Stability Methods for Regulated Loads

Abstract

Power electronic circuits often regulate load power and present a constant power profile to the utility or other electrical source. These constant power loads (CPLs) therefore exhibit a negative incremental input impedance and pose stability challenges when present in either dc or ac systems. This thesis presents CPL design techniques to mitigate these issues, as well as tools to determine in which applications CPL instabilities may occur.

For stability analysis of a dc distribution system, an equivalent circuit model for limited-bandwidth CPLs is presented. This model quantifies the intrinsic damping properties of regulated converters and how they relate to the control bandwidth. A control architecture based on this equivalent circuit model is then analyzed. The control scheme limits the destabilizing impact of CPL operation by adding a lossless internal damping component and shaping the effective input impedance to resemble that of a CPL operating under reduced control bandwidth. An intermediate, internal energy buffer is used to support high-bandwidth output load regulation while enabling the programmable or selectable input impedance on desired time scales. The effectiveness of this strategy on system stability is shown using a stability analysis and experimental results. In addition, for a collection of loads on a dc distribution system, a participation factor analysis is presented which can help identify likely sources of instability. Finally, an ac application is presented in which an adaptation of the previously discussed control architecture can be used to promote stable interaction between ac CPLs implementing power factor correction and an ac source.