As the size, complexity, and functionality of systems we need to model and simulate con-tinue to increase, benefits such as interoperability and reusability enabled by distributed discrete-event simulation are becoming extremely important in many disciplines, not only military but also many engineering disciplines such as distributed manufacturing, supply chain management, and enterprise engineering, etc. In this dissertation we propose a distributed simulation framework for the development of modeling and the simulation of complex systems. The framework is based on the interoperability of a simulation system enabled by distributed simulation and the gateways which enable Com-mercial Off-the-Shelf (COTS) simulation packages to interconnect to the distributed simulation engine. In the case study of modeling Virtual Test Bed (VTB), the framework has been designed as a distributed simulation to facilitate the integrated execution of different simulations, (shuttle process model, Monte Carlo model, Delay and Scrub Model) each of which is addressing differ-ent mission components as well as other non-simulation applications (Weather Expert System and Virtual Range). Although these models were developed independently and at various times, the original purposes have been seamlessly integrated, and interact with each other through Run-time Infrastructure (RTI) to simulate shuttle launch related processes. This study found that with the framework the defining properties of complex systems - interaction and emergence – are realized and that the software life cycle models (including the spiral model and prototyping) can be used as metaphors to manage the complexity of modeling and simulation of the system. The system of systems (a complex system is intrinsically a "system of systems") continuously evolves to accomplish its goals, during the evolution subsystems co-ordinate with one another and adapt with environmental factors such as policies, requirements, and objectives. In the case study we first demonstrate how the legacy models developed in COTS simulation languages/packages and non-simulation tools can be integrated to address a compli-cated system of systems. We then describe the techniques that can be used to display the state of remote federates in a local federate in the High Level Architecture (HLA) based distributed simulation using COTS simulation packages.

This thesis explores the application of the Virtual Test Bed (VTB) and its real-time extension, VTB-RT, for protective relay modeling, simulation, and testing. An instantaneous overcurrent relay model was developed in VTB for a transmission line protection. The same relay model was built in Matlab/Simulink for validation purposes. Both models were tested for various fault conditions on a radial power system and results were compared. Moreover, a low cost real-time Hardware-In-the-Loop (HIL) simulation platform was implemented for relay model testing using VTB-RT and public domain software packages such as Real-Time Application Interface (RTAI), Comedi, and Comedilib, and notebook computer hardware. The applicability of VTB-RT was verified through an open-loop simulation and a HIL simulation of a simple dynamic system using dSPACE as the control hardware and NI DAQCard-6062E as the input/output interface. Simulation results are presented showing the effectiveness of the VTB-RT platform for model testing.