AC/DC power system small-signal impedance measurement for stability analysis

Considerable research work has shown that a dc power system is small-signal stable if the Nyquist contour of product of the source impedance and load admittance satisfied the Nyquist stability criterion. Methods of extracting impedance in dc systems have been well established. However, for ac systems, the traditional current-injection techniques are not suitable, especially for the high power systems. The first contribution of this work is the unique process of injecting a line-to-line current between two phases of a three-phase ac system for the purpose of measuring source and load impedance. This technique details a practical current injection device which is simpler to construct and requires less power electronic components; resulting in significant cost reduction. The impedance measurement device can also be used to measure impedances in a dc or single-phase ac system. The second contribution is analysis and impedance measurement of single-phase ac power systems. In this analysis, a fictitious phase signal is created and combined with the original single-phase signal to build synchronous reference frame quantities. After this transformation, the same criteria used in the three-phase systems can be applied to the single-phase ac system. The third contribution is extending the application of the proposed techniques to extracting synchronous machine parameters online. It has been mathematically proven that there is a strong correlation between the small-signal impedance and synchronous machine parameters. The advantages of using this method include the ability to obtain the parameters under real operating conditions in a simple, practical way --Abstract, page iv