Dynamical Characterization of Multi-Converter System: Simultaneous Measurement of Bus Impedance and Control Loop Gains

Interconnected power-electronics converters connected to a common bus have become increasingly important in various power-distribution systems. Due to interactions between converter subsystems the multi-converter system can often have stability issues even though each converter is standalone stable. Recent studies have presented a passivity-based stability criterion with which the stability of a multi-converter system can be analyzed by measuring the system bus impedance. The technique provides the stability of the complete system but does not reveal the dynamics of a single converter. The dynamics of single converters can be studied by other methods such as loop-gain measurements. The loop gains provide direct information on the operation of single converters and their stability margins but not the information about the global stability. This article combines these techniques to simultaneously analyze the single converters and the complete multi-converter system. In the method, several orthogonal perturbations are injected into the converter control loops. The current and voltage responses are measured in the loop and from the converter outputs. After this, Fourier techniques are applied to obtain the spectral information of the loop gains and the bus impedance. The applied perturbations can be designed to have a very small amplitude, and thus, the process does not cause the system to deviate too much from its normal operation. Therefore, the method is well suited, for example, in online analysis and adaptive control. Experimental measurements are presented from a complex multi-converter system. The work is a revised and extended version of a presentation at ECCE2022 (Roinila et al., 2022).