A Grey Box Method for the Stability Analysis of Multi-Inverter-Fed Power Systems using Bode Diagrams

Abstract

A multi-inverter-fed power system is susceptible to small-signal instability owing to weak grid influence. Using the impedance-frequency (IF) method to analyse system stability at engineering sites has the following limitations: 1) The internal information of a grid-connected inverter is unknown because of technical confidentiality. Thus, its impedance model should fit in a grey box. 2) The IF method defaults to the subsystem aggregation impedance without right-half-plane (RHP) poles, which may result in incorrect stability analysis. 3) The data used for drawing the IF curve should be calculated per frequency, which is time-consuming. To address these limitations, this study proposes a grey box method for small-signal stability analysis of a multi-inverter-fed power system based on Bode diagrams. First, grey box impedance fitting for a single grid-connected inverter and impedance aggregation for a multi-inverter-fed power system are discussed. Second, the principle of the proposed method is elaborated. Using the Bode diagram, the number of RHP poles of the aggregated impedance and actual number of circles for Nyquist curves are identified. Third, a multi-inverter-fed power system is constructed, and the effectiveness of the proposed method is verified using case analysis and hardware-in-the-loop real-time experiments based on RT-LAB. Finally, the advantages of the proposed method are revealed by comparing it to the IF method.