Soft Pre-charging Procedure for LVDC Microgrids

Low-voltage dc (LVDC) microgrids offer, in the case of locally increased use of dc-based generation and loads, the possibility of saving energy by reducing the number of conversion processes. For the grid operator of a LVDC microgrid, the pre-charging process of voltage-stabilizing capacitors during load plug-in is an essential aspect of the operating concept. In addition, standardization at an early stage is important for the development of the operating concept in order to ensure interoperability and plug-and-play functionality for (future) products and infrastructure. For this reason, the Current/OS foundation is developing specifications for the standardization of dc infrastructure. This paper presents a pre-charging system based on a dc-dc converter topology with appropriate modulation and control, which realizes the pre-charging of the capacitor between the dc bus and the load according to the specifications of the Current/OS foundation. It is of particular importance that a constant charging current is applied and the transitions from and to the base charging current are realized by linear ramps to avoid uncontrolled high inrush current peaks potentially leading to nuisance tripping. The presented pre-charging procedure is suitable for achieving the desired current and voltage profiles.