Model-Based Systems Engineering for Offshore HVDC Grid Integration Systems

Abstract

To achieve the 1.5 °C target of the Paris Climate Agreement that came into force in 2016 [UNI15], the conversion of the entire energy system to renewable energy sources is essential. Strong and stable wind conditions offshore as well as a high growth potential make offshore wind energy far away from shore an important energy source. The integration of such Offshore Wind Farms (OWF) has to be safe, flexible, efficient, and affordable, which makes High Voltage Direct Current (HVDC) transmission systems often the preferred choice for their grid connection. HVDC transmission systems consist of many subsystems, components, and interfaces e.g. to the OWF, which characterise them as complex systems. The complexity, many and diverse requirements and a large number of system design alternatives require grid planners, grid operators and manufacturers to employ a systematic approach to develop the most suitable design that meets all requirements and functions as expected in its environment. Systems Engineering with its procedure based on the so-called V-model, has proven itself for many years outside the energy industry as a suitable methodology for the development of complex systems. The extension to Model-Based Systems Engineering (MBSE) allows the specification of a system in a universal system modelling language (SysML) based on the activities in the V-model, which enables the collaborative system development across disciplines. Based on the MBSE methodology, a HVDC transmission system for a case study consisting of three OWFs with a total power of 3 GW is designed in this thesis. The requirements are derived from stakeholder needs and categorised before the system design process. Stakeholders are for example the OWF owner and the onshore grid operator. This thesis considers electrical engineering requirements and the objective of a low levelised cost of energy (LCOE) design. The system integration and verification of the prior defined requirements are performed, before a validated HVDC transmission system design is available following the system development process with the V-model. MBSE enables early identification of design errors through verification activities when they are still easy and cheap to correct. The SysML model forms the core element for the HVDC transmission system design as all relevant requirements, constraints, specifications, and the system objective are included in the model. The most suitable system architecture and component definition is developed with a Multi-Criteria Decision Aid assessment and selection method based on the specifications in the SysML model. In this thesis, a step-by-step guideline for the development of HVDC transmission systems based on MBSE is developed to improve the system design with an interdisciplinary and model-based approach. This approach leads to a detailed understanding of system elements and their interactions inside and outside the HVDC transmission system. The result for the case study is a HVDC configuration consisting of two symmetrical monopoles, which can be interconnected as a multi-terminal HVDC system in the event of a monopole failure. This arrangement is identified as the preferred architecture for the case study and assumptions. The converter transformer is chosen as an example to demonstrate the identification of the number and type of components as part of the Systems Engineering process. Discipline-specific models and simulations provide details to the design. At the end of the system design activities, the SysML model represents a detailed specification of the HVDC system and thereby replaces many documents with specifications. The advantage of the developed SysML model is to facilitate the design analysis and to identify the impact of requirement changes or changes in the system environment on the system design. The SysML and discipline-specific models allow the verification of system requirements, e.g. of the availability, which has a high influence on LCOE and is used as an assessment criterion on system and component level. The validation procedure is also introduced. Based on the proposed approach for HVDC transmission system development, the potential for the extension of the methodology for energy systems in general is presented to support the realisation of the energy system conversion to renewable energy sources.