Resistive Superconductive fault current limiters for the protection of MTDC grids

(RSFCL) have been introduced in distribution (i.e. 23 kV section of Seogochang substation,

South Korea) and transmission networks (i.e. 220 kV Mnevniki substation in Moscow, Russia) in order to cope with increasing fault currents level using their instantaneous and self-triggering current limitation properties. Research has also been carried out in the field of DC transmission with, in particular, the construction and installation of one 160 kVDC pilot RSFCL in Nan’ao converter station.

In a Multi-Terminal DC grid (MTDC grid), the reaction time of the protection system in case of fault is of utmost importance to avoid voltage collapse. The RSFCL then appears particularly advantageous in case of such event: the ultra-fast opposition of a resistive load to the converters prevents voltage collapse. Using this property, RSFCL installed at each end of the faulty line also enables identification of the faulty line, allowing a selective triggering of the protection circuit breakers. Additionally, the use of RSFCL practically limits the current to approximately two to three times the rated current and allows for a longer current interruption time.

In this paper, a fully selective protection strategy, with the target of avoiding MMC blocking, is implemented in a 4-Terminals benchmark MTDC grid and evaluated, based on an ElectroMagnetic Transient software (EMT) simulation. A design of a MTDC grid protection system under fault conditions is described. A DC reactor based and a RSFCL based protection are compared on the same basis. In both cases, the technology of the DC CB is of the mechanical type, using a vacuum interrupter (VI) and a forced counter current injection loop. The response of the converters, the DCCB and the RSFCLs, in case of fault are assessed. This highlights additional key advantages of the RSFCL combined with a mechanical DCCB: no requirement for a series DC reactor, no need for ultra-fast interruption capability, and reduced or negligible transient overvoltage across both devices, indeed, unlike stand-alone DCCB, energy dissipation by zinc oxide surge arresters is not required.

To show the feasibility of this technology, this paper also presents the design of a RSFCL prototype comprising its main components: the active parts, consisting of windings of superconductive tapes that can be parallel and / or series connected, are dipped into a cooling liquid inside a grounded tank. The operating temperature being in the range of -200°C, liquid nitrogen (LN2) is used to maintain the superconductive properties of the high temperature superconducting (HTS) tapes. To manage the operational parameters of the LN2 bath, a cryostat and its cryogenic system are used. Experimental test results, namely dielectric test results and short circuit current test results achieved on the prototype are presented. In particular, a 50 kVDC limitation and interruption test series was performed. Some of the results will be discussed, illustrating the response of a combined RSFCL + DCCB to various prospective DC fault currents.