Control of a Multi-terminal VSC-HVDC system - A general Control System Structure

Sammanfattning: Environmental impact and security of supply has fuelled a shift in the power mix of power systems today. Traditionally used fossil fuel gives more and more space to the development of renewables in an effort to comply with strict international standards regarding CO2 emissions and secure energy supply. Wide geographical spread of renewable resources indicates that HVDC technology is most suitable for transmitting power from the isolated points of generation to the points of consumption. This is also supported by the fact that the most advanced among renewable technologies is wind power technology which tends to expand offshore where higher wind potential is available and projects are more immune to public opposition. Integration of renewables and their upgraded role in the power system together with the ambition of an integrated energy market trigger visions of a highly controllable and reliable, continent-wide DC grid based on multi-terminal HVDC technology. Recent developments in converter technology make this vision realistic. Voltage Source Converter (VSC) technology shows great controllability facilitating the connection to the AC system compared to Current Source Converter (CSC) technology. VSC-HVDC technology is suitable for multi-terminal system arrangements but several issues need to be investigated before this becomes reality. While the development of large multi-terminal VSC-HVDC depends on the functionality of a fast and reliable DC breaker, such component may not be indispensable for the development of smaller systems. Nevertheless, concerns exist for the development of smaller, regional multi-terminal VSC-HVDC systems, especially if they are expected to expand and interconnect to form a larger DC grid in the future. Lack of field experience is a source of concern but most importantly, lack of standardization and absence of a control system to perform the coordinated operation of the multi-terminal VSC HVDC system. The focus of this thesis is the control system that will allow automated and coordinated operation of a multi-terminal VSC - HVDC system. It is perceived that the control system can contribute in the standardization in software level with the intention to allow uniform interfacing with equipment coming from different suppliers. The transition from small, regional multi-terminal VSC-HVDC systems to a large DC grid will most likely happen gradually expanding and interconnecting the individual, small multi-terminal systems to form a larger system where coordinated operation is considered necessary. A well designed control system already in this stage can contribute in keeping up with this evolution assuring at the same time safe system operation both in normal conditions and under disturbances. The intention is to describe the structure and features of such system and provide an implementation that can be validated by simulations. In this thesis the structure and features of an overall control system for a multi-terminal VSC-HVDC system are described. Following this outline an implementation is proposed that is validated through simulations on mainly 3-terminal VSC-HVDC systems. Uniform interfacing is used and a minimum necessary data set is suggested. The expandability of the proposed control system is tested as well as its behaviour in normal operation and operation under disturbances, such as communication loss events and AC faults.