Communication-less protection algorithms for meshed HVDC grids.

For the large-scale integration of renewable energy sources into the power system, transmission corridors with power ratings and lengths greatly exceeding those in the existing power system will be needed. To realize these corridors, Voltage Source Converter High Voltage Direct Current (VSC HVDC) offers several advantages over the currently widely used ac technology. The use of VSC HVDC in a large-scale meshed grid can provide the major reinforcements to the power system needed for the integration of massive amounts of renewable energy sources.

Selective protection against dc side faults is essential to safely and reliably operate meshed HVDC grids. Since required operating times for HVDC grid protection are ten to hundred times faster than existing ac protection, HVDC grid protection algorithms are fundamentally different from those used in ac systems. Furthermore, the limited number of HVDC grid protection algorithms reported in the recent literature were only tested in specific small-scale test systems. For a generally applicable and reliable HVDC grid protection, a more fundamental approach towards the development of protection algorithms is needed.

This work provides the necessary concepts to develop communication-less protection algorithms for meshed HVDC grids. A detailed overview of dc fault phenomena is provided and fault clearing strategies proposed in the literature are discussed and classified. The fault current contribution of the half-bridge modular multilevel converter is characterized and a reduced converter model for dc fault studies, is proposed. Guidelines for the design of fault detection methods, based on fundamental traveling wave theory, are provided. Furthermore, signal processing requirements for protection algorithms, in particular required sampling frequency and digital filtering, are investigated. Finally, fast and selective HVDC grid protection algorithms for primary and backup protection are developed. These algorithms are tailored for selective fault clearing in VSC HVDC cable grids with inductive cable termination.