Coupled Inductor Based Hybrid DC Circuit Breaker Topologies for DC Grid Application

Reliability of DC grid is measured in terms of fault interruption time of the switchgear. Unlike AC systems, DC systems do not encounter zero crossing of fault current. Hence the DC circuit breaker should be capable of very fast fault interruption and isolation. Solid state circuit breakers (SSCB) exhibit fast fault interruption at the expense of substantially large conduction power loss due to on-state drop of semiconductor devices. Hybrid circuit breakers yield better performance in terms of efficiency although the principal hurdle remains in demagnetization of network inductance after mechanical breaker is completely turned off. This paper proposes two hybrid DC circuit breaker (DCCB) topologies using coupled inductor to mitigate this issue. The coupled inductor forms a resonant circuit with a commutation capacitor during the fault to turn off the mechanical switch at zero current. Commutation capacitor is charged through the circuit elements; hence the requirement for external pre-charging circuit is eliminated. Moreover, the capacitor is charged to unipolar DC voltage only, enabling use of electrolytic capacitor. Proposed topologies do not require surge arrester to demagnetize the DC network once fault is cleared. One of the proposed circuit breaker topologies is designed for bidirectional power flow, implying its use in multi terminal DC transmission system or DC micro-grid. Detailed analysis and design of the proposed circuit breakers are presented in this paper and validated through simulation and experimental results.