Decentralized Impedance Specifications for Paralleled DC Distribution Power System With Multiple-Voltage-Level Buses

The dc distribution power system (DPS) has witnessed rapid development in recent years, and it is gradually characterized by multiple voltage levels, multiple dc buses, and multiple converters. The complex topology of the system leads to some serious system stability problems. Compared with the cascaded dc DPS with multiple-voltage-level buses, the paralleled dc DPS with multiple-voltage-level buses is of outstanding advantage because of the higher efficiency in the case of more dc buses. To study the small-signal stability of the paralleled dc DPS with n dc buses, a simplified small-signal model of the system is first established in this article. Then, a stability criterion based on the equivalent loop gain of the system is proposed. Furthermore, the decentralized impedance specifications for the studied system are proposed to ensure the system's stability and improve the scalability of the system. First, the stability conditions for the design of impedance specifications are given. On this basis, the impedance specifications for the Z-type converter and dc transformers are proposed, which are only related to the dc bus voltage, the rated power, and the desired stability margin. Finally, sufficient case studies and experiments were conducted to validate the proposed criterion and impedance specifications.