DC power systems have been discussed for connection of renewable, small-scale power generation units. The investigated DC power system is based on a ring bus structure with voltage source converters connected back-to-back (Fig. 1). This paper addresses small-signal analysis of DC distributed power systems with emphasis on converter modeling and DC bus voltage control. Droop control (Fig. 2) is utilized to distribute the load between the source converters. The dynamic properties in terms of small-signal stability are investigated both for the converters and for small DC distribution systems. First a two-converter system and then a three-converter system are investigated. Stability is investigated in terms of frequency responses and pole-zero maps. It is found that the converter models do not have to be taken into account. It is also found that the negative equivalent small-signal resistance of constant power loads does not cause instability, for the proposed converter and investigated DC bus parameters. Load sharing is also investigated with frequency responses. Simulation results based on the analyzed small-signal model is compared to both large-signal simulations and experimental results.
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