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IEEE open access journal of power and energy 9, 76-87 (2022). doi:10.1109/OAJPE.2022.3148777, Published by IEEE, [New York, NY]

Energies : Energies : open-access journal of related scientific research, technology development and studies in policy and management 14(11), 3046 (2021). doi:10.3390/en14113046 special issue: "Special Issue "Design and Validation of Smart Energy System Concepts and Methods" / Special Issue Editors: Dr. Thomas Strasser, Guest Editor ; Prof. Dr. Sebastian Rohjans, Guest Editor ; Prof. Dr. Graeme Burt, Guest Editor", Published by MDPI, Basel

One can define an energy system as a system that converts one or more energy fluxes into other energy fluxes of a different kind. This definition may describe a relatively small system, for instance, a power plant, a chemical plant, or the heating and cooling apparatus of a single-family house, as well as one covering larger energy needs, for example, those of a city, a country, or even a continent. As energy systems are developed through the centuries, the way we structure these systems goes through changes affected by contextual conditions. Recently, concerns about the availability of traditional fossil energy sources and their environmental effects are revolutionizing the way energy systems are planned, designed, and operated.

IEEE power electronics magazine 5(3), 35-44 (2018). doi:10.1109/MPEL.2018.2850698, Published by IEEE, New York, NY

Electrical distribution grids are undergoing a tremendous transformation process. More and more renewable energy sources are installed and the power demand increases due to the coupling with different sectors such as mobility and heating. These changes require a different operation of the grid. In the past, distribution grids had only the task to deliver power to the end consumer. Moving the power generation to the distribution level leads to a bidirectional power flow. As a result, voltage violations are more likely to occur. New concepts addressing the arising challenges of balancing power generation and consumption are required. A key concept to address the mentioned challenges is the exploitation of flexibility. Flexibility is made available by shifting the operation of a power producer or consumer in time. In addition to active power flexibility, many components in the distribution level are connected to the grid via a converter enabling a flexible reactive power behavior as well. Besides the functionality, a flexibility management concept has to be scalable and fault-tolerant. This dissertation aims at developing and assessing a flexibility management concept for grid services in the distribution level. Moreover, a modular deployment system enabling scalability and fault-tolerance is developed. In particular the work aims at satisfying the following requirements: (1) Voltage control: Flexibility is used for local voltage control in the distribution grid. (2) Transmission system support: Additional flexibility, which is not used for local voltage control, is offered to the transmission system. (3) Scalability and fault-tolerance: The concept is highly scalable and it tolerates faults of single flexibility providers and parts of the ICT system. (4) Modularity: The management concept can be rolled out step-wise enabling a smooth transition from the current status. Moreover, it is self-configuring and highly automated. The four requirements are considered in a three step approach. These steps also depict the major scientific contributions of this dissertation. In a first step, the requirements 1 and 2 are addressed by a multi-agent based flexibility management concept, called SwarmGrid-X. In SwarmGrid-X each power producer and consumer is represented by one agent. Producer and consumer agents constantly negotiate the amount of power that is produced and consumed, aiming at balancing production and consumption as locally as possible. Multiple voltage levels are integrated in the concept by means of a holonic architecture. Also the transmission system is represented by one agent, which participates in the negotiation process to drive the entire distribution grid towards a desired behavior. Several scenarios representing possible developments of future distribution grids are simulated. Results indicate that SwarmGrid-X is capable of using flexibility for the purpose of voltage control. Specifically shifting electric vehicle charging processes and the use of reactive power during times of high PV power generation turn out to be key flexibility sources that are successfully exploited by SwarmGrid-X. Also the support of the transmission system by means of a specific set-point for the reactive power exchange at the point of common coupling is demonstrated. The second step deals with the implementation and deployment of the concept targeting the requirements 3 and 4. While multi-agent systems (MAS) generally enable high scalability and fault-tolerance, a poor implementation might destroy these features. In fact a literature review reveals that popular multi-agent platforms (MAP), such as JADE, are not well suited for large-scale MASs comprising thousands of agents or even more. That is why a novel MAP based on cloud computing techniques is developed as part of this dissertation: cloneMAP. It utilizes a microservice architecture, which enables horizontal scalability and fault-tolerance of platform components. Also on agent level fault-tolerance mechanisms are implemented. A performance comparison with JADE based on different benchmarks reveals a massively improved scalability of agent messaging and platform components. Fault-tolerance is demonstrated successfully as well. In a third and last step SwarmGrid-X is implemented using cloneMAP. The resulting software is executed as a real system and coupled with a scalable software-in-the-loop simulation methodology to simulate the distribution grid. An assessment shows that the functionality of SwarmGrid-X is not affected by the integration with cloneMAP and that the performance is sufficient for the purpose of flexibility management.

Electrical power systems are becoming interdisciplinary with power electronics and more digital control algorithms entering the field. This development demands for comprehensive testing of new equipment and algorithms before deployment since power systems must be highly reliable. Pilot projects provide valuable insights but they do not offer the flexibility and reproducibility of simulation based testing environments. Hence, the concept of digital twins, an equivalent software simulation of physical assets, is becoming increasingly relevant in product development. Executing the simulation in real-time further broadens the spectrum of development stages that can be supported by simulation because the digital twin can be interfaced with hardware prototypes in Hardware-In-the-Loop experiments and with automation systems. However, the increase of power electronics introducing wide bandwidth signals and larger system sizes related to the interconnection of national power systems complicate the simulation of modern power systems in real-time. The maximum system size can be increased by running a distributed real-time simulation, which is challenging due to the small time steps required for ElectromagneticTransient (EMT) simulations typically used when considering network dynamics. An alternative is to simplify the simulation model and consider different time constants in order to reduce the required computation resources. Current simulation solutions though are highly specialized to one or few of the time constants present in power systems and the associated modelling domain, for example EMT or quasi-stationary phasors. Transferring simulation models is difficult due to the variety of modelling domains, computing technologies and input data formats. This thesis applies the dynamic phasor approach to real-time power system simulation to remove the requirement of proportionality betweenthe simulation time step and the highest frequency considered in the simulated signals. Especially for power electronics and geographicallydistributed real-time simulation, this is an interesting feature. However, the real-time execution and large scale simulation are rendered moredifficult by the increase of variables when using multiple dynamic phasors to represent a single physical signal. To address this challenge, a new power system simulator is developed in the scope of the thesis, which integrates traditional power system components and power electronics, two domains that are usually treated separately in dynamic phasor related literature. The simulator decomposes the system model into subsystems, each featuring a subset of the network nodes and the considered frequency bands. Consequently, it executes a data dependency analysis to determine a schedule for solving these subsystems and take advantage of parallelization. The scalability of the simulator is presented for models featuring a large number of electrical nodes and a wide frequency spectrum related to detailed power electronics models. Further examples demonstrate the advantage of dynamic phasors with respect to EMT simulations in terms of accuracy for larger simulation time steps. Eventually, the developed solution offers the user the flexibility to optimize for smaller simulation time steps and detailed results or large system size without having to replace models and input data of the simulation.

Electrical power systems undergo a paradigm change from unidirectional to bidirectional power flow. This change is caused by an increasing number of distributed energy resources installed especially in distribution systems. System operators are forced to rethink the traditional ways of system operation since these are not designed for a bidirectional power flow. Research has identified information and communication technology as a key enabler for the integration of distributed energy resources in the system operation through communication and data processing. The coupling of an electrical power system with a communication and data processing system results in a cyber-physical power system (CPPS). The overall behavior of a CPPS emerges from the interactions of its components through physical coupling and communication. Modeling and simulation of CPPS have to consider these interactions as the source and the means to obtain the overall system solution. The potentially large number of components and their heterogeneity constitute severe challenges that require solutions for both the modeling and the computation of the simulation itself. This dissertation identifies the following four major requirements of CPPS simulation based on a review of existing approaches and their shortcomings. The requirement of scalability (1) is motivated by the expected number of system components. The system solution has to emerge from the interactions of its components (2). Large-scale simulations require a scalable and flexible management of input and result data (3) that is not available in existing solutions. Interoperability (4) is required to enable the coupling with other simulation tools or data processing systems for CPPSs. As the main contribution of this dissertation, the simulation method Distributed Agent-based Simulation of Complex Power Systems (DistAIX) is proposed. It enables the simulative assessment of CPPSs at an unprecedented scale without neglecting interactions through simplification or aggregation of components. It utilizes parallel computing to achieve scalability and to overcome the limited processing power and memory resources of single computing nodes. Agent-based modeling and simulation addresses the requirement of emergence of the system solution from interactions. System components are modeled as agents that include an electrical model and a specific behavior. They are distributed to processes for the computation of the simulation. DistAIX is embedded in a framework facilitating a flexible scenario creation and a database-supported result storage. Results can be post-processed in a dedicated application that abstracts from the agent-based modeling approach and the database storage. A generic communication interface is integrated to exchange data with external systems during a simulation. The DistAIX method is introduced and assessed comprehensively throughout this dissertation with respect to all four identified CPPS simulation requirements.

In the Renewables Directive of the European Union, in effect since 2009, the member states agreed that the share in renewable energy should be 20% of the total energy by 2020.The concomitantly growing number of renewable energy producers such as photovoltaic systems and wind power plants leads to a more decentralized power generation. This results in a more complex power grid management. To ensure a secure power grid operation even so, there is a transformation from conventional power grids to so-called smart grids where, for instance, not only status information of power producers but also of consumers (e.g. heat pumps and electrical vehicles) is included in the power grid management. The utilization of flexibility on generation and demand side and the use of energy storage systems for achieving a stable and economic power supply requires new solutions for the planning and operation of smart grids. Otherwise, manipulations of the systems in the public energy sector (i.e. power grid, ICT infrastructure, energy market, etc.) can lead to unexpected problems such as power failures. Computer simulations therefore can help to estimate the behavior of smart grids on any changes without the risk of negative consequences in case of immature solutions or incompatibilities. The main objective of this dissertation is the application and analysis of HPC and computer science methods for improving power system (co-)simulation software to allow simulating more detailed models in a, for the particular use case, appropriate time. Through more automation and control in smart grids, the higher demand on flexibility, and the need of stronger market integration of consumers, the power system models become more and more complex. This requires an ever greater performance of the utilized computer systems. The focus was on the improvement of different aspects of state-of-the-art and currently developed simulation solutions. The intention was not to develop new simulation concepts or applications that would make large-scale HPC on super-computers or large computer clusters necessary. The dissertation presents the integration of modern direct solvers for sparse linear systems in various power grid simulation back-ends and subsequent analyses with the aid of large-scale power grid models. Furthermore, a new method for an automatic coarse-grained parallelization of power grid system models at component level is shown. Besides such concrete applications of HPC methods on simulation environments, also a comparative analysis of various HPC approaches for performance improvement of Python based software with the aid of (just-in-time) compilers is presented, as Python - usually an interpreted programming language - becomes more popular in the area of power system related software. Moreover, the dissertation shows the integration of an HPC interconnect solution based on InfiniBand - an open standard - in a software framework for the coupling of different simulation environments to a co-simulation and for HiL setups. The support of a standardized data model for the processing of power system topologies by simulation environments, on which the aforementioned HPC methods were applied, is necessary. Therefore, the dissertation concerns the CIM as, i.a., standardized by IEC61970/61968, which can be used for the specification of data models representing power system topologies. At first, a holistic data model is introduced that was developed for co-simulations of the power grid with the associated communication network and the energy market by extending CIM. To achieve a sustainable development of CIM related software tools, an automated (de-)serializer generation from CIM specifications is presented. The deserialization from CIM is a step needed for the subsequently developed template-based translation from CIM to simulator-specific system models which is also covered in this dissertation. Many presented findings and approaches can be used for improving further software from the area of electrical engineering and beyond that. Moreover, all presented approaches were implemented in open-source software projects, accessible by the public.