Fault mitigation in Multiterminal DC systems using DC Breakers

Abstract:
The German Grid Development Plan newly introduces so-called "DC hubs", via which various HVDC converters can be connected to each other on the DC side. The resulting Multiterminal DC systems allow a more flexible grid operation, e.g. in terms of congestion management, and thus increase the operational efficiency. However, operational experiencewith the HVDC technology has so far been largely limited to point-to-point DC systems. Multiterminal DC systems callfor a re-evaluation of the impact of DC-side faults on the overall system, ensuring system security and system stability.This is challenging because DC systems are fundamentally different in nature to AC systems: In AC systems, the faultcurrent is limited by the inductive elements in the fault current path, which at the same time reduce the fault-relatedvoltage decline. In DC systems, on the other hand, the (stationary) fault current is only limited resistively, the resultingvoltage decline is much more distinctive and affects all connected HVDC converters. This paper analyses technologicalconcepts for coping with DC-side faults in Multiterminal DC systems, whereby the focus is on DC breakers. The effectof a DC breaker is modelled using a simplified but suitable simulation model. The technological implementation is disregarded in such a way, that effects valid for all types of breakers are deduced. Interactions that arise between DC breakers,the DC grid and the connected HVDC converters are investigated. The analysis deduces system aspects which have to beconsidered when integrating DC breakers and derives requirements regarding the protection of Multiterminal DC systems.