Review of different fault detection methods and their impact on pre-emptive VSC-HVDC dc protection performance

Multi-terminal voltage-sourced converters(VSC)high-voltage direct current(HVDC)transmission system is expected to play a vital role in future power systems.Compared with ac power transmission,dc transmission is more vulnerable to faults due to low dc-side impedances and sensitive power electronics in the converters.Dc protection issues must be tackled before any multi-terminal VSC-HVDC grid can be built.The multi-terminal VSC-HVDC system is studied in detail using switching models for two-level converters,detailed equivalent models for the modular multi-level converters,detailed hybrid circuit breaker switching models and frequency-dependent phase models for dc cables.Using such high-fidelity system models,a systematic study of HVDC fault protection methodologies in more detail than previous studies is conducted.This is the first comprehensive study that includes pre-emptive circuit breaker operation.The results presented in this study underline the benefits of such a detailed treatment of the breaker,and of considering it as part of a fast power electronics system rather than isolated dc equipment.The study identifies the best existing fault detection method and tests it extensively.In order to further improve post-fault system recovery response,which is a key but often neglected part of previous studies,a novel bump-less transfer control has been implemented in the converters.