Analysis and protection of converter-side AC faults in a cascaded converter-based MVDC link: ANGLE-DC project

Medium-voltage direct-current (MVDC) technology exhibits advantages over ac transmission due to its enhanced capacity, controllability, and flexibility. A practical example is the ANGLE-DC project from Scottish Power Energy Networks, which converts existing 33 kV ac circuits into dc operation to improve the system transmission capacity in North Wales, UK. Specially designed cascaded power converters are employed to build up the ±27 kV dc link voltage. Within the converter stations, converterside single-phase ac faults may threaten the safe operation of the converters. This paper presents an analysis of such faults for an MVDC link based on ANGLE-DC. It is shown that the capacitors of the converters will be overcharged without suitable protection in place. A protection strategy using a bypass thyristor-based branch is presented to relieve overvoltage issues following converter-side faults, and it is verified through simulations conducted in PSCAD/EMTDC. For completeness, case studies considering post-fault operation are also included