Impedance Modeling for Line-Frequency Rectified Constant-Power Loads

Constant-power loads (CPLs) represent a growing presence on utility grids and microgrids. These power electronic loads are frequently configured as rectified loads operating from an ac input to produce a controlled dc output at a constant power level. This constant-power demand creates a well-known incremental negative input impedance that “fights” changes in grid voltage by demanding opposing changes in input current to maintain constant power operation for a load. Rectified CPLs can, therefore, create instabilities with relatively unique perturbations in the envelope of ac-side current and voltage. A system’s envelope response, characterized by the envelope impedance, has allowed for the application of traditional stability analysis techniques to rectified CPLs where complex nonlinear dynamics prevent the use of, e.g., small-signal approximations. This paper presents clear and experimentally useful definitions for envelope impedance, explains the limitations of traditional single-frequency perturbation stability analysis for rectified CPLs, develops new techniques for estimating rectified CPL input impedance using a double Fourier integral method, and demonstrates these techniques with both simulations and experimental results.