Simultaneous Measurement of Bus Impedance and Control Loop Gains in Multi-Converter Systems

Power distribution systems are often dependent on the operation of a large number of power-electronics converters that are connected to a common bus. The converters may be standalone stable but the system may still exhibit stability issues due to interactions among multiple converter subsystems. Recent studies have presented methods such as passivity-based stability criterion, where the system stability can be monitored by using bus-impedance measurements. The method provides the dynamic performance of the complete system but does not reveal the effect of a single converter on system dynamics. Other studies have presented methods based on loop-gain measurements, where the system is analyzed through single converters. These methods provide more direct information on the operation of single converters and their stability margins compared to the bus-impedance approach but the loop gains do not provide direct information on the overall system operation and global stability. This paper proposes a method for simultaneously measuring the bus impedance and all the loop gains of a multi-converter system, thus providing an efficient tool to combine the two analysis methods. In the technique, several orthogonal broadband perturbations are simultaneously injected into the control loops of single converters. The resulting responses are measured from both sides of injection points and from the output voltages and currents of single converters. Then, cross-correlation and Fourier methods are applied to extract the spectral information of the measured responses for obtaining the bus impedance and the loop gains. The proposed technique allows a straightforward online method to fully characterize the stability of a multi-converter system. Experimental results are presented and used to demonstrate the effectiveness of the proposed method.