Requirements on MV DC circuit breakers

Direct Current (DC) power transmission offers many benefits over Alternating Current (AC) and it is therefore making fast inroads in electrical energy systems all over the world. High voltage direct current transmission High-voltage Direct Current (HVDC) offers efficient transmission over long distances particularly when cables must be used, for instance for subsea links. DC is also used at low voltage, for instance for grids supplying the many servers in data centres. Also at medium voltage (>1500 V – 50 000 V) DC potentially offers significant advantages in many applications. The power conversion chain can be simplified and power flow can be more accurately controlled.

The HYPERRIDE project (HYbrid Provision of Energy based on Reliabilty and Resiliency via Integration of Dc Equipment) contributes to the field implementation of DC and hybrid AC/DC grids. Starting with the definition of most relevant fields of application for DC grids (local microgrids, grid enforcement to overcome congestions, coupling of AC grid sections, etc.), the enabling technologies shall be specified in detail on different levels. One such enabling technology is Direct Current Circuit Breakers (DCCBs) for Medium-Voltage Direct Current (MVDC) grids. Even though MVDC DCCBs are available for DC railway systems e.g., MVDC distribution grid applications have different requirements, mainly with respect to the speed of operation. In recent years different DCCB prototypes for grid applications for MV have been developed using state-of-the-art (mechanical) techniques but also novel solutions, such as the VSC-assisted resonant current (VARC) concept from the project partner SCiBreak AB.

In this report, the most important grid parameters (e.g. grid configuration, converter topology, DC reactor size) impacting the requirements for DC circuit breakers are analysed by means of simplified circuit models. The results of grid simulation studies concerning the consequences of short circuit scenarios for DCCBs in real a MVDC demonstrator grid as well as in a fictional
14 kV grid are also presented.

A list of requirements for MVDC circuit breakers is put together in the report. Moreover, a review of HVDC standards and standards concerning DC railway applications that may be relevant for establishing MVDC standards in the future is given, with the indication of relevant technical brochures and normative documents that are likely to be published in the upcoming years.