DC Transmission Grid With Low-Speed Protection Using Mechanical DC Circuit Breakers

This paper introduces a dc transmission grid with fault-tolerant inductor-capacitor-inductor (LCL) voltage-source converters (VSCs) and using a slow protection system based on mechanical dc circuit breakers (CBs). LCL VSC inherently regulates dc fault current to levels that converters can sustain for prolonged periods which avoids insulated-gate bipolar transistor tripping and brings significant advantage to security and reliability aspects. Simple mechanical dc CBs are used at dc busbars and connecting points of each dc cable, in the same manner as it is normal practice used with ac transmission protection. The protection logic is based on differential methods which gives excellent selectivity and reliability. The fault clearing time is in the order of 30-60 ms which allows for reliable protection decision making. The simulation results obtained from a four-terminal dc grid modeled on the PSCAD platform confirm successful dc fault isolation and grid recovery for a range of severe dc fault scenarios.