Constant Power Load Modeling for a Programmable Impedance Control Strategy

Expanded use of power electronics opens the possibility that input characteristics presented to the conventional utility as well as ac and dc microgrids could be controlled in ways that help support utility operation. Power factor correcting interfaces are one wide-spread example, used around the world to meet regulatory requirements for both power factor and also harmonic content injected to the utility. More support for ensuring healthy utility operation might be demanded from these power electronic interfaces. For example, the potentially destabilizing effects of constant power loads (CPLs) could be remediated on time scales useful for supporting utility operation. This article presents an equivalent circuit model for limited-bandwidth CPLs that quantifies the intrinsic damping properties of regulated converters. A control architecture based on this equivalent circuit model is then analyzed. The control scheme limits the impact of CPL operation by emulating the internal damping of CPLs through programmable input behavior. An intermediate, internal energy buffer is used to support CPL operation while enabling the programmable or selectable input impedance on designed time scales. The effectiveness of this strategy on system stability is shown using a stability analysis and experimental results.