Considerations on the impedance character and impedance criterion in two-stage dc distributed power system

Impedance character as well as relevant impedance criterion is important in the small-signal stability analysis of multi-stage dc distributed power systems, where subsystems are integrated. At the same time, we usually approximately simplify a tightly controlled load-converter as constant power load. In this paper, we focus on the impedance character and impedance criterion in two-stage DC distributed system, which is in fact inter-dependent dynamical system with both interaction loop and many individual regulation loops. We verify that minor loop gain method can be directly extended to inter-dependent subsystems, while original impedance criterion cannot. Furthermore, we develop a substitutional impedance criterion, which is more efficient on small-signal stability analysis. With this substitutional impedance criterion, we discuss the influence of regulation loop gain, load resistance, input voltage and various duty-cycle ratios on system small-signal stability. In addition, we analyze the impedance shift process of the input impedance of load-converter and build the relationship between it and regulation loop gain. Negative resistor approximation of load-converter is only regarded reasonable when high regulation-loop gain is assumed and low-frequency character of load-converter is considered. Simulation results illustrate the validity of the substitutional impedance criterion.