, and modern types of loads (such as EV charging stations) with high or medium voltage AC or DC grids. Such grid-connected converters rely on control algorithms and synchronization systems, and (as of late) on communication infrastructures, with the aim of providing smart grid functionalities. It has been demonstrated that the widespread use of grid-connected converters may lead to a scenario where the grid is largely decreasing its inertia. If a network features a lack of inertia as well as a topology and line characteristics that result in operation close to voltage collapse, it is classified as weak. In a weak network, the stability of the system is a major concern. This project focuses on stability issues in weak microgrids. Currently, there are no quantitative methods that would allow assessing the stability margin as a function of the system topology, the system state, and the primary control laws. In particular, the influence of the synchronization elements and the communication infrastructure on the stability has not been well investigated in the existing literature, although it has been demonstrated that they do indeed have an impact. Due to the general lack of investigations in this field, adequate models to describing such effects as well as standardized approaches for thoroughly validating such models are currently missing. This project aims at filling these major gaps in the existing works by developing a general framework for investigating stability issues in hybrid grids. To start with, formal methods for quantifying the static and dynamic stability margin of hybrid grids, while taking into account the effects of synchronization and communication, shall be elaborated. Such methods are an enabling factor for real-time stability assessment and the design of robust controllers. Moreover, a benchmark library of accurate time-domain models of hybrid distribution grids shall be developed. In doing so, special attention will be given to modeling the finite bandwidth of synchronization elements and the finite latency communication network. Finally, a thorough validation of the developed stability assessment tools and time-domain models shall be conducted using a combination of power-hardware-in-the-loop and real-scale microgrid experiments. Thereby, close-to-reality experimental conditions can be achieved while ensuring a minimum level of approximation. This framework is expected to grant deeper insights into the stability issues en-countered in hybrid distribution grids, and how they can be modeled and detected.

Development of Novel Models and Control Methods for Multilevel-VSC Multiterminal HVDC-Systems for Improving the Stability of Interconnected AC- and DC-Grids

The suggested work program comprises the development of models of novel multilevel-VSC multiterminal HVDC and the appropriate innovative control concepts in order to give a significant contribution to both the stability of the AC- and DC-grid. Against the background of the German ‘Energiewende’ the interconnected system is transforming to a hybrid and multimodal energy system. The high penetration of renewable energies, the shut-down of conventional and nuclear power plants, the consequently resulting lack of inertia, the novel hybrid and multimodal structure and the bidirectional flow of energy across all network levels must therefore require a change of the control strategy of the entire energy systems.In order to ensure the stability of the future grid the novel multilevel Voltage-Source-Converter can make a valuable contribution due to the existence of many degrees of freedom in its control. Therefore, the approach of the weighted droop-constants, which applies a frequency droop to the AC-grid and a voltage droop to the DC grid at the same time needs investigation in order to provide the proper method for the selection of the droop constants and weighting factors. Hence, optimization approaches as the Particle Swarm Optimization and the Bacterial Foraging Algorithm will be used since they promise very good results for large scale systems. An implementation of the models in the test and integration environment of the Priority Programme is intended in order to compare and validate the results with the other contributors. Since the topology of the multilevel converter allows a decoupling of the AC- and DC-side, the energy stored in the cells of the converter plays a very important role. A power deviation between the AC- and DC-side does not directly influence the DC-voltage, but affects the converter energy. Therefore, the advancement of the voltage droop- to an energy droop-method is carried out. In order to guarantee a proper contribution of the method to the system stability, the optimization approaches have to be applied to the energy droop-method as well. Once the different droop-methods are intensively investigated and deep insights could be gained, the second funding period should focus on nonlinear control approaches. Droop-methods always provide a linear characteristic between active power deviations and frequency, voltage or energy support respectively. But, as the transformation of the energy system is continuously advancing, linear control approaches might not sufficiently fit to the novel topology of the grid anymore. Nonlinear control approaches could therefore gain better stability effects and more sophisticated system service.

Principal Investigator:
Prof. Dr. Matthias Luther

Distributed Dynamic Security Control in Next-Generation Electrical Power Systems

Ensuring secure and reliable operation of complex energy systems represents one of the most essential tasks for the well-functioning of modern industrial societies. The unprecedented pace at which the operating environments have changed during the last decade highlights the need for fundamental reconsideration of adopted practices in almost all parts of the industry. In this context, one of the most critical aspects represents the way how the increased levels of uncertainty related to large-scale integration of intermittent renewable energy sources would affect the definition, evaluation, and provision of system security in highly integrated structures near real time. The existing approaches for modeling, optimization, and control of electrical power systems mainly consist in solutions relying on assumptions that are increasingly questioned by the industry itself. For example, profound changes develop through the dramatic rise of renewable power generation and the advances in communication technology. In order to account for the expected technical challenges and also to respond to changing regulatory paradigms, the present project aims at the creation of a dedicated set of novel system-theoretic computational methods towards distributed security assessment and enhancement of power systems. Taking into account predictions regarding the future availability of fast communication and in addition relying on a specialized descriptor format managing the set of differential- algebraic equations, the proposed methods form a scalable and adaptive framework called DistDSA. The latter is to be suitable for deployment in vertically and horizontally integrated heterogeneous power systems and their operation near real time. Thanks to a computational stage of structure-preserving model order reduction, DistDSA for the first time optimizes short-term electromechanical system dynamics online and in a distributed way in their pure and approximated forms. The methods are designed to reflect the call for a multilateral provision of system security in electrical power systems of the next generation.

Principal Investigators:
Prof. Dr. Volker Mehrmann
Prof. Dr. Kai Strunz

Development of Novel Control Strategies and Equivalent Models for Wide-Area Interconnected Hybrid and Multimodal AC-DC Power Systems

In order to reach the German and European policy to achieve an electrical power supply of 80% renewable energy resources, the entire power system needs to be restructured and new control strategies also need to be developed. This project aims (i) to conceive a new power system structure under consideration of existing assets and network infrastructures by the introduction of a new concept termed Smart Power Cell (SPC), which makes the coordination of a large numbers of flexible loads, multimodal interfaces and DGs manageable, (ii) to design control strategies for SPCs in order to enable their coordinated operation to interact with the electrical transmission network, other SPCs and gas as well as district heating networks in a supportive way, and finally (iii) to design a methodology for the development of reduced dynamic equivalents of SPCs based on hybrid identification approaches to make the time domain simulation of future wide area, power-electronics-based decentralized hybrid and multimodal AC-DC power systems possible. The work program comprises six major working packages (WP). In WP1, state of the art modeling and simulation methods for power system stability studies are reviewed and preliminary simulation studies using available models are realized. In WP2, SPC components are modelled and individually tested via time domain simulations. The resulting models are used in WP3 to build a detailed SPC model. Additionally, control schemes for the SPC are designed and tested in WP3. Next, in the WP 4, the dynamic behavior of the developed SPC model including the designed control schemes are analyzed with respect to its response to changing control signals and physical inputs. Further in WP5, hybrid identification techniques are used to develop a reduced dynamic equivalent of an SPC, suitable for time domain simulations of wide-area power-electronics-based decentralized hybrid and multimodal AC-DC-power systems. Finally, WP6 will consist of the continuous documentation of project results and presentation in journals, technical reports as well as in conferences proceedings.

Principal Investigator:
Prof. Dr. Johanna M.A. Myrzik

Novel Methods and Models for the Analysis of Harmonic Instabilities in Distribution Grids with a High Penetration of Power Electronics

Analysis of Controller Conflicts in Multimodal Smart Grid Systems using the Concept of Emergence in Technical Systems

Four transition driving forces characterize the ongoing and upcoming changes in the transition of separate energy systems to multimodal Smart Grids:

1. The generation of electricity is shifted from large capacities in the high voltage grid to small units in the distribution grid, prone to forecast errors and operational uncertainty.
2. Whereas in the past flexible loads where mainly allocated in industrial production, the flexibility potential of small loads in the distribution grid can be added to compensate for missing stabilizing capacity in the high voltage grid.
3. More and more autonomous controllers for grid stabilization purposes and the integration of DER are allocated in the electrical energy system on all voltage levels.
4. Additional flexibility potential will be unlocked by interlinking the infrastructures of different energy systems.

The first three transition driving forces enlarge the degree of distribution regarding electricity generation, usage and grid operation. The pervasion of the energy system with autonomous controllers leads to the development of a distributed adaptive system. From an engineering perspective, it shows the desired characteristics regarding scalability and self-stabilization. Unlike these desired aspects, other effects can emerge: Autonomous controllers might counteract, thus leading to unintended system behavior like oscillations and instable system states within a multi-controller system. From the perspective of distributed systems research, these controller conflicts can be understood as unintended effects of emergence that might extend and multiply when interlinking the infrastructures. As the reliability and resilience of energy systems is of crucial importance for industrial societies, approaches are needed to increase the confidence in the upcoming largely adaptive distributed system.

In the proposed project, the evolving multimodal Smart Grid is modelled as agent-based self-organized system to explore and analyze controller conflicts originating from the emergent properties of the interconnected system to answer the question, if this modelling approach can help to detect upcoming controller conflict states of the system. The project follows the following research steps:

• Modelling of an exemplary interlinked energy system comprising electricity and heating systems
• Modelling of autonomous controllers and relevant component using a multi-agent based approach by combining agent architecture from agent-based control systems and multi-agent system architecture from self-organizing systems
• Deterministic identification of instable system states with undesired emergent properties using the concept of co-simulation
• Development of metrics for the identification of instable system
• Analysis of the simulated instable system states by application of the derived controller conflict metrics for the identification of upcoming controller conflicts.

Principal Investigators:
Prof. Dr. Astrid Nieße

Analysis of Long-Term Voltage Stability in Hybrid Power Systems under Consideration of Changing System Dynamics and Underlying Multi-Modal Active Distribution Networks

Due to the changing conditions in power systems caused by decreasing number of conventional synchronous generators, the installation of HVDC+links and simultaneously the large-scale penetration of renewable energy sources, the dynamics and (voltage) stability of future power systems as well as the suitability of emergency controls for voltage critical situations need to be investigated. This projects aims at analysing the impact of increased renewable energy sources penetration and their different control possibilities, advanced active distribution networks control strategies and fast-controllable HVDC converters on overall power system dynamics and stability – in particular long-term voltage stability. Further, control opportunities for the prevention of voltage collapse, e.g., load curtailment or reactive power support of active distribution networks will be simulated in order to estimate their impact on stabilizing fast decreasing bus voltages when faced with these changing grid situation.

Principal Investigator:
Prof. Dr. Christian Rehtanz

Loop Circle Arc Theory (LoCA) – New Method for Comprehensive Evaluation of Cross-Sectoral and Cellular Organized Energy Systems

The present use of renewable energy sources (RES) will increase strongly in the future. The actual systems of electricity, gas and heat supply have grown over the last 100 years and it was designed to transmit the energy from large, central power plants to distributed con-sumers. To handle a large number of fluctuating feed-in by RES it must not only be rein-forced but transformed fundamentally. A constantly recurring adjustment is only a sympto-matic patching of vulnerabilities. To ensure a proper transformation, a referring target must be set. The new Loop-Circle-Arc-theory (LoCA) finally offers a method to design and evaluate this transformation target.
The LoCA-theory combines cellular design, fractal organization and joint energy carriers. The cellular design understands energetic balance groups as self-organizing and self-optimizing energy cells. The fractal organization describes the self-similarity between the whole and its individual parts. Energy carriers are not further considered and traded separately, but they rather complement each other. The joint of electricity, gas and heat increases synergy effects.
The central element in the LoCA-theory, which is used again and again, is the energy cell, which is called Cell from this point onwards. Each Cell has exactly one Port, one Source, one Storage, one Converter and one Sink. The Cell provides algorithms for self-management and operation optimizationThe focus of the project is the formulation of this theory and the development of a suitable tool, as basis to design and evaluate concepts of future energy system. Within the project the theory is translated into a mathematical model, which is the basic for a first exemplary implantation of the LoCA-theory in a software tool. The following simulation starts with small clearly arranged energy systems to test and review the implementation. At the end, bigger and complex energy systems will be simulated to demonstrate the ability of this method. The knowledge gained feed back and refine the theory.
The final LoCA theory offers a method to develop and evaluate innovative energy systems and a tool for simulating different configurations.

Principal Investigator:
Prof. Dr. Peter Schegner

Stochastic Optimisation for Secure Dynamic AC-load Flow in Hybrid Multi-Modal Energy Systems under Uncertainty

Grid-bound resources, in the present project foremostly electricity, but also gas and water, will have a critical role in a world of increased economic activity, and so will the grids themselves. For their design and operation, the growing complexity of decision making under data uncertainty poses challenging optimisation problems for whose solution the advancement of the existing methodology is indispensable. The latter starts with conceptual work on the proper treatment of risk, nonlinearity, and the large-sale nature of resulting models. Optimal power flow (OPF) in alternate-current (AC) networks has a pivotal role, both since it is indispensable for realistic modeling and also mathematically rewarding. Another crucial aspect is the proper inclusion of external uncertainty due to volatile input from renewablesIn the core of the planned investigations there is the development of an appropriate framework of new integrated mathematical models and tailored methods of “engineering value” supporting research into those energy systems prone to forecast errors and uncertainty. In particular, the following topics are addressed:

• stochastic contingency management
• network stability
• management of congestions in AC networks under renewable infeed uncertainty
• risk aversion in multimodal energy systems

Principal Investigator:
Prof. Dr. Rüdiger Schultz

 Distributed Optimization in Smart Grids

The goal of this project is to find a structural approach to distributed optimization and regulation in Smart Grids, which will take into account not only specific functional properties of the network levels, but also realistic interconnections between these levels. The focus will be on a multi-component model of Smart Grids. The choice of the model consisting of three levels is motivated by recently formulated challenges for the analysis of Smart Grids by means of complex system theory. The idea is to formulate global objectives at each level by means of game theory and distributed optimization. In this project we will develop the corresponding game-theoretic and distributed optimization approaches applicable to the different levels of the Smart Grid model. This work promises to answer the following questions: How should objectives at different structural levels of Smart Grids be formally defined to meet realistic applications as good as possible? What mathematical tools can take communication technologies and information restrictions of large-scale energy systems into account and, thus, provide the methods enabling us to handle the most general payoff-based equilibria learning and constrained distributed non-convex optimization problems?
Under which assumptions the theoretic methods proposed in the project will provide some global improvement of the Smart Grid operation in comparison with the techniques presented in the literature so far? How should the proposed theoretic methods be synchronized to achieve a stable and efficient operation of Smart Grids?
The main questions addressed in this project will be studied by means of engineering and mathematical tools including game theory, distributed optimization theory, consensus-based methods, stochastic martingale processes, including ergodic Markov chains, and stochastic approximation methods.

Principal Investigators: 
Dr. Tatiana Tatarenko
Dr. Volker Willert