An Impedance-Based Stability Assessment Methodology for DC Distribution Power System With Multivoltage Levels

Impedance-based methods are effective for stability assessment of cascaded systems. However, the dc distribution power system (DPS) is becoming increasingly complex, incorporating multivoltage levels, many dc buses, and numerous converters. Consequently, conventional stability criteria, such as Middlebrook criterion and its extensions, may not have the ability to judge stability very succinctly. Thus, more research work related to stability evaluation of dc DPS with multivoltage levels needs to be carried out. This article proposes an impedance-based stability assessment methodology. First of all, a unified form of the system is obtained regardless of its structures and operating modes, based on classification of any converter as either a bus voltage-controlled converter or a bus current-controlled converter. Then, according to the two-port small-signal model, the equivalent loop gain of the system is derived precisely. It should be noted that intermediate bus converter could adopt output current or output voltage control modes, which will have influence on the equivalent loop gain expression. After that, the stability requirement of the system is given by introducing the Nyquist criterion to the equivalent loop gain. Finally, an experimental prototype is established to validate the effectiveness of the proposed criterion.