Medium voltage direct current interruption principles and testing methods in DC and AC circuits

Medium voltage direct current (MVDC) networks are attracting interests in distribution, and pilot applications appear recently. In comparison to alternating current (AC) networks, DC offers number of advantages. But drawbacks of DC exist. Beside measurement and protection, the main drawback is related to maturity of equipment: switchgear, circuit-breaker, switches. The paper describes development of MVDC switchgear, which will be put in service in “OPHELIA” project in France in 2025 [1]. Nominal voltage will be ±5 kV with a load current up to 100 A. The network will collect power from photovoltaic (PV) of around 0,9 MW and will be installed linearly over one kilometre. Fault current protection in the network will be realized by a medium voltage current-limiting fuse. A standard AC MV fuse was characterized to interrupt DC fault. Load current will be interrupted by oscillating principle and tests have been performed in two different laboratories. Firstly, under DC circuit conditions supplied by a rectifier. Secondly, in an AC circuit, with a short-circuit generator operating at low frequency. Dimensioning of all passive components, i.e. capacitor, reactor, resistor and fuse was performed, under the premise of their size. This enabled to develop MVDC switchgear, based on architecture of MVAC ring-main-unit and therefore combining proven technology of vacuum circuit-breaker and switches.