Analysis of Plug-In and Plug-Out of Converters in Droop Controlled DC Microgrid Systems

This article presents methodologies to achieve seamless plug-in and plug-out of converters in droop-controlled DC MG systems. It is shown that the typical approach of pre-charging the converter to the nominal voltage before plugging it into a droop-controlled DC MG system can result in high transients in converter currents, which can trigger the system's shutdown. New and effective methodologies are presented to achieve seamless plug-in and plug-out of a converter from the DC MG system. The seamless plug-in is achieved by controlling the reference voltage of the converter and plug-out is achieved by controlling the droop resistances. The proposed methodologies act as damping techniques to minimize the transients in converter currents. The methodologies are implemented using the conventional droop control and thus are simple to implement. An analytical model is developed for a generic DC MG system to analyze the performance of the system during a hot swap of a converter. The model is also used to propose guidelines for controller design to achieve seamless plug-in and plug-out of a converter. Extensive experimental results on different configurations and operating points of MG systems are provided to validate the proposed methodologies.