Output Voltage Control-based Soft Pre-charging for Downstream Capacitors in LVDC Microgrids

With regard to the increasing amount of dc-based sources and loads to be connected to the ac utility grid, the concept of dc microgrids is gaining in relevance. One of the reasons for this is that in certain constellations, where facilities have several dc-based systems to be connected to the grid, but also interacting with each other, dc microgrids provide higher efficiency. Compared to ac technology, the different conditions of dc fault currents, the lack of standardization and the relatively limited experience entails intensive investigation in dc protection. One major systemic difference is the large number of dc link capacitors directly connected to the microgrid. In case of fault, the stored energy discharges into the short-circuit, significantly increasing the fault current, which is to be prevented by fast and sensitive protection devices. High inrush currents, which can unintentionally trigger protective devices, also occur when these capacitors are pre-charged in an uncontrolled manner, e.g. when loads are mechanically connected to the grid without further interventions. For a CurrentOS-compliant pre-charging procedure, it is, unlike presented in existing research, required to ensure a predefined pre-charge current profile at the connection point to the dc bus. Therefore, this paper presents an output voltage control-based pre-charging approach of downstream capacitors, that controls the capacitor voltage level and avoids voltage overshoot, while simultaneously allowing for limiting the peak and setting the average input current. Thereby, nuisance tripping is prevented. The control method is verified by simulation results and measurements.