A frequency analysis of the small-signal voltage model of a MVDC power system with two cascade DC-DC converters

Innovative Medium Voltage DC (MVDC) distribution starts to play a crucial role in the future shipboard power systems. The analysis of stability conditions and the study of the frequency behavior of these systems are of primary importance nowadays. The authors have already proposed a model of a power system with one controlled load converter, achieving some stability conditions based on the load controller bandwidth in relation with the system parameters. In this paper, the power system, composed by two cascaded DC-DC converters, is analyzed starting from a different point of view. After the linearization around its equilibrium point, it is modeled according to a feedback Lure scheme. The input variable of the feedback loop is the output voltage of the feeder controller, while the output variable is the duty cycle of the load buck. The stability and the frequency behavior are studied both with the Nyquist criterion and the margin phase method. Furthermore, the influence on stability of both the feeder filter and the control bandwidth, together with their mutual interaction, is discussed. An example taken from a shipboard power system is used to validate the proposed model.