Design Considerations for Load Droop Control in DC Systems

In DC power distribution systems, droop control is typically applied on the source side to facilitate power sharing; however, it can also be implemented on the load side to enable dynamic demand response by adjusting consumption per the available source capacity. In particular, load droop control offers the added benefit of transforming a constant power load (CPL), which exhibits a negative impedance, into a voltage-dependent load with positive impedance behavior, thereby mitigating the well-known CPL instability. Today, most commercial off-the-shelf CPLs do not include built-in droop functionality. Consequently, implementing load droop in practice introduces a non-negligible delay, which can diminish its stabilizing effect and potentially reintroduce instability. In this work, we investigate how such delays influence system stability and demonstrate that system behavior is highly sensitive to these delays. Further, we analyze the impact of ramp-rate control in commercial converters by modeling it using a Filter-Based Ramp Rate Controller (FRRC). We derive stability conditions for the droop-FRRC-controlled system and determine the minimum capacitance to maintain stability. The theoretical findings are validated through hardware-in-the-loop (HIL) tests.