The lack of interrupting capacity of the available DC Circuit Breaker (DCCB) hinders the further development of Multiterminal HVDC technology. Fault current limiters (FCLs) are able to compensate the limitation of DCCBs.

The functional requirement of an FCL for a Multiterminal HVDC system is substantially different from that of an AC network or that of a two terminal DC transmission system. Conventionally, resistance is regarded as essential and sufficient, while inductance is less related and optional. However, pure resistance cannot effectively suppress the rising rate of fault current at the very early stage of a fault in a Multiterminal HVDC network, resulting in unnecessary blocking of a Modular Multilever Converter (MMC). Adequate inductance is required to curb the rapidly increasing fault current within the first few milliseconds, which is critical for a DCCB to isolate the fault without interrupting other DC transmission lines in a Multiterminal network. In this presentation, we analyze the performances of FCLs with different resistances and inductances in a four-terminal MMC HVDC system. The results show that both resistance and inductance are indispensable to realize an effective function to protect a multiterminal HVDC network.

In addition, the structural design and working principle of a new kind of superconducting FCL (SFCL) suitable for being applied to multiterminal HVDC networks is proposed in this presentation. The SFCL presents low impedance and enables a smooth reclosing operation after an interruption. It is able to make a maximum use of the permeability of the iron core to offer a large inductance to cub the rising rate of the fault current in the early few milliseconds as a fault takes place. In addition, it can have an adjustable current limiting threshold to match the different requirements of different grid protection schemes. The experimental results of a concept-proofing prototype are reported and analyzed in this presentation.

Keywords: Superconductivity, fault current limiter, HVDC, multiterminal HVDC network