An Investigation into the Limitations of the Combined dv/dt and di/dt Protection Technique for Compact d.c. Distribution Systems

d.c. distribution networks interfaced by two-level voltage source converters (VSCs) are characterised by rapid fault current development, which places severe time constraints on d.c. protection. A proposed solution to ease this is the implementation of advanced converters with current management capabilities such as modular multilevel converter (MMC). However, the limited fault current caused by MMC will make the widely used LV current-based protection less effective. A new method, targeted utility-scale LVDC distribution networks at conditions with limited fault current, combining dv/dt and di/dt has been proposed in literature with promising results for fast and selective protection. However, further investigation must be carried out to determine its applicability and limitations for protecting future compact d.c. distribution systems. This paper presents an investigation into the selective capabilities of the combined dv/dt and di/dt protection technique for a range of fault scenarios. The study considers the effects of sampling frequency, fault locations, and the network architecture with the aim of determining the limitations of the protection method. A simplified converter model with active fault current limiting and blocking capabilities is built in PSCAD/EMTDC and implemented in a radial and ring compact d.c. distribution network to facilitate the study.