Large-scale and increasingly software-defined systems in power and factory automation are very long-lived. Longevity requires sustainability-economically, environmentally and last but not least in terms of usability. Sustainability therefore requires continuous change. In this talk we look at handling requirements, models, and implementations in a model-driven formal way that lends itself to a more systematic change tracking than traditional software development approaches and languages, but that also crosses boundaries of software-controlled physical equipment models (so-called cyber-physical systems), distributed digital control (networked systems) and software services. The industry is currently seeing a rapid development of cyber-physical system products. The systems that are developed have increasing demands of sustainability, dependability and usability. Moreover, lead time and cost efficiency continue to be essential for industry competitiveness. Extensive use of modeling and simulation - Model-Based Systems Engineering (MBSE) tools - throughout the value chain and system life-cycle is one of the most important ways to effectively target these challenges. Simultaneously there is an increased interest in open source tools that allow more control of tool features and support, and increased cooperation and shared access to knowledge and innovations between organizations. In this talk we briefly present technology and open source tooling for MBSE based on the Modelica and UML standards, supported by tools such as OpenModelica and Papyrus respectively. Modelica is a modern, strongly typed, declarative, equation-based, and object-oriented language for modeling and simulation of complex cyber-physical systems, whereas UML is a wide-spread industrial standard for software modeling. We present the OpenModelica open source MBSE environment including the ModelicaML Eclipse plug-in integrating Modelica and UML, covering the development process starting from business processes, via requirements, to models, which can be compiled to simulations or to product code. An important question is whether a particular system design fulfills or violates requirements that are imposed on the system under development. We give examples of case studies starting with natural-language requirements and show briefly how they are translated into models. Then, designs and verification scenarios are modeled, and simulation models are composed and simulated automatically. The simulation results produced can then be used to draw conclusions on requirement fulfillment. Other features of the environment are meta modeling for efficient model transformations, the Functional Mockup Interface for general tool integration, model-based optimization, as well as generation of parallel code for multi-core architectures.