Beat Frequency Oscillation Analysis for Power Electronic Converters in DC Nanogrid Based on Crossed Frequency Output Impedance Matrix Model

The interaction of power electronic converters in a microgrid can introduce system instability and power quality issues. Existing investigations focus on interactions either in low-frequency regions, such as the constant power load, or in very high-frequency regions like electromagnetic interference. However, interactions of power converters around their switching frequency range are not included. In fact, the interaction of dc-dc converters with different switching frequencies can introduce beat frequency oscillation in certain cases. Since additional frequency component (beat frequency) is generated, traditional impedance concept is no longer the tool for beat frequency oscillation analysis and new models need to be developed. In this paper, a crossed frequency output impedance matrix model is proposed to describe the terminal characteristics of a dc-dc converter around its switching frequency range. A high-frequency equivalent circuit model for the converter is then developed to predict the beat frequency oscillation in parallel and cascaded systems, which results from the interaction of power converters with different switching frequencies. Finally, design guidelines are proposed to avoid potential beat frequency oscillation in a dc nanogrid. Experimental results for both parallel and cascaded systems validate the accuracy and effectiveness of the proposed prediction method and design guidelines.