Capacitive Element for Limiting VSC Fault Current in DC Grids

Maintaining continuous voltage source converter operation under DC faults is one of main challenges in protection of DC transmission grids. In order to limit converter fault current below overcurrent tripping threshold (2-3 p.u.), impractically large inductors would be required in series with DC circuit breakers which would cause control-related issues. This paper proposes a novel shunt-connected protection device which, utilizing fault current sharing property of DC buses, reduces fault current contribution from the converter and improves DC bus voltage stability. Converter blocking can consequently be avoided with practically acceptable inductors. A case study is performed on a three-terminal offshore DC grid model developed in PSCAD. Peak fault current increase and bus voltage drop indicate greatly improved performance. Critical inductor size and critical fault neutralization time are analyzed for a range of protection system configurations and feasibility of such arrangements discussed.