Atiyah Elsheikh

Simulating Cyber-Physical Energy Systems: Challenges, Tools and Methods

The energy system of the future is expected to be composed of a large variety of technologies and applications. However, the diverse nature of these components, their interlinked topology, and the sheer size of the system lead to an unprecedented level of complexity. Industry is confronted with severe problems in designing interoperable grid components, analyzing system stability, and improving efficiency. This paper describes the main challenges of continuous time-based and discrete event-based models of such cyber-physical energy systems. Using a characteristic test model, the scalability of the two approaches is analyzed. The results show the strengths and weaknesses of these two fundamentally different modeling principles that need to be considered when working with large scale cyber-physical energy systems.

The high level architecture RTI as a master to the functional mock-up interface components

Recently many commercial and non-commercial Simulation Packages (SPs) have agreed to use the Functional Mockup Interface (FMI) as the medium of interoperability. FMI presents numerous opportunities to utilize highly specialized SPs for modeling, and simulating multidisciplinary applications with several components of various types. However there is one thing missing in the FMI; the master algorithm. The paper proposes to use, the High Level Architecture (HLA) compliant Run Time Infrastructure (RTI), as a master for the FMI compatible simulation components. The ultimate goal is to provide a completely generic and standalone master for the FMI, making FMI-based simulation components usable as plug and play components, on variety of distributed environments including grids and clouds. Towards this promising goal an initial methodology is outlined.

Co-simulation of components, controls and power systems based on open source software

There exists no universal tool to analyze the increasing complexity in smart grids. Domain specific simulation and engineering tools partly address the challenges of complex system behavior. Different component technologies, customer behavior and controls in the power networks are interacting in a highly dynamic manner. Results of isolated simulations may be not accurate enough on the system level. Free and open available tools like GridLAB-D, PSAT, OpenModelica and 4DIAC are well known and widely used because of their excellent domain specific expertise. With co-simulation approaches the individual strengths of each tool can be exploited to model and simulate the various aspects of complex smart grids. The achieved level of detail and realism potentially surpasses the results that the individual analyses would gain. This paper demonstrates a local smart charging control strategy implemented with the IEC 61499-based standard for distributed control systems. It is simulated with different electric vehicle driving patterns, modeled with the multi-agent environment GridLAB-D. Battery models are defined in OpenModelica and embedded as individual dynamic loads. The power system is simulated using PSAT. This work shows that boundaries and restriction in terms of modeling cross-domain specific problems can be overcome by coupling these open source applications.

Modeling Intelligent Energy Systems: Co-Simulation Platform for Validating Flexible-Demand EV Charging Management

Energy systems experience a rise in complexity: new technologies, topologies and components, tighter links to other systems like markets and the increased usage of information technology. This leads to challenging questions that can not be answered via traditional methods. The goal of including renewable energy and clean technologies in the grid, however, requires solutions for the resulting complex problems. This paper investigates dynamic demand response for intelligent electric vehicle charging as a use-case for detailed hybrid models that cannot be properly handled by traditional tools alone. Universal modeling languages and specialized domain-specific modeling solutions are brought together via standardized co-simulation interfaces to achieve maximal flexibility and minimal implementation efforts. This combination of previously numerically incompatible modeling paradigms enables a detailed look into the dynamics of hybrid component models while keeping the comfort and the strength of established tools. This coupling of a Modelica-based physical simulation engine, a commercial power system simulation tool and an agent-based discrete event simulator for energy grids results in a novel co-simulation platform. This visionary concept provides the high level of detail, scope, flexibility, scalability and accuracy in simulations needed to analyze and optimize energy systems of the future.

The FMI++ library: A high-level utility package for FMI for model exchange

The success and the advantages of model-based design approaches for complex cyber-physical systems have led to the development of the FMI (Functional Mock-Up Interface), an open interface specification that allows to share dynamic system models between different simulation environments. The FMI specification intentionally provides only the most essential and fundamental functionalities in the form of a C interface. On the one hand, this increases flexibility in use and portability to virtually any platform (even embedded control systems). On the other hand, such a low-level approach implies several prerequisites a simulation tool has to fulfil in order to be able to utilize such an FMI component, for instance the availability of adequate numerical integrators. The FMI++ library presented here addresses this problem for models according to the FMI for Model Exchange by providing high-level functionalities, especially suitable for but not limited to discrete event simulation tools. The capabilities of this approach are illustrated with the help of several applications, where the FMI++ library has been successfully deployed. This approach intends to bridge the gap between the basic FMI specifications and the typical requirements of simulation tools that do not primarily focus on continuous time-based simulation. In other words, this enables such models to be used as de-facto stand-alone co-simulation components.

Modelica-enabled rapid prototyping of cyber-physical energy systems via the functional mockup interface

Modelica has achieved a great success in the last decade. Universal modeling concepts, object-oriented facilities and large set of libraries in several physical domains allow for rapid prototyping of multidisciplinary applications. A larger community can benefit from these capabilities if Modelica-based components can be integrated into their favourite simulation tools. This work addresses the impact of transferring Modelica prototyping capabilities into different classes of simulation tools: general-purpose modeling tools, domain-specific tools and academical research-oriented simulation environments. In particular, it shows that the realization of model-based research of cyber-physical systems shall benefit from the convergence of such efforts using the functional mockup interface.

Evaluation of two approaches for simulating cyber-physical energy systems

Simulation-driven design has become an important design process in many technological domains. It allows a more rapid deployment of innovative technology in products that have to fulfil high quality standards. In view of the success of this approach it is also becoming an increasingly important tool for the development of the future energy system. Due to the size and complexity of such systems this is however a challenging task. Energy systems combine not only a multitude of physical domains, related directly to the processes of generation, storage, distribution and consumption, but will in the future also increasingly rely on communication technologies and software in- frastructure for information exchange and control purposes. This article evaluates two distinct software tools, Simulink/Simscape and Ptolemy II, that nevertheless have the potential to serve as a framework for modelling, simulating and analysing such cyber- physical energy systems.

Simulating complex energy systems with Modelica: A primary evaluation

Rapid physical modelling of complex energy systems is a requirement for coping with rapid changing technologies demanded by this field. The variety of model domains and component types required for describing such complex systems makes the modelling task very challenging. Moreover, while a wide set of advanced tools for modelling highly-specialized tasks and perspectives already exists, a tool that covers all perspectives and components of a complex energy system does not exist. For this purpose, a rather primary but comprehensive evaluation of employing the universal modelling language Modelica in modelling applications of complex energy systems is presented. The advantages of utilizing such an approach is emphasized on an abstract model that is composed of several typical components within a complex energy system.

Distributed hybrid simulation using the HLA and the Functional Mock-up Interface

High Level Architecture (HLA) and Functional Mock-up Interface (FMI) are two simulation interoperability standards. The HLA is older, well established and popular in industry. The FMI is a new standard, with plenty of support from the open source community and scientists. In this paper, it is presented how the strengths of both, the HLA and the FMI, can be utilized to realize a distributed hybrid (or heterogeneous) simulation platform. Two different algorithms are proposed for such a platform. To demonstrate the correctness of algorithms, and their performance comparison, a simulation example is chosen from the domain of complex energy systems.

Using the HLA for Distributed Continuous Simulations

Distributed computing offers many advantages for all types of computational applications. Realizing heterogeneous simulation platforms may benefit from many facilities of distributed computing. However, distributing simulation components over a network raises many challenges concerning communications, data exchange, numerical stabilities and others. A well-known solution that addresses some of these challenges is the High Level Architecture (HLA). The HLA is an industry standard for distributed simulation and interoperability. So far the HLA has been used in industry for Discrete Event Simulations (DES). In this paper it is presented how the HLA can also be employed for continuous simulations. Two HLA specific algorithms for distributing explicitly coupled continuous simulation components over the network are presented. The simulation components follow the Functional Mock-up Interface (FMI) specification. The FMI is a specification for model exchange and co-simulation among simulation tools. Any simulation component conforming to FMI, generated from any of the more than forty simulation tools that are supporting or planning to support the FMI, can be considered in the proposed architecture.