In this paper we propose a modular realization of the active capacitor, where each module is composed of a bidirectional converter and a buffering capacitor. This modular capacitor is a two-terminal device that can automatically adapt itself to different DC bus voltages, and is meant to filter out a wide range of low frequency ripples. Our plug-and-play control approach means that the modular active capacitor can be directly connected to the DC bus without any extra connections, just like a passive capacitor. The proposed modular design improves the overall reliability, providing fault tolerance operation. We investigate the application of our type of active capacitor in DC microgrids, where multiple active capacitors are distributed at different nodes. It is shown that ripple power can be shared in proportion to the storage capacity of the active capacitors without any communication between them. We also offer a mathematical stability analysis of a DC microgrid in which the admittances of all the devices satisfy a certain condition that is less restrictive than being positive-real (and which is satisfied by our proposed design). The modular active capacitor concept has been verified in simulations and experiments.