D-Q Frame Impedance Measurement and Small Signal Stability Improvement in a 3-ϕ System with Constant Power Loads by an Active Control Method

This paper presents a method for d-q frame impedance measurement and the small signal stability analysis of 3-ϕ power electronic converter based system with constant power loads (CPLs). The method is based on the Newman’s multi-tone low crest technique in which the perturbed frequencies are added at the optimal phase angle. The resultant perturbation signal has the lowest crest factor compared to the conventional multi-tone sinusoidal signal. The optimal perturbation signal is suitable for impedance measurement of strong/weak grids. In this work, a multi-tone current signal is generated based on the Newman’s optimal phase angle method to measure d-q frame impedances and subsequently the system stability is analyzed using general nyquist stability criteria based on the source and load impedances. Once the stability is analysed, an active control method is applied to improve the stability margin of the system based on the grid impedance conditions by reshaping the impedance profile of the load subsystem contains the CPL. The effectiveness of active control method has been tested in MATLAB/Simulink simulation environment for a three-phase motor drive system feeding through an active front end (AFE) rectifier, which behaves as the CPL.