Small-Signal Stability Assessment and Interaction Analysis for Bipolar DC Microgrids

Abstract

Bipolarity in dc microgrids is desirable as it enhances the system's reliability and efficiency. However, the stability assessment for a bipolar dc microgrid is challenging due to the integration of a three-wire dc distribution line and numerous connected power converters, which is different from the stability analysis of a conventional unipolar dc microgrid. In this article, the basic form is arranged by a generalized voltage source or current source, and the simplified form is derived by looking into the different bus ports of a bipolar system in detail. Then, an impedance sum criteria-based stability conditions of different bus ports in the bipolar dc microgrid are proposed. To explore the mutual influence of the stability among different bus ports and investigate the stability issues caused by unbalanced loads connected to the symmetrical bus port, three cases are studied: 1) ± <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> bus ports are connected with balanced and unbalanced loads; 2) − <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> bus port introduces the photovoltaic unit and the energy storage unit; and 3) the 2 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> bus port introduces the photovoltaic unit and the energy storage unit. Finally, a ±24 V bipolar dc microgrid platform is set up to conduct experiments, verifying the accuracy and effectiveness of the stability evaluation method.