Design of DC-line terminating inductors for enhancement of protective functions in MTDC grids

This study presents a detailed DC-side fault analysis considering inductive termination of lines within a high-voltage multi-terminal direct current (MTDC) grid. The analysis aims to provide design guidelines for DC-side inductors, taking into account important aspects of protection such as the required speed of operation of relays and the performance characteristics of current interruption devices (i.e. of DC circuit breakers). Moreover, the impact of current limiting inductors on the fault signatures is investigated. In particular, it has been found that DC-side inductors not only limit the fault current level, but also the resulting signatures in voltage and current, can assist to enhance the speed of operation, stability and selectivity of protective functions for DC-side faults. The analysis has been extended to include the impact of inductive termination on fast transient phenomena known as travelling waves. Specifically, DC-side inductors can form a significant reflection boundary for the generated travelling waves. A deeper insight into the faults has been achieved by utilising wavelet transform.